Engineering Metallurgy

PITMAN METALLURGY SERIES

FRANK T. SISCO, Advisory Editor

Engineering

Metallurgy

Engineering

pitmanPUBLISHING CORPORATION

>*

Metallurgy

By

THE COMMITTEE ON METALLURGY

-4 collaborative writing group

of metallurgy professors.

NEW YORK • TORONTO • LONDON

Copyright ©, 1957

BY

PITMAN PUBLISHING CORPORATION

All rights reserved. No part of this bookmay l>e reproduced in any form withoutthe written permission of the publisher.

1.2

Associated Companies

Sir Isaac Pitman & Sons, Ltd.London Mcllraurne Johannesburg

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PRINTED in the United States of America

Coauthors

Theodore Allen, Jr., M.S.M.E., Associate Professor of Mechanical En-

gineering, University of Houston, Houston, Texas; Engineer Associated

with Anderson, Greenwood and Co., Bellaire, Texas

Lee L. Amidon, M.S.M.E., Professor and Head, Department of Mechani-cal Engineering, South Dakota State College, Brookings, South Dakota

John K. Anthony. M.S.. Associate Professor of Physical Metallurgy, Uni-versity of Arizona, Tucson, Arizona

Robert E. Bannon, S.M., Professor of Metallurgy, Newark College of

Engineering, Newark, New Jersey

Francis William Brown, Ph.D., Associate Professor, Clarkson College

of Technology, Potsdam, New York

Frederick Leo Coonan, D.Sc, Professor and Chairman, Department of

Metallurgy and Chemistry, U.S. Naval Postgraduate School, Monterey,

California

Howard P. Davis, M.S., Associate Professor, Department of Mechanical

Engineering, University of Wyoming, Laramie, Wyoming

Harold Vincent Fairbanks, M.S., Professor of Metallurgical Engineering,

West Virginia University, Morgantown, West Virginia

Mars G. Fontana, Ph.D., Professor and Chairman, Department of Metal-

lurgical Engineering, The Ohio State University, Columbus, Ohio

Arthur R. Foster, M.Eng., Associate Professor of Mechanical Engineer-

ing, Northeastern University, Boston, Massachusetts

Arthur C. Forsyth, Ph.D., Associate Professor of Metallurgical Engi-

neering, University of Illinois, Urbana, Illinois

Richard Edward Grace, Ph.D., Associate Professor of Metallurgical En-

gineering, Purdue University, Lafayette, Indiana

Leonard B. Gulbransen, Ph.D., Associate Professor, Washington Uni-versity, St. Louis, Missouri

Joseph Gurland, D.Sc, Assistant Professor, Division of Engineering,

Brown University, Providence, Rhode Island

Walter R. Hibbard, M.S., Adjunct Associate Professor of Metallurgy,

College of Engineering, University of Bridgeport, Bridgeport, Con-necticut

vi Coauthors

Walter M. Hirthe, M.S.M.E., Assistant Professor of Mechanical En-

gineering, College of Engineering, Marquette University, Milwaukee,

Wisconsin

Abraham Eldred Hostetter, Ph.D., Professor of Metallurgy, Kansas

State College, Manhattan, Kansas

John J. Kaufman, Metallurgy Department, Academy of Aeronautics,

Flushing, New York

J. Edward Krauss, M.S., Head, Department of Mechanical Technology,

New York City Community College, Brooklyn, New York

Hollis Philip Leighly, Jr., Ph.D., Chairman, Department of Metallurgy,

University of Denver, Denver, Colorado

IrvingJ. Levinson, M.S., Professor of Mechanical Engineering, Lawrence

Institute of Technology, Detroit, Michigan

Jules Washington Lindau, III, M.E., Associate Professor of Mechanical

Engineering, The University of South Carolina, Columbia, South

Carolina

James R. MacDonald, Ph.D., Chairman, Department of Mechanical En-

gineering, School of Engineering, The University of Mississippi, Uni-

versity, Mississippi

Omar C. Moore, M.S., Associate Professor of Chemical Engineering,

Alabama Polytechnic Institute, Auburn, Alabama

Don R. Mosher, B.S., Assistant Professor of Mechanical Engineering,

University of Colorado, Boulder, Colorado

Charles Arthur Nagler, Ph.D., Associate Professor, Department of

Chemical and Metallurgical Engineering, Wayne State University,

Detroit, Michigan

Richard O. Powell, College of Engineering, Tulane University, NewOrleans, Louisiana

Oran Allan Pringle, M.S.M.E., Associate Professor of Mechanical En-

gineering, University of Missouri, Columbia, Missouri

Kenneth E. Rose, M.S., Professor of Metallurgical Engineering, Uni-

versity of Kansas, Lawrence, Kansas

Philip C. Rosenthal, M.S., Professor of Metallurgical Engineering, Uni-

versity of Wisconsin, Madison, Wisconsin

Robert E. Shaffer, M.S., Associate Professor of Engineering, University

of Buffalo, Buffalo, New York

Coauthors vii

Walter E. Short, M.S.M.E., Associate Professor of Mechanical Engineer-

ing, Bradley University, Peoria, Illinois

Floyd Sheldon Smith, M.S., Associate Professor of Mechanical Engineer-ing, Alabama Polytechnic Institute, Auburn, Alabama

GEORGE V. Smith, Ph.D., Assistant Director for Metallurgical Engineer-ing, School of Chemical and Metallurgical Engineering, College of

Engineering, Cornell University, Ithaca, New York

Sicmund Levern Smith, M.Met.E., Professor of Metallurgy, College ofMines, University of Arizona, Tucson, Arizona

Joseph William Spretnak, Ph.D., Professor of Metallurgical Engineer-ing, The Ohio State University, Columbus, Ohio

Rocer Greenleaf Stevens, Ph.D., Head, Department of Chemical En-gineering, Southwestern Louisiana Institute, Lafayette, Louisiana

William H. Tholke, B.S., Instructor of Metallurgy, University of Cin-cinnati, Cincinnati, Ohio

John Stanton Winston, M.A., M.S., Chairman, Department of Metal-lurgy, Mackay School of Mines, University of Nevada, Reno, Nevada

Preface

Engineering Metallurgy was developed to present in a concise, under-

standable manner the principles of ferrous and nonferrous metallurgyfor all engineers—student and practicing. Both graduate and undergradu-ate student engineers need a fundamental knowledge of the metals they

will employ in their work. The emphasis, throughout therefore, is onmetallurgical principles rather than on handbook information; however,specific data are given so as to provide a realistic structure to reinforce

the theoretical presentations. The practicing engineer who has had little

contact with the field of metallurgy, or who has had no formal work in

metallurgy, will find in this book a sufficiently complete summary of all

of the essentials he needs to know to obtain a broad understanding of

the field.

Keeping up with metallurgical developments in all branches of the

art, as reported in the technical literature of the world, is difficult for

those actively engaged in the manufacture, processing, or the industrial

use of engineering metals and alloys. For the thousands of such indi-

viduals, who work with or who use metallic materials but who cannotpossibly find time to read everything, summaries such as this book havea well-defined place in the scheme of things.

Because metallurgy is such a dynamic and diversified art and science,

the preparation of a definitive, up-to-date, authoritative work in this field

required a bold approach. Forty professors actually engaged in teachingengineering metallurgy in universities across the country were asked to

pool their knowledge and research to produce this text. Through inten-

sive questionnaire techniques, the scope and content of the book werefirst defined and outlined by the group. Once the basic content had beenselected, ideas for all chapters were channeled to experts selected to

serve on individual chapter committees. From these ideas and their owncombined experience and research, each committee built chapter out-

lines. Overlaps and omissions were detected by the editorial staff andreferred to the committees for alteration and preparation of rough draft.

The coauthors read and checked the smoothness of presentation of thechapter, adding to and refining the draft. Thus were built the twenty-three chapters of Engineering Metallurgy by the forty coauthors.

Chapters 1 through 6 deal with the general principles of metallurgyas they are related to engineering. Chapter 3 (Factors Affecting En-gineering Properties) will be especially useful to the student in gaining

ix

x Preface

an appreciation of the over-all study of engineering metallurgy. In Chap-ter 6 (Phase Diagrams and the Simply Alloy Systems) , the student is

introduced to basic problems of equilibrium and alloying. Chapters 7

through 10 treat of the nonferrous metals and alloys. To these important

materials, a generous amount of space has been devoted so that com-plete coverage could be obtained. Chapters 1 1 through 20 arc concerned

with iron and steel—with special attention given to the subjects of heat

treatment and ferrous alloys. Machinability, corrosion, and the effects

of temperature are fully covered in Chapters 21 to 23.

All technical terms are defined as they are introduced, and stress is

laid upon fundamental concepts. At the end of each chapter there is

a set of questions and exercises constructed to help the student focus at-

tention on the important definitions and principles presented in the

chapter.

Principles, definitions, and illustrative examples are set down with

precision and clarity. Drawings and photographs are used without re-

serve to amplify the discussion. In certain chapters, detailed tables are

included for the convenience of the reader.

The Committee on Metallurgy is aware that the usage of this text anddevelopments in the field will indicate areas needing revision. Professors

and students are therefore urged to send comments on chapters to the

publisher or chapter committees so that appropriate changes may be

made in the next edition.

The Editor

Contents

Preface v

Chapter 1. Metallurgy and Engineering 1

1.1. Metallurgy as an Art 1

1.2. Metallurgy as a Science 2

1.3. Metallurgy and Engineering 3

Chapter 2. Fundamental Structure of Metals and Alloys 5

2.1. Building Blocks of Matter 5

2.2. The Building-Up Principle 9

2.3. Types of Bonding in Solids 9

2.4. Assemblages of Atoms 11

2.5. Defects in Crystals 14

2.6. Polycrystalline Aggregates 16

2.7. Interactions in Metallic Solutions 18

2.8. Liquid Solutions 19

2.9. Solid Solutions 20

2.10. Intermediate Phases 20

Chapter 3. Factors Affecting Engineering Properties ... 23

3.1. Grain Size Control 24

3.2. Effect of Grain Size on Properties 29

3.3. Deformation of Metals 32

3.4. Slip in Single Crystals 33

3.5. Twinning 36

3.6. Deformation of Polycrystalline Metals 37

3.7. Hot Working 38

3.8. Cold Working 40

3.9. Annealing Cold Worked Metal 42

3.10. Factors Affecting Rccrysiallization Temperature and Grain

Size 45

xi

xii Contents

3.11. Summary of Hot and Cold Working : 46

3.12. Solid Solution Effects 47

3.13. Polyphase Structures 49

3.14. Allotropic Transformation 51

3.15. Precipitation Hardening 52

Chapter 4. Static Properties of Metallic Materials ... 57

4.1. Properties of Metallic Materials 57

4.2. The Relative Standardization of Static Tests 60

4.3. Tensile Strength 61

4.4. Elastic Limit, Proportional Limit, and Modulus of Elas-

ticity 63

4.5. Yield Point and Yield Strength 66

4.6. Elongation and Reduction of Area 67

4.7. Hardness 69

4.8. Comparison of the Various Hardness Tests 72

4.9. Relation among Hardness Tests and between Hardness

and Tensile Strength for Steel 73

4.10. Relation among Hardness Tests and between Hardness

and Tensile Strength for Nonferrous Alloys .... 74

4.11. Shear, Compression, and Bend Tests 77

4.12. Sonic Testing 78

Chapter 5. Dynamic Properties of Metallic Materials . . 81

5.1. Notch Brittleness and Transition from Ductile to Brittle

Fracture 81

5.2. Value of the Notched-Bar Impact Test 85

5.3. The Mechanism of Fatigue 86

5.4. The Endurance Limit 87

5.5. Relation of the Endurance Limit to Other Properties . . 88

5.6. The Effect of Notches on the Endurance Limit .... 89

5.7. Corrosion Fatigue 92

5.8. Increasing the Endurance Limit by Shot Pcening ... 92

5.9. Significance of Endurance Data 93

5.10. Damping Capacity 94

Chapter 6. Phase Diagrams and the Simple Alloy Systems 97

6.1. Solid State Thermodynamics and Thermostatics ... 97

Contents xiii

6.2. Concept of Dynamic Equilibrium 97

6.3. Cooling Curves 98

6.4. Solid State Mass Transfer 99

6.5. The Phase Rule 100

6.6. Solid State Solubility 101

6.7. Binary Systems 101

6.8. Intermetallic Compounds 106

6.9. The Peritectic Reaction . • 108

6.10. Closure 109

Chapter 7. Heat Treatment of Alloys by PrecipitationHardening Ill

7.1. Alloy Requirements . Ill

7.2. Step I—Solution Heat Treatment 112

7.3. Step II—Precipitation Heat Treatment (Aging) .... 1 14

7.4. Theory of Precipitation Hardening 114

7.5. The Effect of Time and Temperature During Precipitation

Heat Treatment 117

7.6. Precipitation Hardening of Aluminum Alloys .... 119

7.7. Precipitation Hardening of Magnesium Alloys .... 121

7.8- Precipitation Hardening of Copper Base Alloys .... 124

7.9. Precipitation Hardening in Low Carbon Steel .... 125

7.10. Strain Aging and Its Consequences in Low Carbon Steel 126

7.11. Precipitation Hardening in Alloy Steels 126

Chapter 8. Light Alloys as Engineering Materials .... 129

8.1. History of Aluminum 129

8.2. Economics of the Aluminum Industry 130

8.3. Aluminum Ores—Occurrence and Concentration . . . . 131

8.4. Manufacture of Aluminum 132

8.5. Physical Properties of Aluminum 133

8.6. Chemical Properties of Aluminum 134

8.7. Aluminum Alloys 135

8.8. The Wrought Alloys 136

8.9. Aluminum Casting Alloys 140

8.10. Functions of Alloying Elements 143

8.11. Cold and Hot Working of Aluminum Alloys 147

xiv Contents8.12. Heat Treatment of Aluminum Alloys 148

8.13. Corrosion Resistance of Aluminum Alloys 150

8.14. Joining of Aluminum Alloys 152

8.15. Magnesium and Its Alloys 155

8.16. Beryllium 158

8.17. Titanium 159

Chapter 9. Copper and Copper-Base as Engineering Materials 161

9.1. Properties and Uses of High-Purity Copper 164

9.2. Constitution of the Common Copper-Rich Alloys . . . 165

9.3. Nomenclature of the Copper-Rich Alloys 167

9.4. Characteristics and Uses of the High Brasses 168

9.5. Characteristics of the Low Brasses and the Nickel Silvers 170

9.6. Properties of the Wrought Brasses 171

9.7. Cast Brass and Cast Nickel Silver 172

9.8. The Copper-Base Bearing Metals 172

9.9- The Tin Bronzes 174

9.10. Aluminum Bronze and Copper-Silicon Alloys 176

9.11. Copper-Beryllium Alloys 177

9.12. The Copper-Rich Copper-Nickel Alloys 184

9.13. Copper and Copper-Base Alloys in Powder Metallurgy . . 186

Chapter 10. Miscellaneous Heavy Nonferrous Metals andAlloys 190

10.1. The White Metals 191

10.2. Lead and Tin as Engineering Materials 196

10.3. Zinc and Zinc Base Alloys as Engineering Materials . . . 197

10.4. Nickel and Nickel Base Alloys 199

10.5. Cobalt Base Alloys 201

10.6. Other Miscellaneous High Temperature Metals and Alloys 201

10.7. The Precious and Semi-Precious Metals and Alloys . . . 202

10.8. Metals and Alloys in Atomic Power Applications .... 203

Chapter 11. The Manufacture and Composition of Carbonand Alloy Steels 208

11.1. Definitions of Ferrous Engineering Materials 210

11.2. Iron Ore and the Manufacture of Pig Iron 212

11.3. Acid and Basic Processes 216

Contents xv

11.4. Bessemer Processes 21711.5. Open-Hearth Processes 220

11.6. Manufacture of High-Quality Steels by the Electric

Processes 226

11.7. Wrought Iron 227

11.8. Special Steel-making Processes 22911.9. Mechanical Treatment of Steel 231

11.10. Harmful Elements in Carbon and Alloys Steels .... 23311.11. Manganese in Carbon and Alloy Steels 236

11.12. Carbon Monoxide, and Rimming and Killed Steels . . . 237

11.13. Silicon and Other Degasifiers 239

11.14. Other Elements 239

11.15. Low-Alloy Steels. . 240

11.16. High-Alloy Steels 241

Chapter 12. The Constitution of Steel 24412.1. The Allotropy of Iron 24412.2. Iron-Carbon Phase Diagram 245

12.3. Phase Changes and Microstructures of Slowly Cooled PlainCarbon Steels 247

12.4. Isothermal Transformation in Steel 251

12.5. The Effect of Cooling Rate Upon the Resulting Structure 254

12.6. Effect of Alloying Elements Upon the Iron-Carbon Dia-

g™™ 256

12.7. Effect of Alloying Elements on the Isothermal Transfor-mation of Steel 257

12.8. Effect of Hot Working on Structure 25812.9. Effect of Cold Working on Structure 259

Chapter 13. Fundamentals of Heat Treatment of Steel . . 263

13.1. Grain Size and Grain Growth 264

13.2. Controlling and Classifying Grain Size 265

13.3. Effects of Hot Working on Grain Size 267

13.4. Hardenability 26913.5. Grain Size and Hardenability 27013.6. Quenching and Properties of Martensite 270

13.7. The Instability of Quenched Carbon Steels 273

XVI Contents

13.8. Retained Austenite and Cold Treatment 274

13.9. Structural and Other Changes in Tempering 274

Chapter 14. The Operations of Heat Treatment 278

14.1. Heating Cycle 279

14.2. Annealing 280

14.3. Normalizing 280

14.4. Spheroidizing 281

14.5. Quenching for Hardening 282

14.6. Tempering 284

14.7. Isothermal Treatments 285

14.8. Case Hardening Processes ' 289

14.9. Flame Hardening and Induction Hardening 293

Chapter 15. Carbon Steel as an Engineering Material ... 297

15.1. Carbon-Steel Castings as Engineering Materials .... 298

15.2. Factors Affecting the Properties of Carbon-Steel Castings 300

15.3. Hot-Worked Carbon Steels as Engineering Materials . . 302

15.4. Effect of Composition on Static Properties of Hot-WorkedCarbon Steels 303

15.5. Effect of Composition on Other Properties 305

15.6. Cold-Worked Carbon Steels as Engineering Materials . . 306

15.7. The Important Properties of Cold-Worked Steel ... 307

15.8. General Effects of Cold Working on Strength and Ductility 308

15.9. Variables Affecting the Properties of Cold-Worked Wire . 311

15.10. Effect of Cold-Working on Dynamic Properties . . . . 312

15.11. Heat-Treated Carbon Steels as Engineering Materials . . 312

15.12. Effect of Section Size on the Properties of Heat-Treated

Medium-Carbon Steels 315

Chapter 16. Low-Alloy Steels as Engineering Materials . . 318

16.1. Balanced Compositions in Low-Alloy Steels 319

16.2. General Effects of the Alloying Elements on Carbon Steel 320

16.3. Effects of Phosphorus, Manganese, and Silicon .... 322

16.4. Effects of Nickel and Chromium 324

16.5. Effects of the Other Common Alloying Elements .... 325

16.6. Low-Alloy Structural Steels as Engineering Materials . . 326

Contents xvii

16.7. Composition and Properties of the Low-Alloy Structural

Steels 327

16.8. The S.A.E. Low-Alloy Steels 328

16.9. The S.A.E. Low-Alloy Steels as Engineering Materials . . 331

16.10. The New Metallurgy of Low-Alloy Steels 333

16.11. Similarity of Properties of Heat-Treated S.A.E. Low-AlloySteels 334

16.12. The Engineering Properties of the S.A.E. Low-Alloy Steels 337

Chapter 17. Hardf.nability 342

17.1. Hardness and Hardenability in Carbon Steels .... 343

17.2. Hardness and Hardenability in Low-Alloy Steels ... 344

17.3. Cooling Rate and Hardenability 344

17.4. Time Delay and Hardenability 346

17.5. Variables Affecting Hardenability 346

17.6. Methods of Determining Hardenability 348

17.7. The Jominy End-Quench Hardenability Test 349

17.8. Relation of the End-Quench Test to Actual Cooling Ratesand the Selection of Steel by Hardenability 350

17.9 Virtues and Shortcomings of the Jominy End-Quench Test 352

17.10. Hardenability Bands 353

17.11. Relation of Hardenability to Engineering Properties . . 354

17.12. Relation of Tempering to Hardenability 356

17.13. Fundamentals of Calculated Hardenability 356

17.14. The Accuracy of Calculated Hardenability 360

17.15. The Effect of Boron on Hardenability 361

Chapter 18. Special Purpose Steels 364

18.1. Classes of Stainless Steels 364

18.2. Constitution of High-Chromium Steels 366

18.3. Relation of the Constitution of High-Chromium Steels to

Their Heat Treatment 367

18.4. Mechanical Properties of High Chromium Steels ... 369

18.5. Corrosion and Oxidation Resistance of High-ChromiumSteels 370

18.6. The Constitution of 18-8 372

18.7. The Role of Carbon in 18-8 374

xviii Contents

18.8. Properties of 18-8 376

18.9. Recent Developments in Stainless Steels 378

18.10. Superstainless Steels 381

18.11. High-Nickel Steels and Special Iron-Nickel Alloys ... 384

18.12. Austenitic Manganese Steel 385

Chapter 19. Tool Steels, Die Steels, and Cemented HardCarbides 389

19.1. High-Carbon Tool Steels 390

19.2. Low Alloy Tool Steels 395

19.3. Medium Alloy Tool and Die Steels 396

19.4. High-Alloy Tool and Die Steels 397

19.5. High Speed Steels. 398

19.6. Function of Alloy Additions in Tool and Die Steels . . 402

19.7. Cast Alloys 404

19.8. Cemented Carbide Tools 405

Chapter 20. Cast Iron 408

20.1. White Cast Iron as an Engineering Material 410

20.2. Malleable Cast Iron as an Engineering Material .... 410

20.3. Engineering Properties of Malleable Cast Iron .... 410

20.4. Gray Cast Iron as an Engineering Material 412

20.5. Structure of Gray Cast Iron 414

20.6. Relation between Properties and Structure of Gray Cast

Iron 415

20.7. Effect of Cooling Rate 416

20.8. Effect of Graphite Size on Structure and Properties . . 417

20.9. Evaluation of Gray Cast Iron for Engineering Applications 419

20.10. Nodular or Ductile Cast Iron as an Engineering Material 420

20.11. Melting and Casting of Irons 422

20.12. Ternary System of Iron, Carbon, and Silicon 425

20.13. Heat Treatment of Cast Iron 430

20.14. Normal and Alloy Elements in Cast Iron 434

Chapter 21. Machinability, Wear Resistance, and Deep-Draw-

ing Properties 438

21.1. Variables Affecting Machinability 438

21.2. Evaluation of Metallic Materials for Machinability . . . 438

Contents xix

21.S. Free Machining Steels 440

21.4. Relative Machinability of Steel and Nonferrous Alloys . 441

21.5. Types of Wear 442

21.6. Variables Affecting Wear Resistance 443

21.7. Evaluation of Steel for Wear Resistance 444

21.8. Importance of Deep-Drawing Properties 445

21.9. Evaluation of Steels for Deep Drawing 446

21.10. Yield-Point Elongation, Stretched Strains, and Deep Draw-ing Properties 448

21.11. Deep-Drawing Properties of Nonferrous Alloys .... 449

Chapter 22. Corrosion and Oxidation 452

22.1. Electrochemical Corrosion 45222.2. EMF Series and Passivity 454

22.3. Uniform Corrosion 456

22.4. Galvanic or Two-Metal Corrosion 456

22.5. Concentration Cell Corrosion 458

22.6. Pitting 459

22.7. Intergranular Corrosion 460

22.8. Stress Corrosion 462

22.9. Dezincification. 466

22.10. Erosion-Corrosion 468

22.11. Methods for Combating Corrosion 469

22.12. Corrosion Testing 471

22.13. Liquid-Metal Corrosion 472

22.14. High-Temperature Oxidation 472

22.15. Formation of Oxides 473

22.16. Dependence of Oxides Growth upon Gas Pressure . . . 474

22.17. Dependence of Oxide Growth Upon Time 477

22.18. Dependence of Oxide Growth Upon Temperature . . . 479

22.19. Oxidation Prevention 480

Chapter 23. Effect of Temperature on Mechanical Propertiesof Metals 485

23.1. The Importance of Creep 486

23.2. The Engineering Significance of Creep 486

23.3. The Creep to Rupture Curve 488

xx Contents

23.4. Determination of Creep 489

23.5. Effect of Variables on Creep 491

23.6. Fatigue and Combined Fatigue and Creep 493

23.7. Structural Changes During Creep 495

23.8. Variation of Other Properties with Temperature . . . 498

23.9. Design for Elevated Temperature Service 499

23.10. Variation Mechanical Properties at Reduced Tempera-

tures 500

23.11. Effects of Metallurgical Variables 502

23.12. Design for Low-Temperature Service .... ... 503

Index 507

Engineering

Metallurgy

CHAPTER

Metallurgy andEngi nee r i ng

1 Don R. Mosher, B.S., Assistant Professor of Mechanical

Engineering, University of Colorado, Boulder, Colo-

rado

1.1. Metallurgy as an Art

METALLURGICAL knowledge had its begin-

nings in the stone age when some ancient craftsman first recognized the

difference in behavior amongst certain of the "rocks" with which he wasworking. The stones familiar to this primitive artisan were hard andbrittle, and were capable of being fashioned into tools and weapons onlyby tedious shaping, chip by chip. His first encounter with metal then,

undoubtedly native copper or gold, must have been an exciting exper-ience. Here was a substance which, instead of fracturing, yielded to the

blows of his stone hammer. Here was a material which could be formedmuch more precisely, which was pleasing to the eye, which could beworked to a keener edge, and which, moreover, somehow acquired greater

strength the more it was worked.

From the use of native metal to the deliberate reduction of an ore bycarbon is a long step, and one which is the subject of much interesting

speculation. It seems likely that the first such reduction was accidental-possibly the result of a fortunate combination of circumstances in whichthe heat of a campfire, together with carbon from the charred logs

succeeded in reducing copper ore contained in the surrounding stones,

On other occasions the stones may have contained additional metals suchas tin, and the result was a remarkably stronger metal. In time, the rela-

tionship between the fire, the charred logs, and the particular types of

stones was recognized, and the discoverer, the first metallurgist, began to

produce metals at will.

He and his progeny of the next several thousand years have accumu-lated a mass of information concerning the effects of variables in pro-cessing upon the properties of the final product.

1

2 Engineering Metallurgy

1.2. Metallurgy as a Science

Questions concerning the reasons why these variables resulted in the

observed effects must certainly have been present in the minds of these

pioneers long before the means were available to answer them. With the

advent of the microscope and the X-ray, these inquiring minds began to

supply the answers.

The science of metallurgy really began when Sorby, a British scientist,

reported in 1864 the results of his investigations on the use of the micro-

scope to study the structure of meteoric iron. This was followed bystudies in the same general field by Martens in Germany, reported in

1878. The work of these two scientists, however, aroused little interest

at the time, and nothing further was accomplished until Sorby showedto the British Iron and Steel Institute in 1886 some photomicrographs of

iron and steel. This aroused much interest in the internal structure of

metals, and from 1890 to 1920 many distinguished metallurgists devoted

themselves to developing a science of physical metallurgy.

The paramount early problem of metallurgy, which fairly cried for

solution, was that of the hardening of steel—why steel containing con-

siderable carbon was soft when cooled slowly from a red heat but hardwhen cooled rapidly from the same temperature. This problem occupied

most of the workers in the science of metals for more than two decades.

Despite the efforts of many brilliant minds, which resulted in a volumeof published literature that amazes present-day metallurgists, little that

was wholly decisive was accomplished until confirmatory X-ray crystal-

lography methods came into use about forty years ago. Although somecynics say that the X-ray has created more problems than it has solved,

X-ray crystallography has been a useful tool in the study of the structure

of metals and the constitution of alloys.

In the past three decades the science of physical metallurgy haschanged remarkably. Always closely related to chemistry and physics, it

has been greatly affected by the revolution that has occurred in these

two sciences since 1920. The gap between chemistry and physics has

been largely eliminated, and, as these sciences came together, the science

of metallurgy changed from simple speculations on the structure of

metals and alloys, as affected by composition or mechanical or thermaltreatment and as observed by the microscope, to speculations whichinvolve such complex abstractions as spinning electrons, statistical me-chanics, electromagnetic theory, quantum theory, wave mechanics, andthermodynamics.

Present-day physical metallurgists are inclined to smile condescendingly

at the battles over beta iron, cement carbon, and amorphous metal which

Metallurgy and Engineering 3

filled the transactions of the metallurgical societies forty or fifty years ago.It is not at all certain that even broader smiles will not be in order thirtyyears from now over the discussions of free energy, entropy, and mosaicstructure which are filling our journals at the present time. Especiallyapropos in this connection are the words of a venerable man of science,Sir Ambrose Fleming, who presented a paper to the first meeting of thePhysical Society of London in 1873 and who, in a formal address to thesame body of scientists on "Physics and Physicists of the Eighteen Seven-ties,"* summarized his seventy years of experience by saying:

When we come to look back then on the world of physicists during theeighteen seventies, what we find is that their inventions, discoveries of fact, andascertained principles remain with us today of permanent value, forming partof our useful knowledge. But their theories and speculations as to underlyingcauses and nature have nearly all passed away. Perhaps it will also be thesame with our present-day work. If some sixty years hence a fellow of thePhysical Society gives a talk on the physics of the nineteen thirties, he will haveto record the great additions then made to knowledge of physical facts. But hemay also have to say that our explanations and theories concerning them haveall vanished, or at least been replaced by others also destined in turn to passaway.

1.3. Metallurgy and Engineering

Until about forty years ago there was little need for the engineer toknow anything about metallurgy since untreated carbon steel, hot-rolled or cold-drawn, was used for at least 95 per cent of steel structuresand machines. The engineer was interested primarily in four properties-tensile strength, yield point, elongation, and reduction of area-and inhaving available an ample supply of cheap steel which, in addition tomeeting specifications for tensile properties, would machine easily andfabricate readily. It was considered sound engineering practice to buildmachines and structures that would carry a much higher load than wasanticipated; weight was synonymous with quality, and the heavier thestructure the better the design. High factors of safety were used; con-sequently slight variations in quality, such as lack of structural homo-geneity, surface irregularities, and numerous others, made little or nodifference in designing.

This is no longer true. Weight and the strength-weight ratio (tensile

strength divided by specific gravity) have become very important. Underthe leadership of the automotive and aircraft industries engineers havecome to realize that excess weight not only indicates poor design butis an inexcusable economic loss. The experience of the automotive and

•Nature, v. 143, 1939, pp. 99-102.


the aeronautical engineer in designing lightweight structures and ma-

chines stimulated similar efforts in other fields of engineering. This is

shown by the recent developments in machine tools, in lightweight rail-

way rolling stock, and even in bridge and building structures.

It is, of course, self-evident that the present-day emphasis on light

weight in engineering design as exemplified by the automobile, airplane,

and the streamlined train is directly related to the development of newtypes of steels and light nonferrous alloys and to new treatments for

these materials. It is a moot question whether the metallurgist or the

engineer was responsible for most of this development. Enthusiastic

metallurgists insist that engineering progress has been the direct result

of metallurgical progress; that engineers only improved their tools, ma-chines, and structures because metallurgical art and science had produced

new metallic materials for the engineer to use. There is no doubt that

many engineers are too conservative and that engineering progress has

at times lagged behind progress in metallurgy. On the other hand, ex-

amples could be cited where the metallurgist did not improve his product

until insistent engineering demand forced it upon him. A discussion of

whether this advance was pioneered by the metallurgist or the engineer

is as futile as arguing whether the egg or the chicken came first; the

essential fact is that important changes have taken place and that the

engineer should know something of the metallurgical progress that has

accompanied his changes in design.

It is, therefore, the purpose of this book to outline the recent develop-

ments in metallurgical art and in metallurgical science. This does not

mean that there are long descriptions of melting and refining, or of

mechanical and heat treatment, or of thermodynamics and wave me-

chanics; it does mean, however, that sufficient details of the present state

of metallurgical art and science are given so that engineers mayrecognize the importance of the variables, inherent in the manufacture

and treatment of metals and alloys, that affect the engineering properties

and the suitability of these materials for engineering applications.

QUESTIONS

1. Distinguish between the art and the science of metallurgy.

2. What research tools have most profoundly influenced the development ofmetallurgical science?

3. To what other sciences is metallurgy closely related?

4. How is progress in engineering and in metallurgy inter-related?

5. Why is it becoming increasingly important for engineers to understand the

fundamentals of metallurgy?

CHAPTER

Fundamental Structure

of Metals and Alloys

Leonard B. Gulbransen, Ph.D., Associate Professor,

Washington University, St. Louis, Missouri

Joseph William Spretnak, Ph.D., Professor of Metal-

lurgical Engineering, The Ohio State University,

Columbus, Ohio

In RECENT years the study of the solid state

and the application of wave mechanics to solid state physics and metal-lurgy has resulted in a much clearer picture of the structure of metals andalloys. Physical and mechanical properties of the metals, such as tensile

strength, ductility, electrical conductivity, diffusion, etc are dependenton structure, sometimes to a marked degree. For this reason it is desir-

able to discuss the structure of solids, and in particular the structure ofmetals and alloys.

2.1. Building Blocks of Matter

All metals are aggregates of atoms. Atoms consist of a nucleus and oneor more planetary electrons. In general, except for applications of thenuclear reactions, it can be assumed that the atomic nucleus consists ofpositively charged protons, and neutrons with no electrical charge. Mostof the mass of the atom is concentrated in the nucleus and is equal to thesum of the masses of the protons and neutrons in the nucleus. Negativelycharged electrons sufficient to balance the positively charged protons inthe nucleus, resulting in an electrically neutral atom, are found outsidethe nucleus. Electrons may be thought of as point particles, with a definitemass and electrical charge; however, their motion must be described interms of an associated wave. The part of science that deals with thisproblem is known as wave mechanics. The fundamental equation de-


scribing the motion of an electron and accompanying wave is De Broglie's

equation:

A = h/mV

where h is Planck's constant (6.62 x 10—27 erg-sec) , m is the mass of the

electron, V is the velocity of the electron, and A is the associated wave-

length. Practical use of this equation is made in electron diffraction

equipment. Application of De Broglie's equation, and Heisen berg's un-

certainty principle, which states that the position and momentum (mV)

of a particle cannot be exactly determined simultaneously, result in a

somewhat different idea of atomic structure than the classical picture of

a nucleus and planetary electrons in definite fixed orbits. Application

of the equations of wave mechanics results in a picture of the atom as a

positively charged nucleus, with electrons in discrete, but "smeared out"

orbits. The limits of the orbit can be described in terms of a probability

function, in which, the probability of finding the electron at the center of

the orbit is maximum, but finite and small probabilities exist for finding

the electron at the limits of the "smeared out" orbit. This description of

atomic structure is sometimes described as an electron cloud picture.

1 s State 2 p State 3 d State

Fig. 2.1. Electron cloud diagrams of hydrogen atom.

In the solution of the wave equation for a given atom, the electrons are

characterized by four quantum numbers; n, I, mtand m s. The quantum

number n is called the principal quantum number, and is related to the

total energy of the electron. Number n may have any integral value from

+ 1 to infinity. The quantum number / is related to the angular mo-

mentum of the electron and may have any integral value from to (n-1) .

The quantum number m, is related to the magnetic moment of the elec-

Fundamental Structure of Metals and Alloys 7

tron and may have any value from to ± I. The quantum number m, is

related to the rotation of the electron about its own axis, and may havevalues of ± i/

2 . The four quantum numbers determine the energy of the

electron in various states, with the result that an atomic system may be

fully described by specifying the values for the quantum numbers for

each electron.

A further result of wave mechanics is the Pauli Exclusion Principle

which states that no two electrons in the same atom can have the samefour quantum numbers. This principle in effect restricts a given electron

to one and only one energy slate in a single atom. If then, two atoms are

brought together to form a molecule, the electrons of each atom mustoccupy different energy states or energy levels. The idea of energy levels

is important with regard to molecule formation and also to the forma-tion of the solid state.

MAXIMUM NUMBEROF ELECTRONS

5P

to5s

Up3D

Us

IS

1062

"i

Fig. 2.2. Energy level diagram of an atom.

As an example of the above principles, consider the hydrogen atom.The hydrogen atom consists of a proton, + 1 charge, and a mass of ap-

proximately one atomic unit. The hydrogen atom must then possess oneelectron. In the lowest energy state, this electron would have an energydescribed by the quantum numbers n = 1, / = 0, m ;

= 0, and m, = + \/2 .

The next atom in the periodic system, helium, with a + 2 charge on the


nucleus must possess two electrons. The quantum numbers describing

these electrons, in order to fulfdl Pauli's exclusion principle, would then

be: n, =1, I, = 0, mj, = 0, m, l= + \/z , and n, = 1, l„ = 0, wijj = 0, and

m sl =— \/z . These two electrons have very nearly the same energy, the

only difference being related to the spin quantum numbers m„, and m tl .

Both electrons are confined to the same major energy level with n = 1, and

only these two electrons can occupy this level. The next atom in the

periodic system, lithium, must possess three electrons. Two of the elec-

trons have the same quantum numbers as those for helium, but the third,

n = 2, I = 0, m = 0, and m, = + i/2 .

A more common method of representing the electronic configuration of

an atom is to use the terminology of the spectroscopist, in which the

principal quantum number n is listed, followed by a letter designating

the quantum number /, and indicating by a superscript the number of

electrons in this particular energy state. For most metallurgical applica-

tions, the difference between energy states with different values for m ( and

m, is so small that it may be neglected. According to this method of

representation the hydrogen atom in its lowest energy state may be

described as being in a (Is) 1 state; the helium atom in a (Is) 2 state; the

lithium atom in a (Isf (2s) • state etc. The quantum number I is desig-

nated by s for I = 0, p for / = 1, d for / = 2, and / for / = 3. By applying

Pauli's exclusion principle, the s state may contain a maximum of two

electrons, the p state a maximum of six electrons, and the d state a maxi-

mum of ten electrons.

ATOMIC NUMBER ELEMENT ELECTIION CONFICUR ATION

1 Hydrogen (Is)1

2 Helium (Is)'

3 Lithium (Is)' (2s) 1

i Beryllium (Is)' (2s) 1

5 Boron (Is)' (2s)'(2p)'

6 Carbon (Is)' (2s)'(2p)'

7 Nitrogen (Is)' (2s) '(2p)3

fi Oxygen (Is)' (2s)'(2p)<

9 Fluorine (Is)' (2s)'(2p)s

10 Neon (Is)' (2s)'(2P)«

ii Sodium (Is)' <2s)'(2p)« (3s) 1

12 Magnesium (Is)' (2s)'(2p)« (3s)»

Fic. 2.S. Table of atoms through atomic number 12 with energy states.


2.2. The Building-Up Principle

Using the principles outlined in the previous section, the electron con-figuration of the elements can be determined through argon, atomicnumber 18. However with the element potassium, atomic number 19,

instead of progressing from a completed 3p energy state to the 3d state,

the next electron falls into a 4s state. Calcium follows with two electronsin the 4s state. This discontinuity in the building up principle results

from the fact that as electrons are added to energy levels further removedfrom the nucleus a resultant shielding of the positively charged nucleustakes place due to the intervening electrons, causing the energy of the 4sstate to actually have a lower value than that of the 3d state. Followingcalcium in the periodic table, the building up of the 3d state occurs in a

normal manner, inside of a completed Is state. The electron configura-tion of nickel, atomic number 28, being (Is) 2 (2s) 2

(2p) " (3s) 2 (3p) 6

(3d) 8 (4s) 2. The electron configuration of element atomic number 29,

copper, then becomes normal with a completed 3d state and a (4s) 1

valence state. Elements with atomic number 21 through 28 are known astransition elements and are characterized by incomplete 3d energy states.

In the second long period of the periodic table a similar series of elementsoccur with incomplete 4d energy states. The group of transition ele-

ments is important to the metallurgist because it includes many of ourmost important metals, such as iron, titanium, vanadium, chromium,manganese, cobalt, and nickel.

ATOMIC NUMBER TRANSITION ELEMENT ELECTRON CONFIGURATION

21 Scandium (Is)' (2s)'(2p)* (3s)'(3p)«(3d)> (4s)'

22 Titanium (Is) 1 (2s)2(2p)' (3s)'(3p)«(3d) 2 (4s)*

23 Vanadium (ls)« (2s)*(2p)« (3s)»(3p)«(3d)' (4s) 8

24 Chromium (Is) 1 (2s)*(2p)° (3s) 2(3p)«(3d) s(4s)'

25 Manganese ' (Is)' (2s)'(2p)° (3s) 2(3p)«(3d)» (4s)'

26 Iron (Is) 2 (2s)'(2p) 6 (3s)'(3p)"(3d)« (4s)2

27 Cobalt (Is) 2(2s) 2(2P)

6(3s) 2(3p)«(3d)' (4s) 2

28 Nickel (Is) 2 (2s)*(2p)« (3s)'(3p)°(3d) 8(4s) 2

Fig. 2.4. Table of atoms of the transition elements.

2.3. Types of Bonding in Solids

All solids can be classified in groups according to the bond formedbetween atoms in the solid state. The four types of bonds that may beformed in solids are:


(1) The Van der Waal's bond

(2) The ionic bond

(3) The covalent bond

(4) The metallic bond.

These various types of bonds may all be analyzed in terms of electron-

electron, electron-proton, proton-proton interaction between atoms.

Van der Waal's bonding occurs between atoms or molecules in solids

due to an induced polarity on one molecule with respect to another. This

type of bond is electrostatic in character and the binding force is relatively

weak. Solids formed by Van der Waal's bonding have low melting and

sublimation temperatures, and low strength. Solids formed by the atoms

or molecules of neon, argon, and iodine are examples of Van der Waal's

solids.

Ionic bonds can be considered to be the result of the transfer of an

electron, or electrons from one atom to another with the net result that

one atom becomes positively charged, and the other negatively charged.

On the basis of the building-up principle, such bonds should tend to

form between atoms with one or two electrons in the valence state and

atoms with one or two vacancies in the valence state. More stable elec-

tronic structures occur for completed energy levels; thus, when two

atoms such as sodium and chlorine are brought together to form a solid,

the valence electron of sodium is accepted by the chlorine atom to com-

plete the valence state of chlorine, leaving a net charge of + 1 on the

sodium atom, and resulting in a net charge of— 1 on the chlorine atom.

The ionic bond is electrostatic in nature and results in a strong bond.

Examples of ionic solids are sodium chloride, magnesium fluoride,

magnesium oxide, and calcium sulfide.

The covalent bond is formed by forces arising from the sharing of

electrons between atoms. The simplest example of this type of bond is

the formation of the hydrogen molecule from two hydrogen atoms. The

lowest energy state for the hydrogen atom is the h state in which m„ =± \/2, and since this state can be occupied by two electrons, it follows that

if a molecule is to be formed between two hydrogen atoms, their elec-

trons must share this lowest energy level. Many elements with more than

one valence electron form crystal structures in the solid state based on

this type of electron bond. To complete an octet of electrons, required

for a stable structure, electrons are shared with 8-N neighboring atoms,

where N is the number of valence electrons in a given element. The

covalent bond is a strong bond, because of the completion of an energy

level; so that covalent solids are generally speaking hard and strong.

Examples of covalent solids are diamond, silicon, and graphite.


The metallic bond has some of the features of both the ionic and co-

valent bond. When metal atoms are brought together to form a solid, a

broadening and possible overlapping of energy levels occurs; so that,

although a discrete number of electrons must occupy a given energy

level, according to the Pauli principle, the electrons in a given energy

level may have more freedom to move throughout the solid lattice underan applied electric field.

Solid

Decreasing interatomic distance

Fig. 2.5. Broadening of energy levels in a solid.

The metallic state is sometimes visualized as a structure consisting of

positive ions arranged periodically in space, with a negatively charged

cloud of electrons surrounding the positively charged core. An explana-

tion of the high electrical conductivity of the metals is based on the idea

of unfilled or overlapping broad energy levels. An applied electric field

in either case may increase the energy of electrons in such bands andpromote them to a conduction band, resulting in a net flow of electrons

in the direction of the applied field. Insulators on the other hand are

characterized by filled and empty levels. Thus, a normal electric field

does not possess enough energy to promote electrons onto a conductionband.

2.4. Assemblages of Atoms

Of the seven crystal systems used by the crystallographer, the twomost important to the metallurgist are the cubic and the hexagonal


systems. Nearly all of the common metals and alloys of commercial im-

portance belong to these two systems. Before discussing metal structures,

it is important to define certain terms used in describing solid structures.

The Space [.attire. A space lattice is defined as a lattice of lines divid-

ing space into equal sized prisms which stand side by side with all faces

in contact, so as to fill space with no voids. All of the points of a space

lattice have identical surroundings. It can be shown that there are only

fourteen possible space lattices.

The Space Group. Starting with a given space lattice, there are manyways of building up actual crystals by placing atoms or molecules at

various points on the space lattice. Actually there are 230 such ways,

called space groups, of forming actual crystals based on the fourteen

space lattices.

The Unit Cell. The unit cell is defined as a network of lines through

the points of a space lattice, dividing it into prisms. Each unit cell in a

space lattice is identical in size, shape, and orientation with every other

unit cell. The unit cell is the building block from which the actual

crystal is constructed by repetition in three dimensions. The three most

important unit cells for the metallurgist are the body-centered cubic,

the face-centered cubic, and the hexagonal close-packed.

/' / 7y y «/

\ i

/ . f // > /

/* / ry / y

/ ¥/• 7

Fie. 2.6. The three unit cells.

In the cubic system all of the cube edges of the unit cell are of equal

length and stand at 90° to one another. In the hexagonal system, three

axes on the basal plane of equal length stand at 120° to one another with

the third axis of different length at 90° to the basal plane.

Special planes and special directions in crystals are of great importance

in plastic deformation of metals, hardening reactions, and other aspects

of the behavior of metal crystals; therefore, it is necessary to have some

method of describing such planes and directions.


The most common method is to use Miller indices and indices of direc-

tion to describe the position of a plane and its direction from a fixed

reference point. The Miller index system is based on the space lattice

and unit cell of the structure in question, and is derived from the inter-

cepts of the plane along the axes of the space lattice. The three steps in-

volved in the determination of the Miller indices of a particular plane

are as follows:

(1) Determine the intercepts of the plane on the three-crystal axes

as a multiple or fraction of the unit cell edge.

(2) Determine the reciprocals of these three numbers.

(3) Reduce the reciprocals to the smallest integers that are in the

same ratio.

The three indices are then enclosed in parentheses. Planes that have the

Plane I

Intercepts x axis = a,,

y axis = a„

z axis = ao

Reciprocals x axis =— =1

y axis = = 1

a„

z axis — 1

Plane II

x axis = a„

y axis = oo

z axis = oo

x axis =— = 1

a.

y axis = - =oo

/. axis =-r— =oo

Miller Index (111) (100)

Fig. 2.7. Determination of Miller indices.

same integers, either positive or negative, but in a different sequence

belong to the same family of planes. The planes (010) , (OlO), (100) etc.

belong to the same family of planes, the [100] family.

Indices of direction are derived by considering the motion that must

be imparted to a point at the origin to move it in a required direction.


The required indices are found by moving the point first along the Xaxis, then along the Y axis, then along the Z axis, in terms of the unit

distance along these axes. The three smallest integers in the same ratio

that will impart die required motion to the point are the indices of

direction. Indices of direction are enclosed in square brackets. Direc-

tions having the same indices, positive or negative, but in a different

sequence belong to the same family. The indices of direction [100],

[O'TO], [001] belong to the same family of equivalent directions <100>.

Note that reciprocals are not used in finding indices of direction.

2.5. Defects in Crystals

Tn idealized representations of metal crystals, a model of perfect crystals

is generally used. In this model, a unit cell is used that possesses no

imperfections and is repeated indefinitely through space with no dis-

registry among the unit cells. Such a model clearly is not representative of

a real metal crystal. If it were, we should commonly experience tensile

strengths of the order of magnitude of millions of pounds per square

inch.

Real metal crystals and |>olycrystalline aggregates are characterized by

the presence of several types of defects. It is convenient to classify these

defects in three categories:

1. Surface imperfections

2. Line imperfections

3. Point or atomic imperfections.

Surface imperfections include the free surface of a crystal, the grain

boundary or interface between two adjoining grains in a polycrystalline

aggregate, interfaces between two solid phases, the interface of a twin and

the parent crystal, and sub-boundaries of the mosaic block structure.

Metal crystals are made up of cubic crystallites about 10 * cm. edge

length, called mosaic blocks. There is a slight disregistry among the

blocks, giving rise to interfaces formed by adjacent blocks.

Line imperfections are termed dislocations. There are two basic types

of dislocations, the "edge" type and the "screw" type. These dislocations

are illustrated in Figure 2.8. The dislocation concept is the basis of the

modern theories of plastic flow in metals.

Atomic imperfections include several types. The lattice vacancy is a

normal lattice point from which the atom is missing. Interstitial defects

consist of either a foreign atom in the interstitial space between normal

lattice sites, or an atom from a normal lattice site removed to an intersti-


POSITIVE DISLOCATION

SCREW DISLOCATION

Fie. 2.8. Dislocations in Metals.

tial site at a considerable distance from the vacancy so formed. Finally aforeign atom may substitute for a host atom on a normal lattice site,

giving rise to a substitutional defect.

The defects listed above account for the flow and fracture character-

istics, electrical properties including semi-conducting behavior, diffusion

mechanisms, and creep characteristics of real metals and alloys. Thesedefects are the basis of the "prior history" effect so well known in metalsand alloys. The study of these defects by metal scientists has a two-foldpurpose; namely (1) a better understanding of them and how theyaffect properties, and (2) exploration of possibilities of minimizing oreliminating these defects. For example, metal whiskers of diameter ofabout 10—

' cm. exhibit essentially theoretical strengths and arc thoughtto be free of dislocations.


Fie. 2.9. Schematic representation of crystallization. (Rosenhain)

2.6. Polycrystalline Aggregates

In technical freezing processes such as the solidification of ingots in

metal molds or castings in sand molds, many nuclei or starting sites be-

come active. A nucleus consists of a crystallite which is large enough to

be thermodynamically stable in the presence of the liquid. Each nucleus

grows until it impinges on its growing neighbors. This process is illus-

trated schematically in Figure 2.9. The impingement process yields metal

grains at the completion of freezing, since on the average, two neighbor-

ing nuclei have different orientations in space. The structure developed

is generally termed a polycrystalline aggregate.


The junction of two neighboring grains gives rise to a zone of dis-

registry termed the grain boundary. The grain boundary is a few atomsin thickness and is essentially disordered in structure, showing viscous

properties since the atoms in this zone belong to the lattice of neitherof the two neighboring grains. Some important manifestations of grainboundaries are the following:

(1) They are barriers to the propagation of slip, the basic mechanismof plastic deformation. The smaller the grain size, the harder is the

metal.

(2) Grain boundaries are high energy sites; thus, they are favored sites

for nucleation of solid state reactions. They may also attract solute atomsin some cases, resulting in a segregation of the foreign atom at theboundary.

(3) Diffusion is faster along grain boundaries than in the bulkof the grain.

(4) At elevated temperatures, grain boundaries may contribute to de-formation in creep by grain rotation.

The grain size range usually encountered is from 10 to 250 microns(micron = 10—* cm.) in diameter. The grain structure is usually studiedwith the aid of a microscope. In certain cases such as cast structures andelectrical sheet, grains are easily discernible by the naked eye. An ex-

ample of a polycrystalline metal is given in Figure 2.10.

Fie. 2.10. Polygonal grains of high purity iron, etched. 500x-


2.7. Interactions in Metallic Solutions

The initial addition of solute atoms of species B to a pure liquid or

solid metal of species A results in a liquid solution or solid solution

respectively. The solubility of B in A may be extensive or restricted.

If B is completely soluble in A, then it is said that B is completely miscible

in A. If B is not completely miscible in liquid A, then a second liquid

phase will appear. In solid metal A, limited solubility will give rise to the

appearance of a second phase, which may occur in the form of another

solid solution, or as an intermetallic compound with a characteristic

crystal structure.

Two characteristics mainly determine the miscibility of B in A, namely

the size factor and the chemical interaction among the species. Theratio of the diameter of the solute atom B to the diameter of the solvent

atom A must be such that B can either (1) substitute for A at a lattice

site forming a substitutional solid solution, or (2) such that B ran fit in

the interstitial space of the A lattice, giving rise to an interstitial solid

solution. The size factor is illustrated in Figure 2.11.

I. Favorable

Favorable for

for isubstitutional

interstitialisolid solutions

solid solutionsj

i

0.59 0.85 1.0 1.15

Ratio of size of solute atom to solvent atom—

»

Fie. 2.11. The size factor in the formation of solutions in metals.

Chemical interaction has to do with the strength of the bond between

atoms. In an assemblage of A and B atoms, the possible pairs of nearest

neighbors are A-A, B-B, and A-B. We can also refer to bond strengths by

these symbols. In solutions, three general types of behavior may be ex-

perienced: ideal solution behavior, positive deviation from ideality, and

negative deviation from ideality. In the case of ideal solution behavior,

the situation in bond strengths is A-A — B-B — A-B. This is to say that

a given A atom has no preference as to an A or a B atom as its nearest

neighbor. (The number of nearest neighbors depends on the symmetry

of the space lattice, and is expressed as the coordination number. Values

for the common lattices are 12 for the face centered cubic and hexagonal

metals, and 8 for the body centered cubic) . In this case complete mis-


cibility in the liquid and solid state is expected, yielding the simple

A A 1 HHisomorphous diagram. For the situation AB > , positive devia-

tion is experienced, meaning A atoms prefer to cluster with other Aatoms, and B atoms prefer to cluster with B atoms. This gives rise to the

tendency for phase separation. With increasing positive deviation, the

sequence of binary diagrams is the following:

Isomorphous —» phase separation in the solid state —

»

peritectic with a minimum —» peritectic -h> eutectic

-» eutectic with a miscibility gap in the liquid region.

For the situation AB < , negative deviation from ideality is

experienced, meaning A's and B's prefer to be associated. This leads to

the tendency for compound formation. The sequence of phase diagramswith increasing negative deviation is the following:

Isomorphous —» ordering —» compound formation.

An ordered solid solution is one in which the solute occupies specific

lattice sites and is not distributed randomly.

2.8. Liquid Solutions

The concept of liquid solutions of metals is easy to grasp since we havecommon counterparts in our everyday experiences such as sugar dissolved

in coffee, salt dissolved in water. A liquid solution has some importantdifferences from the solid solutions. Probably the most important differ-

ence is that only "short-range order" exists. In a true solid, the crystalline

arrangement is repeated indefinitely in space giving rise to "long-range

order." When a metal melts, an increase in volume occurs (only bismuthand gallium among the pure metals contract on melting) . This increase

in volume is accounted for by the formation of additional lattice vacancies

and an increase in the interatomic distances, with a loss of long-rangeordering. It is believed that the liquid configuration is 12-15 atomsclustered about a lattice vacancy. As the temperature of the liquid is in-

creased, the short-range order begins to disappear and the structure ap-

proaches complete randomness.

Liquid metals and solutions in general have no ability to sustain

shearing stresses. The thermal and electrical conductivities are about50% of the values for the solid structure. The solubility of gases such as

hydrogen increases with increasing temperature. When immiscibilityoccurs in the liquid region of an alloy, generally two liquid layers areformed because of difference in density of the two liquid phases.


2.9. Solid Solutions

The concept of solid solutions is somewhat harder to grasp. Thesesolutions are in reality solute atoms dissolved in the crystal lattice of the

host, or solvent metal. As previously mentioned, the solute atom maysubstitute for a solvent atom on a lattice site, giving a substitutional solid

solution, or it may lodge in the interstitial space between the solventatoms, giving an interstitial solid solution. These two types of solid

solution are illustrated schematically in a two dimensional array in

/' 7 ss / y— < i

/ ,<- '

/ } /

/• / },/ / y

2yC

/*

yt

B

Fig. 2.12. Schematic representation of solid solution.

Figure 2.12. Solid solutions are produced either by the freezing of the

liquid solution, or by the diffusion of the solute atom into the lattice of

the solvent atom in the solid state.

The introduction of solute atoms into the lattice of the solvent metal

brings about some important changes in properties:

(1) The electrical and thermal conductivities are reduced.

(2) The thermoelectric power is reduced.

(3) The strength and hardness are increased.

(4) The ductility is decreased.

The formation of solid solutions is one of the fundamental methods of

hardening a metal.

2.10. Intermediate Phases

When the solid solubility of a solute species is exceeded, a second

phase will appear in the microstructure of the alloy. These intermediate

phases can be classified into six categories:

(I) Intermediate solid solutions.

These solid solutions exhibit a range of composition and lie inter-

mediate to the components A and B. Terminal solid solutions, on the

other hand, start with pure A or pure B.


(2) Normal Intermetallic compounds.These are often called valency compounds since they show a definite

stoichiometric ratio of A to B and a definite chemical composition. Ex-amples are Mg2 Pb, Mg3 Bi2, PtSn 2 , AlSb, MgTe, MnSn, and AuGa.

(3) Electron Compounds.These compounds are characterized by a definite valence electron to

atom ratio, a definite structure, and usually a range of composition. Theelectron/atom ratios encountered are 3/2, 21/13, and 7/4. A commonexample of electron compounds is the Cu-Zn, or brass series of phases.These electron compounds are found in alloys of copper, silver, and gold.

(4) The Sigma Phase.

The sigma phase is a hard, brittle phase found in certain alloy systemssuch as for example Fe-Cr. It has a tetragonal structure with 30 atomsin the unit cell.

(5) Interstitial Compounds.These compounds involve small solute atoms, usually hydrogen, nitro-

gen, and carbon, which lie in orderly arrays between layers of the largersolvent atoms. They are characterized by their high melting points,high hardness, and metallic characteristics.

(6) The Laves Phases.

The Laves phases are compounds which form when the difference inatomic size between solute and solvent is intermediate. They show thecomposition AB2 with a size ratio of 1.2/1.

The formation of intermediate phases is another important method ofhardening metals. The intermediate phases can further be classifiedas to being coherent or non-coherent. Coherent phases are those whosestructure is continuous with the matrix structure and lead to precipita-tion hardening (age hardening) . Non-coherent phases are those whoselattice structures are discontinuous with that of the matrix and are simplymechanical mixtures. This class of intermediate phase leads to dispersionhardening. The hardness of such a mixture is mainly dependent on theaverage distance between the precipitate particles, or expressing itdifferently, the mean free path in the matrix.

QUESTIONS

1. Calculate the wave-length of an electron (m = 9.1 x 10-28 grams) with avelocity of 1.8 X 109 cm/sec.

2. Calculate the wave-length of a particle of mass 2000 pounds and a velocityof 50 miles per hour. Would it be necessary to use wave mechanics tointerpret the motion of this mass? Explain.

3. Write the electron configuration of silver, molybdenum, and tungsten.4. Discuss the difference between the ionic and Van der Waal's bond.


5. How do the metallic bond and the covalent bond differ?

6. Show how two different space groups may be constructed on a body-

centered cubic lattice.

7. Determine the number of atoms in a body-centered cubic unit cell; a face-

centered cubic cell. (Note that corner and face-centered atoms are shared

by other unit cells.)

8. Determine the Miller indices of a plane in the cubic system with intercepts

'A a, V-i a„, and \i a . (a is the length of the unit cell edge)

9. Determine the intercepts of a plane in the cubic system with Miller indices

of (235).

10. Sketch the direction [101] in a face-centered cubic cell.

11. Enumerate the three classifications of defects encountered in real metal

crystals and give examples of each type.

12. What is the influence of these defects in metals and alloys?

13. Define a grain boundary and list some important manifestations of grain

boundries.

14. Describe the two types of solid solutions formed in metal systems.

15. What is meant by the size factor in solid solutions? Give the favorable

size factor range for the two types of solid solutions.

16. Discuss the arrangement of atoms in solid solutions exhibiting (a) ideal

behavior, (b) positive deviation from ideality, and (c) negative deviation

from ideality.

17. Give the sequence of phase diagrams expected for increasing (a) positive

deviation from ideality, and (b) negative deviation from ideality.

18. Discuss the properties of liquid solutions of metals.

19. What changes occur in physical and mechanical properties when B atoms

arc dissolved in the crystalline lattice of A, forming a solid solution?

20. Enumerate the six categories of intermediate phases and describe the char-

acteristics of each.

CHAPTER

Factors AffectingEngineering Properties

3Philip C. Rosenthal, M.S., Professor of Metallurgical

Engineering, University of Wisconsin, Madison, Wis-

consin

IF A "perfect" single crystal of metal is tested,

it is found that its strength is in the neighborhood of a million poundsper square inch, exceeding by a hundred-fold or more the strength of the

same metal in its ordinary polycrystalline form. According to dislocation

theory, this great difference in strength between the "perfect" crystal andthe ordinary polycrystalline metal can be attributed to the presence in

the latter of dislocations within the grains, and to the grain boundaries

themselves, which in effect, represent a series of dislocations. Therefore,

the properties of polycrystalline metals are limited at the start by this

characteristic atomic disregistry; and whatever means we have available

for controlling their properties cannot eliminate this situation.

In addition to the inherent atomic imperfections that are present,

other microscopic or even macroscopic imperfections may be introduced

during the manufacture of the metal part. These are such imperfec-

tions as segregation of alloying elements, presence of non-metallic inclu-

sions, seams, porosity, voids, dissolved gas, etc., which will alter the

properties of the metal and determine its "quality." These "built-in"

factors cannot always be prevented and must be recognized as affecting

the engineering properties to a greater or lesser degree.

Starting with a material that is already limited in its properties be-

cause of these imperfections, we can alter its properties within limits,

through the utilization of one or more of the following processes or "unit

operations" of engineering property control that are available to the

engineer for his manipulation. These are:

23


1

.

Grain size control

2. Mechanical working

3. Alloying

4. Heat treatment

5. Nuclear radiation.

The final engineering properties of a metal may therefore be thought

of as resulting from a combination of positive and negative factors. Thenegative factors are the dislocations and the additional imperfections of

segregation, inclusions, porosity, and any other undesirable factors that

are picked up by the metal during the course of its preparation and that

detract from the properties of the metal. The positive factors are those

combinations of the first four of the process variables listed above that

are used to obtain the properties desired. To be most effective, any oneor combination of these operations should be performed on metal having

a "quality" consistent with the application and costs of the part.

To illustrate the extremes possible, the metal going into an ordinary

boat anchor and that going into a highly stressed landing gear of an air-

plane can be compared. In the first instance, the most important require-

ment is shape and weight, and strength is decidedly secondary. Metal

going into this part could contain a number of the imperfections pre-

viously listed without seriously detracting from its serviceability. Further-

more, no strengthening would be required by any further processing.

On the other hand, the highly stressed part of an airplane landing

gear would need to be very carefully produced to avoid any imperfections

and excessive inclusions. In addition, the metal would be alloyed andproperly heat treated and possibly mechanically worked to get the best

possible serviceability for the minimum weight of the part. Here the

negative factors would be kept at a minimum and the positive factors

derived from the processing developed to the highest degree.

While knowledge concerning the control of all the variables affecting

engineering properties is important to the engineer, it is with respect to

the processes used to manipulate the engineering properties* that he is

generally concerned and these will be discussed primarily herein. Refer-

ences to the factors affecting the "quality" of the metal will be made whennecessary.

GRAIN SIZE AND ENGINEERING PROPERTIES

3.1. Grain Size Control

Although other factors such as alloying or heat treatment may have a

more profound effect on metal properties, nevertheless control of the

• The metallurgical engineer, of course, is expected to be familiar with all.

Factors Affecting Engineering Properties 25

Fie. 3.1. Partially solidified iron crystals showing their dendritic appearance (Howe).

grain size of the metal can be important because such properties astoughness, high temperature strength, machinability, and formabilitycan vary appreciably with the grain size. How grain size can be con-trolled will be discussed first, following which some of the more im-portant engineering aspects of grain size control will be discussed.

Casting Process. When metal is melted and poured into castings, themetal solidifies as a mass of interlocked grains. The way the metal freezesand the final form it takes is related to its composition. It is not possibleto consider here all the ramifications of solidification, and further con-sideration of this point is given in Chapter 6, but those factors affectingthe final grain size can be considered.

As the temperature of the molten met;il is lowered by loss of heat to themold walls, freezing commences by the nucleation of solid metal grainsalong the sides of the mold. The grains so nucleated are probablyrandomly oriented but a number will be so oriented that growth inward


Fig. 3.2. Completely frozen gold alloy showing dendritic character of each grain andgrain boundaries. 100X.

will be favored, i.e. in the direction of the temperature gradient. At the

same time that they grow inward, these grains will also grow at lower

rates in their other dimensions as well. The growth, however, will not

be like that of an expanding block but more on the order of a growing

tree, with the main stem being represented by the growth inward andthe branches representing the lateral expansion. This tree-like structure

is related to the crystallographic structure of the metal since the lateral

growth is not random (as in a tree) but confined to definite preferred

directions. In the growing crystals illustrated in Figure 3.1, for instance,

the lateral growth in any one grain is confined to two major directions at

right angles to each other. In addition to the secondary branches, there

are also tertiary branches extending therefrom. Crystals which grow in

this manner are referred to as dendrites. The tree-like or dendritic growthrather than a wall-like growth in these crystallographic directions is

attributed to the fact that heat can diffuse away from a protuberance

much more readily than from a flat surface; consequently, there will be a

natural tendency toward local extension of the growing interface.

The crystals in Figure 3.1 are only partially solidified. If allowed to

freeze completely, and assuming enough liquid metal available to satisfy

their needs, the grains would eventually fill out until all available space

would be occupied by solid metal and each grain would be separated fromthe other by an irregular grain boundary (Fig. 3.2)

.


Fie. 3.3. Polygonal or equi-axed grains of high-purity iron etched. 500 x.

If the metal is pure, the dendritic structure displayed in Figure 3.2

will not be visible, but the grain boundaries will still be present. Thedendritic structure shown in Figure 3.2 occurs because chemical hetero-geneity is created during freezing and can be revealed by etching thesolidified alloy. Such chemical heterogeneity would not be possible in a

pure substance; therefore, evidence of the dendritic structure can onlybe obtained by draining liquid away from a partially solidified metal as

was necessary to reveal the dendrites in figure 3.1.

Whenever grains grow more rapidly in one direction than the others,

they are called columnar grains. If they grow equally fast in all direc-

tions they are called equi-axed grains. Equi-axed grains of iron areshown in Figure 3.3.

The final size of these grains is dependent on two factors: (1) the rate

of nucleation, N, or in other words, the number of growth centers thatform in a given amount of time, and (2) the rate of growth, G, of thesenuclei. It should be quite obvious that if conditions are adjusted to givemany nuclei and their growth is at a moderate rate, grain size will besmall. On the other hand, only a few nuclei, growing at an equal rate,

will produce a coarser grain structure. Both rates of nucleation and therates of growth can be represented by complex thermodynamic functionsinvolving a number of factors. The rate of nucleation in particular is

usually affected by impurity particles or the container walls. Practically,


however, both of these rates are dependent on the cooling rate of the

metal. As the cooling rate increases, the greater is the degree of super-

cooling of the liquid and the greater the rates of nucleation and growth.

The relative changes of these two factors, however, is such that for most

metals, rapid cooling produces a finer grain structure than slow cooling.

It is apparently impossible to repress completely nucleation in metal as is

the case for glass which represents a material which does not develop an

appreciable rate of nucleation until it is cooled to a temperature where

the rate of growth is too low to be effective and the glass remains a super-

cooled liquid.

Because nucleation of metals can be affected by impurities, some metals

can be produced in a fine grain structure by the addition of certain

nucleating or "inoculating" agents. The addition of sodium to analuminum-silicon alloy, for example, produces a fine grained alloy.

Mechanical vibration can also induce nucleation.

During the cooling of liquid metal in a mold other changes occur

that may have a bearing on the properties and behavior of the solid.

Most metals contract on freezing. Since this normal contractual process

occurs simultaneously with dendritic growth, interdendritic void areas

may develop that cannot be fed properly.

Also as metal cools, the solubility for certain dissolved gases or solids

may change, resulting in their precipitation and entrapment within the

grains or on grain boundaries depending on the degree and time of these

changes. These conditions affect the "quality" of the metal as discussed

earlier.

Mechanical Working. The previous section has described how grain

size of cast metals is controlled. Many metal parts, however, are produced

from metal that has been cast into an ingot and then subsequently rolled

or otherwise mechanically worked into a desired shape such as a forging,

bar stock, plate, angle or some other form. This mechanical working

can also change the grain sue of the metal.

If the working is done at an elevated temperature (hot working)

the combined effects of the heat and working action will cause recrystalli-

zation, the size of these new grains being dependent on the amountof this mechanical action and the temperature at which it is performed.

As long as the temperature remains high enough to assure spontaneous re-

crystallization, the lower that temperature, the finer will be the grain size.

If the metal is worked at room temperature (or below for some metals) ,

the original grains are severely deformed and distorted. This is called

cold working. However, insufficient energy is available to cause this

metal to seek a new orientation (nucleate) and grow into a new set of


grains. By raising the temperature of the metal (annealing) , a tempera-ture is reached where recrystallization docs occur and a new set ofgrains develops as a result of nuclcation and growth in the solid state.

The size of these grains can be controlled, as described later in thischapter.

Powder Metallurgy. Metal powders can be produced by reducingoxides or other compounds, by electro-deposition, grinding, or by othermeans. If these are subsequently compressed and heated (sintered) ametallic compact is produced by interdiffusion of atoms between theparticles, producing ultimately a metallic mass composed of grains havinga grain size that is determined by the processing variables involved.

Allotropic Transformation. In metals which undergo an allotropictransformation in the solid state a new set of grains is produced as aresult of the transformation. Thus, in iron, which is the best exampleof this situation, cooling it from the high temperature face-centered cubicform to the room temperature body-centered structure results in nuclea-tion and growth of the body-centered structure from the original face-centered structure. The size of the grains is again determined by therelative rates at which nucleation and growth occur. Slow rates of coolingpromote large grain size.

Other Methods for Controlling Grain Size. In addition to the fore-going methods, it is also possible to produce a solid metal of a certaingrain size by deposition from a vapor or by electrodeposition from asolution. In the latter case the characteristics of the plating bath deter-mine the grain size of the deposited metal.

3.2. Effect of Grain Size on Properties

Table 3.1 gives selected values showing the influence of grain size onthe strength and ductility of pure iron.

Table 3. 1. Effect of Grain Size on Tensile Properties of Iron. *

Grain Size, Grains Tensile Strength Yield Strength Elongation in 2 in.per sq. mm. psi ps; m

2.5 to 9.7 23,900 to 33,700 5,900 to 6,500 28.8 to 35.3

35.6 to 77.5 38,200 to 41,600 8 , 300 to 20 , 200 44.8 to 50.7

109 to 135 42,200 to 43,300 15,100 to 22,200 42.8 to 45.3

• Metals Handbook, ASM, 1948 ed., p. 433.


These data show that there is a substantial increase in strength as a

result of decreasing grain size, with an accompanying rise in ductility.

However, once the grain size reaches approximately 100 grains per sq.

mm. little additional change in properties is apparent.

Since metal failure usually occurs by a process of slip (described later)

along certain crystallographic planes of the grains, followed by eventual

fracture, the effect of decreasing grain size is to cause interference to

slip among the many grains composing a fine-grained material thereby

raising its strength and altering the ductility, as compared to coarse

grained metal where each individual grain is freer to slip and distort with

less interference from neighboring grains.

Figures 3.4 and 3.5 show further examples of the effect of grain size

on the strength and ductility of metals. In the case of pure copper, very

little change in properties is evident, but for the 65-35 Cu-Zn brass the

strength and ductility are noticeably affected by grain size. In this case,

the strength decreases and ductility increases with increase in grain size.

In addition to affecting strength and ductility, grain size is also im-

portant in forming operations. Table 3.2, for example, shows the appli-

cations for brass of various grain sizes. The coarser grained material is

more readily deformed but gives a rougher surface than the fine-grained

material. Similar effects are noticed in deep-drawing of low-carbon steel

where grain size in excess of about 0.0012 inches average diameter maylead to a surface roughness after drawing referred to as an orange peel

effect (Figure 3.4) . On the other hand, extremely fine-grained or annealed

low-carbon steel may show another type of defect after deep-drawing

called stretcher strains. This defect is characterized by fine wavy lines

that appear on the surface of the part after the deformation (Figure

3.5) . Also, a grain size that is too small is likely to make deep-drawing

difficult because of high strength and low ductility.

Table 3.2. Grain Size of Cold Worked and Annealed Copper Alloys. *

Nominal Grain Size, mm. Typical Use

0.015 Slight forming operations.

0.025 Shallow drawing.

0.035 For best average surface combined with drawing.

0.050 Deep drawing operations.

0. 100 Heavy drawing on thick gages.

• Loc, cit.


Fie. 3.4. Microstructure and surface structure of a fine-grained steel (A) and acoarse-grained steel (B). Orange-peel eficct is revealed in (B) after Erichsen cuppingtests. (F

.Korber, Staht & Eisen, v, 47, 1927, p. 1158).

Machining operations are also influenced by grain size. Coarse-grainedsteels, for example, allow for higher cutting speeds, but if finish is im-portant a fine-grained steel is recommended.

Metals under stress at elevated temperatures tend to elongate slowlywith time (creep)

. This behavior is associated with grain boundarymovement, hence where resistance to stress at high temperatures is im-portant, a coarse-grained metal will be superior to a fine-grained metal.The tungsten filament in a lamp bulb, for example, is produced as acoarse-grained wire because of its better life. In other high temperatureapplications, such as turbines, jet engines, etc, the grain size of the metalcomposing the high temperature components will be a factor in determin-ing its resistance to creep. This is discussed in Chapter 23.

32 Eiigineering Metallurgy

Fie. 3.5. Stretcher strains in low-carbon steel. % actual size (1955 Supplement

to Metals Handl>ook).

EFFECT OF MECHANICAL WORKING ON ENGINEERINGPROPERTIES

In a previous section the effects of mechanical working on grain size

were described. In addition to these effects there are other important

effects of mechanical working on the engineering properties that should

be considered. Before these can be discussed, however, it is necessary

to review briefly the principles dealing with the deformation of metals

because some knowledge of this subject is helpful for an understanding

of the engineering applications of metal working.

3.3. Deformation of Metals

When a stress of a magnitude to cause only temporary displacement

is applied to a metal, the resultant strain is termed elastic strain. If the

stress exceeds the limiting stress for elastic behavior, the permanent

strain resulting is termed plastic strain. With increased application of

stress beyond the elastic limit, metals continue to deform until they fail

by fracturing. Most nonmetals and occasionally some metals are totally

lacking in this plasticity and fail by fracturing with no permanent de-

formation. This is referred to as brittle fracture. Metals, in fact, are

unique among structural materials in their ability to be deformed and

it is only under special circumstances that they display a brittle fracture.

Demonstrations of elastic and plastic behavior in tensile testing will be

given in Chapter 4.

From an atomistic viewpoint, a stress that causes elastic strain results

in a temporary and small displacement of atoms from their normal posi-


Fig. 3.6. An octahedral or (111) plane. Eight such planes can be constructed in agiven cube.

tions in the lattice so that the lattice is slightly distorted. Release of thestress causes the atoms to return to their normal positions and the lattice

regains its original shape.

In the case of plastic strain, two basic processes of atomic movementare possible—$/»'/> and twinning. These deformation mechanisms are dis-

cussed in the following sections, first with respect to single crystals andthen with respect to polycrystalline metal. It will be impossible to coverall phases of this subject here, but the interested reader can find gooddiscussions of this topic elsewhere.

3.4. Slip in Single Crystals

In plastic strain there is a permanent displacement of the atoms,the atoms moving from one atomic position in the lattice to another.The particular crystallographic structure of a single crystal restricts themovement occurring during this straining to certain preferred planes.In the face-centered cubic metals such as copper, aluminum, or nickel thepreferred. plane for straining by slip is normally the (111) • plane. Thisis the octahedral plane that cuts diagonally through the unit cell as

illustrated in Fig. 3.6. When this plane is viewed from a position at rightangles to it, the atoms are close-packed much in the manner shown in Fig.

3.7b. On the other hand, it should be apparent from the left-hand illus-

tration in Fig. 3.7 that other planes such as those parallel to the faces ofthe cube have a lower atomic density. Because of its high atomic density

•A system of crystallographic indexing which indicates displacement of unity inthe three crystallographic directions as measured from a reference point.

34 Engineering MetallurgyV

\

(111) plane

\(III) plane-^\

(a)

Fig. 3.7. Arrangement of atoms in the octahedral plane of the face-centered cubic

unit cell as viewed from the side of the cube (left) and perpendicular to the face of

the plane (right).

the octahedral plane has the greatest interatomic attraction and there-

fore is said to be a strong plane. As a consequence, slip occurs parallel to

this plane rather than along some intersecting plane of lower atomic

density. For slip to occur along a plane of lower atomic density would

require that the stronger interatomic attraction existing on the octa-

hedral planes would have to be overcome—an unlikely situation.

The direction of slip is likewise limited. These directions are shown by

the arrows in Fig. 3.7 and correspond to the directions parallel to the

lines of greatest concentration of atoms, and the lines of least resistance to

movement of one row of atoms past another. Any other directions within

the octahedral planes would require a greater disturbance of atoms since

the direction of movement would be tangent to some atoms but pass

directly through others.

The combination of slip planes and slip directions constitutes the

slip system. In face-centered cubic metals, four slip planes are possible as

mentioned in Figure 3.6, each having 3 slip directions. There are there-

fore 12 slip systems. Since slip depends on exceeding a critical resolved

shear stress that is specific for a given metal, metals crystals will not flow

or deform by slip unless the applied force can be resolved into shear

stress parallel to a slip plane and in one of the slip directions. This is

always possible in face-centered cubic crystals; therefore flow always

precedes fracturing in these crystals (unless a unique condition of equal

triaxial stresses exists)

.

A hexagonal close-packed metal such as zinc or magnesium can slip

only on the basal or (0001) plane. This plane represents the plane of

greatest atomic density in this type of crystal. As in the face-centered

metals, there are also three directions of slip. In fact, there is nothing to


distinguish the basal plane of a hexagonal close-packed metal in atomic

arrangement from the octahedral plane of the face-centered cubic metals.

The differences come in the way atoms in adjacent planes are stacked

above or below the reference plane. Because it has only the one slip

plane and three slip directions, a hexagonal close-packed crystal has only

three slip systems, and it is possible to orient such a crystal with respect

to the applied load so that the slip planes are perpendicular or parallel

to this load. In such cases, no flow occurs and a brittle failure ensues.

Only when the basal plane is so oriented with respect to the applied

load that a shear stress can be resolved in one of the slip directions canflow occur. Metals having this crystallographic structure, therefore, are

not so ductile as the face-centered cubic type.

The slip planes in body-centered cubic metals are not so well defined

as in the preceding two cases, and one of several possible planes canrepresent the plane of major slip. The slip direction, however, is always

the close-packed direction represented by the cube diagonals—four in

number. Around each of these four directions there are arranged 12

possible slip planes; consequently, 48 slip systems exist in these metals.

Although this represents a comparatively large number of slip systems,

each system is less regular and less densely packed than those of the

face-centered cubic metals. In general, then, the body-centered cubic

metals require somewhat higher resolved shear stresses to initiate slip,

and less permanent deformation is possible in advance of fracture.

Manifestation of Slip. The visual evidence of slip is the appearanceof numerous slip lines or slip bands upon microscopical examination of a

polished metal surface after the specimen has been deformed plastically.

These slip bands represent a permanent displacement of one part of ametal crystal with respect to the rest of the crystal. The resulting micro-scopical difference in elevation accounts for the appearance of lines

traversing the crystal faces when viewed under a microscope. Repolishingand etching will remove this evidence of slip because the surface displace-

ment which produced the slip lines is now removed thereby. In cases of

severe distortion, however, the disturbance is so great that slip will beevident even though the distortion preceded the polishing and etching.

This is demonstrated by the cold worked brass in Fig. 3.1 1.

Strain Hardening. The amount of stress required for slip increases

with an increase in the amount of prior deformation. In other words,a strain-hardening action occurs which lends to retard or restrict furtherslip on a given slip plane. As the stress increases, slip is transferred to

other slip planes. This is the basis for hardening metals by cold work-ing as described later. This mechanism is unique to metals and


represents one of its outstanding characteristics. Putty or clay, for ex-

ample, is readily deformed and worked, but no hardening action occurs.

Although much has been written about this hardening action and manytheories proposed to explain it, the theory of dislocations is generally

accepted today as offering the best explanation for the relatively low

stress required to initiate slip and increased resistance to the slip as

deformation proceeds. The low stress required to start slip is explained

on the basis of a wave-like movement of a small dislocation "zone" across

the crystal rather than a block-like displacement of the entire grain. Theincreased resistance to slip as deformation progresses is attributed to aninteraction between dislocations created during the slip process. Since

the dislocation theory is still in the formative stage, any explanation of

strain hardening based on this theory is still somewhat speculative.

The theory does, however, offer the best overall explanation for metal

behavior yet available.

3.5. Twinning

Another mechanism accounting for plastic deformation of metal

crystals is twinning. This process has certain features similar to those of

slip and other features which are dissimilar.

Like slip, twinning occurs on certain crystallographic planes andin certain directions, but the planes and directions are not necessarily

the same as those for slip. Also like slip, a certain level of shear stress is

required.

Twinning differs from slip in that the atomic movement is not fromone atom position to the next but rather a fractional atomic distance.

This results in a new orientation in part of the grain. This orientation is

such that the atoms in the area undergoing twinning present a mirror

image of those in the original part of the grain when viewed along the

plane dividing the twinned area from the untwinned region as shownin Fig. 3.8. Although the figure depicts an atomic movement which in-

creases the greater the distance from the twinning plane, in reality the

movement, of any one atom need only be less than one atom spacing withrespect to a neighboring atom in order to accomplish the repositioning

necessary for twinning.

Twins produced by mechanical action as just described are referred to as

mechanical twins. Cold worked metals that are annealed also will exhibit

annealing tivins which probably originated from mechanical twins.

Mechanical twins are usually narrow and may be confused with slip lines.

Slip lines should appear on an unpolished surface that has been strained;

whereas, twin lines are usually evident only after etching. Furthermore,


Twinning plane*

Original lattice of face-centered cubic metal as

viewed parallel to twinning plane (111).

Mirror image lattice as

created by atomic move-ment shown by arrows.

Fig. 3.8. Diagram showing atomic movement required for twinning.

repolishing and etching would destroy a slip line but a twin would re-

appear. Slip usually shows up as lines; whereas, twins normally appear as

bands.

Twinning is not capable of contributing as much permanent deforma-

tion as slip. In face-centered cubic metals, therefore, it represents only

a small part of the total deformation. On the other hand, in hexagonalclose-packed metals, twinning is important not because of the deforma-

tion it permits but because twinning facilitates slip by aligning moreslip planes in a favorable direction.

3.6. Deformation of Polj crystalline Metals

The metals used in engineering are, of course, polycrystalline rather

than single crystals. The mechanisms previously described apply to each

of the single crystals composing the aggregate. However, the extent of

the movement is governed by the presence of the surrounding grains.

Consequently, the situation is a good deal more complex than in the ideal

case of a single crystal.

Cold working or stressing a polycrystalline aggregate composed of

randomly oriented grains causes slip to occur in the most favorably

oriented grains and on the most favorably oriented slip planes. As the

stress increases, additional grains less favorably oriented are similarly

deformed. Severe distortion in one direction will cause the following

effects in addition to the slip and twinning.


1. The lattice of each grain that is deformed tends to rotate into the

direction of the applied load.

2. In addition, the constraint by surrounding grains may cause the

lattice to be bent.

3. Deformation bands representing different degrees of orientation

within a grain appear in the microstructure.

4. A preferred orientation of the grains develops. This is sometimes

referred to as crystallographic fibering. The type of fibering that

develops depends on the metal being worked and the nature of the

stress system that is operating. This effect is of importance in the

cold working processes applied to metals, because the properties

are similarly rendered anisotropic and when the metal is recrystal-

lized by annealing (discussed later) a "recrystallization texture,"

which is also of a preferred orientation, may develop.

3.7. Hot Working

If metals are mechanically worked at a high enough temperature,

they do not strain harden, but simply recrystallize into new sets of

grains. When this occurs the working is referred to as hot working.

Hot working temperatures depend on the characteristics of the metal

being worked. Thus, lead is essentially hot workable at room tempera-

ture because it will recrystallize during the working operation. Steel, onthe other hand, must be heated to above a red heat before the operation

can be classified as hot working. In either case recrystallization occurs

simultaneously with the mechanical working, and the properties, with

the exceptions noted in succeeding paragraphs, are not changed much.

During the working process, the shape of the metal part is usually

changed drastically. As an example, in forming bar stock, the metal starts

out as a bulky ingot and ends up as an elongated bar. The same relative

changes occur to a greater or lesser degree in forming the many other

wrought shapes by rolling, forging, extruding, or by other metal working

processes. The coarse ingot structure is refined by this operation and

voids and porosity that might have been present in the original ingot are

largely eliminated, but the heterogeneity originally existing in the ingot

as a result of the changes occurring during freezing is not completely

irradicated. In fact, the metal working tends to elongate areas showing

chemical segregation, and the inclusions are also strung out in the rolling

direction. This has a tendency of producing a fiber-like structure in the

metal so that the properties when tested in the direction of rolling will

not be the same as those obtained from tests made at right angles to the

rolling direction. This type of directionality developed by the rolling


>— n* -'iMMH

Fig. 3.9 Elongated slag inclusions in rolled steel, uneiched. IOOX.

Fie. S.10. Elongated sulfide inclusions in steel and evidence of chemical segregationin the region of these inclusions, etched. 100X.

process is sometimes referred to as mechanical fibering to distinguish it

from the preferred orientation effect which may occur as the result of cold

working and referred to as crystallographic fibering. Examples of the

elongation of inclusions and evidence of chemical segregation are given in

Fig. 3.9 and Fig. 3.10.

The principal effect of this directionality is to lower the ductility andtoughness in the transverse direction as compared to the direction of


rolling. The data in Table 3.3 illustrates how the V-notch Charpy impact

values (which serve as a measure of toughness) vary with the selection of

the test piece. .Similar differences but not so pronounced are also en-

countered in the reduction of area values obtained in tensile testing.

TABLE 3.3. Influence of Directional Effects on V-Notch Charpy Impact Values at

70-F. (9).

Nature of Steel Tensile

Strength

Specimen Transverse

to Principal

Rolling Direction

Specimen Parallel

Principal

Rolling Direction

137,500 psi. 34-34 ft-lb. 67-64 ft-lb.

Straightaway 142,000 36-37 70-71

rolled steels* 134,000 43-44 75-76

Cross rolled 132,000 54-49 62-61

steels* 155,000 44-47 46-45

Cast steel, as-cast 110,000 53-56 ft-lb.

Same steel forged * * 110,000 102-122 ft-lb.

* Notch parallel to plate surface.** From 2 x 3}^ in. section to XA in. square bar.

Outside of the effects previously noted, hot working does not alter the

tensile properties of metals appreciably, particularly when compared to

the effects of cold working, which can effect a substantial change in

properties.

3.8. Cold Working

Cold working results in a distortion of the grain structure as depicted

in the series of photomicrographs of Fig. 3.11. Beginning with the hot

worked or soft tempered metals containing equi-axed grains, the succes-

sive stages of cold reduction produce increasingly greater distortion and

elongation of the grains and generation of slip lines. Because of the grain

fragmentation and disturbance of the lattice, there is an accompanying

hardening action as shown by the accompanying Rockwell B hardness

values. Tensile strength increases in about the same manner as hardness,

and there is a simultaneous decrease in ductility and toughness. Changes

in the tensile properties of brass and copper with percentage reduction in

drawing are shown in Fig. 3.12. This hardening action is attributed to

internal barriers to flow such as precipitated particles, impurity atoms,

etc., to interactions between dislocations moving on different slip sys-

tems, and to formation of subboundaries within the grains (polygoniza-

tion)

.


ml&E&fmiife s& m

M i,- '-'

- 1*3;

S .a. .-=H:'- :i ----.::

-J - • -^- ;*- «<

Fie. 3.11. Microstructure of 70% Cu-30% Zn sheet brass of various tempers.

Soft temper: 47 Rockwell B; i/4 hard: 73 Rockwell B; i/

2 hard: 82 Rockwell B; hard:

89 Rockwell B; spring temper: 94 Rockwell B. (Courtesy of R. A. Ragatz, University of

Wisconsin)

Cold working therefore provides a means for hardening pure metals

or alloys. It is a particularly useful process where the composition is

such that hardening by heat treating cannot be used. The terminology

used in Fig. 3.11 (such as quarter hard, half hard, "spring temper") are

commercial expressions for the degree of cold reduction obtained. De-

pending upon the final finishing processing operations required, a metal

hardened by cold working can be ordered from the mill in any one of the

"tempers" indicated. Where heavy subsequent forming operations are

intended, the annealed or partially hardened metals are specified. Butif little further forming is to be done and a metal of high hardness andstrength desired, the full hard or spring tempers can be specified. Theactual hardness and strength for each of the tempers would vary depend-

ing on the base metal used.


Q.

OOQ

140

120

t£ 100

c

Soft 10 20 30 40 50 60 70 80 90Percentage Reduction by Drawing

Be. 5.12. Eircct of cold reduction on the tensile properties of copper and brass.

(ASM Handbook.)

Steel can also be cold worked, and cold finished steel has certain

advantages not obtainable by other means. For example, not only does

cold finishing provide a smooth, oxide-free finish, but the metal itself is

readily machinable due to the increased brittleness induced by the rolling

action. This effect makes chip formation easier as compared to a steel of

the same hardness obtained in other ways.

3.9. Annealing Cold Worked Metal

Because of its strained condition, cold worked metal can be re-

crystallized by reheating or annealing. In practice, this probably should

be considered as essentially a fabrication procedure inasmuch as the

accompanying recrystallization and softening may be necessary to allow

further forming operations on the metal, but it produces properties con-

trary to the usual requirements of engineering materials. However, it is

important to be acquainted with the principles of annealing cold worked

metals because this knowledge is necessary wherever cold working is

used for fabrication.

As cold worked metals are heated, eventually enough energy is supplied

to form nuclei for recrystallization. These nuclei grow at the expense of

the deformed metal until the entire structure is composed of a new set of

grains. It is convenient to separate the process into three distinct stages:

(1) Recovery, (2) Recrystallization, and (3) Grain growth.

Recovery. Recovery represents a relief of internal stress and a decrease

in thermelectromotive force and electrical resistivity. Internal stresses


c

CO

QJ

.**

co

.cD>~

CD

Wmm^/AalS^^^r^Rtcot/cry

'Recrysloll'I notion

I

GroinG'owin

Annealing Temperature

Fin. S.13. Schematic changes in recovery, strength and recrystallization on annealingcold worked metal. (ASM Handbook)

were originally developed during the cold working process. They repre-

sent a balance of compressive and tensile stresses within the metal. Theirmagnitude and distribution are determined by the degree and type of

cold working treatment used. These stresses are detectable only indirectly

through the distortion that accompanies annealing or in the loss in

"spring-back" tendencies. "Spring-back" is the increase in diameter of a

roll of strip after it has been removed from a mandrel around which it washeld. The presence or absence of internal stress is one of the factors

that an engineer must constantly bear in mind in design, and these stresses

may be beneficial or harmful depending on how they occur within a part.

For example, shot peening a helical spring can markedly improve its

service life, i.e., its resistance to fatigue failure. Distortion occurring after

machining a cold-worked metal is an example of the harmful effects of

residual stresses.

Fig. 3.13 shows schematically what happens to residual stress, strength,

and grain size when a metal is heated in the recovery range.

Recrystallization. When the metal is heated sufficiently high, recrystal-

lation occurs. As shown by Fig. 3.14 this results in a rapid decrease in

strength and increase in ductility. This process is both temperature andtime dependent, but the former variable has the more potent effect.


BO

._... > Lokt T.P., a Pnoipnorued

° Elee T.P worncAnneolinq Temperature, deq Fohr

200 HOC 600 800 1000 1200 1400

vrI. *-jt ^ Contract-.™ at A'CQ

rr.Trrytr**.,' ^.l—,-

70

6

SO

40

30

10

(Of -j

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014

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OlO

06

? «3

006 ««JE o5 S

004 O IS J

«f0.02 25

?;"

1. 65 ?

S5

(00 ?00 300 4O0 tOO 600 700 800Annealing Temperature, deq Cent

Fic. 3.14. Elfect of annealing on tensile properties and grain size of four commercialgrades of copper. (ASM Handbook)

Grain Growth. Once a new set of grains is established, the growth of

these grains will continue if the temperature is raised or the time of heat-

ing prolonged. The effect on the mechanical properties as well as on grain

size is shown for copper and brass in Figs. 3.14 and 3.15.

140

Annealing Temperature, deg FahrZOO 600 1000 1400

GrainSite- 4Q

20

— 63XHard- in—43%Hard

24%Hord

70 .g

CM60 g

50 .§

§,

so Ci

Hard ZOO 400 600 800

Annealing Temperature, deg Cent

Fic. 3.15. Effect of annealing on tensile properties and grain size of yellow brass

wire. (ASM Handbook)


3.10. Factors Affecting Recrystallization Temperature and GrainSize

There are a fairly large number of variables affecting the recrystalliza-

tion temperature and the grain size after recrystallization, and since these

effects may be encountered in the engineering utilization of metals it is

important that they be recognized.

800 1200

i-00 400 600 800

Annealing Temperature, deg Cent

Fig. 3.16. Influence of the amount of prior cold work on the recrystallization

temperature of high-purity copper. Annealed one hour at temperature. (Koster, ASMHandbook)

Percentage of Cold Work. Increasing the percentage of cold worklowers the recrystallization temperature. This is more apparent in Fig.

3.16 than in Fig. 3.15. Increasing the percentage of cold work also de-

creases the size of the new grains (Fig. 3.17) . There is, however, anintermediate amount of deformation as illustrated by Fig. 3.17 that will

lead to excessive grain growth. Apparently, this amount of deformationrestricts nucleation to but a few sites, and these nuclei therefore grow to

large grains. In an actual part where a variation in the amount of cold

working might be experienced, annealing could result in a region of ex-

gE

in

.c

I

i

f-iq. 7

77% JWOTTO20 40 60 80

Degree of Deformation, %

Fic. 3.17. Influence of degree of deformation on the grain size developed at 950 C(1742 F) in low-carbon steel. (Hanemann, ASM Handbook)


cessively large grains which would effect the properties adversely. A cold

punching operation would be a good example of a situation where this

could occur. If annealing is required, its temperature should be con-

trolled to keep it below the recrystallization of the less severely worked

metal but above that of the more severely deformed areas to avoid large

grains in the regions of critical deformation.

Composition. The actual temperature at which recrystallization occurs

depends on the composition. Thus some metals like zinc may recrystalli/.e

by working at room temperature. Other alloys such as those used in high

temperature applications may not recrystalli/.e even when heated to 650°C

(1200°F) or above.

Annealing Temperature, deg Fohr

J00 400 500

too iso koo eso soo

Fie. 3.18. Effect of annealing lime on the softening of copper wire reduced 93%by cold drawing. (Alkins & Cartwright, ASM Handbook)

Effect of Time. Time-tern (jerature relations for the annealing of copper

previously cold reduced 93% are shown in Fig. 3.18. This shows that

increasing time lowers the recrystallization temperature.

Effect of Initial Grain Size. The larger the original grain size, the

greater the amount of cold deformation to give equivalent recrystalliza-

tion temperatures and time.

3.11. Summary of Hot and Cold Working

Hot working is used primarily for fabrication purposes and does not

cause a large change in tensile properties. A directionality or fibering

effect is introduced, however, which has a tendency to reduce ductility

and toughness when tested transversely to the rolling or forming direc-

tion.

Cold working raises strength and hardness and lowers the ductility. It

offers an important method for hardening alloys that cannot be hardened

otherwise or as an auxilary treatment along with other hardening

processes. The effects of the cold rolling can be removed by annealing or


tempering and can only be reintroduced by additional cold rolling. Thecombination of cold rolling and annealing has an effect on the re-

crystallizcd grain size of the metal, and the various factors affecting the

grain size and recrystallization temperature must be recognized for a

proper utilization of this process.

EFFECT OF ALLOYING ELEMENTS ON ENGINEERINGPROPERTIES

When one or more metals are added to another metal in the liquid

state, it is found that upon solidification the atoms of the added ele-

ments will either be a part of the lattice structure of the base metal as a

solid solution or another phase will appear in the microstructure. Either

of these effects will alter the engineering properties of the base metal;

therefore, alloying represents one of the important methods for con-

trolling them. Frequently, the effects of alloying are combined with those

of heat treatment to further influence the properties.

3.12 Solid Solution Effects

A binary alloy that exists as a solid solution has grains containing

atoms of two sizes in the lattice structure. This strains the lattice and in-

creases resistance to deformation, thereby serving to strengthen andharden the alloy. For two metals that are completely soluble in each other

in the solid state, the maximum hardening occurs at about 50 atomic

percent of each. Copper—nickel alloys are good examples of this case

and Fig. 3.19 shows how several properties of these alloys change with

composition. Although the maximum strength occurs at about 70%nickel, the maximum hardness (not shown) occurs close to 50 atomic

percent (48 wt.%) nickel.

In addition to the mechanical properties, other physical properties are

altered by solid solution alloying. Fig. 3.20, for example, illustrates howelectrical resistivity and the the temperature coefficient of electrical re-

sistance vary with nickel content in copper-nickel alloys. This illustrates

the important principle that the maximum heat and electrical conduc-

tivity are obtained from pure metals and that the presence of alloys

(impurities in this case) is definitely detrimental in those instances wherehigh conductivity is desired.

Other alloy systems may have only limited rather than complete solid

solubility of the alloying element in the base metal. Such is true for manyof the copper-base alloys, for example. In these alloys the solid solution

hardening action increases with the amount of the alloy up to the limit

of solid solubility, following which the properties are affected by the

presence of another phase in the microstructure as described in the next


40 60 80

Nickel, percent

Etc. 3.19. Effect of nickel on the mechanical properties of copper-nickel alloys.

(Wise)

. 40 60Nickel, per cent

Fie. 3.20. Effect of nickel on the electrical properties of copper-nickel alloys. (Wise)


section. The degree of hardening is usually inversely proportional to therange of solid solubility, so that those alloys of very limited solubilityexhibit the greatest percentage-wise influence on the properties. As anillustration of this point, it takes 0.6% Sb, 4.6% Si, 4.7% Al, 12.1 % Znor 13.2% Ni to increase the hardness of pure copper by an increment of14 Vickers DPH. (diamond pyramid hardness number) . The order inwhich these elements are listed is also the order of increasing solubility ofthese elements in copper.

Besides the aforementioned effects, it might also be mentioned thatduring the free/ing of alloys that exist as solid solutions at room tempera-ture, there is a tendency for chemical segregation to occur within thegrowing dendrites as freezing progresses. Also, voids or a general porositymay develop or inclusions may segregate between or within grains. If

the metal is further processed by working, the segregation and porosityare fairly well eliminated and the inclusions more or less uniformlydistributed and broken. If the metal is retained in the cast condition,however, these imperfections may have some bearing on the properties ofthe casting. These are examples of some of the factors affecting the qualityof the metal.

3.13 Polyphase Structures

If an alloy is prepared which exceeds the limits of solid solubility atroom temperature, the second phase may appear as:

1. The second component in essentially pure form. Alloys of aluminumand silicon are of this type, with the silicon phase interspersed inthe aluminum phase. The effect of silicon on the properties ofAl-Si alloys, is shown in Fig. 3.21.

2. An intermetallic phase, in itself a solid solution, interspersed in theoriginal solid solution. Copper-zinc alloys, with zinc exceedingabout 38%, illustrate this behavior. The alloy at room temperatureconsists of separate grains of alpha brass, the original solid solu-tion, and beta brass, an intermetallic phase containing about 45%Zn. (In the absence of the alpha phase, the beta phase can range incomposition from about 45 to 50% Zn., which indicates its solidsolution characteristics)

.

3. An intermetallic phase which is essentially an intermetallic com-pound. Carbon in steel precipitates as the compound Fe.-,C so thatat room temperature steel consists of a mixture or iron and ironcarbide.

No matter what these extra phases are, they will influence the engineer-ing properties. There is a tendency to generalize by staling that the effect


£ 4 6 8 K)

% Silicon

Fie. 3.21. Effect of silicon content in annealed and hard rolled aluminum-silicon

alloys. (ASM Handbook)

of the second phase is proportional to the amount present, the properties

consequently being determined by the properties and amounts of the

individual phases present. This is only true if the second phase is rather

uniformly distributed in the parent phase; however, there are so manyexceptions to this rule that it is perhaps better to generalize that the

effects of the second phase are determined by (1) its properties, (2) the

amount present, and (3) its distribution. As will be learned when con-

sidering steel, for example, the way the iron carbide is distributed will

have a profound effect on the properties of the steel. Present as a net-

work, around the grains it can lead to brittleness and poor machinability.

Finely distributed throughout the steel, it contributes to high strength

and toughness.

Occasionally, a very small quantity of a given phase can be very harm-

ful to properties due to the fact that it may be distributed in a manner to

produce extreme brittleness. Examples of these effects are found with

bismuth in copper, iron sulfide in steel, and graphite in cast iron. In the

latter case, whether the graphite is distributed as essentially spherical

particles or as elongated shapes referred to as flakes will determine

whether the metal is strong and ductile or weaker ami brittle.

There are so many different possibilities that it is impossible here to

cover each of the situations that may be encountered. However, it can

be stated that the presence of a second phase can produce a decided

strengthening effect or can be decidedly harmful depending on the three

factors listed previously. In any event, by alloying to produce a second

Factors Affecting E?igineering Properties 51

phase we have available to us another method for controlling engineering

properties, and in succeeding chapters, examples will be given to illustrate

its importance.

To have a full understanding of the consequences of alloying it

is necessary that one become familiar with the principles of alloying

as enunciated in Chapter 6.

EFFECT OF HEAT TREATMENT ON ENGINEERINGPROPERTIES

The use of heat treatment to alter mechanical properties dependsupon:

1. An allotropic transformation as in the case of steel, or

2. A decrease in solid solubility of a second phase with decrease in

temperature as exemplified by a fairly large number of binary sys-

tems having copper, aluminum, magnesium, iron, or a number ofother metals as the base metal. Heat treating based on this effect is

referred to as age hardening or precipitation hardening.

3.14. Allotropic Transformation

Iron undergoes an allotropic transformation from face centered cubic

to body centered cubic structure at 910°C (1670°F). When ironis alloyed with carbon up to about 0.8%, this transformation is depressedto lower temperatures and occurs over a temperature range. The normalproduct on slow cooling steels in this range of carbon content is a mixtureof iron (ferrite) and a product referred to as pearlite which is actually amixture of ferrite and iron carbide. Rapid cooling, however, tends toretard or repress this transformation with the result that other micro-structural products are produced, and in the instance of very rapidcooling a new metastable phase called martensite is produced instead ofiron and pearlite. This phase is the hardest product obtainable for agiven carbon content. It is fairly brittle as produced and is usuallysoftened and toughened by a subsequent tempering treatment: thehigher the temperature of tempering, the softer and tougher the resultantstructure. Utilization of the various heat treating possibilities based onthe allotropic transformation in iron produces a variety of microstruc-tural products and properties. Hence, heat treatment represents a basic

method for controlling the properties of steel. Inasmuch as this subject is

discussed fully in subsequent Chapters 12, 13, and 14, no details need bepresented here.

However, it should be pointed out that in addition to the operationof heat treatment, the engineer also has at his disposal the benefits avail-


able from alloying so that in steel we are usually utilizing a combination

of alloying and heat treatment. For example, by varying the carbon

content (alloying) the base hardness of the steel can be changed; whereas,

varying some of the other alloying elements will alter its susceptibility to

heat treatment. Combinations of these effects, therefore, provide a

degree of control and versatility not obtainable by either method alone.

Superimposed on these effects of alloying and heat treatment are those

resulting from the quality of the metal as originally produced in the ingot

and from the mechanical processing the steel may have had. These mat-

ters must also be kept in mind when designing a part.

a'

68

|.

-

105 F1

—//OT___

t— CHANCEIN SCALE

-J75

I

1 ;

'

140 F. • ^

VVL /F

1

32 F1

r~ £210 F-' P

'

'0 10 20 30 40 SO 60 75 100 ZOOTime al Aging Temperature. hours

Fig. 3.22. Quench-aging of a 0.06% carbon steel. Hardness after quenching from

717C (1325 K) and aging at indicated temperatures. (ASM Handbook)

3.15. Precipitation Hardening

The principles of precipitation hardening are discussed in Chapter 7

and will therefore not be considered in detail here. The process involves

heating an alloy of selected composition to a temperature where it is a

single phase and then cooling it rapidly to a temperature where it

normally would be a two-phase structure as a result of solubility changes

with temperature. The rapid cooling, however, retains it as a single phase

metal. It is then subsequently "aged" at room temperature or above to

initiate a start toward the equilibrium two-phase structure. As a result

of the atomic adjustments that occur during this treatment, certain

alloys can be hardened appreciably. This hardening action is a function

of time and tem|>eraturc as illustrated by Fig. 3.22. The maximum hard-

ness reached is decreased as the aging temperature is raised. Also, the in-

crease in aging temperature results in a rapid drop off in hardness which

is referred to as overaging. Fig. 3.22 illustrates the fact that even in iron-

carbon alloys, which were previously used to describe the principle of

hardening by allotropic transformation, age-hardening can also be en-

countered. This aging effect depends on a slight solubility of carbon in


body-centered cubic iron. This solubility increases with increasing tem-

perature. Consequently the necessary {but not exclusive) condition for

age hardening is present.

In addition to the hardness change illustrated by Fig. 3.22, there are

also corresponding changes in strength, inverse changes in ductility andtoughness, and changes in electrical conductivity and other properties.

Aging may also be combined with metal working treatment to provide

additional control over engineering properties.

EFFECT OF NUCLEAR RADIATION

For materials going into the construction of nuclear reactors, it is

necessary to know about their relative absorption coefficient for neutrons.

Materials with low neutron absorption coefficients are necessary whereneutron passage is desired, or in other words, where the maximum utiliza-

tion and minimum waste of neutrons is desired. On the other hand, a

high coefficient is necessary for shielding or control purposes. Theabsorption coefficient, then, is one of the engineering properties that mustbe established. As examples, elements like boron, cobalt, and cadmiumhave high absorption coefficients, but elements such as beryllium, sodium,magnesium, aluminum and zirconium have low coefficients. The property

of neutron absorption plus related properties of strength, formability.

UNIT CELL OF URANIUM WITH PERTINENT PLANES INDICATED

PREDOMINANT PLANES PREDOMINANT PLANES""*

'

EFFECTS OF PREFERRED ORIENTATION ON DIMENSIONAL STABILITY OF URANIUM DURING IRRADIATION

Fic. 3.23. Effect of nuclear radiation on •'growth" of a nuclear fuel material.

(Courtesy of General Electric Co. Hanford Works)


corrosion resistance, heat resistance, etc. must be evaluated for the design

of nuclear reactors, and the metallurgy of the materials used to construct

the reactor is therefore a very vital part of design.

Besides the aforementioned requirements, it is also necessary that the

susceptibility of the metal to radiation damage be known. Fig. 3.23 shows

what happened to a nuclear fuel material exposed to an intense neutron

atmosphere. The original smooth cylindrical specimen increased in size

or elongated and warped depending on crystal orientation. Proper

metallurgical treatment can considerably improve resistance to this type

of deterioration.

Structural metals subjected to neutron radiation are hardened or

strengthened and sometimes embrittled by such treatment. It has been

found that the face-centered metals are least damaged by such exposure,

whereas the hexagonal and body-centered cubic metals tend to become

brittle. Raising the temperature causes less damage. Heat treating by

annealing can be used to eliminate the embrittlement.

SERVICE FACTORS

In the preceding sections the factors affecting the engineering properties

have largely been those concerning the processing and preparation of the

metal for service. There are, however, additional effects that may have

an influence after the part is placed in service.

Lack, of attention to the problem of residual stresses as briefly dis-

cussed under cold working could lead to a premature failure of a part of

fatigue. Besides originating from cold working, these stresses can also be

thermally induced as by welding or heat treating. Without going into the

details of this point, since it requires a knowledge of the static and

dynamic properties as discussed in Chapters 4 and 5, it should be em-

phasized nevertheless that the stress distribution created by design or

metallurgy is unfortunately often overlooked and that many of the failures

which have been recorded can be attributed to this omission.

Another service factor of extreme importance to be covered in Chapter

22 is corrosion. It should be quite obvious that when a part is designed

for corrosive conditions the properties of that part will deteriorate in

time and cause failure unless a sufficiently resistant material is used or it

is replaced frequently enough to avoid damage. By the same token, un-

expected corrosion can seriously detract from the load-carrying ability

of a part in service and possibly cause premature failure. These are addi-

tional factors, therefore, that the engineer must observe.


SUMMARY

There is a very close relationship between the structure and properties

of metals. Part of the structure is established during the manufacture of

the metal, and this can carry over in part at least to affect the final

properties. Most of the structural modifications, however, depend on

subsequent processing, and such operations as grain size control, metal

working, alloying, heat treatment, and/or nuclear radiation are available

to the engineer for controlling the properties. In certain applications,

these operations may continue while the part is in service as for example,

at high temperatures or under corrosive conditions. Then it is necessary

for the engineer to recognize the effect this will have during the service

life of the part. It can be said, therefore, that unlike gases and liquids,

the properties of solid metals depend on past history, rather than on the

variables existing at the moment.

In order to gain an appreciation of the relationships and interrela-

tionships involved in producing a given set of properties, it is necessary

for the engineer to learn about the fundamental structure of metals as

discussed in Chapter 2, about the measurements of behavior as discussed

in Chapters 4 and 5, and about the principles of alloying and heat treat-

ment as discussed in succeeding chapters.

With the exception of corrosion behavior which is dependent on surface

conditions and chemical reactions, the behavior of a metal is governedby its internal structural condition.

QUESTIONS

1. What are the processes available to the engineer for controlling metalproperties?

2. How does the factor of metal "quality" enter into the problem of propertycontrol?

3. Describe the freezing of a dendrite.

4. Why is the dendritic structure absent on etched surfaces of pure metals?

5. What two factors affect the final grain size of a freezing metal?6. How arc these two factors affected by cooling rate and what additional

variables beside cooling rate may be used to control grain size?

7. Beside the growth of dendrites, what additional changes take place duringfreezing that may affect the quality of a metal?

8. Describe several ways besides the casting process which can be used tocontrol the grain of metals.

9. Of what practical importance is grain size control? Cite a number ofexamples.

10. Compare the slip behavior in the face-centered cubic, hexagonal close-packed and the body-centered cubic crystals.


11. How does the twinning mechanism differ from the slip mechanism and in

what respects is it similar?

12. Why is the twinning mechanism important in some metals?

IS. Describe a number of the structural changes accompanying cold working of

polycrystalline metal.

14. Compare the effects of cold working and hot working on such factors as

(a) tensile strength, (b) directionality of properties, (c) toughness, and (d)

ductility.

15. Suggest possible applications for hard or spring temper brasses.

16. Why is the relief of internal stresses during the recovery period when

annealing cold worked metal of industrial importance?

17. Why is annealing a cold-worked metal sometimes considered a fabrication

procedure?

18. What damage may result from annealing at too high a temperature for a

metal that has experienced a wide variation in the amount of cold work?

19. What are the possible effects on properties when one metal is added to

another to (a) form a solid solution? (b) cause another phase to appear?

20. What basic ways are available for altering mechanical properties by heat

treatment?

21. Explain how design for nuclear reactors involves factors not encountered

in other types of applications.

CHAPTER

Static Properties ofMetallic Materials

Lee L. Amidon, M.S.M.E., Professor and Head, Depart-ment of Mechanical Engineering, South Dakota State

College, Brookings, South DakotaWalter E. Short, M.S.M.E., Associate Professor of

Mechanical Engineering, Bradley University, Peoria,

Illinois

4.1. Properties of Metallic Materials

IHE properties of metallic materials are com-monly divided into three general classes: (1) physical constants, (2)electrical and magnetic properties, and (3) engineering properties.

The first class includes elastic constants, density and related properties,

thermal expansion, vapor pressure, emissivity, and thermal conductivity.The thermal properties are affected by temperature and phase changes.

Dimensions change and heat is absorbed or discharged at such times.

Problems in heat-treating operations may be anticipated as a result.

These properties are of interest to the physicist as well as to the metal-lurgist. Usually, however, they are of interest to the physicist as a stepin his study of theories of matter rather than as properties of metallicmaterials. Some of the physical constants are of interest to the engineer:the elastic constants surely are, because they are important in the designof structures; density is of interest if the strength-weight ratio must beconsidered, as in aircraft construction, high speed railway trains, andmotor-haulage vehicles; thermal expansion and thermal conductivitymay be of importance for some engineering applications and of none forothers. The essential fact for the engineer to know about the physicalconstants as a group is that in general they are determined by standardizedmethods that yield fairly precise, readily reproducible values that canbe used in engineering design without trepidation regarding their mean-ing and accuracy. Values for the constants are well established and canbe obtained in handbooks of physical tables.

57


Electrical and magnetic properties include thermoelectric properties,

resistivity, magnetization, induction, permeability, and coercive forces.

These properties vary widely between different metals and their alloys

and must be investigated thoroughly before making final selection for

any given service. Aluminum on a weight basis may be a better conductor

than copper and cheaper as well, but its tensile strength may not besufficient to make it safe for the long spans desired on high-tension trans-

mission lines so we compromise by using a steel core to provide strength

and the result is an efficient carrier at low cost. Table 4.1 gives typical

values of resistivity and the manner in which these values will change

with temperature. The resistor elements for a dynamometer will call

for a material with great resistance to the flow of electricity while in a

Table 4.1. Resistivity of Typical Metals and Alloys

Material

Copper

Aluminum

Iron

Constantan (55% Cu, 45% Ni)

.

Nichrome (80% Ni, 20% Cu) . .

transmission line the 1 2R loss should be as near zero as possible.

Magnetic properties are essential in the design of motors, generators,

transformers, metering equipment, and huge lifting magnets. Themagnetic properties are the result of the plus and minus magnetic fields

set up by the electrons whirling in their orbits. Ferromagnetic materials

such as iron, nickel and cobalt exhibit this property to the greatest extent.

Single atoms will not form a permanent magnet but several atoms

grouped together will form a magnetization unit. This small group is

known as a domain. There will be several domains in one metal grain.

Ordinarily the domains of a metal bar will have random orientation in

which case the bar will not be magnetic. An external magnetic field will

align the orientations found in the many domains and the bar is mag-

netized. If upon removal of the external field, the domains remain

oriented we have a hard permanent magnetic substance. On the other

hand if they tend to lose orientation then we have a soft or temporary

magnetic substance.

Resistivity at 20° C.

Microhm-cm.Temperature Coef.

Increase per ° C.

1.7241 .00393 (0-100° C)

2.654 .00429 (20° C)

9.8 .0056 (0-100° C)

-19 .0002 (20-100° C)

107.9 .0019 (20° C)

Static Properties of Metallic Materials 59

Temperature has a very decided influence on ferromagnetic materials.

Increase in temperature causes the magnetization to decrease and finally

to disappear altogether at what is known as the Curie temperature. Thistemperature is 1414°F for iron, 2039°F for cobalt, and 665°F for nickel.

The domains disappear at these temperatures, which for iron is below the

point at which it becomes austenitic. These temperatures may be altered

drastically by small additions of alloying elements or impurities.

Electric properties are extremely important to the electrical engineer,

and the methods for their determination are precise; but because of their

limited application to general engineering, further discussion is beyondthe scope of this book.

From the standpoint of engineering in general and of those fields of

engineering which deal specifically with the design, erection, and main-tenance of structures and machines, the last class, engineering properties,

is of vital importance. For convenience, they may be divided into three

classes: static mechanical properties, dynamic mechanical properties, andmiscellaneous properties. There is a very close relationship between the

engineering properties and the metal lographic structure, which will bemade clear in later chapters of this book. Because of this relationship

between micro-structure and physical properties, metallic materials are

structure sensitive. In other words, they are affected by changes in the

structure even though the chemical composition is unchanged. As welearn how to produce space lattices which are undistorted during the

mechanical processing of our materials, much improvement in strength

and physical properties may be anticipated.

Precise instructions for the physical testing of engineering materials

have been established by the American Society for Testing Materials andsummarized in the Handbook of the American Society for Metals. It

must be understood that the properties established by these tests are use-

ful primarily in maintaining uniformity of material once a satisfactory

design has been established. The most useful form of testing is that madeupon a completed structure under actual service conditions. Time andcost usually limit this procedure; therefore, the following pages will

evaluate the advantages and shortcomings of the various tests of a given

material for use in engineering machines and structures.

It cannot be emphasized too strongly that the data from a small test

specimen should be accepted with caution unless it is certain that the test

piece is representative of the entire lot of material whose properties are

being determined. A large number of specimens reduce the chances forerror of even the most accurate and reproducible tests. The shape of the

designed part may determine to a large extent the distribution of stress


in a given cross-section. The presence of stress raisers (holes, sharp

changes in cross-section, scratches, inclusions) may increase the stress 3 to

5 rimes the average value. Ductile materials under static loading will yield

and little harm will be produced. However brittle materials and parts

under repeated or reversed loading will start to tear and fatigue failure

of the part thus ultimately occurs.

Many of these stress concentrations can be readily corrected by prop-

er design changes. Frequently the magnitude of stresses at concentra-

tion points cannot be readily determined by analytical methods. The use

of brittle lacquers applied to the structure under simulated service con-

ditions will reveal trouble spots which can then be corrected or eliminated.

This design technique known as stress coating will supplement the

standard test data. The use of strain gages in a similar manner will

determine the location, direction, and magnitude of the unit strain at

locations where stress concentrations are suspected. The unit strain can

then be used to calculate the actual stresses rather than the average stress

that might be determined by the ordinary methods of design. Another

method of determining stress concentrations is based on the optical

properties of transparent materials. Some of these materials are optically

isotropic when in an annealed condition, but become optically aniso-

tropic when subjected to external stress. This method requires a plastic

model, which, of course, limits its use to a certain degree. Hidden stress

raisers such as blow holes, slag inclusions, malformed space lattices or

grain structures may be revealed by sonic testing described later in this

chapter. Other methods frequently used to determine surface and hidden

defects are radiography and magnetic particle detection. All of the above

mentioned methods have the advantages of being non-destructive tests.

Physical properties, then, must always be considered in relation to the

structure itself and sound engineering judgment used in its selection,

considering always the limitations of the material for a given service or

condition.

4.2. The Relative Standardization of Static Tests

Static mechanical properties, which are a measure of the resistance of a

material to a steadily applied load, include tensile strength, compres-

sive strength, torsional strength, bending strength, hardness, and their

related properties, such as yield strength, elongation, and others deter-

mined incidentally to the first named. The determination of some of

these properties has been given considerable attention by testing societies

in the principal metal-using countries with the result that methods and

specimens are fairly well standardized.


The most important and the two most common tests for the evaluation

of the static properties of metallic materials are tensile and hardnesstests. The former is well standardized throughout the world. If the

tensile strength of a certain steel is reported as 60 kg. per sq. mm. in

Germany, it is certain that a standard specimen of the same steel will

have a strength of about 85,000 lb. per sq. in. in the United States andthat either value represents accurately the stress necessary to break a

i/4-in. specimen of that particular steel. The tensile strength of a steel of

certain composition, made by a particular process, and of otherwise satis-

factory quality can be used in the design of a bridge in Japan, even if

the steel was made and the strength determined in France. Because hard-ness tests are made rapidly and cheaply and because they do not destroy

the material, they are used widely. That hardness is not a definite prop-erty but a combination of several properties makes the interpretation of

some of the tests difficult. Hardness tests are not standardized, althoughthe measure of a specific kind of hardness, namely, resistance to indenta-tion, is in a fairly advanced state of development. Hardness tests tookon added importance during the war years as new and untried low-alloy

steels were substituted freely for some of the older alloy steels on the

basis of the hardness of an end-quench hardenability test.

Static tests used less frequently, and for which standardization has

not proceeded so far, include compressive, torsion, and bend tests. Noneof these is used so widely as are tensile or hardness tests, although the

transverse test, a form of bending, is the most widely used and is prob-

ably the most valuable single test for the evaluation of gray cast iron as

an engineering material. With this one exception, none of these tests is

standardized, and it is not always easy to interpret the values obtained or

to apply them to the design of engineering structures.

4.3. Tensile Strength

The commonly determined tensile properties, including tensile

strength, yield strength, proportional limit, elongation, and reduction of

area, are obtained on a single test specimen (see Fig. 4.5) . The accuracy

and reproducibility of the results depend upon the dimensions of the

specimen, the sensitivity of the machine, the alignment of the specimen

in the machine, and above all on how accurately the specimen represents

the whole section. Considerable attention has been paid to these factors

by the testing societies and the builders of machines in all countries,

with the result that reported values are about as accurate as for anyengineering property.


17.000

16.000

15.000

14,000

13,000

12,000

11,000

« 10.000TS

§9000o°:8O0o

| 7000

ta 6000

5000

4000

3000

2000

1000

c 0.002

/^y/'

//

c/hn

/,/ /

1 1 /1

1

1

p

I

Is1/"1/

11

i

i

r

1

1

1

_i A ^/

1

11 1

0.004 0.006 0.002

Strain, inch per inch0.004 0006

Fie. 4.1. Stress-strain curves in tension for (A) annealed low-carbon steel and (B)

cold-worked low-carbon steel.

As is well known, the tensile strength of a specimen is the maximumload before rupture (point C, Fig. 4.1 A) divided by the original cross-

sectional area. It is reported in pounds per square inch, tons per square

inch (England) or kilograms per square millimeter (continental

Europe) . Specimens and other variables have been standardized by the

American Society for Testing Materials *so that further discussion is

unnecessary.

Owing to the inherent variations in structural constituents and in

composition which may cause variations in tensile strength of as much as

± 1,500 lb. per sq. in., there is a growing tendency to test full-sized struc-

tural members with large machines. This type of test is especially valu-

able for structures that are riveted or welded.

If a tensile test is made on a larger or a smaller specimen than is com-

monly used (0.5 in.) , the tensile strength may be considerably higher or

lower than that obtained on the standard bar. For ductile materials,

which include most rolled, forged, or heat-treated carbon and alloy

• A. S. T. M. Standards, 1955.


\STRAIN HARDENING BEGINS

Fic. 42. Greatly enlarged view of stress-strain diagram in region near yield point.

steels, the error is not great; for a brittle material, such as cast iron, the

variation in strength may be considerable if differences in cooling rates

of cast specimens of various sizes result in differences in structure.

For a number of years, structural design has been based on either the

tensile strength or the yield strength with the use of appropriate factors

of safety. For example, a working stress for carbon steel may be of the

order of 18,000 psi. although the yield point of this material may be ap-

proximately twice this value. It might be noted that the portion of the

stress strain curve shown in Fig. 4.1A at point B would have a consider-

ably different appearance if the curve were plotted with a different scale.

It will be noted from Fig. 4.2 that a large amount of yielding at a rela-

tively constant stress will occur before strain hardening begins. Thisphenomenon is the basis of recent work in the field of plastic design.

4.4. Elastic Limit, Proportional Limit, and Modulus of Elasticity

There is occasionally confusion among engineers and steel men con-cerning the meaning of the terms Elastic Limit, Proportional Limit, YieldPoint, and Yield Strength. The elastic limit is the maximum stress that

a material withstands without permanent set. Although it is practically

impossible to determine the true elastic limit experimentally, it is pos-sible with present-day extensometers to detect a relatively slight vari-

ation from Hooke's law,* and by careful testing the point of this variationcan be fixed with considerable exactness on the stress-strain diagram(point P, Fig. 4.1A) . The load which this point represents, divided by

• Hooke's law states that in clastic materials stress is proportional to strain. It is

generally held that in metallic materials the application of stress, within the rangewhere deformation is elastic, actually produces some plastic deformation. The amount,however, is so small that for all practical purposes, and especially for engineeringdesign, Hooke's law is valid.


0.004 0.012 0.016 0.020

Strain, in./in.

0.024 0.032

Fic. 4.3. Stress-strain diagrams of common metals. (H. W. Gillett, Batelle Memorial

Institute, "The Behavior of Engineering Metals," p. 15.)

the cross-sectional area, is the proportional limit. It is frequently deter-

mined in testing laboratories but is seldom used in the design of struc-

tures. In most annealed and heat-treated carbon and low-alloy steels, the

proportional limit is practically identical with the elastic limit and, as

Fig. 4.1A shows, is slightly below the yield point. If the proportional

limit of such steels is found to be much below the yield point, it is likely

that internal stresses are present. The determination of proportional

limit is frequently made to detect the presence of such stresses.

Modulus of elasticity in tension is the ratio of stress to strain with-

in the elastic range. It is given by the slope of the stress-strain curve

(see Fig. 4.1 A) up to point P. For carbon and low-alloy steels of any

composition and treatment, the value is approximately 29,000,000 lb.

per sq. in.; for aluminum alloys, the modulus in tension is about

10,000,000 lb. per sq. in.; for copper and copper alloys, about 15,000,000

lb. per sq. in. Modulus of elasticity may be determined in compression as

well as in tension; values are about the same for either method. Another

important quantity is the shear modulus, or modulus of rigidity, which

is invariably smaller than the modulus of elasticity (Young's modulus)

.


-*«A!

KM^se-r'*

0.002 0.004 0.006

Strain, in. per in.

0.008

Fig. 4.4. Stress-strain curves for two gray cast irons tested in tension. (/. W. Bolton,Ptoc. Am. Soc. Testing Materials, v. 32, 1932, II, p. 477)

The modulus of elasticity is a measure of the stiffness of the metal.It is not appreciably affected by heat treatment. A part too springy inthe annealed state will not have the condition improved by hardening.The modulus decreases with increases in temperature moderately at first,

but may change very rapidly when some specific high temperature is

reached. The rate of change varies with the alloy chosen. Some typicalvalues of stress-strain diagrams and moduli of elasticity are shown in Fig.

There are some metallic materials-gray cast iron and austenitic alloysteels are outstanding examples-which when tested in tension have astress-strain curve that is curved from the origin (Fig. 4.4) . Such ma-terials have no readily determinable proportional limit and yield point.Moreover, they have no true modulus of elasticity; reported moduli arearbitrary values that measure the relative stiffness under specifiedconditions of loading, usually with a stress equivalent to 25 per cent ofthe tensile strength.


4.5. Yield Point and Yield Strength

Engineering structures and machines subject to static loads are designed

on the assumption that the stresses that must be withstood will not cause

appreciable plastic deformation (permanent set) . As plastic deformation

probably starts in a large number of independent locations, it is difficult

to determine where it actually begins. Plastic deformation does not be-

come measurable until a large part of the section has been affected, andeven then very sensitive equipment is necessary to locate this point—the

proportional limit. Fortunately, practical experience has shown that, at

ordinary temperature, the limit of usefulness of a metallic material is that

stress which produces a plastic deformation that is readily detected.

Higher stresses than this may cause considerable damage; with lower

stresses the damage is negligible. This stress is the yield point. In ductile

materials, it is indicated by a marked elongation of the specimen with

no increase in load. It is readily detected with a pair of dividers or by the

drop of the beam of the testing machine. The yield point is of con-

siderable value to engineers in the design of structures, but it has the

disadvantage that it is not well marked in hard materials, some of which

have no true yield point. To overcome this disadvantage the American

Society for Testing Materials recommends the use of yield strength in-

stead of yield point.* This is the maximum stress that produces a

specified set, usually 0.1 or 0.2 per cent of the gage length. In hard

materials, the point can be readily determined from the stress-strain

curve (point B, Fig. 4. IB); in soft materials, it corresponds closely to

the yield point determined by drop-of-beam. The yield strength is an

important and readily determinable value and should replace the yield

point in all reports of tensile properties.

The value for yield point is affected by the rate of strain, that is, by

the speed of the cross head of the machine. Generally, however, the error

is not great for the speeds used in testing to determine whether a ma-

terial is within specifications. For example, increasing the rate of strain

tenfold, as from 0.2 to 2 per cent per min., increases the yield point only

about 2,000 lb per sq. in. The American Society for Testing Materials

specifies that the cross-head speed should not exceed i/8 in. per min. for a

2-in. gage length.

Very soft materials, especially annealed low-carbon sheet, frequently

have two yield points: an upper point, at which there is a decrease in

stress (A in Fig. 4.1 A) marked by a sudden drop of the beam of the test-

ing machine, and a lower yield point (B in Fig. 4.1A) , where strain

1 A. S. T. M. Standards, 1955.

Static Properties of Metallic Materials

Radius not less U gj „.'

than'/8" \| Parallel sectionNole : - The gage length,

parallel sect/on, andfillets shall be asshown, but the endsmay be ofany shapeto fit the holders ofthe testing machinein such a way that theload shall be axial

67

o— fiO.Ol" -

,k

—

2"i0.00S"—AGage length

for elongationafter fracture

h- ?gm/h mgnps-*^*- 3f mih between grips >j<- 2g/n/ningrips-

"1

w+0.003°to0.0OS"(See Note!

w <=o.soo"io.o/o" I^r

,„,» .

x to +0.003 to 0.0051 Zj mm. >-} \ , „Ifeducedsection— 6'min.

''Radius 0.5"to3"

. -t

t = Thickness ofmaferial

Note: Gradual taper from ends ofreducedsection to middle.

Allmachining dimensions are shown below andtestingdimensions above specimenFig. 4.5. Standard round and flat tensile specimens. (AS.TM. Standards E8-46)

hardening begins and where additional stress is necessary to producefurther strain. The difference between the upper and the lower yieldpoint is considered to be an indication of the deep-drawing capacity ofannealed low-carbon sheet.

4.6. Elongation and Reduction of Area

In the United States, the standard round tensile-test specimen has adiameter of 0.505 in. (cross-sectional area 0.2 sq. in.) in a gage lengthof 2 in. (Fig. 4.5 top) . Elongation values determined on round speci-mens larger or smaller than the standard are directly comparable with thestandard if it has a gage length equivalent to

G = 4.5vTwhere G is the gage length and A the cross-sectional area in squareinches. Unfortunately, since the standard tensile specimens used inGreat Britain, Germany, and France have a different relation betweengage length and cross-sectional area from that expressed by the aboveformula, values obtained in those countries, while always reproduciblewhere determined, cannot be compared directly with elongation valuesdetermined in the United States.


Fie. 4.G. BrineU hardness machines: left, hand-operated laboratory model; right,

high-speed production testing model. (Courtesy of Tinius Olsen Testing MachineCompany)

For some products, especially for plate and thin sheet, specimens are

used that may differ in size and shape from the standard (Fig. 4.5 bot-

tom) . Gage lengths for tensile specimens of such products vary from 2

to 10 in., depending upon the material and its size. As elongation is re-

ported as a percentage of the original length, the percentage elongation

of a given material is higher the shorter the gage, owing to the fact

that most of the elongation takes place in the "necking-down" that occurs

at the point of fracture. When tensile properties are determined on speci-

mens that differ in dimensions from the standard, the gage length is

always given. Even if a standard specimen is used, most metallurgists

report the gage length (2 in.) so that there will be no misunderstanding.

Reduction-of-area values are always reported as a percentage of the

original area so that values for this property should be comparable.


Elongation and reduction of area give a rough, but usually a fairly

clear, idea of the ductility of the metal tested. Although engineers gen-erally demand a relatively high elongation, metallurgists place moreemphasis on reduction of area as a criterion of ductility and believe thatthe importance of elongation is usually overemphasized. It is doubtful if

there are any applications for which an elongation of more than 8 or 10per cent in 2 in. is needed. As an example of the exaggerated importancesometimes given to ductility, the crankshaft of the internal-combustionengine may be cited. For years crankshafts were made from highlyalloyed steels which were carefully heat treated so that they would beas ductile as possible coincident with the high hardness required. Thecast-iron crankshaft now used successfully in some automotive enginesshows very little elongation in the tensile test and has practically noresistance to single-blow impact.

It is generally true that in carbon steels and many nonferrous alloyshigh strength values are accompanied by low values of elongation andreduction of area and vice versa. This general relation does not alwayshold for alloy steels. One of the principal reasons why alloy steels areused in place of carbon steels is that a better combination of yieldstrength and ductility-as shown by elongation and reduction of area(and impact resistance) -justifies their higher cost for many applications.

4.7. Hardness

Physicists do not recognize hardness as ordinarily determined as afundamental property of matter; to engineers it is a property of greatimportance. Everyone thinks he knows what hardness is, but no onehas been able to define it satisfactorily. Probably as good a definition asany is that hardness is resistance to permanent deformation. Hardnessis complex. First and most important, it is the resistance of a material toindentation. It is also the resistance to cutting with a tool, which involvesresistance to indentation plus tensile strength and toughness. It is alsoresistance to abrasion, which likewise involves indentation resistance,strength, and toughness.

There are at least eight methods for the determination of hardness, ofwhich two-Brinell and Rockwell-are the more frequently used. Theliterature on hardness testing is extensive. Since standards for makingBrinell and Rockwell tests have been set up by the American Society forTesting Materials,* the methods for making these tests can be dismissedwith a few words.

* A. S. T. M. Standards, 1955.


Fie. 4.7- Rockwell hardness lesliug machine. {Courtesy of Wilson Mechanical In-

strument Company)

In the Brinell test, a definite load is applied to the piece to be tested

by means of a hardened steel (or tungsten carbide) ball. The diameter

or the depth of the resulting impression is measured, and the Brinell

hardness number is calculated by dividing the load applied (in kilo-

grams) by the area of the impression (in square millimeters) , which is as-

sumed to be spherical. Owing to a slight deformation of the steel ball

and to the fact that the impression is not truly spherical, the hardness

number varies somewhat with the load applied and with the diameter

of the ball. These have been standardized in this country: a 10-mm. steel

ball and a load of 3,000 kg. are used for steel and a few nonferrous alloys;

and the same ball and a load of 500 kg. are used for soft materials. Theload is applied for 20 to 30 sec, the diameter of the impression varies

from about 6 mm. for soft materials to about 2.5 for hard materials and

is measured with a micrometer microscope. The Brinell machine is shown

in Fig. 4.6.


The Rockwell hardness machine (Fig. 4.7) is designed to test materialsof widely varying hardness by interchangeable indenters. For hard ma-terials, a diamond cone is used; for softer metals and alloys, y16 -in. andl/g-in. hardened steel balls are employed. In the Rockwell test, a minorload of 10 kg. is applied to force the penetrator below the surface of thematerial. The major load is then applied. The hardness, which is in-versely proportional to the depth of penetration, is read directly on ascale, several of which are used.* A Rockwell hardness number is mean-ingless unless the scale symbol accompanies the value. Loads and scalesused with the Rockwell machine are as follows:

Scale PenciratorMajorload,

kg-

Materials tested

A

BC

D

E

Diamond cone

Hi-in. ball

Diamond cone

Diamond cone

H-in. ball

60

100

150

100

100

Tungsten carbide and other very hard materials.Thin, hard steel, such as razor blades, nitridedsteel, and the like

Soft steels, hard nonferrous alloys

Steels of 90,000 to 350,000 lb. per sq. in. tensilestrength

Materials with thin, hard surface layer; case-

hardened and nitrided steel

Soft materials, aluminum alloys, bearing metals,and the like

There is a third hardness test, which is coming into relatively wideuse, especially in the laboratory; this is the Vickers test in which a dia-mond pyramid is pressed into the surface of the material. Other hardnesstests include the scleroscope, now rarely used, which determines hard-ness by measuring the rebound of a diamond-pointed weight falling froma fixed height; and a number of scratch tests in which needles of definitehardness or diamond or sapphire points are dragged over the polishedsurface. Scratch tests are sometimes useful in the research labora-tory to detect differences in the hardness of the various constituents inan alloy but are rarely used industrially. A new laboratory hardness testerhas been recently developed by Frederick Knoop at the National Bureauof Standards. A carefully prepared diamond pyramid is forced into a• There are a number of other scales in addition to the ones listed in the tablewhich are used with i/e -, i/4 -, and i/2-in. ball indenters; these are used occasionally for

special hardness determinations. There is also available a special Rockwell machinefor the determination of superficial hardness. All of these are discussed in A. S. T. M.Standards, F.18-42.


polished metal surface and the dimensions of the resulting impression

are read with a microscope. The Knoop tester is flexible and apparently

gives precise results, but at present it is adapted only to the laboratory.

A practical qualitative method widely used for the determination of the

hardness of steel is the file test. In the hands of an experienced man,

relatively slight differences in hardness can be detected by rubbing the

material with a file of known hardness. Thus, a steel with a Rockwell Chardness of 54 can be differentiated from one which has a Rockwell Chardness of 60. Obviously, too much depends upon the personal equa-

tion for the file test to be useful except in the hands of an experienced

man.

4.8. Comparison of the Various Flardness Tests

In making hardness tests by any method a number of precautions are

necessary. In the first place, the specimen must be rigidly supported,

and the load must be applied perpendicularly. In the second place, the

surface tested must be flat and relatively smooth, and any surface scale

or decarburized areas must be removed. In the third place, specimens

must be thick enough so that the material of the anvil on which the

specimen rests does not affect the indentation in the specimen. In general,

hardness impressions on thin sections are not accurate unless the material

has a thickness equivalent to 10 times the depth of the impression.

All tests for hardness by indentation cause strain hardening to some

degree. The Brinell test is as close to an ideal test as possible as it distrib-

utes strain hardening over a relatively wide area, but it lias the dis-

advantage that the amount of strain hardening may not be uniform and,

further, that the method is too restricted in application. With a 500-kg.

load, the Brinell test is quite satisfactory for aluminum alloys and for

brass and bronze; with a 3,000-kg. load its use is restricted to steels with a

tensile strength between 45,000 and 250,000 lb. per sq. in. When the test

is used on steels that are harder than 500 Brinell (tensile strength greater

than 250,000 lb. per sq. in.) , there is likely to be some permanent defor-

mation of the steel ball that causes all subsequent hardness determina-

tions made with this ball to be inaccurate. For steels with high hardness

a special tungsten carbide ball is used.

The Rockwell machine is designed to overcome some of the limitations

of the Brinell test. It is flexible and can be used for hard materials, it

makes a small impression that is not likely to spoil the appearance of the

piece, and it can be used to measure the hardness of relatively thin sec-

Static Properties of Metallic Materials 7:5

20

Scleroscope hardness

24 30 40 50 60 70 80 90 100100

150

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-10 10 20 30 40 50 60 70Rockwell C hardness

Fie. 4.8. Relation of hardness values toeach other and to tensile strength for carbonand low-alloy steels. (French and Sands)

accurately. The Vickers test, ideal as aso well adapted to routine testing as the

tions. For example, on steel of

150,000 lb. per sq. in. tensile

strength the minimum thickness

that can be tested accurately by

the Brinell method is 0.15 in.;

with the Rockwell diamond cone

(C scale), it is 0.06 in. TheRockwell machine also has the

advantage that hardness num-bers are read directly from the

scale; it is unnecessary to meas-

ure the width of an impression.

The Rockwell machine is, there-

fore, widely used for routine

testing and for the inspection of

heat-treated parts.

The Vickers hardness test,

which is rapidly becoming stand-

ardized, is the most flexible of

all hardness tests and can be

used without change of pene-

trator on materials of widely

varying hardness by varying the

load applied. The impression is

small and, owing to its diamondshape, can be measured very

laboratory tool, at present is not

Rockwell.

4.9. Relation among Hardness Tests and between Hardness andTensile Strength for Steel

All indentation hardness methods fail to measure the hardness of theundeformed metal. An exact mathematical correlation of the varioushardness numbers is therefore impossible as the amount of strain harden-ing varies from test to test and, furthermore, varies from metal to metal.A large amount of time and effort has been expended in studying therelation among the hardness numbers as obtained by the Brinell, Rock-well, Vickers, and scleroscope machines. One of these correlations forcarbon and low-alloy steels is shown in Tig. 4.8* A word of caution is

• H. J. French and J. W. Sands (editors), Nickel Alloy Steels, The InternationalNickel Co., Inc., New York, 1934.


necessary concerning such a correlation. Brinell values may be converted

to Vickers or Rockwell C values, or vice versa, with an expected error of

not more than 10 per cent, except for hardness numbers over 500 Brinell

where the uncertainty becomes greater. The error in converting Brinell

or Rockwell to scleroscope may be considerably greater than 1 per cent.

The most recent conversion table and probably the most accurate for

carbon and low-alloy steels was compiled by a committee of the Society

of Automotive Engineers and the American Society for Testing Mate-

rials.* These values are reproduced in part in Table 4.2.

The resistance to indentation is directly related to compressive strength.

For most carbon and low-alloy steels there is a fairly close relation be-

tween compressive strength and tensile strength; hence, indentation

hardness values for such steels can be converted into tensile strength

with reasonable accuracy. Results of a large number of investigations on

this relation indicate that the ratio of tensile strength (in units of 1,000

lb. per sq. in.) to Brinell hardness number varies from 0.45 to 0.58, and

heat treatment apparently has no effect on the ratio. The ratio obtained

most frequently is 0.47 to 0.50. Thus, if the Brinell hardness number is

given, tensile strength can be calculated by multiplying the hardness

number by 480; the result should be accurate within =p 5,000 lb. per sq.

in. The relation between the various hardness numbers and the tensile

strength for carbon and low-alloy steels is shown in Fig. 4.8 and in Table

4.2. Although this relation is fairly accurate, there may be considerable

variation in individual specimens. The actual ratio depends upon struc-

ture, with composition a possible complicating factor, and although in

most cases the conversion may be made with an error of ± 5,000 lb. per

sq. in., errors as great as ± 12,000 lb. per sq. in. may be encountered.

4.10. Relation among Hardness Tests and between Hardness and

Tensile Strength for Nonfcrrous Alloys

In contrast to carbon and low-alloy steels, which all harden approxi-

mately the same amount when cold worked (strain hardened) and which

show, therefore, a fairly close relation among hardness tests and between

hardness and tensile strength, the nonferrous alloys differ considerably

in their capacity for strain hardening and no such relation can be worked

out for these materials as a class. For example, a high-strength duralumin

that has been heat treated will have a tensile strength of around 70,000

lb. per sq. in. and a Brinell hardness (500 kg.) of 115; whereas a cast

manganese bronze containing 55 to 60 per cent copper and 38 to 42 per

cent zinc will have a tensile strength of 90,000 lb. per sq. in. and a Brinell

1 A. S. T. M. Standards, 1955.


Table 4.2. Approximate Hardness Conversion Numbers for Steel

Brinell. 10 mm. ball, Rockwell hardness nun ber1

3,000-kg. load Vickersdlamond- Approxi-

mate1 pyramidhardnessnumber

Sclero-

Diameter A-scale.B scale.

100-kg. C-scale. D-scale.scope

hardnesstensile

strength.of Hardness 60-kg. load. 150kg. 100 kg. (DPH) number lb. per

impression number load. Hi-in. load, load. *,

sq. in.mm. diamond ball diamond diamond

2.40 653 * 81.2 60.0 70.7 697 81

wn2.45 627 • 80.5 58.7 69.7 667 79 323,0002.50 601 • 79.8 57.3 68.7 640 77 309,0002.55 578 * 79.1 56.0 67.7 615 75 297,000 'JfWUf2.60 555 * 78.4 54.7 66.7 591 73 285,000

2.65 534 * 77.8 53.5 65.8 569 71 274,000'>--

2.70 514 • 76.9 52.1 64.7 547 70 263,0002.75 495 ' 76.3 51.0 63.8 528 68 253,000 -

2.80 477 75.9 50.3 63.2 516 66 247,000

***/tf2.85 461 75.1 48.8 61.9 495 65 237,000

2.90 444 74.3 47.2 61.0 474 63 226,0003.00 415 72.8 44.5 58.8 440 59 210,000 '-

3.10 388 71.4 41.8 56.8 410 56 195,0003.20 363 70.0 39.1 54.6 383 52 182,0003.30 341 68.7 36.6 52.8 360 50 170,000

3.40 321 67.5 34.3 51.0 339 47 160,0003.50 302 66.3 32.1 49.3 319 45 150,0003.60 285 65.3 29.9 47.6 301 43 141,0003.70 269 64.1 27.6 45.9 284 40 133,0003.80 255 63.0 25.4 44.2 269 38 126,000

3.90 241 61.8 100.0 22.8 42.0 253 36 118,0004.00 229 60.8 98.2 20.5 40.5 241 34 111,0004.10 217 96.4 17.5 228 33 105,0004.20 207 94.6 15.2 218 32 100,000 '0*i</»'

x.

4.30 197 92.8 207 30 95,000

4.40 187 90.7 196 28 90,0004.50 179 89.0 188 27 87,0004.60 170 86.8 178 26 83,0004.80 156 82.9 163 24 76,0005.00 143 78.7 150 22 71,000

* By tungsten carbide ball.

hardness of less than 100. Copper hardened with arsenic, in the cold-

rolled (hard) condition, will have a Rockwell B hardness of 95 to 100and a tensile strength of about 60,000 lb. per sq. in.; whereas admiraltymetal containing 71 per cent copper, 28 per cent zinc, and 1 per centtin and of the same Rockwell B hardness will have a tensile strength of

95,000 lb, per sq. in. or higher.


The only nonferrous alloy for which hardness conversions have been

worked out is cartridge brass. Some of these values are collected in

Table 4.3.* All the thousands of other alloys, with the possible exception

of the high-nickel alloys, which apparently harden much like steel, are

quite evidently a law unto themselves, and each must be considered

as a special case.

Table 4.3. Approximate Hardness Conversion Numbers for Brass (70 Per Cent

Copper, 30 Per Cent Zinc)

Brinell, 10-mm. ball Rockwell hardness numberVickcrs

diamondpyramidhardness

number(DPH)

Diameter

of

impres-

sion,

mm.

Hardness numberB scale.

100-kg.

load,

F scale.

60-kg.

load,

Tscale. t

30-kg.

load,3,000-kg. 500-kg.

load* load Ke-in. ball Ho-in. ball Hs-in. ball

2.00 158 91.0 108.0 75.5 183

2.10 143 85.5 105.5 72.5 165

2.20 130 79.5 102.5 69.5 149

2.30 119 74.0 99.0 66.0 135

2.40 653 109 68.5 96.0 62.0 123

2.50 601 100 63.5 93.5 59.0 113

2.60 555 93 59.0 91.0 55.5 105

2.70 514 86 54.0 88.0 52.5 98

2.80 477 80 47.5 84.4 48.0 90

2.90 444 74 40.0 80.0 43.0 82

3.00 415 69 33.5 77.0 38.5 77

3.10 388 65 28.7 74.0 34.0 73

3.20 363 61 18.5 68.5 29.0 66

3.30 341 57 12.5 65.0 23.0 62

3.40 321 53 61.0 18.0 58

3.50 302 50 56.5 12.0 54

3.60 285 48 53.5 52

3.70 269 45 47.0 48

3.80 255 42 40.0 45

3.90 241 40 43

* For steel, included for comparison.

t Rockwell superficial hardness: initial load 3 kg.; major load 30 kg.

If the proper precautions are observed, indentation hardness tests are

accurate and readily reproducible; and if judgment is used, the tests

give a reasonable approximation of the tensile strength and a rough idea

of machinability and wear resistance for most steels but not for most

• A. S. T. M. Standards, 1955.


nonferrous alloys. Their greatest value lies in their speed and cheapness,which make them ideal for checking uniformity of heat treatment andas an acceptance test for hardened steels, especially tools and dies, andfor a number of other metallic materials.

4.11. Shear, Compression, and Bend Tests

The determination of the resistance of a material to failure by shearor torsion is important for many engineering applications, especially forshafts subjected to twisting, for rivets holding two plates together, forkeys in shafts, and the like. Neither specimens for such tests nor themethods of making the tests are standardized. Stress-strain diagramsmay be plotted from a torsion test, from which the maximum torque(torsional or shearing strength)

, yield point or proportional limit, andthe modulus of elasticity in shear (modulus of rigidity) may be deter-mined. There is no accurate relation between torsional strength andtensile strength, but for carbon steels, the torsional strength is generallyabout 75 per cent of the tensile strength.

Compressive strength can be determined accurately on brittle mate-rials that fracture; on ductile materials, which flatten without fracturing,it can be determined only when a limiting amount of deformation is

specified. Compressive strength is a common test for cast iron, as thismetal is frequently used in compression, but is rarely made on steel. Thecompressive strength of cast iron is 3 to 4 times its tensile strength.Bend tests are becoming increasingly valuable for some of the new

low-alloy steels. These steels are used in structures primarily to saveweight, and the bend test is useful to determine the relative stability ofthese materials under load. When ductile materials are tested, the speci-men does not fracture when bent; hence, the test can also be used todetect deep-seated flaws and other internal defects or to discover brittleareas in and around a weld. A variety of the bend test-the transversetest-is a useful measure of the quality of cast iron and has been used sowidely for this purpose that virtual standardization has been effected.

Transverse tests of cast iron are made on standardized bars. As dis-cussed in Chap. 20, the results are reported as the load necessary tofracture a specimen of definite diameter held on supports a specifieddistance apart. In the United States this is usually reported as transversestrength in pounds; abroad, and to some extent in the United States, it isreported as modulus of rupture in pounds per square inch. The test is

usually made on a cast bar without machining; removing the surfacelayers of the bar may cause a slight change in values. The deflection ininches (that is, how much the bar bends before breaking) is also re-


Fie. 4.9. Oscillograph screen showing diagram obtained by sonic tester.

ported. The transverse test gives an indication of toughness, flexibility,

surface condition, and even machinability and is the most valuable single

test for cast iron.

4.12. Sonic Testing

Large forgings, castings, and weldments are inspected for hidden flaws

by x-ray radiography. Recently the sonic tester has been developed for

non-destructive testing of all types of machine elements. This apparatus is

portable and can be used on a part even when it is completely assembled

in the machine. This makes possible inspection during manufacture, but

it also can reveal fatigue cracks and defects in highly stressed machine

parts after the machine has been placed in service.

High-frequency compression waves are produced by a quartz crystal

placed directly on the work surface or the crystal may be carried on an

oil film. The beam of vibrations flows through the metals and are re-

flected back from the bordering surfaces. The reflections are then record-

ed on an oscilloscope, A on Fig. 4.9 being true top surface reflection and

C the bottom surface reflection. An internal crack would also send back

a reflection as shown by the pip at B. Location of the defect can also be

determined from die oscillograph. The length from A to C represents the

work depth. AB is then the distance from top surface to defect on the

same scale. This ability to measure depth makes it possible to gage the


SEARCH UNITAT POSITION 1

SEARCH UNITAT POSITION 2

Fro. 4.10. Surface Wave Testing of a Shaft. (Courtesy Sperry Products, Inc.)

progress of corrosion in piping and pressure vessels even when filled withfluid.

The majority of ultrasonic testing is done by x-cut quart/, crystals whichsend waves directly through the specimen. However, Y-cut crystals orplastic wedges may be used to propogate surface waves which can followcurved surfaces and thus reach otherwise inaccessible locations. Fig. 4.10shows this technique being used to check the fillet areas of a shaft forfatigue cracks, such a crack being revealed by the surface wave unit inposition 2.

QUESTIONS

1. Define static mechanical properties. What are the five most useful staticproperties? What engineering property is standard throughout the world?

2. What materials, because of their magnetic properties, are most useful inthe design of motors, generators, and transformers? Define domain. DefineCurie temperature. What happens to the domain when this temperature isreached?


S. How accurate is the tensile strength of a metallic material as usually deter-

mined? What variables affect this accuracy? What factor is most important

in using tensile-test data in engineering design?

4. What are the advantages and disadvantages of testing a completed structure

under actual service conditions?

5. Discuss the use of aluminum as an electrical conductor.

6. Distinguish between elastic limit and proportional limit. What is modulus

of elasticity, and what does it represent? Can proportional limit and

modulus of elasticity be readily determined on all metallic materials? Give

the reason for your answer.

7. Give approximate values of modulus of elasticity for (a) steel, (b) aluminum

alloys, (c) copper.

8. Describe three methods of testing which will allow the designer to deter-

mine the location and magnitude of stress concentration in a given speci-

men or structure.

9. What modulus is a measure of the stillness of a metal. Can a metal part

which is not stiff enough in the annealed state be improved by hardening?

10. What is the difference between yield point and yield strength? What is the

importance of these values to engineers? How does the speed of testing

affect the yield point?

11. What phenomenon occurs on the stress-strain curve which has developed

into a new science known as plastic design?

12. Define elongation and reduction of area. How are these values determined?

What is the effect of gage length on elongation?

13. What properties are components of hardness? What precautions are neces-

sary in the accurate determination of hardness?

14. Describe briefly the method of determining hardness by the Brinell, Rock-

well, and Vickers machines, and give the principal advantages and dis-

advantages of each.

15. When a part is so designed that stress concentration is present, how will a

ductile material react as compared with a brittle material? What will be the

reaction if the stress is released and then reapplied?

16. Convert a hardness of 40 Rockwell C to Brinell and Vickers values and to

tensile strength. How accurate are these conversions? If the tensile strength

of an annealed carbon steel is 85,000 lb. per sq. in., what are its approxi-

mate Brinell, Vickers, and Rockwell C hardness values?

17. What do we mean when we say a material is structure sensitive?

18. What hardness tests are commonly used for nonferrous alloys? Why is it

impossible to convert hardness values to tensile strength for most non-

ferrous alloys?

19. What is the value of the shear test? What is the value of the bend test (a)

for ductile materials? (b) for brittle materials? What is the relation be-

tween the strength of a metallic material in torsion and in tension?

20. How docs the sonic tester function? What is the advantage of this method

of testing as compared with x-ray analysis? How is a defect in the specimen

located?

CHAPTER

Dynamic Properties ofMetallic Materials

Oran Allan Pringle, M.S.M.E., Associate Professor of

Mechanical Engineering, University of Missouri,

Columbia, Missouri

iHE design of engineering structures andmachines that are subject to static loads is based upon tensile strengthand yield strength with an ample factor of safety. Few failures of suchstructures and machines occur. This, however, is not the case with movingparts of machines which fail so frequently that it has been estimated that

95 per cent of all metal failures are caused by dynamic loads. For this

reason, metallurgists and engineers are now paying more attention thanever before to the resistance of ferrous and nonferrous materials to

dynamic stresses and to methods of determining the dynamic properties.

The principal dynamic properties — impact resistance, endurance(fatigue) strength, and damping capacity—measure the ability of the

material to withstand suddenly applied or pulsating loads. Endurancestrength has been the subject of much study and in some respects mighteven be called a standardized property. Impact resistance and dampingcapacity are not standardized; hence, the values obtained by impact anddamping tests may mean very little unless the method and the specimenused are known. Such tests are more valuable as a method of comparingvarious materials or various grades of the same material than as a meansto obtain values which can be used in design.

5.1. Notch Brittleness and Transition from Ductile to Brittle

Fracture

In applications involving impact loads it is necessary to know the in-

fluence of speed of loading or strain rate on the mechanical properties of

81


. , Stress Stress

atMN

M-,

Fie. 5.1. Typical notches or stress raisers occurring in machines and structures.

(Moore)

the materials. According to present knowledge, there is always a con-

siderable increase in yield strength of metals at high strain rates; for

example, over a range of strain rates from 10—3 to 102 in. /in. per sec.

the yield point for mild steel increases by as much as 100 per cent. Theultimate strength also increases at high strain rates but to a lesser

degree, and at very high strain rates the yield point of mild steel ap-

proaches the value of the ultimate strength. These considerations might

lead one to conclude that there is less likelihood of failure under impact

loads than static loads. Unfortunately however, it is now well known that

some commonly ductile materials can fail in a brittle fashion when sub-

jected to certain insidious combinations of high strain rates, low tempera-

tures, and notch effects.

A ductile fracture is preceded by a considerable amount of plastic

deformation; since the force required for fracture thus acts through a

comparatively large distance, a relatively large amount of energy is

absorbed. A brittle fracture is preceeded by little or no plastic deforma-

tion; therefore very little energy is required to produce this type of

fracture. Some materials, like glass, always fracture in a completely

brittle manner. On the other hand, a few metals, among them the plain-

carbon and low-alloy steels, behave in a ductile manner under some con-

Dynamic Properties of Metallic Materials 83

Charpy Impact specimen-„?--

—Y-45"•/--

0.J/5" ^0.01-Bad.

ajs.*'

Alternate Charpy impact specimen

*SM -o.OOO--l.l024~iO.OI9P

0.3I5~£0.00I- '-O.OI'Rad..

0.395?'1 0.001\)

Izod impact specimen

Fie. 5.2. Standard impact specimens. (American Society of Testing Materials)

ditions, such as in the ordinary tensile test; however, they can break in analmost completely brittle manner under certain other conditions. Thethree most important conditions which tend to bring about this transi-

tion from ductile to brittle behavior are high strain rates due to shockloads, low temperatures, and the presence of notches (Fig. 5.1). Thusa certain steel part may bend without breaking under a static load at roomtemperature if smooth, but may snap with a brittle fracture if a sharpnotch is located in the region of stress, and/or the load is suddenly ap-plied, and/or the temperature is sufficiently lowered.

In testing, it is often convenient to hold constant the notch conditionand strain rate and to use the temperature zone or transition temperatureat which ductile behavior ceases as an index of the susceptibility of the

material to brittle fracture. Many different types of tests and specimenshave been used. The Charpy and Izod tests are common in the UnitedStates. The specimens used are shown in Fig. 5.2. The pendulum of the

testing machine falls from a definite height and breaks the specimen at

the notch. The energy absorbed during fracture is measured from thebackswing of the pendulum. Typical results are shown in Fig. 5.3. It

can be seen that a drastic decrease in ability to absorb impact energyoccurs when the temperature is below the transition range.

In order to understand how brittle fracture can occur in ordinarilyductile materials, it must be realized that several entirely different

mechanisms can cause fracture. The brittle type of fracture occurs whenthe highest tensile stress reaches a critical value, and is almost indepen-

84

-I7S

70 r1

Engineering Metallurgy

Temperature, deg.C.-150 -I2S -100 -75 -50 -25tJ ,J 1 A, !_, I '

+25

-300 -250 -200 -150 -100 -50

Temperature, deg. F.

+50 +100

Tig. 5.3. Transition temperature curves for various steels: (A) 18-8 austenitic

stainless steel; (B) annealed low-carbon 3.5 per cent nickel steel; (C) annealed low-carbon 13 per cent chromium steel; (D) normalized 0.35 per cent carbon, 15 per centnickel, 0.6 per cent chromium steel; (E) annealed 0.35 per cent carbon steel; and (F)

normalized 0.35 per cent carbon steel. Specimens for steels B and E had keyhole notch;

others had V notch. (The International Nickel Company)

dent of other types of stress. On the other hand, the ductile type of

fracture occurs only after plastic yielding has taken place, which begins

when the maximum shear stress reaches a critical value. As the load on a

ductile material is increased, the critical value of shear stress is ordinarily

reached before the critical value of tensile stress, consequently plastic

deformation and ductile fracture result. If by some means the critical

shear distress could be raised so high that the critical tensile stress

were reached first, brittle fracture would occur. This is exactly whathappens at high strain rates, since increasing the strain rate raises the

yield strength. Low temperatures have a similar effect, since the yield

strength of most metals increases by a factor of around 2 as the tempera-

ture is lowered from room temperature to the boiling point of hydrogen.

The brittle tensile strength is also affected by strain rate and temperature,

but to a lesser degree.

Since brittle fracture results from exceeding a critical tensile stress

and ductile fracture from exceeding a critical shear stress, any means by


which shear stress in a part is reduced or suppressed will tend to promotebrittle fracture. This is what is accomplished by notches. Consider acylindrical tensile specimen with a circumferential notch or groove. Asthe load is increased, the first effect of the notch is to produce elastic

stress concentration at the root of the notch. With further increase in

load, the elastic limit is reached, and plastic yielding relieves the stress

concentration. As the core of the circumferential notch undergoes plastic

elongation, it must also contract in a transverse direction, pulling in onthe metal surrounding the notch. The surrounding metal then pulls

back out on the notch core, subjecting it to biaxial tension in the trans-

verse direction. Actually, however, the notch core is in a state of triaxial

tension, as the external axial load on the specimen is still acting. In asystem of equal triaxial tensions, the shear stress is zero. Thus the effect

of the notch is to suppress all or part of the shear stress which wouldordinarily be present in the tensile specimen, leaving the way open for thetensile stress to become sufficiently high to exceed the critical value for

brittle fracture. This effect of the notch is called plastic constraint.

In addition to high strain rates, low temperatures, and notches, numer-ous other factors tend to a lesser degree to promote the transition fromductile to brittle fracture. Some of these factors are: increased size of

part, large grain size, pearlitic rather than martensitic microstructure,

certain deoxidation practices, and prior cycles of fluctuating load.

Although the ferritic steels are the most important alloys which exhibit

the phenomonon of a transition from ductile to brittle fracture, othermetals, such as zinc alloys, tin alloys, and magnesium have been shown to

behave similarly. On the other hand, aluminum, copper, nickel, andaustcnitic steels (Fig. 5.3) have thus far failed to show a transition tem-perature.

5.2. Value of the Notched-Bar Impact Test

It is evident from the foregoing discussion that the chief value of thenotched-bar impact test is in classifying materials according to their

tendency toward brittle fracture. It is rapid and cheap and gives informa-tion that no other test gives at present, provided that common sense andexperienced judgement are used in its evaluation. In the present state

of development it has a number of disadvantages. Transition tempera-tures obtained with one type of specimen and machine cannot be directlycorrelated with values obtained with different specimens and machines;similarly, test results cannot be directly correlated with results on full size

parts under service conditions. Furthermore, it is not a reliable test forinherently brittle materials like cast iron and hardened tool steels. De-


Fig. 5.4. Typical fractures of fatigue specimens with unclistortcd area (A) in center

(Moore) and (B) at side. (Johnson)

spite these disadvantages, the engineer and metallurgist have found the

notched-bar impact test to be a valuable tool. It is used primarily as an

exclusion test to detect steels with high tendency toward brittle fracture.

In industry it is used by manufacturers of ordnance, aircraft, and auto-

mobiles, who have found as a result of experience, that steel which ex-

ceeds a certain minimum impact resistance in the laboratory gives satis-

factory service in applications where low temperatures and shock loads

are encountered.

5.3. The Mechanism of Fatigue

Ever since man has used metal for moving parts, he has noticed that

after a period of satisfactory service such a part would sometimes fail

even though the stresses were relatively low. Usually the failure would

be sudden: an axle that had been forged from steel of excellent ductility

would snap off short as if it were brittle. Examination of the fracture

would show a dull area usually surrounding an oval-shaped area of bright

crystals (Fig. 5.4)

.

Although all details of the mechanism of fatigue failure are not

thoroughly understood, the main events which occur during this pro-

gressive type of failure may be summarized. Under repeated stress the

metal yields at one or more local areas of weakness, usually at some surface

defect where elastic stress concentration occurs. Eventually the atomic

bonds are broken, resulting in submicroscopic cracks at the ends of which


I'io. 5.5. R. R. Moore high-speed machine for rotating-hcam fatigue testing.

there may be high concentrations of stress. As the stress on the piece is

repeated, the crack or cracks spread. After a time there is so little soundmetal left that the normal stress on the piece is higher than the strengthof the remaining material, and complete fracture occurs.The appearance of a typical fatigue fracture is explained by the fact

that as failure proceeds the severed surfaces rub and crush each other andproduce a dull appearance; while the remaining unfractured portionpreserves the normal grain structure up to the moment of fracture.

5.4. The Endurance Limit

Fatigue is perhaps not the best word to use in referring to the type offailure under discussion, but it is too firmly entrenched in metallurgicaland engineering terminology to be dislodged. The term endurance isalso commonly used.

The endurance limit of a metal or alloy is the stress below which thematerial will not fail if subjected to an infinitely large number of cyclesof stress. It is determined in a number of ways, the most common of whichis by alternate flexure, usually known as the rotating beam method. Apolished specimen of suitable size, which is rotated in a specially de-signed machine (Fig. 5.5) , is subjected by dead weights to alternatetensile and compressive stresses.

A number of identical specimens are prepared from a single material.I he first is run at a stress which is considerably higher than the expectedendurance limit, and the number of cycles causing failure is noted. Thestress is reduced for the next specimen; this process is continued until,for steel, there is no fracture after 107 cycles of stress. The stress (S) for


'104 105 106 I01

108

Cycles

Flo, 5.6, Typical S-N curves for carbon steels. (Moore and Kommcrs)

each number of cycles (N) is plotted; the result is the S-N diagram(Fig. 5.6) . The value for S which the horizontal portion of the curve

represents is the endurance limit.

Although steels have a definite endurance limit, it is doubtful if the

same is true for most nonferrous alloys. At any rate, the S-N diagram for

nonferrous alloys usually does not show an abrupt knee at the endurancelimit, but rather consists of a gradual curve with the portion at lower

stresses becoming approximately horizontal only at a very large number of

cycles of stress. In reporting results of fatigue tests on nonferrous alloys

the endurance strength (or fatigue strength) corresponding to a certain

number of cycles of fatigue life, such as 500,000,000, is usually given, since

a well denned endurance limit is not found.

The importance of determining the endurance limit or endurance

strength, even though the test takes a long period of time, is evident whenit is recalled that airplane propellers, turbines, etc. may easily run a

billion cycles during their useful life.

5.5. Relation of the Endurance Limit to Other Properties

If the endurance limit is determined by the rotating-beam methodusing carefully polished specimens of proper design, the value obtainedfor most carbon and alloy steels is about one-half the tensile strength

(Fig. 5.7) . As there is a fairly close relation between tensile strength andhardness, it follows that there is also a close relation between endurance

limit and hardness; the former is about 250 times the Brinell hardness

number (Fig. 5.7) . There seems, however, to be no correlation between

endurance limit and any other property of ferrous alloys.


Brinell hardness number

,,0 100 200 300 400 500 600

20 40 60 80 100 120 140 160 180 200 220 240 260 280

Tensile strength,thousand lb per sq. in.

Fig. 5.7. Relation of endurance limit of steel to tensile strength and Brinell hard-

ness. (Moore)

The above relations among endurance limit, tensile strength, andhardness hold for carbon and alloy steels of any composition and heat

treatment except those treatments which produce very high hardness,

and may be used as approximations in design. For refined design, how-

ever, the actual endurance limit of the material must be determined, as

this may in individual cases vary between 35 and 65 per cent of the

tensile strength. Cast steel (not heat treated) has an endurance limit

of about 0.4 times the tensile strength, somewhat below the average for

wrought steel. Cast iron has a ratio of 0.3 to 0.35. Attempts to correlate

endurance strength of nonferrous alloys with tensile strength are not very

successful; the endurance strength may be less than 25 per cent or morethan 50 per cent of the tensile strength.

5.6. The Effect of Notches on the Endurance Limit

The endurance limit of a carefully polished specimen may be reducedas much as 50 per cent by a notch. In actual machines and structures

notches of some sort are almost invariably present in some form such as

rough surfaces, tool marks, holes, screw threads, keyways, fillets, inclu-

sions, gas cavities, etc. (Figs. 5.1, 5.8) . The effect varies with the kind of


Fie. 5.8. Failure of a crankshaft starting in a sharp fillet. The crack started in the

fillet about \/2 in. above and to the left of the arrow.

notch, its location, and the notch sensitivity of the material. For hard

steels the effect of the notch can be approximately predicted by the elastic

stress concentration as calculated by mathematical theory or as determined

by photoelastic studies. For ductile steels the effect of the notch is usually

less severe. The effect of a number of different surface conditions is shown

in Fig. 5.9.

As a general rule the damaging effect of a notch in carbon and alloy

steels and in some nonferrous alloys increases with its sharpness, that is,

with increase in the ratio of its depth to the radius of its root. Usually a

notch becomes harmful of this ratio exceeds 28 or 30. When shallow

notches are present, hard steels with their relatively high endurance limits

are superior to low strength steels. On the other hand, with sharp notches

present, a soft steel with low notch sensitivity may be superior to a

high-strength high-endurance steel with high notch sensitivity. Gray cast


IC

GO

Tens

80

le strength .thousand lb. per sq

100 120 140 160'

1 1 1

in.

180 200i_

a-

b

20

t in

„c

\s s

d

*40J

P<0

\ s

\S.

-

\e

5

i"fin

\f

1o 70 . 9Bo

flO

V- notch

" -»£

y\l O.lmm.

- 7.5mm. tj>

1 1 1

so

inn

i 3 4 i i J 6 i i

Tensi

i) ae str<

3 9>

ngth3 K

kg.pj

er sq.

15

mrn.D E 3 U > m

Fig. 5.9. Decrease in endurance limit for carbon and low-alloy steel specimenshaving the following surfaces: (a) polished, (b) ground, (c) roughened, (d) circum-ferential V notch, (e) decarburized, (f) corroded in tap water, and (g) corroded insalt water. (Z. Ver. deut. Ing., v. 77, 1933)

iron, although inherently brittle, is relatively insensitive to notches.Another important fact to remember is that a decarburized surface, whilenot acting as a notch, lowers the endurance limit of the whole section to

that of a steel whose carbon content is the same as the surface layer,

since fatigue failure usually originates at the surface. As Fig. 5.9 shows,the effect of a decarburized surface may be greater than the effect of a

sharp notch.

Chafing or fretting the polished surface of steel or nonferrous alloys,

a form of wear which occurs in parts which are intended to fit tightly

but which slip slightly under fluctuating loads, reduces the endurancelimit.


5.7. Corrosion Fatigue

If a polished metal specimen becomes corroded before testing, the en-

durance limit is lowered somewhat, an effect which would be expected in

view of the roughening of the surface. On the other hand, if a polished

specimen is corroded simultaneously with the application of repeatedstress, the endurance limit is lowered to a much greater degree. As a mat-

ter of fact, there is probably no true endurance limit in corrosion fatigue,

since with sufficient time the corrosion alone would produce failure even

at very low stresses. Some data indicate that carbon and low-alloy steels

subjected to simultaneous reversed stress and corrosion will fail at stresses

as low as 5000 lb. per sq. in. if they are run long enough (100,000,000

cycles or more) . The data given in Fig. 5.9 indicate that a combination

of corrosion and fatigue causes a loss of about 50 per cent of the endurancelimit if the tensile strength is low, and a loss of about 90 per cent if the

strength is high. The corrosion fatigue strength of most nonferrous alloys

is likewise only a small fraction of their endurance strength in air.

For high resistance to corrosion fatigue a material must have an oxide

skin which is tightly adherent and at the same time flexible enough to

withstand repeated stressing without breaking. Among ferrous alloys, only

the high-chromium stainless steels normally have such a skin. Corrosion

of carbon and low-alloy steels results in corrosion products which, being

brittle, fracture readily and expose the underlying surface to further

attack. Since all ferrous materials, except some highly alloyed steels, are

reduced by simultaneous corrosion and repeated stressing to a commonlow level of resistance, there are only two things to be done in such appli-

cations: use a stainless steel, or protect the surface by plating with somemetal that will form a continuous corrosion-resistant coating. According

to present knowledge, electroplated zinc or cadmium coatings are best.

5.8. Increasing the Endurance Limit by Shot Peening

Shot peening, if properly controlled, can increase the endurance limit

or fatigue life 25 to 100 per cent or more. Shot peening consists of sub-

jecting the surface of a finished metal part to a hail of metallic shot that

has been released by the blades of a revolving wheel or by an air blast.

The shot, in striking the surface with force, cold work the outer layer,

increasing the strength and hardness and decreasing the ductility of this

region. At the same time, the cold working introduces longitudinal com-pressive stresses near the surface. Surface-rolling is an alternate methodthat produces the same effects.

The resulting increase in endurance limit is due to two factors: (1) the

increase in strength of the surface layer due to the cold work; and (2)


the residual compressive stresses set up in the surface layer. Opposed to

these favorable circumstances, however, are the roughening of the surface

and the formation of a large number of shallow pits that may causelocalized stress concentration. If the shot peening is carefully done andif the size of the shot and the velocity of impact are controlled, the netresult of the operation is a marked increase in endurance limit or fatiguelife. Shot peening is now used regularly by some automotive and aircraft

manufacturers for improving the life of crankshafts, camshafts, connect-ing rods, steering knuckles, transmission shafts, gears, and other parts.

5.9. Significance of Endurance Data

Endurance limits that have been obtained under controlled laboratoryconditions on carefully designed polished specimens are valuable in thedesign of machines and structures subject to repeated stresses, but theyshould be used only with full realization of their somewhat limited signi-

ficance. Such endurance data give information of one kind only; condi-tions in service are rarely so simple that these data can be used withoutconsidering other factors.

Most structure and machine parts are subjected to more or less randomfluctuations of load; however, most fatigue tests are run under conditionsof constant maximum and minimum stress. At the present time there is

no completely satisfactory method of correlating the behavior of a speci-men under cycles of varying stress amplitude and the behavior of anotherspecimen under cycles of constant stress amplitude. It is certain, how-ever, that the endurance limit of a specimen may be affected by its stress

history. If a specimen is understressed, or subjected to a few millioncycles of a stress slightly lower than its ordinary endurance limit, its finalendurance limit as determined later will be considerably increased. Ifa repeated stress slightly above the endurance limit is applied for asmall number of cycles, an improvement similar to that produced byunderstressing is sometimes observed. On the other hand, higher valuesof repeated stress and larger number of cycles may damage the material,and its endurance limit as subsequently determined will be reduced.Another important factor is surface condition. Parts in actual service

are rarely as highly polished as most laboratory fatigue specimens, andfull consideration should be given to the character of the surface, includ-ing the possibility of corrosion and accidental defects, and to how mucha roughened surface may reduce the endurance limit. All in all, a greatmany complicating factors including surface condition, variable stresses,

external and internal stress raisers both intentional and unintentional,notch sensitivity, possible decarburization during heat treatment, residual


Cast iron

mmmtmHmmmmi

Carbon s+eel

Aluminum alloy

Fie. 5.10. Damping curves for cast iron, carbon steel, and an aluminum alloy.

(Pohl)

stresses, and combined stresses must be considered in problems involving

fatigue.

5.10. Damping Capacity

If a metal or alloy bar is supported at one end and if the free end is

struck with a hammer, the bar will vibrate for varying lengths of time

depending upon the composition, the stress imposed, and other factors,

but at a uniformly decreasing rate owing to the internal friction of the

material which dissipates the energy as heat and sound. The dissipa-

tion of energy per unit volume of the material for one cycle of stress is

the damping capacity. It may be reported as inch-pounds per cubic inch,

or as centimeter-kilograms per cubic centimeter, per cycle. Dampingcapacity is usually measured by the area of the stress-strain hysteresis

loop. The ratio of this value to the work of deformation is the dampingratio. This increases with the capacity of the material to damp out

vibration.

There is apparently no direct relation between damping and any other

mechanical property. In most cases the ability of a steel to damp vibra-

tion decreases as the tensile strength increases. Of ferrous materials, the

strong alloy steels usually have the lowest, and cast iron and low-carbon

steels the highest damping capacity. There are, however, many excep-

tions to this generality as steels of the same composition and tensile

strength may have very different damping capacities. Typical curves


showing free vibrations clamped only by internal friction are shown in

Fig. 5.10 for cast iron, carbon steel, and duralumin.

It has been suggested that the endurance limit might be predicted fromdamping capacity, although attempts to correlate the two have not beensuccessful. There is, however, some evidence that damping capacity is

related to notch sensitivity. For, example, cast iron has a high dampingcapacity and is relatively insensitive to notches; duralumin, with a highnotch sensitivity, has a very low capacity to damp vibrations. There is noquestion, however, that the damping capacity of a material is an im-portant criterion for its usefulness in many applications. For example, in

crankshafts in internal combustion engines resonant vibrations fre-

quently occur. If a material with high damping capacity is used, theenergy of vibration is dissipated by internal friction and the peakamplitude of the vibratory stresses is held to a low level, thus minimizingthe possibility of fatigue failure.

QUESTIONS1. Define the terms notch brittleness and notch sensitivity. Give an example

to illustrate the importance of each of these properties in engineering.

2. What is the chief value of the information gained from notched-bar im-pact tests?

3. What are some disadvantages of the common methods of notched-bar im-pact testing?

4. Why is a fatigue fracture usuallv characterized by a bright area surroundedby a dull area?

5. Define the terms endurance limit and endurance strength.

6. Sketch typical S-N curves for a steel and an aluminum alloy.

7. How many cycles of testing arc usually necessary to determine (a) the en-durance limit of a steel, and (b) the endurance strength of an aluminumalloy?

8. A recent expedition exploring Greenland experienced considerable difficultywith breakage of almost new vehicle tracks. What was the probable causeof these failures?

9. What is the approximate endurance limit of (a) quenched and temperedalloy steel with a tensile strength of 150,000 psi and a ground surface, (b)ultra-high strength steel with a tensile strcngdi of 280,000 psi and a groundsurface, and (c) annealed steel with a Brinell hardness of 200 and a V-notch?

10. If a suspension spring on your automobile breaks after 60.000 miles ofservice, what is the most probable cause of failure?

11. What is the approximate endurance limit of a steel having a tensile strengthof 140,000 psi if the surface is (a) ground, (b) heat treated with a smoothbut decarburized surface, and (c) heat treaded with a rough, pitted,oxidized surface?


12. The gear teeth of an automobile transmission may fail in fatigue. Assumingthat the endurance limit of the steel lias been determined on a rotating*

beam testing machine, what factors would you consider in deciding upona suitable operating stress in the gear teeth?

13. During preliminary testing of a crankshaft, fatigue failure occurs at anabrupt fillet. Due to dimensional restrictions it is undesirable to increase

the fillet radius. Can you suggest another method by which the fatigue-

strength of the fillet may be substantially increased?

14. What advantage would gray cast iron have over other materials for use as

a frame for a precision machine tool?

15. Railroad car axles are frequently surface-rolled over the region where the

wheel is pressed on. What benefit may be gained by this operation?

16. The elastic stress concentration factor of a notch or similar discontinuity

can frequently be determined by photoclastic studies of plastic models. Towhat extent can information of this type be used in predicting fatigue

behavior of metal parts with similar discontinuities?

17. Although fatigue data for standard specimens of many different materials

are available, fatigue testing of actual parts under simulated service condi-

tions is widely practiced. Can you suggest some reasons for this?

18. Under what conditions might there be no advantage in using a high-

strength steel for a part, even though it is heavily loaded?

19. For certain applications, such as bridge cable and helical springs, cold-

drawn steel wire strengthened by work-hardening is preferred over steel

wire strengthened by quenching and tempering. Why?20. Why can the shank area of a bolt be turned down to considerably less dian

the root area of the threads without decreasing die strength of the bolt

under fluctuating loads?

CHAPTER

Phase Diagrams andthe Simple Alloy Systems

Theodore Allen, Jr., M.S.M.E., Associate Professor of

Mechanical Engineering, University of Houston, Hous-

ton, Texas; Engineer Associated with Anderson,

Greenwood and Co., Bellaire, Texas

John K. Anthony, M.S., Associate Professor of Physical

Metallurgy, University of Arizona, Tucson, Arizona

6.1. Solid State Thermodynamics and Thermostatics

1 HE study of physical metallurgy would hardly

be complete without the mention of solid state physics; indeed, it is just

a small portion of that vast field. For reason of simplicity the metallurgist

often refers to a "system." A system may be defined as an arbitrary por-

tion of matter which has been isolated from the rest of the universe by

prescribed boundaries. The system is merely separated by these bound-

aries from its environment for the purpose of analysis. A phase may be

defined as a portion of homogeneous matter in a system. When a system

undergoes a phase change, this change must take place at a rate which is

infinitesimally slow, or equilibrium can not be maintained during the

change. This is not novel, for thermodynamic processes which are

reversible necessarily require the state point to pass through a series of

quasi-static or quasi-equilibrium states. In this sense, then, reversible

or equilibrium changes should be classified as thermostatics while actual

changes which proceed at a finite rate are truly thermodynamic changes

of state. The study of metallurgy includes such non-equilibrium phase

changes as those that produce coring, segregation, and other irreversible

processes as well as those processes which are carried out slowly enoughto produce a "near equilibrium" change of state.

6.2. Concept of Dynamic Equilibrium

From what has been said in the preceding paragraph, one might infer

that this concept of equilibrium is a static sort of thing; such is not the

97


case. From a macroscopic point of view this assumption would be toler-

able, but from the microscopic point of view the picture is a dynamic one.To be specific, two phases of a pure substance may be in equilibriumwith one another, and the molecules of one phase may be escaping intothe other phase, while those of the second phase are escaping into thefirst, ff this exchange were taking place so that the net transport of mass,momentum, and energy is zero, then the two phases would be in equili-

brium. It is a well established fact (the Second Law of Thermodynamics)that nature tends to equalize its potentials; a system, then, would be in

equilibrium if no unbalanced tendency existed which would bring abouta change of state provided this tendency was active.

With respect to the escaping tendency of a component from one phaseto a second phase in chemical equilibrium with the first, it may be saidthat the fugacity of a component in one phase must be equal to thefugacity of that same component in the second phase for equilibrium to

persist.

6.3. Cooling Curves

Useful in equilibrium studies are cooling curves. These curves presentthe temperature history of a system while it is being heated or cooled.Consider the cooling of liquid HaO. The (temperature-time) curvefor it, starting from some arbitrary temperature, say 80°F, is a line witha negative slope. The temperature decreases with time until a tempera-ture of approximately 32° F (corresponding to standard pressure of14.696 psi, abs.) is reached. At this point the formation of another phase(ice) occurs. The cooling curve is then a horizontal line until all of the

liquid is frozen (the delay is explained by the phase rule in section 6.5of this chapter)

. Upon completion of solidification, further cooling re-

sults in a drop in temperature with time. Such a cooling curve is thesimplest possible because only one component, H,0 is present in the sys-

tem.

The concentration and nature of a component is required to specify

the composition of a phase. In the above example, there was one phase(liquid, composed of 100% HaO) at the start of cooling. Next, twophases coexisted (liquid and solid, each composed of 100% H,0) . Finally,

a single phase (solid, composed of 100% FLO) existed. Frequently asystem may contain two or three components. Such systems are referredto as binary and ternary systems, respectively. The number of com-ponents is not limited to three or any number in particular.

Suppose the water had been cooled very rapidly from SOT. Chancesare very good that ice would not have formed at 32°F, but at a somewhat

Phase Diagrams and the Simple Alloy Systems 99

lower temperature. It is possible to cool the water below 32°F without the

formation of ice. However, the addition of a small crystal of ice to serve

as a nucleus causes solidification. Freezing can also be brought about bydisturbing a super-cooled liquid. Super-cooled liquid water in this state

is not characterized by stable equilibrium. This illustration serves to

point out the influence of cooling rate on equilibrium. The only way to

freeze liquid water at exactly 32°F (if this be the "absolutely correct"

freezing point) is to cool it infinitesimally slowly; otherwise the solidifi-

cation will begin at a temperature lower than 32°F, the amount of de-

parture being a function of the cooling rate. The depression of the freez-

ing point is brought about because of time necessary for solidification to

begin after the freezing point is reached. The student of thermodynamicswill again recognize the influence of time on the irreversibility of proces-

ses.

When ice is melted, the melting begins at a temperature slightly

above 32°F if the heating rate is greater than zero. A way frequently

employed to determine the "correct" freezing or melting point is to

construct a heating and a cooling curve based on a minimum rate of

temperature change and to interpolate between the "depressed" freezing

point and the "elevated" melting point.

Cooling curves for alloy systems can be determined experimentally for

various concentrations of the components, and these curves may be used

to construct equilibrium or constituent diagrams such as presented in

Section 6.7 of this chapter.

6.4. Solid State Mass Transfer

The rate of mass transfer or diffusion, as it is commonly referred to,

is given by Fick's Law in the following fundamental form

m=-DAax

in which m is the mass transfer rate, ?£ is the change of concentrationdx

in the direction of the diffusion path, per unit of path length, A is the

cross sectional area of the path (this area is normal to the path) , and D is

a coefficient of mass diffusivity. The minus sign indicates that the masstransfer takes place in the direction of decreasing concentration. It is

through the process of diffusion that a solid solution of several metals maybe homogenized. If a large concentration of one component exists in a

localized form in a solid solution, it will diffuse. The concentration pro-

vides a gradient which gives rise to the mass transfer.


The process of diffusion is very slow for solids at ordinary roomtemperatures. However, the coefficient of mass diffusivity varies ex-ponentially with the absolute temperature of die material being diffused.

Because of this increase in diffusivity at elevated temperatures, a con-centration is diffused in a matter of minutes at 1500°F that would prob-ably take several centuries at room temperature. For this reason, alloys

which are heterogeneous because of rapid and irreversible cooling are

homogenized at elevated temperatures in annealing ovens.

By the process of dilfusion (a "self-homogeni/.ing action") solid aggre-

gations become homogeneous (and thence become phases)

.

6.5. The Phase RuleIn the latter portion of the nineteenth century, the phase rule was

advanced byJ. Willard Gibbs. It is largely because of the work of Gibbs

that our knowledge of phase equilibria is as advanced as it is today. Thefollowing is a short explanation of how the phase rule may be derived.

Equilibrium is assumed; therefore, the fugacity of a component in

one phase is equal to the fugacity of the same component in any otherphase. If there are N p phases in the system, then equilibrium provides(N p-1) equations per component. In a system of N components, thereare Nc (N

p-1) fugacity equations. The concentration of all the com-ponents in a particular phase must sum to unity. This fact provides Np

independent equations. The number of variables that are independent,(also called degrees of freedom) N f are then equal to the difference be-tween the total number of variables and the number of independentequations relating these variables. The total number of variables are N c

N„ due to composition plus any two thermodynamic coordinates. It fol-

lows then that

Nr= [Nc Np + 2] — [N. (N p-1) ] — N„

or

Nf =N„-N„ + 2

This is the phase rule of Gibbs. By way of illustration, attention is

again directed to the freezing of the water. When two phases of the onecomponent were coexistent, the phase rule indicates that there was onlyone independent variable. This is evidenced by the fact that once thepressure (a thermodynamic coordinate which was arbitrarily taken as14.696 psi, abs.) is independently chosen, the temperature is fixed.

(in this case at 32°F) for two phases and one component. Upon com-pletion of freezing, one phase existed. Now there are two variables thatare independent according to the phase rule. After solidification is

complete, the temperature varies although the pressure has been arbi-trarily selected.


To examine further application of the phase rule, the following ques-

tion is advanced. Can three phases of a single component exist simul-taneously? The phase rule indicates that this is possible, but that therewould be no independent variables. This means that the state where thethree phases coexist is fixed ( that is, there is a definite temperature andpressure for a given component where triple phase coexistence can mani-fest itself)

.The temperature and pressure are not arbitrary once the

component (the system in this case) is selected. The reader will recog-nize this concept as the "triple point" in physics.

The phase rule is a valuable tool in the field of experimental metal-lurgy. It is useful in predicting the phase equilibria once the system is

fixed. The phase rule does not apply to the prediction of state configura-tions that result from non-equilibrium effects, nor is the rule applicableto a vacuum. The phase rule is based on the assumption of equilibriumand the existence of a system.

6.6. Solid State Solubility

Solid phases form solutions as do liquid and gaseous phases. Themechanism of explanation here is much the same as that for the familiarliquid solutions. Solid solutions can be formed by atoms of the solutereplacing atoms in the crystalline structure of the solvent. This type ofsolid solution is referred to as a substitutional solid solution. Anothercommon type of solution is the interstitial solid solution, in which theatoms of the solute locate in the interstices of the crystallographic planesof the solvent. Iron, iron-carbide alloys are examples of this type ofsolution. The smaller atoms of carbon are the solute. Carbon is referredto as a metalloid, since it has metal as well as nonmetal characteristics.One of its predominant nonmetallic properties is its negative temperature-resistance slope.

Section 6.7 shows equilibrium diagrams of some binary systems withvarying degrees of solubility between components. It is possible to havesupersaturated solutions in the solid state; martensite is a common ex-ample of a solid solution containing an excess of carbon.

6.7. Binary Systems

The constitution of metallic alloys is studied by preparing a series ofhigh-purity alloys ranging in composition from 100% of one constituentor component to 100% of the other component. Suitable specimens ofthese alloys are subjected to treatments that produce a state of equili-brium. This step is followed by studying the changes in length, electricresistivity, microscopic structure, atomic structure, absorption or libera-


CC = alpha solid solution of Cu+Ni

Cu,

fNi

0%N; 25%Ni 50% Ni 75% Ni lOOZNi100% Cu 75% Cu 50%Cu 25% Cu 0%Cu

Composition (by wt.)

M.P. ofNi

Fig. 6.1. Equilibrium diagram for the Cu-Ni system.

tion of heat (thermal analysis) , and other changes that occur when the

alloy is heated or cooled to room temperature or sometimes to the melting

point and above. The temperatures at which the changes occur are

plotted against the composition; the result is an equilibrium diagram

(also called a constitution or phase diagram) of the series. A diagram

showing changes between two metals is called binaiy equilibrium diagram.

Solid Solution Alloys. A few binary alloys exhibit complete solubility

or miscibility in all proportions and temperatures in the solid state.

These are called solid-solution alloys. Typical of this type is the Cu-Nisystem. Development of an equilibrium diagram for Cu-Ni by thermal

analysis is shown in Fig. 6.1. (The cooling curves shown have been

previously discussed in Section 8 of this chapter)

.

The line, 1-2-3-4-5, is the liquidus line, and everything above this line

is liquid or melt (also indicated as m)

.

The line, l-2'-3'-4'5, is the solidus line, and everything below this line

is solid solution or alpha (also, indicated as <x )

.

(The various solid phases—not necessarily only solid solutions—of anequilibrium diagram are generally shown by letters of the Greek alphabet

starting from the extreme left of the diagram(a. , /?, « , etc.) .

To trace the cooling of an alloy of Cu-Ni, refer to Fig. 6.2. Here alloy

"A" was chosen, containing 63% Ni and 87% Cu, which represents ap-

proximately the composition of Monel metal (shown by the vertical

dotted line)

.


\rm

Tc- _ftrjaTd

Cu NiComposition

Fig. 6.2. Equilibrium diagram for 63% Ni and 37% Cu.

The following stages on cooling may be noted (cooling must be slow

enough so that equilibrium conditions prevail) :

1. At temperature Tm, the alloy is completely molten.

2. The alloy stays molten until the temperature reaches Ta.

3. At Ta, the first crystals of solid solution appear and will have the

composition as shown at a' (by drawing a horizontal line from a

until it intersects the solidus line) . The composition may be read bydropping a vertical line from a' to the composition axis.

4. On further cooling to Tb, more solid solution crystals appear, withthe composition changing along the solidus line from a' to b', andthe composition of the remaining melt is shown by dropping a

vertical line from b on the liquidus line to the composition line.

5. Thus, on cooling between Ta and Td, the composition of the meltwill become richer in Cu from a to d and the composition of the

solid solution crystals will vary from a' to d'.

At any temperature, Tc, between Ta and Td, the relative amounts ofsolid and melt may be calculated as follows:

Let Wt = total weight of alloy,

Wm = weight of melt,

Ws = weight of solid.

Then, Wt = Wm + Ws, (or) Ws = Wt— Wm. [6.1]

And, Wt X % Cu in the original alloy will represent the total amountof Cu present.

Wm x % Cum will represent the amount of Cu present in the melt.

Ws X % Cu, will represent the amount of Cu present in the solid.

(Cu n , & Cu„ represent the composition of the melt and solid re-


spectively as read by dropping vertical lines from c & c', wherethe horizontal line at temperature Tc intersects the liquidus andsolidus lines.

Then: Wt X % Cu=Wm X % Cum Ws X % Cu„

100 100+ " 100

By substituting the value of Ws from Equation 6.1 in 6.2, Ws is elimin-

ated and Equation 6.2 becomes:

Wt X % Cu=Wm X % Cu n , Wt X % Cu, _ Wm X % Cu,

100 100+ " 100 100

(or)

Wt x % Cu- Wt X % Cu 8 = Wm X % Cum-Wm X % Cus

Wt (% Cu_ % CuB) = Wm (% Cu ra- % Cu 8)

At temperature Tc, the composition of the original alloy is still 37% Cu,

and the composition of the melt is 54% Cu,

and the composition of the solid is 28% Cu.

Let 100# represent the total weight of the original alloy; then,

100 (37-28) = Wm (54-28)

;

Wm = 1Q0X9 =34.5# or 34.5%;26

.". Ws = 65.5# or 65.5%.

The Lever Principle. Since the length of the horizontal line is pro-

portional to the composition, the above problem may be solved by the

so-called lever principle.

Illustration

The length of the line c-Tc-c' is 26 units

The length of the line c-Tc is 17 units

The length of the line Tc-c' is 9 units

Therefore, % of solid = -1^- X 100 = 65.5% .— X 100,26 ' c-Tc-c7

q Tc-c'and % of melt = -^ X 100 = 34.5% = X 100.

*6 c-Tc-c'

The ratio for the melt is the distance from the nominal analysis

(fulcrum of the lever) to the solidus line over the total distance between

the lines. The ratio for the solid is the distance from the nominal

analysis to the liquidus line over the total distance.

The lever principle may be used to determine the relative amounts of

each phase when two phases are present (both phases may be solid)

.


The total horizontal distance is always measured between the twoboundary lines of the two phases present, and the fulcrum is taken at

the nominal analysis of the binary alloy.

Application of the phase rule shows that a one-phase region, such as

the cc or m areas, is trivariant: i.e.. there are three degrees of freedom.

N„ + N f= N + 2

1 +N f = 2+2Nf = 3

One degree of freedom is used in selecting a pressure of one atmos-

phere, the second degree is used in selecting a temperature which will

locate the vertical position, and the third is used in selecting the composi-

tion, which locate the lateral position. Thus, the three degrees are

utilized in fixing the position of any point in the single phase area.

When two phases are coexistent, as in the m + <x region, there are twodegrees of freedom.

N p + N f= N c + 2

2 + N f = 2 + 2

N f = 2

One degree of freedom is used in fixing the pressure at one atmosphere.Therefore, if the remaining degree of freedom is used in fixing the

temperature (vertical position) it follows that the compositions of thetwo phases are then fixed; or, if the compositions are selected first, thenthe temperature becomes fixed.

Binary Eutectic Alloys. When two metals arc soluble in all proportionsin the liquid state but only partially soluble in each other in thesolid state, a eutectic will be formed between the limits of solid solubility.

A binary eutectic is a finely divided mechanical mixture of two solid

phases and will exhibit a definite composition and definite melting point.Cu-Ag form alloys of this type as shown in Fig. 6-3.

Consider an alloy containing 6% Cu and 94% Ag. On cooling fromsome temperature above the liquidus line, the first phase change will

occur when the temperature reaches the liquidus line. At this point, a,

crystals of alpha solid solution will start to separate, with an initial

composition as shown by drawing a horizontal line until it intersects thesolidus line at point b. On further cooling, the composition of the alphawill change along the solidus line and the composition of the remainingmelt will change along the liquidus line; with more and more alphacrystallizing out and, consequently, less and less of the melt remaining.At point c the last of the melt will crystallize and the alloy will consist

of solid alpha. On further cooling to the solubility line at point d, betasolid solution will start to precipitate from the alpha as the solubility of


X1200

1000

800

600

Solvus (or)

solubility

line

400

Ao

108.1

Solvus (or)

solubility

line

10 20 30 40 50 60 70 80 90

Wt. % Cu Cu

Fig. 6.3. Equilibrium diagram for the Ag-Cu system.

beta in alpha decreases with a decrease in temperature as shown by the

shape of the solubility line.

Consider an alloy containing 20% Cu and 80% Ag. Again alpha starts

to crystallize at the liquidus line. On further cooling, the composition of

the melt approaches the eutectic composition. When the temperature has

dropped to the eutectic, the remaining melt will have reached the eutectic

composition. At this point, and on further extraction of heat, the melt

will crystallize isothermally to Eutectic.

Therefore, the various areas may be described as follows:

Area 1. Two phases consisting of alpha plus precipitated beta.

Area 2. Two phases consisting of alpha plus eutectic.

Area 3. Two phases consisting of beta plus eutectic.

Area 4. Two phases consisting of beta plus precipitated alpha.

Area 5. A single phase consisting alpha solid solution only.

Area 6. A single phase consisting of beta solid solution only.

Since the eutectic is a finely divided mechanical mixture of both alpha

and beta, the combined area (1, 2, 3, 4) can be designated as consisting

of alpha plus beta, but they will be in different physical states as described

above.

The precipitation of one solid phase from a solid solution is a very

important factor in the heat treatment of alloys such as those of Al, Mg,


10

Atomic % Ca

20 30 40 50 60 70 60 90

Fie. 6.4. Equilibrium digram for the Mg-Ca system.

and Gu. This type of treatment is known as "aging" or precipitation

hardening and will be discussed in more detail in Section 6.8.

Application of the phase rule will show why the eutectic reaction mustoccur at a constant temperature.

Np + N f = N«+ 2

3 + N, = 2 + 2

N, = 1

Again, one degree of freedom is used in fixing the pressure at one

atmosphere; therefore, neither the temprature nor the composition of any

of the phases can change during the course of the eutectic reaction under

equilibrium conditions.

6.8. Intermetallic Compounds

Many alloy systems will exhibit one or more intermetallic compounds.This type of compound is generally very hard and brittle, and, underproper conditions, may impart additional strength ami hardness to analloy. Therefore, intermetallic compounds are very important industri-

ally. Compounds of this type have a definite chemical formula, eg. Cu s Al,

and have a definite melting point that shows as a maximum on the

liquidus line (Fig. G.4) . This melting point may sometimes be higher

than the melting point of either component.

Phase diagrams of this type may be conveniently studied by considering

them as two-part diagrams, i.e. the system Mg-MgXa and the system


a ^=^-

\

I

•7si

—— e

*/< i

' i

«!• is the solidus line.

be is the CuAla solubility line.

2 3 4-56%Cu

Fig. 6.5. Al.-rUh end of Cu-AI phase diagram.

Mg2Ca-Ca. ]Jy this method the diagram simplifies itself into two eutectic

types.

Fig. 6.5 represents the Al rich end of the Cu-AI phase diagram.

If an alloy containing 2% Cu is heated to temperature T, and held for a

sufficient length of time, the alloy will become a homogeneous solid sol-

ution. Rapid quenching will then retain the alloy in this condition at

room temperature. This is known as solution treatment. On reheating

this alloy up to temperature T,, (below the solubility line be) , the inter-

metallic compound, CuAl2, will precipitate from the solid solution. Theamount of this precipitation will depend on time, temperature, and solu-

bility. This step in the heat-treatment is known as aging or precipitation

hardening.

The physical properties of certain alloys may be markedly changed

by the heat-treatments described above.

6.9. The Peritectic Reaction

The peritectic reaction may be called an inverse or fake eutectic re-

action, wherein a liquid and a solid react isothermally to form a newsolid. By examining Fig. 6.6, it will be noted that alloys of Li and Mglying between the points a & P will first contain alpha solid solution

and melt after crossing the liquidus line on cooling. At the peritectic

temperature all of the melt will react with some of the alpha to form the

new solid, beta solid solution. Between points P and b and at the

Peritectic temperature all of the alpha will react with some of the melt

to form beta. If an alloy were chosen such that its composition was ex-


40 50 60

% Wi of Li

Fig. 6.6 Equilibrium diagram for the Mg-Li system.

acily as shown at the peritectic point P, then, on cooling to the peritectic

temperature, all of the solid, alpha, would react with all of the melt to

form beta.

6.10. Closure

In recent years considerable progress has been made in the field of

solids, and these achievements are reflected in no small measure in the

science of metallurgy as well as in other subdivisions of solid state physics.

New developments include transitors, transducers, solid rocket fuels,

high temperature sealants, metal production technology, "light weight"

radiation shields, and a host of other innovations.

The salient feature of the real progress in all of these fields is that

success was accomplished by thorough consideration of the fundamental

nature of the problem. The metallurgist of the future should be well

versed in such items as quantum theory, radiation, and fundamental

rheology. The physicist has helped the engineer to understand aspects

of the solid state phenomena, but there is a need for further investigation

by the research engineer.


QUESTIONS

1. With reference to the "lever principle", would such a rule apply to a twophase region in any plane where extensive properties were used as co-ordinates? Assume equilibrium exists and recall that an extensive propertyof a multiphase system is the sum of that extensive property for all phasesof the system.

2. It is known from thermodynamics that constant pressure boiling of a puresubstance (one component) takes place at constant temperature as long as

the vapor is in contact and in equilibrium with the liquid. Explain this.

S. Can the "triple point" referred to in paragraph 6.5 ever appear as an areaon a plane of thermodynamic properties? Explain.

4. During a certain experiment, pertinent to cooling a binary system, a speci-

men was "isolated" and observed to contain five phases, apparently co-

existing. Give several conclusions and substantiate.

5. With reference to the lever principle in paragraph 6.7, how was equilibriumassumed or implied in the derivation?

6. Using the law of mass transfer as given in paragraph 6.4, derive the partial

differential equation for three dimensional, unsteady state diffusion.

7. Using the equilibrium diagram for copper and nickel calculate the pro-portions of solid and liquid for an alloy containing 30% Ni. at a tempera-ture of 2200 deg.F. What are the concentrations of each component in theliquid and the solid?

8. Compute the volume of a unit mass of mixture containing 80% (by mass)water vapor and 20% liquid water in equilibrium with the vapor. Thepressure of the mixture is 50 pounds per square inch absolute. Referenceto vapor tables will be necessary here. Was the so called "lever principle"employed in the above computation? HINT: See Problem 1 above.

9. For copper-silver phase equilibria discuss the transformations taking placeupon cooling for the following alloys: 5% copper, 20% copper, and theeutectic composition.

10. Some binary alloys exhibit a characteristic such that there exists a composi-tion that has a lower melting point than cither pure component. Showthat this point is a point of tangency for the liquidus and solidus. Wouldthe common tangent to both curves at the above point be a horizontalline on the equilibrium diagram? Substantiate your answer.

1 1. Can water exist in more than one solid phase? Discuss.

12. How many degrees of freedom exist for the following situations:

(a) A diatomic gas.

(b) A pendulum swung by an elastic and extensible cord.

(c) A locomotive making a run between two cities on a specified track.

CHAPTER

Heat Treatment of Alloys

by Precipitation Hardening

Harold Vincent Fairbanks, M.S., Professor of Metallur-

gical Engineering, West Virginia University, Morgan-town, W. Va.

Arthur R. Foster, M.Eng. Associate Professor of

Mechanical Engineering, Northeastern University,

Boston, Massachusetts

X RECIPITATION hardening, sometimes

called dispersion or age hardening, is the most important method of

strengthening nonferrous metals through heat treatment. This treatment

has been used very effectively with the light alloys of aluminum andmagnesium. It has also been applied to a number of heavy alloys of

metals such as copper, iron, nickel, silver, and lead. The susceptibility

of an alloy to precipitation hardening depends upon the manner in whichits component metals combine rather than upon the properties of the

dominant metal.

The process of precipitation hardening consists essentially of two steps.

In the first step, an unstable condition is produced in the alloy structure

through the formation of a supersaturated solid solution. This step is

called solution heat treatment and produces a comparatively soft, ductile

state for the alloy.

The second step requires a certain degree of precipitation of the super-

saturated phase to impart the increased hardness and strength to the

metal. If this step is carried out at room temperature, the process re-

quires several hours or days and is sometimes called age hardening. Onthe other hand, if an elevated temperature is employed, the process re-

quires much less time and is called the precipitation heat treatment.

7.1. Alloy Requirements

There are two main requirements for alloys to be heat treated byprecipitation hardening. The first requirement is that the solubility of the

HI


dissolved constituent decreases with decreasing temperature. The secondrequirement is that there be some coherency between the precipitating

phase and the matrix phase. It is this coherency or lattice conformitybetween the two phases which brings about the internal crystal stresses

required for strengthening of the alloy.

Another influencing factor which governs the degree of hardening is

the quantity of available material for precipitation. The slope of the

solid solubility curve (solvus line) and the composition of the precipitat-

ing phase determines the amount of material available for precipitation.

The closer the precipitating phase is to the solid solubility curve on the

phase diagram, the more the material that is available for precipitation.

Also, the greater the decrease in the solubility with decreasing tempera-

ture, the larger will be the amount of material available for precipitation.

The precipitating material may be either a solid solution or some inter-

mediate phase or compound.

However, it must be remembered that a large quantity of material

available for precipitation does not insure that heat treatment of the alloy

by precipitation hardening is possible. In the Mg-Pb system, magnesiumcan dissolve 46 percent lead at the eutectic temperature and only 2

percent lead at room temperature. This alloy shows but little strengthen-

ing effect of precipitation hardening due to the small amount of co-

herency of the precipitated phase with the matrix material.

7.2. Step I—Solution Heat Treatment

The process of precipitation hardening is divided into two steps: first,

the solution heat treatment, and second, the precipitation heat treatment.

A schematic representation for the process of precipitation hardening is

given in Fig. 7. 1

.

The alloy to be solution heat treated must have a composition that

meets two requirements: (1) that at an elevated temperature a solid

solution is formed, and (2) that the solid solution can be retained uponrapid cooling to room temperature. A typical alloy used for precipitation

hardening is shown as alloy N in Fig. 7.1.

The process for solution heat treatment has two parts: (1) heating, and(2) quenching.

In heating, the alloy is heated to a temperature above the solvus line

but below the solidus line. The alloy is held at this temperature for asufficient period of time to produce a solid solution. For some alloys the

temperature range is fairly narrow between the solvus and solidus lines.

It is very important not to heat above the solidus line as incipient melt-

ing of the alloy will take place destroying the dimensions of the fabricated

Heat Treatment of Alloys by Precipitation Hardening 113

PHASE DIAGRAM PRECIPITATION HARDENING

Temp.

Composition Tlmt »-

Kic. 7.1. Schematic representation for the process of precipitation hardening.

part. Also, too high a temperature can produce excessive grain growth.

On the other hand, the temperature should be above the solvus line in

order to take advantage of the material available lor precipitation harden-ing and to produce as homogeneous a solid solution as possible.

In quenching, the heated alloy is cooled to room temperature rapidly

enough to retain the solid solution. The alloy at room temperature is nowin a state of nonequilibrium as only the supersaturated solid solution

phase is present.

The retention of the potential precipitating phase in solid solution is

the prime requirement of quenching to room temperature. It is im-portant that the alloy be quenched as uniformly as possible as unevenquenching forms areas of high stress in the alloy which will producesoft spots in the subsequent hardening treatment. The stressed areas

will harden first and then become softer during the subsequent heattreatment which is necessary for the main body of the material to obtainmaximum hardness.

The result of the solution heat treatment is a supersaturated solid

solution condition in the alloy. The alloy in this state may be softer

than in the annealed condition, but generally it is found to be harder thanthe annealed state for the alloy, as shown in Fig. 7.6. However, the ma-terial after solution heat treatment is still ductile and may be cold worked.Some alloys start to harden upon standing at room temperature. Any

deformation required after solution heat treatment for these alloys shouldbe done immediately before the hardening begins. In order to arrest this

hardening action, the metal may be placed in a deep freeze which lowersthe rate of atomic diffusion due to thermal activity. This fact was made


use of in the storage of aluminum rivets for assembling of airplane parts

during World War II. However, most alloys are fairly stable at roomtemperature after solution heat treatment.

7.3. Step II—Precipitation Heat Treatment (Aging)

The second step in the process of precipitation hardening brings about

the hardening and strengthening of the alloy. This is achieved by re-

heating the unstable solution heat treated alloy to some temperature be-

low the solvus line, but sufficiently above room temperature to allow

mobility of the atoms held in supcrsaluration.

This precipitation temperature is held for a definite period of time

until the hardness of the alloy has reached the maximum or desired value,

and then the alloy is either quenched or air cooled to room temperature.

Once the alloy has reached room temperature, it will retain the desired

properties due to the lack of mobility of the atoms in the crystals to

migrate.

Attention must be paid to the proper temperature and to the time at

temperature in order to insure maximum hardness and strength. Toohigh a temperature and too long a time will cause softening of the alloy,

and the advantages of this method of heat treatment will be lost.

For some alloys the entire second step is carried out at room tempera-

ture. This is only possible when the mobility of die atoms in the alloys

are sufficient at room temperature to cause the desired hardening and

strengthening of the alloy.

7.4. Theory of Precipitation Hardening

The heat treatment of alloys by the precipitation hardening process

depends upon obtaining an unstable state for the alloy by solution heat

treatment, and then controlling the degree of approach toward the stable

state for the alloy during the precipitation heat treatment. The maxi-

mum hardness and strength are obtained before the equilibrium or stable

state for the alloy is reached.

The actual hardening and strengthening of the alloy occurs during the

precipitation heat treatment step. The mechanism involved may be di-

vided into three stages: (I) the pie-precipitation stage, (2) the inter-

mediate precipitation stage, and (3) the equilibrium precipitate stage.*

Fig. 7.2 shows sketches of the various stages in precipitation.

* For an excellent discussion on Precipitation see "Report on Precipitation" by H.

K. Hardy and T. J. Heal in "Progress in Metal Physics", edited by Bruce Charles and

R. King, Interscience Publishers, Inc., Vol. 5. pp. 143-278. 1954.


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A. Supsrsoturot. d BOlid „ _B. Pit -precipila'e c hlttrmtdletc precipitate D. Equilibrium prtcipitote

solution

Fig. 7.2. Sketches showing the various stages in precipitation. Open circles representmatrix atoms. Filled-in circles represent solute atoms.

In the pre-precipitation stage, the solute atoms segregate along specific

crystallographic planes of the matrix phase in preparation for nucleation.

These clusters of atoms are called knots or Guinier-Preston Zones, havingbeen detected by Guinierf and Preston;}; with X-rays.

In the intermediate precipitate stage, nucleation is taking place andthe resultant new lattice of the precipitating phase is present. This newlattice is in the form of thin plates from 2 to 50 atoms thick and 20 to

several hundred atoms long or wide. There is a lattice coherency present

between the precipitated phase and the matrix phase which results in a

highly stressed area in the matrix surrounding the intermediate precipi-

tate.

In the third stage, the precipitate has grown to such a size that the

coherency forces between the two adjacent crystal lattice systems are over-

come and the precipitate becomes its own independent crystal entity.

This relieves the major stresses in the area and softens the metal.

It is during the second stage in which the intermediate precipitate is

formed that maximum hardening and strengthening of the alloy occurs.

This is due to the fact that a flexible dislocation cannot move easily

through the regions of localized stresses. The greater the number of

these localized stresses per unit volume of material the greater is the

strengthening of the metal due to the increased force required to push the

dislocations through the many stress barriers.

fCuinier, A., Nature 142 (19S8) 569.

j Preston, G. D., Nature 142 (1938) 570.


This precipitation process is one of nucleation and growth of the

solute phase from a supersaturated solution. The driving forces involved

for the diffusion of the solute atoms include: (1) thermal energy, (2) the

supersaturated condition of the matrix phase, and (3) stresses whichwere developed due to quenching in the solution heat treatment step.

The nucleation throughout the alloy does not necessarily start at the

same time in the alloy during the precipitation heat treatment. There-

fore, it is the statistical average effect which is produced at a given time

for a given temperature. There are undoubtedly particles in all three

stages at the same time.

Fie. 7.3. Structure of an alloy containing 3.94 per cent copper, 0.02 per cent silicon,

0.02 per cent iron, and remainder aluminum. (A) as cast, 100x; and (B) quenched,

500x; etched. (Courtesy of Aluminum Research Laboratories)

If the temperature of the alloy is maintained sufficiently low, eventhe first stage cannot take place. On the other hand, if the temperature

for the precipitation heat treatment is too high, only the third stage mayprevail. This is called overaging. The alloy becomes softer and loses

its strength. It may become as soft as in the annealed state. Photomicro-

graphs showing microstructure of an aluminum alloy with various treat-

aents are shown in Fig. 7.3.


Optimum tamp.

Strength

-Tim e

Fie. 7.4. Effect of precipitation heat treatment temperature and time on thestrength of the alloy.

7.5. The Effect of Time and Temperature During PrecipitationHeat Treatment

Both the temperature at which the precipitation heat treatment is

carried out and the length of time at temperature are important. As the

temperature is increased the mobility of the atoms in the supersaturatedlattice structure increases. This means that the growth' of the unstablesubmicroscopic platelets in the transition phase is speeded up. This also

means that they will attain the critical size at which they shear awayfrom the parent alpha phase sooner. The result of this is that optimumhardness and strength are attained more quickly. This is shown in Fig.

7.4. Increasing the time of treatment causes the growth of stable pre-

cipitated particles of the second phase with marked losses in the strengthand hardness of the alloy along with other changes in the properties ofthe alloy as shown in Fig. 7.5.

If the precipitation temperature is lowered, the diffusion of the solute

atoms is impeded. In an extreme case this may mean that it is completelysuppressed and no increase in hardness or strength can occur. In othercases the improvement is less than the optimum amount.The temperature which is required to prevent hardening of the solu-

tion treated metal varies with the alloy. Some aluminum alloys must bestored at sub-zero temperatures to suppress hardening, but berylliumcopper, after solution treatment, remains soft at room temperature in-

definitely.

Some data on hardness of 24S aluminum is included in Fig. 7.6. Thisgraph indicates that increased temperatures will speed the rate of harden-ing. At 240°F it shows that little overaging occurs in a week. It is


Tentile Strength

Propertie*

Log Time

Fie. 7.5. Effect of precipitation heat treatment time on alloy properties-

interesting to note that the furnace cooled specimen is much softer than

the solution treated material. This indicates that the alloy in the solu-

tion treated condition where the solid solution is maintained, is not as

soft as the equilibrium structure which contains relatively large particles

of hard, brittle second phase. This is because, in the case of 24S, the copper

is still in the solid solution with the aluminum in the solution treated

specimen. Since the atoms of copper are of a different size than those of

aluminum, the movement of dislocations along the various planes in the

lattice structure is impeded. This results in greater strength and hardness.

Both the temperature and the length of time at the chosen temperature

must be specified if satisfactory results are to be obtained in the precipi-

tation hardening process. It may be possible to shorten the time in the

heat treating furnace by raising the temperature. Care should be taken

that proper temperature and time be selected from published data or, if

this information is not available, that they be established experimentally.

It can be seen from Fig. 7.4 that although higher than optimum tempera-

tures may reduce the attainable strength slightly, the shortened time mayreduce the heat treating costs sufficiently to justify the slight reduction in

physical properties.


80

70

60

» SO

C0 4Q

20

10

1

240 °F aging temp.

r~~ 75° F aging temp.J

f /8° F agina temp1

Fu noce coo led to re*om temp.

I ? 3 4 5 6 7Time, days

Fie. 7.6. Rockwell B hardness vs. time for 24S. Solution treated 1 hour at 970° K.

Furnace cooled specimen cooled very slowly. Other specimens water quenched prior

to precipitation hardening.

7.6. Precipitation Hardening of Aluminum Alloys

Extensive use is made of precipitation hardening with many alloys of

aluminum. Wrought alloys may be strengthened by either cold work or

precipitation hardening. The latter usually results in a smaller decrease

in ductility for a similar or even greater increase in strength. In castings,

precipitation hardening is especially useful since no cold working can be

performed.

The binary phase diagrams for Al-Cu, Al-Mg, Al-Si, and Al-Zn all showthe characteristic decrease in the solubility of the alloying element in the

aluminum rich alpha phase as the temperature decreases. Few com-mercial alloys are true binary alloys, but the phase diagram for aluminumand the major alloying element usually points the way to possible success-

ful heat treatment.


700

SO SO 10Copper, per cent

Fie. 7.7. The aluminum-copper phase diagram: inset, the aluminum-rich portion.{Dix and Richardson)

The Al-Cu phase is shown in Fig. 7.7. Wrought alloys 1 IS, 14S, 17S,

and 24S all contain 4.0 to 5.5 percent copper plus minor amounts of

silicon, magnesium, manganese, lead, and bismuth, while casting alloys

such as No. 108, 112, 142, 195 and 212 contain 4 to 8 percent copper as the

major alloying element. In alloys containing up to 5.5 percent copper all

the theta phase should go into solid solution with the alpha phase onheating to about 1000°F. A casting alloy such as No. 212 containing

8 percent copper, 1.2 percent silicon, and 1 percent iron cannot dissolve

all the theta phase when solution treated as the alpha phase when heatedto just below the eutectic temperature can hold only 5.5 percent copper.However, this alloy can be hardened just as the other Al-Cu alloys andthe undissolved hard, brittle theta particles will serve to improve the

machineability of the alloy. In the presence of magnesium and silicon,

other intermetallic compounds which are susceptible to age hardeningmay be formed such as Mg,Si, AlXuMg, or AlCuMgSi.The Al-Mg phase diagram shown in Fig. 7.8 is of interest as it shows

the type of partial solubility required for precipitation hardening at

each end of the diagram. The magnesium rich end of the diagram will

be treated in Section 7.7. It can be seen from the phase diagram that

aluminum can dissolve 16 percent magnesium at the eutectic tempera-ture of 844°F and very little at room temperature. On slow cooling of

the alpha phase alloys to room temperature, the beta phase which is anintermetallic compound of Mg2Al 3 precipitates. Casting alloy No. 220containing 10 percent magnesium has the best physical properties of any


of the aluminum base casting alloys. Its major disadvantage is its poorcapability which requires special foundry techniques and inhibitors to

prevent deep surface defects.

The Al-Si phase diagram shows a decreasing solubility of silicon in

aluminum as the temperature is lowered. However, the amount of the

silicon rich beta phase which is suppressed upon solution treatment is

not great enough to produce a commercially important improvementin physical properties. Silicon rich casting alloys are used extensively,

but not in the heat treated state.

When silicon and magnesium are used together in an aluminum alloy,

an intermetallic compound of Mg2Si is formed which allows effective

heat treatment. The solubility of this compound in aluminum increases

from 0.27 percent at 390°F. to a maximum of 1.85 percent at 1100°F.,

which is the eutectic temperature. Silicon in excess of the amount form-

ing Mg2Si does not affect the solubility of the compound in aluminum,but an excess of magnesium does reduce its solubility. Therefore, it is

necessary to be sure no excess magnesium is present.

Wrought alloy SIS (l%Si, .6%Mg, .25%Cr) is of this type. 61S is

slightly more complex (.6%Si, l%Mg. .25%Cu, .2r,%Cr) in that it con-

tains copper, which further complicates the mechanism of precipitation

hardening by increasing the number of intermetallic compounds whichform. The small amount of chromium added to both alloys tends to

offset the ill effects of iron which is present as an impurity. Castingalloys 132, 355, and 356 are heat treatable Al-Si-Mg alloys. They haveconsiderably higher silicon contents (5-12%) than the wrought alloys

just mentioned. This improves their ratability.

Although a high percentage of zinc can be dissolved in aluminum at

elevated temperatures (82.8 percent at 720°F. compared with 5 percentat 248°F.) alloys of these elements show only limited response to pre-

cipitation hardening. With the addition of magnesium, compounds suchas MgZn2> MgZn, and AlMgZn are formed and these alloys will respondvery well to precipitation hardening. The wrought alloy 75S containing5.6 percent zinc, 2.5 percent magnesium, 1.6 percent copper, 0.3 percentchromium and 0.2 percent manganese is essentially a Zn-Mg alloy. It

develops the best mechanical properties of any of the commercialaluminum alloys.

7.7. Precipitation Hardening of Magnesium Alloys

Precipitation hardening is especially useful with magnesium base alloys

since the close packed hexagonal crystal lattice structure will allow onlylimited amounts of cold work. The major alloying elements are alu-


10 20 30

Atomic Per Cent Al

40 50 60 70 80 90

"C

i i 1 1 1 ' i 1 1

600

500

ruA *36*

463"

./ 451" / „..

400Si 5 6 , Mg/I, \

t.V 1«II

300 /

fi

200

_ i i i ' I .1 -L 1

10 20 H 40 50 60Weight Pet Cent AL

70 80 to

Fie. 7.8. Aluminum—magnesium phase diagram.*

• By permission from Structure 8c Properties of Alloys, Brick & Phillips, McGraw-Hill Book Company, Inc., 1949.

minum, zinc and manganese. The manganese strengthens the alloys by

going into solid solution with the magnesium while both the aluminumand the zinc show the decreasing solubility required for successful pre-

cipitation hardening. Fig. 7.8 shows that 12.6 percent aluminum is

soluble in magnesium at the eutectic temperature of 815°F. while less

than 2 percent is soluble at room temperature. Zinc shows virtually no

solubility in magnesium at room temperature and has a maximum of 8.4

percent at 646°F. The precipitating phase is an intermetallic compoundMgZn.Many alloys contain substantial amounts of both aluminum and zinc

so that a ternary diagram is necessary to show the phase relationships in

these alloys. The change in solid solubility and the identity of the phase

or phases in which precipitation is suppressed by solution treatment is

indicated in Fig. 7.9. On this diagram, it can be seen that casting alloy Ccontaining 9 percent aluminum, 2 percent zinc, and 0.2 percent man-ganese should precipitate only Mgi 7Al 12> but alloy H containing 6 percent

aluminum, 3 percent zinc, and 0.2 percent manganese also precipitates

some Mg3Zn2Al 8 . These are the two most commonly used casting alloys.

Solution treatment not only improves the strength of the alloys but also


%i<\

Fig. 7.9. Section of ternary diagram for Mg-Al-Zn system. Isothermal lines showthe limits of solid solubility at the various temperature levels. To the left of thedashed line the structure consists of solid solution and Mg„Al„. The solid dot shownon the diagram represents the composition of Alloy C which shows this type ofmicrostructure. To the right of the dashed line the structure will consist of solid solu-tion, Mg1TAlM ,

and Mg37.njAlj. Alloy H whose composition is represented by the smallcircle on the diagram is an alloy of this type.*

improves the ductility considerably, because the brittle interdendritic

eutectic compounds are dissolved. The precipitation treatment reduces

the ductility, but leaves enough for most applications. It causes a markedincrease in yield strength with little change in tensile strength.

Of the wrought alloys only 0-1 containing 8.5 percent aluminum, 0.5

percent zinc, and 0.2 percent manganese, and ZK-60 containing 5 percent

zinc, 0.7 percent zirconium have enough alloy content to be effectively

precipitation hardened. Extrusions and forgings are usually worked at

high enough temperatures and cool fast enough in the atmosphere to beeffectively solution treated. Thus, they need only to be precipitation

hardened after fabrication is completed.

The precipitation of a second phase with its different crystal lattice

dimensions results in the growth of the piece during precipitation harden-ing. If such dimensional instability cannot be tolerated at the service

• Busk and Miranda Trans. AIME. IG6. 346. 1946.


temperature, a stabilization heat treatment is required. Stabilizing con-

sists of heating to a temperature somewhat higher than the usual precipi-

tation hardening temperature to allow partial precipitation of the

second phase. The degree of transformation is greater than will occur at

the service temperature, and, therefore, there is no further tendency for

precipitation while the part is in use. This is essentially ovei aging and

causes some loss in physical properties. This type of heat treatment maybe applied to other precipitation hardening alloys as well as the mag-

nesium base alloys discussed here.

°C Atomic Percentage Beryllium

5, 10 15 SO IS 30 35 40

1000

700

600

500

300,

- (600

1200

- 1000

Co I B 3 « 5 6 7 8 9 10

Weight Percentage Beryllium

Fig. 7.10. Copper-rich portion ot the copper-beryllium phase diagram. (H. F.

Silliman)

7.8. Precipitation Hardening of Copper Base Alloys

Precipitation hardening is not used as extensively with copper base

alloys as it is with the aluminum and magnesium alloys, but there are

several alloys with which it may be used very effectively.

Commercial beryllium-copper alloys contain from 1 to 2-\/A percent

beryllium plus \/A percent nickel to inhibit excessive grain growth at the

solution treatment temperatures. The phase diagram shown in Fig. 7.10

indicates a maximum solubility of 2.1 percent beryllium at 1590T. Thefigures given in Table 7.1 indicate that precipitation heat treatment is

extremely effective with these alloys and that when combined with cold


working, tensile strengths approaching 200,000 psi may be obtained.The usual heat treating cycle consists of solution treatment at 1450°F.followed by a water quench and reheating to 480 to 620°F. for 2 to 3hours. The high strength, corrosion resistance, and fatigue resistance ofthese alloys recommend their use for springs, diaphragms, and non-spark-ing tools.

Moncl metal which contains 67 percent nickel and 30 percent copperplus small amounts of iron and manganese forms a single homogeneoussolid solution as do all the alloys in the binary Cu-Ni series. However,if 3 percent aluminum is added producing K-Monel or 4 percent siliconis added producing S-Monel, the metal is responsive to precipitation

hardening.

Tin bronzes show little tendency towards precipitation hardeningalthough the phase diagram indicates that it is possible. The low tem-peratures at which the alpha phase becomes unstable which is 570°F.for 5 percent tin and 95 percent copper prevents any appreciable pre-cipitation of the delta phase. The addition of up to 8 percent nickel,however, promotes the precipitation of a Ni-Sn rich constituent whichimproves the physical properties considerably beyond those of the ordi-nary tin bronzes. The data in Table 7.2 indicates the effectiveness ofprecipitation hardening on a commercial alloy of this type.

7.9. Precipitation Hardening in Low Carbon Steel

In Chapter 12 where the fundamental structure of carbon steels will bediscussed, ferrite is defined as alpha iron holding in solution smallamounts of silicon, phosphorus, carbon, and other elements. Of the ele-

ments which may be in solution in very small quantities in alpha iron,three: carbon, oxygen, and nitrogen can, under suitable conditions, causeprecipitation hardening. Fig. 7.11 shows the approximate solubility ofthese in iron. It will be noted that the slope of these curves resembles theslope of the solubility curves of the other alloy systems discussed inthis chapter.

Precipitation hardening in carbon steel is a phenomenon that is com-mon only in the low carbon grades. Since these steels are used rarely in

the quenched conditions, precipitation hardening due to iron carbide oriron nitride retained in solution by quenching, followed by slow spon-taneous precipitation at room temperature or more rapid precipitationat slightly elevated temperature is seldom encountered. It is, however,possible, by quenching a steel containing 0.04 to 0.10 percent carbonand by precipitation treatment at 140°F. for 10 to 15 hr. to increase theBrinell hardness from 120 to 160.


Oxygen, percent.K .04 .os .on .to

irJBOO

%*IS00D§« BOO

g- 400

o

/ *—k.

\.a*iSolidSol. / —

i

V soo SB\l 1

/'

x£4 >i^ a Sum'

,Solution

| |

«w j*

1i

6a: SoiioSolution

j1"^jC

{

aOMM A B e.05 ./<? .« .«>

Carbon, per cent.01 .02.05

Nitrogen, per cent

Fig. 7.11. Iron-rich portions of the (A) iron-carbon, (15) iron-oxygen, and (C) iron-

nitrogen phase diagrams, showing changes in solubility of carbon, oxygen, and nitrogen

with temperature. (Epstein)

7.10. Strain Aging and Its Consequences in Low Carbon Steel

There is another variety of precipitation hardening in low carbon

steels known as strain aging. If steels susceptible to aging are strained by

cold working, a hardening constituent, probably iron carbide, iron oxide,

or iron nitride, precipitates during straining and for some time afterward.

This is accompanied by increased hardness and strength and by de-

creased impact resistance. The effect of the precipitation can be detected

by cold working a rimming steel and a deoxidized steel of the same com-

position and by testing a sample of each immediately and at intervals of

several weeks or months.

Strain aging frequently has undesirable consequences. Low carbon

sheet for automobile bodies and fenders are given a light final cold work-

ing after hot rolling to prevent stretcher strains. This produces conditions

ideal for precipitation of the hardening constituent. Unless the sheet is

deep drawn immediately, the loss of ductility may be so great that the

steel will not withstand the drawing operation successfully. Strain aging

has been known to cause failures in boiler plate in the strained areas

around rivet holes.

7.11. Precipitation Hardening in Alloy Steels

Of the large number of common low alloy steels, only those con-

taining about 1 percent copper are subject to precipitation hardening.

The solubility of copper in alpha iron increases from about 0.4 percent

at room temperature to 1.4 percent at 1525°F. Low carbon steels con-

taining 0.7 to 1.5 percent copper can be hardened by a heat treatment

causing precipitation of the hardening constitutent. This treatment con-

sists of heating to If>50°F., cooling to room temperature, and reheating


for 3 or 4 hr. at 930°F. Precipitation hardening increases the tensile

strength and yield strength about 20,000 psi. Ductility is decreased, butthe precipitation hardened copper steels are not brittle.

A number of investigations have been concerned with the precipita-

tion hardening of highly alloyed steels which contain 2 or 3 percent

titanium and also with stainless steels containing tungsten.

Table 7.1 Typical Properties of Beryllium-Copper Alloy 0.032" Thick Strip

—

2% Be,0.25% Co (or 0.35% Ni).*

Tensile Yield RockwellCondition Strength Strength Elongation Hardness

(psi) (psi) (%) Number

Solution treated 72,000 25,000 50 B 60Precipitation hardened 175,000 80,000 5 C 41Solution treated and cold rolled

37.1% 107,000 70,000 6 B 97.5Solution treated cold rolled

37.1 %and precipitationhardened 195,000 110,000 3 C 42.5

* Metals Handbook, 1948 Edition, p. 934.

Table 7.2 Properties of Type A Ni-Vee Bronze (88% Cu, 5% Ni, 5% Sn, 2% Zn)

Property As Cast Heat Treated*

Tensile Strength, psi 50,000 85,000Yield Strength, psi 22,000 55, 000Percent Elongation 40 10Brinell Hardness Number 85 180

* Heated 6-10 hours at 1400°F., water quenched, and reheated 6-10 hours at 500°F.

Courtesy of the International Nickel Co., Inc.

QUESTIONS

1. Define precipitation hardening. Why is it sometimes called age hardening?2. Discuss the characteristics that an alloy must possess to be precipitation

hardened.

3. Discuss what happens if too high a temperature is used during: (a) thesolution treatment, (b) the precipitation heat treatment.

4. Discuss what happens if the alloy is held for too long a time at temperatureduring: (a) the solution treatment, (b) the precipitation heat treatment.

5. Discuss the constitutional changes which take place during precipitationhardening.


6. How does precipitation hardening effect an alloy in regard to the following

properties: (a) tensile strength, (b) yield strength, (c) hardness, (d) elonga-

tion, (e) electrical resistance, and (f) corrosion resistance.

7. Discuss the effect of precipitation hardening on the movement of disloca-

tions within the metal.

8. Explain why alloys arc sometimes precipitation hardened at temperatureshigher than the temperature at which optimum strength occurs.

9. Explain why wrought magnesium alloys may be precipitation hardeneddirectly after fabrication without a formal solution treatment?

10. What is meant by: (a) overaging, (b) strain aging, (c) stabilizing? Whatuse is made of each?

11. If an alloy were to contain 4% aluminum, 2% zinc and 94% magnesium,what phases would be present in the slowly cooled state at room tempera-

ture?

12. What alloying element added to the following materials will make themsusceptible to precipitation hardening? (a) tin bronze, (b) monel, (c) lowalloy steel.

13. List four elements whose solubility in aluminum decreases with tempera-ture. Can all of them be used effectively as binary precipitation hardeningalloys with aluminum?

14. Why is a moderate amount of Mg used with the major alloying element Znin 75S?

15. What alloying clement produces the strongest commercial casting alloy ofaluminum? What is its major drawback?

16. What three alloying elements present in a plain carbon steel may cause it

to respond to precipitation hardening?

17. Is precipitation hardening used extensively with low carbon steels? Explain.

18. List some practical applications for precipitation hardening.

CHAPTER

8

Light Alloys asEngineering Materials

Francis Wii.uam Brown, Ph.D., Associate Professor,

Clarkson College of Technology, Potsdam, New York

8.1. History of Aluminum

ALUMINUM was first made in 1825 by aDanish scientist named Oersted by means of a reaction between aluminumchloride with a potassium-mercury amalgam. In 1845 Henri Sainte-ClaireDevilie produced globules of aluminum meta] by heating sodium metalwith anhydrous aluminum chloride; later this method was improved byusing cryolite as the flux. The very nature of these raw materials indicatesthe large amounts of energy that are required to make aluminum and whythis metal under these processing conditions was very costly. Aluminumwas produced by Charles M. Hall, in 1886, by the electrolytic decomposi-tion of anhydrous aluminum oxide (alumina) dissolved in moltencryolite. This process has continued since 1886 to the present as theonly economical process for the manufacture of aluminum.The first major industrial demand for aluminum was in the aircraft

field because of aluminum's unusual combination of lightness andstrength. The discovery of the aluminum-copper alloy duralumin andits heat treatment by Wilm in Germany stimulated the use of aluminum.Wilm's early work was done in connection with Zeppelin's developmentof the dirigible. Prior to the First World War, the Aluminum Companyof America supplied the U. S. Navy with duralumin sheet for the Navy'sdirigible. The aircraft industry was primarily interested in the strength-weight ratio of the high-strength aluminum alloys; whereas, most otherindustries were concerned primarily with the cost. It was only abouttwenty-five years ago that these alloys were considered seriously for auto-mobile, truck, and bus construction and for railroad rolling stock; theirconsideration for bridge and building construction is even more recent.

129


8.2. Economics of the Aluminum Industry

For about thirty years after aluminum metal was first produced (1825) ,

it was a laboratory curiosity worth about $100 per pound. In 1857,

Sainte-Claire Deville, as the result of research supported by Napoleon

III, succeeded in bringing the price down to S25; two years later, to $17

per pound. Despite the fact that alumina was a very abundant oxide and

could be produced economically in pure form, cheap aluminum was im-

possible because of the high cost of metallic sodium and the aluminum

chloride. By 1886, the cost price of aluminum was reduced to $8 per

pound, still too high for common use.

The electrolytic reduction process developed by Charles M. Hall in

1886 brought the price down to $1 per pound. In 1943, the price for

aluminum ingots reached $.15 per pound. A question that might readily

arise here is, "can this cost be cut drastically so that aluminum and steel

are on an equal price basis?"

In 1893, less than 100 tons were produced in the United States; ten

years later domestic annual production increased to 3,000 tons. In 1915,

world production was 85,000 metric tons; in 1939 this had increased to

675,000 tons, of which the United States and Germany each were respon-

sible for 175,000 to 200,000 tons. The peak world production of aluminum

during the Second World War, in 1943 was 2,176,000 net tons, of which

considerably more than half was produced in the United States.

This country's consumption of aluminum in 1952 was distributed over

the following fields:

32% Transportation 10% Consumer Durables

15% Building Construction 6% Chemical Uses

14% Miscellaneous 6% Machinery & Equipment

13% Electrical 4% Packaging

It is interesting to note that, despite the fact that during the war

the output of aluminum expanded very much more than iron and steel,

the total production of primary aluminum in the United States for the

year 1943 was approximately equivalent to the average amount of steel

ingots produced in 3 working days during the same year. Reasons for this

great difference in production and cost may be deduced by comparing

the properties of the high grade ores of each of these metals.

Aluminum Iron

Name b Source of Main U. S. Ore Bauxite (Arkansas) Hematite (Minn.)

Secondary ore Cryolite (Greenland) None

Combined water <%) 25% None

Combined metal (%) 32% 50%Density of ore 2.55 4.90

Light Alloys as Engineering Materials 131

The transportation costs for the aluminum ores are higher due to thegreater distances involved and their higher bulking values. The aluminumores have specific gravity values near to that of the siliceous gangue (2.2

—2.3); this means gravity concentration of the ore is difficult. The ship-ping costs of bauxite are decreased by calcination of the bauxite to removewater and other volatile matter. The lower percentage of aluminum inthe ore is another factor that leads to higher processing costs. A compari-son of the Hall Process and the Blast Furnace Process will further bringout the comparative economics of these two commercial metals. A com-parison of the costs of aluminum and steel should be made on the basisof the finished article and not on the relative price per pound of ingotmetal. Because the specific gravity of aluminum is approximately one-third that of steel, less aluminum by weight will be used. In additionthere are economies that result from the unique combination of theproperties of aluminum, namely, (1) ease of fabrication, (2) goodmachinability, (3) good surface color and polish, (4) low maintenancecosts, (5) low cost of distribution, and (6) high scrap value.

8.3. Aluminum Ores—Occurrence and Concentration

Although aluminum occurs in nature in a higher percentage than iron,the major share exists in the form of clay. Bauxite, the principal ore ofalummum, consists of a mixture of the mono- and tri-hydrate of aluminumoxide. Bauxite has been formed in nature by the weathering of clay;bauxite deposits are found primarily in warm countries because hightemperatures and rainfall accelerate this hydrolytic degradation of theclay. The main source of supply of high grade ore (50-65% alumina) inthis country is from Arkansas; in South America it is found in theGuianas. The impurities in bauxite ore are the oxides of iron, silicon,and titanium. Since these impurities are difficult to separate physically,it is evident that high-grade deposits are very desirable.Aluminum is more chemically reactive than iron and silicon; therefore

it is difficult and costly to remove these undesirable impurities fromthe crude metal.

The Bayer process is the most commonly used process for the concen-tration and purification of bauxite. The fundamental principle involvedin this process is "that alumina differs from these other oxides in beingamphoteric, i.e., weakly acidic and basic." Alumina will dissolve in hotconcentrated alkali but is insoluble in cold dilute alkali. The steps in thisprocess are as follows: (1) high-grade bauxite is finely ground; (2) oreis digested with hot, concentrated sodium hydroxide (caustic soda) toform the soluble sodium aluminate; (3) the insoluble (red mud) oxides


of iron, silicon, and titanium are filtered off; (4) the hot sodium alumin-

ate is diluted with water, seeded with crystals of alumina hydrate, and

allowed to cool; (5) pure hydrated alumina settles out; it is filtered off

and calcined to form alumina, (6) the filtrate is concentrated by evapora-

tion and used over again.

8.4. Manufacture of Aluminum

The electrolytic reduction process for the manufacture of aluminum is

dependent on the fact that anhydrous alumina dissolves in molten cry-

olite to give an ionic solution of high electrical conductivity. The passage

of direct current through this molten electrolyte deposits aluminum at

the cathode and oxygen at the anode. Since iron and silicon are objec-

tionable impurities, the iron cell must be properly lined with a material

of high purity and of a conducting nature so as to function as the cathode.

The anodes must likewise have similar properties. Ordinary carbon fulfills

the conductivity requirement but not the purity one. The lining of the cell

and the anodes are made from fabricated parts of petroleum coke. Theoxygen formed at the anode combines with the carbon to form carbon

dioxide gas; therefore, the cost of these electrodes is an important eco-

nomic factor.

When metals are manufactured by an electrolytic process, the weights

of the metals deposited are proportional to their equivalent weights.

The ratio of weights of some common industrial metals deposited by the

same amount of current vary in order listed: aluminum 9, magnesium

12.1, iron 18.6 and copper 31.8. It is evident that the economical manu-

facture of aluminum necessitates large amounts of low cost electrical

energy. Aluminum plants are therefore located near hydroelectric plants.

A measure of the relative manufacturing problems of iron and aluminum

is indicated by the listed data.

Aluminum Iron

(Hall process) (Blast furnace)

Type of operating unit Small, semi-batch large, continuous

Capacity of unit, per day 1000 lbs. 700-1500 tons

Lining of unit Petroleum coke Refractory bricks

Temperature of operation lOOO'C IfiOO'C

Weight of metal per lb. ore 50 lb. .63 II).

Weight of carbon per lb. metal. . . .75 lb. 8"i ll>.

Value of gaseous by-product . . . none 85 Btu/cu. ft.

Agent used to remove impurities. . . Caustic Soda Air, iron oxide

(Bayer Process)

Electrical Energy Requirements per

lb. metal 12 KWHr.Energy requirements ratio

(theory) 5 1


The aluminum produced by the Hall process consists of 99% (mini-

mum) aluminum, the major impurities being iron and silicon. Aluminumof high purity (99.99%) may be obtained by an electro-refining process.

The cell used in the Hoopes process consists of three molten layers, the

bottom anodic layer of crude aluminum-copper alloy, the intermediate

layer of salts, and the top cathodic layer of pure aluminum.

8.5. Physical Properties of Aluminum

The most generally recognized physical characteristic of aluminum is its

low density. The lightest metal, magnesium, is two-thirds as dense as

aluminum (sp. gr. 2.70); whereas, iron and steel are three times as dense.

The density of most aluminum alloys range 2.70-2.80, which means that

aluminum and its alloys roughly weighs .1 lb. per cubic inch. The low

density of aluminum can be attributed to its low atomic weight (27)

and its great chemical reactivity. These two properties in addition gener-

ally indicate such thermal properties as low melting point, high specific

heat, high heat of fusion, and high coefficient of thermal expansion.

The melting point (1220T.) and heat of fusion (94.6 cal.) are slightly

higher than corresponding properties of pure magnesium. Its very high

specific heat and its linear coefficient of thermal expansion are second

to magnesium. The high solidification shrinkage (6.6%) as well as its

"hot shortness" make the casting of pure aluminum rather unsatisfactory.

The electrical conductivity of metals, in decreasing order, are silver,

copper, gold, and aluminum. Pure aluminum has 64% of the electrical

conductivity of copper on an equal-volume basis but has a 212% value

on an equal-weight basis. It is therefore the best commercial electrical

conductor on the latter basis. The best thermal conductors, in decreasing

order, are silver, copper, gold, and aluminum; aluminum is the best

conductor on an equal-weight basis.

This parallelism between electrical and thermal conductivity is ex-

pressed by the Wiedemann-Franz law. Another relationship that appears

to be indicated is that these two physical properties are related to crystal

structure since all four of these listed metals have a face-centered crystal-

line structure.

The low density and melting point of aluminum are related to its prop-

erties of being soft (Brinnell Value 16) and relatively low tensile strength

(8,500 psi).

In view of its low melting point and "hot shortness," the tensile prop-

erties of aluminum rapidly decrease with a rise of temperature; therefore,

aluminum and many of its alloys are not well suited to elevated tempera-


ture applications. Aluminum and all of its alloys become stronger and

tougher at subnormal temperatures.

Aluminum is a very malleable and ductile metal; in fact, it is second

best of the commercial structural metals. These properties are related to

its face-centered cubic crystalline structure, for we note that copper,

silver and gold are very malleable and ductile and have face-centered

cubic structure. Under standard testing conditions, pure aluminum will

stretch 60 per cent of its length before breaking. With the combination of

high ductility, malleability, and strength, aluminum and its alloys are

adapted to working by most methods of forming and fabrication.

Due to the high degree of plasticity and relative chemical inertness to

normal conditions, aluminum can be easily finished to a high polish.

The brilliance of aluminum paint is due to the high reflectivity of the

finely divided flat flakes.

Aluminum is an excellent reflector of radiant energy of all wave lengths.

The high durability of aluminum paint is largely due to the efficient re-

flection of the ultraviolet rays; ultraviolet rays destroy the oil film in

paints and lead to loss of adhesion. Aluminum is used as thin films on the

reflectors of the most modern telescopes. Aluminum foil is important in

the thermal insulation field because of its high reflectivity of infrared

radiation.

Three important requirements of a molten metal for satisfactory casting

arc low surface tension, high fluidity, and relative chemical inertness.

Molten aluminum has a surface tension of 520 dynes per centimeter which

is one third that of molten iron, both at their respective melting temp-

eratures. Pure molten aluminum has a fluidity 50 per cent that of molten

tin; the normal impurities of commercial aluminum increase this value

to 70 per cent. Molten aluminum is protected from oxidation by a thin

oxide film; excessive agitation must be avoided to prevent the removal of

this oxide film. Two other properties of molten aluminum contribute to

its poor castability, first—its reactivity with moisture to form aluminum

oxide and hydrogen, second—the high selective solvency for hydrogen gas

which leads to high-porosity cast metal.

8.6. Chemical Properties of Aluminum

The very high solution potential of aluminum demonstrates that it is

the second most chemically active structural metal. In spite of this great

activity, aluminum offers great resistance to normal weathering. This

relative inertness can be attributed to the presence of a thin protective

and adhering oxide film. Other outstanding features of this oxide film


are its chemical inertness, water insolubility, ability to mend itself, and

its electrical and thermal insulation properties.

The chemical inertness of this film is indicated by fact that aluminumtanks are used for storage and shipment of concentrated acetic acid,

dilute and concentrated nitric acid, and strong hydrogen peroxide.

When the oxide film, is removed, the active nature of the metal as-

serts itself; for example, caustic soda and strongly alkaline solutions

readily attack aluminum. When alkaline degreasing solutions are used to

clean aluminum, a corrosion inhibitor such as silicate of soda is added.

Aluminum in contact with mercury or its salts will show accelerated cor-

rosion due to the removal of the oxide film.

In the melting of aluminum and subsequent handling operations,

precautions should be taken to avoid, as much as practicable, contact with

moisture, water vapor, or other materials that will react with aluminumto form hydrogen gas. The amount of dissolved gas can be kept at a low

amount by using a low temperature, short operating time, minimum agi-

tation, and low absolute humidity of the atmosphere.

The great affinity of aluminum for oxygen is illustrated in its use in

the steel industry. The amount of aluminum required for the deoxidationof molten iron and steel is usually less than 1 lb. per ton. When 1-1 1/2 lbs.

are used, the excess aluminum functions in another manner: it leads to

the subsequent formation of a fine-grained steel.

A similar deoxidation reaction is involved in the thermite welding andrepairing of large ferrous alloy sections, railway and street car rails,

etc. Thermite consists of a mixture of granulated aluminum and mill

scale. It is ignited by means of flashlight powder or another primer, whichleads to localized melting of the aluminum, followed by a violent reaction

to form aluminum oxide and molten iron. The excessively high tempera-ture (4500°F.) is due to the great heat of reaction, the low specific heatof iron, and the absence of any volatile product. A modified thermite re-

action is often used in the manufacture of pure high-melting metals as

vanadium, chromium, molybdenum, and manganese.Atmospheric nitrogen is a very inert gas but will react with aluminum

and chromium, at elevated temperatures, to form metallic nitrides. Thepresence of .85—1.2% aluminum in the nitralloy steel plays an importantrole when being surface hardened by nitriding. The stainless steels andother chromium alloy steels do not require aluminum for this purpose.

8.7. Aluminum Alloys

The important alloying elements used in the manufacture of aluminumalloys are copper, manganese, magnesium, zinc, nickel, chromium, and


silicon. In special purpose alloys the elements titanium, tin, lead, and

bismuth are intentionally added. For example, lead and bismuth are

added to the extent of .5% each in the free machining alloy; those metals

are insoluble in aluminum and are therefore present as a colloidal

dispersion which functions as a lubricant.

The total alloy content of the wrought and casting alloys are usually

7% and 16% maximum, respectively. The three main important strength-

giving elements in the wrought and cast alloys respectively are copper-

magnesium-manganese and copper-magnesium-silicon. Silicon also plays

a very important role in the castability of alloys.

The individual alloys are designated by numbers, the wrought alloys as

a four digit number, and the cast alloys as a two- or three-digit number.

Two high-strength wrought alloys are 5056 (5% Mg) and the 7075 (5%

Zn) ; the alloys that have excellent castability are 43, 355, 356.

There are two classes of wrought aluminum alloys: (1) alloys that are

not heat treated and that owe their properties to strain hardening by

cold work, (2) alloys whose strength may be improved by heat treatment

and whose properties may be altered still more by cold deformation.

8.8. The Wrought Alloys

The various tempers of commercial wrought aluminum alloys until

recently have been designated by a system of numbers and letters. The

new temper designation consists of a letter indicating the basic temper

which may be more specifically defined by the addition of one or more

digits. The letter F means "as fabricated," O—"annealed, recrystallized,"

and H—"strain hardened." The latter may be modified by numbers to

give a detailed meaning as per follows:

Strain hardened only—HI, plus one or more digits.

Strain hardened and then partially annealed—112, plus one or more

digits.

Strain hardened and then stabilized-H3, plus one or more digits.

In dealing with the heat-treatable type of alloys, the letter —T following

the alloy number signifies that it has been hardened by solution treatment.

The specific combination of basic operations is indicated by a number:

6061-T5 means that alloy 60G1 has been hardened by heat treatment

and artificially aged. The various basic operations are as follows:

—T2 Annealed (cast products only).

—T3 Solution heat treated and then cold worked.

—T4 Solution heat treated and naturally aged to substantially stable

condition.

—T5 Artificially aged only.


Table 8.1 Temper Designations for Strain Hardened Wrought Aluminum AlloysI hat Arc Not Heat Treatable (*)

New Temper Designation({)

Strain Hardened Strain HardenedOld Temper Strain Hardened and then and thenDestîahon only Partially Annealed Stabilized

(Alloys 1100 and 3003)

14 H -H12 -H22'/2 H -H14 _H24*A H -H16 -H26H -H18 -H28Extra Hard(Not Standard) -H19

(Alloys 3004, 5052, 5056J)

#{J -HS2%™ -H34

^H -H36

Extra Hard(Not Standard) —H39

(#) Alloys listed also available in -O and —F tempers,

(t) The tempers shown are standard for the alloys listed.

(t) The alloys 3004, 5052 and 5056 may also be obtained in the -HI and -H2 typeConditions.

'~

—T6 Solution heat treated and then artificially aged.—T7 Solution heat treated, then stabilized.

—T8 Solution heat treated, cold worked and then artificially aged.-T9 Solution heat treated, artificially aged and then cold worked.—T10 Artificially aged and then cold worked.—W Solution heat treated-unstable temper.

In Tables 8.2 and 8.3 there are listed the important wrought alloys inorder of their increasing tensile properties, together with other im-portant physical properties. The number of alloying elements and totalalloy percentage for the nonheat-treatable are less than the heat-treat-able alloys.

The principal characteristic of the wrought alloys, especially the heat-treatable type, is their high strength-weight ratio. The typical mechanicalproperties of the annealed and fully hardened alloys are listed in Tables8.2 and 8.3. These properties are applicable to room temperature but aresatisfactory for the range from about 0° to 150°F. All of the aluminumalloys become stronger and tougher at subnormal temperatures. Dueto the fairly rapid decrease in tensile properties, and sometimes the de-


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velopment of hot shortness and/or excessive oxidation, all aluminum

alloys are not well suited for high temperature application.

The Brinell hardness of the wrought alloys (500-kg. load, 10-mm. ball)

varies from approximately 30 to 85 for untreated alloys and from 30 to

120 for the heat-treated material, depending upon the temper. Even the

hardest aluminum alloys are much softer than the softest steels. En-

durance limits (based on 500 million cycles) are low; the endurance ratio

is highest for the annealed alloys and is only about 0.25 for heat-treaied

high-strength sections. It is characteristic of the annealed alloys that the

endurance limit is higher than the yield strength.

The modulus of elasticity for tension or compression, measured in kips

per square inch, ranges from 10,000-10,600, except wherein the high

silicon (12%) increases it to 11,400. The values for the annealed andhardened form of a particular alloy is the same. The modulus of elasticity

for shear, measured in kips per square inch, ranges from 3,800-4000, the

high silicon alloy value being 4300.

The thermal and electrical conductivity of the wrought alloys are less

than the pure metal, the higher the alloy content the greater the lower-

ing. An annealed alloy has higher values than the strain hardened and/orheat-treated alloy. The thermal coefficient of expansion per degree

Fahrenheit at room temperature is almost the same for most wrought alloy

varying from 124-135 x 10—", the only marked exception being when very

high silicon (12%) is present, when the value is 108 x 10-7. The cold

working of alloys does not appreciably change these values; whereas, heat

treatment makes an increase of several per cent.

The resistance to corrosion of aluminum alloys is usually very satis-

factory, the strain-hardening type alloys being generally superior to the

heat-treatable type. The presence of relatively large amounts of coppcr

or zinc reduces the over-all resistance to surface attack. The resistance

to high temperature oxidation is best insured by the presence of the

alloying element nickel.

Maximum resistance to corrosion of the heat-treatable alloys is depen-dent on the solution heat treatment being followed by a very rapid

quench; therefore, thick sections arc not as resistant as thin sections dueto their slower rate of quench. Artificial aging usually effects a markedimprovement in resistance to corrosion.

Some sheets, plates, and tubes arc produced with a surface coating of

pure or low-alloy aluminum that are metallurgically bonded to the core

during hot working. These coatings offer electrolytic protection to the

core against corrosion. This protection includes cut edges and areas

where the coating may be scratched or abraded.


8.9. Aluminum Casting Alloys

The casting alloys are designated by a number of two or three digits. In

some instances the number is preceded by a letter, which indicates a slight

variation in composition. For example, the three casting alloys 214,

B214, and F214 contain 3.8% magnesium but differ in the silicon content.

Casting alloys may be of two types: (1) the nonheat-trcatable alloys, and

(2) the heat-treatable alloys. The condition of heat treatment is designed

by appending some symbol, as 195-T4.

The elements used in aluminum casting alloys are primarily copper,

silicon, and magnesium. The minor alloying elements are zinc, nickel,

manganese, iron, and tin. The requirements for the sand, permanent

mold, and die castings arc not necessarily the same, although many of the

alloys are used for both sand and die. The 5% binary silicon alloy is the

only listed alloy that is used for all three types of castings.

Aluminum finds little use in the production of castings because of its

low strength and inferior casting qualities. Two exceptions are the induc-

tion motor rotors and cable clamps, which require the higher electrical

conductivity of aluminum.

The commercial casting aluminum alloys and their important proper-

ties are listed in Table 8.4. Note that the total percentage of alloying ele-

ments is generally greater than that of the wrought alloys. Some of the

important characteristic properties of molten casting alloys such as good

fluidity, low surface tension, and low pouring temperature necessitate

a high percentage of alloying elements. The low temperature of solidi-

fication leads to low gas solubility and porosity, and fine grain structure.

The important casting characteristics for a sand and a permanent

die casting are (1) freedom from hot shortness, (2) low shrinkage

tendencies, and (3) pressure lightness. Low gas absorption is a very

important property for a sand-casting alloy. A die-casting alloy should

have freedom from hot shortness and mold-filling capacity.

Sand castings of almost unlimited size and shape can be made from

scores of aluminum alloys, each having unique properties or foundry

characteristics. Where only a few castings are required, or where the cast-

ing is relatively large, sand castings are indicated. On the other hand,

when large numbers of relatively small parts are required, or if a smoothsurface and a minimum amount of machining are desired, permanent-

mold or die castings are to be preferred. Some typical permanent-mold

castings are shown in Table 8.4. The method of casting, too, affects the

properties to be expected from a given composition, permanent-mold anddie castings usually having a somewhat higher strength than sand castings.

In general, when the number and the size of the castings to be made would


justify the use of any of the three methods, die castings are the cheapestand can be cast to the closest tolerances, while sand castings will cost themost and will require the broadest tolerances.

A comparison of the mechanical and tensile properties of the sandpermanent mold, and die castings is best made by listing the typicalvalues for alloys 43.

PermanentSand Mold r)ie

Tensile strength 19,000 23000Tensile yield strength 8,000 9 000Elongation (% in 2") 8

'

10Compressive yield strength 9,000 9,000

30,000

16,000

10 9

Brinell hardness •10 45

19,000Shearing strength 14,000 16,000

Although the casting alloys have a greater alloy content than thewrought, the improvement as compared with pure aluminum is primarilygreater in the casting properties rather than the mechanical properties.The maximum tensile strength of the commercial aluminum alloys in

the "as-cast" condition for the sand, permanent mold, and die castings areapproximately 27,000, 35,000, and 46,000 psi. respectively.

The tensile properties of some casting alloys are further improved bysolution heat treatment. This type of alloy contains at least one con-stituent whose solid solubility is substantially greater at elevated tempera-tures than at room temperatures. The usual amount of this alloyingelement ,s such that it is completely soluble near the melting point ofthe alloy. Heat treatment finds limited application with die castings be-cause of blistering and possible loss in dimensional accuracy.The yield strengths in tension and compression for the cast alloys are

practically equal. The yield strength in compression is very importantin the design of some aircraft structural members. Some special treat-ments are available that raise the compressive yield strength to a valuealmost as high as the tensile strength. For example, sand casting alloy

*LZ ,

T6 3nd T62 condition hav* ê respective tensile strengths36,000 and 40,000 psi; the compressive yield strengths under these condi-tions are respectively 25,000 and 38,000 psi.

The modulus of elasticity of most aluminum casting alloys is approxi-mately 10,300 kips; therefore this property is about one third that of steelThe Brinell hardness of "as-cast" alloys range 40-70 (500-kg. load, 10-mm.ball)

;whereas, for the heat-treated alloys the hardness value is 75-100

The endurance limits for the alloys range 5,500-9,500 psi. This gives anendurance ratio of .25-.38 for cast alloys and .17-28 for the heat-treated


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alloys, based on a rotating-beam test of 500 million cycles. These ratiosare one third to one half of those usual for carbon and low-alloy steels.

Inasmuch as the total percentage of alloying elements is usually greaterin casting alloys than wrought, the melting point, and the thermal andelectrical conductivity, are much lower for the former class. The meltingrange and the specific gravity range (2.66-2.92) are wider for the castingalloys because of their higher alloy content. The specific gravity and thethermal and electrical conductivity values are lower for the die castingsthan the sand and permanent mold casting alloys.

Centrifugal castings have been very popular for many ferrous and non-ferrous alloys because the molten metals and alloys are cleansed of oxidesand inclusions by the centrifugal action thereby improving the mechanicalproperties. The small differences in the specific gravity between the usualoxides and inclusion present in molten aluminum alloys makes this type ofcasting of a limited advantage.

8.10. Functions of Alloying Elements

The advantages and limitations of commercial aluminum as a wroughtand cast metal may be summarized as follows. Its advantages as awrought metal are high electrical and thermal conductivity, excellentmalleability and ductility, excellent formability, good machinability, andunusual resistance to normal corrosion. The limitations include lowtensile properties, poor high-temperature tensile properties, and hightemperature brittleness. Molten aluminum has the desirable propertiesof low surface tension and good fluidity, the undesirable properties arehigh selective hydrogen gas solubility, high solidification shrinkage, andgreat chemical reactivity towards air and moisture. The cast metal hasthe unsatisfactory properties of high thermal coefficient of expansion, hotshortness, gas porosity, and coarse grain structure.

The addition of the alloying elements is primarily to increase thetensile properties and improve certain fabricating qualities of thewrought metal; they function to increase tensile strength and castabilityof the alloy casting. Such alloying always results in the improvementof some properties and in the further limitation of others. The changesinvolved depends upon the type and amount of alloying elements, theresulting grain structure, size and distribution of insoluble constituents,homogeneity, etc.

Copper. The most important and commonly used alloying element is

copper. The intermetallic compound CuAU forms a eutectiferous serieswith aluminum; the eutectic alloy contains 33% copper and melts at548°C. The maximum solubility of copper in solid aluminum at elevated


temperatures is 5.65% and less than .25% at room temperature. Thecopper content of the high-strength wrought alloys usually ranges between

4-4.5% copper. Although tensile properties increase progressively above

this range, higher copper alloys are not commercially practicable because

of difficulty in working. The high-strength alloys of this type include

alloys 2011, 2017, and 2024.

The electrode potentials of commercial aluminum and the intei metallic

compound CuAl2 are —.84 and —.53 volts respectively. This relatively

great difference means that the corrosion resistance of the copper alloys is

much less than those of the other alloying elements. Improper heat treat-

ment of the copper-aluminum alloys can further decrease their resistance

to corrosion.

Copper used as an alloying element in the casting alloys leads to in-

creased fluidity and decreased surface tension of the molten metal. Thecast metal is improved by the freedom from hot shortness, decreased

porosity, increased tensile properties, and finer grain structure.

Silicon. The second most important alloying element in aluminum al-

loy is the metalloid silicon. It differs from the other soluble alloying ele-

ments in not forming an intermetallic compound with aluminum. It

forms a eutectic having the composition of 11.6% silicon by weight and

melts at 577°C, The solubility of silicon in solid aluminum at the

eutectic and room temperatures are 1.65% and less than .05%. Although

silicon is a rather hard constituent, it is evident that due to its low

solubility and inability to form an intermetallic compound, the binary

silicon alloys are not markedly strong. A comparison of the strength

giving properties of silicon and copper is given by the tensile yield

strength values of the sand casting alloys #43 (5% Si) and #195 (4.5%

Cu) , namely 8,000 and 34,000 psi respectively.

Silicon plays an extremely important role in the casting alloys. Its im-

portant contribution to the molten alloy are very high fluidity and

low solidification shrinkage. The shrinkage of an 8% silicon-aluminum

binary alloy is 5.6%; the 11.6% silicon-aluminum alloy (eutectic) is

3.8%. The presence of silicon influences the properties of the solid cast

metal in having a lower coefficient of thermal expansion, absence of hot

shortness, and a lower degree of porosity. An unusual property of silicon

in a casting is to form a sound outer layer or skin which leads to pres-

sure tightness. The silicon alloys 43, 355, and 356 give excellent sand and

(>ermanent mold castings. The two die-casting silicon alloys, #13 and

#360 are very satisfactory.

The ability of silicon alloys to "undercool" leads to the modification

of the normal eutectic composition and its properties. In 1921 A. Pacz


discovered that very rapid cooling or the addition of .05% metallic

sodium (or sodium fluoride) results in the formation of eutectic alloys

containing as high as 14% silicon, having a freezing point 10-15 degrees

lower than normal. This modified alloy has a finer grain structure; it is

stronger and more ductile than the eutectic alloy.

The silicon content of wrought alloys is usually less than 1%: one

exception is the forging alloy 4032. The presence of silicon in the heat

treatable wrought alloys generally increases their response to artificial

aging.

The presence of silicon in aluminum alloys does not materially decrease

its corrosion resistance, probably due to the formation of a protective

siliceous film. The 5% binary casting alloy #43 and wrought alloy

4032 resist atmospheric weathering, chemicals and food acids. Inasmuch

as silicon and aluminum are both amphoteric, this type of alloy is not

very resistant to alkaline conditions and solutions.

The strength and corrosion resistance of silicon-aluminum alloys

are markedly increased by the presence of magnesium due to the forma-

tion of the intermetallic compound Mg^.Si.

Magnesium. The eutectic of magnesium and aluminum contains ap-

proximately 33% magnesium and the melting point is 451 °C. Thesolubility of magnesium in solid aluminum at the eutectic and room temp-

eratures is 14.9% and less than 4% respectively. This variation in the

solid solubility with temperature renders some of the alloys susceptible to

improvement in tensile properties by heat treatment.

The electrode potential of 4% magnesium-aluminum solid solution is

approximately equal to that of aluminum; therefore this type of alloy

shows excellent corrosion resistance. In addition, these alloys show the

highest resistance to alkaline corrosion, such resistance increasing with

magnesium content.

Binary alloys containing more than 4.5% magnesium are susceptible to

stress-corrosion cracking particularly when they are in the cold worked

condition. This corrosion is due to the fact that the insoluble phase is

highly anodic to the 4% magnesium-aluminum solid solution. Theelectrode potentials of the two components are respectively —1.07 and

—.87 volts. This stress corrosion is prevented by the addition of small

amounts of manganese and chiomium.

Magnesium is second to silicon in effectively increasing the fluidity of

molten alloys. The fluidity of the binary 6% magnesium alloy is approxi-

mately the same as the binary 5% silicon alloy.

Magnesium is a more reactive metal than aluminum; therefore it reacts

more readily with air and moisture. Molten casting alloys of magnesium-


aluminum form considerable dross, a tendency that increases with mag-

nesium content. Alloys containing 5% or more magnesium cast in ordi-

nary green sand molds show surface discoloration and porosity, particu-

larly when thick sections are involved. High magnesium-aluminum

should be heat treated in a controlled atmosphere or for a minimum of

processing time so as to avoid surface oxidation.

Magnesium ir Silicon. The magnesium silicide forms an eutectic with

aluminum containing 13% Mg2Si and melts at 595°C. When an excess of

silicon (over that required to form the magnesium silicide) is present, a

ternary eutectic of aluminum, MgL.Si and silicon results, melting point

550°C. The composition of this ternary eutectic is approximately 13%silicon, 6% magnesium and 81% aluminum.

The Mg,Si constituent in the binary alloy has a maximum solubility in

solid aluminum of 1-85% at the eutectic temperature and less than .2%at room temperature. These alloys have the ability to respond to heat

treatment and have tensile properties comparable with those of the heat-

treated 4% copper alloys.

The corrosion resistance of this type of alloy is very satisfactory be-

cause the electrode potential of Mg2Si (—.83 volt) is almost the same as

aluminum. Alloys of this type, such as alloys 6053, 6061 and 6151, differ

from most other heat treatable alloy in showing no appreciable decrease

in corrosion resistance after heat treatment.

Manganese. Manganese and aluminum form an eutectic containing

2.2% manganese by weight and melting at 657°C. The intermetallic com-

pound of these metals has the formula MnAle . The solubility of man-

ganese in the solid aluminum at eutectic and room temperatures is .65

and less than .1 respectively.

This low solubility indicates that the most common manganese alloy

3003 (1.25% Mn) is a non-heat-treatable alloy. The electrode potential

of this intermetallic compound is the same as aluminum, therefore this

alloy has exceptional corrosion resistance. Inasmuch as manganese and

its compounds are not toxic, this alloy is used in equipment for preparing

and processing of foods. It is also used extensively for the manufacture

of cooking utensils, conduit and pipes, pressure vessels, architectural ap-

plications and builders hardware.

Nickel. Nickel forms an eutectic: with aluminum which contains 6.5%nickel and melts at 640°C. The composition of the intermetallic com-

pound is NiAlt. The solubility of nickel in solid aluminum at the

eutectic and room temperatures is .06%, and less than .04% respectively.

The addition of nickel to the copper-aluminum type alloys definitely

improves the high temperature properties and retains the high thermal


properties. The forging alloys 2018, 2218 and 4032 are ternary alloys

of aluminum, copper and nickel. Their uses are for forged cylinder heads

and pistons, jet engine impellers and compressor rings and other applica-

tions wherein high strength at elevated temperatures is necessary.

Zinc. Aluminum and zinc form an eutectic consisting of 95% zinc and

5% aluminum, melting point 440°C. The solubility of zinc in solid

aluminum at the eutectic and room temperatures is 70% and 18% respec-

tively.

Zinc is a low melting and highly volatile metal. One important limita-

tion of binary and complex zinc-aluminum alloys is their great loss in

strength at elevated temperatures. For example, castings made from A612and C612 are not recommended for use at temperatures greatly in excess

of 200°F.

These casting alloys have the marked advantage that without heat

treatment they have tensile properties comparable with the aluminum-copper and aluminum-silicon heat treated alloys. They offer the advan-

tage that they can be heated to the temperature required for brazing

and will regain their original properties after a few days. The zinc

wrought alloy 7075 has very good strength and good resistance to cor-

rosion. This wrought alloy in the T6 condition in extrusions has a tensile

and yield strength of 88,000 and 80,000 respectively.

8.11. Cold and Hot Working of Aluminum Alloys

The purpose of cold working in the fabrication of sheet, rod, wire,

tubing, extrusions, and like basic products are (1) increased tensile

properties, (2) grain refinement, (3) improvement in surface appearance,

and (4) improvement of dimensional properties.

Aluminum and its alloys on cold working undergo strain hardening.

The tensile properties increase with the amount of work, in some in-

stances almost proportional to the fractional reduction of the initial

Alloy

3003-O3003-H-12

3003-H-I4

3003-H-16

3003-H-18

Alclad202-1-T42024-T3

2024-T362024-T81

2024-T86

ElongationTensile Strength Yield Strength % in 2 in.

1G.0OO 6,000 30

19,000 17,000 10

21.000 19,000 8

25,000 22,000 5

29,000 26,000 4

64.000 42.000 1964.000 44,000 18

67.000 53,000 11

65,000 60,000 670,000 66,000 6


cross section. The elongation decreases with increasing amount of cold

work. As a general rule the initial cold work produces a relatively greater

effect than subsequent reductions of equal amounts. The effect of cold

work is listed for the two types of wrought alloys.

The degree of strain hardening in aluminum and its alloys has an im-

portant effect on their temperature of recrystallization. It is usual to make50% or higher reduction in cross-sectional area between annealing or heat

treating operations in order to prevent the development of excessively

large grain structure.

In the fabrication of the commercial basic products there is an improve-

ment of the surface properties due to tool surface and shape, lubricants

used, and the burnishing effect obtained by the cold work. The greater

the degree of reduction and the inherent hardness of the alloy, the

brighter the surface.

Straightening and flattening operations are performed cold and are

usually conducted so as to introduce a minimum of strain hardening.

Plate, rod and bar are hot worked nearly to the desired thickness, followed

by cold work to obtain greater dimensional accuracy.

The cast aluminum ingots are hot worked to convert them to wroughtstructures. Hot working is carried out during the manufacture of extru-

sions and forgings. The hot working of aluminum alloys require about

30-40% more power than for steel. Extrusions are made in the shape of

rods, bars, and complicated cross sectioned materials. The extruded

section is highly fibrous in structure which is parallel to its direction of

extrusion, therefore the tensile properties are often much higher than

other hot- or cold-worked products of same alloy.

Another difference between aluminum and steel is that aluminum does

not form scale during heat treatment; therefore a suitable lubricant

should be used during hot rolling. Die lubricants are generally used

during hammer and pressing forging; extrusions usually do not require

lubrication.

8.12. Heat Treatment of Aluminum Alloys

The three basic types of thermal processing that are commonly applied

to aluminum alloys are (1) annealing, (2) solution heat treatment, and

(3) precipitation heat treatment.

The strain-hardening which results from cold working of the aluminumalloys may be removed by annealing. The annealing process likewise re-

moves most of the temper resulting from solution heat treatment. Thisthermal treatment permits recrystallization to take place; the higher the

temperature used, the greater the rate. The rate of recrystallization is


also dependent upon the alloy involved and the severity of the cold work.Complete annealing is almost instantaneous for alloys 1100 and 5052 at

650°F., and for alloy 3003 at 750°F. The rate of cooling thereafter is notcritically important although rapid cooling may lead to some distortion.

The temperature used for annealing the heat-treatable type of aluminumalloy must be carefully controlled and as low as practicable so as to

prevent the dissolving of the intermetallic compound. Castings of nonheat-treatable alloys are often annealed for the purpose of relieving in-

ternal strains, increasing their ductility and shock resistance.

Some alloys are improved in strength and hardness by heating in the

range of 900-1000°F., given a rapid quench, and then aged at room temp-erature and/or at slightly elevated temperatures. The alloy after quench-ing has fair ductility and malleability and is subjected as soon thereafter to

forming or other fabricating processing. Some alloys spontaneously in-

crease in tensile properties within a short period of time; other alloys age

so slowly that they are artificially aged by heating in the range 250-400°F.

Such heat-treatable alloys contain a principal alloying constituent

that has a marked increase in solid solubility at elevated temperature.In most instances this alloying constituent is an intermetallic compound.The 4i/

2% copper-aluminum alloy 2024 has CuAlj as its hardeningagent; its solid solubility at room and elevated temperatures is .25% and5.65% respectively. All this intermetallic compound goes into solid

solution at elevated temperatures and continues to remain in solution

after rapid quenching due to the marked decreased mobility of the sys-

tem. This unstable supersaturated solution has good ductility and mal-leability; therefore it is formed into proper shape as soon as practicable.

During the next few hours fine precipitation of submicroscopic particles

of this hard intermetallic compound takes place, progressively increasing

the tensile properties.

Other heat treatable alloys such as 6061 undergo a similar transition

during heat treatment and quenching, but differ in the respect that therate of precipitation of submicroscopic particles is extremely slow at

room temperature, therefore it is artificially aged by heating in the rangeof 250-400°F. The alloys normally hardened by room temperature agingmay be further increased in tensile properties by this artificial aging.

The time required to reach the maximum strength by artificial agingdecreases as the aging temperature is raised; the value of the maximumstrength, in turn, decreases. The precipitation of the hardening con-stituent from solid solution that results from artificial aging is accom-panied by small changes in specific gravity and therefore dimensionalproperties. The 5% and 6% copper-aluminum alloys after artificially


aging show a decrease in length of approximately .01% and .15%respectively.

The response to artificial aging for alloy 2024 is greatly enhanced if

the material is strain hardened after solution heat treatment and prior

to precipitation heat treatment. In this instance the increase in tensile

and yield strengths is more than doubled by a reduction of 6% by cold

working prior to artificial aging.

The percentage of alloying elements in casting alloys is higher than

for wrought alloys, therefore the solution heat treatment time for cast-

ing is much longer. The greater uniformity and the finer grain structure

of wrought alloys results in a greater degree of improvement in tensile

properties by heat treatment for the wrought alloys.

The solution heat treatment of alloys containing high amounts of

magnesium should be done with proper atmosphere control so as to

prevent possible oxidation along the grain boundaries. This oxidation

condition is manifest by minute surface blisters, decreased tensile prop-

erties and embrittlcment. The time of solution heat treatment of clad

alloys should be kept at a minimum so as to prevent changes in composi-

tion and electrode potential properties of the surface coating due to alloy

diffusion from the core.

The cooling rate during quenching is often critical. The alloy 2024,

improperly quenched, forms a non-uniform precipitation of microscopic

and larger size particles of AIXu which makes the alloy susceptible to

localized corrosion.

8.13. Corrosion Resistance of Aluminum Alloys

The excellent corrosion resistance of aluminum and its alloys is due

primarily to the adhesion and chemical inertness of the oxide film.

Inasmuch as this aluminum oxide is a nonconductor of electricity it

differs from iron rust (cathodic) in not causing galvanic corrosion.

Galvanic or "battery action" type of corrosion is encouraged wherein

the alloying metals or their intermetallic compounds have electrode

potentials much different than the base metal. The solid solutions andthe insoluble intermetallic compounds of manganese, chromium, mag-nesium and MgoSi have electrode potential values almost equal to alu-

minum; therefore alloys involving these constituents show good corrosion

resistance. The solid solutions of copper, and the insoluble constituents

FeAls , NiAl3 , CuAl2 and Si, have much lower solution potentials than

aluminum, therefore alloys of these constituents show only fair resistance

to corrosion.


The poor corrosion resistance of the improperly quenched aluminumcopper alloys is due to its heterogeneous nature. Alloy 2024 on solutionheat treatment and properly quenched consists of a uniform supersatu-rated solid solution; its homogeneous nature makes it very resistant to

corrosion. The subsequent aging of the alloy leads to the uniform precipi-

tation of submicroscopic intermetallic compound which also shows goodresistance to corrosion. When the alloy is quenched slowly, the CuALprecipitates out at the grain boundaries as non-uniform microscopic andlarger sized particles. The phase adjacent to the cathodic intermetalliccompound consists of areas of depleted solid solution (anodic). Thelocalized corrosion that lakes place in this alloy is termed intergranularcorrosion.

When a mild quench is necessary, the resistance to corrosion may beincreased if the alloy is subsequently artificially aged. In the case of thicksections, the rate of cooling, even by immersion in cold water, is oftennot great enough to produce material completely free from susceptibilityto this intergranular attack. This condition may also be discouragedby the use of alclad materials.

One of the most effective methods for increasing corrosion resistance ofan aluminum alloy is to use alclad material or by applying a surfacecoating of pure or low alloy aluminum. The surface layer must beanodic to the base metal, hence the protection is both mechanical andelectrolytic. The present products available with this type of claddingare sheet, tube and wire. One limitation of this type of product is thatduring heat treatment, temperature and time should be very carefullycontrolled so as to avoid excessive alloy diffusion into the clad layer,thereby decreasing its effectiveness in protecting the core from corrosion.

Alclad 2024 sheet improperly heat treated should be checked by propertests for excessive "copper diffusion."

Another commercial process used to protect aluminum alloys fromcorrosion is by the surface oxidation by electrolytic means. The processis known as anodization because the aluminum is made the anode andthe iron tank the cathode, the processing solution being dilute sulphuricor chromic acid. The anodic film is thicker and more uniform than thenatural oxide film; therefore it has increased corrosion resistance, electri-

cal insulation, abrasion hardness and chemical inertness. An unusual prop-erty of the anodic film, especially right after its formation, is its greatpower of adsorption. The excellent anchorage power of anodized alu-

minum for paint films and dyes is dependent upon this great power ofadsorption.


Chromic acid dip treatment, anodi/ation, painting with zinc chromate

primer, and other like processing and treatments of aluminum alloys are

based on the passivation action of chromic acid and chromates, andtheir adsorption in the oxide film. When an aluminum alloy in use

shows excessive corrosion, this condition may be stopped by cleaning

area, painting with chromic acid solution, drying and then painting.

It is important that aluminum assemblies should consist as much as

possible of the same alloy or alloys of like solution electrode potential.

Rivets should be of the same alloy as the base metal, and if different,

should be of an alloy that lias a slightly lower potential so that the

rivets would be cathodic. Galvanic corrosion can be markedly retarded

if the rivets can be electrically insulated by anodic treatment. Welding-

rods should likewise be of the same alloy or of slightly lower potential

than the base metal. Inasmuch as brazing and soldering involves foreign

metals, adjacent areas must be carefully protected from galvanic corrosion.

When two metals are involved that have marked difference in potential,

it is good practice to coat one with a metal intermediate in electrode

potential value. The steel cable used in aluminum cable for transmission

of electric current is plated with zinc to discourage galvanic action.

8.14. Joining of Aluminum Alloys

Aluminum and its alloys are usually joined by fusion and electric-

resistance welding, brazing and riveting. Brazing differs from welding

in that the filler material has a much lower melting point that that of the

parent material so that the latter does not melt.

The common types of fusion welding are the gas and the arc processes.

The three brazing processes are classed by the methods used in applying

the brazing heat, namely, furnace, torch, and dip brazing. The electric

resistance welding is classified as spot, seam, and flash.

The main difference in the welding technique and procedure of alu-

minum and its alloys as compared to the iron alloys is primarily due to the

very high thermal coefficient of expansion and high temperature oxida-

tion of aluminum, and thermal and electrical insulating pro]>erties of its

oxide coating.

The high thermal expansion of aluminum and its alloys necessitates

the use of properly designed parts and carefully prepared joints so as to

minimize buckling and distortion. The filler rod and the base metal

are both provided with a flux coating to maintain an oxide-free metal

surface during the welding process. The commercial flux materials

consist of mixtures of saline chlorides-fluorides. -Proper ventilation must

be provided in the welding area because the fumes of the flux are both


disagreeable and irritating to the welder and highly corrosive to nearbymetal structures and materials. The welded part must be given a mechani-cal cleaning or chemical pickle treatment so as to completely removeresidual saline material due to its toxic and corrosive character.

Gas welding involves the use of oxyhydrogen and oxyacetylene torches,the flames in both instances being neutral in character. The gas weldinghas its main application with materials ranging from .040—1 inch in thick-ness.

The main arc processes include metal, carbon, atomic hydrogen, andtungsten arcs. The metal-arc is particularly suitable for heavy material;thicknesses up to 21/g inches have been welded satisfactorily. Difficulties

are experienced when the metal-arc is used with material less than 5/64inch thick. The carbon-arc is used for manual and automatic welding;the produced welds are comparable in structure and appearance to gaswelds but with less distortion. The atomic hydrogen process is appliedin much the same manner as gas welding but differs in the respect that it

is possible to control the heated zone by moving the torch away from ortowards the work. One limitation of the atomic hydrogen-arc processis the difficulty of removing the residual flux from the welded assembly.The tungsten arc uses an inert gas shield to protect the arc and the weldarea; this method has two advantages: (1) satisfactory welds can bemade with equal facility in vertical, flat, and overhead positions, and(2) no flux is required.

All commercial alloys are not readily adaptable to fusion or electric-resistance welding. The alloys that are commercially fusion welded are1100, 3003, 5052, 6053, and 6061. In some instances the welding is

difficult and/or costly. One limitation of the heat-treatable type of alloys

is that the high temperature of welding tends to decrease the mechanicalstrength and the corrosion resistance.

The proper choice of welding rod is very important particularly whendealing with alloys having large amounts of alloying elements. The twomost weldable alloys 1100 and 3003 use 1100 welding rod. The high-strength alloys 5052, 6053, and 6061, fairly difficult to weld, are weldedwith 5% silicon-aluminum alloy filler rod. This 4043 alloy rod has amelting point much lower than pure aluminum, excellent fluidity andwetting properties, and low shrinkage contraction, thereby insuring aminimum of shrinkage stresses and cracks.

One of the important applications of fusion welding is the salvagingof aluminum alloy castings that have foundry defects, and the repairingof cracked or broken castings. Castings should be preheated in a furnaceto approximately 800°F. This thermal treatment avoids thermal stress,


reduces the welding time and amount of welding gas required. It is

important that careful temperature control is maintained during this

preheat treatment so as to avoid "burnt" material and possible collapse

of the casting.

The areas adjacent to the welded sections of the strain-hardened alloys

are annealed by the welding heat. The weld metal has tensile properties

almost equal to the annealed metal but lower in ductility. Assemblies of

heat-treatable alloys should be welded prior to heating or given a reheat

treatment so as to improve the tensile and corrosion resistance properties.

Electric-resistance welding has been a large factor in reducing the cost

of making joints, improving products, and speeding up production.

This type of welding is usually applied to strain-hardened type of alloys,

but is often used with the heat-treatable alloys particularly of the alclad

type.

The technique and equipment employed for the resistance welding of

aluminum and its alloys is considerably different than that used for steel.

In addition the electrical capacity required for aluminum is several

times greater. The three types of equipment are classified on the basis

of the electrical system used for supplying the welding current, namely,

(1) alternating current, (2) magnetic energy storage and (3) condenser

energy storage. The proper choice of electrode shape and its proper

maintenance is very important in obtaining high quality and strength

spot welds. The welding electrode must have high electrical and thermal

conductivity and fairly good strength; electrodes are usually made of

copper-silver alloys. Weld pick-up and the softening of the tip are pre-

vented by having cold water circulating through the electrode. Somemachines use refrigerated water circulating at a rate of 2 gallons per

minute and to within 3/8 inch or closer of the tip face.

The material to be welded must first be freed of oil and grease, dirt,

and surface oxidation because these substances have electrical insulating

properties. The surface contamination may be removed by an alkaline

degreasing bath whereas the oxide coating necessitates mechanical abra-

sion or chemical etch methods. The metal must be spot welded within

a few hours of chemical etch because the oxide film will reform.

Bra/ing has been used in production on certain alloys such as 1 100 and3003, and to lesser degree on the heat treatable alloys 6053 and 6061.

The copper-aluminum alloys and 5056 show poor workability during

brazing operations; the resulting joints and adjacent areas have decreased

tensile strength and corrosion resistance. The advantages of brazing as

compared to gas and arc welding are as follows: (1) it can be used with

thinner material, (2) the costs are lower, and (3) the surface appearance


is superior. The torch-brazed joint has the tensile strength comparable to

a weided joint and has almost the same degree of corrosion resistance.

Riveting is the most commonly used industrial method of joining thestructural aluminum alloys, particularly the heat-treatable type. Therivets are made of commercial aluminum, the magnesium-silicon alloys

6053 and 6061, and the copper alloys 2017, 2024, and 2117. All except thefirst are received from the manufacturer in the hard, heat-treated condi-tion. Rivets of 2017 and 2024 are solution heat treated, quenched, anddriven as quickly as possible; whereas, the other type of rivets are driven"as received."

It is advantageous from a strength point of view to use rivets havingabout the same mechanical properties as the base metal; whereas, fromthe driving standpoint it is usually advisable to have the rivets somewhatsofter. From the corrosion resistance angle it is advisable to have therivet of the same composition or electrode potential as the material inwhich it is driven. For example, alclad sheet alloys are usually rivetedwith 6053 or 6061 because of their excellent corrosion resistance andnearness to aluminum in their electrode potential value.

Rivets that are solution heat treated prior to driving arc generallyprocessed in large quantities, quenched in cold water, washed withalcohol, and placed in refrigerated cabinets. Solution heat treated 2017and 2024 alloy rivets harden at room temperature over a two-hour period,making them unsuitable for driving thereafter. The rivets will remainsoft for 36 hours if kept at 32°F., or lower, after quenching. Small in-

sulated boxes of cold rivets are kept in refrigerated cabinets near theindividual work areas.

Large rivets of 2017 alloy are usually driven in the hot condition. Theyare solution heat treated at 930-950°F. for approximately fifteen minutes,inserted in the hole and driven. The relatively cold driving tools and thebase metal function as quench. Steel rivets in aluminum-alloy structuresare driven hot in the customary fashion; they are heated to I800°F. anddriven as quickly as possible.

When the service conditions are severe or dissimilar metals are in-volved, special treatments should be applied to prevent corrosion. Vari-ous methods are used, such as application of zinc chromate primer priorto assembly, use of good grade of joint compound before assembly, useof anodized aluminum alloy rivets; and if steel rivets are involved, theyare metallized with aluminum or zinc after driving.

8.15. Magnesium And Its Alloys

Magnesium is a very unusual metal: its properties do not run parallel toany other commercial metal, and its behavior is not as predictable. Mag-


nesiuni holds the unique position among the structural metals in having

the lowest density, the highest machinability, and has an inexhaustible

source of supply.

The low specific gravity of magnesium (1.74) means that it is 2/3 as

dense as aluminum and 1/4 that of iron and steel. The cutting speed for

magnesium and its alloys is twice that of free-cutting brass and four times

that of cast iron. Seawater contains 1 part in 770 of magnesium therefore

one cubic mile of seawater could produce nine million tons of magnesium.

There is also a vast supply of magnesium in magnesite and dolomite.

Magnesium is similar to aluminum in its low melting point, high

solidification shrinkage, hot shortness, low tensile strength near the melt-

ing temperature, good thermal and electrical conductivity, low density,

high strength-weight ratio, and non-magnetic and non-sparking prop-

erties. From a chemical point of view these two metals have low atomic

and equivalent weights, and high electrode potentials. It markedly

differs from aluminum in its crystal structure, (H. C. P.) , fair malleability

and ductility, fair cold workability, poor notch sensitivity, non-protective

oxide coating, and lower corrosion resistance. Magnesium is much morechemically reactive than aluminum due to the physical and chemical

nature of its oxide coating.

The manufacturing procedures and processes of magnesium are very

much like that of aluminum in the respect that fl) the impurities must

be removed from the ore rather than from the produced metal, (2) re-

quires large amounts of energy, (3) requires high temperatures, and (4)

reduction involves nonaqueous reactions.

The most important commercial process for making magnesium is by

the electrolytic reduction of molten solutions of magnesium chloride.

The formed molten magnesium floats on the surface and must be pro-

tected by an inert atmosphere since it chemically absorbs both oxygen

and nitrogen. Another limitation of this (Dow) process is that the

chlorine constitutes a health and corrosion hazard.

Two lesser important processes are the chemical reduction of mag-nesium oxide by carbon and silicon respectively. In the carbothermic

(Hansgirg) process, briquets of magnesium oxide and carbon are

heated at 2000°C. to form magnesium vapor and carbon monoxide gas.

This reaction mixture on cooling would reverse to the original rawmaterials; this is prevented by rapidly cooling and diluting with natural

gas or other inert gases. The fine magnesium powder that results by this

"shock" action must be purified by distillation. The Pidgcon process

depends on the fact that silicon in the form of ferrosilicon will remove

the oxygen from magnesium, that the formed silicon dioxide is a non-


volatile material, and metallic magnesium under reduced pressure canbe distilled at 1150°C. The Hansgirg and Pidgeon processes are moreexpensive than the Dow process, but have the advantage of not dependingon electric power.

The improvement of magnesium by addition of alloying elements is

not very great. The main alloying elements are aluminum, zinc, andmanganese. Zinc is more effective than aluminum in the improvementof mechanical properties but offers the disadvantage of decreasing its

corrosion resistance. A compromise is often made by using both. Thealloy AZ-63 consists of 6% aluminum and 3% zinc. The main functionof manganese is to increase the corrosion resistance.

The minor alloying elements are beryllium, calcium, cerium, silicon,tin, and zirconium. Beryllium decreases the tendency of burning duringthe melting and casting. Calcium reduces oxidation during melting andheat treatment, and increases the reliability and formability of mag-nesium sheet. Cerium increases the high temperature strength. Themain function of silicon is to increase the fluidity of molten magnesiumand to decrease the hot shortness and cracking tendencies of the solidmetal. Tin increases ductility, and zirconium leads to grain refinement.The very undesirable elements and their approximate maximum toleranceare copper (.1%), nickel (.01%) , and iron (.005%)

.

Magnesium alloys are somewhat difficult to roll; therefore structuralmembers are usually extruded. Cold working, bending, and deep drawingcan be satisfactorily accomplished under carefully controlled conditionsand with frequent annealing. Most mechanical processing is done attemperatures above 225°C.

Pure commercial magnesium sheet in the annealed and hard-rolled con-ditions have the respective tensile strength values of 27,000 and 33,000psi. Most magnesium alloys have tensile strength in the range of 40-50thousand psi, and the maximum of sixty thousand. Special costly alloyscontaining 3% silver or 6% cerium have tensile strength of sixty-fivethousand.

The heat-treatment processes of magnesium alloys is similar to that ofaluminum except the temperatures involved are considerably lower. Thereadiness with which magnesium oxidizes in the air means that specialprecautions and new techniques are involved. Although aluminum andits alloys may be heat treated in molten sodium nitrate baths, violent ex-plosions may take place with the alloys of magnesium.One outstanding advantage of magnesium and its alloys is its excellent

machinability. Cutting fluids are used primarily as a coolant to preventthe magnesium from catching on fire. The turnings from machining


operations also represent a potential fire hazard. Dry sand should be

available in these areas because water or the common fire extinguishers

are unsatisfactory for putting out magnesium fires.

The casting of magnesium likewise requires special conditions.

Molten magnesium is usually protected by an inert atmosphere of sulphur

and sulphur dioxide. Green sand castings can not be used unless inhibitors

are present; otherwise the magnesium will react with the moisture. Thesubsequent finishing of the casting involves metallic dust that represents

a fire hazard.

The corrosion resistance of magnesium and its alloys under normal con-

ditions is very satisfactory. Under highly humid conditions and other

severe conditions, magnesium and its alloys should have a protective film

or coating. Magnesium alloys prior to fabrication are usually shipped

with a film of oil or a chrome pickle treatment coating. The chromepickle is superior to the oil film for protection, but the former is easier to

remove prior to spot welding.

Magnesium alloys may be joined by gas and arc welding, electric-resist-

ance welding, and by riveting. Most of the wrought alloys can be joined

by gas welding using neutral flames of oxyhydrogen, oxyacetylene andoxycarbohydrogen

.

The filler rod should have the same composition as the parent metal

or have a lower melting point. A flux must be used during welding dueto the ease of oxidation of magnesium. Magnesium alloy sheet, extru-

sions, forgings, sand and permanent mold castings are often welded by

using an inert gas-shielded arc method. These alloys may be resistance

welded after they have been cleaned of surface contamination and oxide

coatings. Magnesium may be riveted by the most conventional methods.Magnesium-alloy rivets cannot be used because of their rapid workhardening at room temperature. The rivets generally used are the 5%magnesium-aluminum alloy 5056, which has a like electrode potential.

Most processed magnesium alloy parts and assemblies that will be used

where corrosion is greater than normal are given a final chromate treat-

ment. The chromate treatment results in the removal or passivation of all

foreign metals and produces an adhering protective coating. This coating

offers good anchorage for a paint film.

8.16. Beryllium

Beryllium resembles magnesium in its low density (1.84) , its hexagonalclose-packed crystalline structure, and its poor cold workability. Its

similarity to aluminum is shown by its low equivalent weight, its ampho-teric properties, and the refractory and protective nature of its oxide film.

The outstanding property of beryllium is its great power of producing


hardness and strength in other metals; it suffers the two great limitations

of high cost and toxicity. Its cost is high because (1) its ores are notcommon, (2) the concentration of beryllium in the ores is often less

than 4% average, (3) its manufacture by the modified thermite or Gold-schmidt process is a costly one.

The most important alloy of beryllium is the beryllium-copper alloys,

which have high fatigue and corrosion resistance, good electrical andthermal properties, and are virtually non-sparking. The 2% alloy onheat treatment, quenching, and aging develops a hardness equal to Rock-well C-40.

Amounts in the order of .01% beryllium in molten aluminum andmagnesium decrease their oxidation and combustibility markedly, andreduce their viscosity, particularly in the case of aluminum. Small amountsof beryllium may be used to case-harden iron and steel.

The vapors and powdered beryllium are very toxic and give rise to lungdiseases that are often fatal. Beryllium is available commercially in theform of 12% beryllium-copper alloy.

8.17. Titanium

Titanium in one of the most abundant metals in the earth's crust, themain ores being ilmenite (iron titanate) and rutile (titanium oxide)

.

It is made by the modified thermite or Goldschmidt process.* The veryimportant properties of titanium as a structural metal are its high meltingpoint (1820°C.) , its high tensile strength at ordinary and elevated temp-eratures, its relatively low density (4.5) , and its high resistance to ordinarycorrosion. In the annealed and cold-worked (60% reduction) conditionthe commercial metal has the respective tensile strengths 78,700 and111,500 psi. Exposure to accelerated corrosion tests using salt spray for30 days has little effect on commercial titanium.Titanium, like magnesium and beryllium, has the hexagonal close-

packed crystalline structure; therefore its workability likewise at ordinarytemperatures is poor. When titanium is heated to 875°C, it undergoesa transition to an body-centered cubic structure; hence its workabilitymarkedly increases at this temperature. There is a 5.5% volume changeduring this allotropic transformation. One limitation of titanium is

that it will readily unite with and burn in oxygen at 600°C. and withnitrogen at 800°C. At elevated temperatures titanium will likewise unitewith carbon and hydrogen.

Titanium is a very important additive element in the steel industry.The great affinity of titanium for carbon is taken advantage of andcarbide precipitation avoided in 18-8 stainless steel by the presence oftitanium, the amount being about five times that of the carbon content.

* And now produced widely by the Kroll process.


The presence of .5-1% titanium in 17-7 stainless steel leads to a ferritic

structure. The grain refinement of aluminum and its alloys by the

presence of small amounts of titanium is due in part to its great affinity

for dissolved oxygen, nitrogen and hydrogen. Titanium acts likewise as a

scavenger for oxygen and nitrogen in steel.

QUESTIONS1. What is the .most abundant metal in the earth's crust? What is the metal

that is inexhaustible in its supply?

2. Magnesium oxide has been reduced industrially by the action of carbonto form metallic magnesium. Why cannot aluminum be economically madein the same manner?

3. What impurity as a metal oxide, if present in bauxite ore, could not beremoved in the Bayer Process?

i. Aluminum is often melted in iron pots which are usually protected by a

coating of whiting or other insulating material. Why is this protective coat-

ing important?

5. The general rule regarding gases is "they decrease in solubility in liquids

with rise of temperature." How would you explan why the solubility of

hydrogen increases with rise of temperature when present in moltenaluminum?

6. Accurate control of temperature is very important during the solution heat

treatment of aluminum alloys. What are the relative advantages of a moltenbath and an air furnace?

7. What test solution would you use on a large assembly to determine whetherit was an aluminum alloy or a magnesium alloy?

8. What are the main obstacles of manufacturing aluminum from clay?

9. Excessive rates of heating metal lead to cracking. In view of the fact that

aluminum alloys are good conductors of heat and that the nitrate bath is a

fair conductor, why is the heating in this bath still rather slow? (What in-

sulation film is involved?)

10. The processing room wherein the Hall process is conducted must be pro-

vided with good ventilation. What gas and vapor must be conducted away?11. Why does the presence of high silicon in aluminum alloys lead to superior

fluidity? To lower degree of contraction?

12. How do you explain why magnesium has the highest machinability amongthe commercial metals? Why arc its turnings so much more combustible

than those of aluminum?13. Why should the heat treatment of magnesium be made in an air furance

rather than a molten nitrate bath?

14. Aluminum sheet arrives at the plant with fine (tissue) paper between each

sheet. Give three reasons why this has benefical results.

15. After aluminum has been anodized, and then given cold and hot rinses, it

should be painted within one hour thereafter. Why should this time beimportant?

16. What precaution should be made regarding machining or processing of

beryllium alloys?

17. How do you account for the fact that titanium has high strength at elevated

temperatures?

18. Chlorine gas is passed into molten aluminum on two occasions. What are

two conditions that are eliminated by this action. Give reasons for both.

CHAPTER

Copper and Copper -BaseAlloys as Engineering Materials

Waiter R. Hibbard, M.S., Adjunct Associate Professor

of Metallurgy, College of Engineering, University of

Bridgeport, Bridgeport, Connecticut

Roger Greenleaf Stevens, Ph.D., Head, Department of

Chemical Engineering, Southwestern Louisiana Insti-

tute, Lafayette, Louisiana

1 HESE metals are such an integral, but com-monly unnoticed, part of our daily life that even the housewife in the

kitchen and the clerk in the store would find it difficult to establish a

new routine of existence if copper suddenly disappeared from the world.

Judged solely from the standpoint of absolute value to mankind, copperand its common alloys, the brasses and the bronzes, cannot compete withcarbon steels, but each is of vital importance to our civilization andcould hardly be eliminated without destroying or at least seriously

crippling most branches of industry.

It has been said with justification that electricity is the elixir vitae of

machine civilization. It is difficult to conceive of an electrical industryif copper and its ores had never been known, unless nature had providedman with a substitute of equal conductivity, plasticity, and cost. Secondonly to high conductivity in importance is the resistance of copper andsome of the copper-rich alloys to corrosion in certain environments,especially sea air and sea water.

The average automobile uses 45 lb. of copper and its alloys, theaverage steam locomotive 4,500 lb., and the modern steamship more than3 million lb. as castings, in pipes and tubes and in electrical equipment.No metal has proved so suitable for steamship propellers as manganesebronze; four of these, weighing 37 tons each, drove the Queen Maryacross the Atlantic Ocean in four days.

With two or three exceptions, the copper and copper-rich alloys dis-

cussed in this chapter have been important engineering materials for

161

'62 Engineering Metallurgy

years and, unlike some of the light alloys discussed in the previous

chapter, are covered by specifications of the American Society for Test-

ing Materials.* In addition, most of them also conform to specifications

adopted by the Society of Automotive Engineers, the Association of

American Railroads, the American Society of Mechanical Engineers

(Boiler Construction Code) , the American Standards Association, the

U.S. Navy, and the U.S. Army, and to general federal specifications.

These standards—especially those issued by A.S.T.M. -should be con-

sulted for exact chemical composition and for minimum mechanical-

property and other requirements which must be met before the material

can be considered to be of good commercial quality.

Although copper is present in the earth's crust to the extent of only

0.01 per cent, ore deposits of commercial importance are fairly wide-

spread. Compared with iron ores, copper-ore deposits of commercialquality are lean. The ores of the United States contain on an average

somewhat less than 2 per cent copper. In the important ores, copperoccurs as the oxide, carbonate, silicate, and sulphide, of which the last

named is the most abundant. In some parts of the world, notably in the

vicinity of Lake Superior, long weathering of copper ores followed byreduction of the resulting oxide or the carbonate, or the deposition of

copper from solution, has produced large deposits of relatively pure

native metal, which was distributed far to the south by the glaciers.

Native copper was the only metal (except small quantities of meteoric

iron) used by the American Indians for tools and weapons until the

white man arrived.

The production of high-purity copper from lean sulphide ores is a

complex process, but copper exceeding 99.9 per cent purity can be pro-

duced cheaply. In brief, the process consists of concentrating and roast-

ing the complex sulphide ore (copper, plus iron, plus other metals) to

form oxides and to eliminate some of the sulphur and arsenic as the

oxide. The roasted concentrate is smelted with coke and a flux in a

reverberatory furnace to remove part of the iron. The copper and iron

sulphides fuse into a matte containing about 40 per cent copper. This

matte is charged into a Bessemer converter and is blown in a mannersimilar to the refining of pig iron to produce Bessemer steel. The copper

and the remaining iron sulphides are oxidi/ed, and the iron oxide com-

bines with silica to form a silicate slag. The result of these reactions is

"blister copper" containing 95 to 97 per cent copper and considerable

copper oxide.

'Sec A. S. T. M. Standards, Part 2, 1955.

Copper and Copper-Base Alloys 163

In addition to copper oxide, blister copper generally contains smallamounts of sulphur, iron, lead, arsenic, antimony, selenium, tellurium,

nickel, and other elements, usually including some gold and silver. Insome ores, in fact, the silver and gold are worth so much that the coppercould be called a by-product of the refining of the more precious metals.Although the amount varies, a typical anode melted from blister copperfrequently contains 30 oz. of silver and nearly i/

2 oz. of gold per ton.

The final step in refining is melting and casting the blister copper intoblocks which are electrolyzed. Electrolytic copper contains 99.95 percent or more copper, with sulphur as the principal impurity. About 25per cent of the electrolytic copper produced is used for making copper-base alloys; the remainder is melted in a large reverberatory furnace,oxidized to remove sulphur, and then "poled." This consists of insertinggreen wood poles beneath the surface of the molten metal. In the re-

action between the wood and the molten copper oxide enough oxygenis removed so that the remainder, 0.03 to 0.04 per cent, when evolvedduring solidification, exactly balances the normal shrinkage of the metal.Poled copper is known as "tough-pitch." Its density in the cast conditionis approximately 8.5 gm. per cu. cm., compared with 8.94 for the puremetal. Tough-pitch copper is used for wire for electric conductors, forsheets, and for many other purposes.

Lake (Superior) copper, which constitutes about 15 per cent of thecommercial copper in the United States, is-as previously mentioned-ahigh-purity native copper and does not usually need electrolytic refining.It differs from electrolytic copper in that it may contain a small amountof silver (which does not affect conductivity) and from 0.002 to 0.04per cent or even more arsenic, which, in the higher amounts, may reducerelative conductivity as much as 40 per cent.

A recent development in copper refining is the production of oxygen-free copper. Oxygen in tough-pitch copper is present as cuprous oxideparticles, which are almost completely insoluble in the solid metal. Formost purposes this oxide is not harmful, but if extremely high ductilityis essential, the copper should be practically free from oxygen. The oxideis also harmful if the copper is heated in an atmosphere containing hy-drogen as this gas diffuses into the metal, reacts with the oxide, andcauses brittlencss. There are two types of oxygen-free copper. The morecommon one is produced by deoxidizing the molten metal with phos-phorus or some odier element having a high affinity for oxygen. Theresidual phosphorus, although only about 0.02 per cent is present, reducesthe electric conductivity as much as 20 j>er cent. To produce high-con-ductivity oxygen-free copper (known as OFHC) , very pure copper is


melted and cast in an atmosphere free from oxidizing gases. A recentinnovation consists of granulating brittle copper cathodes, which havebeen deposited electrolytically and which are free from oxygen, followedby "coalescing" these in a press at high temperatures, which welds the

particles together and extrudes the metal as a solid bar.

Copper is malleable and is hot and cold worked into sheet, plates,

tubes, and a variety of other shapes. As in the case of most other metalsand alloys, cold working increases strength and reduces ductility.

9.1. Properties and Uses of High-Purity Copper

Copper has the highest electric conductivity of all the common metals.

On a strength basis, its conductivity is more than twice that of high-purity aluminum. About one half of the 800,000 tons of copper consumedin the United States in the average peacetime year is used for electrical

equipment and for wire in light and power lines. When annealed, high-

purity copper has a conductivity of 100 per cent according to a standardset up in 1913 by an international commission. Recently improvedmethods of purification have resulted in conductivities as high as 102per cent.

Copper is also the best heat conductor of any of the commercial metals,

which makes it (and its alloys) useful as cooking utensils and as radi-

ators and other heat-dissipating apparatus. In general, copper and the

copper-rich alloys have excellent resistance to corrosion by the atmos-phere and by water. Owing to this corrosion resistance and to a reason-

able price, copper and some of its alloys are widely used by the buildingindustry for roofing, down-spouts, gutters, screens, drainage and waterpipes, and for hardware and interior fixtures. When exposed to the

atmosphere for long periods, copper oxidizes and forms a green patinawhich is ornamental in addition to being adherent and protecting the

underlying metal from further attack. Approximately 10 per cent of the

copper consumed annually is used by the building industry.

Copper and copper-rich alloys have a wide range of mechanical prop-erties. Annealed high-purity copper has a tensile strength of approxi-mately 30,000 lb. per sq. in., an elongation in 2 in. of about 60 per cent,

and a reduction of area of as much as 90 per cent. It is, therefore, weakerand more ductile than high-purity iron. At the other extreme, precipita-

tion-hardened copper-beryllium alloys may have a tensile strength of

190,000 lb. per sq. in. with accompanying low ductility.

The tensile strength of high-purity copper may be increased to 65,000or 70,000 lb. per sq. in. by drastic cold working; elongation is lowered


to about 5 per cent. According to Gillett,* the endurance limit of an-nealed tough-pitch copper is approximately 10,000 lb. per sq. in., andcold working increases it to as much as 20,000 lb. per sq. in. The modulusof elasticity is approximately 16 million lb per sq. in. The dampingcapacity is almost as high as that of cast iron.

For some applications the low strength of high-purity copper wire is

a distinct disadvantage. Unfortunately, most alloying elements addedto increase strength reduce conductivity to a fraction of the value forhigh-purity copper. A satisfactory compromise between increase instrength and decrease in conductivity results from the addition of smallamounts of cadmium.

1100

40 50 60Zinc, per cent

% V 100

Frc. 9.1. The copper-zinc phase diagram. (A. Phillips and R, M. BricKft

9.2. Constitution of the Common Copper-Rich Alloys

None of the four series of important binary copper-rich alloys—namely,copper-zinc, copper-tin, copper-aluminum, and copper-nickel-can beheat treated in the sense that sterling silver or many of the aluminum-rich alloys can be heat treated. Sections of the copper-zinc and thecopper-aluminum diagrams arc given in Figs. 9.1 and 9.2. Most of theuseful alloys of copper with zinc (brass) and of copper with aluminum

• H. W. Gillett, Metals and Alloys, v. 3, 1932, p. 200.

\ Metals Handbook, American Society for Metals, Cleveland, 1939, p. 13G7.


(aluminum bronze) contain less than 40 per cent zinc and less than 10

per cent aluminum respectively and are, consequently, ductile single-

phase alloys consisting of a solid solution of zinc or of aluminum in

copper. If the zinc in brass exceeds about 35 per cent, beta phase

appears, reducing ductility and impact resistance. Only one such alloy,

Muntz metal, containing -10 ]>er cent zinc, is used widely, and in this

the amount of beta is so small that ductility is not seriously affected.

Similar to conditions in the copper-zinc system, most of the wroughtaluminum bronzes are single-phase alloys containing 5 to 9 per cent

aluminum (see Fig. 9.2) . If the alloy contains more than 9.5 per cent

aluminum, beta phase appears, reducing the ductility. Although the beta

phase cannot be retained in solution by quenching, there is the possibility

of securing transition structures by quenching a wrought alloy containing

10.5 to 1 1 per cent aluminum. The result is a tensile strength of 100,000 to

110,000 lb. per sq. in. and an elongation of 10 to 15 per cent, as com-pared with 65,000 to 85,000 lb. per sq. in. and 35 to 50 per cent elonga-

% Atomic Percentage Aluminum_5 K> JS 20 2S SO

Weight Percentage Aluminum

Fig. 92. The copper-rich portion of the copper-aluminum phase diagram.t (W. L.

Fink, L. A. Willey, and C. S. Smith)

ilbid., 1948, p. 1160.


tion for the alpha-phase alloy. No precipitation hardening is possible

in the binary copper-rich copper-aluminum alloys.

The principal commercial copper-tin alloys are alpha-phase alloys

and contain 1 to 10 per cent tin and usually also zinc, lead, or both. Theonly important binary alloy is phosphor bronze which contains 4 to 10

per cent tin and which is deoxidized in melting with a small amount(less than 0.5 per cent) of phosphorus. Tin increases the hardness andstrength of copper. A cold-rolled phosphor bronze used for springs has atensile strength of about 100,000 lb. sq. in. with 5 per cent tin; this

increases to 120,000 lb. per sq. in. with 10 per cent tin. No heat treatmentis possible with commercial copper-tin alloys.

Copper and nickel form a continuous series of solid solutions free fromany phase change. The strength of the solid solution of nickel in copperincreases with the nickel from about 35,000 lb. per sq. in. for very lownickel to 80,000 for 60 per cent nickel; elongation decreases from 70 to

45 per cent in 2 in. There are a large number of important copper-nickelalloys which, owing to their controllable color, to their corrosion resist-

ance, or to their thermoelectric properties, are used for coinage, orna-mental metal work, ship construction ( especially in parts exposed to salt

water),thermocouples, and in other special applications. An alloy higher

in nickel, Monel metal-containing 65 per cent nickel and 35 per centcopper-is widely used because of its high resistance to many corrosivemediums.

9.3. Nomenclature of the Copper-Rich Alloys

Although the copper industry is no worse than the steel industry inthe use of trade names and in confused nomenclature, so many mis-named alloys are commonly used that some attention to this subjectseems advisable. The most common examples of confused nomenclatureare calling a copper-zinc alloy bronze if it has a color resembling that ofthe copper-tin alloys and calling a copper-zinc-nickel alloy nickel silver

if it has a silvery-white color. This confusion is regrettable, but in mostcases nothing can be done about it, as the names are firmly fixed byyears of usage. The important fact to remember about the copper-richalloys is that the name may be more descriptive of color than of composi-tion, and that a "bronze" may contain no tin and may even be a brass;

likewise, nickel silver contains no silver.

The usual name and the typical compositions of some common copper-rich alloys are given in Table 9.3. It is evident that manganese bronze(item 43) , hardware bronze (item 19) , and commercial bronze (items

9 and 10) are in reality brasses. The only true bronzes given in Table


9.3 are items 49 to 59, which are copper-tin alloys deoxidized by phos-

phorus. Nickel silver, formerly known as German silver, is the nameof a group of brasses, containing 17 to 42 per cent zinc, in which a con-

siderable part of the copper is replaced by nickel. The composition of

four of these alloys is given in Table 9.3. Another example of confusednomenclature is the casting alloy containing 85 per cent copper and 5

per cent each of zinc, tin, and lead. As this alloy contains 5 per cent

tin, it is as much of a bronze as it is a brass, but it is called red brass*

and is, consequently, sometimes confused with wrought red brass whichcontains 85 per cent copper and 15 per cent zinc.

9.4. Characteristics and Uses of the High Brasses

As shown by the diagram in Fig. 9.1, 39 per cent zinc dissolves in

copper at room temperature. Alloys containing less than this percentage

of zinc—when in a state of equilibrium—are, therefore, single-phase solid

solutions and are known as alpha brasses. As shown also by Fig 9.1,

alloys containing about 36 to 39 per cent zinc may have a duplex structure

if heated to a high temperature but should be single phase at roomtemperature. Actually, however, owing to a phenomenon known as cor-

ing, cast alloys usually have a duplex structure if the zinc is higherthan 31 or 32 per cent. A zinc content of 35 per cent is about the maxi-mum for an alloy that is to be fabricated as a single-phase material.

Although there is a continuous series of alpha brasses containing from5 to 39 per cent zinc, these alloys are commonly and arbitrarily dividedinto two classes: high brass, containing 30 per cent or more zinc, andlow brass, containing approximately 25 per cent zinc or less (Table 9.3)

.

Since zinc is cheaper than copper, it follows that high brass is used as

widely as possible, and that the zinc content is usually close to the upperlimit, which is about 35 per cent.

Although the alpha brasses can be hot worked commercially if they

are free from lead, they are not so plastic at elevated temperatures as

the beta alloys. In common with most solid solutions they are, however,plastic at room temperature and can be readily drawn, rolled, or spuninto a variety of shapes. In fact, these alloys combine a degree of plas-

ticity, strength, ductility, corrosion resistance, pleasing color, and lowcost unattainable in any other material of engineering importance. Alloys

13, 14 , 2 9 are used for a large variety of cast and wrought sections,

especially hardware and pipe fittings, for tubes and pipe, for ornamental

• The casting alloy containing 85 per cent copper and 5 per cent eacli of tin, zinc,and lead is also known as composition metal and in many foundries as ounce metalbecause the composition corresponds approximately to 1 lb. of copper. 1 oz. of tin.

1 oz. of zinc, and 1 oz. of lead.


sections and building trim, for cartridge cases, automobile radiator cores,

grillwork, springs, screens, and tor many kinds of nails, screws, andrivets.

Many copper-zinc alloys are used because of their color, as statedpreviously. They change from warm red copper color of copper to com-mercial bronzes (Tables 9.3 items 9 and 10) which have a bronze color tored brass (item 11) which is reddish hence its name. Some of these alloysin the above range duplicate the color of gold alloys and are used formanufacture of gold filled metals. Then comes low brass (item 12)whose color is pinkish, to cartridge brass (item 13) which is yellow, toyellow brass (items 14 and 15) which are greenish-yellow and finally toMuntz metal (item 16) which has the color of cast statuary bronze.

Muntz metal and its modification, manganese bronze (item 16 and 48,Table 9.3)

,usually contain beta phase at room temperature (see Fig.

9.1),which reduces plasticity when the alloys are cold. These materials

are, however, characterized by extreme plasticity at a red heat and canreadily be extruded or rolled. Owing to their resistance to atmosphericand water corrosion, these two alloys, especially manganese bronze,are used extensively for extruded or hot-rolled pipes and condensertubes. Muntz metal is now being replaced, particularly for condensertubes in marine equipment, by a brass known as admiralty metal. Thiscontains not less than 70 per cent copper, 1 per cent tin, and the re-mainder zinc. Other than for condenser tubes, admiralty metal, socalled because it was developed by the British Admiralty, is used forpreheaters, evaporators, air-conditioning equipment, and in other appli-cations where high resistance to salt water, fresh water, oil, or steam is

required. Admiralty metal is an alpha-phase alloy and is not so easilyhot worked as Muntz metal and manganese bronze, but it is readilyworked when cold.

In genera], the machinability of the annealed, non-leaded brasses isabout the same as that of low-carbon steel, in other words, they are toosoft and gummy to be cut readily. Cold working, however, improves ma-chinability, which is also greatly improved by adding lead. Item 29(Table 9.3) can be machined readily in automatic machines. If thisalloy is given a machinability rating of 100, the same brass containingonly 1 per cent lead will have a rating of 70 (determined under the sameconditions), and item H without lead-will have a rating of only 30.However, lead reduces the yield strength; it also makes the brass moredifficult to deform cold and makes it virtually impossible to work thealpha-phase alloys at elevated temperatures.


9.5. Characteristics of the Low Brasses and the Nickel Silvers

As the amount of zinc in brass is reduced, the yellow color changes to

red. Items 8, 9, 10, 11, and 12 (items 9 and 10 are called bronzes

because of their color) are used for hardware, ornamental and architec-

tural sections, wire for screens, and for costume jewelry, in addition to

being used for tubes, pipe, radiator valves, and other fittings for low-

pressure water and steam. An advantage of the low brasses is that, whenthe copper exceeds 80 per cent, the alloy is not subject to "season crack-

ing," a phenomenon prevalent in the high brasses when they are exposedto a combination of corrosion and stress. Season cracking is the spon-

taneous failure of the stressed material after a considerable time in a

corrosive environment, especially in atmospheres containing ammonia.The time element depends upon the severity of the stress and the severity

of the corrosion. Stress caused by cold working, or applied externally, is

necessary for failure of this type.

The low brasses, like the high brasses, are readily deformed cold pro-

vided the lead content is low. Also like the high-zinc alpha brasses, they

are difficult to hot work although commercial bronze (item 9, Table

9.3) can be rolled or pierced and, to a limited extent, pressed or ex-

truded, provided that more than traces of lead are absent.

Most nickel silvers have a copper content of 45 to 65 per cent. Afairly common alloy is the one containing 75 per cent copper, 20 per

cent nickel, and 5 per cent zinc, usually sold under the trade nameAmbrac. All the nickel silvers can be classed as low brasses containing

nickel. If the copper plus nickel exceeds 63 per cent, the alloys contain

only alpha phase. They can be hot worked readily only if the zinc is

about 40 per cent or less than

10 per cent, but they can becold worked easily.

Nickel silvers are valuable

because of their white color, a

corrosion resistance that is

generally superior to that of

the common brasses, goodmechanical properties, andmoderate cost. The effect of

composition on color is shownby Fig. 9.3. The nickel silvers

are used chiefly for ornamen-tal work as a base for silver-

and gold-plated ware.

f * *Nickel.per cent

Fig. 9.3. Effect of composition on the color

of nickel silver. (Kihlgren)'


9.6. Properties of the Wrought Brasses

Some typical tensile properties of wrought brass and nickel silver are

collected in Table 9.3. As the commercial copper-zinc alloys do notrespond to heat treatment, variations in properties are obtained by coldworking and annealing which results, as in the case of some of the alumi-

num alloys, in various tempers, ranging from soft to half-hard, to hard,and to spring temper. With the possible exception of manganese bronzeand nickel silver, the tensile strength of all the annealed brasses is verysimilar to that of annealed or hot-rolled low- or medium-carbon steel,

but the yield strength is much lower. As the brasses cost five to ten timesas much as low-carbon steel, the chief justification for the widespreaduse is, of course, then ease of working cold, their pleasing color, andtheir corrosion resistance, especially to water and steam. In general, thelow brasses are more corrosion-resistant than the higher zinc alloys,

especially in impure water and in salt water. The high brasses, in addi-tion to being subject to season cracking are also subject to "dezincifica-

tion," or removal of the zinc, when the alloy is exposed to impure wateror salt water. The action is apparently electrolytic and leaves a spongymass of copper. Dezincification can cause failure in a relatively shorttime. Alloys containing 20 per cent or less zinc are not usually subjectto this form of corrosion. Manganese bronze has high resistance to salt

water but may be subject to season cracking and dezincification.

The endurance limits of the brasses are low, ranging between 10,000and 20,000 lb. per sq. in. for 100 million cycles, which is equivalent toan endurance ratio of 0.20 to 0.35, or about half that of steel. Annealedbrasses have a definite yield point (as evidenced by the drop of thebeam) but cold-worked alloys have not. Values for yield strength reportedin the literature are usually the stresses causing an extension of 0.5 percent under load. These values are usually much lower than the yieldstrength of steel which corresponds to a permanent elongation of 0.2per cent of the gage length. The modulus of elasticity (secant modulus,see p. 65) varies from 15 million lb. per sq. in. for the copper-zincalloys and for manganese bronze to about 19 million lb. per sq. in. fornickel silver. The mechanical properties of brass and of nickel silverare affected by grain size which, in the case of worked material, dependson annealing temperature. Material with large grains has lower strengthand higher elongation than fine-grained material (except in very thinstrip)

.

• T. E. Kihlgren, Metals Handbook, American Society for Metals, Cleveland 1939pp. 1443-1445.


9.7. Cast Brass and Cast Nickel Silver

There are at least a thousand varieties of brass that are melted andpoured into sand molds. Most of these, however, fall into the nine gen-

eral classes of alloys for which specifications have been issued by the

American Society for Testing Materials, the Society of Automotive En-

gineers, or by the United States government. Some typical compositions

and properties of the most commonly used alloys are collected in Table9.1* The properties of cast brass are determined on test bars cast

separately from the same metal (A.S.T.M. specification B208-46T) andtested either with or without machining their surface.

As shown by the data in Table 9.1, the cast brass, with the exception

of manganese bronze, are relatively weak and ductile and have a very

low yield strength. Nickel silver has better properties than the copper-

zinc alloys containing no nickel, and when the zinc is omitted completely,

as in the case of cupronickel, the properties are even better. The nickel

silvers are, however, considerably more costly than the other brasses.

The cast brasses are easily machined—especially when they contain

considerable lead, as most of them do—and have a high resistance to

corrosion in many environments. They are, therefore, widely used for

architectural and ornamental castings, especially hardware and statuary;

fittings and other parts exposed to sea, air, and water or steam; plumbing

fixtures for water; valves and other parts in chemical equipment; laundry

and dairy equipment; and many others. Nickel silver is a favorite casting

alloy for automobile and household hardware and for industrial use

where alkaline waters or chemicals are encountered.

9.8. The Copper-Base Bearing Metals

The principal copper-base bearing metals (bearing metals are dis-

cussed more fully in Chapter 10.) are those in Class C of Table 10.2.

On a tonnage basis this group is more widely used than the others.

According to Clamer.t about 10 million lb. of alloy CI (either deoxi-

dized, or not, with less than 1 per cent phosphorus) is used annually in

the United States. The leaded bronzes, alloys CI to C5 and C8 and C9,

are employed by the railroads and the automotive industry; alloys C2and C3 are suitable for small bushings and bearings; others are better

• Cast Metals Handbook, American Foundrymen's Association, Chicago. 3rd ed,,

1914, pp. 604-648.

t G. H. Clamer, Metals Handbook, American Society for Metals, Cleveland, 1939,

pp. 1428-1429.


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fitted for general-service bearings; alloy C5 is suitable for high speedsand low bearing loads, and CI for moderate speeds and heavy loads.

The properties of the copper-tin-lead alloys vary with composition.

At room temperature, the property ranges for alloys CI to CIO, inclusive,

are as follows:

Property Range of values

Tensile strength, lb. per sq. in 28,000 to 40,000Yield strength, lb. per sq. in 14,000 to 18,000Elongation in 2 in., per cent 1 to 20Compression, lb. per sq. in., for a deformation of 0.001 in 10,000 to 13,000Brinell hardness (500 kg.) 55 to 75

The hard bronzes (alloys C6 and C7, and alloy CI as well) are usedin castings for movable bridges and turntables which act as bearings

subject to slowly or intermittently applied heavy loads. Most of the

alloys in group CI to C7 are used as large bearings ("brasses") in loco-

motives. Alloys CI, C8, and C9 are used extensively by the automotiveindustry.

The high-lead alloys C10 and Cll are a fairly recent development and,

as steel backed, are used in internal-combustion engines for aircraft andautomobiles, trucks, and tractors. These alloys are inferior in tensile

properties to the other copper-rich alloys: tensile strength varies from12,000 to 22,000 lb. per sq. in.; yield strength from 5,000 to 10,000 lb.

per sq. in.; elongation from 5 to 12 per cent; and Brinell hardness (500

kg.) from 25 to 40. In general, however, they are superior to the whitemetals; their melting temperatures are much higher, and their properties

are not so much affected by temperature. They are also resistant to re-

peated flexure which is important with present-day engine speeds. Mostimportant, if lubrication fails momentarily, the heat generated by the

friction sweats out a film of lead which acts as a lubricant and preventsseizure.

The railroads were the first to use the high-lead alloys on a general

scale, adopting alloy C5 for a large number of bearings. In airplane

and automobile engines, these alloys are now used for crankshaft, drive-

shaft, and connecting-rod bearings. The most recent development is the

addition of 1 to 2 per cent nickel, or up to 1.5 per cent silver, for con-trolling segregation and sweating of the lead, which is insoluble in copperboth in the molten and the solid state.

9.9. The Tin Bronzes

The most important industrial copper-tin alloys are the phosphorbronzes which contain from as little as 1 per cent to as much as II per


cent tin. All wrought phosphor bronzes are alpha-phase alloys, deoxi-dized by adding enough phosphorus to remove all but a trace of oxygen.The two most commonly used of these alloys (items 51 to 57, Table9.3) are the ones containing approximately. 5 and 8 per cent tin; anotherpopular alloy contains about 10 per cent tin. Owing to their high re-

siliency and endurance limit, these bronzes are used for springs anddiaphragms.

Phosphor bronze is hot worked with difficulty, but cold working is

easy and the alloys are available in six tempers. These, and the range oftensile strengths for each temper and each alloy, are given in Table9.2. The actual tensile strength depends on the thickness of the sheet.Elongation (in 2 in.) varies from 40 to 60 per cent for soft sheet to less

than 10 per cent for the harder tempers. The endurance limit in thehard temper is approximately 25,000 lb. per sq. in., and the modulus ofelasticity is 15 million lb. per sq. in., about the same as that of othercopper-rich alloys.

Table 9.2. Tensile Strength of Phosphor-Bronzc Sheet

TemperReduction

by cold

rolling,

per cent

Tensile-strength range, lb. per sq. in.,

for bronze containing

5% tin 8% tin 10% tin

Soft

Half-hard...

HardExtra hard .

.

Spring

Extra spring

.

20.7

37.1

50.0

60.4

68.6

40,000 to 55,000

55,000 to 70,000

72,000 to 87,000

84,000 to 98,00091,000 to 105,000

96,000 to 109,000

53,000 to 67,000

69,000 to 84,000

85,000 to 100,000

97,000 to 112,000

105,000 to 118,000

110,000 to 122,000

58,000 to 73,000

76,000 to 91,000

94,000 to 109,000

107,000 to 122,000

115,000 to 129,000

120,000 to 133,000

Another important copper-tin alloy-which is used in the cast condi-tion-is one containing a small amount of zinc. This alloy, known vari-

ously as government bronze, zinc bronze, or more commonly as gunmetal (in the late Middle Ages it was used for casting cannons) , contains88 per cent copper, 10 per cent tin, and 2 per cent zinc-or 88 per centcopper, 8 per cent tin, and 4 per cent zinc. It is cast 'readily in thefoundry, has a red color, and combines fine grain with considerablestrength, toughness, and resistance to salt-water corrosion. Gun metal is

used for joints and fittings, bolts, nuts, valve and pump parts, bushingsand bearings for steam lines, and especially in naval construction. Thetensde strength of cast and unmachined specimens varies from 30,000 to45,000 lb. per sq. in., the yield strength from 13,000 to 15,000 lb. per sq.


in., and the elongation (in 2 in.) from 15 to 40 per cent. Impact re-

sistance is high. The modulus of elasticity is 15 million lb. per sq. in.

9.10. Aluminum Bronze and Copper-Silicon Alloys

The copper-rich copper-aluminum alloys combine the strength of amedium-carbon steel with high resistance to corrosion by the atmosphere,

by salt water, by a large number of neutral and acid salts of low con-

centration, and by sulphuric acid. They are more resistant than manga-nese bronze to salt water and have good strength and ductility. Thesingle-phase commercial alloys contain 10 per cent or less aluminum. It

was noted in § 9.2 that, if the aluminum is higher than this, the structure

and properties can be changed by thermal treatment. This is also true of

the complex alloys containing 5 to 10 per cent aluminum and several

per cent of iron, nickel, or manganese. By controlling the compositionand by heat treatment the tensile strength of the wrought alloys can be

varied from 65,000 to 125,000 lb. per sq. in., with elongations varying

inversely with the tensile strength from 5 to 70 per cent. The composi-tion and typical properties of alloys in common use are given in Table9.3, items 60 to 63.

Aluminum bronze is hard to handle in the foundry. The alpha-phase

alloys can be hot and cold worked readily, but they are hard to machine.

Owing to its corrosion resistance, its strength and toughness, and its lowcoefficient of friction against steel, aluminum bronze is used for gears,

valve stems, bolts and nuts for propellers, and for various small parts;

as cast, it is used for gun mounts, gears, bearings, propellers, pump parts,

and ornamental pieces, especially for service in salt air and salt water.

The copper-silicon alloys, usually known as bronzes, of which some 20

to 25 are available under such brand names as Duronze, Olympic, Ever-

dur, Herculoy, and others, contain 90 to 98 per cent copper, 1 to 4.5 percent silicon, and small amounts of zinc, iron, or manganese, or—if easy

machinability is desired—0.25 to 0.50 per cent lead. Strictly speaking,

the copper-silicon alloys are copper strengthened by 1 to 5 per cent of

other elements. The strength and ductility of these alloys are approxi-

mately the same as those of low-carbon steel, and the corrosion resistance

and ease of hot and cold working are about the same as those of copper.

Strength as well as ductility increases as the silicon increases up to about

4 per cent. This is shown by the typical properties in Table 9.3 (items

66 to 69) . Thermal conductivity and electric conductivity are 8 to 10

per cent of the corresponding values for copper, and the modulus of

elasticity is 15 million lb. per sq. in.


The principal advantage of the copper-silicon alloys is their corrosion

resistance in both marine and industrial atmospheres, in sea water andin many industrial waters, in sulphuric and hydrochloric acids, sulphatesand chlorides, and in most alkalies and alkali salts. They are also re-

sistant to alcohol and to many organic acids but are readily attacked bynitric acid and other strong oxidizing agents. The alloys can be used for

working stresses up to 10,000 lb. per sq. in. at temperatures of 250°F.(120°C.) or below, and they can be readily welded by oxyacetylene,

metallic-arc, and resistance methods.

The copper-silicon alloys are now used extensively for bolts, screws,

rivets, boilers, tanks, equipment for laundries and sewage disposal, parts

for fans, ducts, and other ventilating and air-conditioning equipment,chemical and electrical equipment, containers for gases and chemicals,

and for many other purposes.

9.11. Copper-Beryllium Alloys

Copper-beryllium alloys are a development of the past twenty-five

years. Their strength and endurance limit, after age hardening, are highwhen compared with other copper-rich alloys. This, combined with acorrosion resistance that is practically the same as of high-purity copper,

and comparatively high electric conductivity, would be responsible for

fairly extensive use if the cost were lower. Beryllium metal, which cost

$1,000 a pound in 1927, had been reduced to less than $40 by 1937, andit is estimated that the price could be lowered further to S5 to $10 apound if there would be more demand for the metal. Assuming, how-ever, that the cost of beryllium is $15 a pound, an alloy of 97.5 per centcopper and 2.5 per cent beryllium would cost between $0.55 and $0.75

a pound in ingot form, which is still too high for extensive commer-cial use.

At present, copper-beryllium alloys contain 1.85 to 2.25 per centberyllium. To some 0.35 per cent nickel or cobalt is added to refine thegrain. The alloys can be hot and cold worked, but not easily, and areavailable as sheet, rods, wire, and tubes. As indicated, they respond to

precipitation-hardening treatments. If hot worked or welded, they aregiven a solution treatment by heating to 1425 to 1475°F. (775 to 800°C.)for 30 min., followed by quenching in water. The alloys are usually coldworked after the solution treatment. They are aged at about 600°F.(315°C.) for 1 to 4 hr. depending upon the properties desired. Typicalproperties are as follows:


Table 9.3. Composition, and physical

NOTE: The tabulated values arc average and, because of manufacturing limitations, should not be utei for•petiftcation purpoui. Within manufacturing limitations and when «o tpceifitd, these products willbe manufactured to applicable current ASTM Specifications. |s

l!Nominal Composition, per cent

Item NameNo.

Copper Zinc Tin LeadAlumi-num

Phos-phorus

Man-ganese Others

<

C0PPER81. Electrolytic Tough Pitch Copper-100 99.9+

99.9+

99.9

99.

9

99.6899. JO99.00

20

20

20

20

208080

0.02

3. Silver Bearing Coppcr-112 10 to IS oz. Silverper Tont

25 to 30 ot. SilverPit Ton'

Arsenic, 0.30Tellurium, 0.50.

.

4. Silver Bearing Copper-113

6. Phosphorlzed Arsenical Copper-108 0.028. Tellurium Copper-127

1.00

BRASSES8. Oilding-4 95.00

90.00

87.50

85.00

80.00

70.00

66.00

63 00

60.00

5.00

10. 00

12.50

15.00

20.00

30.00

34.00

37 00

40.00

20

20

20

30

10

30

30

40

40

14. Yellow Brass-59

LEADED BRASSES17. Leaded Commercial llronzo-201 90.00

88.5085.0066.5066.50(10.50

66.0065.0064.5004.5064.0062.5061.5061.50

9.50 0.502.251.750.250.501.601.000.250.501. 002.002.503.251.800.601.002.002.50

509.25 90

19. Hardware Bronze-267 13.2533.2533.0031.9033.0034.7535.0034.5034.0035.0035.25.SvTil

90SO. Leaded Tube Brass-220 5021. Leaded Tube Brass-218 6022. Fre- Cutting Tube Drass-282 90

7023. Leaded Tube Brass-25724. Threading Brass-223 50

607020. Medium-Leaded Brass-229

27. High-Leaded Brass-235 8028. Extra-lllgh-Leaded Hrass-238 10029. Freo Cutting Brass-271 10030. High-Leaded Brass-243 90

6070

31. Leaded Muntz Metal-274 60.00 39 in

32. Free Cutting Muntz Metal-293 60.0060.0056.00

39.0038.0041.50

33. Forging Brass-2S0 8034. Architectural Bronce-280 80

1 0.034 to 0.051% Silver. •0.085 to 0.102% Silver.

NOTESThe values given are to be considered only as a general guide, and variations In properties due to manufacturing variables

must bo expected.Higher values for strength and hardness arc obtained by a greater amount of cold working but are limited In general to rela-

tively narrow sheet, small-diameter rod and wire, and smalt-diameter thin-wall tubo. Wide sheet and plate, large-diameterrod, and large sizes of wire and tube have. In general properties between those given for hard and soft tempers,

(a) These relative vbIucs are given in comparison with Free Cutting Brass-271, arbitrarily given a value of 100, and areonly an approximate Indication, there being no ready means for accurately comparing the machinablllty of metals.They arc a reasonable guide to relative tool life and amount of power required for cutting.

Copper and Copper-Base Alloys

properties of standard copper alloys

179

= o>.

•3^-Elongation, Yield Strength > .

5 fc 5*>Tensile % in 2 In. @ 0.5 % elon- Rockwell r, g ^Form(b)

ApplicableASTMSpecifi-

Strength, psi. ( Unlessothe wisenoted)

gation underload, ]>si.

HardnessNo. 1

O <5 o

•ciZ-n

cations - -;==£l£89

oa

5

i

iz v a.13

1

Mis?£Hard(c)

Soft Hard(c)

Soft Hard(c)

Soft Hard(c)

Soft mi(SheetJRodiWire

B152 4fi. 000 33,000 8 35 40,000 10,000 B51 F35)B124, B133BI. B2, B3

45.00066,000

32,00035,000 1(e)

4535(e)

40.000 10.000 B50 F35 ! 1083 0.322 .000009b 101. c 226

B42, B75 '1

'

(Tube

Sheet

B88, Blll.lt

B302B152

45,000 35,000 10 45 40.000 10,000 B50 F40 1083 0.323 .0000098 85.0 197

46,000 33,000 5 35 •10,000 10,000 B51 F3 I 1083 0.322 .0000098 100.5 223

Sheet B152 46,000 33,000 5 351 40,000 10.000 B5I F35 1083 0.322 .0000098 100. f 223

TubeRod

B75. BillB301

45,000 35,000 10 45 40,000 10,000 B50 F40 1080 0.323 .0000098 45.0 11245,000 32, 000 18 45 40,000 10.000 B50 F35 1070 0.323 .0000098 90.0 215Rod 45,000 32,000 12 45 40,000 10.000 B50 F35 1079 0.323 .0000098 99.0 222

Sheet B36 55,000 35,000 6 38 44,000 11.000 B61 F45 1065 0.320 .0000101 56 139/Sheet\WIrc

B36, B130 62.000 37.000 6 40 47,000 12,000 B70,

B,.|

B3B134 80, 000 38, 000 J(e) 40(e) 1045 0.318 .0000101 43.0 108

Sheet(Sheet

65.000 38,000 r. 42 50,000 13,000 B73 1035 0.317 .0000102 40.0 100B36 69,000 40,000 7 45 55,000 15.000 B76| B5|(Wire B134 88,000 42,000 Kc)

1042(c)50(Tube B-13. Bill,

B13569,000 40.000 55,000 15,000 B76 1)5

j

1025 0.316 .0000104 37.0 92

(Sheet B36 73,000 43,000 8 50 00,000 16,000 B81 B10IWiro B134 100,000 47,000 1(e) 45(e)

f

iooo 0.313 .0000106 32.5 81(Sheet B19, B36 76,000 47.000 10 65 62,000 17,000 B83 'B20i(Wire 1)131 110.000 50,000 1(e) 50(e) 1 955 0.308 .ooooiii 27 70.Tube B135 76,000 47.000 10 60 62,666 17.000 B83 B20Sheet(Rod(Wire

B36 73,000 45,000 10 60 60,000 17,000 BS0 BIS 930 0.306 .0000112 26.0 6965,000 46,000 20 60 50,000 17,000 B75 B20'

B134 105,000 50,000 1(e) 50(e) 920 0.305 .0000114 26.0 69Sheet 54.000 45 20,000 B«' 905 0.303 .0000116 28.4 73

SheetRod

B121 62,000 37,000 6 40 47,000 12,000 B70 Bl 1045 0.318 .0000101 42.0 104B140 51,000 37.000 IS 40 45,000 12,000 B58 Bl 1040 0.319 . 0000102 42.0 105Rod BI40 52.000 40,000 20 45 43.000 15,000 B.V, B5 1025 0.317 .0000104 311.0

28.093Tube

TubeB135 73.000 45,000 10 55 60,000 17.000 B80 B15 935 0.307 .0000112 68B135 73,000 45.000 10 55 60.000 17.000 B80 1)1.' 935 0.307 , , v Q] | g 26.0

26.068Tube

TuboSheetSheet

B135 73.000 45.000 60,000 17,000 B80 B15 935 0.308 0000113 6873,000 45,000 9 55 60,000 17,000 B80 B15 935 0.307 0000112 26.0 68

iii2i

" 73,000 45,000 10 55 60,000 17,000 B80 B15 930 0.306 0000113 20.0 6873,000 45.000 9 55 60,000 17,000 B80 B15 925 0.300 0000113 26.0

26.068Sheet

SheetSheet

B121B121

73,000 45,000 S 55 60.000 17.000 B80 B15 923 0.306 on mis 68i3.W»l 45,000 8 52 60.000 17.000 B80 B15 920 0.307 00OO1I3 26.0 683121 73,000 45,000 / 50 60,000 17,000 B80 B15 905 0.307 0000114 26 68Rod B16 58,000 47,000 18 60 42,000 18,000 B70 B20 895 0.307 0000114 25.526.0

62SheetSheet

B121 73,000 45,000 7 50 60,000 17,000 B80 B15 908 0.305 0000113 68B171B13SB124

54,00054,000

„.... 4540 6o,'66o'

20,00020.000 B86

" B45B45

900900

0.3040.304

00001160000116

20.026.0

TubeRodRod

80.666 1

6868

J

54,000Ml rm

4525

20.000 BI5B65

895

8840.305 0000115 26.5 82

1

0.305 0000116 28.0 7i

Courtesy American Brass Co.

NOTES (Continued)

(b) SS&r ?SSmES&VSST Sl"*t"

'M0 ta " thleki Rod ' '" ia ' dlttraeter: wlro'008° ta - d,ametcr; Tub,, '

'

"'•

(c> grantsERRsrâwasssssr-* reduowi about 37% in thioknoss by »m rm^- and

r< J:!PClrlc

? 1 conductivity: given for soft annealed metal; in hard temper conductivity Is slightly lower.(e) Klongation of wire, percent in 10 in.

»-»-•..

YOUX 1'S MODULUS OF ELASTICITY for copper is about 10,000,000 psl. The addition of zinc to copper reduces themodulus, to about 14,000,000 psl. for Yellow Brass. Pot most bronzes, the value is about f°WOOO0" TheEdition of nickelto copper Increases the modulus, Cupro Nickel, 30%-702 showing about 20.000,000 psi.»»•»>"> ""™"™ ™ ™™>


Table 9.3

—

Continued.

NOTE: The tabulated values are average and, because of m«nnhctoring limitations, should not be used torspecification purposes. Within manufacturing limitations and lehen so specified, these products willbe manufactured to applicable current ASTM Specifications.

>

Nominal Composition, per cent 11

$9ItemNo. Name

Copper Zinc Tin LeadAlumi-num

Phos-phorus

Man-ganese Others

SPECIAL BRASSES94.97

90.2588.0085.0082.0081.0077.00

71.00

70.00

60.00

60.0060.0060.0058.50

4.00

0.9010.0014.0017.0018.0020.98

27.96

29.00

39.25

39.2538.5537.6039.25

1.00

1.75

0.03

0.10

2036. High Strength Commercial Bronze-

28637. Ambronze-421

Nickel, 1.00 8020

38. Manganese Red Brass-507 1.00 3039. Silicon Red Hrass-102740. Trumpet Brass-43541. Ambraloy-927 (Aluminum -Brass).. .

1.002.00

Silicon, 1.00

Arsenic, 0.04

303030

42. Arsenical Admlralty-439 1.00 30

1.00 30

44. Naval Brass-450 0.75

0.750.750.751.00

30

45. Tobin Bronze"-452 306.701.76

5047. Leaded Naval Brass-612 80

0.25 Iron, 1.00 30

PHOSPHOR BRONZES49. Phosphor Bronze-356 98.70

98.24

95.9595.7595.17

94.75

93.9093.20

91.75

89.75

87.90 4.00

1.251.754.004.004.00

5.00

5.006.50

8.00

10.00

4.00

0.050.01

0.050.250.08

0.25

0.100.30

0.25

0.25

0.10

2050. Phosphor Bronze-361 2051 . Phosphor Bronze, (A )-302 2052. Phosphor Bronze, (A j-303 20S3. Phosphor Bronze-314 0.25 Iron, 0.50 20

54. Phosphor Bronte, (A )-351 20

65. Leaded Phosphor Bronze, (B)-379. ..

.

1.00 SO20

57. Phosphor Bronze, (C)-353 20

58. Phosphor Bronze, (D )-354

4.00

20

59. Free Cutting Phosphor Bronze-610. .

.

90

ALUMINUM IBRONZE8,

60. Ambraloy-901 95.00

92.00

89.25

82.00

5.00

8.00

9.26

9.50

20

61. Ambraloy-928 20

62. Avlalitc'-915

63. Ambraioy-917

0.40

1.00

Nickel, 0.50;

Iron, 0.60Nickel, 5.00;

Iron, 2.50

20

20

' Trade-Mark Reg. V. S. I'at. Off. t 1.25" and over.


must be expected.Higher values for strength and hardness arc obtained by a greater amount of cold working but arc limited in general to

relatively narrow sheet, small-dlamcler rod and wire, and small-diameter thin-wall tub*;. Wide sheet and plate, large-diam-eter rod, and large sizes of wire and tube have, In general, properties between those given for hard and soft tempers,

(a) These relative values are given in comparison with Free Cutting Druss-271, arbitrarily rfiven a value of 100, and areonly an approximate Indication, there being no ready means for accurately comparing the machinabllity of metals.They are a reasonable guide to relative tool life and amount of jwwer required for cutting.


= ~Q5%

i?£*Êlongation, Yield Strength • 3h

Tensile % In 2 In. @ 0.5 % elon- Rockwell 5 —Applicable Strength, psl. (Unless gation under Hardness

1^S-c

Korm ASTM otherwise loud, psl. No. ii 1..a

8*j«n(b) Specifi- noted 1 J3 £ £ J.

E = S

cations a. °fi§£KM*

_M R

S1a

CO i

fcjiHard(0)

Sort Hard(e>

Sod Hard(c)

Soft Hard(o)

Soft

*3

2 >S ttti

Sheet no, ooo 40,000 6 40 50,000 15,000 B70 BS 1060 0.320 .0000101 38.0 95

Rod 70,000 12 60,000 B80 0.3200.316

32.027.0

8169Tube 46,000 58 20,666 B15 1030 .0000102

Sheet 69,666 40,0001 7 46 55,000 15,000 B76 135 1015 0.316 .0000104 22.0 57

Sheet 90,000 55,00048,00052,000

8 60 50,000 20,000 BOO B35B25B30

9901005.

970

0-310.0.3130.301

.0000105. 11000 10S

.0000108

14.727.522.6

397058

TubeBUI

80,00085,000

1010

6065Tube 60,000 20,000 B85

(Sheet\Tube

B171Bill

48,00086, 000 52, 000 io

6565

18,00020,000

B25\B30I

935 0.308 .0000112 24.7 64

Sheet B291 7(1,000 47,000 10 65 62,000 17,000 B83 B20 945 0.308 .0000111 16.0 431Sheet

IRodB171B21. B124 63.000

56,00056,000

22.00022,000 B65

B501B50f 885 0.304 .0000119 25.830 40 35.000 67

Rod B21, B124 63,000 56.000 35 45 35.000 22.000 B65 BSD 885 0.304 .0000117 25.8 67Rod B21 63.000 56,000 28 38 35.000 22,000 B6S B50 890 0.305 .0000119 26.0 67Rod B2I, B124 63,000 56,000 2i 35 35.000 22,000 B66 B50 l 890 0.305 .0000118 26.0 67Rod B124, B138 75, 000 60,000 20 30 45,000 30.000 B85 890 0.302 .0000119 23.0 58

Sheet 65,000 40,000 6 48 50.000 14,000 B75 F60 10751070

0.3210.321

.0000099

.000009943.037.8

126

85Wire BIOS 105,000 45,000 UO 40(c)Sheet moo 80,000 48,000 8 SO 65.000 20,000 B86 B28 1050 0.320 .0000099 18.4 47Tube 70,000 48,000 15 50 58,000 20,000 B80 B2S 1050 0.320 .0000100 18.0 47Rod B139 (15,000 48,000 30 50 55,000 20,000 B75 B28 1055 0. 322 .000010 16.4 44[Sheet 11103 80,000 48,000 8 50 65.000 20,000 B86 B281{Rod B139 65,000 48,000 30 50 55,000 20,000 B75 B28} 1050 0.320 .0000099 18.0 47(Wire III.'.9 110.000 52,000 1(e) 40(c)

1

Rod B139 65.000 48.000 25 40 55,000 20.000 B75 B25 1050 0.322 .0000099 18.4 48Wire 120,000 57,000 He)

1040(e)65

1040 0.319 .0000100 12.0 33(Sheet B103 93,000 00,000 68,000 24,000 B94 B50Rod B139 80,000 60,000 30 AS 24,000 BflOl 1025 0.318 .0000101 13.0 30iwire B159

B103130,000102,000

62,00006. 000

He)12

40(c)65Sheet

[Rod70,000 28,000 B98 B55

B139 85,000 65,000 25 65 1000 0.317 .0000102 11.0 29.Wire B159

B139145.00060,000

68,000 He)20

40(c)

1000 0.320 .000010 12.2Rod 45,000 B76 33

Sheet B169 92,000 55,000 7 65 65,000 22,000 B92 B3.ilRod 55,000

60,00065.000

6560

22,000 B35j 1060 0.295 .0000099 17.0 46TubeSheet

BillBI69 105,000 7 60 65.000 25,000 BM BSOl

Rod BI24, B150 80,000 65,000 30 65 50,000 25.000 B50!101(1 0.281 .0000099 14.8 41

Rod 1)124, B150B171B124, B150

95,000

ios.ooo

80,00090,000

16

12

2212

55.000 40,000 1042

1055

0.274

0.274

.0000094

.000009

12.8 85(PlateRod

1

60.000 BIOS } 7.S 22

NOTES (Continued)

(b) Form for which properties are given: Sheet, 0.040 in. thick; Rod, 1 in. dlumeter; Wire, 0.080 in. diameter; Tube, 1 In.diameter x 0.065 In. wall thickness.

(c) Hani temper: values arc for soft sheet that has been subsequently reduced about 37% In thickness by cold rolling; andfor Rod, w ire and Tube of commercial hard drawn temper.

(d) Electrical conductivity: given for soft annealed metal; in hard temper conductivity is slightly lower.(e) Elongation of wire, per cent In 10 in.

YOUNG S MODULUS OF ELASTICITY for copper is about 16,000,000 psl. The addition of zinc to copper reduces themodulus, to about 14,000,000 psi. for Yellow Brass. For most bronzes, the value is about 15,000,000. Tho addition of nickel tocopper Increases the modulus, Cupro Nickel, 30%-702 showing about 20,000,000 psl.


Table 9.3

—

Continued.

NOTE. The tabulated values are average and, because of manufacturing limitations, should not he met lorspecification purposes. V Itliin manufacturing limitations and when so specified, these products willbe manufactured to applicable current ASTM Specifications.

•22

Nominal Composition, per cent II

Item NameNo.

Copper Zinc Tin Lead NickelAlumi-num Silicon

Man-ganese Others

MCADMIUM BRONZES

64. Hltenso'-901 99.00

98.60

Cadmium, 1.00...

Cadmium, 0.80.

20

2066. 11 ltenso--965 0.60

COPPER-SILICON ALLOYS

66. Everdur"-1010 95.80

95.60

98.25

90.75

3.10

3.00

1.60

2.00

1.10

1.00

0.26

30

60

30

60

67. EvordurM012 0.40

68. Everdur'-1015

69. Evcrdur'-IOM 7.25

CUPRO NICKELS70. Cupro Nickel, 10%-755 88.35

68.90

10.00

30.00

0.40

0.60

Iron, 1 25 20

2071. Cupro Nickel, 30%-702 Iron. 0.50

NICKEL SILVERS72. Nickel Silver, 10%-76I 65.00

64.50

55.00

45.00

25.00

17.50

27.00

42.00

10.00

18.00

18.00

10.00

20

73. Nickel 8llver. 18%-71920

2074. Nickel Silver, lS%-724

7S. Leaded Nickel Sliver, 10%-«25 1.00 2. CO 60

8PECIAL ALLOYS76. Calsun Bronze*-951 95.50

99.05

2.00 2.50 20

2077. Chromium Copper-999 0.10 Chromium, 0.85.

.

• Trade-Mark Reg. \>. 8. Pat. Off.


Higher values for strength and hardness are obtained by a greater amount of cold working but are limited In general to

SSfSR SfEZi SS H&t^fffiS^ ""f "** ft1d srooll-dlamcter thin-wall tube. Wide sheet and plate largeân,

eter rod, and large sizes of wlro and tube have. In general, properties between those given for hard and soft tempers(a) These relative values are given in comparison with Free Cutting Brass-271, arbitrarily given a value of 100,' and are

only an approximate indication, there being no ready means for accurately comparing the macliinabllity of metalsThey are a reasonable guide to relative tool life and amount of power required for cutting.


i "3 § JSd"-Elongation, Yield Strength

6"31*.

2---

Tensile % in 2 in. @ 0.5 % elon- Rockwell o i. *Applicable Strength, psi. (Unless gation under Hardness

S. = 2 ISform ASTM otherwise loud, psl. N 0. a w;(1)) Specifi- noted) o JS SS o' 3 S~cations & -#=§K _«

6fiC >,

Hard Soft Hard Soft Hard Soft Hard Soft

"3

s Q •<3wS.St"*

(c) (c) (c) (o)

IShcet\Wirc B105

B105

55.00090.00095,000

37,00040.00042. 000

1(e)1(c)

5040(e)40(e)

48.000 12,000 B65 F471 1076

1070

0.321

0.321

.0000098

.0000098

87.0

60.0

199Wire

13S

Sheet 1)96, B97 95.000 58,000 7 60 60,000 22,000 B92 B35'.Rod B98, B124 90,000 58.000 18 70 60,000 22,000 B90 B35 1019 0.308 .0000100 6.5 19Wire B99 145.000 60.000 Me) 50(e)

IRod B98 90.000 18 60,000 1)90 1019 0.308 .0000100 6.5 ISSheet B97 65,000 40,000 8 46 50.000 15.000 B77 F55Rod B98 70,000 40.000 IS 50 55,000 15,000 1)80 F55

' Wire B99, BIOS 122,000 42,000 Ke) 40(c) 1055 0.316 .00000911 11.0 31Tube 65,000 40.000 8 50 50,000 15,000 B71 F55Rod BJ24, BlW 95,000 25 53.000 B87 1005 0.278 .000010 9.2 as

Tube Bill 60,000 44,000 15 40 57,000 22,000 BOS B25 U45 0.323 .0000093 9.2 261SheetRod

1 Wire

B122, B171 77,000 55, 000 5 40 70,000 22,000 B84 B3570, 000 55, 000 25 40 60,000 22.000 B80 B35

BUI95.00070.000

58,00055,000

Ke)10

35(e)45 B80 B35.

1225 0.323 .0000090 4.6 17

iTuhe 60,000 22, 000

Sheet B122 88,000 55,000 7 42 70.000 20,000 B87 B30 1010 0.313 .0000091 8.4 27Sheet B122 85.000 58,000 4 40 70.000 B85 B40Rod lll.il 70.000 58,000 20 45 22.000 B40 1110 0.316 .0000090 6.0 IBWire B206 110.000 60,000 Ke) 40(C)Sheet B122 99,000 00,000 4 45 75,000 22,000 B93 B45Rod B151 80,000 60,000 20 45 60. 000 22,000 B85 B45 1055 0.314 .0000093 5.3 17Wire B206 130, 000 65,000 Ke) 40(0)Rod 70,000 IS 40,666 B70 925 0.303 .000011 7.0 21

Wire B105 135, 000 52, 000 Ke) 40(e) 1054 0.308 .0000099 19.0 50•Rod\Rod (()

02, 000 35, 00072,000 • 03.000

2025

4025

57,66661,000

15.00045.000

B70B77

FgqB65

1075

10750.3210.321 .0000098

40.0 u»80.0 187

NOTES (Continued)

(b) Form for which properties are given: Sheet, 0.040 in. thick; Rod, 1 in. diameter; Wire, 0.080 in. dia ncter; Tube, 1 In.diameter x 0.065 in. wall thickness.(c) Ilard lemper: values arc for soft sheet that has been subsequently reduced about 37% in thickness by cold rolling; and

for Rod, \\ Ire and Tube of commercial hard drawn temper.(d) Electrical conductivity: given for soft annealed metal; In hard tempor conductivity is slightly lower.(e) Elongation of wire, per cent in 10 In.

(f) Properties after heat treatment (alloy 77).YOUN'O'S MODULUS OF ELASTICITY for copper is about 16,000,000 psi. The additon of line to copper reduces the

modulus, to about 14,000,000 psl. for Yellow Brass. For most bronrts, the value Is about 15,000,000. The addition of nickel tocopper increases the modulus, Cupro Nickel, 30%-702 showing about 20,000,000 psi.


Typical Propcrlics of Copper-Beryllium Alloys

ConditionTensile

strength, lb.

per sq. in.

1

Yield Elongation

strength, lb. in 2 in.,

per sq. in. per cent

Brincll

hardness

Soft

Average treatment

Maximum treatment ....

70,000175,000

193,000

31,000

134,000

138,000

45

6.3

2.0

110340

365

The endurance limit is usually between 35,000 and 45,000 lb. per sq.

in. depending on the treatment. The endurance ratio varies from approxi-

mately 45 per cent for the unhardened alloy to about 20 per cent for fully

hardened material. The modulus of elasticity averages 18 million lb. per

sq. in. In these properties the copper-beryllium alloys are far superior to

any other copper-rich alloy. They also have relatively high wear resistance

and corrosion resistance and are superior to phosphor bronze in resisting

metal-to-metal wear when run against steel. At present they are used

to a limited extent for springs, gears, diaphragms, bearings, and for

other small parts in the electrical and the aircraft industries.

Chromium copper (Table 9.3, item 77) is a heat-treatable alloy

which is extensively used for welding electrodes, grid support rods,

and for lateral wires in electronic vacuum tubes.

A copper-silver-iron alloy containing 6 per cent silver, 1 per cent

iron, copper remainder, is also a heat-treatable alloy used by the Signal

Corps of the U. S. Army for field telephone wire during the Korean war.

9.12. The Copper-Rich Copper-Nickel Alloys

Copper and nickel dissolve in each other in all proportions in the solid

state, and their alloys are free from phase changes. Some of the most

widely used industrial alloys contain from 2 to 30 per cent nickel (the

cupronickels) or 45 per cent nickel (Constantan) . Of the important

nickel-rich alloys, the best known is Monel, containing 68 per cent nickel.

This group is discussed briefly in the next chapter.

Nickel increases the tensile strength, the yield strength, and the en-

durance limit of copper, as shown for annealed alloys in Fig. 9.4.

Even the strongest alloys are ductile and have an elongation of 45 to 50

per cent in 2 in. As noted in the discussion of nickel silvers, the addi-

tion of increasing amounts of nickel to copper changes the red color

to white. A 15 per cent nickel alloy has a faint pink tinge, and a 20 per


cent nickel alloy is practically

white. The five-cent coin of

the United States, the

"nickel," is an alloy of 75 per

cent copper and 25 per cent

nickel.

The most common indus-

trial alloys contain 15, 20, and30 per cent nickel. The 15 and

20 per cent alloys are used for

bullet jackets and for a large

variety of parts in which the

white color, high ductility,

and corrosion resistance are of

value. They were formerly

used for condenser and other

tubes for marine engines andoil refineries, but in these ap-

plications they have largely

been replaced by the 30 per

cent alloy. This material can

readily be hot and cold

worked and can be soldered,

brazed, and welded. It is oneof the most resistant of the

copper-base alloys to salt

water and to many of the cor-

rosive solutions of the chemical industry. The cupronickels are generallyresistant to the atmosphere, to industrial waters, and to many acids andalkalies. The 30 per cent alloy, containing also 1 per cent manganeseand 1 per cent iron, is cast into a large variety of couplings, ells, tees,

and other fittings for valves, pumps, and other similar applications.

The cupronickels do not respond to heat treatment. As in the case of

most solid-solution alloys, high strength can be attained only by coldworking with concomitant loss of ductility. Cold working with a reduc-

tion of 60 per cent increases the tensile strength of a 30 per cent nickelalloy from 50,000 to 85,000 lb. per sq. in. and reduces the elongation from50 to 4 per cent in 2 in.

Some of the copper-nickel alloys have important electric properties.

As shown by Fig 9.5, the electric resistance is at a peak and the tem-

40 SO

Nickel, per cent

Fig. 9.4. Effect of nickel on the mechanicalproperties of copper-nickel alloys (Wise)


perature coefficient of resistance is practically zero in the alloy contain-

ing 45 per cent nickel. This alloy, known as Constantan, has high anduniform thermoelectric force against copper and iron and is used ex-

tensively for thermocouples for the accurate measurement of tempera-

tures below 1800°F. (985°C.)

.

40Nickel, per cent

Fie. 9.5. Effect of nickel on the electrical properties of copper-nickel alloys. (Wise)

It should be evident from the discussion in this chapter that the en-

gineer has available a wide variety of wrought and cast copper-rich alloys

for use in his structures and machines. There are so many of these alloys

that an intelligent choice of a specific one is sometimes difficult, espe-

cially in view of the fact that a large number of them are proprietary

alloys for which extravagant claims are occasionally made.

9.13. Copper and Copper-Base Alloys in Powder Metallurgy

Powder metallurgy is a very old art and a very young science. Sponge

iron was formed into weapons a thousand years ago, but when man dis-

covered how to melt steel, he discarded the more cumbersome methodof powder metallurgy, and the art was lost until relatively recently. Withthe exception of filaments for the electric-light bulb, which have been


made by powder-metallurgy methods for fifty years, and the perfectionof hard carbide tools twenty-five years ago, most of the development in

powder metallurgy has come in the past fifteen years.*

The process underlying the production of powder compacts is wellknown and need not be reviewed here. Broadly it consists or preparingmetal or nonmetal powders of the proper size and physical character-istics, forming them into the desired shape, compacting under highpressure (10 to 100 tons per sq. in.), and sintering at the proper tem-perature. Close tolerances can be maintained and no machining of thefinished part is ordinarily necessary. Despite the high cost of the equip-ment for carrying on the various operations, great savings result in manyapplications, especially for a large number of identical parts that aredifficult or expensive to machine from solid metal.

The properties of a part made from powder depend on the characterof the raw material and upon the pressing and sintering-in other words,on the homogeneity and the absence (or presence) of voids in the finishedpart-and they range from about 50 per cent to practically the sameas for the solid metal or alloy. One of the great advantages of powdermetallurgy is that dissimilar metals, or metals and nonmetals, thatdo not alloy in melting can be readily combined into an article havingunique and often very desirable properties. Of the several millionpounds of copper, for example, that are compacted by the methods ofpowder metallurgy a large proportion goes into porous "oilless

1

or "self-

lubricating" bearings for electrical machinery, automobiles, machines,and household appliances. More than a billion such bearings are in use.

The most common oil less bearingf is made by compacting 87.5 to 90.5per cent copper powder, about 9 per cent tin powder, and about 1.5 percent graphite. These bearings are made with a controlled number ofvoids (by regulating the density) , so that, when installed, they contain15 to 35 per cent impregnated oil; they are, therefore, both bearing andlubricant and, if properly made, will function as such for the life of themachine. Sometimes 3% lead is added for improved properties.

'The first general technical meetings in the field were held at MassachusettsInst.ti.te of Icchnology in 1940 ami 1941. The papers presented were published in

,L°o !LUrgy'" edited by John Wu,ff

'Am"i(an Society for Metals, Cleveland

1942, 622 pp. The first conference of the Powder Metallurgy Committee of the

foTT^KIn* titute of Mini "S a"* Metallurgical Engineers was held in February,

1944. The 12 papers presented were published in Trans. Am. Inst. Mining MetEngrs.,

y. 161, 1945. pp. 524-634. See also ••Seminar on Theory of Sintering" and otherpapers in rrans. Am Inst. Mining Mel. Engrs., v 166, 1946, pp. 474-587. See alsoProceedings of Com. B-9 of A.S.T.M.fA^.T.M. specifications B202-51T.


Copper and copper-base alloys compacted with graphite are also used

successfully for brushes and other parts of motors and generators, to

take advantage of the antifrictional character of the graphite and the

current-carrying capacity of the copper. Depending on the strength re-

quired, the amount of graphite may be 50 per cent or more. Thousandsof contacts for making and breaking electrical circuits are produced by

compacting copper and refractory metal, such as tungsten or molyb-

denum or the carbides of these metals.

The principles underlying the art of producing powder compacts are

not simple. Not only must such variables as particle size, pressure, andsintering temperature be controlled carefully to secure the density, hard-

ness, and strength desired (all of which arc directly related) , but struc-

tural changes resulting from diffusion must also be taken into account.

The structure should resemble that of the solid metal or alloy, the grain

size should be fine, and the voids (if present) should be uniform in

size and distribution. Powders of an alloy may, when compacted, have

a structure and properties different from those resulting when the pow-

ders of the metals are used and alloying is accomplished during sintering.

On the other hand, prealloying may cause increased resistance to com-

pression and result in less dense compacts. Whether the compact is pre-

pared by hot or cold pressing, whether the powders contain gas, whether

they oxidize easily—all of these are important factors and none has been

thoroughly studied.

In general the metallurgy of producing metal parts from powder is in

its infancy and considerable advance can be expected in the next few

years. Other than for the manufacture of hard carbide tools, discussed

in a subsequent chapter, the process has been used successfully in produc-

ing parts of iron, steel, iron phis graphite, steel plus copper, stainless

steel, copper and copper-base alloys, aluminum and aluminum-base al-

loys, and iron-nickel alloys—and the list is growing rapidly. Whereeconomic factors are favorable the future of powder metallurgy seems

secure.

QUESTIONS

1. What are the properties of copper that make it useful commercially?2. What are the composition and properties of common brass?

3. What is Muntz metal? Commercial bronze?

4. Name two methods for making copper alloys free-machining.

5. Define dezincification.

6. What are nickel silvers and their most valuable property?

7. What is intended when the term "high brass" is used in the brass industry?

8. Give the eutectic composition of the copper-zinc alloy system.


9. What is red brass? Admiralty metal?10. What is manganese bronze, and what is its principal use? Is it a true bronze?11. Compare the composition and properties of cast brasses, cast nickel silvers,

and cast manganese bronze.

12. What are the principal metals in the copper-base" bearing alloys? Namesome of their principal characteristics and uses. What are the uses for high-lead, copper-base alloys? Mow do those compare as bearings with the whitemetals?

13. What is the effect of phosphorus on the properties of copper-tin bronzes?What arc the phosphor bronzes? Name the properties and some uses of phos-phor bronzes.

14. What is the effect of varying amounts of cold working and varying amountsof tin on the tensile strengths of phosphor bronzes? What is the approxi-mate composition of gun metal? What are the properties and uses of gunmetal?

15. What is the general composition of aluminum bronze? Indicate some of its

characteristics and uses.

16. Indicate the approximate composition and some of the applications ofcopper-silicon alloys.

17. Indicate the approximate composition, properties before and after precipita-tion-hardening, and applications of the copper-beryllium alloys. What is thereason for their limited use?

18. What is the effect of increasing amounts of nickel on the tensile propertiesand endurance limit of copper? What are the properties of copper-nickelalloys? Name some of the copper-nickel alloys.

19. What general classes of materials may be combined by powder metallurgymethods? Name and give the properties of some articles made by powdermetallurgy methods from copper combined with other materials.

20. How are metal-powder compacts made? What variables determine the prop-erties of articles made by the methods of powder metallurgy?

CHAPTER

10

Miscellaneous HeavyNonferrous Metals and Alloys

William H. Tholke, B.S., in Metallurgical Engineering,

Instructor of Metallurgy, University of Cincinnati,

Cincinnati, Ohio

1 HE preceding chapters on the light metals,copper, and copper base alloys have dealt with the two largest nonferrousmetal industries. Before taking up the complex field of iron-carbon alloys,alloy steels, cast irons and the metallurgy associated with the workingand manufacturing of these materials, it would be well to look briefly

at the remaining commercial nonferrous metals and their alloys, how theyare used, and the part they play in the overall metals picture.

In terms of density or specific gravity most of these metals are heavierthan iron. Table 10.1 lists these metals in alphabetical order with theirrespective specific gravities. Some categorizing may be done in terms of thephysical characteristics of the metals, and the metal elements with whichthey are alloyed. Some of these metals such as Pb, Sn, Sb, Bi, Cd, and Inhave low melting temperatures. The alloys of Pb and Sn are often re-

ferred to as the white metals. Then, there are those metals which havehigh melting temperatures as well as good corrosion resistance. These arethe metals Ni, Cr, Co, W, Mo, and Re and their alloys. A third groupcomprises those metals and alloys which by virtue of scarcity of supplyare known as the precious and semi-precious metals. These arc the metalsAg, All, Pt, Pd, and Rh. And, lastly, a new. group which has come intoprominence through the development of atomic energy. These are theelements Th, Tm, U, and Zr.

190

Miscellaneous Heavy Nonjerrous Metals and Alloys 191

Table 10.1. Miscellaneous Nonfcrrous Heavy Metals and their Specific Gravities

Specific Melting Specific MeltingGravity Temp. °F. Gravity Temp. "F.

Antimony (Sb) 6.62 1167 Nickel (Ni) 8.9 2651Bismuth (Bi) 9.80 520 Palladium (Pd) 12.0 2830Cadmium (Cd) 8.65 610 Platinum (Pt) 21.5 3224Chromium (Or) 7.19 3430 Rhenium (Re) 20.0 5740Cobalt (Co) 8.90 2725 Rhodium (Rh) 12.4 3570Columbian (Cb) 8.57 4380 Silver (Ag) 10.5 1761

or Niobium (Nb)Copper (Cu) 8.96 1981 Tantalum (Ta) 16.6 5425Gold (Au) 1 9 . 32 1945 Thorium (Th) 11.5 3300Indium (In) 7.31 314 Thulium (Tm) 9.35Iridium (Ir) 22.5 4450 Tin (Sn) 7.30 449[Iron (1*0 7.87 28021 Tungsten (VV) 19.3 6170Lead (Pb) 11.34 621 Uranium (U) 18.7 2065

Vanadium (V) 6.0 3150Mercury (Hg) 13.6 -38 Zinc (Zn) 7.13 787Molybdenum (Mo) 10.2 4760 Zirconium (Zr) 6.7 3353

LOW MELTING METALS AND ALLOYS

10.1. The While Metals

The term "white metals" is usually reserved for alloys of lead andtin in which one or both of these metals may comprise the major per-centage of the total alloy content. The remaining alloying elementspresent may be one or more or the following: antimony, arsenic, bismuth,cadmium, copper, indium, silver, and zinc. The white metal alloys areoften more commonly known by their trade names such as pewter, terneplate, babbits, and silver solders. Their applications find greatest usagein the fields of (1) bearing materials, (2) type metals, and (3) low-melt-ing fusible alloys such as the solders and safety fuse plugs.The bearing metals must meet many requirements before they can

qualify as a good bearing material.

These may be divided into three principal classes. First in importanceis proper structure, that is, a soft matrix with sufficient plasticity to con-form to slight irregularities in the machining and alignment of the shaft,to allow any abrasive particles in the lubricant to become embedded inthe bearing metal (to prevent scoring of the steel journal) , and to retainoil (to prevent mctal-to-metal contact) , combined with a number ofuniformly distributed particles which resist wear. Some metallurgists,however, now claim that wear-resisting particles are not necessary forgood bearings and that the structure is of minor importance. Such claims


are strongly supported by the successful use of the steel-backed silver

bearings and by the cadmium bearings.

Second, the mechanical properties must be satisfactorily balanced. Thebearing must resist the shock of impact loads, and it must have sufficient

strength, ductility, and resistance to compression not to crack or to

squeeze out under heavy loads, especially at the operating temperatures,

which may be 200 to 300°F. (95 to 150°C.) and in the modern airplane

even higher.

The third requirement is easy melting and casting and, most important,

good bonding properties. In other words, when cast in a strip as thin as

0.01 in., the metal must adhere firmly to the steel or bronze piece used

as a backing and should not spall off or separate from the backing during

operation. For some purposes, especially for large railroad bearings, it

must be possible to remelt and recast the bearing metal without exces-

sive loss by oxidation.

In addition, the bearing metal should have high thermal conductivity,

resistance to corrosion by the lubricants used, and it should be cheap.

There are four classes of bearing metals. Typical compositions of somecommon alloys in these different classes, with the specification numbersadopted by the American Society for Testing Materials and the Society

of Automotive Engineers, are given in Table 10.2.

The white metals include the tin-base (class A, Table 10.2) and the

lead-base (class B) alloys. The 14 alloys in these two groups and the

numerous unimportant modifications that have been advocated fromtime to time are old and well known. The tin-base alloys, commonlycalled babbitts, are widely used in automobile engines and in a large

variety of machines where speeds are high and loads are light or at most

only fairly heavy. Lead, added (alloys A4 and A5) primarily to reduce

the cost, is objectionable if operating temperatures are high as it forms

a fusible eutectic with tin and lowers the temperature at which incipient

melting takes place. The following illustrates the effect of lead: the tem-

perature at which alloy A5 (18 per cent lead) is completely liquid is

about 565°F. (295°C.) ; the corresponding temperatures for alloys Aland A2 (no lead) are 680 and 670°F. (360 and 355 °C).

The properties of the tin-base alloys vary somewhat with composition

but are usually within the following ranges:

Properly Range of values

Tensile strength, lb. per sq. in., at 70"F. (20°C.) 12.800 to 17,500

Tensile strength, lb. per sq. in., at 200°F. (95°C.) 6,700 to 10,000

Yield strength, lb. per sq. in., 70°F. (20°C.) 4300 to 6500Yield strength, lb. per sq. in., at 200°F. (95°C.) 2.100 to 3,200

Brincll hardness (500 kg.) at 70°F. (20"C.) 17 to 27

Brincll hardness (500 kg.) at 200-F. (95°C.) 8 to 14

Miscellaneous Heavy Nonferrous Metals and Alloys 193

Table 10.2. Typical Compositions of Some Common Bearing Mclals

Specification Nomina] composition, percent

Class A.S.T.M. S.A.E. Sn Sb Pb Cu Zn Cd Others

Al B23-46T1 10 91 4.5 4.52 2 110 89 7.5 3.53 3 83.3 8.3 8.34 4 75 12 10 35 5 65 15 18 26 11 87 6.8 5.7

Bl B23-46T6 20 15 63.5 1.52 7 14 10 15 753 8 5 15 804 10 2 15 83S 11 15 856 12 10 907 15 15 1 15 848 19 5 9 86

CI B144-46TA 792 10 10 80 0.0 to 0.9 P2 B 7 7 83 33 C 5 9 85 1

4 D 7 15 785 E 5 25 706 B22-46TA 19 80 0.0 to 0.9 P7 B 16 83 0.0 to 0.9 P8 791 4 4 89 39 793 4 8 85 310 48 30 70n 480 35 65

Dl2

17P* 99.75+ Ag17C* 1 98.50+ Ag

3

•1

17R* 99.75+ Ag18 98.5+ 1.2 Ni

5

6180 0.6 98.3 + 0.7 Ag

98+ t

* 17P is plated, 17C is cast, and 17R is rolled,

t 0.5% Ca, 0.5% Na, 0.1 % Li + Al.

In the case of these alloys, the "tensile strength" is the stress producinga deformation of about 25 per cent, and the "yield strength" is the stress

producing an elongation of 0.125 per cent of the gage length.

The lead-base white metals (class B, Table 10.2) are cheaper than thetin-base alloys but are not so satisfactory for operation at high speeds and


medium loads. They are used extensively by the railroads for freight- andpassenger-car bearings, in electric motors, and in some machines. Alloys

B2 and B4 are used in a number of bearings in automobiles. The proper-

ties of this group of alloys also vary with composition, usually within the

following ranges:

Property Range of values

Tensile strength, lb. per sq. in., at 70"F. (20»C.) 12,800 to 15,500Tensile strength, lb. per sq. in., at 200°F. (95°C). .' 5.00 to 8,000Yield strength, lb. per sq. in., at 70-F. (20"C.) 2,800 to 3.800Yield strength, lb. per sq. in., at 200°F. (95-C.) 1,200 to 2.000Brinell hardness (500 kg.), at 70»F. (20"C.) 15 to 22Brinell hardness (500 kg.), at 200°F. (95«C.) 6 to 11

It is evident from these values that the lead-base alloys are inferior in

properties to the tin-base metals.

As the temperature increases above 200°F., the tensile and the yield

strength of the white metals decrease still more; values of 2,000 to 4,500

lb. per sq. in. are common for tensile strengths at 300° F. (150°C.)

.

Elongation and reduction of area increase with temperature as, for ex-

ample, from 15 to 30 per cent and from 18 to 40 per cent respectively.

The constitution of the white-metal alloys is complex and is imperfectly

known. Essentially, the structure (Fig. 10.1) consists of a solid-solution

matrix in which needles or. cubes of a hard constituent, probably an inter-

metallic compound such as SbSn, are embedded.

Considerable work has been done recently to determine the effect of

small amounts of cadmium on the tin-base alloys. Apparently, 1 per cent

cadmium increases the tensile strength and hardness 10 to 20 per cent at

normal temperature and at 200 to 300°F. (100 to 150"C.)

.

The copper base bearing materials as listed in class G of Table 10.2

have found wide application particularly in the automotive and other

transportation fields. The leaded bronzes (alloys of copper, tin, andlead) in general are slightly superior to the white metals; their melting

temperatures are much higher and their properties less affected by the

operating temperature range encountered by bearings of this type. Theyalso have better fatigue life which is affected by repeated flexure.

Recent trends in bearing materials have been in the direction of silver

or cadmium based alloys, the oilless bearing as made by powder metal-

lurgy, and aluminum base alloys with tin as one of the principle alloying

elements. For a more complete picture of bearing alloys, their composi-

tion and properties, the reader is referred to the ASM Metal's Handbook.The type metals are a series of lead based alloys with minor amounts of

tin and antimony which arc used in the printing industry. The choice of


Fie 10.1. Structure of tin-base bearing metals (A) containing 86 per cent tin,6 per cent antimony, and 8 per cent copper; and (B) containing 82 per cent tin, 10per cent antimony, and 8 per cent copper: etched. 150x- (Courtesy of O. W. Ellis)

alloy depends on the printing method used. In linotype machines it is

advantageous to have an alloy that freezes at nearly a constant tempera-ture, hence the use of the ternary eutectic mixture of 84% lead, 12%antimony, and 4% tin. This alloy freezes at 462°F. and makes possibleclose temperature control as well as good delineation of the type charac-ters in the cast strip. In other printing methods a harder alloy may benecessary and this may be obtained by adding copper in small amountsto the type metal or by solution heat treating and precipitation harden-ing an alloy which has the necessary amounts of tin and antimony.The artificial aging operation often times can be accomplished in onehour at 185°F. or 1 minute at 212°F.

These alloys are ideal for these applications since they must have(1) low melting temperatures, (2) low cost, and (3) good casting charac-teristics. Tin imparts to the alloy fluidity which makes for ease of casting,and the antimony increases the hardness and decreases contraction duringfreezing. Both elements together lower the melting point of the alloyand makes possible precipitation hardening if needed.The fusible alloys are those which are noted principally for their

low melting temperatures usually in the range of 125 to 450°F. Thesolders, which are primarily mixtures of Pb and Sn, could be included


in this category; although their melting range is usually considered as

being from 360 to 570°F. However, when alloying elements such as

antimony bismuth, cadmium, and Indium are alloyed with lead andtin, the melting temperature of the resulting alloy may be as low as 1 I7°F.

These alloys are used in the vital parts of safety mechanisms in fire andexplosion safety equipment. They are alloyed to melt or yield at

specified temperatures thus releasing fire doors or sprinkling equipment.However, caution should be exercised in using these alloys where static

tensile loads are applied. Since these alloys have low melting tempera-

tures, they similarly have low recrystallization or cold working tempera-

tures. Hence, most of these alloys will under go creep at room tempera-

tures under static loading. Other uses for these alloys include patterns in

precision casting methods, forming dies for aluminum and magnesium,filler material inside thin walled tubes during forming operations, cores

and forms on which other metals may be electroplated, and safety plugs

in tanks and cylinders of compressed gas.

10.2. Lead and Tin as Engineering Materials

While the preceding discussion has dealt with the alloys of lead andtin, both metals are used in considerable quantities in commercially pure

form.

Lead is a soft metal (tensile strength of 2,000 psi) which is easily

formed by most fabricating methods. Part of this ease of fabrication re-

sults from the crystal structure of the metal which is face centered cubic,

and part from the fact that it will recrystallize at room temperature.

Thus all forming operations at room temperature results in hot working

the lead. Aside from its formability, lead is primarily noted for its cor-

rosion resistance both to atmospheric conditions and to the acids sulphuric

and hydrofluoric. As such it is used in relatively pure form as sheet andpipe in plumbing and building applications, in acid plants, and in the

handling of chemicals. Lead or its alloys is rarely used in strength appli-

cations where static loads exist, since it will undergo creep at roomtemperatures. The principal uses for lead are (1) in storage batteries (2)

as tetraethyl lead in gasolines (3) as a cable sheathing material (4) as

alloying element in solders and (5) as a building material. This is in the

order of the total yearly amounts used. Lead may be strengthened byalloying with small amounts of cither antimony or calcium which pro-

duces a precipitation hardenablc alloy. The resulting room temperature

tensile strengths are as high as 12,000 psi.

Terne Plate, which is a lead based alloy containing 15 to 25% tin hot

dipped onto sheet steel, is a competitor to zinc-coated products. It has


the good corrosion resistance of both lead and tin and the bright ap-pearance and formability of tin plate at a much lower cost than tin plate.

Tin in the commercially pure form has essentially only one application,the tin can. This involves depositing a thin layer of tin uniformly overa steel sheet by either hot dipping or electroplating. The latter, whilemore expensive, has helped conserve tin by producing a thinner coat.Tin has good corrosion resistance particularly to foods and fruit juiceshence its use in the packaging and canning industries. However, onsteel the coat must be continuous to be protective. Otherwise an electro-lytic cell would be established and the steel would corrode preferentially.

10.3. Zinc and Zinc Base Alloys as Engineering Materials

Zinc may also be considered as one of the low melting point metals,having a melting temperature of 787°F. This plus its good atmosphericand salt water corrosion resistance are the characteristics which giveszinc its place in the metals industry. The largest use of commercially purezinc is in the galvanic protection of steel products-ga/wmzzmg sheet,tube, and wire. The protective coat is applied either by hot dippingthe steel in molten zinc or by electroplating. The latter produces athicker coat providing longer protection and without forming a brittleintermetallic compound at the interface between the zinc and the steelwhich will crack and flake off when the product is bent. In recent years,over 40% of the zinc used in the United States has gone into galvanizedproducts.

Second, in importance in the use of zinc is the die casting industry.Here, zinc alloys containing up to a combined 6% of aluminum, copper,and magnesium are used. An increase in strength is achieved by thisalloying and some dimensional stability is attained in those alloys con-taining copper. The die casting alloys have a characteristic of shrinkingin size apparently due to some phase change which occurs upon aging.Approximately two-thirds of the total shrinkage occurs in the first month,and this is often times made to occur at an accelerated rate by heating theparts 3 to 6 hours at 212°F. The automotive industry is probably thelargest user of zinc die castings. Many of the accessories, functional parts,and decorative trim are produced by this method. It is an easy and eco-nomical way of producing precision parts which need little in the way offinishing operations. Quite often they will be chrome plated to enhancetheir appearance. The other applications where zinc die castings areused are in household appliances, office equipment, radios, toys, andtelephone apparatus.


The mechanical properties of the zinc and zinc base alloys are

similar to those of aluminum and magnesium. Pure zinc has a tensile

strength of about 20,000 psi. The high strength die casting alloy has

a tensile strength of approximately 52,000 psi. However, this is not

meant to imply that zinc may be used for strength purposes. For at

room temperature, zinc and its alloys undergoes creep at loads exceeding

6,000 psi. This is as may be expected from its low melting temperature.

The fatigue strengths for these metals are of the same order being from700 to 8500 psi depending upon the alloy.

The fabricating characteristics of zinc are good considering this metal

has a hexagonal close packed crystal structure. Some of the ease of fabri-

cation arises from the fact that pure zinc is hot worked when fabricated

at room temperature. Zinc and its alloys may be fabricated by practically

all of the known methods—rolled, drawn, spun, extruded, and cast. Somedifficulties are encountered in the joining of zinc, but with proper techni-

ques it may be gas welded or soldered.

Other uses for zinc include (1) use as an alloying element in brasses,

(2) in photoengraving plates, (3) dry batteries, (4) weather stripping

and building materials, and (3) more recently as expendable anodes for

cathodic protection of underwater steel structures.

HIGH TEMPERATURE METALS AND ALLOYS

The early use of metals for high temperature usage was in steam

power generation and in the petroleum industry. In these applications

the operating temperatures were in the range of 1000° to 1200°F. Other

uses at slightly higher temperatures were in ore roasting furnaces. How-ever, in these applications corrosion resistance and economy are of pri-

mary consideration while mechanical strength of secondary importance.

More recently with the development of the gas turbine, jet engines, and

atomic energy, the operating temperatures have been edging upward andthe emphasis is more on mechanical strength along with corrosion re-

sistance. The early high temperature metals and alloys involved the

alloy steels and the nickel base alloys. As temperature and strength re-

quirements have increased the trend has been toward nickel, cobalt and

chromium based alloys. With the development of the cermets a newapproach to high temperature alloys has been made. In cermets, the

high temperature strength is attained from an intermctallic compoundusually a carbide of tungsten, titanium, chromium, tantalum, or noibium

embedded in a matrix of cobalt or nickel. Other examples involve

metal oxide embedded in a metallic matrix. The problem here is one

of improving the creep resistance of the metal or alloy. To do this it is


necessary to produce a continuous matrix or network which has verygood strength and corrosion resistance, hence the carbides or metaloxides. This then is surrounded by a metal which has good oxidationand corrosion resistance.

10.4. Nickel and Nickel Base Alloys

The metal nickel is in many respects similar to iron. Its mechanicalproperties, modulus of elasticity, and atomic diameter are quite similar.

Hence it will alloy with many of the same elements found in steels. It

differs principally in its corrosion characteristics. It resists oxidizingconditions very well but will be susceptible to attack in reducing condi-tions particularly if sulfur is present. It resists atmospheric corrosionquite well and often, where price will permit it, is used as a cladding ma-terial on other metals for this purpose. Nickel is also often used as an in-

termediate layer in the chrome plating of steels.

Nickel Alloys. As aforementioned, nickel will alloy with many of theother metal elements. It forms a continuous series of solid solution alloyswith copper. It has a high solubility for such other metals as Co, Cr, Fe,Mn, Mo, Ta, W, and Zn. The elements Al, Be, Si, and Ti have onlylimited solubility and alloys of Ni containing small quantities of these

metals often exhibit precipitation hardening characteristics. Some of thebetter known nickel base alloys are as follows:

Monels are a series of nickel-cop[>er base alloys containing approxi-mately 65% Ni and 30% copper with small amounts of silicon, aluminum,manganese and sulfur. The Monels are primarily noted for their strengthand corrosion resistance.

The Inconels contain appreciable quantities of chromium and iron withsmaller amounts of titanium, aluminum, and niobium. These alloysfind application where high temperature corrosion and oxidation re-sistance is important.

The Nimonic alloys were developed as a series which had high strengthat high temperatures. An early alloy containing 80% Ni and 20% Cr hadbeen used quite successfully as a heating element in electric furnaces and•appliances. Modifications of this alloy gave rise to the present Nimonicseries. The latest developments in this series has large quantities ofcobalt in place of the nickel and small amounts of molybdenum, titanium,and aluminum.

The Hastelloys are another series of high temperature alloys developedby the Haynes Stellite Company and originally were nickel base alloyscontaining appreciable quantities of molybdenum and iron. However,the newer alloys showed decreases in the iron content with this being


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replaced by larger amounts of chromium, tungsten, and cobalt. Someprecipitation alloys have been made containing small amounts oftitanium and aluminum. The Hastelloys are fabricated by the normalworking methods but due to their high strengths and good oxidationand corrosion resistance require heavier fabricating equipment or highertemperatures to obtain the desired deformation. Table 10.3 lists some ofthe typical alloys falling in these various categories. They find applica-tion where operating temperatures are in the range of 1300 to 1800°F.

10.5. Cobalt Base Alloys

The element cobalt finds its greatest application in alloy form. It isused in other metal systems to increase strength and corrosion resistanceand, combined with chromium forms a series of alloys known as Stellites.The stellites will have cobalt contents ranging from 67% down to 37.5%,and with chromium contents ranging from 20 to 27%. The remainingelements present will consist of varying amounts of the following ele-ments, Ni, W, Mo, Nb, Ti, and Fe. In addition to the above elements,these alloys will all contain carbon percentages from 0.1 to 0.5%, and thecarbon will be present in carbide form making these alloys very hard andbrittle. As such, with one or two exceptions, these alloys are usuallycast directly to their final shape. By virtue of their physical propertiesthey are primarily used where high strength and abrasion resistance isrequired at high temperatures. Their normal operating temperaturerange ,s from 1350 to 1800°F. Other uses for cobalt are in magneticmaterials where it is alloyed with Al, Fe, and Ni in "Alnico," and Fe plusV in Vicalloy.

r

10.6. Other Miscellaneous High Temperature Metals and AlloysRecent experiments have indicated the direction in which future re-

search must be aimed in the development of alloys for future highertemperature applications in the range of 1800 to 2000°F. Some of thesemetals and alloys will only be mentioned briefly here but will serve toshow possibilities and shortcomings. Recent work with alloys of iron,chromium and aluminum; or iron, molybdenum, and aluminum whosealloys are known as "Ironals" have shown promise as good high tempera-ture alloys. It is important that little or no carbon be present. Operatingtemperatures will be limited to approximately 2100 to 2300°F. Chromiumbase alloys are also possibilities, but chromium is difficult to melt due toits high vapor pressure and is very reactive with oxygen and nitrogenwhich makes the alloy very brittle. Experimentation with the metalsmolybdenum, niobium, rhenium, tantalum and vanadium all of which


have high melting temperatures, as can be seen in Table 10.1, are for

the most part limited in use at high temperatures due to their oxidation

reactions. The oxides of Mo, V, and Re all volatilize at temperatures

below 1200°F., while Ta and Nb form spongy oxides quite rapidly at or

above this temperature. It therefore appears that if these are to be used

for high temperature applications they will have to be clad with another

metal or ceramic which will keep oxygen away from the base metal.

OTHER METALS AND ALLOYS

10.7. The Precious and Semi-Precious Melals and Alloys

The metals and alloys which find themselves in this group are here by

virtue of their scarcity in nature and their usage as coinage and jewelry.

They are the metals silver, gold, platium, and the allied platium group-

palladium, iridium, osmium, rhodium, and ruthenium. Most of the

metals are found in nature in the metallic form or alloyed with the ores

of copper, lead, nickel, or zinc. Usually they are present in very minor

amounts and thus are obtained as by-products and only when the eco-

nomics of the operation make it worth while.

Silver. The element silver in the pure form is a soft white metal with

good corrosion resistance and high electrical conductivity. Since it is

quite soft, it is rarely used as a pure metal except perhaps as a plating

on jewelry items. When used as a base metal, it is alloyed with copper,

the common alloys being with 10% copper in coinage, and with 7.5%

copper as sterling silver. The silver solders have still higher copper con-

tents. Silver is also used in an amalgam in dental alloys and is combined

with tungsten and molybdenum by powder metallurgy for use as heavy

duty electrical contacts. As aforementioned, it is used rather widely in

electroplating of hardware, light reflectors and costume jewelry. Theamounts used here are quite small and their cost is negligible. However,

they add materially to the eye appeal and reflectivity of the base metal

and improve the oxidation resistance of the part.

Gold, like silver, is quite soft and corrosion resistant. Similarly, it

is rarely used in pure form due to its softness, the one exception being

as gold leaf in sheet form with a thickness of approximately 0.00005

inches. In this form it can be used in lettering, display, and decorative

applications. More often it is alloyed with copper, silver, or platium for

jewelry applications. When alloyed with platinum, palladium, and nickel

it takes on the characteristic white color and becomes white gold. Gold

can also be plated on jewelry either by dipping on the molten metal or

by electroplating. The latter yields heavier coats. Still thicker coats are


obtained by cladding with sheet material by rolling, soldering or brazingthe sheet to the base metal. The other application for gold is as amedium of exchange on an international basis, for it is accepted through-out the world.

The platinum metals while better known for their jewelry applications,

in actuality, find greater application in commercial uses such as cata-

lysts, high temperature corrosion resistant materials, electronics, andthe chemical industry. With the exception of palladium, the other metals—platinum, iridium, osmium, rhodium, and ruthenium—all cost more perounce than gold. Of this group, only platinum is used in pure form. Theothers being used as alloying elements in platinum. Platinum is used in

making chemical and medical apparatus usually as an alloy containingiridium. In pure form, platinum wire is used for a platinum-rhodiumalloy wire as a high temperature thermocouple. This couple readilyresists the high temperatures and oxidizing atmospheres that cause othermetals to fail rapidly. Rhodium is sometimes used as a replacement forsilver on plated articles which will be subjected to sulfur atmospheres.Rhodium has a silvery appearance but will not tarnish under these con-ditions.

10.8. Metals and Alloys in Atomic Power Applications

While considerable interest has been shown in recent years in theapplication of atomic energy for power purposes, much of the informationhas of necessity been withheld due to security reasons. However, thepart metals play in this new industry can be broken down into four fields:

(1) as fuel elements, (2) as structural materials or moderators, (3) ascontrol materials, and (4) materials which can be irradiated and bercome gamma ray emitters.

In the first category, as fuel elements, it is necessary that the materialundergo atomic fission. While many of the elements high in the periodtable in terms of atomic number are radioactive, only uranium and per-haps thorium may be used as fuel elements. It is necessary that the fuelmaterial be rather plentiful in nature and undergo fission at a rapidenough rate to make it economically feasible as a source of energy. Inthe second category, the structural materials in an atomic reactor shouldbe able to withstand the temperatures involved, should not trap or absorbneutrons, and should have good corrosion resistance. The metals whichmeet these requirements are aluminum, beryllium, and zirconium.Zirconium seems to be the preference particularly where high tempera-tures are encountered.


In the third category are those metals which actually control the fission

rates in a reactor. The boron steels are the materials commonly used by

virtue of availability and ease of fabrication. Hafnium, which has a

greater capture cross section than the steels, is not as prevalent in nature

and hence is not as widely used. However, this is a field which is being

thoroughly investigated and other alloys will in all probability be de-

veloped.

In the fourth category are the metals cobalt, cesium, thulium, and

iridium. These metals when irradiated become radioactive and isotopes

of these metals are used as gamma ray emitters. The metal isotopes and

their halflife periods are listed in Table 10.4.

Tablc 10.4 Radioactive Isotopes

Isotope Av. Energy* Radiation Output mr/hr* Half Life

Cesium 137 670 KV 3750 27 yrs.

Cobalt 60 1200 KV 14000 5.3 yrs.

Iridium 192 375 KV 5950 75 days

•Approximate**Milli-roentgcns per hour at a distance of 12 inches.

These metals find wide usage in radiographic examination of metal

parts for internal defects. In many applications they have advantages

over x-rays. In general they are less costly, have greater portability, and

can be used in smaller places. The gramma ray source usually is smal-

ler than a cube 14 inch on each side. Exposure times for the radio-

graphic films are about the same as for high voltage x-rays.

QUESTIONS

1. List five metal elements which are noted primarily for their low melting

temperatures?

2. What are the "white metals," and for what are they principally used?

3. What are the principal requirements of a bearing alloy?

4. Why is a eutectic alloy used in linotype machines?

5. How does tin and antimony affect the casting characteristics of alloys con-

taining these elements?

6. What are the principal uses of lead, tin, and zinc?

7. Which of the metal elements do we consider as being high-temperature

materials

8. What is a cermet? How is strength at high temperatures attained in a

cermet?


9. What are the better known nickel-base alloys, and what are the principalalloying elements in them?

10. In what ways is the element cobalt like nickel? What other uses is made ofcobalt?

11. Many of the elements have high-melting temperature; why can they notbe used as high temperature metals or alloys?

12. Why are the metals gold, silver, platinum, palladium, etc. so expensive?13. What are the four categories for metals with respect to atomic energy?14. What are the requirements of metals that arc used as fuel elements in

atomic reactors?

15. Why is it important that the structural materials in atomic reactors notabsorb neutrons? How do they differ from the control materials?

16. Which of the elements are commonly used as gamma ray emitters in radio-graphic examination?

17. How do gamma rays differ from x-rays?


Manufacture of Metals 207

CHAPTER

The Manufacture andComposition of Carbon

and Alloy Steels

11 Howard P. Davis, M.S., Associate Professor, Depart-

ment of Mechanical Engineering, University of

Wyoming, Laramie, WyomingFloyd Sheldon Smith, M.S., Associate Professor of Me-

chanical Engineering, Alabama Polytechnic Institute,

Auburn, Alabama

MAN has been using iron for at least 5,000

years. For almost half of this time he used it as a precious metal which

came from heaven—as meteorites—and which was seized when found and

laboriously worked into jewelry, charms, and amulets. These he wore for

personal adornment or to ward off the evil spirits that harassed him daily.

For the last 2,500 years, he has had terrestrial iron, reduced from its ores,

and it is characteristic that the first use he made of it was to fashion

weapons. Damascus swords, made by the Moslems in the Middle Ages

from steel imported into the Near East from India, and by the famous

families of Japanese sword makers between a.d. 500 and 1500, are the

finest specimens of the early steelmakers' art which have come down to us.

They were made by welding alternate strips of high- and low-carbon steel

into a bar, forging out this composite material, doubling it over and re-

forging again and again until a laminated strip of the finest quality re-

sulted. This was drawn out into a sword or dagger blade, heat treated,

sharpened, and finally etched with a dilute acid to bring out the lamina-

tions. Such a blade is shown in Fig. 11.1, in which the blade and scab-

bard are photographed against a background of the damask pattern near

the dagger point, magnified eight times. Man's next use of iron was to

fashion tools, so that he could make life easier for himself.

Since the beginning of large-scale manufacture about 1870, steel pro-

duction has increased greatly each decade; by 1953 the world was using

over 258 million tons a year. Of this the steel industry of the United

States has been producing about 100 million tons annually.

208

The Manufacture and Composition 209

Fie. 11.1. Damascus steel dagger. The background is a photograph of the etcheddamask pattern of the blade. 8x- (Ziegler, Mining and Metallurgy, v. 20, 1939, p. 69)

The use of chemical analysis for determining the composition of steel

and cast iron did not become general until about seventy years ago,more than a decade after large-scale steelmaking by the Bessemer andthe open-hearth processes had been introduced.

For many years the analysis of steel and cast iron consisted of thedetermination of carbon, manganese, sulfur and phosphorus, silicon, if

present, and alloying elements when these were specially added. Only in


the past few years has it been realized that these common elements tell

only a part of the story of the composition and that in addition some ten

or twelve other metals, metalloids, and nonmetals may be present. As a

result, the chemical composition of unalloyed steel is no longer as simple

as it seemed thirty years ago; on the contrary, it is very complex andworthy of considerable attention.

Despite the fact that commercial carbon steels and cast irons contain

many other elements in addition to carbon, metallurgists look upon themas alloys of the metal iron and the metalloid carbon because carbon has

the greatest effect on their structure and properties.

Commercial carbon steels contain, in addition to carbon, varyingamounts of manganese, silicon, sulfur and phosphorus, and small per-

centages of oxygen, nitrogen, and hydrogen. Occasionally they also con-

tain small amounts of copper, nickel, chromium, molybdenum, lead,

arsenic, aluminum, and tin. Some of these are added intentionally to

improve the quality, but most of them are picked up from the rawmaterials used in the various processes. Steel men commonly divide these

elements into three classes: harmful, beneficial, and neutral.

11.1. Definitions of Ferrous Engineering Materials

For many years committees of metallurgists in English-speaking

countries have been trying to define carbon steel, alloy steel, cast iron,

pig iron, wrought iron, and other products of the industry precisely andconcisely, but with little success.

It would be a waste of space, and probably even confusing, if the

complex definitions for the numerous products of the iron and steel

industry were all set down at once.

Carbon steel may be defined as a commercial alloy of iron and carboncontaining less than 2.0 per cent carbon, less than about 1.30 per cent

manganese, less than 0.5 to 0.6 per cent silicon, small amounts of ad-

ventitious elements, and no intentionally added special element. If the

alloy contains more than 2.0 per cent carbon and is poured direct fromthe blast furnace into cast iron molds of uniform size, or is used in the

molten condition as an intermediate product, it is called pig iron; and if

this pig iron (usually with the addition of scrap) is remelted and pouredinto a sand mold, thereby assuming the desired shape of the finished

section, it is known as cast iron. This means that whether an iron-carbon

alloy containing more than 2.0 per cent carbon is called pig iron or cast

iron depends upon whether the material is an intermediate product or a

finished product.


Pig iron and cast iron have been differentiated here chiefly to indicate

some of the necessary distinctions in formulating definitions of ferrous

products. Pig iron is of little interest to the engineer; cast iron is an

important engineering material and is discussed in some detail later.

Ingot iron is a commercially pure iron. It contains between 99.75

and 99.90 per cent iron. The common elements, carbon, manganese,

silicon, sulfur, and phosphorus, account for about half the impurities;

the occasional elements make up the other half. Ingot iron resembles

carbon steel in that it is melted and refined by the basic open-hearth

process, but it is lower in carbon (about 0.02 per cent) than the lowest

carbon steels made industrially, which range from 0.04 to 0.08 per cent.

Ingot iron is used chiefly in the form of galvanized and enameled sheet.

Wrought iron is a low-carbon material and is no longer widely used.

The nature of the refining is such that this material contains a larger

proportion of solid nonmetallic inclusions than low-carbon steel madeby the usual processes. Owing to the nature of the process and the com-position of the inclusions, wrought iron when rolled has relatively lowstrength but is ductile and tough, and is easily welded. It is used for

staybolts, rivets, some grades of pipe, boiler tubes, and especially for

heavy chains and hooks. Wrought iron has well-marked directional

properties.

There is no precise and concise definition of alloy steel, but practically

there is seldom any difficulty in distinguishing between carbon and alloy

steels. The practical criterion is: if an element, not used primarily for

deoxidation or degasification, is added to carbon steel or is present ad-

ventitiously in the amount required to produce a desired specific effect,

the resulting material is an alloy steel.

It may be asked legitimately why the dividing line between carbonsteel and cast iron is placed at exactly 2.0 per cent carbon. Although this

percentage is fixed primarily for convenience, it has considerable scientific

justification. The 2.0 per cent carbon concentration marks the maximumsolid solubility of carbon in iron at very high temperature, and belowthis concentration graphite—a common form of carbon in cast iron— is

not ordinarily present under normal conditions of treatment.

For convenience, the commercial iron-carbon alloys may be divided

into four classes:

Low-carbon steel Less than 0.25 per cent carbonMedium-carbon steel 0.25 to 0.65 per cent carbonHigh-carbon steel 0.65 to 1.70 per cent carbonCast iron and pig iron 2.50 to 4.50 per cent carbon


WOT

800'f

11.2. Iron Ore and the Manufacture of Pig Iron

The four essential raw materials of the iron and steel industry arc iron

ore, coke and coal, limestone, and scrap steel and scrap cast iron. The iron

ore used in the United States is chiefly hematite, an impure ferrous oxide

(Fe2Og) containing between 45 and 55 per cent metallic iron and be-

tween 20 and 35 per cent earthy matter, known as gangue, which is madeup of silica, alumina, calcium oxide, magnesium oxide, and water, plusvarying small amounts of phosphorus and occasionally sulfur. Hematitesand other usable iron ores are widespread throughout the world; the com-

mercial ores in the United States are

estimated at more than 10 billion tons,

of which 90 per cent is located near

Lake Superior and in Alabama. Iron ores

are graded by phosphorus content into

Bessemer and non-Bessemer (basic)

ores. As phosphorus is not removed in

the blast furnace, the ore must not con-

tain more than approximately half the

amount of this element permitted in

Bessemer steel.

Iron ore is reduced by coke in the

blast furnace, limestone being used to

flux the gangue and the ash of the coke.

The raw materials, in the approximate

ratio of 2 tons of ore (frequently includ-

ing some scrap steel or cast iron) to 1

ton of coke and i/2 ton of limestone, are

charged into the top of the furnace, andair, preheated to 1000° F. (540°C.) or

above, is blown into furnace through

a number of nozzles (called tuyeres)

near the bottom. The blast furnace,

shown in cross section in Fig. 11.2,* is a

steel shell lined with firebrick; 90 to

100 ft. high, 17 to 20 ft. in diameter at

the top, and 24 to 28 ft. in diameter

at the bosh (just above the tuyeres)

.

Operation is entirely continuous: Theraw materials, carefully weighed andanalyzed, are charged into the top at

* Based on J. M. Camp and C. B. Francis, The Making, Shaping and Treating ofSteel, Carnegie Steel Co., Pittsburgh, 5th cd., 1940, p. 288.

Fig. 11.2. Cross section of amodern blast furnace showing tem-

perature and chemical reaction

levels. (Based on Camp and Fran-

cis)


Row ,

materia!^

Limestone 774

Coke 1954't5fi«8

B^^3gfi Ore, elc. 3925

Air 8500+

5000

Products <

10000

Sasesl2,000±

_]

Kj Slag 867

Ftc. 11.3. Relative proportions by weight of the raw materials and products ofthe modern blast furnace. (Boylston)

frequent intervals and slowly settle to the bottom, the downward pressure

being slightly greater than the upward blast pressure plus the friction of

the burden on the walls of the furnace.

As the charge settles, the temperature increases (Fig. 11.2) until near

the bosh the iron ore is practically all reduced and the spongy iron is

heated to incandescence. Here it starts absorbing carbon, which lowers

the melting point. The iron then becomes pasty, absorbs carbon morerapidly, and finally melts, trickling down over the remaining unburnedincandescent coke into the hearth. The calcium oxide in the limestone

forms a fusible slag that is much lower in specific gravity than the iron

and that floats on top of the molten metal in the hearth from where it is

drained at regular intervals. The molten high-carbon iron is drained fromthe hearth into ladles every 5 or 6 hr. and then is either poured into

molds to solidify as pig iron or used in the molten condition in the

manufacture of Bessemer or open-hearth steel. A modern blast furnaceproduces between 800 and 1 ,200 tons of pig iron every 24 hr. and, barringshutdowns for economic conditions, will operate continuously for years.

The average proportion of the raw materials entering the blast furnaceand of the resulting products is shown graphically in Fig. 11.3.* The slag

is mostly waste and must be disposed of, a considerable problem in someplants. As Fig. 1 1.3 shows, an enormous volume of gas is given off. Thisgas, which totals more than 100 million cu. ft. a day for a single furnace,

contains 20 to 25 per cent carbon monoxide and has a heating value of

• H. M. Boylston, Iron and Steel, John Wiley & Sons, Inc., New York, 1935, p. 71.


90 to 100 B.t.u. per cu. ft. It is, consequently, used to heat the blast andto generate power. Thermally the iron blast furnace is not efficient.

Owing, however, to the value of the gas as a by-product and to the

operation of large units, the over-all efficiency is relatively high, and pig

iron is made cheaply.

Chemistry of the Blast Furnace. The preheated air, which is blowninto the furnace under a pressure of 15 to 20 lb. per sq. in., comes into

immediate contact with incandescent coke forming large volumes of

carbon monoxide gas. This gas, together with the carbon in the coke,

reduces the iron oxide by two general reactions:

Fe2Os + 3 CO *± 2 Fe -f 3 C02 , (1)

Fe2Os + 3 C *± 2 Fe + 3 CO. (2)

Both reactions are reversible and may proceed in either direction de-

pending upon the temperature and other conditions prevailing at the

various levels in the furnace. The net result, however, is reduction of the

oxide to metallic iron. Both reactions are progressive: the Fe2O s is reducedfirst to Fe3 4 , then to FeO, and finally to Fe. Reaction (1) predominates

in the upper part of the furnace (see Fig. 11.2) , beginning at a tempera-

ture of about 400°F. (210°C.) ; reaction (2) begins at a temperature of ap-

proximately 1200°F. (650°C.) and continues until the iron oxide is com-pletely reduced in the zone of the bosh.

The chemical reactions in the blast furnace are essentially reducing

so that any easily reducible oxides in the iron ore or the coke (in addition

to the iron oxide) are also reduced. Thus, all the phosphorus and mostof the manganese will be found in the iron. Oxides of silicon and sulfur

are more stable but are partially reduced. The remaining oxides—of

calcium, magnesium, and aluminum—are very refractory and are notacted upon by the carbon or carbon monoxide even at the highest

temperature of the furnace. To flux these refractory oxides, the necessary

amount of limestone (CaCO.,) must be charged. The limestone calcines in

the upper part of the furnace and, together with the calcium and mag-nesium oxides in the ore, combines chemically with the alumina and with

part of the silica to form a fusible slag composed of calcium, magnesium,and aluminum silicates.

The pig iron, tapped from the furnace, contains about 4 per cent

carbon, all the phosphorus, and most of the manganese in the raw ma-terials. The amounts of silicon and sulfur in the iron are controlled to

some extent by a careful selection of raw materials but chiefly by regu-

lating the chemical characteristics of the slag, that is, by controlling the

proportion of calcium oxide to silica, and by controlling the tempera-


ture. The latter depends upon the amount of coke, the temperatureof the blast, the amount of water vapor in the blast (the humidity)

,

and other variables. The operation of a modern blast furnace and theproduction of cheap pig iron of satisfactory quality is an art which de-mands long experience and skill of a high order.

Operation and Use of the Cupola. The cupola is a vertical, cylindrical

type of furnace, consisting of a steel shell lined with firebrick. Usuallythe charging door is located 10 to 25 feet above the bottom of the furnace.Air enters the cupola through the tuyeres, located near the lower end ofthe cupola. Fuel, metal, and flux enter the cupola through the chargingdoor. The initial charge of coke is known as the "bed charge." Themetal is deposited on the coke. Then alternate layers of coke and ironare added. Generally a limestone flux is charged on the coke. The pur-pose of the flux is to form a slag with the dirt, impurities in the metal,and the coke ash.

The metal charge ordinarily consists of local gray iron scrap, purchasedgray iron scrap, pig iron, and steel scrap, where increased strength is

desired.

The proportions of various metals in the charge are governed bythe type of castings desired. The base of the charge is pig' iron. Sincethe pig iron is of known composition, the greater the amount of pig ironand the smaller the amount of scrap, the more uniform will be thecomposition of the resulting castings. For high grade castings, such asautomobile cylinders, the percentage of pig iron will range from 25 to 50per cent of the charge, while for low grade castings, almost no pig ironwill be used. Fluorspar, soda ash, and a number of proprietary fluxes aresometimes used in combination with the limestone to thin the slag andalso to aid in the removal of sulphur.

Air is introduced into the cupola to aid in the combustion of thecoke, which in turn melts the metal. Under ordinary conditions the blastpressure will range from 8 to 20 ounces per square inch, depending on theheight of the cupola and the number and character of the metal charges.

The ratio of the coke and metal charges are ordinarily one pound ofcoke to eight pounds of iron. The charges are very carefully weighed.As the metal charge becomes molten it trickles down through the bedcharge of incandescent coke and is collected in the crucible zone of thefurnace. The liquid slag which collects above the molten metal is drawnoff through the slag notch located just below the tuyeres. The molteniron is drawn off through the tap hole, located at the bottom of thecupola, into crucibles and poured into sand molds to form gray castiron castings.


11.3. Acid and Basic Processes

The reduction of iron ores and the manufacture of steel are chemical

processes and are unique in large-scale industrial operations in that the

chemical reactions involved take place at high temperatures, nearly al-

ways above 2000°F. (1100°C.) and usually in the range 2500 to

3500°F. (1370 to 1925°C). Few refractory materials withstand such

temperatures. The only ones that are available cheaply arc the oxides

of silicon, calcium, and magnesium. These are relatively inert at normaltemperatures but become active chemically at steelmaking temperatures:

silica (Si02) becomes an active acid, and calcium and magnesium oxides

become active bases, reacting at high temperatures; thus,

CaO + SiOs -» CaSiOs , (3)

MgO + Si0 2 -> MgSi03 . (4)

It is characteristic of silicates such as these that they fuse at temperatures

much lower than the fusing temperatures of the oxides.

Molten metals are refined by slags which are essentially silicates of

calcium, magnesium, and iron and are formed by a reaction between

definite amounts of a basic oxide and the acid oxide silica. Depending on

whether the basic or the acid constituent is in excess, the slag will have

a basic or an acid reaction at steelmaking temperatures. The character

of the refractories used in the furnace—which is decided by economicconsiderations—determines the character of the slag and thus determines

whether the process is basic or acid.

It is obvious that, if a furnace has an acid lining, the slag must be acid

in character or it would attack and destroy the furnace lining. Thus, in

an acid process the slag must contain an excess of silica, and in a basic

process, where the lining is magnesium oxide, the slag must contain an

excess of calcium oxide. The advantages of acid and basic steelmaking

processes may be summed up as follows:

Advantages of Acid Process. Acid slags contain less free oxide than

basic slags. Consequently, if all other things are equal, acid steels will be

cleaner and of higher quality than basic steel. Acid refractories are muchcheaper than basic refractories.

Advantages of Basic Process. The basic slags will remove phosphorus

and sulfur from the steel. Therefore, lower priced scrap and pig iron

can be used since it is not necessary to avoid small amounts of phos-

phorus and sulfur.

Fluxing materials used in the steelmaking process are classified as

basic, acid, or neutral according to their chemical activity at elevated

temperatures. Silica is the only substance that is classified as a strictlv


acid fluxing material. Sand, gravel, and quartz are available in large

amounts and in sufficiently pure state to be used as acid fluxing materials,

but normally they are not used because the lining of the furnace will

furnish all that is required. In basic processes, silica might be used as a

flux if excess lime has been charged or where the raw materials are too

low in silicon.

Iron and manganese oxides act as basic materials when present in acid

processes, but they are not normally added for this purpose and therefore

are not classified as fluxing materials. The most common basic fluxing

materials are limestone and dolomite, limestone being mostly CaO, while

dolomite is a double oxide of calcium and magnesium. "Burnt" lime,

a calcined limestone, is usually added in the basic open hearth when morethan the amount furnished by charged limestone is found to be needed

at a later stage of the heat.

In order to make the slags less viscous without changing their acid,

basic, or oxidizing properties, a neutral substance with a low melting

point such as flourspar (mostly CaF2) is most commonly used.

11.4. Bessemer Processes

Forty years ago, 25 to 30 per cent of the so-called tonnage steels were

made in the Bessemer converter. The price differential in favor of

Bessemer steel, averaging S2 to $4 a ton, disappeared some years ago owingto the increasing scarcity and higher cost of low-phosphorus ores. Thiscondition, coupled with the disadvantage that Bessemer steel containing

more than about 0.20 per cent carbon is considered by many metallurgists

to be inferior in quality to steel of comparable carbon content madeby the basic open-hearth process, has resulted in the decline of Bessemer-

steel production to about 3 to 4 per cent of the total. During the past

15 years it has leveled off to between 3 to 6 million tons per year.

The Acid Bessemer Process. The acid Bessemer converter is a pear-

shaped steel vessel lined with silica brick and mounted on trunions by

which it can be tilted 180 degrees. The bottom contains a large numberof tuyeres to admit the blast. In making steel by the Bessemer process

the converter is tilted forward, and 15 to 25 tons of molten pig iron are

poured into the belly of the vessel. It is then turned upright, and at the

same moment the blast is turned on. Refining begins immediately. Noexternal source of heat is needed as sufficient is generated, by the reaction

of oxygen with the carbon and other elements in the pig iron, to raise the

temperature several hundred degrees.

The blast of air entering the converter forms large volumes of iron

oxide as soon as it strikes the molten metal, which immediately oxidize


Fie. 11.4. Bessemer converter in operation. (Courtesy o\ Jones and Laughlin Steel

Corporation)

the manganese and silicon. The phosphorus is oxidized; but as there is

no excess of lime, stable phosphate is not formed, and none of this ele-

ment is removed from the metal. Sulfur is also unaffected.

The progress of the chemical reactions is followed by observing the

color and length of the flame shooting from the mouth of the vessel.

Refining is complete in 10 to 15 minutes when the manganese, silicon, and

carbon in the pig have been reduced to a low percentage, after which the

converter is tilted forward and the metal is poured into a ladle from


which it is poured into ingot molds. Harmful oxides are eliminated as

far as possible, and the proper amounts of carbon and manganese are

secured by adding an alloy of iron, manganese, and carbon to the ladle as

the contents of the converter are transfered to the ladle.

No slag-making materials are added; the slag formed is composedof compound silicates of iron and manganese—oxidized from the pig

iron—plus some silica eroded from the lining. It is acid in character.

Owing to the rapidity with which die reactions occur and to the large

volume of iron oxide formed, acid Bessemer steel usually contains moredissolved iron oxide than basic open-hearth steel. The production

from a Bessemer plant of three vessels is 5,000 to 6,000 tons in 24 hours.

To produce the same tonnage of basic open-hearth steel, a battery of

twenty 100-ton furnaces is needed.

For many years, the Bessemer process has been important in the steel

industry, but, in contrast to the open-hearth process, little attention has

been paid to the chemistry or metallurgy of refining. In 1940, however,some of the larger plants making Bessemer steel instituted a study of the

process, and some noteworthy developments resulted.* A photo-electric

cell is now being used to determine precisely the end of the blow, and aprocess has been developed for removing phosphorus by treating the

molten metal in the ladle. Very recently a method has been worked outfor deoxidizing Bessemer steel, so that it is apparently equal in qualityto open-hearth steel of approximately the same composition, with theobject of using this deoxidized material for seamless tubes and pipes.

The Basic Bessemer Process. This process is not used in the UnitedStates, mainly because the iron ore being used, and consequently the pigiron being produced from it, is not suitable for this process. It is

necessary that there be at least 2% phosphorus present in the pig ironin order that the oxidation of the phosphorus during the afterblow canmaintain the temperature of the bath high enough to prevent solidifica-

tion. The silicon content must be low in order that the expensive basiclining last as long as possible.

The operation of the basic process is similar to that of the acid processexcept that lime is added, so the slag formed is basic and the phosphoruswill be removed. After the manganese, silicon, and carbon have beenoxidized out there is the additional afterblow during which the phos-phorus is oxidized out of the steel.

• These developments in Bessemer steel arc described in papers by Graham. Work.McGinley and Woodworth, Yocum. lulton. Bowman, Wright, and Dunkle in Transac-tions, Iron and Steel Division, American Institute ot Mining and Metallurgical En-gineers, 1941, 1942, 1944. 1945.


The cost of the basic process is higher than the acid process due to the

higher cost and shorter life of the basic refractory lining, the longer

blowing time and the fact that lime has to be added to the process. InEurope, where this process has been used with success, the high phos-

phorus ores necessary are available, and there is also a market for the

slag as a fertilizer, which helps overcome the higher cost of the process.

Fie. 11.4A Cut-away model of a modern open-hearth furnace showing roof con-

struction (upper left), bottom and port construction (lower left), cutaway checker

chamber showing brick construction (lower center), and stack (right). (Courtesy

H. W. Graham, Jones and LttUgMitt Sled Corporation)

11.5. Open-Hearth Processes

Description of the Modern Plant. The typical open-hearth furnace has

a capacity of 100-150 tons, with the largest furnace in the United States

having a capacity of 350 tons. The furnace is a chamber about 80 ft.

long and 18 to 20 ft. wide. The hearth is shaped like an elongated saucer,

the inside dimensions being about 40 ft. long, 14 to 18 ft. wide, and20 to 30 in. deep at the center. The hearth lining consists of a mixture of

burned magnesite and basic slag sintered into place. The furnace is fired


tesy

ftC, I1.4B Pouring molten blast-furnace iron into the open-hearth furnace. (Coxr-\y of Carnegie-Illinois Steel Corporation)

with gas, oil, tar, or powdered coal, using preheated air, and if a lean gasis used, this is also pre-heated. At the center of the back wall is the tappinghole, 8 in. in diameter, in such a position that when opened it will drainall the steel from the hearth into the ladle. There is a hole for the slagrunoff at the top level of the hearth and toward one end of the furnace.The walls are made of silica brick, 13i/£ in. thick, and extend 8 ft. abovethe charging floor level. The charging doors make up most of the frontwall of the furnace and are lined with silica brick and have water cooled


frames, the bottom of the doors being just a few inches above the slag line.

The arched roof is 12 in. thick, made of silica brick, and is built in-

dependent of the walls so that it can be replaced without completely

rebuilding the furnace.

To attain the high temperature necessary to make steel, the regenera-

tive principle is used. White-hot exhausi gases from the furnace are led

through a series of brick checker-work chambers on the way to the stack.

After about 15 minutes, these exhaust gases are diverted to another set of

checker chambers at the other end of the furnace, and the cold air is led

through the heated checkers on the way to the furnace. The direction

of the exhaust and incoming gases is reversed every 15 minutes initially,

and the time interval is reduced as the temperature of the furnace ap-

proaches the tapping temperature. The reversal of the gases is automatic-

ally controlled depending on the temperature of the checkers and the

furnace. The regenerative process makes it possible for the gases to

attain a temperature of approximately 3500°F. (1925°C.) as they sweep

across the hearth, in addition to creating a saving in the amount of fuel

used in the furnace.

A hot metal mixer is usually located in the same area with the bankof open-hearth furnaces. It is a large refractory-lined vessel capable of

storing as much as 1500 tons of molten metal from the various blast

furnaces until it is needed in the open hearth. Some are equipped with

heating devices but mostly they depend on the heat from the hot iron as

it comes from the blast furnace to maintain a molten condition in the

mixer. The mixer makes it possible to have a molten metal with a more

consistent chemical composition available to the open hearth, so that a

wide variation in the charge is not necessary, and also to decrease the time

from charge to tap of the furnace, because of the retained heat. This also

tends to cut down on the fuel used.

Operation of the Process: Charging. The order of charging the ma-

terials into the furnace is quite important. The limestone is charged first

in order that it will not enter into the slag reaction until the desired

time. Any ore charged into the furnace is placed on top of the limestone.

The limestone and ore protect the hearth from damage during the

charging of the scrap iron which follows immediately. II the scrap is

light and bulky it may be necessary to melt part of it down in order to

charge the rest into the furnace. When the furnace reaches a temperature

somewhat higher than the melting point of the pig iron, the molten

iron is poured into the furnace from a ladle, using a spout placed in

one of the charging doors.

The ratio of the amount of steel scrap to pig iron in the charge

varies according to the price of the scrap. When the price of scrap is low,


the charge might consist of 75% or more scrap and 25% or less pig iron,

but if the price of scrap is high, then a charge of 25% scrap and 75% pigiron is possible.

Melting the Charge. As soon as the steel scrap is charged into the

furnace, it is heated with a hot oxidizing flame that will cause the scrap

to melt and become oxidized. This oxidized material later becomes in-

strumental in the purification of the pig iron by oxidizing out the

impurities. If enough oxides are not provided by the oxidation of the

scrap, additional oxidation is obtained by the ore that is charged in ontop of the limestone in the charge. This oxidation usually takes place

for about two hours after the start of charging, when at that time thehot metal is added to the furnace.

The Ore Boil. As soon as the hot pig iron is added to the furnace theoxides present begin to react with the impurities in the iron. First to beremoved are silicon and manganese, forming SiOa and MnC) which, alongwith FeO, combine to form the early slag. At the same time the oxygenis combining with the carbon in the pig iron to form CO which, whenescaping, causes the slag to "froth" and increase in volume to such adegree that a considerable amount will overflow through the slag run-off hole. In this manner much of the silica that would have to beneutralized by the limestone is eliminated, requiring less limestone to becharged into the furnace. It is interesting to note that a good deal of ironis lost here (as FeO in the slag) , and that sometimes this slag is re-

charged into the blast furnace to recover both the iron and the man-ganese from their oxides.

The Lime Boil. After the ore boil has subsided, the temperature of thefurnace has reached the point where limestone will calcine, giving off largevolumes of C0 2 . This characterizes the "lime boil," which is much moreviolent a boil than the "ore boil." At the same time the lime formed bycalcination rises from the bottom of the hearth and replaces the iron andmanganese oxides in the slag, with any excess of lime tending to make theslag more basic. The basic property of the slag makes it more capable ofretaining phosphorus in the slag as a phosphate. This lime boil alsotends to mix thoroughly the contents of the hearth so that the chemicalcomposition and the temperature are more uniform from top to bottom.The Working Period. During the working period there are several

specific aims. These include: (a) lowering of the phosphorus and sulfurcontent below the maximum values specified, (b) eliminating the siliconand manganese residue not oxidized out during the melting period, (c)

eliminating the excess carbon as quickly as possible, (d) conditioningthe slag so that it will be of the proper viscosity and have a basic chemical


composition, and (e) raising the temperature of the bath to approxi-

mately 150°F. above the melting point of the steel to be tapped.

Tapping the Heat. When the carbon content has been reduced

to the desired amount and the temperature of the steel has been raised to

the tapping temperature, the heat is ready to tap. The clay plug and a

portion of the dolomite that fills the taphole is removed. The rest of the

dolomite is burned out with an oxygen torch, and the taphole is cleared byusing tapping rods from the front of the furnace. The center of the ladle

is placed off center in order to cause a swirling action and to insure rapid

melting and good mixing of the ladle additions.

Ladle Additions. Usually included in the ladle additions are alloys

of manganese to remove oxygen and sulfur, alloys of silicon that also

remove oxygen, and aluminum which is added to further the deoxidizing

process and also to control the grain size. In order to increase the carbon

content of the steel after tapping, some carbon is recovered from the

manganese alloys, and if more is needed it can be obtained by addingground anthracite coal to the ladle. Other intentionally added elements

may be added either in the furnace or in the ladle. All ladle additions

should be added before any slag comes from the furnace, otherwise they

become mixed with the slag causing high alloy losses and a possible

phosphorus reversion. By using varying amounts of these alloying ele-

ments, high quality steel of any desired carbon and manganese content

can be made.

Chemistry of the Basic Process. The basic open-hearth is an oxidation

process. Oxygen in the air or in some form of iron oxide (iron ore)

is used to oxidize the carbon, silicon, manganese, phosphorus, and part

of the sulfur in the charge.

In the basic open-hearth process limestone (CaCOs) is charged with the

scrap and pig iron; and when the metal is melted, this limestone, calcined

to lime (CaO) by the heat, rises to the top of the molten bath to form

slag. Iron oxide in the form of iron ore is added to augment the iron

oxide which, in the form of rust, was on the scrap and pig iron. Theiron oxide oxidizes the silicon and manganese first, according to the

reactions

Si + 2 FeO '-* SiO a + 2 Fe, (5)

Mn + FeO -> MnO + Fe, (6)

MnO + Si02 -» MnSiOg, (7a)

FeO + SiO, -* FeSiO.,. (7b)

The manganese and iron silicates, of low melting point and low specific

gravity, rise to join the slag. The phosphorus is then oxidized:


2 P 4- 5 FeO -» Pa 8 + 5 Fe, (8a)

Pa 5 + 3 FeO -» (FeO) aP3 B,(8b)

(FeO) ,PaOB + 3 CaO -» (CaO)3PsO., + 3 FeO. (8c)

The iron phosphate formed by reaction (8b) is unstable, but in the

presence of an excess of lime, reaction (8c) takes place, and the more

stable calcium phosphate is formed and also leaves the metal and joins

the slag.

Reactions (5) to (8) are exothermic and take place at relatively low

temperatures, as the charge is melting or just after melting is complete.

The oxidation of carbon,

C + FeO - CO + Fe, (9)

by reaction (9) is endothermic and takes place only at high temperatures.

When all the impurities have been eliminated as far as possible or

desirable, the molten metal usually contains less than 0.10 per cent

carbon, less than 0.20 per cent manganese, less than 0.01 per cent silicon

and usually less than 0.03 per cent phosphorus. Sulfur drops from

above 0.04 or 0.05 to 0.025 or 0.035 per cent. As the result of these oxidiz-

ing reactions the molten metal also contains a relatively large quantity of

dissolved iron oxide which, as discussed in Section 11.10, is harmful.

It also contains too little manganese to combine with the sulfur. Toremove harmful oxide and to ensure the presence of sulfur as manganese

sulfide, manganese is added to the molten metal in the furnace or as it

is tapped from the furnace. This reacts with dissolved iron oxide,

Mn + FeO -» Fe + MnO, (10)

to form manganese oxide which is insoluble in the molten metal andwhich rises to the top to join the slag.

The Acid Open Hearth Process. The open-hearth used in the acid

process is similar to that used in the basic process except that it is usually

smaller, and since the slag is acid, the lining of the furnace hearth is madeof acid material also. The initial charge consists of cold pig iron andscrap. No ore is added because the iron oxide is basic and would react

with the lining of the hearth. For this same reason scrap alone is not

used as a charge, for it would oxidize and have the same detrimental

effect on the furnace lining. Only manganese, silicon, and carbon can

be removed readily; therefore a premium grade of pig iron and scrap

that is low in both phosphorus and sulfur is necessary. This tends

to increase the cost of acid steel over the basic type. The acid slag tends to

be less oxidizing than the basic slag, and for this reason there are fewer

oxide inclusions in the steel when it is poured, resulting in a "cleaner"

steel. The acid steel is usually tapped at a higher temperature, in

order that the steel may stand in the ladle for 30 minutes or more, thus


Fig. 11.5. Heroult electric furnace tilted for tapping, showing arrangement ofelectrodes (Courtesy of American Bridge Company)

giving what inclusions are present a chance to rise to the surface andenter the slag.

11.6. Manufacture of High-Quality Steels by the Electric Processes

High-quality carbon and alloy steels are made by the acid open-hearthand the acid and basic electric process (Table 11.1). Most carbon tool

steels and special alloy steels are made in the basic-lined electric furnace.

Selected steel scrap is melted in an arc or induction furnace. The arc

furnace, shown in Fig. 11.5, produces most of these materials in lots of


5 to 70 tons; the induction furnace, with a capacity up to 5 tons, is used

chiefly to melt high-alloy steel scrap without loss of costly alloying metals.

The large arc furnaces are used chiefly for the production of low-alloy

steels by duplexing, that is, by refining further a charge of molten steel

that has been partly refined by the acid Bessemer or the basic open-hearth

process.

If the scrap contains too much carbon and phosphorus, these elements

are oxidized together with the silicon and manganese, under conditions

similar to those outlined for the basic open-hearth process on p. 224.

When oxidation is completed, the slag is removed and a new one is

added, composed of lime, fluorspar, and silica sand or crushed ferro-

silicon. This slag, which consists principally of calcium silicate whenmelted, is deoxidized, and strongly reducing conditions are established

in the furnace by scattering powdered coke or ferrosilicon over the sur-

face. Since slag and metal tend to be in equilibrium, eliminating iron

oxide from the slag also deoxidizes the metal. Furthermore, a strongly

reducing slag will remove sulphur almost completely from the metal.

Owing to the complete deoxidation of the slag and to the absence of

oxidizing gases—a condition attained only in the basic electric process-

much sounder steel can be made by this process than by either the acid

Bessemer or the basic open-hearth process where gases and slag are

strongly oxidizing at all times.

The acid electric process combines the nonoxidizing character of arc

heating with the lining and slag of an acid process. Acid electric steel is

usually not so well deoxidized as basic electric steel, but it is cheaper.

The small acid electric furnace is especially suitable for intermittent

operation and, owing to the high temperatures of the arc, is widely used

for melting steel for small high-grade castings.

The acid process is also used to produce ingots, high-quality forgings,

ordnance and alloy steel. The furnace charge is usually all scrap, andpractically the same purifying procedure is used as in the acid

open-hearth. The steel can be more thoroughly deoxidized since the

oxidizing effect of the furnace gases on the slag is eliminated. Theprocess is used mainly to melt and refine carefully selected scrap low in

phosphorus and sulfur, as the process will remove neither.

The general features of the various steelmaking processes arc sum-

marized in Table 11.1.

11.7. Wrought Iron

Wrought iron is a ferrous material of highly refined iron incorporated

with 1 to 4 percent of a ferrous silicate slag. Two primary methods are



used in its manufacture: (1) the puddling process, and (2) the Aston or

Byers process.

In the puddling process, a charge, weighing about 600 pounds, of iron

scrap, pig iron, iron ore, and mill scale are melted and refined in the

puddling furnace. The fuel used for heating and refining the charge is

bituminous coal burned on grates at one end of the furnace. After the

charge is molten, it is mixed by the operator, using the puddle bar. Dur-

ing the refining period, the oxidization of the carbon, silicon, and man-ganese is brought about by reaction with the iron oxide. As the refining

progresses, the melting temperature rises, so the temperature within the

furnace is not sufficiently high to keep it molten, and it becomes a plastic

mass known as a puddle ball. The puddle ball is then removed from the

furnace, squeezed and rolled into bars to eliminate much of the en-

trained slag. The bars, known as "muck" bars, are cut into short lengths,

fastened together in piles, reheated to a welding temperature and rolled.

If this process is repeated a second time, the product is known as

double-refined iron.

The Aston or Byers process consists of melting Bessemer grade pig iron

in a cupola and refining it in a Bessemer convenor until the carbon,

silicon and manganese are removed. At the same time, an iron silicate

slag of the same composition as that produced in the puddling operation

is prepared in an open-hearth furnace.

The refined metal is poured at a predetermined rate into a ladle, called

a thimble, containing the molten slag. The temperature of the metal is

higher than that of the slag and the former rapidly solidifies. Gases are

liberated during the solidification period and the metal disintegrates into

small globules sinking to the bottom of the thimble to form a spongymass. This operation is known as shotting. The excess slag is then pouredoff and the solidified ball of metal, containing slag, is transferred to the

squeezer and rolls where the same procedure follows as in the case of

puddled iron. Wrought iron is a highly ductile material and is used for

pipe and pipe fittings, bolts, sheets, bars, and plate.

11.8. Special Steel-making Processes

The Duplex Process. The acid Bessemer and the basic open-hearth

are used in succession. Most of the carbon and all of the silicon andmanganese are oxidized in the converter and then the metal is transferred

to the basic open-hearth where the phosphorus, sulfur, and the remainderof the carbon are removed. Under good conditions an open-hearth mayproduce up to twice the tonnage per week by the duplex process.


The Super-refining Process. This is a special duplex process in whichpurified basic open-hearth metal is transferred to a basic electric furnace,

deoxidized, and finished into high-quality electric steel. Care is taken

to have the residual manganese as high as possible in the open-hearth

product. Carbon is added to bring this component near the required

finishing range. Since no further oxidation is required the basic carbide

slag is immediately charged, and dcoxidation and de-sulfurization are

carried out by the carbide and the residual manganese in the metal.

The Triplex Process. This process is carried out in two ways: (1) a

combination of the acid Bessemer and two open-hearths in succession, or

(2) a combination of the duplex and super-refining process. The first

is used when pig iron is very high in phosphorus. The open-hearth is

tapped and charged into another open-hearth where a new slag is built

up and the rest of the phosphorus and carbon removed.

The Talbot Process. This is a process that results in a very large

tonnage of metal in a very short period of time. The increase in capacity

is obtained by using a bath of metal forty or more inches in depth. Thefirst charge, usually consisting of pig and scrap, is worked down to the

desired composition, and about one third of the metal is tapped. Oreand limestone are added to the bath to produce a very basic oxidizing

slag. Molten pig iron, equal to the metal tapped, is poured through the

slag. The reaction is so rapid that the silicon, manganese, and most of the

phosphorus is removed. The charge is then worked down to the required

carbon and phosphorous content as the temperature is raised. For a period

of from 3 to 6 hours, the bath is purified, and another heat is tappedThe slag is reoxidized, more pig added, and the procedure repeated. Atthe end of a week the furnace must be drained and the lining patched,

making the process very expensive.

The Duplex Talbot Process. This process is a combination of the

Talbot and Duplex processes. Metal from the Bessemer converter, free of

carbon, silicon and manganese, is poured through the oxidized slag, the

phosphorus being removed almost immediately. The addition of pig iron

raises the carbon content and aids in deoxidation.

The Monell Process. Limestone, ore, and sometimes steel scrap, are

charged upon a basic hearth and heated until pasty. Molten pig iron is

then added. The low temperature will remove the phosphrous very

rapidly with a violent reaction. The silicon, manganese, and carbon is

worked down in the usual manner.

The Campbell Process. A charge of pig and scrap is melted in a Camp-bell tilting basic furnace and the phosphorus, silicon, manganese andpart of the sulfur are removed. The furnace is tapped and the metal


charged into an acid open-hearth furnace. An acid slag is built up and

the heat is finished by the usual acid procedure.

The Crucible Process. The crucible process consists of melting properly

proportioned scrap, fcrro-manganese and charcoal in a closed crucible.

This is primarily a mixing and refining process. Each crucible holds 50

to 100 pounds of metal, which is heated in a gas fired regenerative

furnace for approximately four hours for complete refining.

11.9. Mechanical Treatment of Steel

Ingot molds: Types, Sizes, Shapes. Most of the steel made in the

United States is cast into ingot molds of one type or another, irrespective

of the type of steel, for further fabrication into desired shapes. This is

done, rather than casting into desired shapes directly, because of the

better physical properties obtained, and because it can be done muchmore cheaply. Two general types of ingot molds are used in the United

States: the big-end-up and the big-end-down. As their names imply, the

big-end-up mold is larger at the top, and the big-end-down is larger at

the bottom.

Most steel is solidified in the big-end-down type mold, since it is suit-

able for the cheaper semi-killed steels and for the rimming-type steels.

These steels avoid shrinkage or actually increase in volume in the molds

by the evolution of gases on cooling, thereby avoiding the formation of

"pipe." By the use of a "hot-top," a refractory collar added to the mold,

killed steels may be poured into big-end-down molds because the steel

in the hot-top section is kept molten and will fill any cavity produced by

shrinkage.

Big-end-up molds tend to produce less pipe since there is a larger

volume of steel at the top of the ingot, and it remains molten longer.

However, when used with killed steels, a hot top is generally used.

Big-end-up ingot molds are more costly and are generally reserved for

high-quality and alloy steels.

Bottom-poured ingots are those in which the metal is introduced fromthe bottom, usually into big-end-up molds with refractory hot-tops. Dueto the complicated arrangement for pouring, they are more expensive andare usually reserved for high-quality and high-alloy killed steels.

Hot Working. After the ingot has solidified, it is transferred to a

soaking pit or heating furnace to equalize the temperature and to heat

it to a temperature at which the steel is very plastic. It is then hot

worked by rolling, pressing, or forging it into a finished or semifinished

product. A few special carbon and alloy steels used in very heavy sec-


tions, such as large guns, axle shafts, armor plate, and the like, and a fewtonnage products are worked from the ingot directly into the finished

section. It is, however, considered better practice, whenever the size

of the finished product permits, to reduce the ingot to a section knownas a bloom, slab, or billet, which is reheated, either with or withoutintermediate cooling and inspection, and is rolled into finished products.These may be rails, structural shapes, plates, sheets, strip, pipe and tubes,

bars, or rods of a wide variety of sizes and forms. Finished sections fromthe large rolling mill, especially bars and rods, may be fabricated furtherinto a multitude of small articles.

Hot working has two primary objects: (1) to produce various shapes

and sizes economically, and (2) to improve the structure and properties

by breaking up the coarse crystal structure of the ingot.

Cold Working. A large tonnage of hot rolled bars, rods, sheets, piercedtubing, and other hot rolled products are worked at room temperature.This operation is known as cold working. Cold working is a finishing

operation and the articles coming to this stage have been roughly shapedby hot working. The general methods of cold working are cold rolling,

cold drawing, cold pressing, or stamping. Hot rolled shapes are finished bycold rolling and cold drawing processes to produce smooth surfaces, ac-

curate dimensions, and increased strength. The drawing of hot rolled

rods through dies is used principally in making wire and in finishing

seamless steel tubing. In the cold pressing or stamping process, plate andsheet for various purposes are often shaped between dies by means of a

heavy hydraulic press.

Cold working also has two primary objects: (1) to produce sections,

sizes, and shapes that cannot be produced economically by any othermethod, and (2) to produce certain combinations of properties, especi-

ally very high strength accompanied by considerable ductility, that also

cannot be secured economically by any other process.

The mechanical treatment of steel is an art that has been perfected

mechanically until it is now in a relatively high state of development.Hundreds of millions of dollars have been invested in rolling mills andforging plants that are marvels of mechanical efficiency and low-cost

production. Modern rolling mills, such as the recently developed con-tinuous strip mill, are, in fact, such marvels of costly perfection.

Although the mechanical equipment used and the methods of hot andcold working are a fascinating field for discussion, they are of little con-cern to the engineer who uses steel and who does not care whether strip

is rolled on a continuous or a hand mill so long as it has the properties

he requires and is cheap enough for him to use economically. The effect


of hot and cold working on the structure and properties, a subject of great

interest to most engineers, is given more consideration elsewhere in this

book.

Steel and other metallic materials that have been hot or cold worked(or, in some instances, cast) frequently must be joined together to formthe finished structure. The principal methods of joining are welding andriveting. The equipment and the methods used for joining are of muchinterest to engineers but are hardly wilhin the scope of a book on ele-

mentary metallurgy and are, therefore, not discussed further here.

11.10. Harmful Elements in Carbon and Alloys Steels

The five so-called harmful elements present in all steels in varying

amounts, but mostly in small fractions of 1 per cent, are phosphorus,

sulfur, oxygen, nitrogen, and hydrogen.

Phosphorus. In the amounts usually present in carbon and alloy steels

phosphorus is combined with the iron as iron phosphide (Fe3 P) , whichdissolves in solid iron as sugar dissolves in coffee. Although metallurgical

opinion is not wholly unanimous, most available data, plus many years of

experience with acid steels containing 0.07 to 0.12 per cent phosphorus,indicate that this element makes steel cold short, in other words, brittle

when cold, particularly in its resistance to impact. This brittleness is moremarked in high-carbon steels than in low-carbon grades and is apparentif the amount of carbon plus phosphorus is above 0.30 per cent. Thestatic ductility is not appreciably affected.

Because of the brittleness caused by phosphorus, the maximum amountpermitted is usually given in engineering specifications. This varies froma maximum of 0.10 to 0.12 per cent in low-carbon acid Bessemer steels

to a maximum of 0.045 per cent in rails and railway materials, structuralshapes, sheet, strip, and other products of the basic open-hearth process.

In tool steels and other high-grade high-carbon and alloy steels, themaximum permitted is usually 0.03 and occasionally even 0.02 per cent.The average amount of phosphorus in various grades is shown in

Table 11.1.

Despite the fact that, in general, phosphorus has a bad name and fornearly a hundred years has been considered a nuisance that should bekept as low as possible, it has been discovered lately that it has its goodpoints. It raises the tensile and the yield strength and improves otherproj>erties, including the resistance of steel to some varieties of corrosiveattack. Some of the new low-alloy high-strength steels contain about0.10 per cent phosphorus used together with small amounts of copper,nickel, or chromium. The carbon, however, is low.


Sulfur. In a manner analogous to that of phosphorus, sulfur will com-

bine with iron to form iron sulfide (FeS), which dissolves in molten iron.

However, manganese, if present, having an affinity for sulfur which it

does not have for phosphorus, will rob the iron of its sulfur to form

manganese sulphide (MnS) . This compound is almost completely insolu-

ble in solid iron. Consequently, when the iron solidifies, manganese sul-

fide is present in the mass of metal as discrete particles. These particles,

together with oxides and silicates, are known to the metallurgist as solid

nonmetallic inclusions or, more simple, as inclusions, and to the man in

the mill by the more expressive term dirt. If insufficient manganese is

present, some of the sulfur remains as iron sulfide. This compound melts

at a temperature lower than the usual temperatures for rolling or forging,

with the result that the steel is likely to crack during hot working. This

brittleness or fragility at elevated temperatures is known as hot shortness.

Practically all commercial steels contain plenty of manganese, so that

engineers who use steel at elevated temperatures need not worry about

hot shortness caused by sulfur. Of much more interest to engineers is

the fact that manganese sulfide particles are always present and that

they may be present in large amounts if the sulfur is high. If these

particles segregate in vital areas, they may have a deleterious effect on the

ductility of the steel; in fact, impact resistance may be greatly reduced.

Any particle of dirt in the metal, if large enough, if the shape is favorable,

and if it is strategically located, may act as a stress raiser and cause

failure, especially by fatigue, much sooner than would normally be

expected.

Sulfur has few virtues when present in steel. Compared with phos-

phorus, it is a real nuisance and is, therefore, kept as low as possible.

Its only benefit, so far as is known now, is that, if the manganese sulfide

is present in the proper amount and is well distributed, it makes the

steel easier to machine. Steels that are machined automatically at high

speeds and that are used for parts that are not subject to high impact

stresses contain 0.080 to 0.150 per cent, or even more, manganese sulfide.

With higher sulfur percentages the manganese is frequently increased

from the usual amount in low-carbon steels, namely 0.30 to 0.60 per cent,

to O.fiO to 1.65 per cent to ensure that no iron sulfide is present. Theaverage sulfur percentages for various grades of steels made by the

different processes are shown in Table II. 1. In general, the higher

amount of the range is the maximum permitted by the usual specifica-

tions.

Oxygen. All steel, except that made by some special variants of the basic

electric process, is melted and refined by gases and slags that are essen-


tially oxidizing. At the high temperatures used, oxygen combines avidly

with iron, carbon, manganese, silicon, and some other elements that maybe present, forming a variety of gaseous or liquid oxides. Some of these

dissolve in molten steel and are thrown out of solution when the steel

solidifies, others are deoxidized by later slags or by alloys added especially

for this purpose. The result of these deoxidizing reactions, which are

more effective in some processes than in others (see Table 11.1), is to

produce a steel which is more or less free from oxygen and its reaction

products. Even under the best conditions, however, the steel is never

wholly cleansed; hence, all steels when solid contain a larger or smaller

quantity of gas cavities or solid inclusions bearing oxygen in some form.

These oxides or combinations of oxides (silicates) form, together with

sulphides, the most common source of the solid nonmetallic inclusions or

dirt found in commercial' steels. Like sulfide inclusions, oxides andsilicates, if segregated or if present in large particles, may act as stress

raisers or as loci of weakness where, under favorable conditions, failure

may start.

Inclusions that are entrapped in the steel ingot (or casting) when it

solidifies arc usually in the form of small rounded particles varying in

size from those which are submicroscopic to those which can be seen

on a polished surface with the unaided eye. At high temperatures mostof the inclusions are plastic. Consequently, when the ingot is rolled or

forged, they elongate into stringers or threads. It should be emphasizedthat inclusions, once they are entrapped in the steel, may be distorted,

elongated, or otherwise changed in form by hot or cold work, but they

cannot be removed or even diminished in amount by any treatment. Nocommercial carbon or alloy steel is wholly free from dirt. The amountpresent and the degree of dispersion of the particles depend to a con-

siderable extent upon the nature of the melting process. As indicated in

Table 11.1, basic electric steel is inherently the cleanest, acid electric steel

ranks next, and acid Bessemer steel ranks lowest. The process itself is,

however, not the only factor. Also important are quality of the rawmaterials used and the skill of the man who makes the steel.

It has been well known for many years that most ferrous materialsexhibit directional properties, that is, some properties, notably impactresistance, are lower if determined on a specimen cut at a right angleto the direction of rolling than if determined on a specimen taken longi-

tudinally. Elongated inclusions are an important factor in causing this

difference in properties.

Owing to the impure raw materials available to the iron and steel

industry and to the necessity of using-except in a few restricted cases-


oxidizing reactions to get rid of these impurities economically, one of the

chief problems of steel men has been to eliminate the harmful effects of

oxygen and its compounds by deoxidizing as thoroughly as possible. Formore than sixty years one of the principal tenets of metallurgists has been"the more complete the deoxidation, the better the steel." Within the

past twenty or twenty-five years, however, it has been found that the

phrase "for certain steels" should preface this doctrine of deoxidation.

McQuaid, Bain, and other investigators in this field have shown that

a certain degree of oxidation may be very desirable in some carbon andalloy steels in which a control of the grain size is advantageous.

The amount of oxygen in solid steel varies from a trace to as muchas 0.02 per cent: few steels contain less than 0.005 per cent; the average

amount in commercial carbon steels is probably between 0.01 and 0.015

per cent, with the higher amounts found normally in the lower carbon

grades. Most of the oxygen—probably all of it in most steels— is present

as an oxide or silicate. Small amounts of oxygen are difficult to deter-

mine accurately by chemical analysis, and it is even more difficult to

determine in what form it exists in the metal.

Nitrogen and Hydrogen. Although the mechanism is not well under-

stood, it is generally conceded that the presence of either nitrogen or

hydrogen in carbon and alloy steels will tend to increase their brittle-

ness.

11.11. Manganese in Carbon and Alloy Steels

Manganese is an essential and a beneficial element in nearly all

grades of steel because it performs a vital dual role: It combines with

sulfur to form manganese sulfide, a less obnoxious impurity than iron

sulfide; and, as noted previously, it deoxidizes the .metal by reacting

with oxygen to form an oxide which is less soluble in molten steel than

iron oxide and which, therefore, will leave the metal more readily. Theamount used varies with the grade of steel and with the amount of

oxidation in melting; enough is added to most steels so that, after its

purifying action has been completed, 0.30 to about 0.75 per cent, occa-

sionally as much as 1.00 per cent, will be left in the metal.

A small amount of manganese also increases the strength of steel.

Either alone or, preferably, in combination with relatively small amountsof other alloying elements, it produces such a favorable combination of

properties that the so-called intermediate-manganese alloy steels are

finding increasing use in some fields of engineering, especially for rail-

road rolling stock and in bridges. They were also widely used during

the war as a substitute for low-alloy steels containing considerable nickel.


The amount of manganese in these steels ranges from about 1.00 to 1.90

per cent. There is considerable confusion, even among metallurgists,

about the dividing line between these steels and carbon steels as somesteels that are made and used as intermediate-manganese alloy materials

may contain 1.00 per cent or even less, and a few steels that have for

years been classed as plain carbon steels—rails are an example—containas much as 1.00 per cent or even more manganese. If carbon steel andintermediate-manganese steel cannot be differentiated by their manga-nese content, they can sometimes be classed according to the industrial

application for which they were made. This is a glaring example of the

slipshod terminology that has grown up over the years in the iron andsteel industry and that confuses maker and user, metallurgist and engi-

neer, alike.

11.12. Carbon Monoxide, and Rimming and Killed Steels

As a result of the reaction between oxygen and carbon in the moltenmetal large volumes of carbon monoxide gas are formed. Most of this

gas is given off during refining; some of it, however, remains entrappedand, unless removed by silicon or other degasifier as described in § 11.13,

will be still in the metal when it starts to solidify. Most of this remainingcarbon monoxide escapes during solidification. Owing, however, to theviscosity of the metal just before it solidifies, some of the gas remainsentrapped to form cavities of varying size in the ingot or casting. Thesecavities-blow holes as they are commonly called-will usually weld inrolling or forging, especially in low- and medium-carbon steels, and willdisappear. If, however, the inner surface of the cavity becomes oxidized,which frequently happens in heating for rolling when the cavity is nearthe surface or if the steel is high in carbon, the surfaces of the cavity do

not weld and the result is a defect

known as a seam (Fig. 1 1 .6). Seamsare usually located on the surface of

the rolled section where they can be

detected and removed by machining,

chipping, or grinding; but they maybe internal, and if so they may, like

inclusions, act as loci where prematurefailure will start.

Carbon steels are frequently classi-

fied according to the method of re-

moving gas cavities or rendering themKic. 11.6. Scams on the surface of a innocuous, as killed, or rimming

shaft. 2x- types.


Killed steels, which nearly always contain more than 0.25 or 0.30 per

cent carbon, are those which must be thoroughly sound and free from

gas cavities. The descriptive adjective comes from the action of the

molten metal when poured into the ingot mold: it lies perfectly quiet

with no evolution of gas; there is neither bubbling nor churning of the

upper surface of the metal. Killed steels are degasified by silicon, alumi-

num, or other degasifiers. All plain carbon forging steels, all rails, all

high-carbon tool and spring steels, and all alloy steels are thoroughly

killed.

Rimming steel—so called from the rim

of solid metal next to the mold wall

(Fig. 11.7)—is low-carbon basic open-

hearth steel in which deoxidation is partly

completed in the furnace or in the ladle

but which is not degasified. When the

steel is poured into the ingot mold and

begins to solidify, there is a brisk but

controlled evolution of gas which results

in an ingot having a sound gasfrec surface

and in locating the blow holes so far be-

low the surface that there is no danger of

their becoming oxidized when the ingot is

heated for rolling (Fig. 11.7). The blow

holes will weld completely in rolling, and

the result is a material with a clean sur-

face free from seams. Rimming steels are

especially suitable for sheet and strip,

notably for thin, deep-drawing stock used

in large tonnages for automobile bodies

and fenders. Close control of the slag

composition (particularly the amount of

iron oxide) , slag viscosity, and the pouring

temperature, is necessary to produce a

steel that rims properly when it solidifies.

Most basic open-hearth steels contain-

ing less than 0.15 per cent carbon are

made so that they rim; steels containingFig. 11.7. Cross section of an in- 0.15 to about 0.25 per cent carbon, used

got of rimming steel about one-jd , f structural shapes and plate,

twentieth natural size. (Courtesy of ' < "'

American Rolling Mill Company) are killed or are partly killed.


11.13. Silicon and Other Degasifiers

Like manganese, silicon is beneficial and is added to carbon steel as

a purifier and, in larger amounts, as both purifier and alloying element.

It is effective in removing oxygen and is, therefore, added to those grades

of steels (usually containing more than 0.30 per cent carbon) in which

gas cavities do not weld readily or are otherwise harmful. Silicon is so

used in a fe"' medium-carbon and high-carbon basic open-hearth steels

(for example, rails) , in practically all acid open-hearth and acid electric

steels, and in all basic electric steels. It is added rarely to such low-

carbon basic open-hearth steels as structural material, wire, sheet, plate,

and strip or to those low-carbon Bessemer steels which are used for

pipe and tinplate. The reaction product of silicon with carbon monoxidegas is silica (Si0 2), which reacts readily with manganese oxide and iron

oxide, forming silicates. These are insoluble in the molten metal and are

of such low specific gravity that they readily leave the steel. Silicon thus

carries deoxidation farther than manganese and is in addition an effective

degasifier. Enough is added to complete the purifying reactions and leave

0.10 to 0.40 per cent in the finished steel.

There is occasionally some confusion—but not so much as in the case

of manganese—in distinguishing between a silicon-treated carbon steel

and a silicon alloy steel. Some specially deoxidized open-hearth steels

used in bridge construction, containing around 0.25 per cent silicon, havebeen and still are termed silicon steels by civil engineers. This is un-fortunate, and it is hoped that the tendency to call a material silicon

steel only if the silicon is 0.50 per cent or more will become more wide-spread. Steels containing 1.50 to 2.25 per cent silicon and about 0.75per cent manganese are used widely for springs, and steels containing0.5 to 5.0 per cent silicon and small amounts of other elements are usedin the construction of dynamos and other electric apparatus.

Small amounts of aluminum and titanium are added to some grades ofcarbon steel as final deoxidizers and degasifiers. It has been discoveredrecently that aluminum, probably through the agency of minute alumi-num oxide particles, can be used to control the grain size of carbon andalloy steels. As this is one of the major recent developments in ferrousmetallurgy, it receives detailed attention elsewhere in this book. Bothaluminum and titanium are also used, in much larger amounts than is

necessary for deoxidation and degasification, as alloying elements.

11.14. Other Elements

Since the introduction of alloy steels some fifty years ago there hasbeen a gradually increasing contamination of carbon steel by nickel.


copper, and other alloying elements owing to the inadvertent mixture of

alloy-steel scrap with the other scrap. In the case of elements that oxidize

readily, for example, chromium, vanadium, aluminum, titanium, and a

few others, the contamination is not serious, as they are largely oxidized

and removed during melting and refining, leaving usually not more thantraces in the steel. In. the case of such alloys as nickel and copper, whichdo not oxidize, the amounts present will normally increase slowly over aperiod of years. The amount of these elements in the scrap is not of

such serious consequences for steels made by a process that uses also a

large proportion of pig iron, which is normally free from nickel and:opper, as it is in steel made using a large percentage of scrap (Table

11.1). In the latter the scrap may contain an unsuspected 0.10 to 0.25 percent nickel and 0.05 to 0.10 per cent copper, sometimes even enough to

affect the properties materially.

A recent development in free-machining steels is the addition of lead

to low-carbon material. This element is insoluble in carbon steel, but byspecial methods of addition to the molten metal it may be incorporated

as a suspension so finely disseminated throughout the metal that it is noteasily visible in an unetched section with the microscope. These sub-

microscopic particles of lead apparently act as an internal lubricant andin addition cause the chips to break up readily. Recently reported data

indicate that a lead content of 0.10 to 0.25 percent greatly increases ease

of machining but has no appreciable effect on mechanical properties.

Carbon steel may be contaminated with small amounts of arsenic, tin,

and antimony, traces of which may be present in iron ores and maypersist through melting and refining. These metals may also be picked

up from scrap containing bearing metals or tin cans. The amounts present

are small, rarely exceeding 0.05 per cent. Very little is known about the

effect of small amounts of most of these adventitious elements. According

to the present state of our knowledge, their effect, with the possible

exception of that of antimony, which is assumed to be harmful, maybe called neutral. During the war, when some detinned scrap was used

in the manufacture of open-hearth steel, considerable attention was given

to the effect to tin. It was found that small amounts of this metal do not

cause hot shortness in rolling; they do, however, increase hardness andreduce toughness. Tin, therefore, may be considered definitely harmfulin some grades of steel, especially in those used for deep drawing.

11.15. Low-Alloy Steels

A large number of metals (and some metalloids and nonmetals) havebeen alloyed with carbon steel, either alone or in various combinations,

and the resulting properties have been studied more or less completely.


These include manganese, silicon, nickel, chromium, vanadium, tung-

sten, molybdenum, copper, and phosphorus as the more common alloy-

ing elements and cobalt, aluminum, zirconium, titanium, nitrogen, lead,

boron, and selenium as the less frequent additions. Steel may also be

coated or plated with zinc (galvanizing) , tin, lead, chromium, or nickel,

but such products are not classed as alloy steels.

There are two general classes of alloy steels: (1) the low-alloy engi-

neering steels and (2) the high-alloy tool and die steels, corrosion- and

heat-resistant steels and special-purpose alloys. In tonnage, the first class

is the more important; in value to man, both are probably of equal im-

portance, for both have played a vital role in the development of our

present-day machine-age civilization.

Before World War II there were several hundred low-alloy engineering

steels, of which some 70 to 80 were considered as standard by the Society

of Automotive Engineers. These steels were divided into eight main

classes: intermediate-manganese, silicon-manganese, nickel, nickel-chro-

mium, molybdenum (including chromium-molybdenum and nickel-mo-

lybdenum), chromium, chromium-vanadium, and tungsten steels. Therewere also a large number of low-alloy structural steels, most of themdeveloped between 1933 and 1939, that contained small amounts (usually

less than 1 to 1.5 per cent) of nickel, copper, chromium, manganese,

silicon, phosphorus, and molybdenum in various proportions and com-

binations. These were developed to satisfy a demand for a material of

better properties than carbon steels but cheaper than the S.A.E. grades.

These have been nicknamed "Irish stew steels" by Gillett, a happy andaccurate designation.

11.16. High-Alloy Steels

There are almost as many varieties of the high-alloy steels as there are

of the low-alloy grades. These special materials can be divided into three

classes. The first includes the well-known high-speed steels and the

closely allied die and valve steels. The primary requirement is hardness,

especially at elevated temperatures, which is attained by large amounts

(10 to 25 per cent) of tungsten, chromium, and occasionally cobalt, plus

smaller amounts of vanadium, molybdenum, or silicon, together with

relatively high carbon percentages.

The second class includes all the highly alloyed steels used primarily

because of their corrosion resistance or scale resistance at normal andat high temperatures. The basic element in these steels is chromium, in

amounts ranging from 10 to 35 per cent, either alone or together with

varying amounts of silicon, manganese, nickel, copper, or molybdenum.The best known steels of this group are the cutlery steels, containing


about 0.35 per cent carbon and 14 per cent chromium, and the soft

stainless steels, containing low carbon, 15 to 25 per cent chromium, and6 to 20 per cent nickel, which are used widely for building trim, hardware

and fixtures on buildings and automobiles, various kinds of utensils andmore recently, in the construction of railroad equipment and aircraft.

Not so well known but just as important are the highly alloyed steels

containing nickel, chromium, molybdenum, cobalt, and other alloying

metals, used in steam plants, oil refineries, gas turbines, including jet

engines, and in other applications where high stresses and high tempera-

tures are encountered. Also in this second class are the alloys containing

up to 65 per cent nickel, 15 to 20 per cent or more chromium, the re-

mainder being iron. These are the heat-resistant alloys of high electric

resistivity that are used for the heating elements of all our domestic

and industrial heating appliances.

The third class includes a large number of alloys of highly specialized

but important uses. They arc the steels with special magnetic and electric

properties, alloys with special expansion characteristics, steels with ex-

ceptional wear resistance, and many others. Although these materials

are not manufactured in large tonnages, some are of such importance

that by means of them the communication and other industries have

experienced a veritable revolution in the past twenty or twenty-five years.

Nearly all the high-alloy materials are made in electric arc or induction

furnaces. Alloy steels are much more costly than carbon steels so that

care is exercised to keep the quality as high as possible. Selected scrap is

used, deoxidation is carried farther, and sulfur and phosphorus are usually

required to be lower than in carbon steels.

QUESTIONS

1. Name the four principal raw materials of the iron and steel industry. Namethe two main classes of final products.

2. What are the general principles underlying the reduction of iron ore in

the blast furnace? Name the two principal reactions in the blast furnace,

and give the approximate temperatures at which each reaction takes place

most readily.

3. In the manufacture of pig iron, describe how the amounts of phosphorous,

sulfur, and silicon are controlled in the product.

4. What is blast furnace slag, and where does it come from? What are its

principal (unctions?

5. Describe the acid Bessemer process. What are die raw materials used? De-scribe the principal oxidizing reactions. How does the cost and quality of

the Bessemer process compare with the basic open hearth?


6. Sketch briefly the general features of the basic open-hearth process, and

compare it with the acid open-hearth process in (a) raw materials, (b)

slags, (c) chemical reactions, (d) general quality of the product.

7. Give two advantages of acid open-hearth steel production over basic open-

hearth production. Give two advantages of basic open-hearth production

over acid open-hearth production.

8. Describe the regenerative principle of the basic open hearth. Give two

reasons for its use.

9. Differentiate between the "ore boil" and the "lime boil" by describing the

reactions in each.

10. Describe how the impurities, such as phosphorous, sulfur, manganese,silicon, and carbon are eliminated in the basic open hearth.

11. Give four advantages of the electric processes for making steel. Do they

have any disadvantages?

12. Describe the manufacture of wrought iron. What are its uses? How doeswrought iron differ from ingot iron?

13. What stcelmaking process would you recommend for the production of

steel for the following: (a) wire for suspension cables? (b) sheet steel for

tin cans? (c) alloy-steel crankshafts for airplane engines? (d) rails for

mainline railway track? (e) armor plate for a battleship? (f) small high-strength castings? (g) tools for machining automobile connecting rods?

14. Compare the usual amounts and the principal effects of sulfur and phos-

phorous in carbon and alloy steels.

15. Describe the principal functions of manganese and silicon when added to

carbon steel. Why is manganese a valuable alloying element?16. Describe the essential differences between rimming and killed steels. How

do these two classes ordinarily differ in carbon content?

17. Give a practical definition of low-alloy steels. Define high-alloy steels andgive the characteristic property of the classes of high-alloy materials.

CHAPTER

12

The Constitution of S tee I

Hollis Philip I.eighly, Jr., Ph.D., Chairman, Depart-

ment of Metallurgy, University of Denver, Denver,

Colorado

John Stanton Winston, M.A., M.S., Chairman, Depart-

ment of Metallurgy, Mackay School of Mines, Univer-

sity of Nevada, Reno, Nevada

1 HE most common group of alloys in use today

are those referred to as steels. These alloys consist primarily of iron plus

carbon in varying amounts, to which may be added various alloying ele-

ments to achieve the desired properties. Steel is used in tremendous

quantities to produce necessities and luxuries; without steel our civiliza-

tion would be quite primitive. It is not intended to indicate, historically

speaking, that steel is a new material. On the contrary, steel was pro-

duced as early as 1200 B.C. However, it is only within the past century

that methods have been developed to produce steel in large quantities

cheaply enough to make it a material of very common usage.

12.1. The Allotropy of Iron

The effects of carbon additions to iron are intimately associated with

the allotropy of iron. In the process of freezing and cooling to atmospheric

temperature, iron undergoes two allotropic transformations. Upon solidi-

fication at 2802°F. (1539°C.) iron assumes a body-centered cubic struc-

ture (8 Fe). At 2552CF. (I400°C.) this structure transforms into a face-

centered cubic structure (y Fe). This structure persists clown to 1670°F.

(910°C.) where it transforms back to the body-centered cubic structure

(a Fe). Actually, 8 and a iron are the same form of iron. They are often

regarded, however, as distinct modifications because of the two ranges in

temperature in which the body-centered cubic structure is stable.

The transformations of iron from one crystal structure to another are

accompanied by changes in various properties; such as, density, electrical

conductivity, and thermal change. The thermal change consists of an

244

The Constitution of Steel

1245

Time

Fig. 12.1. Cooling curve for pure iron.

absorption of heat during heating and an evolution of heat during cool-

ing. Fig. 12.1, the cooling curve for pure iron, illustrates how the measure-ment of the changes in a given property at transformation may be usedfor determining the temperature of transformation.

12.2. Iron-Carbon Phase Diagram

In the previous chapter it was shown that iron is reduced from its

ore by means of carbon and that the resulting product, pig iron, containsapproximately 4% carbon. Two effects of the carbon immediately evidentare that it lowers the melting point of the iron and makes the resultingalloy brittle. It was also shown diat the chief function of the steel makingprocess is to lower the carbon content to the proper level in order toobtain a certain combination of desired properties. The effects of carbonare so strong that its content is specified in hundreths of a per cent.The best approach to understanding the nature of the iron-carbon

alloys is to study the phase diagram of the system shown in Fig. 12.2.In pure iron-carbon alloys, the carbon not in solid solution usually occursas cementite (Fe3C) , which contains 6.68 per cent carbon by weight.Although cementite is metastable and decomposes, given sufficient timeat elevated temperature, into iron plus graphite, conditions are generallysuch that in steels and pure iron-carbon alloys the carbon is present ascementite. Therefore the phase diagram chosen for study concerns theequilibria between iron and cementite; and thus, in reality, is the iron-iron carbide diagram. Although this diagram covers a small proportion ofthe possible combinations of iron and carbon, it includes the entire rangeof commercial iron-carbon alloys (plain carbon steels and cast irons)

.


Atoms per cent, carbon

4 6 8 10 12 14 16 IS 20

\"Hypoeutectoid\Hyptreutectoid

/rons-r* Stee/s

Fie. 125. Iron-carbon equilibrium diagram.

Carbon is soluble to a limited extent interstitially in each of the allo-

tropic forms of iron. Inspection of the iron-iron carbide diagram shows

that a-iron can dissolve a maximum of 0.025 per cent carbon, y-iron up to

2 per cent and 8-iron up to 0.1 per cent. These solid solutions of carbon

in alpha, gamma, and delta iron are named ferrite, austenite and l-ferrite,

respectively. The greater solubility of carbon in face-centered gamma iron

than in body-centered alpha or delta iron is probably due to the larger

size and more favorable location of the interstices in the face-centered

structure.

Since delta iron has little to do with the final microstructure of iron-

carbon alloys, transforming as it does to austenite, it will not be con-

sidered further. It should be noted, however, that in alloys between 0.1

and 0.5 per cent carbon content austenite is formed by a periiectic re-

action.

Alloys whose compositions fall between 0.5 and 2 per cent carbon

solidify as austenite. For those alloys containing more than 2 per cent

carbon a eutectic reaction occurs producing austenite and cementite.

The typical eutectic structure which forms is known as iedeburite. Alloys

containing this brittle structure cannot be hot- or cold-worked but must

be cast, and are therefore known as cast irons.

The Constitution of Steel 247

All alloys with compositions between 0.025 and 6.68 per cent carbon

undergo a eutectoid reaction at 1333°F. (723°C.) when austenite con-

taining 0.8 per cent carbon decomposes, at constant temperature, into

pearlite, an intimate mixture of ferrite and cemcntite (Figs. 12.3F and

12.4) . A plain carbon steel of this composition is called a eutectoid steel.

Steels having less carbon or more carbon than this are called hypoeute-

rtoid and hypereutectoid respectively.

From the cooling curve for pure iron (Fig. 12.1) it is evident that

arrests are experienced when the transformations occur. Such arrests

are also present in the cooling curves for iron-carbon alloys. The tempera-

tures at which these transformations in the solid state occur are called

critical temperatures or critical points. The loci of these points (lines)

on the phase diagram are given symbols. In hypoeutectoid alloys, A3 and

Aj are the upper and lower critical temperatures (lines GS and PSFig. 12.2); in the eutectoid alloy and hypereutectoid alloys, A.t

and A,

coincide (line SK Fig. 12.2) and, therefore, is designated A ll3 . The zone

between A3 and A, is known as the critical range.

Some of the other lines appearing on the phase diagram have also been

given symbols. The curve separating the fields austenite and austenite

plus cementite (line SE) is designated A,.„, and the line of temperatures of

transformation of B-iron to y-iron (line AHJB) is called A^.

An understanding of the phase relationships shown by the iron-iron

carbide diagram and the final microstructures attained is best obtained by

following the process of solidification and subsequent cooling of several

representative alloys of the system.

12.3. Phase Changes of Slowly Cooled Plain Carbon Steels

At the completion of solidification all steels consist of austenite while

at atmospheric temperature they consist of ferrite and cemcntite. Their

microstructure depends upon the relative amounts of ferrite and cement-

ite and the way in which these are arranged. This, in turn, dependsupon the carbon content.

The phase changes in carbon steels cooled slowly from a high tem-

perature can be illustrated by three steels containing (A) 0.30 per cent

carbon, (B) 0.80 per cent carbon, and (C) 1.30 per cent carbon. Theseare indicated in Fig. 12.2 by lines xx', yy' and zz'.

At 2820°F. (1550°C), the first steel (xx') is a solution of carbon in

molten iron. Upon slow cooling, there is no change until the solidification

temperature 2768°F. (1520°C.) is reached. Here solidification of S iron

begins and progresses with further cooling until at 2720°F. the peritectic

reaction of S + liquid -» austenite occurs with an excess of liquid in this

case. Further cooling causes ihe liquid to disappear so that at 2687°F.


Fig. 12.3. Microstructures of slowly cooled iron-carbon alloys: (A) low-carbon iron;

(B) 0.20 per cent carbon steel; (C) 0.40 per cent carbon steel; (D) 0.80 per cent carbonsteel; (E) 1.3 per cent carbon steel; (F) pearlite at high magnification; etched. A to

E, lOOx: F, 1.000X-


(1475°C.) solidification of austenite is complete. The white-hot steel,

which consists wholly of a solid solution of carbon in gamma iron, cools

unchanged until a temperature of 1512°F. (822°C.) is reached. At this

point on line A 3 (GOS) , the austenite is saturated in iron (ferrite) . If the

temperature falls slightly, the excess iron is precipated in the form of

alpha phase. Since the austenite loses no carbon during the process,

the loss of some iron as alpha phase by the austenite is equivalent to in-

creasing the carbon content of the austenite. As the temperature contin-

ues to fall, precipitation of alpha iron and increase of carbon content of the

austenite (since there is progressively less austenite for a constant amount

of carbon) occur continuously. When the temperature reaches that of the

A i (PSK) , the austenite is saturated with carbon. Further cooling results

in simultaneous precipitation of alpha iron and carbon (as iron carbide)

.

This process goes to completion, which is to say that the austenite dis-

appears completely.

If the cooling is very slow through the temperature interval from line

As (GOS) to just below line A x(PSK), the excess ferrite which is first

formed assumes the shape of discrete polyhedral grains, and the remain-

ing austenite, containing 0.80 per cent carbon, transforms to the pearlite

aggregate in which the ferrite and cementite particles are in the form

of relatively large laminated plates. The structure of the slowly cooled

steel consists of 36 per cent pearlite and 64 per cent excess ferrite.*

If a piece of slowly cooled carbon steel is polished and etched with

dilute acid and examined with the microscope, the relation between the

carbon content and the amount of free ferrite (etches light) and pearlite

(etches dark) is clearly evident. This relation is shown in Fig. 12.3.

Micrograph A shows the structure of iron containing practically nocarbon; it consists wholly of polyhedral grains of ferrite. In B, the steel

containing 0.20 per cent carbon, the dark grains of pearlite are numerous,

and in C, the material containing 0.40 per cent carbon, they are still morenumerous. The parallel plates of cementite and ferrite making up the

pearlite aggregate are shown clearly at high magnification in micrographF, and when viewed by the electron microscope, as is evident from Fig.

12.4.

Referring again to Fig. 12.2 (line yy'), at 2820°F. (1550°C.) a steel

containing 0.80 per cent carbon is a solution of iron carbide in molteniron. As the molten metal cools, it is unchanged until a temperature of

2670°F. (1468°C.) is reached where solidification of the austenite be-

gins. This is not complete until a temperature of 2520°F. (1380°C.) is

• 0.30 per cent carbon is equivalent to 4.5 per rent Fe3C. Since 1 part of KeaC is

associated with 7 parts of ferric to form pearlite. 4.5 per cent Fe,C plus 31.5 per centferrite form 36 per cent pearlile.


reached. It will be noted that with higher carbon the solidification rangeis wider.

After the steel has solidified completely, the solution of cementitein gamma iron cools unchanged until a temperature of 1333° (723°C.)

is reached. Owing to the fact that the steel contains 0.80 per cent carbonand, therefore, the exact amounts of cementite (12 per cent) and iron

(88 per cent) to form the aggregate pearlite, there will be no preliminary

precipitation of alpha iron from the saturated austenite, as was the case

with the 0.30 per cent carbon steel described in the previous section.

As the steel cools through the A x temperature of 1333°F. (723°C.)

,

the austenite undergoes precisely the same change as was described for

the last stage of transformation of the 0.30 per cent carbon steel, that

is, simultaneous precipitation of alpha iron and iron carbide. There is

no further structural change as the steel cools from A x to room tempera-

ture, and if cooling through the Axtemperature is slow, the cementite

and ferrite form well-defined plates as shown in Fig. 12.3D and, at high

magnification, in Figs. 12.3F and 12.4.

In the case of a steel containing 1.3 per cent carbon (19.5 per cent

cementite), represented by line zz' in Fig. 12.2, the changes taking place

during slow cooling are fundamentally analogous to those taking place

in a 0.30 per cent carbon steel. The essential difference is (in addition

to a wider solidification range) that the A cm line marks the preliminary

separation of excess iron carbide rather than excess alpha iron. When a

steel containing 1.3 per cent carbon cools slowly, at 1640°F (893°C.)

the austenite is saturated with carbide. Further cooling results in pre-

cipitation of this excess carbide from the saturated austenite, whichcontinues as the temperature falls from 1640°F. to 1333°F. (the A x tem-

perature) . The remaining austenite, which now contains 0.80 per cent

carbon (12 per cent cementite) , here transforms to pearlite, as in the

preceding case. At room temperature, the structure is made up of grains

of pearlite (etches dark) with a network (or needles) of excess carbide

(light) . This structure is shown in Fig. 12.3E.

The discussion to this point has all been concerned with slow cooling.

In heating carbon steels, the reverse changes take place. For example,

a 0.30 per cent carbon steel that has been cooled slowly consists at

room temperature of a structure of 36 per cent pearlite and (>4 per cent

excess ferrite. If this steel is heated slowly, no change takes place until a

temperature of 1333°F. (723°C.) is just exceeded. Here, the alpha iron

of the pearlite transforms to gamma iron and the cementite goes into

solution. As heating is continued, the excess alpha iron changes to gamma


Fie. 12.4. Electron micrograph of pearlite in eutectoid steel. Polystyrene-silica rep-lica. 20,000x. etched. Approximately i/

A in. on the micrograph = i micron = 0001mm. (Courtesy of C. S. Barrett)

iron and is absorbed by the austenite. At 1512°F. (822°C.) , these

changes are complete, and the steel, now a solid solution of 4.5 per cent

cementite in gamma iron, remains unchanged to the melting point.

12.4. Isothermal Transformation in Steel

Considerable work has been done in studying the effect of continuouscooling upon the structure and properties of steels. The effect of the

various alloying elements was known in a rather general way, however,no good explanation was available. In 1930, Davenport and Bain # pre-

sented a paper describing the results of a study of the isothermal transfor-

mation of steel. This paper resulted in a further extensive study of manyiron-carbon alloys, which gave a better understanding of the mechanismof austenite decomposition and the part played by the alloying elements.The results of these studies are published in the Atlas of IsothermalTransformation Diagrams.^

• E. S. Davenport and E. C. Bain, Trans. Amer. Inst. Min. & Met. Eng V 90 1930p. 117-154.

t Atlas of Isothermal Transformation Diagrams, U. S. Steel Corp., 1951.


1 1 1 1

Avsrtvit aunt

. _

A, TCMPOIAIVfiC

jurtrmret

Ais-

V "

- -

\ \ -

\ \WLHSTC W*J \

1

I [.....!._..

-

130000 ICCOO00

10OMITHMIC TIUC I

Fig. 12.5. Schematic Drawing of a Typical Isothermal Transformation Curve Re-vealing Significant Features.

The results of isothermal transformation studies are presented in the

form of diagrams such as the one in Fig. 12.5. These are usually referred

to as TTT curves (time-temperature-transformation) with time as the

abscissa and temperature as the ordinate. The left curve indicates the

beginning of transformation with the right curve showing the completion.

In the case of high or low carbon steels, the formation of the pro-eutectoid

cementite or ferrite is usually indicated on the diagram as an additional

feature.

To construct a TTT curve, it is necessary to heat thin specimens of the

desired steel into the austenite region for sufficient time to obtain a

homogeneous structure. The specimens are then quenched into a pot of

molten lead or salt which is maintained at some constant temperature, i.e.,

isothermal, below the austeniti/.ing temperature. These specimens are

kept in the molten bath for varying time intervals from 1 second to as

much as 105 seconds or longer. At the end of the desired time interval,

the specimen is quenched into cold water causing the isothermal trans-

formation to be- terminated. Any austenite not transformed isothermally

is transformed by quenching to martensite. The specimens quenchedisothermally at a particular temperature are prepared metallographically

and etched. Microscopic examination will reveal that the isothermal

transformation product appears as dark areas in a white field. The


<

el

'

I

Fie. 12.6. Micrograph of coarse pcarlitc formed by isothermal transformation at

I275°F. 600X. (left)

Ftc. 12.7. Fine pcarlitc formed by isothermal transformation at 1200°F. 600X.(right)

Fig. 12.8. Nodular pearlitc formed by isothermal transformation at 1100°F. Thisspecimen is only partially transformed. 600X. (left)

Fig. 12.9. Bainite formed by isothermal transformation at 850°F. This specimenis only partially transformed. 600 X. (right)


white field represents the untransformed austenite prior to water quench-

ing. The time associated with the specimen showing the smallest amountof dark area is assumed to be the time lor the beginning of isothermal

transformation at the particular temperature while the end of transfor-

mation is determined by the specimen showing a completely darkened

surface. By repeating the experiment at several different temperatures,

enough data are obtained to make possible the construction of a diagram

similar to that shown in Fig. 12.5.

The transformation products obtained arc a function of the transforma-

tion temperature. Near the eutectoid temperature very coarse pearlite

forms as is shown in Fig. 12.6 while finer pearlite forms as the tempera-

ture is reduced (see Fig. 12.7) until at the "nose" of the curve a very fine

pearlite forms in nodules giving nodular pearlite, Fig. 12.8. Below the

temperature of the nose, a product forms, which only occurs during

isothermal transformation, bainite, Fig. 12.9. The exact nature of this

material is not well understood but it appears to be some sort of very

fine mixture of fcrrite and cemeiuite. The above description applies to

those alloys which do not yield a pro-cutectoid constituent. Pro-eutectoid

ferrite or pro-eutectoid cementite forms before pearlite is formed.

12.5. The Effect of Cooling Rale Upon the Resulting Structure

In the previous section, the isothermal transformation of steel was

discussed. However, during the heat treatment of most steel parts, it is

not possible to remove the heat within the extremely short time as is done

for obtaining a TTT diagram. As a result, the surface may be very

severely quenched whereas the interior may cool very slowly. Such varia-

tion in cooling rate will certainly result in different structures.

It was observed that isothermal transformation just below the austen-

itizing temperature caused the austenite to transform very slowly into

coarse pearlite. As the temperature was reduced, the transformation was

more rapid and the pearlite became finer. In Fig. 12.10 there is shown a

schematic drawing of a portion of an isothermal diagram with some

typical continuous cooling curves. Curve A shows very slow cooling, such

as might be expected in furnace cooling. The austenite exists until the

curve reaches the beginning of transformation at which lime coarse pearl-

ite begins to form. It continues to form until the transformation is com-

plete. Now curve B shows moderately slow cooling that might be ex-

pected in still air cooling of steel. Austenite exists until the curve meets

the beginning of the transformation curve at which time medium pearlite

begins to form and continues to form until the transformation is com-

plete. The specimen cooled as shown by curve C would have still finer


i

4vsr/riir£ StablC

1 r

.*00» "\_-A_. _i£ \ \ *l ItltPCHASVBj —

400*--*. -\ •

I \ / \r/stc

-

w\ \

Itf^Rtfl

w - *"%^

«OC* - -

400" -

»00»

.i i r I

-

IMMITHMIfi IIMt ** S«CO*DS

Fig. 12.10. Schematic Representation of an Isothermal Diagram and Various Con-tinuous Cooling Curves.

pearlite. The critical cooling rate as shown by curve D shows that the

austenite would transform into martensite without any pearlite form-

ing. Martensite begins to form at the temperature shown as M 3 and in-

creases as the temperature is reduced. Any cooling rates more rapid than

D will result in complete transformation into martensite, Fig. 12.11.

- w '

-* •«*-..

«. \f1

...... „--"'

Fie. 12.11. Martensite formed by water quenching after austenitizing. 600x-


12.6. Effect of Alloying Elements Upon the Iron-Carbon Diagram

In Section 12.2, the iron-carbon phase diagram was discussed in detail.

Now, the effect of the common alloying elements upon the eutectoid

temperature and composition will be discussed as well as the effect of the

alloying elements upon the physical properties of steel. Alloying ele-

ments tend to divide themselves into two groups: those that widen the

range of gamma stability by increasing the S-y transformation tempera-

ture and lowering the y« transformation temperature and those that

narrow the range of gamma stability by lowering the 8-y transformation

temperature and raising the y« transformation temperature. The com-

mon gamma stabilizers are manganese, nickel, cobalt, copper, nitrogen

and carbon, while the common alpha stabilizers are silicon, chromium,tungsten, molybdenum, phosphorus, vanadium, titanium, aluminum,boron and sulfur. Nickel and manganese are such strong gamma stabi-

lizers that it is possible to have stable austenite at room temperature such

as occurs in the austenitic stainless steels and Hadfield steel respectively.

Certain of the alpha stabilizers reduce the gamma region to a narrowloop and may result in ferrite existing continuously from the liquidus

temperature to room temperature.

By an element being an alpha or gamma stabilizer means that it either

raises or lowers the eutectoid temperature respectively. As was indi-

cated previously, nickel and manganese are gamma stabilizers and thus

reduce the eutectoid temperature, nickel being a bit more effective than

manganese. The other gamma stabilizers have a similar effect howeverrather reduced. By the same token the alpha stabilizers raise the

eutectoid temperature, thereby reducing the region of gamma stability.

Titanium has the greatest effect in raising the eutectoid temperature

followed by molybdenum, tungsten, silicon and chromium in decreasing

order.

Most alloying elements have an effect in reducing the carbon content

necessary for the eutectoid composition. This results in a completely

pearlitic alloy with less carbon than indicated by the iron-carbon diagram.

Again, titanium has the greatest effect in reducing the eutectoid carbon

content with molybdenum, tungsten, silicon, manganese, chromium, andnickel in descending order. This effect reduces the amount of cementite

and causes the steel to be softer and less brittle.

Most alloying elements have a useful effect upon the heat treating of

steel; certain alloying elements have other useful properties. One of the

properties is the ability of strengthening the ferrite to obtain sheet

and plate materials for the fabrication of high strength, lightweight ob-

jects such as pressure vessels, railroad cars, truck bodies, etc. These are


usually referred to as high-strength, low-alloy steels being sold under

several trade names. They rely on the fact that phosphorus, manganese,

copper, titanium and silicon form solid solutions with ferrite and thereby

strengthen it. Nickel, molybdenum, and aluminum are mild strength-

ened.

Another property that alloying elements have is their ability to

form carbides. These carbides may have a grain refining action on the

austenite during heat treatment to prevent excess grain growth. Theyalso may help block creep in high temperature applications. In the case

of certain types of stainless steel, columbium and titanium are used to

form a more stable carbide than chromium and thus enhance the

corrosion resistance of the steel. Columbium, titanium, zirconium, vana-

dium, tungsten, molybdenum, and chromium are considered as strong

carbide formers while manganese is only moderate. Phosphorus, nickel,

sulfur, cobalt, and copper have no effect in the formation of carbides

while aluminum and silicon tend to cause the carbides to decompose,precipitating graphite.

12.7. Effect of Alloying Elements on the Isothermal Transformationof Steel

Referring back to Fig. 12.5, we see that there are regions on the diagramlabeled pearlite and bainite. By comparing this figure with Fig. 12.10, it

is apparent that if thick sections are to be completely hardened, it will benecessary to displace the curve to the right, thus requiring a longer time

to elapse before the beginning of transformation. This is precisely the

effect of alloying elements on the TTT curve. By suitable selection of

alloying elements, it is possible to displace the curve enough to permit

complete hardening of quite large pieces by air cooling.

Usually the effect of an alloying element is determined as a function

of its ability to displace the pearlite or bainite region to the right. In the

case of the pearlite region, manganese, molybdenum, phosphorus,and chromium have the greatest effect in that order, while silicon, nickel,

and copper have much smaller effects. The displacement of the bainite

region to the right is greatest with manganese, phosphorus, and chromium.Silicon, nickel, and copper have reduced ability while molybdenum has

no effect on the bainite region. It is interesting to note that sulfur has a

slight tendency to displace the curve to the left. By careful selection of

proper alloying elements, the beginning curve for transformation can bedisplaced considerably to the right. It should be noted that small addi-

tions of several alloying elements are more effective than one large addi-

2 J° Engineering Metallurgy

tion. This is evident in the alloys that have been developed recently

with their multi-element additions.

The addition of alloying elements has another effect in the thermal

treatment of steel. In Fig. 12.5 it can be seen that there is a horizontal

line labeled M s . This is the temperature at which martensitc begins to

form. It continues to form as the temperature is reduced. Most of the

common alloying elements tend to reduce the M, temperature, carbonbeing quite strong in its ability. The other elements reduce the M, in the

following order; manganese, vanadium, chromium, nickel, copper, molyb-

denum, tungsten. Silicon has no effect while aluminum and cobalt tend

to raise the M5 . Alloy additions may lower the M, to such an extent that

complete transformation of austenite to martensite mav not be possible

by quenching to room temperature. It may require subatmospheric

cooling in order to form martensite.

12.8. Effect of Hot Working on Structure

So far, the discussion in this chapter has been confined to the phase

and structural changes in carbon steels when they are heated or, moreimportant, cooled at varying rates between the temperature where aus-

tenite is stable and room temperature. It was stated that there is no

change in the austenite if it is heated or cooled between the transforma-

tion temperature and the melting point. This is true. There is no change

in the internal structure of the austenite grains, but there may be a great

change in the size of the grains. If carbon steel is heated above the trans-

formation temperature, the austenite grains grow, and the higher the

temperature the larger the grains.

Small-grained steels are generally considered to have better properties

than coarse-grained material of the same composition. Since grain size at

room temperature depends largely upon the grain size of austenite whenit is ready to transform, the grain size of austenite should be small. Oneof the important factors affecting the grain size of austenite, and of

pearlite and ferrite, is mechanical working.

When molten steel cools, solidification usually begins by crystallization

at the wall of the mold, and if the temperature of the steel is high whenit is poured, and if the ingot or casting is large and of favorable shape,

these crystals may become very large (Fig. 12.12). Hot rolling or forging,

which is ordinarily carried out at temperatures at which the metal is

plastic, breaks up the large crystals and elongates the fragments in the

direction in which the work is applied. These elongated grains, together

with elongated inclusions, give carbon and many of the alloy steels their

directional properties.


FIG. 12.12. Large columnar crystals in a steel ingot, natural size. (Hanemann and

Schroder, Atlas Metallographiats, Berlin, 1927)

Austenitc grains, even when plastically broken and deformed, have

the tendency to grow together again. This tendency is the greater the

higher the temperature. Hence, if the finishing temperature of hot work-

ing is much above the transformation temperature, grain growth mayoccur while the steel is cooling to the transformation temperature, espe-

cially if the section is so large that cooling is slow. For this reason, if hot-

rolled or forged steels are to be used without subsequent heat treatment,

and if a fine grain is desirable, the hot-working temperatures and the

amounts of reduction are controlled so that the finishing temperature

will not be much above the transformation temperature. Thus, the steel

will cool below this teni]>erature before appreciable grain growth has

taken place. The effect of hot working in breaking up the coarse grain

structure of cast 0.20 per cent carbon steel is shown in Fig. 12.13B.

12.9. Effect of Cold Working on Structure

Cold working is usually defined as mechanical deformation of steel at

temperatures below the transformation temperature. In practice it is


Fig. 12.13. Structure of a 0.20 per cent carbon steel: (A) as cast, and (B) as hotrolled; etched. 100x-

generally carried out at room temperature. If steel is plastically deformedat these low temperatures—as in drawing wire, cold rolling or extruding

bars, or cold rolling sheet—the pearlite and ferrite grains are elongated

in the direction in which work is applied. This mechanical distortion

results in rapid increase of hardness and brittleness.

At atmospheric temperature, the rigidity of the steel is so great that

the distorted grain fragments cannot rearrange themselves by recrystal-

lizing into their normal equiaxed shape. Hence, if cold working is con-

tinued, the hardness and brittleness accompanying the fragmentation of

the grains will increase with the amount of deformation until soon, unless

the steel is annealed, it will fail. The amount of cold deformation that

steel will stand depends upon its ductility, which, in turn, is related

directly to chemical composition and prior heat treatment. Thus, an-

nealed low-carbon wire will withstand more reduction by drawing before

failure, or before reheating for recrystallization becomes necessary, than

annealed medium- or high-carbon wire.

The unstable structure of cold-worked steel* and its accompanyinghardness and brittleness are caused by the distortion of the ferrite andpearlite grains and not, as in quenched steel, by the presence of sub-

microscopic particles of carbon in a distorted alpha-iron lattice. But, as

• The structure of severely cold-worked steel is unstable in that the fragmentedgrains will recrystallize if given the opportunity; that is, if the steel is heated to theproper temperature, which is considerably below the A

xtransformation. Actually,

however, owing to the rigidity of the material at room temperature, the fragmentedgrain structure will persist indefinitely.


Fig. 12.14. Structure of a 0.20 per cent carbon steel (A) as cold-worked and (B)as cold-worked and annealed; etched. 500 X-

the stability of quenched steel can be increased by tempering (see Chap.13), so can the stability of structure of cold-worked steel be increased byreheating below the transformation temperature. This is known as process

annealing.

Structural changes which occur when cold-worked steel is reheatedare not gradual. Cold-worked steel is softened by heating to 800 to

1200°F. (425 to 650°C), the exact temperature depending on the com-position, si/e of the section, and the amount of cold work. Reheating at

the proper temperature effects complete recrystallization and restoration

of the original structure and properties. The material can then be coldworked again until grain distortion and its accompanying brittleness

have again become so serious that another reheating is necessary. Theeffect of cold working on the structure of a 0.20 per cent carbon steel

(whose structure as hot-worked is shown in Fig. 12. ISA) is illustrated inFig. 12.14A. The effect of reheating on the distorted grain structure is

shown in Fig. I2.14B. Note that after annealing the grains are equiaxed,and all indication of distortion has disappeared.

QUESTIONS

1. Describe the changes taking place in cooling from 3,000°F., iron-carbonalloys having the following carbon contents, 0.3%, 0.8%, 1.5%, 3.0%, 4.3%and 5.0%. When crossing a two phase field, indicate how each phasechanges in quantity and composition.

2. Using the lever rule, give the composition and amounts of the phasespresent in the above alloys at the following temperatures, 2,500°F 2 000°F1,500°F., and 1,000°F.

3. Describe the crystal structures of delta iron, austenite, and ferrite. What isthe crystal structure of pearl ite? How does it differ from the previous men-tioned phases?

262 Modern Metlallurgy jor Engineers

4. Give the range of carbon contents for steel and cast iron. From the iron-

carbon diagram, give one reason that cast irons frequently have limited

ductility.

5. Describe in detail, the method of constructing a TTT diagram. What is the

white constituent which is present in specimens which are only partially

transformed? Where does it originate?

6. Sketch the TTT curve from Fig. 12.5. Superimpose on this diagram curves

representing your opinion of the transformation curves for alloys contain-

ing manganese, silicon, phosphorus, nickel, copper, chromuim, and molyb-

denum. Sketch the position of the M, also.

7. Sketcli the iron-carbon diagram from Fig. 12.2. Superimpose on this dia-

gram, the position of the eutectoid temperature and composition for alloys

containing, titanium, molybdenum, tungsten, silicon, manganese, chromium,

and nickel.

8. Define the term carbide former. Give several examples of carbide formers.

Define alpha and gamma stabilizer. Give examples, why gamma stabilizers

are important.

9. What are columnar crystals and how are they formed? What is the effect of

hot working on the crystals? Why should the finishing temperature of hot

working be just above the transformation temperature?

10. What is cold working? How docs it affect the structure and properties of

hot worked steel? What happens to die structure and properties when cold

worked steel is reheated to 800 to 1,200°F.?

11. Define the terms, hypoeutectoid, hypereutectoid, and eutectoid as used in

steel. Describe the difference that will occur when such steels are slow

cooled from the austenite region.

12. Sketch the unit lattice cells for austenite and ferritc. Point out the differ-

ences. Which is the closest packed cell? Since carbon is an interstitial atom,

indicate where it fits in the austenite unit cell.

13. From the iron-carbon diagram in Fig. 12.2, point out the various reactions

which occur. Indicate the reactants and products.

14. From the iron-carbon diagram, indicate the amount of pearlite present in

alloys containg 0.3%, 0.8%, 1.2%, and 1.9% carbon. What is the other

constituent in each of the alloys?

15. Cast iron is quite extensively used and is rather inexpensive as compared

with cast steel. From the material presented in this chapter, give reasons

to explain this situation.

10. Describe the changes that take place during the solidification and sub-

sequent cooling of molten pure iron. Compare these charges with those

that occur in copper.

17. Define the term ferrite strcngthener. Why are these important? Give ex-

amples of ferrite strengtheners.

18. Describe the formation of martensite. Upon what variables is martensite

formation dependent?

19. When steel is cold worked, how arc the mechanical properties affected? De-

scribe applications where cold worked steel is superior to annealed steel.

20. Describe the solidification of steel in a large ingot mold. Consider the

differences which would occur at the surface and interior of the ingot.

CHAPTER

Fundamentals of

Heat Treatment of Steel

13Abram Eldrkd HoSTETTER, Ph.D., Professor of Metal-

lurgy, Kansas State College, Manhattan, Kansas

JL HE process of quenching and tempering

steel to improve its hardness and wear resistance has been used for at least

3,000 years. Some of the early tools and weapons testify to the skill of the

early metal smiths. Little was known, however, regarding the nature of

the changes taking place in the steel during heal treatment, and the suc-

cess of these early metal workers is a tribute to their patience and persist-

ence in developing the art of heat treating. Medieval literature is filled

with the magic heat-treating processes developed by these early ironworkers, especially for the treatment of swords. The Spanish swords of

Toledo, for example, gained world renown because of the supposedlymysterious properties of the water in which they were quenched;the famous swords of Damascus were heated to the color of the rising sunand were quenched by plunging them into the belly of a fat Nubian slave.

•A favorite quenching medium was urine, and this is understandable since

we know that chloride brines and other salt solutions extract heat fromred-hot steel 10 to 20 percent faster than water alone.

Today our approach to the heat treatment of steel is somewhat morescientific. Much has been learned about the complex reactions takingplace during the heat treatment of steel and various factors such as alloy

content, cooling rate, tempering temperature, etc. are much better under-stood today. Our knowledge of the structural changes taking place in

metals is by no means complete, but with the knowledge we have, heattreatment processes can be prescribed to give desired properties with aminimum of "trial and error" procedure.

263


1650'

F

1530°K

0.6 0.8

Per cent carbon

Fig. 13.1. Schematic representation of the grain size of carbon steel as affected by

heating. (Stoughton)

13.1. Grain Size and Grain Growth

There are two means by which the grain size of carbon steel can be

changed. The first is by cold working the metal and annealing it at a

temperature below the critical range. This is called process annealing.

The grain size thus produced depends mainly on the extent of cold work-

ing and the annealing temperature. The second method is by heating

the metal above the critical range where, as described in Chapter 12,

austenite is produced. With the formation of austenite, recrystallization

takes place, resulting in a fine grain structure (Fig. 13.1)

.

In Fig. 13.1 the change in grain size with raising temperature is

indicated by varying sizes of circles. If a coarse-grained hypoeutectoid

steel is slowly heated, grain refinement starts at the lower critical range

(1330°F.) and gradually proceeds throughout the critical range. Grain

refinement is then complete at the upper critical temperature. This is

not a reversible process however. That is, on cooling, the grain size does

not revert back to the original large grain size, but remains the maximumsize reached while above the critical range. On cooling, the austenite

grains convert to ferrite and pearlite or martensite, depending on the

cooling rate. In either case the grain size observed at room temperature

reflects the prior austenitic grain size. In the case of eutectoid and

hypereutectoid steels, grain refinement takes place at the eutectoid tem-

perature as shown in Fig. 13.1. In case a steel has a fine grain at room

Fundamentals of Heat Treatment of Steel 265

temperature there will be little or no change in grain size when it is heated

through the critical range.

If a steel is heated to produce fine-grain austenite and then heating

is continued several hundred degrees higher, the fine grains of austenite

grow to a larger size as shown in Fig. 13.1. Time also plays a part in

determining the final grain size. At higher temperatures the grain

growth is rapid, while at temperatures just above the transformation

range, there is little tendency for grain growth. The eventual grain size

of steel therefore depends on both the temperature to which it is heated

and the time it is held at this temperature.

The effect of temperature on grain growth is shown in Fig. 13.2. Twopieces of steel containing .40% carbon were heated above the critical

range and cooled slowly. Sample A was then heated to 1550°F. (in the

fine grain range Fig. 13.1) and held there for 30 minutes, and sample Bwas heated to 1900°F for 30 minutes. The samples were cooled slowly,

well below the critical cooling rate, and as a result, both samples are soft

and consist of grains of ferrite and pearlite. The grain size of sample B,

however, is much larger then that of sample A because of the graingrowth at 1900°F.

When austenite is cooled to room temperature, the hardness and the

resulting structure is dependent on the cooling rate, as indicated in

Chapter 12. If cooling is slower than the critical cooling rate, pearlite

would be produced, and if quenched faster than the critical coolingrate, martensite is produced. The grain size in both cases, however, is thesame if they were heated to the same temperature. Thus the grain size

depends on the temperature to which a steel is heated, but the hardnessdepends on the cooling rate and is quite independent of the grain size.

Steel may be hard or soft, depending on the cooling rate, and be fine orcoarse grained, depending on the temperature to which the steel washeated.

13.2. Controlling and Classifying Grain Size

When steel is gradually heated above the critical range, there is acertain range of temperature (Fig. 13.1) where there is little tendency forgram growth. Above this range, grain growth becomes more and moreapparent as the temperature rises. It has been found, however, that byadding definite small amounts of aluminum or other deoxidizers tocarbon and alloy steels, the tendency for grain growth at elevated tem-peratures can be controlled. That is, the fine-grain temperature range(Fig. 13.1) is broadened so that a carbon steel containing the properamount of aluminum can be heated to 1830°F. and still remain fine


grained, whereas a similar steel without the aluminum would quickly

become coarse grained at this temperature.

The research work of McQuaid and Elm, along with other investiga-

tors, has served to emphasize the importance of grain size in the heat

treatment of steel. For many years a coarse-grained structure was con-

sidered to be undesirable and every precaution was taken in hot working

and heat treatment to avoid it as far as possible. This idea is now known

to be wrong in some cases. Coarse-grained steels machine more easily and

harden more deeply than fine-grained steels. On the other hand, fine-

grained steels do not crack so readily when quenched and in general

have greater toughness at low temperatures.

The grain size of steel can be classified in terms of a standard scale

established by the American Society for Testing Materials. The grain-

size class are numbered 1 to 8. In this system the number of grains per

square inch at 100X is 2"—1 where n is the grain-size numbers. Photo-

graphs of each of the grain-size classes are available in metallurgical hand-

books, and the grain size of a sample is readily obtained by comparing

the grain size of these photographs with that of the sample when viewed

or photographed at 100X. Table 13.1 lists the ASTM grain-size numbers

and the corresponding grains per square inch. Three different grain

sizes are shown in Fig. 13.3 as they appear at 100X.

Table 13.1. A.S.T.M. Grain-Size Numbers

A.S.T.M. Grain-Size NumberGrains per Sq. In. at 100X

Average Range

1

2

3

4

5

6

7

8

1

2

4

8

16

3264

128

1 J/£ or less

lj^to 3

3 to 6

6 to 12

12 to 24

24 to 48

48 to 96

96 or more

Another system for classifying grain size is the Shepherd fracture

standards. This set consists of ten broken steel specimens graded from fine

to coarse grain. The grain size of a sample is then determined by break-

ing it and visually matching its grain size with one of standard specimens.

This method is especially useful where metallographic facilities are not

Fundamentals of Heat Treatment of Steel

MM267

5&isi§k;i"i^jjFie. 135. Structure of 0.40 per cent carbon steel (A) heated just above the trans-

formation temperature and slowly cooled and (B) overheated and slowly cooled;etched. IOOx-

available. The fracture standards are prepared by hardening and then

breaking them. The sample being tested or compared to the standards

must also be in the hardened condition to get a valid comparison.

It must be remembered that the grain size referred to above the trans-

formation temperature is austcnitic grain si/e. In most carbon steels, how-ever, little austenite is retained at room temperature, except in highly

alloyed steels, and at room temperature we are concerned with grain size

of the transformation product on cooling (martensite, ferrite, pearlite,

bainite etc.) . By proper polishing and etching of martensite, the previ-

ous austenitic grain size is revealed since the grain size of the martensite

follows that of the previous austenitic grain size. Steel in the martensitic

condition may also be notched and broken to display the grain size.

The broken surface consists of many small facets. The facets follow the

fracture planes through the grains and thus their size gives an indication

of the martensitic grain size. On slow cooling hypoeutectoid steel, the

austenitic grain size is reflected in the grain size of the ferrite andpearlite since the proeutectoid ferrite forms at the austenitic grain bound-aries and the pearlite forms in the body of the parent austenitic grain

as shown in Fig. 13.2b.

13.3. Effects of Hot Working on Grain Size

When steel is hot worked at temperatures around 2000°F., the grains

are broken up into finer grains. This fragmenting process keeps the grain


Fig. 13.3. Typical structure of slowly cooled carbon steel having (A) grain size 2:

(B) grain size 5; and (C) grain size 8; etched. 100X-

size fine even though there is a great tendency for grain growth at this

temperature. The two factors thus offset each other, and the grain struc-

ture remains fine during the hot working. If, however, the hot working

is stopped and the metal held at the hot working temperature for a

short time, grain growth takes place rather fast, and after cooling to room

temperature, the grain would be coarse. In this case the steel should

be reheated to a temperature above the critical range in order to refine

the grain. Hot working may be continued as the temperature falls to a

point just above the critical range. In this case there is little chance for

grain growth, and the grain remains fine. When steel is cast as an ingot,

the grain size is very large and columnar. Hot working is employed

to break up this coarse condition and produce a fine grain structure and

thus greatly improve the physical properties.

T


1600

1400

1200

1000

si 800

600

400

200

Q-

E

K

vV"VKK^V\ MS l) X\

Mf

1600.

1400

(200

1000

TimeA

-T 800o.

E* 600

400

200

\

\

,

\f

CV\1°

x\\ Ms\ *) N

\\ \

;Mf .

—

'

TimeB

Fig. 13.4. A. The TTT curve for a typical carbon steel. B. The TTT curve for analloy steel. Cooling curve O represents the cooling rate on the outside of a bar. Cool-ing curve C represents the cooling rate in the center of a bar.

13.4. Hardenability

If a one-inch bar of carbon steel containing .70% carbon is quenched in

brine, the outside of the bar will be hard (close to Rc65) while the centerof the bar %vill be only Rc30 to 35. Such a steel is said to have low harden-ability. The structure on the outside would be martensite because the cool-

ing rate on the outside portion of the bar was faster than the critical cool-

ing rate as shown in curve O, Fig. 13.4A. The structure of the center of thebar would be pearlite since here the cooling rate is much slower than thecritical cooling rate as shown in curve C, Fig. 13.4A. How deep the outsidehardened shell will be depends upon the critical cooling rate since all of

the steel that is cooled at the critical cooling rate or faster will be hard.If a one-inch bar of an alloy steel is quenched, as just described for a

carbon steel, the bar will be hardened to the center, and only martensitewould be observed. Such a steel has high hardenability (hardens deeply).The actual cooling rate of the two steels at a given distance from the

surface is essentially the same, but the TTT curve for the alloy steel

plots to the right as shown in Fig. 13.4B. In this case the cooling raterepresented by curves O and C are both faster than the critical coolingrate because the TTT curves plots sufficiently far to the right. Thus acomparison of the relative position of the TTT curve for various steels

on the time scale affords a quick qualitative means of comparing their


relative hardenability. The role of alloying elements in determining

the position of the TTT curve has been covered in Chapter 12.

There are applications in industry where only surface hardness is

needed and high hardenability is not important. For such applications it

usually is good economy to use carbon steels. For certain uses, however,

such as aircraft parts, uniformly high strength and hardness is generally

required through the part. In these cases it becomes necessary to use an

alloy steel such as S.A.E. 4140 in order to have sufficient hardenability.

When a steel has high hardenability, a slower quench (oil quench) can

be used to obtain full hardness in a rather large section. Since this is

the case, it is unwise to use a brine quench because such a quench is much

faster than necessary and greatly increases the chances of cracking the part.

Such steels are referred to as oil hardening steels, and steels with low

hardenability arc called water hardening steels.

13.5 Grain Size and Hardenability

When carbon steel is heated to a temperature just above the critical

range and quenched in brine, very shallow hardness is obtained. If, how-

ever, the same steel is heated high enough to produce a coarse grain before

quenching, the hardness is much deeper. The increased depth of hardness

indicates a slower critical cooling rate as explained earlier. 1 his would

mean that an increase in grain sizes makes the transformation more slug-

gish, and transformation is slower in starting at a given temperature. The

TTT curve is thus moved to the right when the grain size is increased,

which is the same effect observed when certain alloying elements are

added to the steel.

Because of the shallow hardening feature of fine-grained carbon steel,

a somewhat coarser grain may be desirable even though coarsening of the

grain may induce some brittleness. The actual grain size desired in carbon

steels may therefore be a compromise of moderately fine grain in order to

avoid the severe brittleness associated with a very coarse grain and

not have the low hardenability associated with the very fine grain. In

alloy steels, however, this dilemna is not encountered since hardenability

is afforded by the proper alloying elements.

13.6. Quenching and Properties of Martensite

As discussed in chapter 12, increasing progressively the speed of cooling,

up to a certain rate, lowers the A, transformation temperature progres-

sively from 1333° F. (723°C.) to approximately 1020°F. (550°C.) .While

this increase in rate affects the speed with which the gamma iron trans-

forms, it does not affect the completeness of the transformation. The


increased cooling rate causes the pearlite lamella to become finer. If,

however, the cooling rate is increased still more, an important phenom-enon occurs. The transformation temperature is suddenly depressed

to 480°F. (250°C.) or lower, and the steel, especially if it contains morethan about 0.30 per cent carbon, becomes very hard and brittle. Therate of cooling that is just fast enough to suddenly depress the A^ tempera-

ture to 480°F. is known as the critical cooling rate and is attained byquenching a high-carbon steel in water or an alloy steel in oil.

The changes taking place when high-carbon steel is cooled at the

critical rate or faster are in general simple, although some of the details

are not wholly understood. In brief the mechanism of quenching is as

follows: At a temperature above 1333°F. (723°C.) , the structure is

austenite, the solid solution of carbon in gamma iron. In cooling moreslowly than the critical rate, the face-centered gamma iron changes to

body-centered alpha iron and the carbon is thrown out of solution as ironcarbide. If cooling is faster than the critical rate, the change of face-

centered gamma iron to body-centered alpha iron is arrested and an in-

termediate crystal structure, the body-centered tetragonal, is formed. Thecarbon, instead of being wholly expelled from solution, is entrapped inthe tetragonal lattice as a supersaturated solution. The result is a highlystrained, very unstable crystal structure containing a multitude of en-

trapped submicroscopic particles.

Steel fails under stress by slipping or gliding along crystallographicplanes of relative weakness; these submicroscopic particles act as keysand help to prevent easy slip (Fig. 13.5). The result is much higherstrength and hardness, accompanied, however, by increased brittleness.

The sudden lowering of the transformation temperature to about480°F. (250°C.) and the increased hardness and strength are accom-panied by the formation of a structural constituent known as martensite,characterized by an angular, needlelike appearance (Fig. 13.6). The

A B n-^CFJ3T

C^:cc

±h =D= <)

\=Q-r^^ 6\-Or Q.0=

Fie. 13.5. Schematic representation of a crystal containing (A) no hard panicles.(B) one large particle, and (C) many small particles.


needlelike formation, while common in rapidly cooled high-carbon steels

of small cross section, is by no means universal. Martensite is a transition

constituent; its composition, hardness, and appearance under the micro-

scope depend primarily upon the amount of carbon, the size of the sec-

tion, and the quenching treatment. Depending upon the carbon content,

the strength of steels which are chiefly martensitic in structure may vary

from 150,000 to more than 350,000 lb. per sq. in., and such steels maybe as brittle as glass or may show considerable toughness.

Metallurgists are generally agreed that the primary cause of the hard-

ness of martensite (and drastically quenched medium- and high-carbon

steels) is the precipitation of submicroscopic particles of carbon or iron

carbide from the gamma solid solution and the retention of these particles

as a supersaturated solution in the tetragonal alpha-iron lattice, where

they act as a multidude of keys effectively preventing slip. Some authori-

ties, thinking that this explanation does not account for all the hardness,

have postulated lattice distortion, internal strain, and the fine grain size as

important contributing causes.* The internal structure of martensite and

Fig. 13.6. Structure of martensite in high-carbon steel quenched in iced brine.

(A) 500x: (B) 3,000x; etched.

• A complete and wholly satisfactory explanation of the hardness of martensite will

not be available until more is known about the atomic forces that hold the crystal

lattice together and about the nature and magnitude of the strains produced by an

entrapped stranger atom in a lattice.


the causes of its hardness are of much interest to the physical metallurgist

but are of only passing interest to the engineer who wants a basic knowl-

edge of heat treatment. Far more important is a realization of the insta-

bility of structure that results from quenching.

13.7. The Instability of Quenched Carbon Steels

When carbon steel is cooled slowly through the transformation tem-

perature, all phase changes take place and the carbon is completely

precipitated as large particles of iron carbide (cementite) , resulting in a

structure, laminated pearlite, that is very stable. If, however, the steel

is cooled very rapidly, phase changes are arrested before completion, the

carbide is entrapped in the crystal lattice as exceedingly minute particles,

and the lattice is strained, resulting in a structure, martensite, which is

very unstable.

As is characteristic of an extremely unstable condition, there is a very

strong tendency for quenched steel to become more stable under the

slightest provocation. Re-

heating to 500°F. (260°C.)

or above will give the atoms

enough mobility so that in-

stability and the strained

structural condition are re-

lieved more or less com-

pletely, depending on the

temperature and the time.

The instability after

quenching is accompanied

by high internal stresses, so

high that frequently a small

piece of water-quenched

high-carbon steel, though

undisturbed, will crack (Fig.

13.8) or will, occasionally, go

to pieces with explosive

violence hours or even days

after quenching. To relieve

these stresses and to produce

at the same time a more

F.G. 13.7. Quenching cracks in the spline endStable «™«ural condition,

of a shaft, natural size, etched. quenched steel is always re-


heated. This Is known as tempering or, colloquially and somewhat in-

accurately, as drawing*

13.8. Retained Austenite and Cold Treatment

When steel is quenched, the cooling must be continued to the M a

temperature before austenite will start to transform to martensite and be-

fore complete conversion takes place the temperature must lower to or

below the M f temperature (Fig. 13.4). If cooling stops between these

temperatures only part of the austenite transforms giving rise to what is

called retained austenite.

The temperature level of the M, and M t are both lowered by the addi-

tion of alloying elements and increase of carbon content as indicated in

Chapter 12. If the M f temperature is below room temperature then

quenching to room temperature will result in only part of the austenite

being transformed and the remainder being retained as austenite. If

however the steel is quenched and then cooled by refrigeration well below

the Mrtemperature the transformation of the austenite to martensite can

be made fairly complete.

Since martensite is much harder than austenite the hardness, strength

and wear resistance of steel can be improved by eliminating the retained

austenite by cooling the steel down to about minus 100°F. This can easily

be done by placing dry ice in a container of acetone and using this as the

cooling medium.

This kind of cold treatment is often used to reduce the retained austen-

ite particularly in the case of tool steels where alloy content encourages

retained austenite and hardness and wear resistance must be especially

high or in steel gages where dimensional stability is important.

13.9. Structural and Other Changes in TemperingWhen a drastically quenched high-carbon steel is reheated, two things

happen: The unstable tetragonal crystal structure of the martensite trans-

forms to the more stable body-centered cubic lattice of the ferrite, andat the same time internal stresses are relieved. To produce such changes,

it is only necessary to reheat to about 200 to 400°F. (100 to 200°C.)

;

for some steels the temperature of boiling water is sufficient. Temperingat these low temperatures produces only a very slight mobility of the

iron and carbon atoms, and it is likely that the only effect, in addition

to relief of stresses, is the escape of the carbon atoms from the tetragonal

lattice as it changes to body-centered cubic. As a result low-temperature

• The terra drawing for tempering originated from the old blacksmith's expression

"drawing the temper." Drawing in its correct sense means pulling a wire through adie to reduce the cross section.


tempering produces no appreciable change in hardness. Tempering at

400°F. (200°C.) or lower is, therefore, commonly used for quenched

high-carbon tool steels which must be very hard but should be free from

internal stresses.

When tempering medium and high carbon steels in this low tempera-

ture range of 180 to 480°F., it has been established that some sort of transi-

tion carbide (not Fe3C) is precipitated. The composition of this carbide

seems to have the composition approaching Fe2C or Fe^C.,. As the tem-

pering temperature goes higher this first carbide is absorbed and re-

precipitated as cementitc (Fe3C)

.

When drastically quenched carbon steel is tempered at 500°F.

(260°C.) or higher, the particle size of the iron carbide increases. Howfar this return to stability proceeds depends upon the tempering tem-

perature and to a lesser extent upon time. Most of the growth of carbide

particles at any definite temperature occurs in the first few minutes; the

reaction then slows down but continues for some time. In practical heat

treatment the time for tempering is rarely less than 30 min. for each inch

of cross section.

As the carbide particles grow larger, it follows that there will be fewer

of them to act as minute keys to prevent slip (see Fig. 13.4) . Hence, hard-

ness and strength decrease, and ductility, as measured by elongation andreduction of area, increases. The higher the tempering temperature, the

larger the carbide particles and the more stable the resulting structure.

Finally, when the tempering temperature is within a few degrees of the

A, transformation temperature, the cementite has grown to particles so

Fie. 1S.8. Structure of high-carbon steel quenched in ice water and tempered (A)at 575-F. (300°C.) and (B) at 1100 o F. (595°C); etched 500x-


large that they are readily visible with the microscope at low magnifica-

tions, and the properties are practically the same as those of a steel of

corresponding carbon content cooled slowly through the A x temperature.

Structural changes in tempering can be followed by microscopic ex-

amination of polished and etched specimens but not so closely as by

dimensional changes and other methods. Martensite which has a struc-

ture similar to that shown in Fig. 13.6 is white after etching, that is,

etches slowly. Tempering at 575°F. (300°C.) produces, for the samedegree of etching, a dark, almost black, structure indicating precipitation

and considerable growth of the carbide particles. After tempering at

1100°F. (595°C.) , the particles have increased in size so that they are

visible in the microstrucfure. These two stages are shown in Fig. 13.8.

Tf quenched steel contains retained austenite and is not cold treated

to transform the retained austenite to martensite, tempering is somewhat

more complex. The martensite will go through the various stages of tem-

pering just described. However, the retained austenite will, at about

450°F. to 500°F., transform to a dark etching bainite, %vhich is quite hard,

and may result in a little increase in hardness.

QUESTIONS

1. What happens to the structure of a small piece of .8% carbon steel con-

taining a mixture of large and small grains when it is subjected to the

following treatments: (a) heated slowly to about 1400°F., held 80 min.,

and cooled slowly to room temperature; (b) heated slowly to 2000°F., held

30 min., and cooled slowly to room temperature; and (c) heated slowly to

2000°F., held 2 hr., and cooled slowly to room temperature?

2. Define grain-growth tendency. Compare grain-sue control by submicro-

scopic particles and by controlling the heating above the critical. Why is the

former method preferable?

3. What ,is critical cooling rate? What is its effect on the austenite-pearlite

transformation and on the resulting structure of high-carbon steel? Is a

cooling rate of 600°F. per sec. more or less than the critical cooling rate

for eutectoid steel.

4. When a high-carbon steel is cooled faster than at the critical rate, what

is the effect on the crystal structure, the appearance under the microscope,

and the properties? What is martensite, and why is it hard?

5. Why is high-carbon steel that is drastically quenched unstable? What unde-

sirable conditions may accompany this instability and how may they be

avoided?

6. Describe the changes in structure and properties that take place whendrastically quenched steel is reheated to (a) 350°F.; (b) 600°F.; and (c)

1200°F.

7. What effects have the alloy elements on the temperature at which austenite

transforms to pearlite? How are these effects related to heat-treatment?


8. Name the three principal variables affecting the hardness of quenchedsteel. If other factors are constant, which will be the harder one as

quenched: (a) steel containing 0.30 or 0.60 per cent carbon? (b) steel con-taining 0.60 or 0.80 per cent carbon?

9. Define hardenability. What is its relation to grain size and to the alloyingelements present? Distinguish between shallow-hardening and deep-harden-ing steels.

10. In what classes of steel is deep hardening important? Why?11. Describe process annealing.

12. How does grain size effect hardenability?

15. What factors determine the grain size of steel after process annealing?14. What is the relationship between grain size and toughness?15. What are two standards for classifying grain size?

16. Describe the effects of hot working on grain size.

17. What factors determine the amount of retained austenite after quenching?

CHAPTER

The Operations ofHeat Treatment

14 Omar C. Moore, M.S., Associate Professor of Chemical

Engineering, Alabama Polytechnic Institute, Auburn,

Alabama

Robert E. SHAFFER, M.S., Associate Professor of En-

gineering, University of Buffalo, Buffalo, New York

1 HE value of heat treatment to mankind, em-

phasized in previous chapters, has little relation to the proportion of the

world's steel tonnage that is treated. This tonnage amounts, in the

United States, to possibly two or three million tons annually, certainly

less than 5 per cent of the total amount of steel used. Heat treatment

is most valuable for tools and dies, which are in turn a very small propor-

tion of heat-treated steel.

Heat treatment is ordinarily an expensive operation, because costly

furnaces, quenching baths, apparatus for controlling the atmosphere in

the furnace, and temperature-recording and -controlling equipment are

necessary. Some treatments cost as much or more than the steel itself, but

this is no criterion of the value of the operation. Moreover, the cost of

steel and of treatment is usually a very small part of the cost of the

finished article and is even a smaller proportion of the value of the article

to industry. One example will indicate this. The steel in a die may cost

$2 and the heat treatment may increase this to $4. On this $4 die, the

machine work frequently costs $100, but the completed die will turn

out thousands of pieces of a finished product at a cost which is a very

small fraction per piece of the cost of a similar product turned out, one by

one, by hand.

Owing to the economic value of a properly heat-treated article and to

the customary expense of the machine work to prepare this article for

use, it is poor economy on the part of the engineer to take chances on

the quality of the steel or the heat treatment so that a few cents may be

278

The Operation of Heat Treatment 279

saved. It is, furthermore, poor practice for the engineer to design ex-

pensive tools, dies, or other parts without requesting the metallurgist to

advise whether the finished article will or will not survive the heat treat-

ment satisfactorily. Thousands of dollars are wasted every year because

tools and other articles, which cost much to machine, crack in quench-

ing. Such waste can usually be prevented by the intelligent selection of

the proper steel and by avoiding, in the design, sharp corners or reentrant

angles where stresses concentrate. This factor is discussed in some detail

later in this chapter.

The operations of heat treatment discussed in this chapter are those

used for carbon steels, most alloy steels, and some nonferrous alloys.

Modifications of these operations—used for a few high-alloy steels, for

certain other iron alloys, and for a number of nonferrous alloys—are

described in the chapters where these materials are discussed or in the

chapter on precipitation hardening.

ColorOrange-

Salmon

Bright-Red

Cherry —

Medium\Cherry ]

Dork l

Cherry )

14.1. Heating Cycle

Uniform and consistent results from any cooling cycle can be obtained

if the proper care has been taken in the heating of the steel prior to the

method of cooling. The steel must be heated slowly to the proper tem-

perature range and held at this temperature long enough for homogeniza-

tion of the austenitic structure. The temperature used and time required

are functions of the chemistry of the

steel, section size and shape, prior grain

900 size, original condition of the steel, prior

850 heat treatments, or mechanical working.

boo The usual time requirement at heat for

750 homogenization of the austenite for a

TOO forging or casting, is one hour per inch

650 of section thickness. This is not a firm

1.60 rule and is frequently neglected. How-ever, adequate time at heat must be

provided for solution of carbides, and

also, if uniformity of product is to be

achieved upon cooling for the diffusion

of carbon and alloying elements from areas of high concentration into

the lean areas.

Modern heat treating equipment should provide some means of sur-

face protection for the steel during the heating cycle. The use of con-

trolled atmospheres in heat treating furnaces is becoming standard prac-

0.40 0.80 120

Percentage Carbon

FlC. 14,1. Temperature ranges

for the heal, treatment of carbonsteels. (Metah Handbook)


tice. If during heating the steel is exposed to combustion gases or air in

the furnace, it will oxidi/e, dccarburize, or both. An oxidized surface is

easily detected and can be removed by machining or grinding. Decarburi-

zation does not show and will result in low surface hardness and poor

mechanical properties if not removed.

Protective atmospheres are provided by several different methods and a

number of different gases are used, including hydrogen, nitrogen, carbon

dioxide, and sulfur dioxide.* With the proper selection and composition

control of the furnace atmosphere it is possible to produce heat treated

parts free of scale and decarburization.

14.2. Annealing

Process annealing is the recrystallization of a cold-worked steel, by heat-

ing below the Attransformation temperature, usually in the range be-

tween 800° to 1200°F., the exact temperature being governed by the

amount of cold working, composition, grain size, and time held at heat.

Heating to the proper temperature affects complete recrystallization andrestores the original structure and properties. Too high a temperature or

too long time at heat may produce a coarsening of the grain size with a

subsequent loss of toughness, hardness, and strength.

For full annealing, the steel is heated slowly to from 50° to 100°F.

above the A3 transformation (Fig. 14.1) , held at this temperature until

uniformly heated, and then slow cooled to room temperature. The cool-

ing may be done in the furnace, or in an outside pit covering the steel with

asbestos, sand, lime or cinders. The rate of cooling is in the range of

5° to 20°F. drop per minute.

The objects of annealing are to soften the steel for machining, to re-

fine the grain structure, to relieve stresses from cooling or machining,

and to increase ductility. In hypoeutectoid steels it produces the poorest

strength and wear resistance of any structure except spheroidizing. An-nealing is commonly used for castings and forgings to reduce the coarse

grain structure prior to machining.

14.3. Normalizing

The steel is heated slowly at least 100°F. above the As or Acm

transformation (Fig. 14.1) for the proper time, and then cooled in still

air. The cooling rate can be varied by changing volume of air or air

• Discussion of controlled atmospheres and equipment is outside the scope of this

book. For a more complete description see: American Society for Metals, Metals Hand-book. The Society Cleveland, 1948.


temperature. The normal rate of cooling is the range of 120° to 200°F.

per minute, depending on section size and air conditions.

Normalizing of low carbon content steels is usually done for stress relief,

grain size refinement, to improve toughness and to increase strength and

ductility.

Steels containing over 1% carbon, slowly cooled from high tempera-

tures, may have the excess carbides forming in a network around the

grains of pearlite. This carbide network is stable and unaffected by re-

heating to the annealing and hardening temperatures (Fig. 14.1). This

carbide formation will increase machining costs and produces an un-

desirable condition in the heat treated part. By heating above the Acm

temperature and normalizing, insufficient time is available for the excess

carbide to form a network around the grains or to form large plates in the

pearlitic structure.

Some finished parts, castings, forgings, and rolled shapes to be used after

a normalizing treatment should be given a temper to relieve cooling

stresses resulting from non-uniformity of cross section. This temper will

usually give a noticeable improvement in ductility with little or no change

in hardness or strength.

14.4. Spheroidizing

The spheroidized structure, ferrite and carbides, is usually produced

by prolonged heating, 3 to 8 hours at a temperature slightly above or

below the A! or A3 2 , transformation. The prolonged heating causes

the carbides to coalesce into spheres, completely destroying the pearlitic

formation and resulting in a matrix of ferrite with the carbon in the form

of spheroidal carbides. The time for spheroidizing is usually lessened bya normalizing treatment prior to spheroidizing.

In medium and high carbon steels the spheroidized structure has the

lowest hardness, strength and wear resistance, but the best machineability,

toughness, and ductility.

Though the spheroidized structure is desirable for machining and form-

ing, it may cause some difficulty in heat treatment. The carbide material,

if in large spheres, may not go into solution when a simple heating andquenching cycle is used, resulting in spotty and low hardness. It is desir-

able to give steels of this type a normalizing treatment prior to the

hardening cycle to help break up and to dissolve the excess carbide areas.

A similar structure of a ferritic matrix with spheres of undissolved

carbides will result if a martensitic structure is tempered at a high tem-

perature 1200— 1250°F. This method of producing the spheroidized struc-

ture is not recommended because of the problems involved in quenchinghigher carbon steels.


14.5. Quenching for Hardening

For hardening, the sieel is slowly heated above the A.t transformation

range (Fig. 14.1). The hardening temperature range for carbon steels is

the same as the full annealing range. The steel is usually held at heatlong enough to insure uniform heat distribution in all sections. It is then

removed from the furnace and immediately quenched in the cooling

medium.

In carbon steels it is generally true that the faster the heat is extracted,

the harder, stronger, and more wear resistant the finished part will be.

High hardness is frequently accompanied by excessive brittleness. Waterquenched high-carbon steels will often crack or shatter if dropped, or if

allowed to stand overnight before tempering.

The common quenching media are air, oil, water, and brine solutions.

Selection of the proper quenching medium is based on the chemistry

of the steel, section size and shape, and the final mechanical and physical

properties required. Air has the slowest cooling rate, and brine solutions

the fastest rate. The cooling rate of a quenching medium will vary with

the amount of agitation, temperature, and composition. Water at 65°F. is

considered to have a cooling rate of one; brine solutions have as muchas twice the cooling rate of water; oils vary from 15% to 50% of the cool-

in rate of water.*

Hardening of a steel is usually required because of the demand for the

maximum of one or more of the mechanical or physical properties that

the steel is capable of assuming. Sometimes because of size, shape, varia-

tion in cross section, or some production problem, a compromise must bemaele. If this must be done, a careful study of the different cooling mediashould be made in order to select the one best suited to produce the best

compromise of mechanical and physical properties.

For lower carbon steels, water is usually used because of its low cost,

easiness of handling, and availability. In medium and high carbonsteels, oil is often used because it produces less warpagc, distortion, andcracking than with a water quench. However, by using an oil quenchinstead of water, maximum hardness, strength, and wear resistance maynot be obtained.

When the heated steel is quenched in a liquid, there are two distinct

steps or stages of heat removal that are passed through before reaching

the temperature of the quenching medium.Step One: The vaporization period. In this period of the cooling cycle

the quenching medium will vaporize when it comes in contact with the

• American Society for Metals Handbook. The Society, Cleveland, 1948. pp. 616-619.


steel. If the vapor is allowed to form a film around the part being

quenched, the cooling rate is slow because the vapor film insulates and

blocks off contact between the cooling liquid and the steel. By breaking

up the vapor film and allowing the liquid to vaporize and be carried away,

the fastest cooling rate is achieved. The breaking up of this vapor film is

accomplished by proper agitation or circulation of the quenching liquid

or by agitation of the parts being quenched, or both.

Step Two: The liquid period. After the quenched pari has lost suffi-

cient heat so that the liquid is no longer vaporized upon contact, the

removal of heat is now a simple problem of heat transfer and the rate of

heat loss is relatively fast as compared with the vapor cycle.

The difficult and critical period in quenching is the elimination of

the vapor film. Prolonging this step may result in unsatisfactory harden-

ing. The part may have areas which arc low in mechanical properties, or

the entire microstructure may be unsatisfactory and fail to produce the

required mechanical properties.

The internal stresses introduced by quenching are the result of (1)

dimensional changes in cooling, (2) large temperature gradients between

surface and center, and (3) the change in volume accompanying the

change from gamma to alpha iron. If the steel is ductile, these changes

cause slight plastic flow of the grains which relieves the stress. In high-

carbon steels where no plastic deformation takes place these stresses maybuild up to a high value.

Stress distribution in quenching is complex and difficult to analyze

accurately. In many instances, it is certain that the sum of the induced

stress and the working stress is near or above the tensile strength of the

steel. Thus, it was found* that in hollow cylinders the stress that was in-

troduced by quenching at the inner surface of the hole was about 150,000

lb. per sq. in. As the working stress on these cylinders was 100,000 lb. per

sq. in., the total stress was 250,000 lb. per sq. in. or close to the actual

tensile strength of the steel.

Internal stresses may affect the strength and ductility and frequently

cause warping and cracking. The magnitude of these stresses is de-

pendent on the size and shape of the piece, the kind of steel, the quench-ing temperature, and the coolant. When the stress builds up to a value

higher than the tensile strength, the steel cracks. There are two types

of such defects: (1) local cracks, which are usually external and start

at sharp comers or deep tool marks, and (2) internal cracks, whichfollow the major axis of the piece. Quenching in water is more likely to

• O. V. Greene, Trans. Am Soc. Steel Treat., v. 18, 1930, pp. 369-403.


cause cracking than quenching in oil or air. Coarse-grained steels crack

more readily than fine-grained material. Localized heating in grinding

frequently causes cracking, as does increasing the temperature from whichthe steel is quenched.

The best procedures in avoiding quenching cracks are as follows: (1)

use generous fillets where the contour changes and avoid tool markswhich may act as a notch; (2) avoid rapid heating, especially through

the transformation range, which may cause cracking because of the un-

even expansion; (3) temper immediately after quenching; and (4)

quench in oil instead of water wherever possible.

14.6. Tempering

The previous discussions of the principles underlying quenching in-

cluded details of structural changes and related changes in properties.

Some of these changes in physical properties are often desirable, particu-

larly where hardness and wear-resistance are of importance. However, this

increase in hardness, strength and wear-resistance is at the expense of

ductility because, in most instances, the brittleness has also increased. Theextent of brittleness, coupled with hardness, etc., depends on the severity

of quench and the percentage of carbon in the steel. Tempering lowers

the hardness and strength and decreases wear-resistance; it also increases

ductility.

The purpose of tempering is not to decrease hardness but rather to

restore ductility and improve toughness. A way of doing this and at the

same time retaining the original hardness and strength would be desirable.

However, since tempering does decrease the hardness and strength, butat the same time restores ductility, it would be well to consider what is

desirable in the way of strength. Generally a structural part is not sub-

jected to deformation until it breaks; it is the stress capacity without

permanent deformation which is important.

Tempering should be carried out at the highest temperature that will

insure retention of sufficient strength so that the part can withstand loads

comparable to its yield strength. The higher the temperature, the

tougher the product, but this will necessarily be at the expense of hard-

ness and strength.

For tempering, the hardened steel is reheated in a furnace, in heavy

oil, in molten salts, or in molten lead, and held long enough for the

heat to penetrate to the center. For most steels, the speed ol cooling after

tempering is of little consequence, provided the steel was not heated

to the transformation temperature. In practice, the cooling after tem-

pering is usually in the air.


The metallographic structure of tempered steel will be further dis-

cussed in Section 14.7, where tempering is compared to other heat treat-

ment processes.

14.7. Isothermal Treatments

One result of the study of the subcritical transformation of austenite,

which also led to the S. curve, by Bain and his associates* was the dis-

covery of austempering, a process of interrupted quenching whereby

high-carbon and low-alloy steels of suitable composition and cross section

can be hardened without cooling to atmospheric temperature, thus elimi-

nating the possibility of introducing high quenching stresses with possible

distortion or cracking. The process cannot be used if a very hard mar-

tensitic structure, as obtained by water quenching high-carbon steels andtempering at 200 to 300°F. (100 to 150°C), is desired; but it may be

used to obtain a structure corresponding to tempered martensite of any

required degree of fineness, such as is usually attained by cooling rapidly

to room temperature followed by reheating to 400°F. (200°C.) or above.

At any temperature below the A\ transformation and above the tem-

perature—about 400°F. (200°C.) —where martensite is formed, the aus-

tenite transforms to pearlitej of definite structure at a definite rate. If,

therefore, small specimens of high-carbon steel are heated above the

transformation temperature and are quenched very rapidly into a bath

maintained at any intermediate temperature, for example, 500 to 800°F.

(260 to 425°C.) , and if the specimen is held in this bath for a definite

time, the transformation of the retained austenite and the resulting struc-

ture may be controlled closely. Steels so treated are more ductile than

similar steels cooled rapidly to room temperature and tempered at a

temperature that produces a corresponding structure. The hardness andstrength of the austempered steel are about the same as if the steel had

been quenched and tempered in the usual manner, but elongation, reduc-

tion of area, and impact resistance are usually higher. The explanation

advanced for the higher ductility in the austempered steel is its freedom

from submicroscopic cracks that are caused by drastic quenching (to

form martensite) and not subsequently healed by tempering.

A schematic representation of austempering is shown in Fig. 14.2. t Toprevent any transformation of the unstable austenite to pearlite,. the

specimen must be small enough to cool past the nose of the S curve very

rapidly; for the steel whose diagram is given in Fig. 14.2, it must cool

•E. C. Bain and E. S. Davenport, U.S. Patent 1.924,099, Aug. 29, 1933.

f Based on diagram by F. J. McMulkin, Iron Age, v. 157, June 27, 1946, p. 58.

% (or Bainite)

.


1400

I30C

Mostly \morfensife

I5min.30min. Ihr.

I I

Bhr. Icty .

1200

MOO

-11000

900,

800"°.

700|E

600"

500

400

300

200

100

10,000 100.0001000

Time, sec.

Fig. 14.2. Schematic T-T-T diagram for carbon steel showing critical-cooling-rate

curve, MB temperature, and temperatures for austempering and martcrnpering. (Mc-Mulkin)

through the temperature range of 900 to 1000°F. (480 to 540°C.) in about

1 sec. The specimen is therefore quenched rapidly into a bath maintained

at the desired temperature-600°F. (315°C.) in Fig. 14.2-and held in the

bath until the transformation to bainite (emulsified ferrite and cementite)

is complete, that is, for about 45 min. Structural changes are now com-plete, and the specimen may be cooled, at any rate, to room temperature.

A process that carries the principles underlying the subcritical de-

composition of austenite farther than austempering is martempering,

which was developed a few years ago by Shepherd.* In this process the

* B. F. Shepherd has described martempering in a number of papers. A luciddescription, which also includes an elementary discussion of the transformation ofaustenite to martensite in quenching and isothermally, as mapped by the S curve, is

contained in two arUcles by Shepherd in Production Eng., July, 1945, p. 438, and Aug.,1945, p. 515.


steel is quenched at its critical rate or faster—so that it will pass the nose

of the S curve without any transformation of the austenite—into a bath

maintained at a temperature slightly above the Ms point, in other words

just above the temperature where martensite starts to form. This tempera-

ture varies with carbon content and slightly with the amount of alloying

elements present, but for high-carbon and many alloy steels it is ap-

proximately 400°F. (200°C.) . The steel is held in this bath just long

enough for the temperature to become uniform throughout the cross

section, after which it is removed and cooled in air. A schematic repre-

sentation of martempering is shown in Fig. 14.2. The specimen is

quenched at the critical cooling rate or faster into a bath maintained at

about 450°F. (230°C.) and held in this bath for a sufficient time to be

heated through (about 1 min. for a small specimen of the steel shownin Fig. 14.2) . It should be noted that the specimen should not be heated

so long that the line where bainite begins to form is crossed by the time

axis at this tempering.

Martcm|>ering is based on the principle that, if austenite is retained

by rapid cooling down to the temperature where martensite begins to

form, the austenite-martensite transformation will proceed, regardless of

the cooling rate from this temperature down to atmospheric temperature;

and that on cooling slowly, as in air, from about 400°F. full hardness will

result without the concomitant high internal quenching stresses whichare characteristic of drastically quenched high-carbon or alloy steels. Ac-

cording to Shepherd, this treatment improves the mechanical properties,

especially ductility and toughness. The method can be used for heavy

sections and for pieces of irregular shape which often crack in the usual

hardening operation.

Martempering resembles austempering only in that the quenching is

interrupted at an intermediate temperature; but the steel is not held at

this temperature long enough for any appreciable amount of the austenite

to transform. In martempering, therefore, the austenite transforms to

hard martensite, while the steel is cooling slowly from this intermediate

temperature to atmospheric temperature; in austempering the steel is

held at the intermediate temperature long enough for the austenite to

transform to a ferrite-carbide aggregate of desired structure and prop-

erties.

Patenting is a very old process used to heat treat medium- and high-

carbon rods which are to be cold drawn into high-strength wire. Therods or the wire (if patenting is used as an intermediate stage in drawingcoarse wire to finer sizes) are heated to a temperature considerably abovethe A 3 transformation temperature, so that some grain growth takes place,


and cooled in air or quenched in a bath of molten lead maintained at

850 to 950°F. (455 to 510°C). The object of patenting is to produce a

fairly coarse austenitic grain that transforms in the air or at the lead-bath

temperature to large pearlite grains which are made up of fine ferrite

and cementite. Such a combination of controlled overheating and finely

dispersed carbide and ferrite in the pearlite is essential for the steel to

withstand the cold work to which the wire is subjected in drafting andis also essential if satisfactory quality is to be secured.

Patented and cold-drawn wire for use in cables in suspension bridges

is fairly high in carbon and has a final strength of around 225,000

lb per sq. in. and an elongation of about 2 to 6 per cent in 10 in. Patented

and cold-drawn wire is the only suitable material found thus far for

suspension-bridge cables. A high-carbon wire, cold drawn and then

quenched and tempered so that it has the same mechanical properties,

is unsatisfactory. The reason for this is not clear, but the first attempt

to use heat-treated wire instead of patented and cold-drawn material

ended so disastrously that it is doubtful whether it will ever be tried

again." Patented and cold-drawn wire has been used for suspension-

bridge cables for many years, and failures are unknown. The most that

can be said is that apparently the equiaxed structure of the heat-treated

wire does not resist so well as the elongated "fibrous" structure of the

cold-drawn material the propagation of minute cracks on the surface,

formed when the wire is bent around the anchorage shoes.

Some emphasis should be given to the advantages of austempering andmartempering as compared to tempering a martensitic steel. A tougher,

more shock-resistant steel is obtained in austempering or martempering.

Tempering involves the decomposition of martensite to granular mix-

tures of cementite and ferrite in which the cementite particles tend to

be round rather than lamellar as in pearlite. Temperature and time,,

primarily temperature, are important factors, along with the amounts of

alloying elements in the case of alloy steel, in determining the size of

the separated carbide particles. At very low tempering temperatures the

resulting decomposition products are hardly discernible under the micro-

scope. Tempering at progressively higher temperatures results in micro-

structures that can be resolved under the microscope; the carbide par-

• The wire used in the construction of the Mt. Hope Bridge, at Providence, and the

Ambassador Bridge, at Detroit, both ot which were suspension bridges, contained 0.80

per cent carbon and was oil quenched and tempered. Although the mechanical prop-

erties were satisfactory, the wires began to fail soon after the cables were spun; the

bridges had to be dismantled and rebuilt with the usual patented and cold-drawnwires in the cables. See E. E. Thum, Metal Progress, v. 21, 1932, No. 6; v. 22, 1932,

No*. 1, 2, and 3.

The Operation of Heal Treatment 289

tides are progressively larger, and eacli micro-structure is softer and

tougher than the preceding one. The structure in which the divorced

cementite, embedded in the ferrite matrix, can be seen with moderate

magnification is termed spheroidite.

In the practice of austempering, the formation of martensite structure

is avoided by suddenly cooling the heated steel to a predetermined tem-

perature at which only bainite forms and by holding it there for sufficient

time to complete the transformation of austentite to bainite. Thus a

product having ductility (toughness) comparable to that of a tempered

steel is obtained in a single operation and without subjecting it to the

quenching necessary to produce martensite.

Martempering, which produces a hard martensitic structure, avoids

the high internal quenching stresses produced in a fully hardened steel

by drastic quenching.

Attention should be called to the commercial limitations of austem-

pering and martempering. In practice, small section sizes (plain steel)

up to y4 inch respond best to the above method because of the necessity

of suppressing the austenite-to-pearlite transformation. The maximumsize that can be effectively treated, though, is determined by the composi-

tion and characteristic S-curve of the alloy.

14.8. Case Hardening Processes

There are many important industrial products, such as gears, cam-

shafts, piston pins, and the like, which should have a hard, wear-resistant

surface and a tough core. Carburizing is the most widely used method

of securing such a combination of properties. In this process, unalloyed

low-carbon or low-carbon alloy steel is heated in contact with a carbon-

aceous material, from which the steel absorbs carbon. The depth of

absorption depends on temperature, time, the alloying element present

(if any) , and the carburizing compound. With a commercial compound,

a case %„ in. deep is obtained by heating for 8 hr. at 1700°F. (925°C).

Two general methods are in use: pack carburizing and gas carburizing.

In the former, the steel article, fully machined and finished except for a

small allowance for grinding, is packed, together with the compound, in

a heat-resistant alloy box. The carburi/ing compound is coke or charcoal

mixed with an "energizer," which is usually barium carbonate. At the

carburizing temperature, the barium carbonate dissociates into barium

oxide and carbon dioxide; the latter reacts with the carbon in the char-

coal to form carbon monoxide gas (CO a + C = 2CX)) . This gas reacts

with the iron at the surface of the steel to form iron carbide. By diffusion,

the carbon then penetrates below the surface. In the other method, used


Fie. 14.3. Structure of slowly cooled carburized steel (A) at the surface and (B)in the transition zone between surface and core. The structure of the low-carbon coreis shown at the bottom of B; etched. 100x-

chiefly for small articles, the steel is heated in a retort furnace into whicha gas composed of hydrocarbons or carbon monoxide is introduced. Thismethod is more rapid than pack carburizing, and the case depth can be

more closely controlled.

After carburizing for a sufficient time at the proper temperature (both

of which have been standardized) , the carbon at the surface varies fromabout 0.8 to about 1.1 per cent. This grades off uniformly with in-

creasing distance from the surface, until at a depth of >/4 or s/A in. the

structure is unaffected. Typical structures of slowly cooled carburized

steel, at the surface (A) and in the transition zone (B) , are shown in

Fig. 14.3.

Carburized steel is always heat treated to produce the required hard-

ness of the case and usually to improve the ductility of the core, whichmay be too coarse-grained from the long heating at high temperature.*

After heat treatment, the case of carburized steel articles is martensitic in

structure and very hard, and the core has the ductility of low-carbon

•Sec Metals Handbook, 1948, pp. 681-G85, for a detailed description. See also S.

Epstein, The Alloys of Iron and Carbon, Vol. I, Constitution, McGraw-Hill BookCompany, Inc., New York, pp. 335-340, for a discussion of principles.


steels. Even greater toughness and shock resistance, combined with a hard

surface, can be secured by carburizing a low-carbon alloy steel, especially

one containing 3 to 5 per cent nickel.

Austenite grain size is important in carburizing steels. After carburiz-

ing and heat treatment, a coarse-grained material has a uniformly hard

surface, but the core may not have such high impact resistance as is

generally desired. Fine-grained steels, on the contrary, are prone to

carburize irregularly and develop soft spots on the surface after heat

treatment. These conditions can frequently be avoided by special quench-

ing techniques, most of which use a spray of water under pressure, or

by the use of alloy steel.

Nitriding, an established process for producing a hard surface on

steel, uses nitrogen as the hardening agent instead of carbon. It was in-

vented by Adolph Fry in Germany about thirty years ago and is now used

widely in applications where a superhard surface combined with consid-

erable corrosion resistance and resistance to softening at elevated tem-

peratures is desired. It is used by the automotive and aircraft industries

for valve seats and for guides, gears, and piston pins of internal-com-

bustion engines and in a number of applications in steam plants and oil

refineries.

The process consists of heating the finish-machined article in a closed

container into which ammonia gas is forced under pressure. At a tempera-

ture of 850 to 1000°F. (450 to 540°G.) the ammonia is mostly dissociated

into nitrogen and hydrogen. The former combines with the iron and

other elements in the steel to form complex nitrides which diffuse into

the surface layers. This diffusion is slow; a case varying in depth from

0.0005 to 0.02 in. is obtained after 5 to 25 hr.

Unalloyed carbon steels cannot be nitrided successfully. Some alloying

element which forms a stable nitride must be present. The steels used

for nitriding, therefore, contain chromium, aluminum, molybdenum, and

vanadium in various combinations. Aluminum and chromium are the

effective elements in producing high hardness, while molybdenum and

vanadium increase the toughness of the steel and also increase the depth

of penetration of the nitrogen. Nitriding steels are usually heat treated

before nitriding to obtain the desired mechanical properties, which are

not affected by the treatment with ammonia, provided that the steels were

tempered prior to nitriding at the same or higher temperatures than used

during the treatment with ammonia.

The outstanding advantage of nitriding is high surface hardness and

wear resistance. This surface hardness cannot be equaled by any other

method known to ferrous metallurgy. It averages a Vickers hardness


number of 1200 as compared with 650 to 700 for quenched high-carbonsteel of maximum hardness.

Another advantage is that no subsequent heat treatment is necessary,as it is for carburized steel. Nitriding is, however, accompanied by anincrease in the size of the section. This growth is dependent upon thetime and temperature of the operation but is constant for a given set ofconditions. It averages 0.001 to 0.002 in. for the usual temperatures andfor nitriding periods of 15 to 20 hr. To allow for this growth, parts thatare to be nitrided are machined so that they are undersize by the amountof the expected growth. If it is desired to prevent the absorption ofnitrogen in certain areas, these are plated with nickel or coated with tin.

Nitriding is a relatively costly process. In the first place, special alloysteels are necessary; in the second, nitriding equipment is expensive. Anitrided camshaft, for example, costs from 5 to 15 times as much as acomparable article which has been carburized and heat treated. Since thecost depends largely upon the quantity treated, an appreciable reductionin cost would result if the demand for nitrided steel would permit large-scale production.

Cyaniding is another method used to produce case hardening. Thisprocess uses molten sodium cyanide, sodium chloride, and sodiumcarbonate as the bath in which the steel is immersed. The case producedis very thin, seldom over 0.002 of an inch, and the temperature requiredis from 1300 to 1600°F. (700 to 870°C).

A modification of the cyaniding process is to bubble ammonia gasthrough the molten salt bath, thus liberating nitrogen faster and in-

creasing the nitrogen content of the case. This modified process is termedchapmanizing.

Carbonitriding involves both carburizing and nitriding in a singleoperation by heating the steel (1 200-1 600°F) in a carburizing gas(CH4 ,

GjH8 , etc.) containing ammonia. This process has gained wideacclaim and many steel parts are being case-hardened by this methodtoday. Lower temperature processing results in a case similar to anitrided steel. Higher temperatures produce a case like a carburizedsteel.

Nitrided cases are very stable; the hardness is unaffected by long heat-ing at temperatures as high as 750 to 850°F. (400 to 455°C.) and is notaffected by heating for short periods at temperatures as high as 1000 to1 100°F. (540 to 595 C C.)

. The surface is resistant to such corrosive agentsas the atmosphere, alkalies, crude oil, tap water, salt water except whenit is moving, ethyl gasoline, and many others. It is, however, not resistantto mineral acids. It has the highest wear resistance of any ferrous material


and is finding increasing use in applications where this property is of

prime importance.

14.9. Flame Hardening and Induction Hardening

There are a number of applications for steel articles that should be

hard only in a relatively small area on the surface. For such uses, it is

not economical to heat treat a section weighing 100 lb. or more by the

usual method of heating in a furnace followed by quenching and temper-

ing, or by carburizing to harden the whole surface. In some cases, heat

treating a complex section by the usual method causes too much

distortion. Also, it is not generally economical to operate a furnace for

the treatment of a single piece. For heat treatment under such conditions

as these, flame hardening and induction hardening have been developed

recently.

Flame hardening consists of heating above the Aa transformation tem-

perature any desired external spot of the finished steel article, as for ex-

ample the teeth of a gear (Fig. 14.4),* with an oxyacetylene torch. This

is adjusted for flame composition so that the steel does not oxidize. Thedistance of the torch from the work and the time of heating are also

controlled so that the steel does not become overheated to the point

where grain growth is serious. As soon as the surface area is at the proper

temperature, it is quenched and immediately tempered to relieve quench-

ing stresses.

Two general methods are used. In the first, the steel piece is fixed and

the torch moves across the area to be heated at a predetermined rate. This

must be controlled carefully as the temperature of the oxyacetylene flame

is approximately 6300°F. (3500°C.) , but the piece to be hardened

should not be heated higher than 1475 to 1550°F. (800 to 850°C.) . In

the second method, the torch is stationary, and the steel piece moves

slowly under the flame. The hot area is usually quenched in water, which

is conveniently conveyed through one tube of the torch and flows out of

an orifice under pressure just behind the flame.

In addition to economy under certain conditions, flame hardening has

other advantages. It does not scale or pit the surface; it has no effect on

the chemical composition; it hardens only at the surface and to a depth

of not more than \/4 in., and distortion is minimized. Its disadvantages are

that it is more costly than the usual methods if large numbers of identical

pieces are to be treated, and that high-carbon steels and very coarse

grained steels cannot be treated, as they arc likely to crack at the surface.

• R. L. Rolf, Trans. Am. Soc. Metals, v. 27, 1939, pp. 43-00.


Fie. 14.4. Flame hardening the teeth of a large gear. (Rolf)

Steels most suitable for flame hardening are carbon steels containing

0.35 to 0.60 per cent carbon and low-alloy steels containing 0.25 to 0.50

per cent carbon. Such articles as alloy-steel gears which have been heat

treated so that they have optimum strength and toughness can be ef-

fectively hardened on the surface of the teeth by this method.

Differential hardening by induction is a recently developed method for

securing high hardness on a relatively small bearing surface and is es-

pecially applicable to heavy-duty crankshafts* and to gear teeth, the ends

of shafts, pins, hubs, and similar sections that act as thrust bearings.

The surface to be hardened is enclosed in a perforated induction block

that surrounds the steel but does not touch it. A high-frequency current

of 2,000 to 200,000 cycles (from 25 to 50,000,000 may be used) is passed

through the block, inducing eddy currents which rapidly (in I to 5 sec.)

* The method is described in detail by M. A. Tran and W. E. BcnninghofT in Trans.Am. Soc. Metals, v. 25, 1937, p. 935. Sec also Induction Hardening, American Society

for Metals, Cleveland, 1946, 172 pp. This is a series of lectures covering theory andpractice.


Fie. 145. Cross section of a crankshaft showing hardened (while) bearing surfaces,

etched. (Tran and BenninghofJ)

heat the surface to be hardened to a temperature above the upper trans-

formation temperature. As soon as this occurs, water under pressure is

sprayed onto the hot surface through the holes in the induction block.

The heating and quenching cycle and other conditions are controlled,

so that if steel of the proper composition and grain size is used, the bear-

ing surface will have a hardness of about 60 Rockwell C (600 Brinell)

to a depth of i/jj in. or more, and decarburization, grain growth, and dis-

tortion will be prevented.

One of the most difficult problems to solve in the manufacture of in-

ternal-combustion engine crankshafts is how to secure a combination of

high hardness and wear resistance on the bearing surfaces and ample

toughness and relatively easy machinability for the rest of the shaft. This

problem has been solved by induction hardening. The shaft can be heat

treated by quenching and tempering at a high temperature, or even by

normalizing, to obtain a structure of fine pearlite, which has satisfactory

toughness and ductility and can be machined fairly easily. Hardening by

induction heating produces a bearing surface with ample hardness and

wear resistance and leaves the fillets, where the stress concentrates,

ductile and relatively stress-free. An etched cross section of a heavy-duty

induction-hardened crankshaft is shown in Fig. 14.5. The hard bearing

surfaces are readily apparent. Heat treatment by induction heating has

expanded rapidly in the past few years; at present approximately one

third of all the quenched and tempered parts used in the United States

are hardened by this method.*

• E. L. Cady, Materials and Methods, v. 24, Aug., 1946. pp. 400-410. This is an

excellent summary with emphasis on the economic aspects.


QUESTIONS

1. Why is complete Austenization of hypoeutectoid steels essential for full

hardening? Why is only partial austenization sufficient for hypereutectoidsteel to be hardened?

2. What is process annealing? What is full annealing? Explain the differencebetween these two annealing processes on the basis of prior processing,temperature required, changes in micro constituents, cooling rates andgeneral usage.

3. What are the objects of normalizing low carbon steels? Of normalizinghigh carbon spheroidized steels?

4. Compare air, oil, water, and brine as quenching media. What is the masseffect in quenching? What other conditions affect the selection of theproper quenching medium?

5. What causes internal stresses in quenched steels? Why should these internalstresses be relieved?

6. How is spheroidized structure obtained? What are the reasons for spheroidiz-ing? What problems arise in the heat treatment of a spheroidized structurefor full hardness?

7. What is the overall objective of tempering? Correlate the effects thattempering has on the hardness, brittleness. strength, and toughnessproperties.

8. List the metallographical structures produced in tempering a steel whenmartensite decomposes at progressively increased temperatures. Comparethe physical properties of these different structures.

9. What is meant by austempering, and what are its advantages and limita-tions?

10. Compare martempering with austempering.11. Why is carburizing limited to low-carbon or low-carbon alloy steel, and how

is carburizing accomplished?12. Compare the processes: cyaniding, nitriding, and carbo-nitriding.

CHAPTER

Carbon Steel as anEngineering Material

15 Arthur C. Forsyth, Ph.D., Associate Professor of Metal-

lurgical Engineering, University of Illinois, Urbana,Illinois

J.Edward Krai.ss, M.S., Head, Department of Mechani-

cal Technology, New York City Community College,

Brooklyn, New York

Jf OR a thousand years before 1850 man's en-

tire stock of ferrous metals consisted of cast iron, wrought iron laboriously

made in small quantities at high cost, and a few high-carbon steel tools

made even more laboriously and at still higher cost. These were valuable

materials and represented a long step forward from the metals of the

bronze age, but they were too costly or too low in quality to be used in

building railroads, bridges, buildings, and ships. Between 1850 and 1860

Henry Bessemer in England and William Kelly in the United States madediscoveries which led to the development of a process for making carbon

steel in quantities large enough and at a cost low enough to spark a cen-

tury of industrial progress.

A reasonably accurate understanding of this progress can be obtained

from the following production figures: 800,000 tons in 1870, 28 million

tons* in 1900, 119 million tonsf average world production between 1935

and 1939 (42 million tons in the U. S.) , and 253 million tonsj between

1951 and 1955 (103 million tons in the U. S.) . This shows an increase of

over 100% in the last twenty years. Before World War II approximately

95 per cent of the steel produced in the world was carbon steel. This per-

• Estimate by Sir Robert Hadfield, The (London) Times Trade and EngineeringSupplement, Jan. 28, 1953.

f Steel, Jan. 4. 1943. p. 357, Average Production of Steel (1935-1939) for Ingots andSteel for Castings, 119,454,600 toss. Same period for the United States, 42.176,600 tons.

t Steel, Vol. 140, No. 1, p. 172, Average World Production of Steel (1951-1955) forIngots and Steel for Castings, 253,032,163 tons. Same period for the United States,

103,065,068 tons.

297


centage has now dropped to about 90.8 per cent (in the United States) •,

but carbon steel is still the most important metallic material on a tonnage

basis.

About 1.3 per cent of the carbon steel produced annually is poured into

castings -)- which are used in the form the metal assumed in the mold with

no further mechanical treatment. The remaining 98.7 per cent is poured

into ingots that are hot worked by rolling, forging, or pressing into a

large variety of finished or semifinished products. Most finished wrought

sections receive no further treatment, but a few are subjected to some

form of heat treatment before use. Some semifinished wrought products

are processed further by additional hot working; the rest are fabricated

into the desired form by cold working. (See Section 1 1 .9.) Cold-worked

steels may or may not be heat treated before being used.

There are, therefore, four major classes of finished products of carbon

steel: castings, hot-worked products, cold-worked products, and heat-

treated products. It is impossible to decide which of these four classes

is the most important in present-day economic life, since each plays a

definite role for which there is no understudy. Without hot-rolled

I beams, there would be no skyscrapers; without heat-treated tools they

could not be built; without cold-drawn wire for elevator cables, they

could not be used; and without castings for steam and water lines,

they would not be habitable. It is the same in most other industries: each

class of carbon steel is indispensable, and the cost or tonnage is no

criterion of its value to man.

15.1. Carbon-Steel Castings as Engineering Materials

Unalloyed steel castings have many industrial uses. Despite the fact

that their mechanical properties are generally inferior to those of hot-

worked steel of the same composition, the proportion of castings to rolled

or forged material remains substantially unchanged year after year. The

reasons for this are twofold: (1) Sections that are so complex as to be dif-

ficult or even impossible to fabricate by rolling or forging can be pro-

duced readily at low cost by casting; (2) castings are cheaper than

forgings if a few pieces of a fairly intricate shape are desired. The eco-

nomical production of such articles by forging is possible only when a

• Steel, Vol. 140, No. 1, p. 174, Average U. S. Alloy Steel Production, Total Ingots

and Castings, 9,488,097 tons (1951-1955).

+ Steel, Vol. 140, Xo. 1, p. 178, Average U. S. Production of Carbon Steel Castings,

(1951-1955) 1,216,948 tons.

Carbon Steel as an Engineering Material 299

Table 15.1. Chemical Composition and Minimum Mechanical Properties for MediumStrength and High Strength Steel Castings *

Chemical Composition

(per cent) Tensile Yield Elonga- Reduc-(maximum) t Strength Point tion in tion in

Grade (lb. per

sq. in.)

(lb. per

sq. in.)

2 in.

(per cent)

Area(per cent)

C Mn Si

Nl 0.25 0.75 0.80

N2 0.35 0.60 0.80

N 3 1.00

U. 60-30 0.25 0.75 0.80 60,000 30,000 22 30

60-30 0.30 0.60 0.80 60,000 30,000 24 35

65-30 65,000 30,000 20 30

65-35 0.30 0.70 0.80 65,000 35,000 J 24 35

70-36 0.35 0.70 0.80 70,000 36,000 22 3080-40 80,000 40,000 18 3080-50 80,000 50,000 22 35

90-60 90,000 60,000 20 40

105-85 105,000 85,000 17 35

120-95 120,000 95,000 14 30

150-125 150,000 125,000 9 22175-145 175,000 145,000 6 12

• A.S.T.M 1955 Standards, Part One, Ferrous Metals, 27-55, and A148-55, pp. 1026

1027 and 1035-1036.

f Maximum sulphur 0.06 per cent; maximum phosphorus 0.05 per cent. For each

reduction of 0.01 per cent carbon below the maximum specified, an increase of 0.04 per

cent manganese above the maximum specified will be permitted to a maximum of 1.00

per cent.

X When agreed upon by the manufacturer and the purchaser, and when full annealing

is required by the purchaser, the yield point value of the 65-35 class shall be 33,000 psi

instead of 35,000 psi.

large number of pieces of a simple shape are produced thus spreading

thinly the cost of the expensive forging dies.

Steel for castings may be melted by any of the processes enumerated

in Table 11.1. Most of it is made by the basic or acid open-hearth or the

acid electric process. The open-hearth process is used for melting steel

for large castings; the acid electric process, owing to its flexibility and

to the high temperatures obtainable, is favored for small castings, espe-

cially those of intricate shape. Steel castings are used for railroad equip-

ment, especially for underframes of cars, for agricultural and excavating

equipment, various parts of machines, electric equipment, and a large

variety of small parts and fittings.

There are several grades of steel castings for which specifications have

been issued by the American Railway Association, the United States


government, and a number of technical societies. Typical specifications

for chemical composition and minimum tensile properties are given in

Table 15.1.

For high-strength castings (above 80,000 lb per sq. in. tensile

strength), the chemical composition, with the exception of maximumpercentages of sulphur and phosphorus, is not specified; the foundryman

is permitted to adjust the percentages of carbon and manganese and to

use alloying elements, if necessary, which, when combined with a suit-

able heat treatment, will result in the desired properties. (See Table 15.2.)

15.2. Factors Affecting the Properties of Carbon-Steel Castings

Since the properties of carbon-steel castings are greatly improved by

heat treatment, many castings are now annealed, normalized and an-

nealed, or normalized and spheroidized. Quenching and tempering of

castings of such shape that distortion is not likely to be serious, are used

more and more frequently.

Table 15.2. Mechanical Properties of Steel Castings*

Steel castings are generally specified by mechanical properties, leaving to the producer

control of the chemistry within certain limits, t

Structural Grades—Carbon Steel

Tensile Strength psi 60,000 70,000 85,000 100,000

Mechanical Properties JMod of Blast in Tension, psi: 30.1 x 10° 30.0 x 106 29.9 x 10" 29.7 xlO6

Tensile Strength, 1000 psi: 60(a) 70(b) 85(c) 100(d)

Yield Point, 1000 psi: 30 38 50 70

Elong in 2 in., %: 32 28 24 20

Reduction of Area, %: 55 50 40 46

Hardness, Bhn: 120 140 175 200

Impact, Izod, Ft-Lb:

(70° F) 30 30 20 30

(-50° F) 8 10 10 15

Fatigue Str, (End Limit), 1000 Psi: 25 31 38 47

* Selected data from "Materials Engineering File Facts." Materials and Methods vol. 38,

No. 4, 1953, pp. 153. Data sheets include current specifications, physical and mechanical

properties, thermal treatment etc. for carbon and alloy steel castings.

f "Summary of Standard Specifications for Steel Castings" may be found in Materials

and Methods, Vol. 36, No. 1, 1952, pp. 121 and 123. Minimum mechanical properties and

maximum chemical compositions are listed.

X Normally expected values in the production of steel castings for the tensile strength

values listed.

(a) Annealed, (b) Normalized, (c) Normalized and Tempered, (d) Quenched and

Tempered.

Carbon Steel as an Engineering Material

V SxSJC

301

^V '&N*S?££

Fie. 15.1. Photomicrographs of Typical Microstructures o£ Small Steel Castings con-

taining 054% C, 0.60% Mn., 0.84% Si., 0.032% P. and 0.036% S. Notice the change in

grain size due to the following heat treatments. A-as cast, B-normalized 1700°F.

(926°C). C-annealed 1700°F. (926°C), and D-quenched from 1700'F. (926°C.) tempered

at 1200°F. (651 °C). Nital etch lOOx. Courtesy of the Mining and Metallurgy Depart-

ment, University of Illinois.

Large castings cool so slowly in a sand mold that a very course angular

structure results. If the casting is not too large, simple annealing will

refine the grain and improve the properties (Fig. 15.1C) , but such

treatment does not effect complete homogenization in large castings.

These are subjected to high-temperature normalizing followed by a grain-

refining treatment. Quenching and tempering produce the finest structure

(Fig. 15. ID illustrates the structure for small castings) and the best com-

bination of properties.*

Grain size is directly related to the rate of cooling from the solidifica-

tion temperature to the A 3 transformation temperature. It follows, there-

•Table 152 shows the normally expected values for mechanical properties in the

production of steel castings for the tensile strength values listed.


fore, that a small casting usually has smaller grain size and better proper-

ties than a large casting; moreover, if the casting is not heat treated, it is

also true that in large castings the properties are better near the surface

than at the center,fThe factor of greatest importance to the properties of cast carbon steel

is carbon content. If other things are equal, increasing the carbon in-

creases the tensile and the yield strength and decreases elongation, re-

duction of area, and impact resistance. When the carbon is higher than

0.50 per cent, the casting is usually so brittle, even after heat treatment,

that it is used only for a few specific applications—for example, rolls anddies—where hardness and wear resistance are the most important prop-

erties.

In general, carbon-steel castings of satisfactory mechanical properties

can be made readily by regulating the carbon content and by using a

simple annealing or normalizing treatment. The minimum properties

given for medium strength castings are conservative and are easily attained

in commercial production (compare properties in Table 15.1 and 15.2)

.

Cast carbon steels containing 0.20 to 0.40 per cent carbon have an

endurance ratio between 0.40 and 0.50. In the cast condition the ratio

is usually between 0.40 and 0.43. Heat treatment (that is, annealing,

normalizing, or quenching and tempering) improves the ratio, but it is

usually somewhat lower than the endurance ratio of a comparable steel

that has been hot worked. Heat treatment also improves the yield ratio,

elongation, and reduction of area of cast carbon steel and increases the

resistance of the material to single-blow impact. The added cost of heat

treatment is usually justified if castings are to be subjected in service

to stresses of considerable magnitude (Table 15.2).

15.3. Hot-Worked Carbon Steels as Engineering Materials

Most hot-rolled or forged carbon steels contain between 0.05 and 0.30

per cent carbon and are used for sheet, plate, strip, tubes, pipe, various

structural sections, tinplate and other coated sheets, and a large number

of semifinished sections that are hot or cold worked into bars, wire, sheet,

and tubes. Also included in this class is the well-known "machine steel"

(containing about 0.20 per cent carbon and 0.40 per cent manganese)

,

which is used for a wide variety of low-stressed machined parts and is

the mainstay of every crossroads blacksmith shop in the world. Medium-

carbon steels containing between 0.30 and 0.70 per cent carbon are used

for railway materials, especially rails, for a large number of forgings, and

fOwing to the mass elfect and lo segregation, it is frequently a serious problem

to cast a test specimen that has properties truly representative of ihe casting.


for high-strength wire. The higher carbon grades, containing 0.70 to 1.30

per cent carbon, are used largely for tools and cutlery and to a lesser

extent for springs and wire. Low-carbon steels are rarely heat treated;

the medium-carbon grades may or may not be heat treated, depending

upon their use; the high-carbon steels are almost always used in the

hardened and tempered condition.

The properties of hot-worked carbon steel are affected by (1) the

composition, (2) the several variables present in hot working, and (3)

the rate of cooling from the rolling temperature. Except for deep-drawing

sheet, this third factor is not important for low-carbon steels; in the case

of small sections of medium- and high-carbon steels, air cooling maycause enough hardening to mask any effect of moderate changes in com-

position or of variations in the hot-working operation. The effect of

composition is discussed in the next section. Of the three variables in-

herent in hot working—direction of work, amount of work, and finishing

temperature—the first is the most important. In general, specimens cut

longitudinally and transversely to the direction of hot working have about

the same tensile strength and yield strength, but the elongation, reduc-

tion of area, and impact resistance of the transverse specimens are lower

than those of the longitudinal specimens. This difference in directional

properties is present even in clean steels, but it is accentuated by in-

clusions which are plastic at hot-working temperatures and which are

thus elongated into fibers by the mechanical work.

15.4. Effect of Composition on Static Properties of Hot-WorkedCarbon Steels

The effect of carbon on the tensile properties of hot-worked basic

open-hearth steels containing between 0.30 and 0.60 per cent manganese

and between 0.02 and 0.04 per cent phosphorus is shown in Fig. 15.2.*

The center line of the hatched area represents the properties to be ex-

pected most frequently. The top and bottom boundaries of the hatched

area give the limits of the properties to be expected at least 95 per cent

of the time in the testing of commercial rolled or forged basic open-hearth

steel of these carbon percentages. The data used for plotting Fig. 15.2

were obtained mostly by statistical analysis. It is safe, therefore, to con-

clude that of 1000 specimens of 0.20 percent carbon steel about 700 will

have a tensile strength of between (53,000 and 65,000 lb. per sq. in., and

•The values in Fig. 15.2 are a summary of all the data given by F, T. Sisco, in

The Alloys of Iron and Carbon, Vol. II, Properties, Chap. 4, § A. anil Include not only

laboratory investigations and other data but also the data obtained in Germany bystatistical analysis of more than 100,000 individual tests.


0.1 0.2 0.3 0.4 0.5

Carbon, per cent

Fie. 15.2. Effect of carbon on tensile properties of hot-worked carbon steels.

about 970 will have a tensile strength (60,000 to 68,000 lb. per sq. in.)

which falls within the hatched area for steel of this carbon content. There

will, however, be about 30 specimens, or 3 per cent of the whole, with a

strength falling above or below the limits shown. These proportions

should hold for steel of any carbon content between 0.05 and 0.70 per

cent.

The elongation values used for Fig. 15.2, if not originally determined

on a 2-in. gage length, were converted to this gage. In connection with

the elongation it should be noted that the usual flat specimen for plate,

sheets, and similar products has a gage length of 8 in.; hence, values

given in Fig. 15.2 are some 20 to 30 per cent higher than would normally

be obtained on an 8-in. gage* The increase in Brinell hardness with in-

•If standard flat specimens of basic steel of about 60,000 lb. per sq. in. tensile

strength and with gage lengths of 2, 4, and 8 in. are tested, the respective elongation

values will be approximately 45, 35, and 30 per cent, respectively.


creasing carbon approximately parallels the increase in tensile strength:

each 0.10 per cent carbon raises the hardness 20 numbers, from about

100 for 0.10 per cent carbon steel to 220 for 0.70 per cent carbon steel.

Although both manganese and phosphorus affect the tensile strength,

the influence of the former is unimportant in most carbon steels. For

example, if a basic open-hearth steel containing 0.20 per cent carbon

and 0.50 per cent manganese has a strength of 65,000 lb. per sq. in., in-

creasing the manganese to 0.80 will increase the strength only 3,000 to

4,000 lb. per sq. in. Phosphorus increases the tensile and the yield

strength about 1,000 lb. per sq. in. for each 0.01 per cent present. In

basic open-hearth steels, this element is commonly so low that it has

little effect; in acid steels, however, there is usually enough present so

that it exerts considerable strengthening action. Thus, acid steel is usually

stronger, with lower elongation and reduction of area, than basic steel

of the same carbon content.

15.5. Effect of Composition on Other Properties

Increasing the carbon of hot-rolled steel decreases the impact resist-

ance; low-carbon basic open-hearth steels have an Izod value of 50 to 60

ft.-lb.; for hot-rolled steels containing 0.50 to 0.80 per cent carbon, it is

usually less than 20 ft.-lb. Increasing the phosphorus decreases the im-

pact resistance; an acid Bessemer steel is not so tough as basic open-

hearth steel of otherwise identical composition. The brittleness induced

by phosphorus is discussed in § 11.10. In connection with impact resist-

ance it should be emphasized that this property is greatly affected by

grain size, direction of hot working, amount and distribution of inclu-

sions, and other factors (aside from chemical composition) related to

the structure.*

These factors may have so much influence on the impact resistance that

the effect of variations in composition may be completely obscured.

The endurance limit of hot-worked carbon steels increases with the

tensile strength. Owing, however, to the fact that high-carbon steels

have less plasticity under load than low-carbon steels, and are morelikely to contain internal stresses, the endurance ratio is usually between

0.35 and 0.42, compared with 0.50 to 0.60 for wrought iron, ingot

iron, and steels containing less than 0.35 per cent carbon.

The modulus of elasticity of carbon steels is between 29 and 30 million

lb. per sq. in and is not greatly affected by composition and heat treat-

ment. Some of the other physical constants and most of the electric and

magnetic properties are affected, more or less, by changes in composition.

•Also see: Temper Kmbrittlement in Plain Carbon Steels, by J. F. Libsch, A. E.

Powers and G. Bhat, Trans. A.S.M. Vol. 44, 1952, pp. 1058.


Since these properties are not of great importance to most engineers,

discussion of them is omitted here.

15.6. Cold-Worked Carbon Steels as Engineering Materials

Cold working has two advantages. First, sections of certain sizes,

shapes, or surface finishes can be produced more readily and more eco-

nomically by this method than by any other. Some important industrial

products, in fact, cannot be produced at all except by cold working; it

would be economically impracticable to produce by any other methodwire small enough to be woven into screens or in quantities large enoughfor modern telegraph and telephone systems. In addition, cold working

is the most effective and the cheapest method of securing a smooth sur-

face and accurate size. Second, certain combinations of properties char-

acteristic of cold-worked material cannot be secured by any other

process; for example a tensile strength of 300,000 to 400,000 lb. per sq.

in., combined with considerable ductility, can be attained readily.

The economic value of cold-worked carbon steel is attested by the

fact thai in an average year about 3 million tons of wire* 12 million tons

of sheet* and strip, and 1.8 million tons of bars* and other cold-worked

products are made in the United States. Sheet, strip, tubes, and bars are

cold worked primarily to obtain certain sizes, shapes, and surface finishes

economically. High strength is usually of secondary importance. Further-

more, most cold-rolled sheet and strip, and some cold-drawn wire, espe-

cially the low-carbon grades which are used for screens, fences, barbed

wire, nails, screws and nuts, telegraph and telephone lines, and for manyother common applications, are annealed before use. Because annealing

completely destroys the effect of cold work, the properties of such ma-

terial need not be considered further here.

Carbon steels that are cold worked to secure high strength as well as

to produce special shapes and sizes and are not subsequently annealed

comprise an important class of engineering materials. The properties of

these steels and how cold working affects them are, therefore, worthy

of considerable attention. This class of material includes some low-carbon

sheet, strip, and wire, but the most important is wire containing about

0.25 to 1.00 per cent carbon used for needles, surgical and dental instru-

ments, music (piano) wire, springs, rope and cable wire, and numerous

other purposes where high quality and reliability combined with high

tensile strength and considerable ductility are required.

•Estimated from -Shipments of Steel Products". Steel Vol. 140, No. 1, 1957, pp. 176.


10 20 JO 40 SO 60 70 SO SO 92

Reduction by cold working, percent

Fig. 15.3. Stress-strain curves of cold-worked low-carbon steel (Kenyan and Burns)

15.7. The Important Properties of Cold-Worked Steel

The static properties ordinarily determined on cold-worked steels are

tensile strength and elongation, usually with a gage length of 10 in. for

wire and of 2 or 8 in. for sheet and strip. Owing to the small cross section

of most cold-worked materials, reduction of area, which is a valuable

measure of ductility, cannot be determined accurately. Yield strength is

rarely determined on cold-worked wire and sheet or strip; but for cold-

rolled bars of large enough cross section so that accurate measurementsare possible, yield strength and reduction of area are frequently reported.

Cold working increases the yield strength; with reductions in cross sec-

tion of 30 to 70 per cent it is at least 90 per cent of the tensile strength,

and with higher reductions it may for practical purposes be the same as

the tensile strength. As shown by Fig. 15.3,* the stress-strain curve of

severely cold-worked steel is curved from the origin; the elastic limit is,

therefore, very low and may be zero.

Several unstandardized tests are used to determine the ductility of

cold-worked wire and sheet. Some of these are crude, but in experienced

hands they give reliable indications of excessive brittleness or local ir-

regularities. The most common of these is the bend test in which a speci-

men of definite size, clamped in a vise and bent 180 deg. over a definite

radius, must not crack. In another test, a specimen is bent back and

•R. L. Kenyan and R. S. Burns, Trans. Am. Soc. Metals, v. 21, 1933. p. 595.


forth 90 deg. over a mandrel until failure occurs. The number of 90-deg.

bends before failure is a measure of the ductility.

Another test of the ductility of wire is the twist. A specimen of suit-

able length is twisted in a machine, which has one revolving head, until

it fails by shear. The number of 360-deg. revolutions of the movable headof the machine is the number of twists. Some metallurgists consider that

the twist is a good measure of ductility; others believe that its principal

value is to detect local irregularities or flaws in the wire.

10 20 30 40 50 60 70 80 90 100

Reduction ofarea in drafting,percent

Fie. 15.4. Effect of cold working on ihe tensile strength of carbon steel, (l.egge)

15.8. General Effects of Cold Working on Strength and Ductility

These particular properties are markedly affected by cold work and the

effect it produces on grain structures. Cold work rapidly increases the

tensile strength while decreasing the ductility. The latter is evident by

the decrease in elongation. The change in these properties is directly


10 20 30 40 50 60 70 80 90 100

Reduction ofarea in drafting .percent

WW. 15.5. Effect of cold working on the elongation of carbon steel. (Legge)

related to the amount of cold work. Figure 15.4* gives an indication ofthe effect of cold working on the tensile strength. The effect of cold workis directly related to the carbon content of the steel. As the carbon contentincreases, the tensile strength increases more rapidly and to a greaterdegree for each increment in the reduction in area.

The converse effect is evident when the elongation is considered. Thisvalue reduces sharply with small amounts of cold work and does not varygreatly with the carbon content (Figure 15.5). As the reduction in area*is increased, the effect on the elongation is very much smaller.

One should bear in mind that the figures referred to only approximatethe properties resulting from cold work. While the amount of cold work is

the prime varible having an effect on strength and ductility, it is not the

•The term reduction of area is used in cold working to signify the amount thatthe cross section has been reduced by the cold working operation. It should not beconfused with the same term used in tensile testing.


sole factor. Composition is important, and the structure of the material

has an effect, particularly for medium and high carbon steels.

Another factor that causes variation in total effect is the cross-sectional

area. Table 15.3 indicates the variations that exist as a result of this

factor. Obviously a 1" bar is less affected at the center than a wire wouldbe.

Table 15.3. Effect of Cold Rolling on the Tensile Properties of a Steel Containing0.14 Per Cent Carbon.

Property AnnealedReduced 30

per cent bycold rolling

Reduced 60per cent bycold rolling

58,800

34,040

27,500

41.7

65.8

80,600

75,300

98,100t)e> inn

15 600 -ininn

2258

10 5

43

Since the degree of cold work cannot be equal in all directions, the

grain shape is changed, mechanical twinning occurs, and preferred orien-

tation of the grains also results. While cold working generally has the

same effect on the properties of sheet and strip as it has on wire, mechani-cal directionality shows up in sheet and is usually undesirable.

Condition

Tensile

strength,

lb. per

sq. in.

Hot-rolled strip

Cold rolled 50 per centAnnealed

Yield

strength,

lb. per

sq. in.

40,000

96,000

33,000

Elongation

in 2 in.,

per cent

28

2

38

The effect on the properties varies with the carbon content. A reduc-

tion by cold rolling of 60 per cent has the following effect on strip:

Carbon,

per cent

0.10

0.20

0.30

0.50

Increase in tensile

strength, per cent

65

6361

57

Decrease in

elongation, per cent

70

70

68

67


15.9. Variables Affecting the Properties of Cold-Worked Wire

There are many variables which affect the properties of cold-drawn

carbon steel wire. Only a few may be referred to at this point. Approxi-

mately 85 per cent of all wire used today is drawn from either acid Besse-

mer or basic open-hearth low-carbon steels. Hard-drawn Bessemer wire is

from 10 to 15 per cent stronger than the hard-drawn open-hearth wire.

The tensile strength of Bessemer varies from 90,000 to 150,000 lb. per

square in. while the open-hearth varies from 80,000 to 125,000 lb per

sq. in. for similar sizes. A slightly increased elongation and a greater

ability to withstand twist is characteristic of the basic open-hearth wire.

The effect of cold work on the grain structure is particularly notice-

able in wire. The fragmentation and distortion of the grains occurs

throughout the material since we are dealing with small cross sections.

The effect of galvanizing low-carbon cold-drawn wire is a reduction

in tensile strength, an increase in elongation, but this is complicated by

the induced brittleness as shown in a twist test. This brittleness is

probably caused by the formation of a brittle iron-zinc compound at

the interface between the steel and the zinc coating. Here failure starts

easily under the torsional stresses.

High-strength wire for cables, ropes, and springs and for music wire

is usually made from high-grade deoxidized medium and high-carbon

steel. The steel is rolled into rods which are patented. The best combina-

tion of strength and ductility results from drawing a lead-patented rod.

Air patenting is cheaper, but the properties of the wire are not as good.

Wire drawn from either lead- or air-patented rod is, however, muchsuperior to that drawn from a hot-rolled rod. Hard-drawn music andcable wire drawn from lead-patented rod has a strength of 160,000 to

220,000 lb. per sq. in. in sizes of 0.2 to 0.3 in. and of 280,000 to

400,000 lb. per sq. in. in sizes smaller than 0.1 in. in diameter. Despite

the high strength and low elongation (0.5 to 1.5 per cent in 10 in.), the

wire is very ductile and withstands many 90° bends and can be wrappedaround itself or rolled into long springs of small diameter without

cracking.

The wire used for the cables of large suspension bridges is an acid open-

hearth steel containing about 0.80 per cent carbon, 0.50 percent man-ganese, and low sulfur and phorphorus. The wire is drawn from patented

rods, and the usual size is 6 gage (0.192 in. in diameter) . It has an average

tensile strength of 215,000 to 225,000 lb. per sq. in., a yield strength for a

permanent set of 0.75 in. in 10 in. of 160,000 to 175,000 lb. per sq. in., anelongation of 3 to 7 percent in 10 in., and a reduction of area of 25 to 30

percent. Rope or cable formed from high-strength wire has, owing to its


construction, a breaking strength which is never more than 90 per cent,

and usually only 70 to 80 per cent, of the sum of the breaking strengthof the individual wires.

15.10. Effect of Cold Working on Dynamic Properties

As has been pointed out previously the transition temperature has aneffect on these properties. Although cold working increases the endurancelimit by about the same percentage as it increases the tensile strength,

this means relatively little because most cold-worked material has such a

small cross section that standard highly polished specimens usually can-

not be tested. For specimens of wire tested with the surface produced bythe die, the endurance ratio varies from about 0.50, for material con-

taining 0.05 per cent carbon, to 0.25 to 0.35, for high-carbon wire having atensile strength of 275,000 to 350,000 lb. per sq. in. The low value for

hard-drawn high-carbon wire is largely due to high internal stresses as

reheating the wire to effect a partial or complete recrystallization of the

grains raises the ratio.

Cold working also decreases the resistance of carbon steel to impact.

The magnitude of this decrease depends primarily upon the amount of

cold work and the carbon content—high-carbon steels are embrittled to a

greater degree than low-carbon materials—and upon other variables.

The actual Izod impact value may be as low as 5 to 10 ft.-lb. In general,

however, notched-bar impact resistance is not an important property of

cold-worked material.

15.11. Heat-Treated Carbon Steels as Engineering Materials

As we approach the problem of heat treatment, it would be well to

review a few definitions. The two treatments that we are particularly

concerned about are annealing and normalizing. Annealing is a process

involving heating and cooling, usually applied to induce softening.

Normalizing is a process in which steel is heated to a suitable temperature

above the transformation range and is cooled in still air at roomtemperature.

With the exception of carburizing and annealing of cold-worked

material, low-carbon steels are seldom heat treated. Quenching generally

improves the properties but hardly enough to justify the cost. Medium-carbon steels are frequently heat treated; the general improvement in

strength and ductility, in machinability, or in some other property due to

thermal treatment is usually well worth while. Since high-carbon steels

are used primarily for tools where high hardness is the chief requirement,

these materials are always quenched and tempered. Frequently they are


also annealed or normalized to improve machinability or the structural

condition prior to quenching.

The effect of heat treatment on the properties of cast steels having

been treated earlier in this chapter and the characteristics of treated high-

carbon steels being more easily presented in the chapter on tool steels, it

remains only necessary to briefly cover the properties of medium-carbon

steels after heat treatment.

Referrring to our definitions as a basis, it is easy to point out that

annealing is used primarily to improve the machinability of medium-

carbon steels. Comparing steel in the 0.30-0.60 per cent carbon range, wefind very small differences in the physical properties. The tensile and

yield strengths are 6,000 to 10,000 lb. per sq. in. lower while the elonga-

tion and reduction in area are a bit higher. While no effect is evident on

the endurance ratio, slight increases in impact strength are apparent.

Hot-rolled and normalized medium-carbon steels have practically

the same tensile properties. This can be attributed to the fact that most

carbon steels after hot working are air cooled from temperatures very

close to the normalizing temperatures. Since normalizing produces a

homogeneous structure, it can be justified, economically, for large cross

sections after forging or rolling. In addition to producing a finely divided

evenly distributed carbide, it eliminates directionality due to mechanical

fibering of inclusions such as oxide, slag, etc.

Owing to the residual stresses introduced by quenching, medium-carbonsteels are seldom used in the quenched and untempered condition. Theproperties of such materials are therefore of little interest. If, however,

the internal stresses are relieved by tempering, quenched medium-carbon

steels become valuable structural materials.

If quenched steel is tempered at a temperature high enough so that

the particle size of the carbide changes, strength and hardness decreases

and ductility increases almost uniformly as the tempering temperature in-

creases. This is shown in Fig. 15.6, which gives the ranges of properties

(included within the hatched bands) that result 95 per cent of the time

from tempering water- and oil-quenched 0.40 to 0.50 per cent carbon steels

at increasing temperatures.

Charts showing the effect of tempering on the tensile properties of

quenched carbon and alloy steels of varying carbon content are widely

available in the literature and in a number of handbooks. It should be

emphasized, however, that these charts, while undoubtedly of value in

indicating the properties to be expected most of the time, should never be

used for specification purposes as individual specimens frequently vary

considerably from the average shown on the chart.


600_l_

800Tempering temperature, deg. F.

1000 1200 600 800

500 600 700 300 400 500Tempering temperature, deg. C.

600 700

Fic. 15.6. Effect of tempering on the properties of 05 in. bars of water-andoil-quenched 0.40 to 0.50 per cent carbon steel (S.A.E. 1045). (Sisco, The Alloys ofIron and Carbon, Vol. II, Properties, McGraw-Hill Book Company, Inc., New York,1937, p. 200.)

Carbon steels containing the usual amounts of manganese (0.30 to 0.90

per cent) are shallow hardening. The time available to quench a carbon

steel past the nose of the S-curve is usually less than a second, with the

result that even on severe quenching martensite forms only in very small

pieces or on the surface of larger sections. Consequently, the structure of

a carbon steel quenched in oil or even in water generally consists of a

little martensite mixed with a relatively large amount of a ferrite-

cementite transition structure. It will be shown in the chapters on harden-

ability that the mechanical properties of a quenched and tempered steel

are in general better if the structure before tempering was almost wholly

martensite rather than a mixture of martensite and one or more transition

constituents. The primary reason why low-alloy steels are favored for

quenched and tempered parts is that the alloying elements increase

hardenability and thus increase the likelihood that martensite will be

formed in large quantities when the steel is quenched in oil.

Carbon Steed as an Engineering Material 315

15.12. Effect of Section Size on the Properties of Heat-Treated

Medium-Carbon Steels

It is well known by metallurgists generally, but by surprisingly few

engineers, that the properties of steel determined on a small heat-treated

bar of steel may not be representative of the average properties on a

large bar similarly treated. The effect of mass on properties is important.

Since in many industrial applications large sections are used, it may be

more essential for the engineer to know the average properties or, in

some cases, the minimum properties than to know the properties at the

surface. It is not always practicable to test full-sized I beams or locomo-

tive axles; hence, data that indicate the effect of increasing size of section

are of considerable value.

A heavy section cannot be cooled as rapidly as a thin section; the in-

terior of a heavy section, cooling more slowly than the surface, transforms

not only at a later time but at a higher temperature and therefore to a

softer product than the surface. This results in differences in properties

between the exterior and the interior, possible distortion, residual stresses,

etc. If we were to heat and quench a series of bars of SAE 1045 steel of

different diameters from \/2" to 5", the smallest piece would cool most

rapidly and each succeeding piece more slowly

Size of Bar Surface Hardness

0.5" 59

1.0 58

2.0 41

3.0 35

4.0 30

5.0 24

A study of the surface microstructure reveals that for the hardest

material the microstructure is predominately martensitc, and as hard-

ness decreases, the amount of martensite decreases rapidly and pearlite in

varying degrees of coarseness appears. If the bars are cut and a hardness

survey taken of the cross section, it would show the variation in cooling

rate from outside edge to center by the variation in hardness. (Fig. 15.7).

In most large sections the cooling rate upon quenching decreases as

the distance from the surface increases and, if other variables do not

enter importantly, hardness and strength decrease and ductility increases

slightly. Data for medium-carbon steel are given in Fig. 15.7. The effect

of mass is more pronounced in water-quenched than in oil-quenched

material, and for the same quenching treatment the effect of mass is

less pronounced with high tempering temperatures than with lower

temperatures.


Diameter

Fie. 15.7. Hardness of series of SAE 1045 steel bars heated and quenched.

The endurance ratio of quenched and tempered carbon steels is 0.50

± 0.05 unless the quenching treatment has introduced stresses whichwere not wholly relieved by tempering. When such stresses are presentthe ratio may be as low as 0.30 or 0.40. Quenching and tempering usuallyhave a favorable effect on impact resistance, which, in general, is muchhigher than that of a rolled steel of corresponding composition. Temper-ing increases the impact resistance of a quenched steel: if the Izod valueof a water-quenched 0.30 per cent carbon steel is 10 ft.-lb., it will be 20to 25 ft.-lb. after tempering at 700°F. (400°C.) and 45 to 65 ft-lb. after

tempering at 1100 to 1200°F. (595 to 650°C.) . The modulus of elasticity

is, of course, unaffected by thermal treatment.*

QUESTIONS1. By what per cent has the steel production in the United States increased in

the last twenty years?

2. Why should the per cent production of carbon steel in the United States beless now than it was before World War II?

* Sec footnote, p. 305.


3. Give two reasons why a large tonnage of steel is melted and poured into

castings every year. How is steel for castings usually melted? What is the

general relation between structure, heat treatment, and properties of cast

carbon steel? Does carbon steel have a low or high hardenability?

4. How does the per cent of carbon in steel castings affect the hardness of the

martensite produced when the castings are quenched?

5. What carbon content in steel castings would be most likely to cause cracking

during the quenching cycle? If cracking during quenching were a real possi-

bility, what heat-treating cycle could be used to prevent cracking?

6. How would the rate of cooling from die rolling temperature affect the

properties of hot worked carbon steel?

7. Name the three general classes of hot-worked carbon steel, and give the

principal industrial uses of each class. Which of these three classes leads in

tonnage?

8. List the principal variables that affect the properties of hot-rolled carbon

steel. What tensile properties would be expected 95 per cent of the time

from hot-rolled steel containing 0.15 per cent, 0.30 per cent carbon? Howdo manganese and phosphorus affect the tensile strength of hot-rolled

carbon steel?

9. What is the effect of carbon on notched-bar impact resistance? on en-

durance limit? on modulus of elasticity?

50. Why are steels cold worked? What properties are ordinarily determined on

cold-drawn wire and on cold-rolled bars? What is the effect of cold working

on the yield point? How is the ductility of cold-worked wire usually deter-

mined?

11. What is the general efTect of cold working on tensile strength and elonga-

tion? If a hot-rolled rod containing 0.15% carbon has a tensile strength of

60,000 lb. per sq. in. and an elongation of 25% in 10 in., what strength

and elongation would be expected in drawing to wire with (a) a reduction

of 40%? (b) a reduction of 70%?

12. What percentage increase in tensile strength can normally be expected if

automobile fender steel is cold rolled 50%? What is the effect of this reduc-

tion on the elongation? Give typcial tensile properties (a) of small sizes of

music wire, and (b) of wire for suspension bridges.

13. What is the general effect of cold working on the impact resistance and on

the endurance limit of carbon steel?

14. What general classes of carbon steel arc heat-treated? What is the advantage

of either annealing, or normalizing, or quenching and tempering such

steels? What is the general effect of tempering on the tensile properties? For

a tempering temperature of about 1000°F., what would be the expected

properties of a 1-in. round bar of 0.45% carbon steel (a) when quenched in

water? (b) when quenched in oil?

15. If a small specimen of hot-rolled carbon steel containing 0.45% carbon has

a tensile strength of 93,000 lb. per sq. in., a yield strength of 54,000 lb, per

sq. in. an elongation of 20% in 2 in., and a reduction of area of 85%. whattreatment can be used to increase these values to 103,000: 70,000; 25; and

60; respectively? With these latter tensile properties for a heat-treated small

section, what would be the corresponding properties at the center of a 4-in.

bar?

CHAPTER

Low - A Hoy Steels as

Engineering Materials

16 Robert E. Bannon, S.M., Professor of Metallurgy,

Newark College of Engineering, Newark, New Jersey

Sic>fUND Lever.v Smith, M.Met.E., Professor of Metal-

lurgy, College of Mines, University of Arizona, Tucson,

Arizona

Al.I.OY steels are about seventy years old. Thepioneer investigations of the effect of the various alloying elements oncarbon steel were made between 1875 and 1890 in England, Germany,France, and, to a lesser extent, the United States. Owing to the high

price of alloying metals, the only use for these materials on a relatively

large scale was in armor plate. By 1910, alloying metals were muchcheaper, and from then on industrial applications multiplied rapidly,

especially between 1915 and 1918 when there was an abnormal demandfor high-quality alloy steel for war materials. This demand, together

with the approximately simultaneous development of the first "stainless"

steel, stimulated metallurgists all over the world to investigate the effect

on carbon steel of all possible combinations of many alloying elements.

The result has been alloy steels by the hundreds—steels containing onealloying element in addition to carbon, and steels containing half a dozen.

Before World War II much of the research work on alloy steels was of

the hit-or-miss type or was carried out to develop a steel that wouldduplicate the properties of a patented composition without actual in-

fringement. One result of this frenzied research between 1910 and 1950

was a phenomenal growth of metallurgical literature; another was the

development of many unnecessary alloy steels which, with slightly dif-

ferent compositions, duplicate the characteristics of some of the older

and better-known steels. However, some remarkable developments haveresulted from this mass of hit-or-miss research, including several newsteels and special ferrous alloys with splendid properties and of muchvalue to industry. It has also shown to engineers that alloy steels make

318

Low-Alloy Steels as Engineering Materials 319

possible the design of certain structures that would be impossible with

unalloyed carbon steels and cast irons. Without alloy steels the modern

airplane, the streamlined railroad train, the modern automobile, and

many other important developments would never have been brought to

their present efficiency.

The most characteristic indication of the increasing value of alloy

steels to the engineering professions is that their production has increased

at a greater rate than total steel production. The output in 1910 was

600,000 tons, or 2 per cent of the total; in 1939 it was more than 3,000,000

tons, or about (i per cent of the total output of the steel industry. During

1944, when total steel production in the United States reached 89,642,000

net tons of ingots, alloy-steel ingot production was 10,630,000 net tons,

which is 12 per cent of the total. The demand for alloy steels in the

average prosperous postwar year, for example 1960, cannot be foreseen

now; probably it will average about 8 or 10 per cent of the total.

The amount of published data on alloy steels and cast irons is so large

that it is now impossible for a metallurgist to know everything that is

going on in this field; it is a hard job even to keep up with developments

in a single branch.* To summarize adequately, in a single book, the

present status of knowledge of alloy steels is a difficult task, and it is,

of course, quite impossible to give many details of their properties in

three short chapters. Consequently, the discussion in this and the next

two chapters is restricted to a brief description of the characteristics of

these materials and to a concise outline of the metallurgical fundamentals

in the relation of the common alloying elements to carbon steels.

16.1. Balanced Compositions in Low-Alloy Steels

A workable definition of alloy steels and a general classification of

these materials into low- and high-alloy steels, including common compo-

sitions for the low-alloy grades, are given in Chap. 11. Many low-alloy

steels, especially the S.A.E. grades, are purchased under specifications

that give desired ranges for the various alloying elements and for carbon

and manganese and usually maximum percentages for sulphur and phos-

phorus. Widely available in the literature and in handbooks are charts

of average, typical, or minimum mechanical properties of the various

steels after definite heat treatments. Such specifications and charts are

the result of long experience and arc of unquestioned value to engineers.

• Alloys of Iron Research, founded in 1930 by The Engineering Foundation of NewYork, is reviewing this literature and correlating and summarizing the most importantof the world's research on alloy steels and cast irons. Results are published in a series

of monographs.


In the past few years, however, there has been a trend toward makinguse of steels of "balanced composition" or of definite hardenability rather

than attaching too much importance to rigidly specified compositions

and the necessity of accepting the data in mechanical-property charts.

Steels of "balanced composition" are those in which the percentages

of carbon and alloying elements, as well as the heat treatment (if any)

,

are varied to produce a desired combination of properties for a particular

application. Assume, for example, that for a structural application a steel

must have a minimum yield strength of 50,000 lb. per sq. in. as rolled,

must weld readily, and must not air harden after welding. A steel con-

taining 0.30 per cent carbon, 0.75 per cent of alloy metal A, and 0.5 per

cent of alloy metal B has the required strength and welds readily, but it

air hardens unless specially treated during or after welding. Because this

composition is not balanced for this particular application, the carbon

is reduced to 0.15 per cent, and the amounts of the alloying metals are

increased to 1.0 and 0.75 percent, respectively. The yield strength is

still well above the required minimum, the material still welds readily,

but owing to a better balance of the carbon and metals A and B the steel

does not harden in air after welding.

Producing steel with a definite hardenability is primarily a problem in

balancing the composition correctly. The effect of carbon and of the

various alloying metals on hardenability is known with considerable ac-

curacy (this is discussed in detail in the next two chapters) , with the

result that steels having a definite response to quenching can be readily

produced by balancing the amounts of carbon and of the one or morealloying metals which are readily available.

The final criterion in the selection of an alloy steel for a specific appli-

cation is economy. Any alloy steel is more costly than plain steel of cor-

responding carbon content and should not be used for a structure or a

machine merely because it contains one or more special metals, or be-

cause the steelmaker says it is better than some other steel. Alloy steel

should be used only if considerable weight can be saved, or if the de-

sign can be materially simplified, and only if its total cost—meaning the

cost of the steel, of its treatment (if any) , and of fabrication—is less,

when spread over the expected life, than the total cost of any competingmaterial.

16.2. General Effects of the Alloying Elements on Carbon Steel

The general effects of the common alloying, elements on the structure

and, consequently, on the properties of carbon steel are discussed in

Chaps. 12 and 13. It is the purpose of the discussion in this and the next


two chapters to expand this and to show how these alloying elements

are used to produce steels which are more satisfactory for certain appli-

cations than the unalloyed carbon steels.

Hot-rolled carbon steel is low in cost and has excellent properties, and

with certain carbon percentages these properties can be still further

improved by heat treatment. Carbon steel also has several disadvantages

as an engineering material. An important disadvantage is the rapid de-

crease of ductility as the carbon (and the strength) increases. Another

is that, because it is shallow hardening, optimum combinations of prop-

erties in heat-treated material can usually be attained only in small sec-

tions. A third disadvantage is that carbon steels, whether hot rolled or

heat treated, suffer marked deterioration of properties when used at

temperatures considerably below or above normal. Alloying elements are

added to carbon steels to overcome, partly at least, these disadvantages.

The principal effect of the alloying elements in low-alloy steels is to

increase hardcnability by heat treatment. In addition, all the commonalloying elements dissolve in the ferrite and strengthen it to some extent.

Five of these elements also form carbides, but of these only molybdenum,

vanadium, and tungsten have a strong carbide-forming tendency. Chang-

ing the properties of carbon steel materially by the formation of such

carbides is important chiefly in the high-alloy and the tool steels, which

are discussed in later chapters.

The ferrite-hardening (and -strengthening) potency of the elements

forming solid solutions with iron varies (increasing from chromium to

2 4 6 e 10 12 It 16 IB 20 22

Alloying element in alpha iron, per cent

Fie. 16.1. Effect of the common alloying elements, when dissolved in iron, on its

hardness. (Bain)


phosphorus), as shown in Fig. 16.1.* None of these, with the possible

exception of phosphorus, is so potent as carbon. In fact, the strengthen-

ing effect is small, provided that the carbon content and the structure are

held constant. This is shown by the curves for the furnace-cooled speci-

mens plotted in Fig. 16.2.f If, however, the structure is altered by chang-

ing the treatment, that is,

by taking advantage of the

effect of the element on har-

denability, small amounts

of an alloying clement may,

as shown in Fig. 16.2, have

a great effect. In fact. Fig.

16.2 shows that chromiumincreases the hardenability

enough so that high tensile-

strength values result from

moderately slow (air)

cooling.

240

230

220

210

200

190

180

4no

I160

£150

§140

8£.130-n

1ri2o

"no

J100

90

80

70

60

5012 3 4 5 6 7

Chromium, percent

Fie. 165. Effect of carbon, chromium, and cool-

ing rate on the tensile strength of carbon steels.

(Wright and Mumtna)

ft*/o\V

f 0/y

c

1i f>

1 / /V

1

/(

/

/ //

*

/tiro10tr</

.curn wee c xjicg

__ __ 0.30

~OJ0

%c

__ —

•

--

.—

-

16.3. Effects of Phospho-

rus, Manganese,

and Silicon

Of the alloying elements

which dissolve in the fer-

rite, phosphorus, as just

noted, is the most potent in

its strengthening action

(Fig. 16.1) . The addition

of 0.01 per cent of phos-

phorus increases the tensile

and the yield strength ap-

proximately 1,000 lb. per

sq. in., that is, by about as

• E. C. Bain, Functions of the Alloying Elements in Steel, American Society for

Metals, Cleveland, 1939, p. 66. Data reported by C. R. Austin (Trans. Am. Soc. Metals,

v. 31, 1943, p. 331) indicate that the strengthening effect of manganese and molybdenumin small percentages is greater, and that of chromium is slightly less, than those

shown by Bain's data in Fig. 16.1. Neither Bain's nor Austin's data arc very precise,

because the iron used was not of high purity and the slope of the curves shown in

Fig. 16.1 may be altered by future work. The ferrite-strengthening cirect of the alloying

elements is, however, not of great importance.

t Plotted from data by E. C. Wright and P. F. Mumma, Trans. Am. Inst. MiningMet. Engrs., v. 105, 1933, pp. 77-87.

Low-A Hoy Steels as Engineering Materials 323

much as these properties are increased by carbon. This strengthening is,

moreover, attained with less sacrifice of elongation and reduction of area

than in the case of carbon addition.f Another advantage is that phos-

phorus, either alone or more intensely if copper is present, increases the

resistance of the steel to atmospheric corrosion. Phosphorus is favored

as an alloying element chiefly in low-carbon steels that are used in the

rolled or normalized condition.

Manganese, added to steel in amounts in excess of that necessary to

combine with the sulphur and to deoxidize the metal—0.1 to 0.25 per

cent for most steels—partly dissolves in the ferrite and partly replaces

iron in iron carbide (FeaC) to form a carbide that may be represented by

(Fe,Mn)3C. If the manganese is below 2 per cent, the amount of this

carbide formed is small. Moreover, this substance is so much like iron

carbide that it has in itself no appreciable effect on structure and prop-

erties. The effect of manganese in solid solution is less than that of phos-

phorus; the amount by which 0.01 per cent manganese increases the

tensile strength varies with the carbon content from about 100 to 500 lb.

per sq. in.; it increases the yield strength somewhat more than this. Ayield strength of at least 50,000 lb. per sq. in., together with good duc-

tility, can be readily attained in hot-rolled structural shapes of 0.20 to

0.30 per cent carbon steel by adding 1.00 to 1.75 per cent manganese.

Manganese in amounts exceeding about 0.75 per cent has a strong

effect on the hardenability of carbon steel. In steels that are heat treated,

this effect is an advantage, since it permits the use of less drastic quench-

ing mediums, which means less distortion or cracking. In hot-rolled struc-

tural steel increased hardenability may be a disadvantage, unless the

carbon is very low, because air hardening may occur in cooling from the

rolling temperature with serious loss of ductility. In general, manganesereduces the ductility of untreated carbon steel, but this is not serious if

the carbon is low.

Silicon in the amounts present in low-alloy steels dissolves in the ferrite.

Its effect on tensile strength is somewhat greater than that of manganese(Fig. 16.1) ; it raises the yield strength and yield ratio and does not

appreciably reduce ductility. Its effect on hardenability is strong but not

so strong as that of manganese. It has no appreciable effect on corrosion

resistance when present in small amounts. In other than the low-alloy

structural steels discussed later in this chapter, silicon is used as analloying element in one important group of spring steels that contain

about 2 per cent silicon, 0.60 to 0.90 per cent manganese, and 0.50 to

f Phosphorus is likely to make steel cold short, that is, brittle when cold, unless

the carbon percentage is low.


140

120

^ 100

5?

=9 80

acc

8 60

40

20

,Reducfion ofarea_per cent

(A) Nickel(with carbon 0.20%)

Tensile strength.,

I

|

Reduction ofarea.percent -

(B)Chromium(withcarbon 0.20%)

70

601

50 „-

40 c

30-

co

I 2 3 4 I 2 3

Alloying element, per cent

Fig. 16.S. Effect of (A) nickel and (B) chromium on the tensile properties of

rolled carbon steels. (Bain and Llewellyn)

0.65 per cent carbon (the S.A.E. 9200 series) and in certain steels for

electric apparatus.

16.4. Effects of Nickel and Chromium

Nickel dissolves in the ferrite of carbon steel and exerts a strengthen-

ing effect that is somewhat less than that due to phosphorus, silicon, or

manganese (Fig. 16.1) . The effect of nickel on the tensile properties of a

0.20 per cent carbon steel is plotted in Fig. 16.3A,* which shows that in

the hot-worked (unhardened) condition nickel has little effect on elon-

gation and reduction of area; it does, however, raise tensile and yield

strengths considerably. In the heat-treated condition, nickel steels are

characterized by high impact resistance, which remains high at low

temperatures.

Nickel does not form a carbide. It increases the hardenability of carbon

steels mildly, being less effective in producing deep hardening than

molybdenum, manganese, or chromium. It also increases the resistance

• Figures 16.3A and B are based on data quoted by E. C. Bain and F. T. Llewellyn,

Trans. Am. Soc. Civil Engrs., v. 102, 1937, p. 1249.


of carbon steel to some forms of corrosion, especially in combination with

copper.

Chromium dissolves in the ferrite and also forms carbides if the carbon

is high. As indicated by Figs. 16.1 and 16.2 its ferrite-strengthening ef-

fect when in solid solution is weaker than that of any of the commonalloying elements, but it increases hardenability greatly, as shown by

the curves for the air-cooled specimens in Fig. 16.2.* The effect of

chromium on the tensile properties of small air-cooled (hot-rolled) sec-

tions of a 0.20 per cent carbon steel is plotted in Fig. 16.3B. In such

sections, the increase in tensile and yield strengths and the decrease in

elongation and reduction of area are much greater than the correspond-

ing effects of nickel. Although chromium is a toughening element and

increases the impact resistance of very low carbon steels, its primary

function is to increase hardenability, decrease the magnitude of the mass

effect, and, of course, increase corrosion resistance, especially if more than

3 per cent is present. Owing to its effect on hardenability, chromiumis a common addition to the S.A.E. nickel steels in the proportion of

about half as much chromium as nickel.

Chromium is used widely as an alloying element in high-carbon steels,

where it forms a carbide and also dissolves in the ferrite. When these

materials are quenched and tempered at a low temperature, they are very

hard and have good wear resistance. They contain about 1.0 per cent

carbon and 0.75 to 1.50 per cent chromium and are used for tools, dies,

and ball bearings.

16.5. Effects of the Other Common Alloying Elements

The other alloying elements used in low-alloy steels are copper, molyb-

denum, vanadium, and, to a very limited extent, tungsten. Copper, in the

small amounts added to carbon and low alloy steels dissolves in the fer-

rite, but its effect on tensile and impact properties is small. If more than

0.6 per cent is present, the steels, as noted in another chapter, may be

precipitation hardened. Copper is most useful as an alloying clement for

its effect on atmospheric corrosion. The addition of 0.10 to 0.40 per cent

reduces this variety of corrosion so much that a large proportion of low-

carbon sheets, especially when very thin, and many of the new high-

strength structural steels now contain copper.

* If the carbon is low (below 0.08 or 0.09 per ceni), chromium up to approximately3.0 per cent has only a small effect on the hardness and strength when steel is air

cooled. For example. Franks (R. Franks. Trans, Am. Soc. Metals, v. 35, 1945, p. 616)

reported an increase in tensile strength from 55,000 to 70,000 lb. per sq. in. in 0.08 per

cent carbon steel when chromium was increased from to 3 per cent. With 6 per cent

chromium, the strength was 197,000 lb. per sq. in.


Molybdenum, vanadium, and tungsten are strong carbide formers. In

the amounts present in low-alloy steels a portion of these three elements

dissolves in the ferrite and probably strengthens it moderately (Fig.

16.1). Vanadium is valuable chiefly for its effect on austenite grain size.

Steels containing 0.10 to 0.20 per cent are usually fine-grained and are

not so readily coarsened by overheating as some of the other low-alloy

steels. Molybdenum, vanadium, and tungsten all contribute to deep hard-

ening, and the first two are very effective in small amounts. Molybdenumor vanadium is used in a number of low-alloy steels, especially the

S.A.E. grades, usually together with another alloying element, such as

chromium, nickel, or manganese. Owing to its strong effect on harden-

ability and to the fact that the world's largest deposit of molybdenumores is located in the United States, the use of molybdenum as an alloy-

ing element has increased greatly in the last few years. The most im-

portant function of tungsten and, to a lesser extent, of molybdenum is

to increase the hardness and stability of the structure of alloy steels, espe-

cially at elevated temperature. For this purpose, relatively large amounts

are required, usually in conjunction with chromium. This is discussed

in a later chapter.

16.6. Low-Alloy Structural Steels as Engineering Materials

As noted previously, there are two broad classes of low-alloy engineer-

ing steels: the structural steels and the S.A.E. steels. The structural steels

have a higher yield strength than carbon steels; they are usually of care-

fully balanced composition and are used for structural purposes without

heat treatment other than natural or controlled cooling after hot rolling.

The S.A.E. steels are, in general, much more costly than the low-alloy

structural steels and should be heat treated to obtain optimum properties.

In contrast to the structural steels, in which the function of the alloying

elements is primarily to strengthen the ferrite and to produce a fine

pearlite when the material is air cooled, in the S.A.E. steels the alloying

elements also affect the response of the steel to thermal treatment and,

in a few cases, harden it by forming carbides.

Low-alloy steels of high yield strength are forty years old. One of the

first contained about 0.25 per cent carbon and 1.50 per cent manganeseand was used between 1900 and 1915 to reduce dead loads in long-span

bridges and steamship construction in the United States and in England.

A little later, a deoxidized carbon steel (misnamed silicon steel) was

developed in the United States. This material contained 0.25 to 0.35 per

cent carbon, 0.70 to 1.00 per cent manganese, and 0.20 to 0.40 per cent

silicon. In the hot rolled condition the yield strength of these two steels


was 45,000 lb. per sq. in. or higher, and a considerable tonnage was used

for bridges between 1915 and 1928. A steel o£ similar yield strength,

containing 0.12 per cent carbon, 1 per cent silicon, and 0.70 per cent

manganese, was developed in Germany in 1925 but, owing to its variable

quality, it was not used long. This German steel is important because

it was the first attempt to produce a steel that had high yield strength

but was low enough in carbon to be welded readily without the trouble-

some brittleness in and around the weld that frequently results from

welding a steel containing more than 0.20 per cent carbon and 0.75 per

cent manganese.

About fifteen years ago, the depression stimulated steel manufacturers

to investigate the possibility of producing a low-cost, easily weldable,

tailormade steel that would have strength properties and resistance to

atmospheric corrosion much superior to those of ordinary structural steel.

The depression also focused the attention of engineers upon the desir-

ability of reducing cost in certain structures by the use of smaller sec-

tions of a steel of higher strength, and by welding instead of riveting.

The result was that the steelmakers produced the steels and the engineers

used them for railway rolling stock, streetcars, trucks, buses, cranes, steam

shovels, and similar structures. One of the essential factors in this ad-

vance was the realization that tensile strength is not always the best

criterion of the suitability of a steel for certain structural uses, that re-

sistance to buckling or crumpling, which is related to yield strength, is

essential, and that increased corrosion resistance is an economic necessity

in many engineering structures.

The low-alloy structural steels have, as rolled, approximately twice

the yield strength of low-carbon steel, combined with high ductility, easy

weldability, and a resistance to atmospheric corrosion which is muchhigher than that of carbon steel. In tonnage lots, they cost 15 to 50 per

cent more than comparable sections of plain carbon steel. Owing pri-

marily to their high yield strength, it is possible to reduce the size of a

section 25 to 35 per cent and still design a structure that has ample stiff-

ness. Despite somewhat higher fabricating and machining costs, it is fre-

quently possible to save 10 to 15 per cent of the cost of a structure bythe use of the properly selected low-alloy steel.

16.7. Composition and Properties of the Low-Alloy Structural Steels

The first of the present crop of some thirty of these steels was made in

1929 and contained 0.30 per cent or less carbon, 1.05 to 1.40 per cent

manganese, 0.60 to 0.90 per cent silicon, and 0.30 to 0.60 per cent chro-

mium. Known under the trade name of Cromansil, this steel attracted


considerable attention, and a number of others followed it quickly under

such trade names as Cor-Ten, Man-Ten, Hi-Steel, Jal-Ten, Konik, Yoloy,

Otiscoloy, N-A-X, Tri-Ten, Carilloy T,, and others.

As a group, the low-alloy structural steels are characterized by low

carbon (to improve ductility and weldability, and to prevent air harden-

ing) and by the presence of at least two, and sometimes as many as six,

of the following alloying elements: manganese, silicon, nickel, copper,

chromium, molybdenum, phosphorus, boron, and vanadium. During

the war titanium was an important minor constituent in these steels. Thepercentage of alloying elements is usually low. Manganese is 1.70 per

cent or less, with an average of about 1.25 per cent. Silicon is 1 per cent

or less, the usual amount being between 0.20 and 0.70 per cent. Nickel

varies from 0.3 to 2.0 per cent, and copper, owing to its favorable effect

on resistance to atmospheric corrosion, is usually at least 0.10 per cent

and may be as high as 1.50 per cent. Molybdenum, if present, is less

than 0.40 per cent, and chromium is less than 1 .5 per cent. Phosphorus,

if used as an alloying element, is between 0.05 and 0.20 per cent, vanadiumup to about 0.1, and boron around 0.003 per cent.

In all the low-alloy structural steels the composition is balanced to

ensure for hot-rolled sections a minimum yield strength of 50,000 lb. per

sq. in., a tensile strength of 70,000 to 105,000 lb. per sq. in., and an elon-

gation of 18 to 30 |>er cent in 8 in., depending upon composition. Theimpact resistance of most of these steels is satisfactory; usually it is higher

than 25 ft.-lb. Claims have been made that the endurance ratio is higher

than for carbon steels, but not enough data have accumulated yet to

accept this as generally true. As a group, the steels have a resistance to

atmospheric corrosion considerably superior to that of carbon steels.

These steels are outstanding in the relation of properties to cost. There

are too many of them now; probably there will be some casualties in

the next few years, but it seems certain that some will survive and that

the survivors will be widely used as structural materials where a saving

in weight is important.

16.8. The S.A.E. Low-Alloy Steels

Of the alloying elements in low-alloy steels, nickel and chromium were

the first used commercially. Steels containing about 3 per cent nickel

were forged into armor plate between 1885 and 1890, causing a revolu-

tion in naval warfare. Chromium was used even earlier: steels containing

0.60 per cent were installed as main members of the Eads bridge over

the Mississippi River at St. Louis in 1874. Extensive use of the steels in-

cluded in the S.A.E. grades began about 1910. It has been claimed with


considerable justification that the availability of these steels at a fairly

reasonable cost was the chief factor in the development of the automo-

tive and aircraft industries, and that without them the weight per horse-

power of internal-combustion engines would be at least twice, and the

efficiency less than half, what it is today.

Nearly fifty years ago, the Society of Automotive Engineers proposed

specifications for the composition of a number of low-alloy steels that

were being used at that time by the automotive industry. These specifi-

cations were adopted in 1911, and since that time the compositions

recommended in the S.A.E. specifications have been used widely in all

branches of mechanical manufacturing in the United States and abroad.

Revision and extension of the specifications have kept pace with progress

in the automotive and steel industries, with the result that engineers

generally look upon the S.A.E. steels as the most satisfactory low-cost,

high-quality steels commercially available for the production of auto-

motive and aircraft equipment and machine tools, and they know that

these steels will give satisfactory service when properly treated.and used.

in 1941, the Society of Automotive Engineers in collaboration with the

American Iron and Steel Institute made major changes in the composi-

tion ranges of their standard steels. These changes consisted primarily

of adopting narrower ranges for chemical analysis as made on a specimen

representing a whole heat, plus an allowance for check analyses on indi-

vidual specimens from the same heat. At the same time the American

Iron and Steel Institute issued standard analysis specifications for low-

alloy steels, which are known as A.I.S.I, specifications and which, in

general, follow the same system of classification as is used by the S.A.E.

The S.A.E. specifications for low-alloy steels for 1956 cover 114 steels

divided into eight series. These are shown in Table 16.1.* The A.I.S.I,

specifications cover 170 steels divided into 13 classes, eight of which are

the same and have the same series numbers as the S.A.E. grades.

The system of classifying these low-alloy steels is evident from Table

16.1. Series 1300 comprises the medium-manganese steels; 2300 contains

3.50 per cent nickel; and 2500 contains 5.00 per cent nickel. The 3100

series contains nickel and chromium but the amounts are different in

• In addition to specifications for low-alloy steels, both the Society of AutomotiveEngineers and the American Iron and Steel Institute have standardized unalloyed

carbon steels. The S.A.E. specifications cover 22 carbon steels (series 10XX) ranging

from less than 0.10 to 1.05 per cent carbon, and 10 high-sulphur free-machining steels

(series 11XX) containing up to 0.15 per cent sulphur and as much as 1.65 per cent

manganese. These specifications are given in the S.A.E. Handbook, 1956 edition, pp.51-55, and in Steel Products Manual, Sec. 2. Carbon Steel, Semi-finished Products,

published by the American Iron and Steel Institute, New York, 3d rev., June, 1943.


the 3100, and 3300 series. The 4000 series is made up of plain molyb-

denum steels, the 4100 series of chromium-molybdenum, the 4600 and4800 series are nickel-molybdenum, and the 4300 series which is popu-

lar in the aircraft industry contains nickel, chromium, and molybdenum.The last two digits of an S.A.E. or an A.I.S.I, number indicate the middle

of the carbon range; thus, a 2330 steel contains 3.25 to 3.75 per cent

nickel and 0.28 to 0.33 per cent carbon. The larger number of A.I.S.I.

steels is due chiefly to more detailed specifications according to carbon

Table 16.1. Basic Numbering System and Composition Ranges for S.A.E. andA.I.S.I. Low-Alloy Steels *

Series

Composition range, per cent

S.A.E.

or A.I.S.I.

Manganese Nickel ChromiumMolyb-denum Other

1300 1.60 to 1.90

23002500

3.25 to 3.75

4.75 to 5.25

31003300

1.10 to 1.40

3.25 to 3.75

0.55 to 0.75

1.40 to 1.75

400041004300

46004800

1.65 to 2.00

1.65 to 2.00

3.25 to 3.75

0.80 to 1.10

0.40 to 0.90

0.20 to 0.30

0.15 to 0.25

0.20 to 0.30

0.20 to 0.30

0.20 to 0.30

51005200

0.70 to 0.90

1.20 to 1.50

6100 0.80 to 1.10 0.15 min. V

8600

8700

0.40 to 0.70

0.40 to 0.70

0.40 to 0.60

0.40 to 0.60

0.15 to 0.25

0.20 to 0.30

9260

93009800

0.70 to 1.00

3.00 to 3.50

0.85 to 1.15

1.00 to 1.40

0.70 to 0.90

0.08 to 0.1

5

0.20 to 0.30

1.80 to 2.00

Si

t86BO0 0.40 to 0.70 0.40 to 0.60 0.15 to 0.25 Boron, .0015to .0020

'S.A.E. Handbook, Society of Automotive Engineers, New York, 1956, pp. 50-55;

Steel Products Manual, Sec. 10, Alloy Steels, American Iron and Steel Institute, New York,

3d rev., June 1945, pp. 13-30.

t xxBxx, B denotes Boron Steel.


percentage. For example, in the S.A.E. series 4100 there are 10 steels, cov-

ering a carbon range from 0.17 to 0.53 per cent; in the corresponding

A. I.S.I, series there are 18 steels to cover the same range of carbon.

In addition to carbon and alloying elements, these specifications give

ranges for manganese (regardless of whether it is an alloying element)

,

silicon, sulphur, and phosphorus. The A.I.S.I. specifications also carry

letters that identify the process by which the steel was made: A for the

basic open-hearth, D for the acid open-hearth, and E for the electric-

furnace process.

Some of the low-carbon S.A.E. alloy steels do not conform exactly to

the ranges shown in Table 16.1. For example, in the low-carbon 4100

steels, the chromium is lower (0.40 to 0.60 per cent) and the molyb-

denum is higher (0.20 to 0.30 per cent) than in the steels containing

more than 0.30 per cent carbon. The manganese in most standard S.A.E.

steels varies widely. The 2300 series is typical; manganese is 0.40 to 0.60

per cent in the low-carbon grades, and 0.60 to 0.80 or 0.70 to 0.90 per

cent in the higher carbon steels.

The standard S.A.E. and A.I.S.I. steels are only a small fraction of the

low-alloy steels used commercially. Some of these other steels vary

slightly, but many vary greatly, from the standard compositions given in

Table 16.1. A few of them, which are widely used and which vary

slightly, usually in only one element, are given S.A.E. numbers prefixed

by the letter X. An example is X3140, the nominal composition of whichis given in Table 16.2. In this steel the chromium is 0.70 to 0.90 instead

of 0.55 to 0.75 per cent.

16.9. The S.A.E. Low-Alloy Steels as Engineering Materials

Composition, series number, and recommended heat treatment for all

the S.A.E. steels are given in the S.A.E. Handbook and in many other

readily accessible sources and need not be considered here. These steels

are more expensive than the low-alloy structural steels, and, further-

more, they must be heat treated to be of the greatest value. They are

widely used in the construction of automobiles and aircraft—especially

engines—and for machine tools, particularly for highly stressed parts such

as crankshafts, camshafts, axle and other shafts, gears, bolts, studs, steer-

ing knuckles, spindels, collets, and valves.

A few of the S.A.E. low-alloy steels are used in the rolled and air

cooled conditions; there is one series, discontinued as standard by S.A.E.

a few years ago, which contained less than 2 per cent nickel, that waswidely used for engine bolts and boiler plate and, to a lesser extent, for

bridge and ship construction. The medium-manganese series 1300 is also

used occasionally as rolled and air cooled, and a few of the low-alloy


structural steels discussed in § 16.7 are essentially S.A.E. 1320 containing

a small amount of copper to increase resistance to atmospheric corrosion.

Most of the S.A.E. steels are used in applications, especially in moving

parts, where stresses are relatively high, and they must be quenched

(usually in oil) and tempered to secure the maximum combination of

strength and ductility. When properly heat treated, they combine high

ductility with high strength. Compared with carbon steels of the same

tensile strength and hardness, heat-treated low-alloy steels have 30 to 40

per cent higher yield strength, 10 to 20 per cent higher elongation, and

35 to 40 per cent higher reduction of area. The impact resistance of the

S.A.E. steels is frequently twice that of a carbon steel of the same tensile

strength.

The most important characteristic of the S.A.E. low-alloy steels as a

class is that they are deeper hardening than carbon steels; in other words,

the mass effect is smaller. It is easier to harden large sections of low-

alloy steels throughout than is the case with carbon steels. Moreover,

as the critical cooling rate is much lower, oil quenching can be used to

attain high strength and hardness with less possibility of distortion or

the introduction of high internal stresses.

Tempering temperature, deg. C.

700 1200 1400900 1I0O 1300 800 1000

Tempenng temperature, deg. F.

Fig. 16.4. Elfect of tempering on the tensile properties of oil-quenched (A) nickel-

chromium (S.A.E. 3140) and (B) nickcl-chroniium-molybdenum (S.A.E. 4S40) steels.

{S.AM. Handbook, 1941)


16.10. The New Metallurgy of Low-Alloy Steels

For many years charts of average or minimum tensile properties and

hardness (occasionally impact resistance as well) of S.A.E. and other

low-alloy steels, as quenched and tempered at various temperatures, were

published in the S.A.E. Handbook and in other readily available sources.

The properties indicated by these charts were, moreover, commonly

used by engineers in the design of machines and other important equip-

ment. Figure 16.4 shows two such charts.

These charts of properties were considered invaluable because of the

commonly held assumption that each alloy steel had a set of mechanical

properties after heat treatment that was characteristic of, and due to, its

chemical composition, and that, in general, these properties could be

reproduced at will by subjecting any steel of the same specified composi-

tion range to the same thermal treatment. This assumption is statistically

valid, but the accompanying and widely held assumption that certain

low-alloy steels have inherently a better combination of strength and

ductility than others (Fig. 16.4) is not valid, at least insofar as the as-

sumption is based on charts of mechanical properties.

Table 16.2. Typical Tensile Properties of S.A.E. Low-Alloy Steels (0.35 to 0.45

Per Cent Carbon) Quenched in Oil and Tempered at 1000° F. (540° C.) *

Elon- Reduc-Nominal chemical Tensile Yield gation tion

Steel composition, per cent strength, strength, in of

No. lb. per lb. per 2 in., area,

sq. in. sq. in. per perC Mn Ni Cr Mo cent cent

1340 0.40 1.75 141,000 120,000 18 522340 0.40 0.75 ! 3.50 137,000 120,000 21 603140 0.40 0.75 1.25 0.60 143,000 119,000 17 57

X3140 0.40 0.75 1.25 0.75 155,000 1 30,000 17 554140 0.40 0.75 1.00 0.20 170,000 147,000 16 564340 0.40 0.65 1.75 0.65 0.35 184,000 160,000 15 524640 0.40 0.65 1.80 0.25 151,000 131,000 19 56

* Data obtained on standard test specimens machined from bars less than 1 .5 in.

in diameter.

Some typical tensile properties of S.A.E. low-alloy steels containing

0.35 to 0.45 per cent carbon as quenched in oil and tempered at 1000°F.

(540°C.) are given in Table 16.2. These properties were collected by

Hoyt* and are undoubtedly representative. For a definite heat treatment.


tensile and yield strengths vary as much as 40,000 lb. per sq. in., and the

variation in ductility as measured by elongation and reduction of area is

considerable. The existence of such lists of properties (and published

charts of properties) makes it understandable that engineers designing

a crankshaft for an internal-combustion engine, for instance, believed

that there is a marked inherent difference in the mechanical properties

of heat-treated low-alloy steels.

Two relatively recent events in ferrous metallurgy were mainly re-

sponsible for the development of what might be called a new philosophy

of low-alloy steels and for changing greatly our ideas about the relation

between composition and properties. They were also primarily respon-

sible for the disappearance of mechanical-property charts from the S.A.E.

Handbook (in 1942) and from many other publications and for the

almost overnight development during World War II of the National

Emergency steels, an accomplishment in which American metallurgists

can take much justifiable pride. These two events were the work of

Janitzky and Baeyertzf on the similarity in mechanical properties of heat-

treated low-alloy steels, and the practical application of the principles

underlying the relation of carbon, alloying metals, and hardenability.

The relation among carbon percentage, the amount of alloying ele-

ments present, hardenability and other variables, and the properties of

steel is discussed in detail in the next two chapters. In brief, this relation

is as follows: If M per cent of nickel and N per cent of chromium pro

duce a structure of 50 per cent martensite at the center of an oil-quenched

2-in. bar of steel containing 0.45 per cent carbon; and if X per cent of

manganese and Y per cent of molybdenum produce the same structure

in an oil-quenched bar of the same size and approximately the same

carbon content, the tensile properties of the two steels after tempering

to the same hardness will for all practical purposes be identical.

16.11. Similarity of Properties of Heat-Treated S.A.E. Low-Alloy

Steels

Some of the data obtained by Janitzky and Baeyertz are reproduced

in Figs. 16.5 and 16.6. These investigators quenched 1-in. round bars of

• S. I.. Hoyt, Metals and Alloys Data Book, Reinhold Publishing Corporation, NewYork, 1943, 334 pp.

+ E. J. Janitzky and M. Baeyertz, in Metals Handbook, American Society for Metals,

Cleveland, 1939, pp. 515-518. The work of Janitzky and Baeyertz on the similarity

of properties of heat-treated low-alloy steels was antedated by several years by work

done but not published by the metallurgical laboratory of General Motors Corpora-

tion which resulted in the removal of the old-style property chart from General

Motors Standards several years previously. The first publication of a part of this

work was by A. L. Boegehold, Metal Progress, Feb., 1937, pp. 147-152.


260

240

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thousand lb.pcrsq.in.

240 20 40 60

Elongation and reduction

ofarea, per cent

Fig. 165. Tensile properties of water-quenched and tempered S.A.E. low-alloy

steels containing about 0.30 per cent carbon. (Janitzky and Baeyertz)

a number of S.A.E. alloy steels in water and oil and tempered them at

various temperatures in accord with recommendations given in the

S.A.E. Handbook. Yield strength, elongation, and reduction of area were

plotted against tensile strength with the result shown in Figs. 16.5 and16.6. For tensile-strength values of less than 200,000 lb. per sq. in., there

is little difference in the steels. At high strength values considerable scatter

is evident; this is undoubtedly due to internal stresses caused by quench-

ing rather than to any inherent difference in the steels.

The data reported by Janiuky and Baeyertz in 1939 have been con-

firmed since by a number of other investigators. Patton,* for example,

plotted tensile properties and Izod impact resistance for 409 tests on 180

heats of 37 grades of carbon and low-alloy steels. The steels used were

mostly N.E. and S.A.E. grades, but a considerable number of others wereincluded; all of the steels, however, were of such a composition that it

was reasonably certain that the test specimen hardened completely in

quenching. For tensile-strength values of 100,000 to 200,000 lb. per sq.

• W. G. Patton, Metal Progress, v. 43, 1943, pp. 726-733.


280

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20 40 60

Elongation and reduction

ofarea, per cent

Fie. 16.6. Tensile properties of oil-quenched and tempered S.A.E. low-alloy steels

containing about 0.40 to 0.45 per cent carbon. (Janitzky and Baeyertz)

in., yield-strength, elongation (in 2 in.), and reduction-of-area values

fell close to a line representing expected values in at least two-thirds of

the tests, and 90 per cent of the values fell within a band that varied

not more than ± 5 per cent from the expected values.

Of the values that varied more than ± 5 per cent from the expected

values the variation was such that no single steel and no group of steels

could be considered superior to any other steel or group of steels. It was

also evident that steels heat treated to a tensile strength above 200,000

lb. per sq. in. contained internal stresses that were responsible for con-

siderable scatter in properties. Impact resistance was more erratic than

other property values, varying 20 per cent or more from the line of ex-

pected values.

The most recent correlation of mechanical properties, for low-alloy

steels containing 0.30 to 0.50 per cent carbon, is shown in Fig. 16.7.*

• Courtesy American Society for Metals.


100 120 140 160 180 200Tensile Strength, 1000 psi

FlC. 16.7. Normally expected mechanical properties (heavy line) and average varia-

tion (hatched band) of quenched and tempered low-alloy steels containing 0.30 to 0.50

per cent carbon (Metals Handbook, 1948, page 458).

16.12. The Engineering Properties of the S.A.E. Low-Alloy Steels

Some typical tensile properties of S.A.E. carbon and low-alloy steels,

including a few triple-alloy (N.E.) steels, collected from various sources

are given in Table 16.3. All the steels whose properties are given in this

tabulation were heat treated to a tensile strength of 150,000 lb. per sq. in.

(Brinell hardness number approximately 300) . The similiarity in proper-

ties of the low-alloy steels is evident; also evident is the inferior yield


strength and ductility of the carbon steels which are inherently shallow

hardening in the sizes treated, namely, 1- to li^-in. bars.

The effect of tempering on the tensile properties of 1-in. round bars

of S.A.E. low-alloy steels containing 0.30 to 0.35 and 0.40 to 0.45 per cent

carbon is shown by the charts of Fig. 16.8. These are the properties that

can be expected 90 per cent of the time if an S.A.E. low-alloy steel of a

size that will harden with a structure of approximately 50 per cent

martensite at the center when quenched in oil is tempered at the tem-

peratures shown.

Table 16.3. Typical Tensile Properties of S.A.E. and N.E. Carbon and Low-AlloySteels Heat Treated to a Tensile Strength of 150,000 lb. per sq. in.

*

Elon- Reduc-Nominal chemica Tempering Yield gation tion

Steel composition, per cent temperature strength, in 2 ofNo. lb. per

sq. in,

in.,

perarea.

per

C Mn N1 Cr Mo V °F. °c. cent cent

1045 0.45 0.75 600 315 114,000 10 351060 0.60 0.75 750 400 113,000 15 451080 0.80 0.40 950 510 111,000 16 47

1340 0.40 1.75 950 510 128,000 16 502340 0.40 0.75 3.50 950 510 132,000 20 593130 0.30 0.65 1.25 0.60 850 455 128,000 17 57

3140 0.40 0.75 1.25 0.60 950 510 127,000 17 56

X3140 0.40 0.75 1.25 0.75 1000 540 130,000 17 55

4065 0.65 0.80 0.20 1100 595 130,000 16 47

4130 0.30 0.65 0.65 0.20 1050 565 131,000 20 61

X4130 0.30 0.50 0.95 0.20 1050 565 130,000 20 574140 0.40 0.75 1.00 0.20 1100 595 131,000 18 604340 0.40 0.65 1.75 0.65 0.35 1200 650 130,000 19 574640 0.40 0.65 1.80 0.25 1000 540 131,000 19 56

5140 0.40 0.75 1.00 1000 540 130,000 18 55

6150 0.50 0.75 1.00 0.18 1200 650 135,000 18 54

8640 0.40 0.90 0.50 0.50 0.20 1050 565 133,000 17 53

8740 0.40 0.90 0.50 0.50 0.25 1100 595 130,000 18 56

8750 0.50 0.90 0.50 0.50 0.25 1200 650 129,000 18 54

8950 0.50 1.00 0.50 0.50 0.40 il200 650 133,000 18 57

9440 0.40 1.10 0.45 0.40 0.12 1050 565 130,000 18 57

* All test data obtained on standard specimens machined from bars 0.75 to 1.5 in.

in diameter. All quenched in oil except 1045, which was quenched in water.


Tempering temperature .deg.C.

400 500 600 300 400

600 800 1000 1200 60O 800

Tempering temperature, deg.F.

1000

Fie. 16.8. Effect of tempering on the normally expected tensile properties of low-

alloy steels containing (A) 0.30 to 0.35 per cent carbon, and (B) 0.40 to 0.45 per cent

carbon.

160

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140

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,Manganese 015VNickel 1.25%

• Chromium 0.157.-

£e<fccf/on ofarea

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B. 8b30"Carbon 0.30°,

Mawnese 0.807.

Nickel. 0.657.-

Chromium 0.507.

Molybdenum 0.257."

Reduction ofarea

^pn0jonin2in.

"6

- c

si

I 2 3

Diameier of section, in.

I 2 3

Diameter of section.in.

Fig. 16.9. Effect of mass on the normally expected tensile properties of (A) a

through-hardening (X3140) steel, and (B) a semithrough-hardening (8630) steel,

quenched in oil and tempered at 1000°F.


The mechanical properties of heavy sections o£ heat-treated low-alloy

steels depend on how deeply the material hardens in quenching and

upon several other variables such as grain size, quenching tenqjerature,

and quenching medium. The higher the carbon (up to about 0.60 per

cent) , the harder and stronger is the material at and near the surface;

and, in general, the higher the percentage of alloying metals promoting

hardenability, the harder and stronger is the steel both at the surface

and throughout the cross section.

The effect of section size on two low-alloy steels is shown in Fig. 16.9.*

The nickel-chromium steel X3140 could be classed as a through-harden-

ing material, and the triple alloy steel 8630 is undoubtedly semithrough-

hardening material. Both steels have a relatively high strength (after oil

quenching and tempering at 1000°F.) in small sections; and in both

steels the strength decreases as the size of section increases, but the

tensile and yield strengths of 8630 decreases to a greater extent and more

rapidly with increasing section size than those of X3140.

The impact resistance of the low-alloy steels depends on a number of

variables including composition, grain size, thermal treatment, and

others. Patton, whose data were discussed in the previous section, plotted

the Izod impact values given for the 400 specimens and found that the

normal expected value decreased from 85 or 90 ft.-lb. for steels with

100,000 to 120,000 lb. per sq. in. to about 20 or 25 ft.-lb. for steels or

200,000 lb. per sq. in., leveling off at this value for higher tensile strengths.

Because impact resistance is very sensitive to slight variations in struc-

ture, Patton found much more scatter than in any of the other properties.

The endurance ratio of polished specimens of low-alloy steels is ap-

proximately the same as, or perhaps slightly lower than, for carbon

steels. Available data in the literature give ratios of 45 to 50 per cent

with the average a little below 50. Because of internal stresses, the en-

durance limits of heat-treated low-alloy steels with a tensile strength of

250,000 lb. per sq. in. or higher do not exceed approximately 100,000

lb. per sq. in.

QUESTIONS

1. Why should low-alloy steels be balanced?

2. Give as many advantages as you can as to why low-alloy steels arc becomingso popular.

8. Explain the effects of adding small amounts of nickel to steels.

4. Explain the effects of adding small amounts of chromium to steels.

• Data on X3140 from International Nickel Co., Handbook on Nickel Steels, § 2,

1934; data on 8630 from Republic Steel Corp., Handbook on N£. Steels, 1943.


5. Explain the effects of adding small amounts of molybdenum to steels.

6. Explain the effects of adding small amounts of vanadium to steels.

7. Explain the effects of adding small amounts of tungsten to steels.

8. Explain the effects of adding phosphorus to steels.

9. What arc the disadvantages of adding phosphorus to steels?

10. Name three alloying elements that arc carbide formers.

11. Why is the carbon content low for low-alloy steels?

12. What advantages do low-alloy steels have compared to plain carbon steels

when quenched?IS. Name twenty specific uses for low-alloy steels.

14. Compare in chart form the physical properties of low-alloy steels containing

0.40 per cent to a plain carbon steel containing 0.4 per cent carbon.

15. Explain why silicon is not used to any great extent in low-alloy steels.

16. List various sources where physical properties of low-alloy steels may beobtained.

17. What do the letters S.A.E. stand for?

18. What do the letters A.I.S.I. stand for?

CHAPTER

Hardenability

17 Walter M. Hirthe, M.S.M.E., Assistant Professor of

Mechanical Engineering, College of Engineering

Marquette University, Milwaukee, Wisconsin

Richard O. Powell, College of Engineering, Tulane

University, New Orleans, Louisiana

r OR more than a thousand years, hardness

has been the most highly valued attribute of iron-carbon alloys; in fact,

the word "steel" has long been an accepted synonym for hardness, dura-

bility, and reliability. This is natural; to early social groups, with limited

knowledge of the properties of the metals then available, unhardenable

wrought iron was no better, and in many instances worse, than properly

made and fabricated bronze. When wrought iron was carburized and heat

treated, however, the resulting steel was far superior to anything pre-

viously available, not only for weapons, but also for plowshares and other

tools essential to an agricultural and pastoral society. It is not surprising,

therefore, that hardness became a supreme and lasting virtue in metals,

and the medieval metal worker who knew how to produce hardened steel

was for centuries considered something of a magician.

Today, as well as a thousand years ago, no one questions the im-

portance of hardness and its accompanying high strength; in fact, the

ease with which hardness and its related properties can be controlled bythermal treatment makes steel man's most important metallic material.

Owing, however, to inherent limitations in our present methods of

thermal treatment, hardness may be only superficial. Metallurgists have

realized for many years that hardenability, which, briefly, is the depth to

which martensite will form in definite sections under definite cooling

conditions, is often as important as actual hardness, and there are manyapplications for which steel should be practically as hard and strong at

or near the center as at the surface. Surface-hardened steels are valuable

for many engineering uses, but if weight is to be saved and high stresses

342

Hardenability 343

withstood, steels hardened throughout are necessary. In addition—andthis is equally important—a steel hardened throughout by quenching has

properties, as tempered to a definite hardness, that are superior to the

properties of a steel tempered to the same hardness but not hardenedthroughout.

17.1. Hardness and Hardenability in Carbon Steels

Among engineers who use heat-treated steel there is considerable con-

fusion concerning the difference between hardness and hardenability,

that is, between the maximum hardness that a steel attains in quenchingand the depth to which it hardens. There is no direct and close relation

between the two, despite the fact that hardness is used to measurehardenability. An example or two should make this clear.

Suppose that two 1-in. bars of carbon steel containing, respectively,

0.30 and 0.70 per cent carbon are quenched from above the transforma-

tion temperature into ice water or iced brine. The surface hardness of

the first will be approximately 55 Rockwell C and that of the other oneabout 65 Rockwell C. Both bars will harden to a depth of about \/A in.

The hardness values of the two steels differ (at the surface), but the

hardenability is the same.

If small specimens of either carbon or low-alloy steels are cooled fromthe proper quenching temperature at the critical cooling rate or faster,

so that no transformation occurs before martensite begins to form, the

0.40 0.5O 0.60

Carbon, percent0.70 0.80 0.90

Fjc. 17.1. Relation between carbon content and maximum martensitic hardness(upper curve), and maximum hardness usually attained (hatched band). {Based ondata by Burns, Moore, and Archer, and by Boegehold)


maximum hardness attainable depends on the carbon content. This is

shown by Fig. 17.1.* It should be emphasized that the upper curve shows

the maximum hardness obtained when the specimen is entirely marten-

sitic in structure. In the commercial heat treatment of carbon and low-

alloy steels this is rarely realized, except with very small sections. Ordi-

narily the martensite is contaminated by varying small amounts of transi-

tion structures that reduce the hardness appreciably. In consequence,

many metallurgists put the highest practical hardness obtainable in the

commercial quenching of these steels at 5 to 10 Rockwell C numbers

below the maximum, as is shown by the hatched area in Fig. 17.1.

17.2. Hardness and Hardenability in Low-Alloy Steels

The difference between hardness and hardenability is also well illus-

trated by Fig. 17.2,* which shows hardness values over the entire cross

section of quenched bars of various diameters of S.A.E. 1045 (0.45 per

cent carbon) and S.A.E. 4140 (0.40 per cent carbon, 1.0 per cent

chromium, 0.20 per cent molybdenum) steels. If 1-in. bars of these two

steels are quenched in water from above the transformation temperature

(Fig. 17.2), their hardness at the surface is practically the same; at the

center, however, S.A.E. 4140 has a hardness of 59 Rockwell C, whereas

S.A.E. 1045 has a hardness of only 34 Rockwell C.

This illustration shows clearly that two steels of about the same carbon

content may have the same hardness at the surface but a different harden-

ability. They may also have a very different hardness at the surface and a

different hardenability, depending primarily upon the relation of the

cooling rate to the location of the nose of the S curve and, consequently,

upon whether the rate of cooling is sufficient to prevent much pearlite

from being formed during quenching.

17.3. Cooling Rate and Hardenability

In many ways steel is a very cooperative metal and responds readily

to the treatments man gives it, no matter how unreasonable or illogical

these may be. In other ways, however, it is as stubborn as the traditional

army mule, and, like it or not, we can do little about it. Heat extraction

is typical of this characteristic. We can put heat into a piece of steel by

•The upper curve of Fig. 17.1 is based on data collected byJ.

I.. Burns, T. L.

Moore, and R. S. Archer, Trans. Am. Soc. Metals, v. 26, 1938, pp. 1-36. The hatched

band is based primarily on data given by A. L. Boegehold, S. A. E. Trans., v. 52,

1944, pp. 472-485.

* Based on diagrams published in U. S. S. Carillory Steels, Carnegie-Illinois Steel

Corporation. Pittsburgh., 1938, 214 pp.

Hardenability 345

1

S.A.E. 04! ,QUENi

i

:hed in

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WAid

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Diameter of bar.in. Diameter of bar, in.

Fig. 17.2. Hardness over entire cross section of bars of various diameters ofS.A.E. 1045 and S.A.E. 4140 steels quenched in water. (U. S. S. Carilloy Steels)

electrical or other methods at almost any rate we choose, but we cannotextract the heat from the interior any faster than thermal conductivity

will permit. No matter how fast we chill the surface, the heat will flow

from the center to the surface of a large section at a slow rate, which is

largely unaffected by the composition of the material. This process is

complicated by the fact that during quenching the steel conforms ap-

proximately with Newton's law of cooling—that heat loss is proportionalto the temperature difference between the bar and the quenching medium—and also by the fact that, quenching mediums vary greatly in their

ability to extract heat from the surface of hot steel.

And it must be remembered that it is not always desirable to quenchsteels drastically in water, as this treatment is likely to result in highinternal stresses which may cause severe warping, cracking, or some other

unhappy consequence.


17.4. Time Delay and Hardenability

The relatively slow heat transfer from the center to the surface of a

section 1 in. or larger severely limits our ability to cool steel rapidly

enough to secure deep hardening, if the critical cooling rate is high.

There is consequently only one method of securing deep hardening:

reduce the critical cooling rate. In other words, what we do is to decrease

the tendency of austenite to transform to pearlite in the temperaturerange of the nose of the S curve, or to bainite at lower temperatures;

this is another way of saying that we push the line of the S curve that

marks the beginning of austenite transformation below the critical tem-

perature to the right so that more time is available for the steel to cool

to 600°F. (315°C.) or below without such transformation.

Below the A t transformation temperature, austenite is, of course, un-stable; and the more the temperature is lowered, the greater becomesthe instability. The tendency of unstable austenite to transform increases

with lowered temperatures; but this is counteracted to a degree by the

fact that as the steel gets colder it gets more rigid; that is, its ability to

transform decreases.

The forces involved in the tendency of unstable austenite to transform

to pearlite, bainite, or martensite are complex, and at present they are

not well understood. The net result, however, is that in all steels there

is a time interval, commonly known as time delay, which elapses before

the unstable austenite begins to decompose. This interval may be very

short or very long, depending upon a number of factors—in addition to

temperature, which is most important—that include among others homo-geneity of austenite, grain size, and chemical composition.

Small differences in time delay are important in hardenability; buteven so, all carbon steels and many low-alloy steels have a time delay at

the nose of the S curve that is rarely more than 3 to 5 sec. (for con-

tinuous cooling), which means that few of them harden to the center

except in very small sections. In other words, changes in the S curve

due to continuous cooling may be important in the heat treatment of a

razor blade or a small tool but not of a 2-in. shaft or a 55-mm. gun tube.

17.5. Variables Affecting Hardenability

There are a number of variables that affect time delay and thus affect

hardenability. Homogeneity of the austenite increases its sluggishness

and thus increases time delay. Heating long enough or at a high enoughtemperature to ensure homogeneity is also likely to change the grain size,

and since grain size greatly affects hardenability it is difficult to untangle

the two variables. It is, however, a fact that, if one or more areas in the

Hardenability 347

austenite are high or low in carbon or in alloying elements, transforma-

tion during cooling will be accelerated. Premature transformation in a

segregated area has a "trigger action" that sets off transformation over

the whole section, thus decreasing time delay and hardenability.

Since homogeneity depends on diffusion and diffusion takes time, one

of the common but insufficiently recognized causes of a variation in

hardenability in steels of the same composition is insufficient time at the

quenching temperature for the austenite to become homogeneous. This

is also the reason why the hardenability of a steel as determined in the

laboratory, where the homogeneity factor is likely to be taken care of,

may differ from the hardenability of the same steel heat treated in large

lots in the plant.

Another and even more important factor is austenite grain size. Trans-

formation begins principally at the grain boundaries; hence, as the num-

ber of grains increases, the likelihood that transformation will start

quickly is increased. Small grain size thus decreases time delay and

lowers hardenability. On the other hand, increasing grain size decreases

the chances for transformation to begin quickly; it lengthens the total

time for the reaction to start and increases hardenability.

Much work has been done in this field, which can be summarized

briefly by stating that if other factors do not enter, increasing the grain

size from about 8 (A.S.T.M. rating) to 5 or 6 will double the time

available for the austenite to cool to a low temperature without trans-

forming to pearlite; and increasing the grain size to 2 or 3 will more

than double the time and it may even triple it.

Grain-size control during melting is relatively simple; it would, there-

fore, be easy to increase the hardenability of shallow-hardening grades

by using a steel that would be coarse grained just before quenching.

Unfortunately, however, the consequences might not be pleasant. Coarse-

grained steels frequently crack if cooling is drastic, and they are likely to

be sadly lacking in toughness (impact resistance) at room temperature

and below. In fact, this lack of toughness is usually so dangerous that

most steels that are to be used in highly stressed parts are deliberately

made fine grained, and deep hardening is secured by the use of alloying

elements.

By a proper balance of carbon content and the various alloying ele-

ments, the critical cooling rate can be lowered so much (that is, the

time delay is greatly increased) that the steel will harden deeply when

large sections are quenched in oil or even on cooling in air. This, and

the methods used for measuring hardenability, are the subjects covered

in the following sections.


17.6. Methods of Determining Hardenability

The first of several tests proposed to measure hardenability directly

is the penetration-fracture (P-F) test developed by Shepherd.* This in-

volves quenching short bars, % in. in diameter, in a brine solution after

homogenizing for 30 min. at 1450, 1500, 1550, and 1600°F. (790, 815,

845, and 870°C). The bars are fractured and the fracture is comparedwith a standard. One half is polished on the end and etched with acid to

determine the depth of hardening. The steel is rated according to the

numerical values of the fracture (grain size) and the penetration (hard-

enability). The test demands considerable skill and experience. It hasnever been used widely except by companies making or using large

quantities of high-carbon tool steel. It is possible by this test to classify

tool steels as sensitive or insensitive to grain growth with increasing

quenching temperature and as deep or shallow hardening.

Unquestionably, the most reliable test for hardenability is made simply

by quenching bars of varying sizes from the proper temperature, cutting

the bars in half, and determining Rockwell C hardness from center to

surface. These values are then plotted as a U-shaped diagram similar

to that shown in Fig. 17.2. For a study of the hardenability complete

enough to rate a steel accurately it is necessary to prepare, quench undercontrolled conditions, and measure the hardness of, the transverse sections

of five to eight or more bars varying in diameter from i/, to 4 in. or more,

which takes 15 to 20 hr. This is the only disadvantage of the test, but it

is a major one.

Twenty years ago, when hardenability was attracting wide attention,

one of the important problems in the field was to devise a simple, readily

reproducible, comparatively accurate test to determine the relative re-

sponse of the commonly used medium-carbon and low-alloy steels to

quenching.

It had been apparent for some time that, if the hardness values at the

center of quenched bars are plotted against the diameter, depth-harden-

ing curves result. Four such curves are shown in Fig. 17.3. The twocurves in Fig. 17.3A are plotted from the data given in Fig. 17.2, and the

difference between the shallow-hardening carbon steel (1045) and the

deep-hardening chromium-molybdenum steel is clear. The curves plotted

in Fig. 17.3B are from hardness data on two low-alloy steels which differ

slightly in response to quenching. The hardenability of these two steels

is practically the same for \/r and s^-in. bars and for bars larger than H/£

in., but it varies considerably with intermediate sizes.

• B. F. Shepherd, Trans. Am. Soc. Metals, v. 22, 1934, pp. 979-1016.

Hardenability 349

3 4 5 'h V4 I

Diameter of bar. in.

Fig. 17.3. Depth-hardening curves for (A) water-quenched shallow-hardening (1045)

and deep-hardening (4140) steels and (B) two low-alloy steels differing in hardenability

in intermediate sizes.

It was, of course, immediately recognized that any test that could be

used to determine hardenability rapidly and cheaply must distinguish

not only between deep- and shallow-hardening steels where differences

are great (Fig. 17.3A) but also between the hardenability of two steels

whose depth-hardenability curves resemble those plotted in Fig. 17.3B.

Such a test was developed by Jominy and his coworkers at the research

laboratory of General Motors Corporation and has now been standard-

ized and is used for specification and control purposes.*

17.7. The Jominy End-Quench Hardenability Test

The Jominy test consists of quenching the end of a 1-in. round bar

under controlled conditions and determining the hardness (Rockwell C)

at intervals along the bar, beginning with the quenched end. The fixture

and the test bar are shown in Fig. 1 7.4. The test bar is rough-machined

and normalized by heating to approximately 150°F. (85°C.) above the

A3 transformation. After holding at this temperature for 30 min. the bar

is cooled in air and finish-machined to the dimensions shown in Fig. 14.6.

It is then heated in a nonscaling atmosphere to about 75°F. (40°C.)

above Az and held 20 min., after which it is rapidly withdrawn from

the furnace by the collar, placed in the fixture, and a column of cold

• The method has been tentatively standardized by American Society for Testing

Materials (A. S. T. M. Standards, Part 1A, 1955, A255-48T, pp. 636-642) and by Society

of Automotive Engineers (Handbook, 1953, pp. 112-120).


45'-.

\Unimpe<kdwaterjti »-/*-—

I

1

+

water passing through a i/£-in. pipe

and under pressure sufficient to rise

2i/£ in. above the opening is directed

against the red-hot end until the

specimen is cold.

Two flat surfaces 1 80 deg. apart and0.015 in. deep are ground longi-

tudinally on the bar, and Rockwell Chardness measurements are taken at

% 6 -in. intervals, beginning at the

quenched end and extending for

about 2 in. The hardness values are

plotted against distance from the

quenched end, which results in curves

similar to those in Fig. 17.5.

Owing to the method of quenching

and to the laws of heat flow, the

Jominy bar when cold has a structure

and hardness that result from cooling

rates which vary from about 600°F.

(3358C.) per sec. on the quenched face

and about 420°F. (235°C.) per sec. at % e in. from the quenched end to

less than 10°F. (5°C.) per sec. at the other end of the bar. These are

equivalent to the center cooling rate of a i/g-in round bar quenched in

agitated cold water as one extreme and a 4-in. round bar quenched in oil

as the other.

Some typical curves obtained by the Jominy test arc reproduced in

Fig. 17.5.* In these the distance is shown as inches from the quenchedend; in many curves it is shown as the number of sixteenths of an inch.

The horizontal hardness value indicated by the short dashed lines is the

average maxima obtained by quenching unalloyed steels of the mediancarbon content indicated.

rl-in. roundspecimen

Waterat 7tttS'

'i-in. inside diamorifice

From quick-opening valve .._'

Fig. 17.4. The Jominy standard

end-quench hardcnability test.

17.8. Relation of the End-Quench Test to Actual Cooling Rates andthe Selection of Steel by Hardenability

Jominy and a number of others, including a Society of AutomotiveEngineers' committee on hardenability tests/ have done much work in

•W. K. Jominy, Metal Progress, v. 38. 940. pp. 685-690.

fS. A. E. Handbook, 1953, "Recommended Practice for Determining Harden-ability," pp. 112-120

Hardenability

Rate of cooling at I300°R, °F. per sec.

351

420190 100 75 55 45

'/4 3/8 '/2 5/8 3/4

Distance from water cooled end, in.

»/«

Fig. 175. Jominy end-quench curves for carbon and low-alloy sleels containing

0.40 per cent carbon. (Jominy)

recent years in correlating Jominy distance and hardness with cooling

rates. Such a correlation is shown in Fig. 17.6, in which is plotted the

cooling rate at center and surface and at various radii, bar size, and the

position on the Jominy bar. By using the values given in Fig. 17.6, we

can readily tell what the hardness should be at the center, at one-half

radius, at three-fourths radius, and at the surface of any steel whose end-

quench curve has been determined.

Suppose, for example, that we want to select a steel for a li^-in. round

shaft that should be quenched in oil and tempered at a fairly high tem-

perature and that should have a minimum Rockwell C hardness of about

35 at the center and 40 at the surface. Tempering at 1000°F. (540°C.)

causes an average reduction in hardness of 15 to 20 numbers at the

surface and 7 to 10 numbers at the center; so the hardness of the steel as

quenched in oil should be 55 to 60 Rockwell C at the surface and 42 to 45

Rockwell C at the center. According to Fig. 17.6, the center cooling rate

in oil for a li^j-in. round bar is 25°F. per sec. and the corresponding

distance on the Jominy bar is % 6 in.; the surface cooling rate is about

90°F. per sec, and the Jominy distance is slightly more than % 6 in.

It is evident from the curves shown in Fig. 17.5 that our requirements

can readily be met by S.A.E. X3140 but not by the other four 0.40 per

cent carbon steels whose curves arc reproduced in Fig. 17.5. There are,

of course, many other low-alloy steels that will serve equally well for this

application.


Goolinq rate.deg. F. per sec. at 1300'F.

4lllll 1 § § 8 ? » ssssa ^350=93^3355*i i i i i i /

// / A

3.c

i/r //

//

/..,

f Mlî/

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42

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\1

m1 Water-quenched rounds

n

2 J 4 5 6 1 8 9 10 12 14 16 18 20 24 32 48

Distance from quenched end, sixteenths ofon inch

FlG. 17.6. Correlation of surface, center, and intermediate cooling rates, bar sizes,

and distance from quenched end of Jominy bar, for oil and water quenching round

bars. (Boegehold)

17.9. Virtues and Shortcomings of the Jominy End-Quench Test

The Jominy test can be made relatively quickly and cheaply, and the

curves for a particular lot of steel obtained in one laboratory check

closely with results from other laboratories. This agreement is fortunate

because even before a tentative standard was worked out by the American

Society for Testing Materials in 1942, it was put to good use in the war-

time development of the low triple-alloy steels.

In brief, the method used when the common alloying elements became

short and had to be conserved, and when insufficient time was available

for extensive mechanical and service testing, was to make up experimental

heats of the new triple-alloy steels, adjusting the composition according

to the expected effects of carbon and the alloying elements, and to deter-

mine the end-quench hardness characteristics. If the end-quench curve of

Hardenability 353

the new steel coincided closely with that of one or more of the standard

higher alloy steels, it was reasonably certain that the new steel could be

substituted for these standard steels with considerable assurance that it

would have the mechanical properties desired after heat treating to obtain

a definite hardness at a definite distance on the Jominy bar.

The Jominy test will tell with accuracy whether the nose of the S

curve has been missed in quenching and that there is no pearlite in the

microstructure of quenched bars of certain diameters. It will not tell,

however, whether the structure as quenched is contaminated by bainite.

There is little difference in the hardness of martensite and that of acicular

bainite which forms just above the M R temperature, but there is a marked

difference in the properties of tempered martensite and those of tempered

bainite of about the same hardness. Consequently, because the end-

quench test does not make this distinction, there is no guarantee that

structure and properties will be exactly what is required.

There are other shortcomings of the end-quench test. These are not

inherent in the test itself; rather, they arc precautions that should be

observed in applying the results too widely. If a steel is segregated or

otherwise heterogeneous (and many large heals are), Jominy curves of

a specimen cut from a segregated bar will be different from the curves

of specimens cut from a bar that is not segregated, even though the two

bars were rolled from the same heat. Another shortcoming is that the

test tells only the hardenability of a round bar. To determine harden-

ability of other shapes, and especially of odd-shaped sections, such as

a gear blank, a rather complicated procedure must be used.

17.10. Hardenability Bands

About ten years ago, the steel industry, represented by the American

Iron and Steel Institute, and the major consumers of heat-treatable

carbon and low-alloy steels, represented by the Society of Automotive

Engineers, worked out hardenability bands for a few steels, to be used for

specification purposes. The composition limits of three of these steels,

designated by the letter H (Table 17.1), are somewhat wider than

ordinarily permitted. Three such bands for the steels given in Table 17.1

are shown in Fig. 17.7. The steelmaker must produce a material whose

hardenability is within the specified limits, and he is, therefore, given

slightly more latitude in chemical composition so that he can make ad-

justments of the analysis while the steel is still in the furnace. Thus, if

the chromium is on the low side and cannot be increased readily, he

can keep manganese, or molybdenum, high to ensure that the harden-

ability will be within the required limits.


Table 17.1 Composition Limits (1955) for Three Low-Alloy Steels for WhichHardcnabihty (H) Is and Is Not Specified

SteelChemical composition, per cent

No.C Mn Si Ni Cr Mo

41404140H

86408640H

87408740H

0.38-0.43

0.37-0.44

0.38-0.43

0.37-0.44

0.38-0.43

0.37-0.44

0.75-1.00

0.65-1.10

0.75-1.00

0.70-1.05

0.75-1.000.70-1.05

0.20-0.350.20-0.35

0.20-0.35

0.20-0.35

0.20-0.350.20-0.35

0.40-0.70

0.35-0.75

0.40-0.700.35-0.75

0.80-1.10

0.75-1.20

0.40-0.600.35-0.65

0.40-0.600.35-0.65

0.15-0.25

0.15-0.25

0.15-0.25

0.15-0.25

0.20-0.300.20-0.30

Incidentally all H steels are made by a practice that ensures fine grain,which means that the hardenability is obtained solely by adjusting thechemical composition.

17.11. Relation of Hardenability to Engineering Properties

If two steels are treated so that their structure is the same, their

important engineering properties will be the same most of the time.»The work of a number of investigators has established this beyond ques-tion, and it is universally accepted as fundamental in ferrous metallurgy.For steels used in highly stressed engineering structures and machines,the primary object of heat treatment is to produce optimum propertiesby controlling the size, character, and distribution of the carbide, andthe chemical composition is the principal variable by which this controlcan be exercised.

Thus, the amount of carbon is controlled to ensure that the martensiteor other phase formed by thermal ueatment-and the structure resultingfrom a change in these constituents by tempering—will have the requiredhardness and strength; and one or several alloying elements are added, if

necessary, to ensure that the desired structure and properties will be at-

tained at the depth beneath the surface that is required for a specific

application. Optimum properties result from a structure that is mostlymartensitic before tempering.

• Theoretically two steels having the same structure should have the same prop-erties all the time. This, however, is an ideal to he approached but hardly attained.There are a number of factors other than structure that may affect properties. Theprincipal one is internal stress, which may be produced by any of a number ofmanufacturing operations. Another is gas, taken up by the metal during melting,refining, or fabrication. These, and possibly others, are undoubtedly responsible forthe fact that the properties of steels having the same structure vary considerablyfrom the normal in about one case in ten.

Hardcnability 355

4 8 12 16 20 24 28 32

Distance from quenched end of specimen.sixteenths ofan inch

Fig. 17.7. Hardenability bands for three 0.40 per cent carbon low-alloy steels.

(Society of Automotive Engineers and American Iron and Steel Institute)

In ordinary heat treatment it is therefore desirable to have martensite

before tempering and, if possible, to avoid cooling at a rate slightly

lower than the critical (commonly known as slack quenching), in which

case bainite or a mixture of martensite and bainite will be formed. In

all but the very shallow-hardening steels, it is relatively easy to obtain

martensite upon quenching sections less than about 1 in. in cross section;

but if optimum properties are required in heavy sections, a steel with

high hardenability should be used. Slack quenching results in a low

yield ratio (low yield strength as compared with tensile strength) and

frequently lower elongation, reduction of area, and impact resistance than

are desired.

In general, a deep-hardening steel in which martensite will form in

quantity even in large sections will, after tempering, have a yield ratio

of 92 to 88 per cent; a shallow-hardening steel in which the structure is

mostly bainite will, after tempering to the same hardness, have a ratio of

85 to 75 per cent or even lower, depending on how much bainite is

formed. The effects of slack quenching are especially apparent in the

notch toughness of low-alloy steel at or below room temperature. Fre-

quently the impact resistance of a steel containing pearlite or bainite as

quenched is only a fraction of the impact value of the same steel when


tempered to the same hardness but quenched to ensure a structure thatwas almost entirely martensitic before tempering.Tempering naturally reduces hardness and strength, but for a given

degree of tempering the reduction is greater if the quenched structurecontains much bainite than if it is primarily martensite. As a result, thecenter portion of heat-treated heavy sections may be too soft and weak.

17.12. Relation of Tempering to Hardenability

Fundamentally, tempering accomplishes two objectives: First, it re-

lieves dangerous internal stresses, which may cause brittleness or evenwarping or cracking; and second, it improves ductility and toughnessby causing transformation of any hard, brittle martensite present to amore ductile aggregate of ferrite and cementite. Concomitantly withthe increase in ductility, hardness and strength are decreased.

Since quenched steel is almost never used in engineering structures

and machines without tempering, it may be justifiably asked why end-quench data are obtained on a quenched specimen instead of on one that

is quenched and tempered. The answer is that, if the Jominy bar werequenched and tempered, we would not get the information we want and,further, that such information as we would get might be misleading.

The final structure and properties of a quenched and tempered carbon orlow-alloy steel depend to a large degree on the quenched structure, andthe end-quench test gives a fairly accurate picture of the hardness and its

accompanying structure (and the corresponding cooling rates) for sections

of various sizes quenched in various mediums. For that reason it is the

as-quenched structure that is the essential variable in determining the

final properties of a heat-treated steel, and it is very important that weknow how this structure may be controlled.

17.13. Fundamentals of Calculated Hardenability

In 1942 Grossmann, a pioneer for many years in exploring the funda-mentals ol quenching and tempering, presented his classic paper on cal-

culated hardenability.* The data reported in this paper showed that thehardenability of most carbon and low-allow steels could be predictedwithin a limit of error of ± 15 to 20 per cent, provided the completechemical composition and grain size were known and thai the austenitebefore quenching was homogeneous and free from undissolved carbides.

The work was based on the concept that "pure" steel has a fundamentalhardenability due to carbon alone, and that the final hardenability can

• M. A. Grossmann, Trans. Am. Inst. Mining Met. Engrs., v. 150, 1942. pp. 227-259.

Hardenability 357

"0 0.10 0.20 0.30 0.40 0.50 0.60 0.10 0.80 0.90 1.00

Carbon, per cent

Fie. 17.8. Relation among ideal diameter, D„ carbon content, and grain size.

(Grossmann)

be calculated by multiplying this basic hardenability by a factor for each

chemical element present. Grain size may be taken into account either

in the fundamental hardenability of "pure" steel or in the final product

of multiplication.

In working out his method Grossmann had to make another and, as

it turned out later, considerably more questionable assumption. This was

that the hardenability should be based on a bar of ideal diameter (Di),

defined as the diameter (in inches) of a bar that would show no un-

hardened core in an ideal quench, which was in turn defined as a quench

in which the outside of the bar is instantly cooled to a temperature of

the quenching medium. This, of course, is a condition that is never at-

tained in practice, but Grossmann believed that it could be used as a

reference point.


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FlG. 17.9. Multiplying factors Cor the live common alloying elements. (Boyd andField)

Using the assumption of ideal diameter and ideal quench, Grossmanndetermined experimentally the hardenability factors for all of the ele-

ments commonly found in steel either as residuals or purposely added.

By using these factors it is easy to calculate the ideal diameter (D,) of

a bar that will just harden in an ideal quench, and by applying the proper

correction it is easy to calculate the diameter (/)) of a bar Uiat will just

harden in any of the commonly used quenching mediums.Basic data for the hardenability in terms of D, of "pure" steel, as de-

pendent on carbon content and grain size, are given in Fig. 17.8. Themultiplying factors for a variety of chemical elements are given in Fig.

17.9.* The factors shown in Fig. 17.9 arc those determined by Gross-

mann, modified in some cases by later data.f Using the factors in Fig.

17.9, the ideal diameter D, can be readily calculated as indicated by the

data in Table 17.2.

It is important to remember that all elements present (except the

gases) should be determined and taken into account in making the cal-

culation. Thus, if only carbon, manganese, and silicon were considered

for steel 1040 in Table 17.2, the ideal diameter would be 0.81 in., instead

• L. H. Boyd and J. Held. "Calculation of the Standard End-Quench HardenabilityCurve from Chemical Composition and Grain Size," Contributions to the Metallurgy ofSteel, No. 12, American Iron and Steel Institute, New York, Feb., 1946, 25 pp.

|- See, for instance, Trans. Am. Inst. Mining Met. Engrs., v. 154, 1943, pp. 386-394: or"Symposium on Calculated Hardenability," ibid., v. 158, 1944, pp. 125-182; or "Sym-posium on Hardenability," ibid., v. 167, 1946, pp. 559-734.

Hardenability 359

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of 0.98 in., an error of 17 per cent, which would increase if the amounts

of residual nickel, chromium, and molybdenum were higher than shownin the table. Sulphur and phosphorus need not be considered if these

impurities are low, and no factors are given for them in Fig. 17.9. Thefactor for 0.02 per cent phosphorus is 1.05 and the factor for about 0.03

per cent sulphur is 0.97 (sulphur decreases hardenability); consequently,

in most high-grade carbon and low-alloy steels the effects of these two

elements cancel out.

17.14. The Accuracy of Calculated Hardenability

Continued work over the past ten years has demonstrated that attempts

to calculate hardenability by the use of multiplying factors represent an

oversimplification of the problem. We now know that additional vari-

ables are involved, variables that are difficult to determine. Nevertheless,

the calculation of hardenability has been used with considerable success

in practice where its limitations are understood.

Recent work indicates that there probably are two hardenabilities,

one affecting the transformation of austenite to i>earlite and the other

the transformation of austenite to bainite, but there are few data which

make it possible to differentiate the two. For example, it has been shown

that nickel has more effect on the austenite-bainite than on the austenite-

pearlite transformation, so that the curve for the multiplying factor for

nickel may have a different location from that shoivn in Fig. 17.9, depend-

ing on the structure that results from the transformation. This un-

doubtedly explains the discrepancies found by several investigators in

attempting to confirm Grossmann's multiplying factors for nickel and for

some of the other alloying elements.

The most important fact 'that was unrecognized by Grossmann and

some of the other early workers in the field of calculated hardenability

is the intensifying effect that one alloying element may have uponanother. For example, if manganese, chromium, or molybdenum is

present, the effect of nickel on hardenability is much greater than its effect

as a single alloying element. Similarly molybdenum is more effective in

increasing hardenability in a steel containing nickel than in one contain-

ing chromium as the second alloying element.

As a result of these recent discoveries it can be concluded that no line,

either curved or straight (see Fig. 17.9), can be drawn that will show

precisely the effect of an alloying clement on hardenability in all types of

low-alloy steels. The calculation of hardenability can be used most

successfully and reliably on alloy steels containing only one alloying

element.

Hardenability 361

Distance from quenched end, in.'1

l'"ic. 17.10. Kffect of boron on the end-quench characteristics of some carbon and

low-alloy steels. (American Iron and Steel Institute)

17.15. The Effect of Boron on Hardenability

One of the most intriguing metallurgical discoveries of the past ten

years or so is that very small amounts of boron greatly increase harden-

ability in some carbon and low-alloy steels. The increase results from the

addition of Q.002 to 0.003 per cent, as ferroboron, or as a complex alloy

containing 0.5 to 10 per cent boron and iron, aluminum, silicon, titanium,

and other elements in various combinations, or even as boric acid or

borax. Some typical end-quench curves for untreated and boron-treated

steels are reproduced in Fig. 17.10.*

• American Iron and Steel Institute. "Report on Special Alloy Addition Agents,"

Contributions to the Metallurgy of Steel, No. 9, The Institute, New York, 1942, 30 pp.


The steel to which boron is added should be fine-grained and thor-

oughly deoxidized as boron is an efficient deoxidizer, and trouble is

occasionally encountered in assuring that the boron is uniformly dis-

tributed in the metal. Very small amounts of boron are difficult to deter-

mine by chemical analysis, and it is not certain what residual percentage

is the most effective, nor have we any definite knowledge of the mech-anism by which this element affects hardenability. We do know, how-ever, that an addition of less than 0.001 per cent is ineffective, and that

if more than 0.003 or 0.004 per cent is added there may be a decided

decrease in hardenability.

The only definite knowledge to date is that boron increases harden-ability by increasing the time available for the steel to cool past the nose

of the S curve—it pushes the nose to the right—but even here its effect

may not be enough to increase the hardenability of shallow-hardening

steels materially as Fig. 17.10 shows. Why it should increase the harden-

ability of 4023 but not of 4042 (Fig. 17.10) is not easily explained.

Data on the effect of boron on mechanical properties are erratic andno definite conclusions can be drawn yet. It has no perceptible effect onthe properties of the steel as rolled, normalized, or annealed, nor on the

maximum hardness after quenching. In heat-treated steels it frequently

is responsible for improved yield ratio and ductility, but whether this is

a direct effect or a by-product of its effect on hardenability is impossible

to judge at present.

QUESTIONS

1. Explain the difference between "hardenability" and "maximum hardness"of a steel.

2. Why is the "maximum hardness" of a steel less than the hardness usually

obtained in commercial quenching for a given carbon content?

3. Compare the hardness at the center of a 5 in. round of S.A.E. 4140 withthe hardness at the center of a 1 in. round of S.A.E. 1045 when both are

quenched in water.

4. How does the "delay time" vary with temperature in the isothermal trans-

formation of austenite?

5. Define "critical cooling rate."

6. What is the effect of a variation of grain size on hardenability?

7. What is the effect of a variation in the homogeniety of the austenite onformation diagram and hardenability?

8. How does deoxidation practice influence hardenability?

9. What is the relationship between the "C-curves" of an isothermal trans-

formation diagram and hardenability?

10. Discuss several methods of determining hardenability.

Hardenability 363

11. What is the hardness at the % radius of a 2 in. round of S.A.E. 4640

quenched in oil?

12. If a steel specification is based on a hardness of 50 Re for an equivalent

Jominy distance of six-sixteenths, which of the steels in Fig. 17.6 would

be acceptable?

13. What are the advantages and disadvantages of the Jominy hardenability

test?

14. What are "hardenability bands" and "hardenability steels?"

15. What are the advantages and disadvantages of "slack quenching?"

16. Define "ideal critical diameter" and "ideal quench."

17. What is the effect of a proeutectoid phase on hardenability?

18. Calculate the ideal critical diameter for the following composition:

C = 0.40 Grain size No. 5

Mn = 0.60

Si = 0.30

19. Calculate the ideal critical diameter for the following steel:

C = 0.40 Grain size No. 8

Mn = 0.60 Cr = 0.50

Si = 0.30 Mo = 0.20

20. What is the effect of boron on hardenability?

CHAPTERSpecial Purpose Steels

18 Frederick Leo Coonan, D.Sc, Professor and Chairman,

Department of Metallurgy and Chemistry, U. S. Naval

Postgraduate School, Monterey, California

Jules Washington Lindau, III, M.E., Associate Professor

of Mechanical Engineering, University of South

Carolina, Columbia, South Carolina

oOME types of service in industry require

unusual properties in metals. Many of the alloys discussed in the previous

chapters are not entirely satisfactory for these specialized requirements;

many alloys that are satisfactory do not conveniently fall into the estab-

lished patterns developed in the earlier chapters. Included in this group

are alloys particularly suitable for (1) tools and dies, (2) corrosion andheat resistance, (3) magnetic applications, (4) wear resistance, (5) low

temperature, and (6) ultra high-strength service.

The alloys in two of these categories will be treated in separate chapters.

Chapter 19 is concerned with tools and dies, and Chapter 21 with wearresistance. Most of the discussion here will be concerned with steels

developed for resistance to corrosion and, to a lesser extent, with alloys

utilized in service involving magnetic effects.

18.1. Classes of Stainless Steels

The element chromium combines readily with oxygen forming anoxide that is thin, tightly adherent, self-healing, and that protects the

metal itself from further attack. When dissolved in iron, it forms an alloy

that has similar stainless and oxidation-resisting properties.

The chromium bearing steels may be divided into three classes: (1)

special purpose steels containing 3.5 to 10 per cent chromium with or

without 0.5 to 4 per cent silicon, nickel, molybdenum, or tungsten as addi-

tional alloying elements; (2) corrosion- and heat-resisting steels con-

taining more than 10.0 per cent chromium and not more than 2 per cent

364

Special Purpose Steels 365

of another alloying element; and (3) corrosion- and heat-resisting steels

containing more than 10 per cent chromium and more than 2 per cent of

one or more other alloying metals.

Chromium steels containing 3.5 to 10% chromium have better cor-

rosion and heat resistance than the S.A.E. low-alloy steels containing

chromium (see Fig. 18.3) and have excellent properties at elevated tem-

peratures. Steel with about 0.10 per cent carbon, 4 to 6 per cent chro-

mium, 0.40 to 0.75 per cent silicon, and about 0.50 per cent molybdenum

is used widely in oil-refining equipment. Another steel in this class is

silchrome, a material commonly used for automobile- and airplane-engine

valves and containing about 0.50 per cent carbon, 7 to 10 per cent

chromium, and 1.00 to 3.75 per cent silicon. Silchrome is resistant to

attack by the exhaust gases of internal-combustion engines and has satis-

factory strength and hardness at a dull red heat. Moreover, it costs

considerably less than the highly alloyed valve materials containing

nickel, chromium, and silicon.

The most important high-alloy steels in which chromium is the only

alloying element in large amounts are those included in class 2. These

may be divided into four groups:

2/1. Low-carbon, high<hromium steel—usually called stainless iron—

containing 12 to 18 per cent chromium and less than about 0.14 per cent

carbon.

2B. Cutlery steels containing 12 to 16 per cent chromium and 0.20 to

0.60 per cent carbon.

2C. Tool and die steels containing 12 to 18 percent chromium and

0.70 to 2 per cent carbon.

2D. Heat-resisting steels containing 20 to 30 per cent chromium, with

less than 0.50 per cent carbon for hot-worked material, and with 0.50

to 2.5 per cent carbon for castings.

Small amounts—generally less than 2 per cent—of nickel or other metals

are occasionally added to the plain chromium steels. With the exception,

however, of 0.15 to 0.50 per cent sulphur to improve machinability, the

addition of small percentages of a second alloying element is not common.

The principal corrosion- and heat-resisting steel of class 3 is the fa-

miliar low-carbon 18 per cent chromium, 8 per cent nickel alloy known

to metallurgists and engineers as 18—8. This is a very important high-

alloy engineering steel. Despite the cost, the amount of 18—8 used per

year has increased enormously. Most of the 18—8 steels contain no

other alloying element. A few that must have exceptional stability at

elevated temperatures contain larger percentages of chromium nickel,


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chromium phase diagram. (Kinzel and Crafts)

plus cobalt, tungsten, or molyb-

denum, and some contain ti-

tanium or columbium, added

to stabilize the carbon and pre-

vent intergranular corrosion.

This is discussed later.

In the past several years newmanganese bearing and disper-

sion hardening types of austeni-

tic steels have been developed.

These will also be discussed in

a later section of this chapter.

! 18.2. Constitution of High-

Chromium Steels

Chromium in amounts less

than 30 per cent dissolves com-

pletely in carbonless iron. It

has marked effects on the tem-

perature of the /f 3 transforma-

tion, where alpha changes to

gamma iron, or vice versa, andon A 4 , where gamma changes

to the high-temperature modi-

fication of alpha (delta) iron,

or vice versa. These effects are

important in commercial high-

chromium steels and should re-

receive some attention.

A portion of the iron-chromium diagram for alloys containing less than50 per cent chromium is shown in Fig. 18.1.+ From this diagram it is

evident that adding 8 per cent chromium to carbonless iron lowers the

At point slightly and lowers the A 4 point nearly 275°F. (15()°C.) . Further

additions of chromium raise /J., and at the same time lower A 4 until, withthe addition of about 12 per cent chromium, these two points merge. Theresult is what is known as the gamma loop and is a characteristic of

several binary alloy systems of which iron is one component.The significance of the gamma loop is plain. If an alloy containing 10

per cent chromium and 90 per cent iron is heated slowly, no change in

+ A. B. Kinzel and Walter Crafts, The Alloys of Iron and Chromium, Vol. I, Low-Chromium Alloys, McGraw-Hill Book Company, Inc., New York, 1937. p. 43.


structure occurs until a temperature of 1650°F. (900°C.) is reached.

Here the alpha solid solutions changes to gamma solid solution. If heating

is continued, the gamma remains unchanged until a temperature of

2350°F. (1200°C.) is reached, where it changes to alpha (delta) solid

solution. On slow cooling, the reverse changes take place. If, now, an

alloy containing 15 per cent chromium and 85 per cent iron is heated,

no change (except grain growth) takes place between room temperature

and the melting point, and, conversely, no structural change occurs in

cooling. Alloys containing more than 12 per cent (and less than 30 per

cent) chromium and very little or no carbon have no allotropic trans-

formations.

The concentration limits of the gamma loop are changed by carbon,

which moves the extreme limit of the loop to the right and at the same

time increases the width of the duplex alpha-plus-gamma field. Thus,

the loop, which with negligible carbon extends to 12 per cent chromium,

extends to about 18 per cent chromium with 0.40 per cent carbon. These

relationships are complicated further by the carbides which are formed

by chromium and which together with iron carbide (Fe3C) form

(Cr.Fc) 7C3 and (Cr.Fe) 4C in the commercial high-chromium steels. Theiron-chromium carbides are partly soluble in the gamma solid solution

but not in the alpha solid solution and may be retained in supersaturated

solid solution by quenching, in a manner analogous to the retention of

iron carbide in the gamma iron of unalloyed high-carbon steel.

18.3. Relation of the Constitution of High-Chromium Steels to

Their Heat Treatment

From the foregoing discussion it is evident that we should know from

the composition whether high-chromium steels will or will not respond

to heat treatment. Thus, a steel containing 12 to 18 per cent chromium

and less than 0.13 per cent carbon (class 2A, § 18.1) will or will not

harden depending upon the relative amounts of chromium and carbon.

If the steel contains 0.12 per cent carbon and 13 per cent chromium,

some austenite is formed if the steel is heated to 180()°F. (980°C.) , and

this transforms to martensite upon quenching. If the steel contains 0.06

per cent carbon and 16 per cent chromium, no phase changes take place,

and the steel does not harden when quenched from 1800°F. (980°C).

Low-carbon high-chromium steels in which no phase changes take place

remain ferritic in structure at all temperatures. They are relatively soft

and ductile, and the only change caused by heating is grain growth

which reduces the ductility.


Fig. 18.2. Structure of (A) oil-quenched cutlery steel containing 0.30 per cent

carbon and 12.7 per cent chromium, and (B) oil-quenched high-carbon chromiumsteel containing 0.70 per cent carbon and 17.5 per cent chromium; etched. 500 X-(Kinzel and Forgeng)

Cutlery steels containing 12 to 16 per cent chromium and 0.20 to 0.60

per cent carbon (class 2B, § 18.1) undergo a phase change: when they

are heated to about 1800°F. (980°C.) , the alpha solid solution changes

to gamma and the carbide goes into solution. These steels respond to

heat treatment very much like unalloyed high-carbon steels. In a steel

containing 12 per cent chromium the structure is entirely pearlitic with

only 0.3 per cent carbon when the steel is cooled slowly; it is, conse-

quently, martensitic when the steel is cooled rapidly (Fig. 18.2) .* These

cutlery steels differ from unalloyed steel containing 0.80 to 0.85 per cent

carbon chiefly in that the chromium lowers the critical cooling rate, that

is, they become as hard when quenched in oil, or even in air, as an un-

alloyed high-carbon steel when quenched in water.

Since steels containing 12 to 14 per cent chromium and 0.30 to 0.40

per cent carbon correspond to unalloyed "eutectoid" steels, it follows

that high-chromium steels containing about 1 per cent carbon (class 2C,

§ 18.1) are hypereutectoid and, when quenched and tempered, have

numerous excess carbide particles in the microstructure (Fig. 18.2B)

,

which increase the wear resistance and make the steels useful for tools

and dies.

Courtesy of A. B. Kinzel and W. D. Forgeng.


The heat-resisting steels containing 20 to 30 per cent chromium (class

2D, § 18.1) may or may not respond to heat treatment, depending upon

the carbon content. The carbon range for this class is 0.5 per cent or less

for steels which are hot or cold worked, and from 0.5 to 2.5 per cent

for those which are used in the form of castings. At room temperature

the steels containing about 25 per cent chromium and less than 0.70 per

cent carbon are ferritic; their structure consists of alpha solid solution

and the carbide (Cr,Fe) 4C, and no phase changes take place in heating.

These steels cannot be hardened; they can, of course, be annealed if

cold worked.

The higher carbon grades respond to quenching, but this treatment

is seldom used as no advantage results. High-carbon castings containing

20 to 30 per cent chromium are very hard as cast and must be annealed

if machining is necessary.

The price of ferrochromium (70 per cent chromium) used in the manu-

facture of high-chromium steel varies inversely with the carbon per-

centage. Ferrochromium containing less than 0.5 per cent carbon, which

must be used to produce high-chromium steels with low carbon per-

centages, is much more costly than ferrochromium containing 4 to 6 per

cent carbon. The economic factor is, therefore, important in selecting a

high-chromium steel for a certain application; the advantages resulting

from the use of a steel containing less than 0.12 per cent carbon, instead

of one containing 0.40 per cent, must be balanced against the higher

cost of this material.

18.4. Mechanical Properties of High-Chromium Steels

As remarked in a previous chapter (§16.4), chromium has little

solid-solution effect, that is, when dissolved in alpha iron, it strengthens

the iron relatively little. This is shown by the data in Table 18.1 for

annealed steels. If the composition is such that the steel undergoes a

phase change, the principal effect of chromium is to increase the harden-

ability, thus producing marked changes in properties with relatively

slow cooling rates.

The tensile properties of high-chromium steels cooled in air or oil

depend obviously upon how much carbon is present and upon what

proportion of the carbides dissolves in the gamma solid solution at high

temperatures. Some typical values for tensile properties of low-carbon

12 to 15 per cent chromium steel and for cutlery steel are given in Table

18.2. The high-chromium steels are more resistant to tempering than

carbon and low-alloy steels. Increasing the tempering temperature has

little effect on the properties until a temperature of 950 to 1050°F. (510


to 565°C.) is reached when there is a sudden decrease in tensile strength,

yield strength, and hardness and a moderate increase in ductility.

The impact resistance of the quenched and tempered low-carbon andmedium-carbon (cutlery) steels is in general satisfactory, except whenthe steels are tempered at, or cooled slowly through, the temperature

range 875 to 1000°F. (470 to 540°C.) . For reasons yet unknown this

treatment causes brittleness, which is manifested by very low notched-

bar impact resistance. The endurance limit of both the hardenable andthe unhardenable high-chromium steels is about one half of the tensile

strength or approximately the same as for carbon and low-alloy steels.

The modulus of elasticity, however, seems to be somewhat higher; most

available data indicate that it is between 31 and 32 million lb. per sq. in.

18.5. Corrosion and Oxidation Resistance of High-Chromium Steels

Although high-chromium steels are more resistant to certain kinds of

corrosion than any other ferrous material, they are not corrosion re-

sistant in the broadest sense of the term. Neither are they always stain-

less. In fact, to some forms of attack, they are no more resistant than the

unalloyed carbon steels.

Steels containing 10 per cent or more chromium are resistant to oxidiz-

ing mediums and to corrosion caused by fruit juices, food products, and

beverages. The low-carbon 12 to 18 per cent chromium steels are re-

sistant to a large number of organic acids and to all concentrations of

nitric acid, except 65 per cent boiling nitric acid. They are also resistant

to strong alkalies and to many salt solutions but are readily attacked by

chlorides and other halides. Sulphuric acid, except when concentrated

and cold or when containing nitric acid, and hydrochloric acid of all

concentrations attack these steels readily.

In general, corrosion resistance is a function of the amount of chro-

mium dissolved in the iron, but the relation is not direct. Corrosion re-

sistance is not important with low percentages of chromium; it becomes

more pronounced as the chromium increases from 4 to 10 per cent and

increases very rapidly with slightly more than 10 per cent. The corrosion

resistance is thought to be due to the formation of a thin, tight, adherent

film of chromium-iron oxide which, if broken, is self-healing unless the

corrosive agent (as, for example, hydrochloric acid or chlorides) dis-

solves the film as rapidly as it is formed. It is generally assumed that in

order to produce a self-healing film there should be at least one atomof chromium to seven atoms of iron. Increasing the chromium from 10 to

30 per cent increases the corrosion resistance, but the increase is not

proportional to that of the chromium.


Table 18.1. Effect of Chromium on the Tensile Properties of Annealed Steels

Containing 0.10 Per Cent Carbon*

Chromium, Tensile strength, Yield strength, Elongation in Brinell

per cent lb. per sq. in. lb. per sq. in. 2 in., per cent hardness

50,000 38,000 44 90

5 65,000 26,000 38 135

12 72,000 35,000 36 140

15.5 75,500 42,500 33 148

17 80,000 47,000 30 155

27 84,500 55,000 26 160

• Based on E. E. Thum, Metal Progress, v. 29, No. 6, 1936, pp. 49-57. 104.

TABU 18.2. Typical Tensile Properties of High-Chromium Steels*

Composition, Quenched and Tensile Yield Reduc-

per cent tempered at strength, strength, Elongation tion of

lb. per

sq. in.

lb. per

sq. in.

in 2 in., area,

C Cr °F. °C. per cent per cent

400 205 146,000 135,000 15 38

600 315 145,000 134,000 15 40

0.09 12.2 800 425 143,000 134,000 16 46

1000 540 135,000 120,000 20 52

1200 650 95,000 79,000 29 60

400 205 170,000 160,000 8 18

600 315 168,000 160,000 10 20

0.09 800 425 166,000 160,000 15 45

1000 540 160,000 148,000 18 46

1200 650 90,000 83,000 21 50

600 315 240,000 208,000 3 5

800 425 245,000 210,000 5 20

0.30 13.0 1000 540 250,000 195,000 7 20

1200 650 160,000 138,000 12 38

1400 760 140,000 105,000 14

i

45

• Based on data by 1). J. Giles, The Book of Stainless Steels, American Society

for Metals, Cleveland, 1935, pp. 267-276; and O. K. Parmiter, Trans. Am. Soc. Steel

Treat., v. 6, 1924, pp. 315-340.


Carbides, if not in solution, have a deleterous effect on corrosion re-

sistance. For this reason, the cutlery steels should be heat treated if op-

timum corrosion and stain resistance is desired. The alloys containing

carbon in excess of that which dissolves in the gamma solid solution at

high temperatures are not so resistant to oxidizing mediums as the lower

carbon steels.

The high-chromium steels rust slowly when exposed to the atmosphere,

but the thin coating can readily be wiped off, leaving the surface under-

neath unaffected. Steels containing 12 to 14 per cent chromium and less

than 0.12 per cent carbon are, therefore, widely used for automobile and

building trim. In industrial at-

mospheres, especially those con-

taining sulphur, the high-chro-

mium steels may pit badly.

The resistance of the high-

chromium steels to oxidation at

elevated temperatures increases

with the chromium but, as

shown in Fig. 18.3, the increase is

not important until more than

10 per cent chromium is present.

With higher chromium percent-

ages the increase in oxidation re-

sistance is fairly uniform until

about 20 per cent chromium is

present, when the resistance is

very high. The data shown in

Fig. 18.3 were obtained* by heat-

ing 0.5-in. cubes with ground sur-

faces for 48 hr. at 1830°F.

(100()°C.) . With the exception

of a few iron-nickel-chromium

and iron-chromium-aluminum

alloys, steel containing 20 to 35 per cent chromium is the most resistant

of the ferrous alloys to high-temperature oxidation.

45

40

35

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Fie. 18.3. Effect of chromium on the re-

sistance of chromium steels to oxidation at

1830°F. (lOOO-C). (MacQuigg)

18.6. The Constitution of 18-8

Nickel (as well as chromium) is soluble in both alpha and gammairon. When in solution it (1) lowers the A3 point, where alpha iron

• C. E. MacQuigg. The Book oj Stainless Steels, American Society for Metals. Cleve-

land, 1935, pp. 351-368.


j-2800uJ

-2400 i-2000 u-1600"°

-1200 J-800 L

•2800 v

-2400 g-2000 «

5 10 150 5 10 150 5 10 15

N ickel .per cent

Fig. 18.4.

4 born)

Effect of chromium on phase changes in iron-nickel alloys. (Bain and

transforms to gamma on heating and where gamma transforms to alpha

on cooling, and (2) makes these allotropic changes, especially the one

on cooling, very sluggish. A small portion of the iron-nickel phase dia-

gram is shown in the upper left corner in Fig. 18.4.+ As noted on a pre-

vious page (p. 366) , chromium restricts the temperature-concentration

region where the gamma solid solution is stable, that is, it produces a

gamma loop; nickel, on the contrary, increases the limits of this area.

When chromium is added to the iron-nickel solid solution it dissolves.

The first effect is a lowering of the delta- (alpha-) gamma transforma-

tion temperature and a widening of the field where delta (alpha) is

stable. This is apparent from the sections of Fig. 18.4 showing phase

relations for alloys containing S and 6 per cent chromium. As the chro-

mium is increased, the area of the delta (alpha) region is increased

f E. C. Bain and R. H. Aborn, in Metals Handbook, American Society for Metals,

Cleveland, 1939, pp. 418-422.


further, and with 9 per cent or more the low-temperature area wherealpha is stable is also increased. Increasing the chromium thus movesthe boundaries of the gamma and the alpha-plus-gamma phase regions

to the right.

Consider now the small diagram at the lower left in Fig. 18.4 for 18

per cent chromium: if 3 per cent nickel is added and the alloy is heated,

no phase changes take place at any temperature. If, however, 8 per cent

nickel is present and the alloy is heated, a phase change occurs at about

660°F. (350°C.) , and some of the alpha solid solution changes to gammasolid solution. If the alloy is heated still further, this allotropic changecontinues until at 1200°F. (650° C.) all the alpha has changed to gamma.There is no further change if the heating is continued almost to the melt-

ing point. Now, if the alloy is cooled, the reverse changes should take

place, that is, gamma should start to transform to alpha at 1200°F. andshould be completely transformed at 660°F. Owing, however, to the

sluggishness of the reactions, only a small amount of alpha is actually

formed in slow cooling, and it is relatively easy to suppress by rapid

cooling the formation of any and thus to obtain at room temperature analloy that is entirely austenitic.

Austenitic steels are strong and ductile and are readily deformed cold

into a variety of useful products. For a combination of maximum re-

sistance to corrosion by certain mediums, austenitic structure at roomtemperature, and other useful properties the most economical composition

is 18 per cent chromium and 8 per cent nickel, with carbon as low as

is commercially possible. With this composition it is advisable to guardagainst the presence of any alpha by heating the material to 1830 to

2010°F. (1000 to 1100°C.) and quenching it in water or in an air blast.

Increasing the nickel to 10 per cent or more increases the stability of

the austenite but also increases the cost.

18.7. The Role of Carbon in 18-8

If 18—8 could be made free from carbon, many of the troubles of the

manufacturers and users of this steel would be over. Unfortunately, how-ever, this is difficult, and even if made by the best commercial prac-

tice, 18—8 contains 0.06 and frequently as much as 0.12 per cent of this

element. • Unfortunately also, only about 0.01 or 0.02 per cent carbonis soluble in the iron-chromium-nickel austenite at room temperature.

This solubility increases with increasing temperature as shown in Fig.

• Carnegie- Illinois Steel Corporation has produced 18-8 with carbon well under0.06 per cent by feeding oxygen into the molten bath during refining. See Electric

Furnace Steel Proceedings, Am. Inst. Mining Met. Engrs., v. 3, 1945, pp. 107, 108, 127.


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nickel austenite. (Kinzel and Franks)

Fig. 18.6. Structure of (A) quenched 18-8 and (B) quenched and reheated 18-8

showing carbide precipitation at grain boundaries; etched. 250 X- (Kinzel and Forgeng)


18.5,f until at 1830°F. (1000°C.) 0.14 per cent, and at 2010°F. (1 100°C.)

0.22 per cent is soluble. One advantage of quenching 18—8 is that the

carbon is held in supersaturated solution at room temperature. This

carbon, however, will precipitate if conditions are favorable. Severe

cold working may cause some precipitation; in addition—and this is mostimportant—on heating to a high temperature, as in welding or in service

in superheaters or other equipment operating at elevated temperatures,

some carbon precipitates from solution.

If this were the only thing that happened, it would not be serious.

However, when the carbon precipitates, it is apparently thrown out of

solution at the grain boundaries as a chromium carbide, thus impover-

ishing the austenite grains adjacent to the boundaries in chromium andmaking them susceptible to corrosion. The usual structure of quenched18—8 showing the polyhedral grains of austenite is pictured in Fig.

18.6A,* the carbides which have precipitated at the grain boundaries

are shown in Fig. 18.6B.

Carbide precipitation occurs frequently if 18—8 is heated in the range

570 to 1470°F. (300 to 800°C.) , making the material very susceptible

to a form of attack known as intergranular corrosion or intergranular

disintegration. It is very hard to detect and was the cause of a numberof disastrous failures before a cure was found.

Carbide precipitation can be prevented by reducing the carbon to

0.02 per cent or less, which is not easily attained commercially, and it

can be reduced innocuous by adding titanium equivalent to 5 times the

carbon content, or columbium equivalent to 10 times the carbon content,

to the steel when it is made. Both of these elements combine with the

carbon to form a stable carbide and prevent the carbon from combining

with the chromium. When 18—8 is "stabilized" with titanium or colum-

bium it can be used at elevated temperatures or can be welded without

danger of premature failure.

18.8. Properties of 18-8

The austenitic chromium-nickel steels can readily be cold rolled into

sheet or strip or cold drawn into wire. They can also be riveted, soldered,

or welded. Welding is now commonly used for fabricating 18—8. Because

of its high electric resistivity and low thermal conductivity, spot welding

has been especially successful. This method of joining is used in the

f A.. B. Kinzcl and R. Franks, The Alloys of Iron and Chromium, Vol. II, High-Chromium Alloys, McGraw-Hill Book Company, Inc., New York, 1940, p. 275.

• Courtesy of A. B. Kinzel and W. D. Forgeng.


construction of the stainless-steel streamlined trains which have attracted

much attention recently.

Austenitic chromium-nickel steel has, as quenched, a fairly high tensile

strength with high ductility and resistance to impact (Table 18.3). It

hardens rapidly by cold work (Table 18.3) . The increase in tensile and

in yield strength is greater and the decrease in ductility is considerably

less than that caused by comparable cold working of low-carbon steel.

Carbon increases the strength (Table 18.3) . The endurance ratio of

quenched 18—8 is 0.40 to 0.45, that is, somewhat lower than the usual ratio

for carbon and low-alloy steels. Peculiarly, the endurance limit is usually

higher than the proportional limit.

Table 18.3. Typical Mechanical Properties of Austenitic Chromium-Nickel Steel

Composition,per cent Tensile Yield Elonga- Reduc-

Condition of specimen strength,

lb. per

strength,

lb. per

tion in

2 in.,

tion of

area,

Izod

impact.

C Ct Ni sq. in. sq. in. per cent per cent ft-Ib.

0.16 18.0 8.0 Water-quenched bars 90,000 37,500 68 78 Ill

0.17 18.0 8.0 Watcr-qucnchcd bars 96,500 40,000 65 75 103

0.07 17.9 8.5 Cold-rolled bars, 15% 142,700 117,800 30 61 88

0.07 17.9 8.5 Cold-rolled bars, 40% 227,600 201,000 11 43 17

0.07 18.0 9.5 Cold-drawn wire, 60% 263,000 4

0.16 16.0 10.2 Cold-drawn wire, 85% 273,000 3

0.07 18.0 8.0 Water-quenched sheet 86,400 33,500 63

0.16 18.0 8.0 Watcr-qucnchcd sheet 93,300 46,600 57

0.06 18.0 14.0 Watcr-qucnchcd bars 80,000 63 75

0.14 18.0 14.0 Water-quenched bars 90,000 53 70

0.14 25.0 20.0 Watcr-qucnchcd bars 90,500 50,000 47 63 90

The coefficient of expansion of 18-8 is about 50 per cent higher than

that of carbon steels, and the thermal conductivity at normal temperatures

is about 0.04 cal. per sec. per sq. cm. as compared with 0.16 cal. per

sec. per sq. cm. for low-carbon steel. The stress-strain curve of 18—8 is

curved from the origin. This steel has, therefore, a low proportional

limit, no yield point, and no true modulus of elasticity. The secant

modulus varies from 26 to 28 million lb. per sq. in. The austenitic chro-

mium-nickel steels have exceptionally high resistance to creep. This is

discussed in a later chapter.

Nickel increases the resistance of low-carbon 18 per cent chromium

steels to corrosion by nonoxidi/.ing mediums, permitting these materials

to be used economically in environments where the high-chromium steels

containing no nickel would be attacked rapidly. These austenitic steels


are resistant to many organic acids, organic solvents, and oils whichattack the high-chromium steels. Their resistance to staining in the at-

mosphere is also higher than that of the plain chromium steels. Theiroxidation resistance is high, especially at 1650°F. (900°C.) , and if the

chromium and nickel are both increased, they may, if not highly stressed,

be used at temperatures as high as 2010°F. (1100°C). The corrosion

and oxidation resistance and the strength and stability at high tempera-

ture can be increased by the addition of silicon, tungsten, molybdenum,cobalt, or columbium.

18.9. Recent Developments in Stainless Steels

There are a number of elements—for example, carbon, nitrogen, andtitanium—that affect the phase boundaries of the 18 per cent chromiumsection of the ternary iron-chromium-nickel diagram. As a result, the

18—8 alloy may have a duplex structure of ferrite and austenitc unless

precautions are taken to prevent it. It is also evident that lowering the

nickel slightly (see the 18 per cent chromium section in Fig. 18.4) will

make it impossible to produce a structure consisting entirely of austenite

even by quenching. In fact, the 18—8 composition is so critical that somemetallurgists believe that with very low carbon and 8 per cent or slightly

less nickel, an austenitic structure is possible chiefly because the small

percentage of nitrogen introduced during metling prevents normal phase

changes. In practice, an austenitic structure is assured by rapid cooling

(Sect. 18.6) and by keeping the nickel well over 8 per cent, especially if

the carbon is less than about 0.08 per cent.

The ferrite chromium-nickel steels, which are magnetic, are muchstronger and less ductile than the austenitic material. They have not

been used industrially, as their properties, including corrosion resistance,

are not enough better than those of low-carbon 16 to 18 per cent chro-

mium alloys( which are also ferritic) to justify the much higher cost.

One of the disadvantages of the austenitic chromium-nickel steels is

that high strength can be attained only by cold working—heat treatment

is used solely to ensure the proper structure. This limits the application

of such cold-worked material as sheet, tubes, strip, and wire products.

Some years ago, Smith, Wyche, and Gorr* discovered that by adding0.50 to 1.00 per cent titanium, which is a ferrite former, to a steel contain-

ing about 0.07 per cent carbon, 17 per cent chromium, and 7 per cent

nickel a ferritic structure can be readily produced; this structure can

• R. Smith, E. H. Wyche, and W. W. Gorr. Trans. Am. Inst. Mining Met. Encrs.,v. 1G7, 1946, pp. 313-345.


be hardened by precipitation of a carbide that is retained in super-

saturated solution in the ferrite matrix by a suitable heat treatment.

The heat treatment consists of heating the alloy to temperatures be-

tween about 1400 and 2000°F. (760 and 1095°C.) to produce a homo-

geneous austenitic structure, followed by cooling to room temperature

(at any rate faster than slow furnace cooling) . The alloy is then reheated

to temperatures between 950 and 1050°F. (510 and 565°C.) to precipi-

tate the hardening constituent.

This steel, known as stainless W, has many desirable properties. Repre-

sentative mechanical properties are given in Table 18.5. In addition to

the high tensile strength, it has a high yield strength and a definite

modulus of elasticity, which is about 28 million lb. per sq. in. The en-

durance limit on polished specimens varies from 80,000 to 90,000 lb. per

sq. in., depending on the treatment; for specimens of sheet having a

normal rolled surface it is about 30 per cent of the tensile strength.

Stainless W has much lower ductility than the austenitic material,

which in view of its strength and structure is not unexpected. The steel

is readily welded. Its corrosion resistance is apparently the same as that of

austenitic 18—8, and it is not subject to intergranular embrittlement.

This steel has been the forerunner of a group developed to meet a need

in industry for austenitic alloys that respond to a hardening heat treat-

ment. Approximate compositions of representative members are given in

the table below.

Table 18.4. Nominal Composition of Prccipitation-Hardenahle Grades

Designation C Cr Ni Other

Stainless W17-4PH

0.07

0.0417

16.5

7

4

0.70Ti, 0.20A1

4.0Cu, 0.35Cb + Ta

17-7PHAM350

0.07

0.07

17

17

7

4

I.IOAI

2.75Mo

17-10P17-14 Cu-MoHNM

0.120.12

0.30

17

16

19

10.2514

9.5

0.25P2.5Mo, 3Cu, 0.45Cb, 0.25Ti

3.5Mn, 0.30P

Data from Roach and Hall, Materials and Methods, April 1956, p. 144.

The steels, except Am350, noted in the table, contain elements forming

compounds that are soluble in austenite at elevated temperatures but

that may be retained in a supersaturated condition in the phase existing


Table 18.5. Typical Room-Temperature Mechanical Properties of Precipitation-Hardcnable Grades

Ten Yld. Elong. Hard- Charpy Endur.Str. Str. in ness, Im- Limit

Designation Condition 1000 1000 2 in. Rock- pact 1000psi psi % well C Ft-lb psi

Stainless W Annealed 135 95 7 25 _Aged 'A hr. at 950° F. 210 195 7 43 — 80Aged Vi hr. at 1050° F. 190 170 8 39 — —

17-4PH Annealed 150 110 12 — — —Aged 1 hr. at 900° F. 195 180 13 43 19 90

17-7PH Annealed 130 40 30 Rb85 ^_ ^_

Treated at 1400° F. 145 100 9 31 — —Aged at 950° F. 215 200 8 45 6 78

AM350 Annealed 186 52 22 Rb94 100 —Subzero cooled 197 110 12 40 — —Tempered at 750° F. 199 159 11 41 51 80

17-10P Annealed 89 37.5 70 Rb82 120'

Aged 137 88 25 30 40' —17-14 Cu-Mo Annealed at 2250° F.

and aged 86 42 45 Rb80 110' —HNM Annealed at 2000° F.

aged at 1200° F. 153 110 26.5 34 18 —


at room temperature. Subsequent heating to temperatures in the vicinity

of 1000°F. will produce a finely dispersed precipitate that is accompanied

by an increase in hardness and strength.

The treatments producing this hardened condition have been described

for stainless W. Alloy 17-4 PH responds in a similar fashion. Mechanical

properties are given in the table. The alloy 17-7 PH on annealing remains

austenitic. It is however unstable and may be converted to martensite on

cold work or by cold treatment. Subsequent heating to about 1000°F. will

produce a precipitate in a state of dispersion that will cause a markedincrease in hardness and strength.

The alloys with appreciably higher nickel, 17-10P, 17-14, HNM, will

remain austenitic on rapid cooling from about 1900°F. and retain disper-

sion hardening compounds in solution, on aging at about 1300°F. the

compounds precipitate. The presence of phosphorus will intensify the

hardening effect.

The continuing expanding use of nickel has led to a shortage of the

available supply of this metal. In an effort to alleviate to some extent this


situation, several low-nickel, high-manganese austenitic stainless steels

have been developed as well as nickel-free, heat-resisting austenitic alloys.

Approximate compositions are given in the table below.

Table 18.6. Composition of New High Manganese Grades

Designation C Mn Cr1

Ni N

A1S1' 201

A1S1' 202

CMN*G-1928

0.15 max.

0.15 max.0.65

0.55/0.65

5.5/7.57.5/10.0

12

8.0/9.0

16.0/18.017.0/19.0

25

21.25/22.75

3.5/5.5 0.25 max.

4.0/6.0 0.25 max.— 0.45— 0.30/0.40

1 Designation of the American Iron & Steel Institute.

2 Crucible Steel Co. of America.8 Allegheny Ludlum Steel Corp.

Data from Roach and Hall, Material/! arts Methods, April 1956, p. 138.

Table 18.7. Typical Tensile Properties of High-Manganese Grades

Designation Form Condition Test

Temp. °F

Ten Str.

1000 psi

Yld. Str.

(0.2%)1000 psi

Elong.

%

A1S1 201 Sheet

Sheet

Annealed

\i HardRoomRoom

115

150

55

105

55

25

A1S1 202 Sheet Annealed Room 105 55 55

CMN Sheet Ann. & Aged 1200 76 — 10

G-192 Bar Hot worked Room 149 84 11


The mechanical properties of types 201 and 202 are essentially the same

as A1S1 301 (17 Cr-7Ni) and A1S1 302. Corrosion tests seem to indicate

that type 201 is comparable in corrosion resistance to 301, and type 202

resembles 304.

18.10. Superstainless Steels

During the early years of World War II, American metallurgists were

faced with an urgent and difficult problem in the metallurgy of

high-alloy steels: to develop a superstainless steel for use in turbosu-


perchargers and gas turbines (jet engines) . In addition to its great

importance for military aircraft and propelled missiles, solution of the

problem would probably have far-reaching peacetime ramifications, be-

cause it has been known for some time that a gas turbine has a high

over-all efficiency for power generation in marine and other forms of

transportation.

The problem was to develop an alloy that would have a relatively long

and useful life as highly stressed wheels, buckets, and other parts of

turbines, when operated at very high temperatures and when exposed

to highly corrosive hot gases. Two methods of attack were used: to pro-

duce a superstainless steel based on the 18—8 analysis—which has excel-

lent elevated-temperature properties—and to develop further existing

nickel-rich and cobalt-rich alloys of the Inconel, Stellite, and Hastelloy

types, which are extremely heat- and corrosion-resistant. Some of the

principal alloys developed and used during the war are given in Table18.8.

Early in World War II, German engineers were ahead of ours in the

design of jet engines, but the lack of suitable alloying elements was re-

sponsible for an average life for these power units of much less than

100 hr. After designs were perfected in the United States, the following

typical requirements for alloys for superchargers were set up: they should

withstand a stress of at least 12,000 lb. per sq. in. at 1500°F. (815°C.)

for at least 1,000 hr. without failure and be resistant to exhaust gases

containing tetraethyl lead. For gas turbines, temperature and stress con-

ditions were somewhat less severe. It is to the credit of American metal-

lurgists that most of the alloys listed in Table 18.8 meet the requirements

satisfactorily.

For industrial gas turbines, economics will be more important than

it was for military aircraft during the war. Some of the elements used

in large quantities during the past three or four years in alloys for turbo-

superchargers and jet engines—cobalt is an example-will be too scarce

or too costly to be used in turbines which must develop power eco-

nomically; further, the life of the alloys must be much longer, up to at

least 100,000 hr., for example, for a marine turbine. Turbine efficiency

increases rapidly with increasing temperalure so that postwar super-

stainless alloys must withstand very high temperatures for long periods

of time. None of the alloys available now will meet this requirement

unless stresses are much below those characteristic of superchargers andjet engines. If high stresses must be withstood at very high temperatures

and for long periods of time, new alloys must be developed. Tt is to be

expected, therefore, that some of the alloys listed in Table 18.8 will


Table 18.8. Chemical Composition of Supcrstainless Steels and

Alloys for Turbosuperchargers and Gas Turbines

Heavy Nonferrous

Alloy

designation

Nominal composition, per cent

C Mn Si ' Cr Ni jCo Mo W Cb Ti

Supcrstainless

steels:

19-9 DL19-9 W-MoS 495

0.30

0.10

0.45

0.45

0.40

1.00

0.60

1.00

1.00

0.60

0.55

0.50

0.50

0.60

0.60

19.00

19.00

14.00

9.00

9.00

20.00

20.00

44.00

20.00

20.00

1.25

0.40

4.00

4.00

4.00

6.25

0.60

3.00

1.35

1.25

1.26

4.00

4.00

4.00

2.25

4.00

1.35

0.40

0.40

4.00

4.00

4.00

4.00

0.60

1.25

1.00

0.30

0.35

S 590 20.00 20.00

20.00 20.00

25.00

20.00

25.00

27.00

4.50

12.00

20.00

24.50

37.00

20.00

1 6-25-6 f 0.08 1.50

0.30 0.60

0.80 16.50

1.00

1.00

1.00

13.00

15.00

15.00

18.50

19.00

20.00

13.00

18.00

25.00

0.40

0.35

0.35

0.10

0.15

0.04

0.03

0.10

1.00

1.25

4.00 0.50

0.75

1.00

0.50

0.70

0.50

0.50

0.50

0.50

0.65

2.00

3.25

3.00 2.00

3.00N 155 MultimctJ..

2.25

Rcfractalloy

25-20-2 Si

3.00 3.00

Heavy nonfer-

rous alloys:

0.15

0.10

0.25

0.40

0.40

0.05

0.15

0.75

0.60

0.60

0.60

0.30

0.75

1.00

0.25

0.60

0.60

0.60

0.25

0.75

1.00

13.00

15.00

27.00

26.00

27.00

16.00

78.00

74.00

2.00

32.00

16.00

65.00

55.00

65.00

35.00

50.00

5.00

5.00

6.00

29.00

17.00 5.00

Inconcl, modified §

.

Stellite 6059

Stellite 30-422

Hastelloy BHastelloy C

* Contains only 4 per cent iron.

t Contains 0.10 per cent nitrogen.

X In development stage, contains 0.15 per cent nitrogen.

§ Contains 0.70 per cent aluminum.

disappear, except for military use, in the next few years, and that other

and more economical or better alloys for industrial gas turbines will take

their place.

The supcrstainless alloys are important almost wholly for their strength

and corrosion resistance at elevated temperatures, and further discussion

is, therefore, deferred to a later chapter. The following data give a gen-

eral idea of the overall improvement in these alloys during the last few

years: The stress which causes failure in 1,000 hr. at 1200°F. (650°C.)

is 1,000 lb. per sq. in. for 3 per cent nickel (S.A.E. 2340) steel; 4,000 lb.


l'i

12

ID

I

O*I0co

S*-, /

//

b°6

1

/1 /

J

"o

£4o

2\J

oC I 9 2() 30 4() 5 ) 60 70 80 90 100

Nickel, per cent

Fie. 18.7. Coefficient of expansion of iron-nickel alloys between and 200° F.

{Marsh)

for the silchrome used before the war; 19,000 lb. for 17W (Table 16.4)

used at the start of the war; 30,000 lb. for gamma Cb used in 1944; and

36,000 lb. for 16-25—6 used at the end of the war.

18.11. High-Nickel Steels and Special Iron-Nickel Alloys

Nickel dissolves in gamma and in alpha iron and has a strong effect

on the position of A 3 , not only lowering it to atmospheric temperature or

below but, in addition, slowing greatly the rate of the phase change and

producing some anomalies in properties which are of great value to in-

dustry. Alloys of iron with 20 to 30 per cent nickel may easily be madenonmagentic (despite the fact that both iron and nickel are magnetic).

Alloys of iron with 30 to 60 per cent nickel have variable and controllable

expansion characteristics which were discovered late in the nineteenth

century in France. The fact that alloys of iron with 45 to 85 per cent

nickel can be treated to produce high magnetic permeability and low

hysteresis loss has led to wide use of some of them in telephone, tele-

graph, and radio equipment.

As shown in. Fig. 18.7,* the coefficient of expansion between and

200° F. (— 18 and 100C C.) decreases to nearly zero as nickel approaches

* Based on data in J. S. Marsh, The Alloys of Iron and Nickel, Vol. I, Special-

Purpose Alloys, McGraw-Hill Book Company, Inc., New York, 1938, p. 160.


36 per cent. Increasing the nickel from 36 to 60 per cent makes it possible

to have any desired coefficient of expansion between 1 and 12 X 10"6 -

As the result, a number of alloys have been developed for use in measur-

ing tapes, watch parts, and parts of machines and instruments which

must remain constant in dimension despite temperature changes. The

use of invar struts in aluminum-alloy pistons is well known. Some of the

commercial alloys with controlled expansion characteristics are given in

Table 18.9.

If chromium and tungsten are added to an alloy containing about 32

per cent nickel, the affect of temperature (in the vicinity of room tem-

perature) on the modulus of elasticity is zero. This alloy is known as

elinvar and is used for hairsprings of watches and for springs of other

precision measuring instruments. The invars and other high-nickel alloys

have the additional advantage that they have excellent corrosion re-

sistance.

The alloys known as permalloy, hipernik, and perminvar are used in

applications where their high or constant permeability produces marked

economics or increased efficiencies in communication. Because under

weak magnetizing forces permalloy has a permeability many times

greater than that of all other ferrous materials except iron of the highest

purity, permalloy is a commercially feasible high-permeability material.

The development of this alloy has made modern long-distance telephony

possible, and its use in the loading of submarine cables has speeded up

the transmission of messages several hundred per cent. Hipernik has

initial and maximum permeabilities much greater than those of com-

mercial grades of high-purity iron and the advantage of relatively high

maximum induction, which makes it valuable for use in radio trans-

formers.

The efficiency of radio loud speakers and other apparatus requiring

powerful permanent magnets has been greatly increased by the recent

development of the iron-nickel-aluminum-cobalt permanent-magnet ma-

terials (see Table 18.9) . In addition to alnico, two Japanese alloys are

noteworthy: the M.K., containing 25 to 30 per cent nickel, 20 per cent

cobalt, and 12 per cent aluminum; and the K.S., containing 10 to 25 per

cent nickel, 15 to 36 per cent cobalt, and 8 to 25 per cent titanium.

18.12. Austenitic Manganese Steel

Of the large number of high-alloy steels that have not been discussed

in this chapter, austenitic manganese steel is of sufficient industrial im-

portance to deserve mention. This steel was discovered a number of years


Table 18.9. Commercial High-Nickel Iron-Nickel Alloys

Composition,* per centName Characteristic properties

Ni Co Cr W Al Cu

36 Low expansions from to

200° F. (-18 to 100° C.)Supcrinvar . . 31 5 Zero expansion near room

temperature32 5 2 Low thcrmoclastic coeffi-

cient

42 For sealing in soft glass

Kovar 29 17 For sealing in hard glassPlatinilc .... 46 Expansion same as platinumPermalloy . . . 78.5 High permeability at low

field strengthsHipernik .... 50 High permeability at higher

field strengthsPerminvar . . 45 25 Constant permeability over

a range of flux densitiesAlnico I . . . . 20 5 12 Permanent magnet of high

magnetic hardnessAlnico II . . . 17 12.5 10 6 Same as aboveAlnico III... 25 12 Same as aboveAlnico IV... 28 5 12 Same as above

* Balance of composition is mostly iron.

ago by Hadfield in England during his pioneer researches on alloys of

iron with manganese, silicon, nickel, chromium, and other elements. Theannual tonnage of high-manganese steel used is small, but, as is the case

with many ferrous materials, production is no criterion of its usefulness

to industry. High-manganese steel contains 1.00 to 1.30 per cent carbon

and 11 to 14 per cent manganese and is made by the basic open-hearth,

Bessemer, or electric process. It may be poured into ingots and rolled into

a variety of sections, or it may be cast in sand molds. Owing to its work-

hardening capacity when cold, it cannot be machined readily; hence,

if shaping is necessary, it usually must be ground. Economically this is

a great disadvantage.

In amounts under 50 per cent, manganese dissolves in alpha andgamma iron. Like nickel it lowers the A3 temperatures where alpha

transforms to gamma in heating and where the reverse change occurs in

cooling, and slows the transformation rate. As noted in a previous chap-

ter, manganese also forms a carbide, (Fe.Mn) 3C, which is closely allied

to Fe8C in structure and properties. Alloys containing 1.00 to 1.30 per


cent carbon and 13 per cent manganese, if cooled very slowly so that all

phase changes take place, contain at room temperature alpha solid solu-

tion plus excess carbide. In the usual cooling after hot working, or in a

mold in the case of a casting, these phase changes, owing to the low

reaction rates, do not take place, and the structure consists of the gamma

solid solution plus, possibly, a little alpha, plus excess carbide as massive

particles or as a network around the grains. With this structure the steel

is hard and brittle. If, however, the steel is heated to 1830 to 2010°F.

(1000 to 1100°C), the carbide goes into solution in the gamma, and

if the steel is now quenched in water, a structure consisting wholly of

austenite with no free carbide is obtained. Unfortunately, tempering to

relieve quenching stresses is impossible because reheating in the tem-

perature range 930 to 1290°F. (500 to 700°C.) or even lower may cause

precipitation of carbide or change of some gamma to alpha and thus

may cause brittlencss.

As water quenched, rolled austenitic manganese steel has the following

properties:

Tensile strength, lb. per sq. in 130,000 to 160,000

Proportional limit, lb. per sq. in 40,000 to 60,000

Elongation in 2 in., per cent 60 to 70

Reduction o£ area, per cent 40 to 60

Brinell hardness 180 to 220

As is characteristic of austenitic alloys, the high-manganese steels have

no well-defined yield point and are nonmagnetic.

High-manganese steel is important industrially chiefly because of its

high abrasion resistance. Although the Brinell hardness of the heat-

treated steel is low, cold working causes a rapid increase to as much as

550 Brinell. As abrasion consists largely of cold working the surface, this

capacity for work hardening has important ramifications. Thus, in steam-

shovel buckets, in crushers, grinders, and other machines, and in rails,

cross-overs, switches, frogs and such sections where pounding, pressure,

or abrasion by rock and sand or by other metals are encountered, high-

manganese steel outwears high-carbon steel by several hundred per cent.

QUESTIONS

1. Name the three classes of high-alloy steels containing chromium as the prin-

cipal alloying element. Give the approximate composition of six commonhigh-chromium steels, and name the class to which they belong.

2. What is the effect of up to about 15 per cent chromium on the transforma-

tion temperatures in high purity iron? Describe the phase changes that take

place if an alloy containing (a) 10 per cent chromium and 90 per cent iron,


and (b) 20 per cent chromium and 80 per cent iron, is heated slowly to

2,500°F. (1,370°C) and cooled again.

3. What is the effect of carbon on the phase changes that take place whenalloys of iron and chromium containing up to about 20 per cent chromiumare heated to a high temperature and cooled again? What effect has carbonon the response of these alloys to heat treatment?

4. What structure would be expected if the following steels were quenchedin oil or water: (a) 0.35 per cent carbon and 14.50 per cent chromium?(b) 0.75 per cent carbon and 16.75 per cent chromium? (c) 0.07 per centcarbon and 15.80 per cent chromium? How does the hardenability of a steel

containing 0.35 per cent carbon and 14 per cent chromium compare withthe hardenability of an unalloyed high-carbon steel?

5. What is the relation between the tensile properties and the carbon contentof heat-treated 14 to 18 per cent chromium steels? Give two peculiaritiesin tempering quenched cutlery steels and the effect of these on tensilestrength and impact resistance. What is the relation between the chromiumcontent and the corrosion (and oxidation) resistance of chromium steels?

What is the effect of carbides on corrosion resistance? To what corrosivemedia are the 12 to 18 per cent chromium steels especially resistant? Why?

6. What is the general effect of nickel on the alpha-gamma transformationsin iron-chromium alloys? What is the effect of adding 5, 8, or 10 per centnickel on phase changes in alloys containing 18 per cent chromium andapproximately 82 per cent iron when they are slowly heated and cooledbetween room temperature and 2,400°F. (1,315°C)? Why are alloys con-taining 18 per cent chromium. 8 per cent nickel, and low carbon usuallyaustenitic at room temperature?

7. What is intergranular corrosion in 18-8? What causes it? What is the under-lying mechanism of the phenomenon? How is it prevented?

8. What are the characteristic mechanical properties of 18-8, and what effect

has heat-treatment on these properties? How does the resistance of 18-8to corrosion and oxidation compare with that of the high chromium steels?

What are fcrritic chromium-nickel steels and what is stainless W?9. What are superstainless steels? What alloying elements in addition to

chromium and nickel are used in these materials? What are the generalcharacteristics of superstainless steels, and what are they used for?

10. Outline the main points of difference between the well-established 18-8

grade of stainless steel and the new group of these materials.

CHAPTER

19

Tool Steels, Die Steels,

and Cemented Hard Carbides

JOSEPH Gi.rland, Sc.D., Assistant Professor, Division of

Engineering, Brown University, Providence, Rhode

Island

Joseph J.Kaufman, Instructor, Metallurgy Department,

Academy of Aeronautics, Flushing, New York

XHE steady, if slow, advance of civilization over

the past ten or twelve thousand years has been largely the result of prog-

ress in the design of tools and in the material from which they are

fashioned: from stone to copper, then to bronze, and then to iron,

and after a long period to carburized iron (steel)-which was the

most important step of all—and finally to high-speed and other alloy-

steel tools and sintered hard carbides, which made possible great reduc-

tions in the cost of finished structures and machines.

All tools can be divided broadly into two general classes: (1) cutting

and shearing tools and (2) forming tools. Requirements differ and should

be considered before the proper steel is selected. Thus, for cutting and

shearing, high hardness, ability to withstand wear, stability of structure

to withstand the heat generated by the friction of the cutting edge

against the work, and a substantial amount of toughness to prevent chip-

ping and breakage are important. For shearing tools, a lack of deforma-

tion or warping in heat treatment is usually also important. Forming

tools consist chiefly of dies, which are used to transfer their impression

to either hot or cold solid metal, or to molten metal. In addition to pos-

sessing the qualities necessary in a cutting or shearing tool, forming

tools must be resistant to distortion in heat treatment and to cracking

caused by sudden changes in temperature during use.

The representative types of tool steels and their principal characteristics

are given in Tables 19.1 and 19.2. The recently adopted AISI-SAE classi-

fication is used. As Table 19.2 indicates, tool steels vary widely in

characteristics.

389


The selection of the proper tool steel for a certain machining operation

is difficult and is a problem in the solution of which the advice of the

tool-steel metallurgist is very helpful.

An accurate evaluation of a steel that is to be used for a specific tool

in a specific machining operation is practically impossible, and as a re-

sult, large quantities of tool steel are, and always have been, purchased

by brand name.

The only reliable performance test of a tool steel is to use it in actual

production and to use enough similar tools of the same steel to make the

results convincing. Large users of tool steel frequently distribute their

yearly or half-yearly requirements among a number of tool-steel manu-facturers and keep accurate records of the performance of each brand.

At the end of this production test the company whose brand makes the

best showing receives 50 per cent or more of the business for the next

period.

19.1. High-Carbon Tool Steels

The steels of class W, Table 19.1, are the cheapest and, despite high

distortion in hardening, low resistance to softening at elevated tempera-

ture, and shallow hardening, the most widely used of the tool steels.

There are four classes, graded according to carbon content as follows:

1. Carbon 0.60 to 0.75 per cent; used for machinery parts, hot-forging

dies, rivet sets, battering tools, large chisels, and set screws.

2. Carbon 0.75 to 0.90 per cent; used for forging dies, boilermakers'

tools, hammers, sledges, mining tools, and miscellaneous blacksmiths'

tools.

3. Carbon 0.90 to 1.10 per cent; used for drills, saws, cutters, taps,

small shear blades and dies, anvils, wood- and stone-working tools of

various kinds, punches, and axes.

4. Carbon 1.10 to 1.40 per cent; used for small drills, taps and lathe

tools, files, cutlery and small edge tools, jewelers' tools and dies, brass-

and copper-working tools, wire-drawing dies, and razors.

The hardness and toughness of the high-carbon tool steels vary with

carbon content; they also vary with heat treatment, that is, with temper-

ing temperature. Since high hardness and considerable toughness are

incompatible in an unalloyed high-carbon steel, it is necessary to sacrifice

part of one or the other depending upon the use for which the tool is

intended. Thus, for razors, which must be very hard and which are not

ordinarily subjected to shock, a 1.20 per cent carbon steel, treated to

give a hardness of 60 to 65 Rockwell C, is used; but for a hammer or a

chisel, which must withstand impact, a steel containing 0.70 to 0.90

Tool Steels, Die Steels, and Cemented Hard Carbides 391

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Tempering femperature.deg.C.

100 300 500

"As 200 400 600 800 1000 I20C

quenched Tempering temperature, deg.F.

Fic. 19.1. Effect of tempering on the hardness of water-quenched high-carbon steel.

(GUI)

per cent carbon, treated to about 55 Rockwell C and with more tough-

ness, will be chosen.

Wear resistance, which usually is not so high in carbon steels as in

some of the alloy steels, is secured by the use of a 1.20 to 1.40 per cent

carbon steel, heat treated so that there will be numerous well-distributed

excess carbide particles in a martensitic matrix. Such a structure (Fig.

19.2) is important for cutting tools and wire-drawing dies. Under the

quenching conditions necessary to obtain high hardness, high-carbon

steels warp considerably. In tools of simple section this may be mini-

mized by quenching in special fixtures; for dies and some tools which

must be finish-machined before heat treatment and for which stability

of dimensions is necessary, special nondeforming alloy steels, which

harden in oil or air, are used.


Fig 19.2. Structure of water-quenched high-carbon tool steel: small carbide particles

in martensitic matrix, etched. 500X-

High-carbon steels, even though of practically the same composition,

may differ in their response to quenching (hardenability). If the steel

has an inherent tendency to become coarse-grained when heated con-

siderably above the transformation temperature, it hardens relatively

deeply and uniformly when quenched but is likely to crack. If it is fine-

grained after such treatment, it is shallow hardening and may have soft

spots on the surface, but it is not so likely to crack in quenching. Un-alloyed high-carbon steels are all shallow hardening compared with tool

steels containing considerable molybdenum or chromium, but the harden-

ability can be controlled to some extent by controlling the grain-growth

tendency during manufacture. The difference in hardenability between

two lots of high-carbon steels of identical chemical composition is shown

in Fig. 19.3.* Bars % in. in diameter were quenched in brine from the

temperatures noted and were not tempered. The fractures of the "fine-

grained" bars are shown at the top, and those of the "coarse-grained"

bars at the bottom. The coarser grain and the greater depth of hardening

for the latter are clearly evident.

Unalloyed high-carbon steels arc apt to be inherently erratic in their

response to heat treatment. Even steels of the same composition, madeby the same manufacturer, may differ in their tendency toward grain

growth and in depth of hardening. To render the response to quenching

more uniform, small percentages of chromium or vanadium are frequently

* Shepherd P-F hardenability test specimens.


Fie. 19.3. Fracture of quenched and untempered high-carbon tool steel. Top,

fine-grained, shallow hardening; bottom, coarse-grained, fairly deep hardening. (Gill)

added to steels of class W. Vanadium inhibits grain growth and chro-

mium increases hardenability; both have a favorable effect which prob-

ably compensates for the increased cost.

As a class, the high-carbon tool steels are characterized by high hard-

ness at the surface and a relatively soft core in all but very small sections.

Owing to this soft core, Gill* rates these steels as tough and resistant to

shock. They are, as previously mentioned, subject to considerable dis-

tortion in quenching—due to the formation of martensite at the surface

and of bainite or pearlite in the center, and to the stresses caused by

differential cooling of surface and center—which means that they are not

suitable for tools of complex or irregular shape.

Small amounts of nickel, chromium, molybdenum, or tungsten are

added to some varieties to increase hardenability and to permit quench-

ing in oil instead of water.

19.2. Low Alloy Tool Steels

Low alloy steels of class L, which include the well-known ball- and

roller-bearing material containing about 1 per cent carbon and 1.2 to 1.5

per cent chromium (S.A.E. 52100) , are used to some extent for dies, rivet

sets, small rolls, and for a few cutting and shock-resisting tools. They are

relatively cheap, at least as compared with some of the highly alloyed

tool steels, and have exceptionally high strength combined with relatively

good toughness. They are also much deeper hardening than the high-

carbon steels. Whether the steels are water or oil hardening depends

chiefly on the manganese content, which in the oil-hardening grades is

0.50 to 0.60 per cent.

• Tool Steels, American Society for Metals, Cleveland, 1944.


The carbon content of these steels depends on the service require-

ments; if toughness is important as in pneumatic hammer pistons or

chisels, it is 0.40 to 0.70 per cent; but if maximum hardness and wear

resistance are necessary as in files, drills, and cutlery, it is between 0.90

and 1.10 per cent. The amount of chromium in the steel depends pri-

marily on the depth of hardening required, which in turn depends on

the size of the tool that is to be treated. Increasing the chromium from

0.50 to 1.50 per cent more than doubles the time available for the steel

to cool past the pearlite nose of the S curve without transforming.

Also in this class are high-carbon variants of some of the S.A.E. en-

gineering steels.

Most of the medium-carbon S.A.E. nickel-chromium or chromium-

molybdenum steels are fairly deep hardening;* increasing the carbon

to 0.60 to 0.70 per cent and maintaining the chromium at well over 1.0

per cent and the molybdenum at 0.30 per cent or more increases the

quenched hardness and the depth of hardening; at the same time the

steel retains considerable toughness. These steels are, therefore, finding

relatively wide use in many machine parts and especially in die blocks

for hot forging. These steels are generally quenched in oil.

19.3. Medium Alloy Tool and Die Steels

Advantage is taken of the effect of manganese or chromium whendissolved in alpha iron of decreasing the critical cooling rate and of in-

creasing deep hardening to produce a valuable and extensively used class

of oil-hardening "nondeforming" tool and die steels (class O) . These

steels are used when the design of the tool or die is such that distortion or

cracking is likely to occur in water quenching. In this group there are

steels containing 0.90 to 1.45 per cent carbon. If the manganese is

1.60 per cent, the steel usually contains no other alloying element

although the addition of 0.10 to 0.25 per cent vanadium to reduce grain

growth is optional. To increase wear resistance the manganese may be

lowered to 1.20 per cent, and about 0.5 per cent chromium plus 0.5 per

cent tungsten may be added; or the manganese may be reduced to 0.25

per cent, and 0.75 per cent chromium and 1.75 per cent tungsten may be

added. These modified steels are not so likely to crack in quenching as

the higher manganese steels, but, on the other hand, they do not becomeso hard. The steels of class O are quenched in oil from 1400 to 1500°F.

(765 to 815°C.) and are tempered at 325 to 500°F. (160 to 260°C.)

.

They are used as stamping dies, thread-rolling dies, dies for moldingplastics, punches, broaches, blanking dies, and especially as master tools

and gages.

• Kor example, the SAE 3100 series and 4100 series.

Tool Steels, Die Sleeb, and Cemented Hard Carbides 397

Silicon-manganese steel (S.A.E. 9255 and 9260) containing 0.50 to 0.60

per cent carbon, 1.80 to 2.20 per cent silicon, and 0.60 to 0.90 per cent

manganese is widely used for springs. Recently it has become popular as

a tool steel, especially for punches, chisels, and shear blades, owing to its

low cost and is fairly high wear resistance and toughness (class S-5)

.

Strictly speaking, silicon-manganese steel is a misnomer, as the amount

of manganese is not greater than that present in many of the S.A.E.

carbon and low-alloy steels. Distortion in quenching is high, but it is

somewhat less than for plain high-carbon steels, and wear resistance is

higher. The steel hardens more deeply than high-carbon steel and is

tougher. To increase deep hardening, 0.20 to 0.40 per cent chromium

or 0.40 to 0.60 per cent molybdenum is occasionally added. Steels of class

S are usually quenched in water (or oil, if enough chromium and

molybdenum are present) from 1575 to 1625°F. (860 to 885°C.) and are

tempered at 350 to 600°F. (175 to 315°C.)

.

The medium-chromium and low-tungsten tool and die steels are fairly

or very deep hardening and, generally, can be hardened with little

distortion. They have higher resistance to wear than unalloyed high-

carbon steels and they have satisfactory toughness.

Steels of class S of low carbon content and therefore of relatively

high toughness, are used for chisels, punches, shear blades, and battering

tools and occasionally for dies. These steels do not have high resistance

to tempering. According to Gill, chisels of this steel have three times the

life of unalloyed chisel steel.

The steels in class A contain 1.0 per cent carbon and 1.0 per cent

molybdenum. Chromium varies from 1 .0 to 5.0 per cent, and manganese

varies inversely with the chromium from 3.0 to about 0.50 per cent. These

steels are air hardening and are widely used for intricate dies that must

maintain their dimensions accurately after heat treatment. They are used

for- blanking and forming dies, rolls, punches, and drawing dies but not

for dies that operate at high temperatures.

19.4. High-Alloy Tool and Die Steels

Dies that are to be used in applications where they are heated to tem-

peratures 600°F. (315°C.) or above are usually rather highly alloyed,

(class H). Steels in these classes are characterized by high hardenability

(the pearlite nose of the S curve of some of these steels is far to the

right of the time axis), very low distortion in heat treatment—which is

natural in view of the fact that most of the steels can be hardened in air

—high hardness at elevated temperatures, and fairly good toughness. Most


Fie. 19.4. Structure of high-chromium die steels (A) forged and annealed, 750X".

and (B) oil quenched and tempered at 400°F. (200°C), 500x: etched. (Kinzel and

Forgeng)

will withstand temperatures of 1 100°F. (595°C.) or even slightly higher.

This is to be expected as some of them approach high-speed steel in

composition and in many characteristics.

The structure pf the high-chromium die steels (class D) as forged and

quenched and tempered is shown in Fig. 19.4. Although, after annealing,

the Brinell hardness of these steels is 200 to 230, they are difficult to

machine. Steels of this type harden deeply with little distortion and do

not soften appreciably when tempered below about 900°F. (480°C.)

.

Their wear resistance is high. Despite high cost and difficulty of ma-

chining, these steels are among the best as dies for forming and trimming,

blanking, thread rolling, and wire drawing, as shear blades, bushings,

and rolls, and as small machine parts where resistance to abrasive wear

is important. They are seldom used as cutting tools.

19.5. High Speed Steels

The majority of today's tool steel cutting tools are made of high speed

steel. The principal types are given in Table 19.3.


Table 19.3. Classification of High-Speed Steel by Chemical Composition

AISI-SAE Type

Nominal chemical composition, per cent

Class

C Mn Si W Cr V Mo Co

T-1 18-4-1 0.700.85

0.80

1.00

0.750,800.80

0.800.85

0.85

0.85

0.851.30

0.85

0.250.25

0.25

0.25

0.25

0.250.25

0.250.25

0.25

0.250.25

0.25

0.25

0.250.250.250.25

0.250.250.250.250.25

0.250.25

0.250.250.25

18.018.0

14.0

18.0

18.0

14.020.0

1.5

2.0

2.0

6.0

5.56.0

4.04.0

4.04.0

4.04.04.5

4.04.0

4.04.04.04.04.0

1.0

2.0

2.03.0

1.0

2.01.5

1.0

2.0

1.3

2.02.04.0

2.0

8.08.0

8.0

8.05.04.5

5.0

T-2 18-4-2

T-7T-3

14-4-2

T-4T-8T-6M-1M-10

M-30M-34M-2

18-4-1 Co14-4-2 Co20-4-2 CoMolybdenum WMolybdenum . .

Mo WCoMo W CoMo W

5.0

5,012.0

5.0

8.0

M-4M-35

Mo WVMo WCo 5.0

The familiar 18-4-1 composition has long been the basic type. But since

World War II, molybdenum is replacing tungsten in these alloys. Today,

80% of all shipments of high speed steel are of types M-1, M-2, and M-10;

whereas type T-1 represents only 10% of all high speed steel being used.

In general, cutting performance increases with tungsten or molyb-

denum. Vanadium is necessary for satisfactory performance and approxi-

mately 1 per cent or more is always present. Chromium is held constant

at about 4 per cent, with carbon varying from 0.50 per cent (if toughness

at the expense of some hardness is desired) to as much as 1.50 per cent.

Cobalt has been used in the regular grades of high-speed steel with con-

siderable success. It increases red hardness and the efficiency of the tool

where temperatures are high as during heavy roughing cuts.

High-speed steel is hard, strong, and brittle at room temperature and

retains its hardness and strength at temperatures up to 1100 to 1200°F.

(595 to 650°C.)

.

The cutting performance of high-speed steel, which is from three to

fifty times that of unalloyed high-carbon steel of the same hardness, is

largely dependent upon the heat treatment. The optimum quenching

temperature depends upon composition, and a variation of ±50°F.

(±30°C.) from the optimum will affect the performance of the tool.


Engineers owe a large debt to two Americans, F. W. Taylor and

Maunsel White, who, while working at Bethlehem Steel Company in

October, 1898, discovered the "weird and novel" heat treatment which

gives high tungsten-chromium steel its property of red hardness. Taylor

and White found that a steel containing 1.14 per cent carbon, 0.18 per

cent manganese, 7.72 per cent tungsten, and 1.83 per cent chromium

performed poorly in rapid machining when heat treated in the usual

way. After considerable investigation the present-day treatment was

developed. This consists of heating the tool slowly to 1500°F. (815°C.)

,

then rapidly to a temperature slightly below the melting temperature—

with modern high-speed steels this is 2200 to 2350°F. (1205 to 1285°C.)

,

which would ruin carbon or low-alloy steel—and cooling it rapidly in

oil, or, sometimes, in air. The tool is then tempered immediately by

reheating to 1000 to 1150°F (540 to 620°C.) . When heat treated in

this way, high-speed steel machines at high speeds and with such heavy

cuts that the point of the tool becomes red hot, without losing any

hardness.

Although considerable work has been done on the constitution of

iron-tungsten-carbon alloys, little is known about the effect of 4 per cent

chromium and 1 to 2 per cent vanadium on these alloys. Studies of

microstructure indicate that high-speed steel when annealed consists of

a relatively soft (probably ferritic) matrix and a large number of par-

ticles of a complex carbide. When the steel is heated to a high tempera-

ture, the matrix changes to austenite and some of the carbides go into

solution. To produce red hardness and satisfactory cutting performance,

the temperature to which the steel is heated must be as high as possible

without causing incipient melting. When the steel is quenched from

this temperature, a structure such as is shown in Fig. 19.5A* results. It

is thought that the polygonal grains of the matrix are composed of

austenite plus untempered martensite. Reheating to 1()50°F. (565°C.)

tempers the martensite slightly so that it etches rapidly (Fig. 19.5B), and

upon cooling from the tempering temperature the retained austenite

transforms to more martensite. The secondary hardening that results

from tempering is shown in Fig. 19.8

The cause of red hardness, the most important property of high-speed

steel, has not yet been explained satisfactorily. It is usually considered

to be related to the high stability of the complex carbide particles and

to the fact that the most favorable size of the particles for stability of

structure is not altered by temperatures as high as 1100°F. (595 °C.)

.

•J. I.. Gregg, The Alloys of iron and Tungsten, McGraw-Hill Book Company, Inc.,

New York, 1934, pp. 294, 295.


&4&&L *3£

,CK-^^^ <8f>*L

FIO. 19.5. Structure of high-speed steel; (A) quenched and (B) quenched and

tempered; etched. 750x- (Gregg)

The most recent development in high-speed steels is the substitution

of molybdenum for most of the tungsten (see Table 19.3).This was

pioneered by Richie* at Watertown Arsenal and by Emmons.t who

developed the steels commercially. Molybdenum has three advantages

in high-speed steel: (1) Since it is twice as effective as tungsten in pro-

ducing red hardness, only 6 to 9 per cent is necessary, as Table 19.3

shows- (2) it is cheaper pound for pound than tungsten; and (3) it is

not a strategic material, as ample supplies of domestic ores are available.

Initial difficulties in the heat treatment of molybdenum grades have been

solved, so that, at present, there is little difference between molybdenum

and tungsten base alloys as to fabrication and tool performance. 1 he in-

creased consumption of molybdenum high-speed steels is due mainly to

their lower price.

Compared to carbon tool steels, the high-speed steels are more expen-

sive and arc not as tough. However, properly used in the right kind

of equipment, the high-speed steels will out-perform tools made from any

other type of steel. It is the only type of steel which will stand up

to the high temperatures generated at the cuttting edge by modern, high-

speed, production machines.

• S. B. Richie, Army Ordnance, V. 11, July, 1930.

fj. V. Emmons, Trans. Am. Soc. Steel Treat., V. 21, 193S, p. 193.


Temperinq temperature , deq.C.SOO 400 soo ' 60

Tempering temperature, deq. F.

Fig. 19.6. Effect of. chromium on the resistance of 0.35 per cent carbon steel to

tempering. (Bain)

19.6. Function of Alloy Additions in Tool and Die Steels

The structure of tool steels consists essentially of a dispersion of hard

carbide particles in a matrix of steel. The purpose of small alloy addi-

tions is to improve the properties of the matrix by increasing the harden-

ability and refining the grain si/e. The most common alloy additions to

low alloy steels are chromium, vanadium and manganese. Chromium in-

creases the depth of hardening and adds some wear-resistance; vanadiumincreases toughness and limits grain growth; manganese permits oil

hardening with much less distortion than water quench. To further re-

duce the likelihood of cracking during cooling, about 1% molybdenummakes a steel air hardened.

Larger amounts of alloy elements, together with increased carboncontents, form hard alloy carbides which are responsible for the superior

hot hardness and wear-resistance of alloy tool steels. The amount and the

dispersion of the carbide particles determine the hardness; the greater

the relative volume of the hard phase, the harder and more brittle the

steel. The principal function of large alloy additions to tool steels is to

increase the amount of the carbide phase.

Tool Steels, Die Steels, and Cemented Hard Carbides

Tempering temperature, deg. C.

403

O 400 600 600 1000 1200

Tempering temperature, deg. F.

Fig. 19.7. Effect of molybdenum on the resistance of 0.35 per cent carbon steel

to tempering. {Bain)

As is evident from Table 19.1, tungsten, molybdenum, and chromium

are the most important alloying elements in medium and high alloy tool

steels, and together with vanadium they are more imi>ortant in high-

speed steel than in the ordinary tool and die steels. All three are soluble

to a considerable amount in gamma and in alpha iron and, more import-

ant, all three form hard and stable carbides. All three, when dissolved

in gamma iron, slow the reaction rate for austenite to transform to pearl-

ite—chromium and molybdenum strongly and tungsten weakly.

The carbides formed by these three metals dissolve to some extent

(depending on the amount of carbon and the temperature) in austenite

and can be retained in supersaturated solution by quenching. This in-

creases hardness and wear resistance. In addition, the carbides in excess

of those dissolved, if favorably distributed, greatly increase the resistance

of the steel to abrasive wear.

In contrast to tungsten, which has a minor cflect on hardenability,

chromium and molybdenum have a powerful effect. This was discussed

in detail in earlier chapters. The most valuable property of tungsten,

molybdenum, and chromium when present in relatively large amounts


Tempering tempcroture.deg. C.

100 200 300 400 500 600'

i

'I ' i

J—

r

1-

48l ~_As J00

quenched

703

500 700 900Tempering femperalure^d

UOO

Fic. 19.8. Effect of tempering on the hardness of high-speed. (Emmons)

in steel containing sufficient carbon is that they make the steel "redhard"; that is, the steel retains its hardness at elevated temperatures,in some cases so high that the steel becomes red in color.

Tungsten, molybdenum, and chromium also increase the resistance to

tempering. This is shown by Fig. 19.6* for steel with 0.35 per cent carbonand increasing chromium, by Fig. 19.7* for steel with 0.35 per centcarbon and increasing molybdenum, and by Fig. 19.8f for high-speedsteel containing 0.70 per cent carbon, 18 per cent tungsten, 4 per centchromium, and 1 per cent vanadium. The increase in hardness of these

steels when tempered between 900 and 1100°F. (480 and 595°C.) is

due primarily to the transformation to martensite of some austenite re-

tained by the quenching treatment.

19.7. Cast Alloys

Alloys rich in cobalt, chromium and tungsten were developed andplaced on the market in 1907 under the name of Stellite. They contain ahigh proportion of a hard constituent (complex chromium-rich carbide)within a cobalt rich matrix phase.

Cutting tools of this type usually contain from 45 to 55% cobalt,

25 to 35% chromium, 12 to 17% tungsten, 1 to 3% carbon. The proper-ties cannot be varied by heat treatment, but are controlled by the com-position, the softer alloys having a higher strength. The tool gradesrange in hardness from 40 to 64 Rockwell C. Considerable hardness and

•E. C. Bain, Functions of the Alloying Elements in Steel, American Society fo*Metals. Cleveland, 1939.

t J. V. Emmons, Trans. Am. Soc. Steel Treat., v. 19, 1932, pp. 289-332.


Fie 19.9. Typical structures of sintered hard carbides. (A) Tungsten carbide with

4 per cent cobalt. (B) Tungsten, titanium, tantalum and columbium carbides with

about 8 per cent cobalt. Polished with diamond dust and etched with alkali ferricyanide.

1.500X- (Courtesy Kennametal Inc.)

strength are maintained at high temperatures, their "red hardness"

making these alloys superior to high speed steels in resisting wear at ele-

vated temperatures. At the same time they are appreciably weaker and

somewhat more brittle.

The alloys are sold commercially under a variety of trade names.

They are used as cutting tools and wear resisting parts. Since they are

expensive and have relatively low strength they are usually cast into

small pieces of the desired size and shape which are used as inserts sup-

ported by a stronger backing material. Or, thin layers can be welded as

a hard surface coating over steel parts.

19.8. Cemented Carbide Tools

Cemented carbides, also called sintered carbides, are used mainly

for tools and dies. They are the hardest of all metallic tool materials,

consisting largely of finely divided carbide particles embedded in small

amounts of a soft ductile metal, usually cobalt (see Fig. 19.9)

.

The hard, abrasive carbides of tungsten, molybdenum, titanium, tanta-

lum, zirconium, and other metals have been known for nearly seventy-

five years, but their use for tools is much more recent. It began about

twenty-five years ago in Germany and about twenty years ago in the

United States.

All cemented carbide tools contain tungsten carbide (WC) as the es-

sential constituent. The tungsten carbide may be used alone for the

machining of cast iron, nonferrous metals and plastics. But steel-cutting

grades, in addition, must contain titanium carbide. The carbides of

titanium and tungsten form a solid solution which has the outstanding

characteristic of resisting the abrasion of the steel chip as it glides over


the tool. Smaller quantities of tantalum, columbium and vanadium car-

bides are also added to some grades.

Cemented carbide tools are made by the methods of powder metallurgy.The finely ground carbide powder is mixed with cobalt, pressed into thedesired shape, and sintered at 2450°F (1343°C) or above. The sinteringtemperature is above the liquidus of the alloy system and densificationoccurs in the presence of a liquid phase. The resulting product is sub-stantially free from porosity.

The properties of sintered carbides vary with composition, particularly

with the proportion of cobalt. In general, hardness is increased buttoughness or shock resistance is decreased by smaller binder contents.Tungsten and titanium carbides are among the hardest materials known;although less hard than diamond, their hardness is of the same order ofmagnitude as that of aluminum oxide, boron carbide and silicon carbide.

Depending upon the amount of cobalt used (which varies from 3 to

about 20 per cent), the Rockwell A hardness (diamond indenter, 60 kg.

load) ranges from 88 to 93, corresponding approximately to a RockwellC hardness of 72 to 76. A good portion of this hardness is retained at the

high temperatures encountered during metal cutting, permitting acutting speed three or four times greater than that of high speed steels.

The material has little ductility, strength is determined by a transverse

rupture test and varies from 150,000 to 400,000 lb. per sq. in. Thecemented carbides have very high compressive strengths (500,000 to

900,000 lb. per sq. in.) . They also have some of the highest elastic

moduli known (up to 90,000,000 lb. per sq. in.)

.

Cemented hard carbides are widely used for wire-drawing dies, ex-

truding dies, and numerous other dies for burnishing or spinning metal

sections, such as metal radio tubes, shell cases, projectile jackets, eyelets,

and many others, and also for gages, thread guides, valve parts for

erosive materials, and the like. The most extensive application of these

materials is in mass-production machining of cast iron, steel (with Brinell

hardness values as high as 500) , and most of the other metals, and also

of many difficultly machined nonmetallic materials including plastics,

rubber, resins, glass, asbestos, and others.

Cemented carbides arc costly and the tool generally consists of a carbide

tip or insert brazed to a steel holder or shank, but great economiesfrequently result from the use of these tools. The greater machiningspeeds and increased tool life permit production increases between 30and several hundred per cent while saving approximately one third of the

production costs.


QUESTIONS

1. Name the two main classes of tool steels. What properties are important

in each class? Give eight essential characteristics that tool steel should have,

and rate these in the order of their importance.

2. How are tool steels usually purchased? Why is it difficult to devise a per-

formance test to evaluate tool steels? How important is die cost of the steel

from which a tool is made?

3. Give the characteristic properties of high-carbon tool steel. What is the

approximate carbon content of the four main classes of high-carbon tool

steels, and whv is the carbon percentage adjusted according to the use to

which the steel is to be put? What carbon percentage would you specify

for the following: razor blade, hammer, drill, anvil, rivet set, small shear

blade, ax, and file?

4. What are the outstanding characteristics of the steels of class L? How is the

hardenability of these steels increased? Compare these steels with the S.A.E.

low nickel-chromium and chromium-molybdenum steels.

5. What are the principal alloying elements in the oil-hardening, nondefonn-

ing tool steels of class 0? Why are these steels widely used? What are they

used for? Give the important properties of the silicon-manganese steels

when used as tools.

6. What are the general effect of (a) chromium, (b) molybdenum, and (c)

tungsten on hardenability, wear resistance, tempering, and hardness at high

temperature?

7. What is the chief advantage of the medium-chromium, low-tungsten tool

and die steels as compared with high-carbon tool steels, and how do the two

groups compare in wear resistance, red hardness, and toughness? What are

the outstanding characteristics of the high-alloy tool and die materials?

What are the main alloying elements used in these high-alloy steels?

8. What is the approximate composition of the principal types of high-speed

steel? How is high-speed steel heat treated? Why is cobalt added to somehigh-speed steels? What are the advantages and disadvantages of molyb-

denum high-speed steel as compared with the usual 18-4-1 material?

9. What is the most valuable property of high-speed steel, and what is the

usual explanation for this property? What variables affect the cutting per-

formance of high-speed steel?

10. What hard carbides are used as tool and die materials? How does the hard-

ness of these carbides compare with that of quenched tool steel? Describe

the structure of these tools.

11. Compare and discuss the compositions, microstructures and properties of a

typical stainless steel and a typical tool steel. Relate the microstructures to

the desired properties. (Note particularly the difference in carbon contents.)

CHAPTER

20

Cast Iron

CHARLES Arthur Naglkr, Ph.D., Associate Professor, De-

partment of Chemical and Metallurgical Engineering,

Wayne State University Detroit, Michigan

Kenneth E. Rose, M.S., Professor of Metallurgical En-

gineering, University of Kansas, Lawrence, Kansas

v*AST irons are alloys of iron and carbon con-

taining more than 2 per cent carbon. Other elements, notably silicon,

are generally present also. Metallurgically, cast irons are characterized by

(1) an eutectic reaction during freezing, and (2) more carbon than can

be dissolved in austenite at any temperature.

In steel, carbon is rarely present in any form other than combined as a

carbide. However, iron carbide, or cementite, is a metastable compoundwhich can be decomposed to iron and carbon (as graphite) . Because of

the embrittling effect of cementite, most of the carbon in commercially

useful cast iron is converted to graphite unless the hardness and wearresistance of the cementite is of primary importance.

Factors tending to encourage the decomposition of cementite are:

1. High temperature.

2. Time (especially at elevated temperature)

.

3. Slow cooling rates during freezing (a corollary of 1 and 2)

.

4. High carbon content.

5. Presence of graphite.

6. Presence of graphitizing agents (including silicon)

.

Cast irons may be classified roughly according to the percentage of

carbon, but because of the various ways in which the carbon is associated

with the iron, it is usually more convenient to make the classification

according to the form of the carbon, rather than the amount. Thefollowing outline suggests a procedure for such a classification.

A. Carbon combined with iron as cementite.

1. White cast iron (usually about 2% carbon).

2. Chilled cast iron (usually 3% or more carbon)

.

408

Cast Iron 409

Fie. 20.1A. Flake graph-

ite in gray cast iron.

Fig. 20.1B. Spheroidal Fig. 20.2. Temper car-

graphite in nodular, 01 hon (graphite) in mallc-

ductile cast iron. able cast iron.

2.

3.

1

B. Carbon both in combined and elemental (graphite) form.

1. Mottled cast iron—a. mixture of white cast iron with graphite,

generally undesirable.

Gray cast iron—graphite as thin, interdendritic flakes which form

during freezing. Cementite is present in pearlite in the matrix.

Gray cast iron is sometimes described as "steel plus graphite."

(See Fig. 20.1 A.)

Nodular or ductile cast iron—graphite as spherulites which form

during freezing of specially treated irons. Massive cementite as

well as pearlite may be present in the as-cast condition. (See Fig.

20. IB.)

Pig iron—a product of smelting in the iron blast furnace, used

primarily as a raw material. Large, smooth, star-like flakes of

primary graphite or "Kish" are embedded in a matrix which is

essentially gray cast iron.

C. Graphite formed by heat treatment, some cementite remaining.

1. Pearlitic malleable cast iron—made by annealing white cast iron

to decompose the excess cementite, leaving a matrix of pearlite.

D. All carbon present as graphite in a matrix of ferrite.

1. Ferritic gray iron—generally undesirable for strength and wearresistance, but may be made by annealing ordinary gray iron to

achieve the maximum machinability.

Ferritic malleable cast iron—made by heat treatment of white

cast iron to decompose both the excess cementite and the

cementite that would otherwise be in pearlite. (See Fig. 20.2.)

Ductile cast iron—made by annealing nodular cast iron to attain

a high degree of ductility. The carbide in such iron is decom-posed as in malleable cast iron.

2.

3.


20.1. White Cast Iron as an Engineering Material

Cast iron containing all or nearly all the carbon in the combined formhas a silvery-white fracture and is known as white cast iron. Very hardand brittle and practically unmachinable, it is important chiefly (1) as

an intermediate product in the production of malleable iron castings and(2) when it is produced, by regulating the composition and by coolingrapidly, as a thin, hard layer on the surface of a softer iron casting. Thelatter product is known as chilled iron and is used where high surface

hardness and wear resistance are important.

The microstructure of white cast iron can be predicted from the iron

iron-carbide phase diagram used for steel; that is, it consists of cementiteand pearlite.

20.2. Malleable Cast Iron as an Engineering Material

Malleable cast iron is a valuable engineering material and is widely

used for machinery, railroad equipment and automobiles, agricultural

machinery, pipe fittings, hardware, household appliances, and in manyother applications. Intricate castings are more readily melted andpoured with white cast iron than with carbon steel and, despite the cost

of malleablizing, are cheaper than carbon-steel castings. Malleable cast

iron compares favorably with gray cast iron in machinability and with

low-carbon steel in properties.

To produce malleable castings, cast iron solidfying with a white frac-

ture is poured into a sand mold; the hard, brittle casting is then annealed

(malleablized) to dissociate the cementite. Malleable cast iron contains

2.0 to 2.5 per cent carbon, practically all of which is graphite (usually

called temper carbon) , 0.7 to 1.2 per cent silicon, 0.40 to 0.60 per cent

manganese, less than 0.20 per cent sulfur, and less than 0.10 per cent

phosphorus.

During the first stage of malleablization, at 1600° to 1700°F., cementite

which is not soluble in the austenite is converted to ragged loosely formed

nodules of "temper carbon." Second-stage malleablization takes place at

or slightly below the lower critical (A,) temperature after cooling from

the first stage treatment. This causes the remaining cementite, including

that which normally would be in pearlite, to gi aphitize. Second-stage mal-

leablization is complete when the structure consists entirely of temper

carbon in a matrix of ferrite.

20.3. Engineering Properties of Malleable Cast Iron

White cast iron for malleablizing is melted chiefly in an air furnace;

some, however, is melted in a cupola. The tensile strength and elongation

Cast Iron 411

(as measured on a test bar) of cupola malleable iron are lower than those

of air-furnace malleable, but the bursting strength of castings is usually

considerably higher. Cupola malleable, therefore, is widely used for pipe

fittings. Most malleable iron is purchased under specifications issued by

the railroads or by the valve- and pipe-fittings industry. Most of these

specifications give the following as minimum properties:

Kind of malleable iron

Tensile strength,

lb. per sq. in.

Yield strength,

lb. per sq. in.

Elongation in 2 in.,

per cent

50,000

40,000

32,000

30,000

10

5

Reduction of area is usually about 20 per cent and is seldom determined.

Malleable cast iron is unique among ferrous materials in that the elon-

gation increases as the tensile strength increases. This is shown by the

data for air-furnace malleable given in Table 20.1. Malleable cast iron,

with a notched-bar impact resistance of 7 to 9 ft-lb., as compared with

1 ft.-lb. or less for gray cast iron, is much tougher than gray cast iron.

The modulus of elasticity is 25,000,000 lb. per sq..in., that is, slightly

lower than that of carbon steel. Malleable cast iron has an endurance

ratio of about 0.5, that is, the same as carbon steel. It is considerably

more sensitive to notches than gray cast iron.

Table 20.1. Tensile Properties of Air-Furnace Malleable Cast Iron*

Tensile strength,

lb. per sq. in.


per cent

Tensile strength,

lb. per sq. in.


per cent

51,000

52,000

53,000

54,000

16.5

17.0

17.6

18.3

55,000

56,000

57,000

58,000

19.0

19.6

20.3

21.0

In malleable iron, as in carbon steel, the tensile strength decreases

with increasing size of section; a typical change is from 55,000 lb. per

• Proc. Am. Soc. Testing Materials, v. 31. 1931, Part II. p. 317. For ranges of com-

position and properties of American malleable iron, see Cast Metals Handbook,

American Foundrymcn's Society, Chicago, 3d ed., 1944, pp. 308-314.


sq. in. for sections 0.25 to 0.50 in. in thickness to 45,000 lb. per sq. in. for

sections of 1.25 to 1.50 in. The yield strength, however, is practically

unaffected by section size. In contrast to steel, in which elongation usually

increases with mass, the elongation of malleable iron decreases; at the

center of a 2- or 3-in. section, for example, the elongation may be onlyabout half what it is at the surface.

Malleable cast iron has poor wear resistance, but in machinability it

ranks about the same as gray cast iron. It is easier to plane, drill, or mill

than carbon or alloy steels, including those containing lead or highsulphur.

Recently, for a few special applications, the graphitizing operation in

the production of malleable has been modified, or alloying elements

have been added and special treatments used so that the annealed

material contains some pearlite. The properties of pearlitic malleable

cast iron depend upon the amount and distribution of the iron carbide;

usually it is stronger and less ductile than fully graphitized material.

A unique and now well-known example of a specially treated pearlitic

alloy malleable cast iron is in the crankshafts of some automotive engines.

The alloy is melted and cast under conditions that produce white cast

iron. It is malleablized by a treatment that produces small graphite

nodules in a matrix consisting almost entirely of pearlite. This treatment

is followed by a complex normalizing and annealing treatment. Thecomposition is approximately as follows: 1.5 per cent carbon, 0.7 per cent

manganese, 0.9 per cent silicon, 1.75 per cent copper, and 0.5 per cent

chromium.

20.4. Gray Cast Iron as an Engineering Material

Cast iron in which most of the iron carbide has dissociated into iron

and graphite is usually soft and readily machinable. It has a gray, almost

black, fracture and is thus known as gray iron. More than 90 per cent of

all the iron castings produced are of gray iron.

Gray cast iron is a low-cost material which is easily melted and cast,

and is economically machined. These are the primary reasons for its wide

use. Of the hundreds of engineering applications for gray cast iron, someof the most important ones, together with the usual compositions, are

given in Table 20.2.*

The properties of gray cast iron that determine its suitability for

engineering uses are the result of controlling four variables: (1) chemi-

• Trans. Am. t'oundrymen's Assoc, v. 39, 1932. pp. 56-64. For a complete list of

specific applications ot plain and alloy cast irons, see Cast Metals Handbook, Ameri-can Foundrymen's Society, Chicago, 3d ed., 1944, pp. 335-594.

Cast Iron 413

Table 20.2. Recommended Analyses of Plain Cast Irons for Common Engineering

Applications

Composition, per cent Typical charge,

per cent

UseCast-

Total Si Mn P S Pig iron Steel

C scrap scrap

3.25 2.25 0.65 0.15 0.10 40 40 20 Automobile cylinders

3.35 2.25 0.65 0.15 0.10 40 40 20 Automobile pistons

3.40 2.60 0.65 0.30 0.10 50 45 5 Automobilccas tings, general

3.50 2.90 0.65 0.50 0.06 60 40 Automobile piston rings

—

individually cast

3.40 2.10 0.60 0.50 0.10 35 55 10 Agricultural implements

—

medium sections

3.50 2.20 0.55 0.70 0.10 40 60 Agricultural implements

—

light sections

3.25 2.25 0.50 0.50 0.09 50 50 Machinery—sections not

over 1 in.

3.25 1.75 0.50 0.50 0.10 50 40 10 Machinery—1.5-in sections

3.23 1.25 0.50 0.50 0.10 50 25 25 Machinery—2-in. sections

3.25 1.25 0.65 0.20 0.10 50 25 25 Pressure castings—air cylin-

ders

3.40 1.75 0.80 0.35 0.09 55 20 25 Gas-engine cylinders—light

3.40 1.50 0.80 0.35 0.09 55 20 25 Gas-engine cylinders—me-dium

3.40 1.25 0.80 0.35 0.09 55 20 25 Gas-engine cylinders

—

3.30 2.00 0.50 0.60 0.10 50 40 10 heavy Valves and fittings

3.50 1.15 0.80 0.10 0.07 70 30 Firepots and ketUes

3.50 1.00 0.90 0.20 0.07 90 10 Ingot molds

3.60 1.00 0.75 0.20 0.07 60 40 Pots for caustic soda3.60 1.75 0.50 0.80 0.08 70 25 5 Light and medium sand-

cast water pipe

3.40 1.40 0.50 0.80 0.08 60 25 15 Heavy sand-cast water pipe

3.40 1.40 0.50 0.80 0.08 70 30 Soil pipe

3.35 0.65 0.60 0.35 0.12 12.5 80 7.5 Car wheels (0.90 per cent

combined C)3.60 1.25 0.55 0.40 0.10 45 40 15 Chilled plow iron

3.75 0.85 0.50 1 0.40 0.10 45 40 15 Plow moldboards

cal composition, (2) rate of cooling from the solidification temperature,

(3) special methods of melting, including superheating and ladle addi-

tions, and (4) heat treatment.

In addition to low cost and easy machinability, gray cast iron has a

number of properties—notably notch insensitivity, high damping capacity,

and high compressive strength—which make it especially valuable in

some applications. In tensile strength, ductility, and toughness, however,


it is much inferior to steel. In addition, the absence of a well-defined

yield point and modulus of elasticity makes it unsuitable for many engi-

neering structures. Despite its disadvantages, however, the production

of gray-iron castings is around 5 or 6 million tons a year in the United

States.

20.5. Structure of Gray Cast Iron

Considering gray cast iron as a carbon steel containing graphite, the

three primary metallographic constituents present at normal temperatures

are alpha iron (ferrite) , iron carbide (cementite) , and free carbon

(graphite). The cementite is usually associated with ferrite as pearlite—

the same constituent found in carbon steel—and the graphite is distri-

buted through the ground mass as irregularly shaped plates or flakes of

varying sizes. However, very rapid cooling from the melt may suppress

the formation of graphite and produce chilled iron, a type of white iron.

Very slow cooling increases the proportion of ferrite to pearlite. Theferrite of gray cast iron, unlike the ferrite of low-carbon steel which is

relatively pure, contains all the silicon and a little of the phosphorus

in solid solution.

Of the elements in gray cast iron, carbon has the strongest effect on

structure and properties. This effect depends on the amount of total

carbon as well as on the relative amounts of combined carbon and of

graphite present. This in turn depends upon the silicon which decreases

the stability of the iron carbide, thus promoting graphitization. Next

in importance is phosphorus, which combines with the iron to form

iron phosphide (Fe3P) . A small amount of this compound dissolves

in the ferrite, the rest forms a eutectic (Fe—FesP) with iron. When the

phosphorus is 0.1 per cent or above, the eutectic (known as steadite) is

visible as a distinct constituent in the microstructure (Fig. 20.3.) Theeutectic melts at a temperature of 1750° to 1800°F. (955° to 980 °C.)

,

much below the melting point of gray cast iron. If about 0.5 per cent

phosphorus or more is present, there is enough eutectic to make the iron

more fluid. High-phosphorus iron is, therefore, used for ornamental

castings and other thin, intricate sections. Owing to the hardness of the

phosphide eutectic, cast irons containing considerable phosphorus maybe harder to machine than low-phosphorus material. In the amounts

usual for American gray cast irons (Table 20.2) phosphorus has little

effect on the strength, but it may increase the brittleness. For cylinders

and various kinds of pressure castings 0.5 per cent is, therefore, usually

the maximum permitted (Table 20.2) . The effect of phosphorus on

graphitization is negligible.

Cast Iron 415

Fie. 20.3. Phosphide cutcctic in gTay cast iron. (A) 100x; (B) l.OOOx- The cutectic

is the irregular-shaped light-colored constituent shown in A. At high magnification its

duplex character is apparent. (Kunkele, Giesserci, v. IS, 19)1, p. 73)

Commercial gray cast iron always contains enough manganese to in-

sure that the sulphur is present as manganese sulphide. Owing to the

notch insensitivity of gray cast iron, more sulphur is tolerated than in

steel because manganese sulphide inclusions are not so likely to be loci

of weakness where failure will start under repeated stress. Manganese in

excess of the amount necessary to combine with the sulphur forms iron-

manganese carbide. This compound is more stable than iron carbide;

hence, manganese inhibits graphitization. In the amount usually present

(less than 1 per cent) the stabilizing effect of manganese is much less

important than the strong graphitizing effect of silicon.

20.6. Relation between Properties and Structure of Gray Cast Iron

There is a close relation between properties and structure of gray

cast iron. The properties are primarily dependent upon the character-

istics of the "steel" matrix and upon the amount and distribution of the

graphite. These in turn are dependent upon the variables already men-


tioned, namely, chemical composition, rate of cooling from the solidifi-

cation temperature, special methods of melting, and heat treatment.

The amount of iron carbide to be decomposed into iron and graphite

or to be left undecomposed depends (if the effect of cooling rate is ig-

nored) upon the amounts of total carbon and silicon. For small speci-

mens cast in sand molds the relation between total carbon, silicon, and

structure is shown in Fig. 20.4.*

?.b J.O i.l> 4.0 4.5

Silicon, per cent5.0 5.5 6.5 7.

Fic. 20.4. The Maurer cast-iron diagram.

A modification of the Maurer diagram by Coylet is shown in Fig. 20.5

in which approximate tensile strengths are plotted against composition.

This diagram shows that for small specimens cooled in sand molds the

highest strengths are attained if the total carbon is 2.00 to 3.25 per cent

and if the silicon is 1 .00 to 2.25 per cent.

20.7. Effect of Cooling Rate

The relation among properties, structure, and composition is affected

greatly by varying die rate of cooling from the solidification temperature

to the gamma-alpha transformation temperature. As the mass is increased,

the cooling rate is decreased, and the dissociation of the iron carbide is

increased. Thus if a 1-in, sand-cast bar has a matrix corresponding to a

• E. Maurer, Kruppsche Monatshelte, v. 5, 1924, pp. 115-122.

+ F. B. Coyle, Trans. Am. Soc. Steel Treat., v. 12, 1927, pp. 146-465.

Cast Iron 417

Z.50 3.00

Silicon. per cent3.50 4.0C "..50

Fie. 20.5. Cast-iron diagram showing cltect of carbon and silicon on tensile strength.

(Coyle)

0.50 per cent carbon steel, the same iron if cast as a 6-in. bar in sand will

ordinarily have a matrix that corresponds to a 0.10 per cent carbon steel.

On the other hand an iron of such composition that, when poured into

a sand mold it will have a matrix of pearlite, will, if poured into a cast-

iron mold, form white iron on the surface next to the cold mold but will

be pearlitic in the interior of the casting.

Thus, the lines of the Maurer diagram (Fig. 20.4) are moved to the

right if the cooling rate is increased, and to the left if the cooling rate is

decreased. The approximate relation among structure, cooling rate, and

properties is shown in Fig. 20.6.*

It is evident from the foregoing that the relation among the composi-

tion, cooling rate, structure, and properties of gray cast iron is not simple.

Another important factor, which adds to the complexity, is the size and

form of the graphite flakes.

20.8. Effect of Graphite Size on Structure and Properties

The strength of slowly cooled carbon-steel castings increases as the

carbon (cementitc) increases. The strength of slowly cooled gray cast

iron would also increase as the cementite increases if no graphite were

present. Graphite destroys the continuity of the grains of the "steel"

matrix and makes it weak and brittle. As the si/.e of the graphite particles

increases and as they become more flaky and platelike, the iron becomes

progressively weaker and more brittle.

* Cast Metals Handbook. Am. Foundrymen's Soc, Third Edition, 1944, p. 360.


350

Strength increases

300

w250

WV

°zoo

co 150

100

sc

S t r u c t u r e

Hard and brittle*\»hite iron. Cemenlite andai/stenife

Mottlediron. Cementite,grapliilerandpearlitc

|

Greatest strength. Hard to machine

— Best nigh-test irons - Closegray iron. Graphite,

andpeartite/air strength. Easymachining yf, ,. . ,4

„ndZrf,w„l -^-p^Med^mgroy.ron.eraph,^peartite ana fernte

and fair finish

.Low strength.

Opengrain.Machines soft

Open gray iron. Graphiteand ferrite"-§

FJO. 20.6. Relation of hardness and strength o£ gray cast iron to structure andsection size. (American Foundrymen's Society)

High-strength cast iron must have a "steel" matrix containing consider-

able pearlite, and the graphite flakes should be small; there must, how-ever, be 2.0 to 2.5 per cent graphite present if easy machinability is

required. The control of the graphite size is accomplished by five meth-ods. The first consists of melting an iron of such composition that it

would normally solidfy with a white fracture and then treating it byadding a special graphitizer, such as powdered silicon or calcium silicide,

just before the castings are poured. The second consists of melting aniron that would normally solidify with a white fracture and mixing with

this a definite amount of a soft gray iron melted in another cupola. Thethird method consists of melting gray cast iron and superheating it con-

siderably above the usual casting temperature. This decreases the size

of the graphite flakes and increases the amount of pearlite in the casting.

The fourth, used considerably in Europe, consists of pouring a low-silicon

iron into a preheated mold. The fifth method consists of adding alloying

elements. To produce high-strength iron by any of these methods the

several variables, including the percentages of carbon and silicon, mustbe controlled carefully.

20.9. Evaluation of Gray Cast Iron for Engineering Applications

The suitability of gray cast iron for engineering structures and ma-chines is judged chiefly by tensile, transverse, and compressive strengths.

Cast Iron 419

Hardness, endurance limit, wear resistance, and other properties may

be valuable criteria of its value for specific applications. Most metal-

lurgists experienced with cast iron consider the transverse test as the

best single source of information on the quality of cast iron.

The transverse test is made by applying a load at the center of a cast

(unmachined) round bar, supported at each end, until fracture occurs.

The size of the bar depends upon the size of the casting and has been

standardized* at 0.875, 1.20, or 2 in. in diameter with a span of 12, 18,

or 24 in. between supports. The load necessary to fracture the bar and

the amount of deflection at the center at the moment of fracture are

noted. In the United States transverse strength is reported as the load

in pounds; in Europe the load is converted by the formula

PicMR =-u

to a value called modulus of rupture. In this formula P is the breaking

load, I the distance between supports, c the distance from the neutral

axis of the bar to the extreme fiber, and / the moment of inertia.

The relation of modulus of rupture to tensile strength varies with the

quality of the iron from 2.5 times the tensile strength for low-strength

irons to approximately the same as the tensile strength for high-strength

irons. Transverse strength or modulus of rupture is of value in design if

cast iron is used as a beam and especially in pipe subjected to heavy

earth loads.

Gray cast irons are graded by (minimum) tensile strength (in thou-

sands of pounds per square inch) into seven classes: 20, 25, 35, 40,

50, and 60. As discussed before, since the stress-strain curves of gray

cast iron are curved from the origin, this material has no proportional

limit nor modulus of elasticity. The secant modulus, representing a

point on the stress-strain curve of 25 per cent of the tensile strength,

varies between 12 and 20 million lb. per sq. in.

In many engineering applications, for example machine beds, columns,

pipes used as structural members, and various other supports, cast iron

is used for its strength in compression. This is determined by subjecting,

sand-cast bars 0.75 to 0.80 in. in diameter and 1 to 3 in. long to a com-

pressive load until failure occurs. The stress-strain curve in compression

is also curved from the origin. Compressive strength is higher than tensile

strength; it varies from about 4 times the tensile strength for low-strength

irons to about 3 times the tensile strength for strong irons.

The other properties of gray cast iron can be summarized in a series

of short sentences. Endurance limits are given in Table 20.2. The en-

•A. S. T. M. Standards, Pari 1, 1955, A48-40, pp. 1848-1854.


durance ratio, determined on highly polished specimens, is more erratic

than for steel. Values between 0.30 and 0.60 have been reported. A very

important characteristic of gray cast iron is the almost negligible effect

of surface on the endurance limit; the same notch which reduces the

endurance limit of steel as much as 75 per cent reduces the enduranceof soft cast iron less than 15 per cent. The impact resistance of cast ironis low and is usually unimportant. Notched-bar tests are meaningless,

and drop tests are frequently erratic. The damping capacity of gray cast

iron is higher than for any other ferrous material. Gray cast iron is readily

machinable unless the surface is chilled; conversely, if the surface is

chilled, cast iron is very wear-resistant and is machined with great dif-

ficulty. It is relatively easy to produce gray-iron castings having a surface

layer in which the carbon is entirely combined as cementite. Chilledgray cast iron is, therefore, widely used for large gears, for a variety of

rolls, for wheels of railway cars, and for many other articles where highsurface hardness and wear resistance are necessary. Machining the sur-

face, if necessary, is, however, a slow and costly operation.

20.10. Nodular or Ductile Cast Iron as an Engineering Material

The size, shape, and distribution of graphite in cast iron has a pro-

nounced effect upon strength and ductility. This may be illustrated bycomparing these properties in malleable cast iron with the same properties

in gray cast iron. The higher ductility in malleable cast iron is related to

the rounded, compact form of the graphite (temper carbon) , which causes

less disruption of the matrix than do the long slender flakes of graphite

in gray cast iron. Until recently, the rounded graphite form could beachieved only by heat treatment of solid metal, but spherulitic or nodulargraphite can now be made to form during the freezing interval by meansof special treatments and additions developed and patented in the UnitedStates and in several other countries. High strength and elongation of as

much as 20 to 25 percent are reported in cast iron made by these processes.

The total carbon in nodular cast iron is about the same as in gray iron.

Magnesium or cerium are most widely used to promote the formation of

nodular graphite. Only a few hundrcths of a percent of these elements is

needed, but the addition must be made shortly before the metal is cast,

and the melt must be very low in sulphur.

Metallurgists do not clearly understand the mechanism of graphitiza-

tion in nodular cast iron. The high shrinkage in nodular iron castings

suggests that little, if any, formation of graphite occurs during freezing.

Furthermore, massive cementite is not uncommon in the as-cast metal,and for maximum ductility this must be eliminated by a separate anneal-

ing heat treatment.

Cast Iron 421

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Engineering Properties. Specifications of the ASTM (A 399-55) recog-

nize two commercial grades designated as 80-60-03 and 60-45-10. The first

of these is in the as cast condition; the second is an annealed grade. Ineach, the first two sets of numbers indicate minimum tensile and yield

strengths in 1000 psi, and the third set indicates minimum percentageelongation.

Nodular iron is more readily machinable than gray irons of similar

hardness. It is somewhat stiffer than gray cast iron, having a modulus of

elasticity of 21 to 25 million psi. The damping capacity is between that

of gray iron and cast steel. Nodular iron has somewhat better resistance

to oxidation, but the resistance to atmospheric corrosion is about the

same as for gray iron.*

20.11. Melting and Casting of Irons

Different methods of melting are used in order to control the chemistryand microstructure of cast iron.

The greatest tonnage of iron is melted in a cupola furnace, Fig. 20.7, a

cylindrical type of furnace mounted vertically on legs and lined withrefractory brick. Depending on the type of foundry operated, the furnaces

can be lined with either basic or acid brick. The lining of the cupola

with basic brick is a relatively new innovation. Cupolas are charged

some distance above the floor of the furnace, and depending on the

capacity, the charging door is located 15 to 30 feet above the tuyeres.

On the lower portion of the furnace is a group of tuyeres attached to

a wind box that supplies the necessary oxygen for combustion. The area

below the tuyeres makes up the molten metal reservoir, which is periodic-

ally tapped. Above the metal tap hole and usually on the opposite side

is the slap tap hole. Materials charged to the cupola are pig iron, scrap

iron or steel, coke, and flux. The materials are weighed accurately andcharged in alternate layers. The function of the cupola furnace is to melt

the various components of the charge, and the molten mass of metal

becomes carburized by being in contact with the hot coke. The carbonsolubility in the iron will depend on the temperature of operation of the

cupola furnace. The carbon solubility will reach an equilibrium valueafter a given length of time. Cupola furnaces are not operated underequilibrium conditions. However, the time that the molten metal is

allowed to remain in contact with the coke is carefully controlled. Whenthe metal is tapped from the cupola furnace, the air is shut off, and

• A. S. M. Metals Handbook. Supplement-Metal Progress—Vol. 66, No. 1-A, July 15,

1954.

Cast Iron 423

FlG. 20.7. Section through a cupola furnace. (Whiting Corp.)

the tap hole is opened to allow the molten metal to flow from the

reservoir in the bottom of the cylindrical shaft furnace. After the tap is

completed, the tap hole is plugged and the air is again turned on and

melting continues until the next tap.

Raw materials that are charged into the cupola furnace ultimately

determine the chemistry of the melt that will be cast. The size of scrap

and pig charged depends on the size of the cupola furnace. Coke,

which is the fuel, is carefully selected on the basis of size, heating value,

chemistry, porosity, and strength, and must necessarily be low in sulfur

and phosphorus content. The molten iron in the cupola furnace has

a high affinity for sulfur and phosphorus, and invariably the sulfur


and phosphorus that are charged with the raw materials will be totally

accounted for in the molten iron. The slag produced in a silica brick-

lined cupola will not effect the removal of sulfur or phosphorus. In the

case of the basic lined cupola, the product will show a considerable

decrease in sulfur content due to the molten iron being in contact with

the basic slag. By far the best method of producing a low sulfur andphosphorus iron is to minimize initially the charging of the two elements

into the cupola. The melting rate in the cupola furnace is controlled

by the amount of air that is admitted through the wind box into the

tuyeres. The air causes the combustion of the coke which in turn sup-

plies the heat for fusion. In controlling closely the operation of the

cupola with respect to air, it is not uncommon to find that the air is con-

ditioned to a constant humidity. Research work on the effect of moisture

on quality of iron has indicated the need for the control of moisture

to insure constant quality of iron. A study on the effect of moisture in

the air feed of a cupola furnace has indicated a definite relationship be-

tween hard spots in cast iron and a critical amount of moisture in the air.

The cupola furnace is considered to be an intermittent type of furnace.

On the basis of 24 hours of foundry operation, the cupola may be con-

sidered to operate 18 hours, and the remaining time allotted to repair of

the lining and to general maintenance on the furnace.

The melting of malleable iron is usually carried out in a reverberatory

or air furnace, Fig. 20.8. A relatively small tonnage is melted in electric

furnaces. The fuel used in the reverberatory furnace is usually powdered

coal or fuel oil. The raw materials added to the air furnace are scrap iron,

pig iron, and flux. Iron of various analyses are selected so that whenfusion and oxidation are completed the resulting melt is close to the

desired analysis. Some adjustments are made in final analysis by addition

of ferro alloys containing desired elements. It is well to mention that the

carbon content of the metal melted in the air furnace is considerably

lower than that produced in the cupola furnace and necessarily has a

higher melting and casting temperature. The feature that differentiates

the product of the cupola furnace from that of the air furnace is that the

fracture of the latter is white and that of the former is gray. The gray

fracture is caused by free carbon being in the form of graphite, and the

white fracture is caused by carbon being in solution. The molten iron is

cast into suitable molds for ultimate solidification into the desired shape

of ultization.

Prior to the casting of the metal from the melting furnace, various tests

are carried out on the molten metal to determine its suitability and

quality for casting. A chill test is carried out on gray cast iron to de-

Engineering Metallurgy 425

Flo. 20.8. Section through a reverberator^ or air furnace. [Whiting Corp.)

termine if the metal when cast will be free from a white cast-iron outer

layer. The test block is fractured and the depth of white layer measured.

A fluidity test is carried out to determine the flow characteristics of the

metal under certain standardized conditions. The test is carried out in a

spiral fluidity mold, the flow of the metal in filling the spiral being a

measure of the viscosity and tendency to produce useful casting. Todetermine the quality of the iron after casting, physical tests such as

tensile strength, transverse breaking strength, and hardness tests are

carried out. The alloys are studied to determine the chemical analysis

with specific interest in such elements as carbon, silicon, manganese,

phosphorus, sulfur and others that may be present. Tests on iron that

are cast from the air or reverbratory furnace are as follows: study of

fracture to determine how much white cast iron is formed; chemical

analysis to determine the presence of such elements such as carbon,

silicon, manganese, sulphur, phosphorus, and others. In the case of white

cast iron the chemical analysis will predetermine whether the alloy will

satisfactorily be annealed to malleable iron. On a tonnage basis the

largest amount of iron cast as white cast iron is ultimately annealed into

malleable iron.

20.12. Ternary System of Iron, Carbon, and Silicon

Cast irons can be considered on a simplified basis as alloys of iron,

carbon, and silicon. In reality, the alloys contain, in addition to the

alloying elements mentioned, small amounts of phosphorus, sulfur,

manganese, and other elements. In order that we be able to interpret the

various changes that take place in the cooling of a cast iron from its

liquidus temperature to room temperature, it is imperative that we be-

come familiar with the ternary system of iron, carbon, and silicon.

The ternary system allows the plotting of die composition on the basal

temperature plane in three dimensions of carbon, silicon, and iron; the

vertical axis is temperature. Drawn in Fig. 20.9 is a corner of the

ternary system iron-carbon-silicon. The familiar iron-graphite constitu-


Fie. 20.9. Iron-rich corner of the ternary system of iron, carbon, and silicon.

tion system is recognized; the other section is for the iron-rich corner of

the iron-silicon system. The iron-silicon system contains a gamma loop,

which terminates at the transformation temperature from alpha to

gamma, and gamma to delta iron. A section taken through the system

at constant amounts of silicon illustrates the effect of silicon on the

temperature and composition of the eutectic and eutectoid of the system.

In Fig. 20.10 is illustrated sections taken through the system at constant

silicon content of per cent, 2 per cent, 3.8 per cent and (i.5 per cent.

It is readily noted in Fig. 20.10 that the transformation which takes

place at the solidus temperature line for the 2 per cent carbon alloy in

the iron-carbon system is simply a transformation of austenite-plus-

liquid to austenite-plus-cementite. In the case of the 2 per cent silicon

section of the iron, carbon, and silicon system, the transformation at 2

per cent carbon involves a three-phase region changing from liquid-plus-

austenite to austenite-plus-graphite. A region of austenite, cementite,

Cast Iron 427

1600

a-

i 1200

o« 1000

800 v

600

#—7'

[

L

OX Si7*0 \ L J>^ -^ /IsCa

—r- -/

«V>

.^ r+Ca

a + ca

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Carbon, percent

6000.5 1.0 1.5 2.0 2.5 3.0 35 4.0 45 5.0

Carbon, per cent

60005 1.0 15 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Carbon, per cent

1400

o°.I200

- a-

a 1000

\ 800

600

—zl

—

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a\ ^Sfcs^p ^ *tc*L3' t \-

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•L-a?

££-a >Ca

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a*La

0.6 1,0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Carbon, per cent

Fig. 20.10. Sections through the iron, carbon, and silicon system at per cent silicon,

2 per cent silicon, 3.8 per cent silicon, and 6.5 per cent silicon. {Alloys of Iron and

Silicon, Greiner, Marsh, and Stoughton, New York: McGraw-Hill Book Co., 1935.


3% Total Carbon. 2% Si

Liquid

A*L

A*C

-A,PtC

A+C+G A+G

AiP,C*G A rP-,G A.t>+$+F

P*C(WhiliIron)

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FoilCooling

ModeratelyFoil

Cooling

ModerateCooling

Moderate',

SiowCooing

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A, Austenite

P, Pearlite

C.CementiteG G, Graphite

F. Ferrite

K

Fie. 20.11. Transformation occurring during the solidification and the cooling of

cast iron. (Lorig)

and graphite is passed through prior to the formation of the solid phase.

This region is used in explanation of the formation of graphite from the

melt on solidification. The explanation of the freezing of iron, carbon,

and silicon alloys is complicated by the presence of two systems—the

stable system of iron and graphite and the metastable system of iron andcementite. In both systems prior to reaching the liquids temperature, the

carbon is in solution in the iron. Considerable changes occur during the

solidification of alloys according to the stable and metastable system. Al-

loys that freeze according to the stable system will give structures that con-

tain graphite plus transformation products of austenite upon cooling.

In the case of the metastable system, the products of transformation oncooling contain iron carbide (cementite) plus transformation products of

the austenite. Hence, it is evident that an alloy has different microcon-

stituents formed, depending on the rate of cooling from above the

liquidus to room temperature. Fig. 20.11 above illustrates the rela-

tionship of cooling to microstructure for a 3 percent carbon, 2 percent

silicon cast iron. It is noted that on fast cooling the microstructure is

basically pearlite and cementite, and on slow cooling the microstructure

is ferrite and graphite. At an intermediate cooling rate, it is possible to

form a structure of cementite, pearlite, and graphite which is referred to

as mottled iron. In Fig. 20.11 notice that the rate of cooling determines

whether the stable or metastable constitution system is applicable in

considering the process of freezing. When alloys of iron, carbon, and

Cast Iron 429

Fig. 20.12. Types of graphite flakes in gray cast iron. (Tentative recommended

practice for evaluating the microslructure of graphite in gray iron, ASTM designation

A247-41T).

silicon solidify according to the stable system, the carbon is present in the

uncombined form of graphite. Depending on the conditions of cooling,

nucleation, and other factors, the graphite will assume a variety of sizes

and shapes. The flake-type chart for graphite is reproduced in Fig. 20.12.

Depending on the mode of formation, any one of five different types of

graphite patterns can be formed, namely, uniform distribution random

orientation, rosette grouping random orientation, superimposed flake

size random orientation, interdenderitic segregation random orientation,

and interdendritic segregation preferred orientation. The size of graphite

flakes is usually rated on the basis of length measured at 100 magnifica-

tions. All types of graphite flake patterns may be present in a section of

gray iron cast, with the exception of the superimposed flake size, which is

found typically in hypereutectic irons. It is generally agreed that the

flake type of graphite forms directly from the melt on cooling below the

region of austenite plus liquid or austenite plus carbide of the stable sys-

tem of iron and carbon. The formation of graphite from the melt is

explained on the basis of the binary system of iron and carbon for

simplification purposes only. In explanation of the formation of the

interdendritic graphite pattern, which contains fine graphite, it is as-

sumed that the eutectic transformation takes place at a lower tempera-

ture than that of the large graphite flakes. This tends to indicate that

the fine graphite structure is formed during the undercooling of the melt

(and the absence of residual nuclei in the melt to initiate the formation

of the graphite) . It can be generally concluded that all forms of graphite

found in cast irons form directly from the melt on solidification. The

innoculation of cast irons is practiced in many foundries to control the

size of graphite flake formed in the cooling of various section sizes. The

amount of innoculant added will control the size and distribution of the

graphite flakes. Any one of a number of innoculants can be used, such

as ferrosilicon, calcium silicide, combinations of titanium, aluminum


and silicon, mill scale, and others. In specially controlled heats of cast

iron it is possible by innoculation to alter the shape of the graphite. Thegraphite formed on cooling after innoculation assumes the shape ofspheres. The chemical composition of the cast iron must be carefully

controlled with respect to sulfur, which must be present in an amountless than 0.02 per cent. The spheriodizing additive may be cerium or

magnesium in alloyed forms. The addition of the innoculants results in

exothermic reactions, and care must be taken in making the alloy addi-

tion. The mechanism of the formation of the nodular graphite particle

on the cooling of the cast iron takes place in the following manner. Theaddition of the cerium or magnesium to the cast iron, low in sulphur,

causes the graphite to crystallize in the form of nodules.

The constitution diagram of iron, carbon, and 2 per cent silicon canbe used to explain the formation of temper graphite during the long-

time annealing of white cast iron to form malleable cast iron. Theformation of temper carbon takes place in two stages during the mal-leablizing of white cast iron. The first stage is carried out at a relatively

high temperature of 1600° to 1700°F. by slowly heating to this tempera-

ture and holding until the cementite has decomposed into gamma iron

and temper carbon. The gamma iron dissolves in the austenite and the

carbon will be uncombined. The amount of carbon that remains in solu-

tion can be determined from the solubility of carbon in iron as deter-

mined in the constitution system of iron, carbon, and 2 per cent silicon.

Prior to the second stage of malleablization the alloy is slowly cooled

from 1600° - 1700°F. to 1200° - 1300°F. and held at this temperature

until the austenite that had transformed on cooling into pearlite decom-poses into ferrite and temper carbon. The amount of carbon that the

austenite will yield can be determined again from the carbon solubility

line. At approximately 1200°F. the solubility of carbon in iron is less

than 0.10 per cent. The alpha iron formed dissolves in the ferrite, and the

uncombined carbon deposits on the temper carbon that was initially

rejected during first stage of malleablization.

20.13. Heat Treatment of Cast Iron

There are a number of methods whereby the heat treatment of cast

iron can be carried out. Briefly we can summarize the various type of heat

treatment under the headings of stress relief and growth, annealing andnormalizing, hardening and tempering, isothermal heat treatment, flame

hardening and induction hardening.

Stress Relief and Growth. Cast irons unless given a stabilizing heat

treatment are susceptible to growth on remaining at room temperature

Cast Iron 431

for extended lengths of time. It is common practice when time permits

to store gray cast-iron parts for a length of time of six months or more

to allow the natural growth to occur on aging at room temperature.

Growth in cast irons can be accelerated by heat treatment, but the

heat-treating temperature is kept at a minimum to lessen graphitization

of the ccmentite phase. Cementite is unstable in the presence of silicon

and when held at slightly elevated temperatures, tends to break down into

graphite and ferrite. The breakdown results in a decrease in mechanical

properties of the iron. The room temperature aging of the gray cast iron

is most desirable in that no conditions are offered for the graphitizing

action. The temperature range used for the aging to promote growth is

between 700° and 1300°F. It is well to mention that all gray cast irons do

not show growth on aging, and this problem has been investigated by

metallurgists for a number of years. There has evolved no acceptable

answer as to why some compositions of irons show the growth

phenomena and others do not. Suggestions have been made as to mechan-

ism of growth, and they embody some of the following concepts: due to

graphitization of some of the cementite, a volume expansion takes place;

penetration of oxygen at the grain boundaries and internal oxidation

occurs with an increase in volume; thermal gradients and volume changes

during heating and cooling. Depending on the type of heat treatment

cycle given the gray cast iron for dimensional stabilization, any or all of

the mentioned factors may apply. Oxidation is a growth factor from roomtemperature up to about 800°F. Cementite break-down is a factor in

temperature ranges from 800° to 1300°F. Thermal gradients and volumechanges occur over all of the heat-treating ranges.

Annealing and Normalizing. The heat treatments of annealing and

normalizing are usually applied to gray cast irons for the purpose of

increasing machinability. The cooling rate from above the critical

temperature will affect the coarseness or fineness of the pearlite—ferrite—

cementite aggregate associated with the graphite. As the coarseness of

the pearlite of the gray cast iron increases, the hardness of the material

decreases. It is possible to over-anneal a gray cast iron. During machining

of such material, tearing takes place, resulting in a poor surface finish.

During the annealing or normalizing cycles, graphitization may take

place with a lowering of the combined carbon content of the cast iron,

resulting in a decrease in strength. In determining the temperature above

which the cast irons must be heated to promote austenization, the usual

iron, iron-carbide diagram cannot be used. The proper section through

the ternary diagram of iron, carbon, and silicon should be consulted.


Hardening and Tempering. The gray cast iron, the pearlitic malleableirons, and the pearlitic nodular cast irons respond to heat treatment muchin the same manner as do plain carbon steels. The ability to respondto heat treatment stems from the presence of carbon in the combinedstate. The carbon present in the combined state causes the formation of

austenite when the ferrite-carbide aggregate is heated above the uppercritical temperature. Some graphitization may take place during the

heating to the austenitizing temperature. The cast iron is held at the

proper austenizing temperature until the desired degree of solubility of

carbon has taken place, and the alloy is then quenched. The usual

precautions taken in the heat treatment of cast iron are the same as

those for steels. To cause the formation of a completely martensitic

matrix from the austenite, it is necessary that the section be quenchedat, or faster than, the critical cooling rate. If the cooling is some-

what slower than the critical cooling rate, some of the soft pearlitic

constituents are formed and associated with the martensite. When con-

ditions of service require properties other than high hardness, the irons

are given a tempering treatment. It is quite common to give the heat

treated cast iron, to be used in the martensitic condition, a stress-relief

heat treatment to minimize cracking during grinding and to minimize

the brittleness of the martensite. The tempering curves of hardness versus

time are similar to that obtained for steels. The hardness of the

martensite decreases as the tempering temperature is increased. Theresulting microstructure formed by the tempering of the martensite is

similar to those produced in hardening and tempering of steels. In study-

ing the hardenability response of cast iron, the Jominy hardcnability

type of bar can be used, the hardness being determined along the edge

of the bar in a manner similar to that for low alloy steels and plain

carbon steels. As one may anticipate, the hardness of the fully heat-

treated cast iron quenched at or faster than the critical rate is some-

what less than a steel of the same combined carbon content, due to the

presence of graphite. The presence of graphite in the martensite matrix

decreases the hardness considerably. The use of a light-load hardness

tester will verify the fact that the hardness of the martensite between

the graphite flakes is as great as the martensite in a lull heat-treated

steel. The presence of graphite in the matrix of the cast iron during

heat treatment to full hardness acts as a cushion and tends to minimize

the formation of quench cracks that many occur during the considerable

volume changes that accompany the transformation of austenite to

martensite.

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C - 3. 14

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P - 0.219

S - 0. 064

Si - 2. 15

Fig. 20.13. Time. Temperature. Transformation (TTT) diagram for a cast iron.

(Nagler and Dondelt).

Isothermal Heat Treatment. The various types of cast irons such as

gray cast iron, pearlitic malleable iron, and nodular cast irons can be heat

treated by any of the known isothermal transformation methods. The

TTT curve for cast irons does not differ greatly from that of low alloy,

high alloy, or plain carbon steels. Fig. 20.13 is a typical TTT diagram

for a cast iron. The combined carbon in the cast iron is an influencing

factor in determing the position of the upper nose of the TTT diagram.

As the carbon content increases, the upper nose of the TTT diagram is

shifted to the right. The maximum shift to the right is produced by a

eutectoid composition of combined carbon. Amounts of carbon above

or below the euctectoid composition tend to shift the nose of the TTTcurve to the left. The cementite or the ferrite, whatever the excess pro-

eutectoid constituent may be, nucleates the transformation of austenite

to pearlite at the position of the upper nose of the curve. The products

of transformation produced isothermally are identical to those produced


in steels. Starting at a temperature just below the lower critical tempera-ture (Aij), coarse pearlite is formed; fine pearlite is formed at the uppernose of the TTT curve; below the upper nose the upper bainite is

formed; at the lower range the lower bainites are formed; at the M,temperature manensite is formed from the austenite on cooling to roomtemperature. The isothermal method of heat treating cast irons produces

increased toughness of the matrix of cast irons.

Flame Hardening and Induction Hardening. The gray cast iron, pearl-

itic malleable cast irons, and the nodular cast irons respond to induction

and flame hardening. The principle of this process of hardening is to

heat rapidly above the upper critical temperature the surface of the parts

to the desired depth and immediately quench. The heated portion of the

iron is changed to an austenitic condition initially, and when quenched,the austenite is converted to a martensite. Depending on the thickness

of the section size, there may be no alteration in the properties of the

core. The intent of this form of heat treatment is to produce a markedincrease in hardness on the surface of the part with retention of un-

altered core properties. The hardened portion of the cast irons illustrates

the presence of a martensite matrix with graphite. This heat treatment

produces a hard-wearing case on the tough shock-damping core of the

cast iron, and the depth of hardening can be controlled very closely.

20.14. Normal and Alloy Elements in Cast Iron

The normal elements in cast iron include such elements as carbon, sili-

con, manganese, phosphorus, and sulfur. The commonly used alloying

elements include copper, chromium, molybdenum, vanadium, and nickel.

Carbon. Carbon in cast iron is present in the uncombined and com-bined forms. In the uncombined form the carbon can assume the shape

of flakes or nodules. In the combined form the carbon can be present as

a simple iron carbide (cementite) , binary carbide, and other complex

alloy iron carbide forms- The presence of the carbon in the uncombinedstate is controlled by such factors as chemical analysis of the iron and the

cooling rate from the casting temperature. In the malleablizing of white

cast iron, an additional type of carbon is formed called tempered carbon.

Silicon. Silicon is generally considered as a graphitizing element. Theamount of silicon present in a cast iron determines whether the carbonis present in the combined or uncombined state for a given cooling con-

dition. The graphitizing effect of the silicon can be somewhat offset

by the addition of carbide forming elements. Silicon dissolves in the fer-

rite and acts to toughen the iron.

Cast Iron 435

Manganese. Manganese combines with the sulfur and forms manganese

sulfide, which is insoluble in iron and forms non-metallic inclusions. Some

of the manganese may form manganese silicate, also present in the iron

as a nonmetallic inclusion. The manganese remaining is dissolved in the

ferrite and acts as a ferrite toughener.

Phosphorus. Phosphorus is present in cast iron in the form of steadite,

a ternary eutectic of iron, carbon, and phosphorus. Steadite is a low melt-

ing constituent contributing to the fluidity of cast iron. The steadite

melts at about 1750°F. The presence of phosphorus adds to the brittle-

ness of cast iron and tends to lower the impact strength of it.

Sulfur. Sulfur is present in cast iron in the combined form of man-

ganese sulphide or iron sulphide. Iron sulphide, when form usually

occurs in the grain boundaries and reduces the impact toughness of the

iron.

Chromium. Chromium has a two-fold effect on cast iron: it dissolves in

the ferrite and is a carbide former. The chromium that dissolves in the

ferrite acts as a toughener. Chromium that is present in an amount over

that necessary to saturate the ferrite occurs in the pearlite as a massive

iron-chromium carbide. Chromium has a tendency to produce a uni-

form fine-fracture grain size. As the per cent of chromium added to the

cast iron is increased, there is a tendency for the carbon to remain

in a combined form, and the fracture will be white. Chromium in the

amounts of from 0.15 to 2.00 per cent may be added to cast iron and still

provide gray fractures if suitable graphitizing elements are present.

Additions of chromium to the cast iron will increase tensile strength,

hardness, wear resistance and depth of chill, and corrosion and oxidiza-

tion resistance. Chromium will delay the isothermal transformation of

cast iron during heat treatment.

Molybdenum. Molybdenum dissolves in the ferrite and, to a limited

degree, is a carbide former. Molybdenum does not act as a graphitizing

agent. The addition of molybdenum to cast iron increases tensile

strength, hardness, transverse strength, and resistance to deflection on

transverse loading. The addition of molybdenum to cast iron when cast

into large section sizes tends to promote uniform structures. This results

in an iron of uniform strength, hardness, and density throughout the

large section size. Molybdenum additions to cast iron promote machin-

ability resulting from structural uniformity and a minimum occurrence

of large carbides. Molybdenum has little or no effect on retarding

isothermal transformation over the range of baintic transformation prod-

ucts. The isothermal transformation to pearlite products is retarded by

the addition of molybdenum.


Copper. Copper dissolves to a limited degree in the ferrite and acts

as a toughener, and is a mild promoter of the formation of graphite.

Copper tends to increase strength, hardness, wear resistance, and corro-

sion resistance. The addition of copper to cast iron reduces the tendency

to form a chill layer, hence increases machinability. Copper is not used

as an individual additive element but is usually added with such elements

as chromium, molybdenum, nickel, and vanadium.

Nickel. Nickel dissolves in ferrite and is a graphitizer. Nickel addi-

tions reduce depth of chill and tend to refine the grain size. Nickel is

effective in retarding the transformation of austenite over the whole

range of transformation. If the graphitizing effect of nickel is disregarded,

nickel is considered to increase the strength, hardness, resistance to de-

flection, toughness, corrosion resistance, and machinability. In addition

to using the nickel in gray cast iron, it is utilized in martensitic cast iron

containing comparatively large percentages of nickel and chromium. Thealloy has superior wear and corrosion resistance. A grade of cast iron

termed austenitic is cast with the addition of nickel and chromium for

service conditions requiring lo wexpansion and stainless qualities.

Vanadium. Vanadium when added to cast iron performs the role of a

strong carbide former. Vanadium retards graphitization and stabilizes

cementite and has a high affinity for carbon and combines to formvanadium carbides. The addition of vanadium to cast iron tends to pro-

duce small uniformly distributed graphite flakes and decreases fracture

grain size and increases the tensile strength, transverse strength, hardness,

and machinability. Vanadium is alloyed in cast iron with elements such

as nickel, copper, chromium, and molybdenum. Vanadium, when addedto cast iron, acts as an effective deoxidizing agent in the removal of dis-

solved oxygen from the iron. Little to no effect can be found on the

isothermal transformation of cast irons because at ausieniting tempera-

tures the vanadium carbide is quite stable.

QUESTIONS1. What is the general appearance of the fracture of gray, mottled, and white

cast iron? What is responsible for the different fractures of these three

classes of cast iron? What is their relative industrial importance?

2. What is the relation of gray cast iron to steel? In what respects are the

properties of gray cast iron inferior to those of steel? Give die variables that

affect the properties of gray cast iron. What characteristics make gray cast

iron a valuable engineering material?

3. How is a gray cast iron melted, and what raw materials are used? How is

the desired composition attained? How much carbon, silicon, and phos-

phorus are usually present?

Cast Iron 437

4. Name the primary metallographic constituents in gray cast iron? Describe

the role played by carbon and silicon in iniluencing the structure as seen

with the microscope. What is steadite? Describe its appearance.

5. What is the general relation between carbon and silicon, and the structure

and appearance of the fracture? If a small specimen of cast iron, poured

in a sand mold, contains 3.5 per cent carbon, what is the structure of the

matrix if the silicon is (a) 1.0 per cent? (b) 1.5 per cent? (c) 2.25 per cent?

For small castings, what percentages of total carbon and silicon would you

specify to obtain a tensile strength of (a) 33,000 lb. per sq. in.? (6) 38,000

lb. per sq. in.?

6. What is the effect of cooling rate on the structure of gray cast iron? If

the matrix of a small sand-cast bar has a structure corresponding to a steel

containing 0.60 per cent carbon, what would the matrix correspond to if

the casting was a block 6 to 8 in. square? What is the general effect of the

size of the graphite flakes on the properties? How is control of graphite-

flake size accomplished? Why is cast iron frequently annealed?

7. What tests are commonly used to evaluate gray cast iron for engineering

applications? Which is the best of these tests, and how is it made? How are

results reported? What is the approximate secant modulus of gray cast iron,

and how is it determined? What is the average compressive strength?

8. What properties would you consider to be representative of (a) a high-

test gray cast iron? (b) a low-strength iron? (c) an average American gray

cast iron? How do the endurance limits, impact resistance, and damping

capacity of gray cast iron compare with those of carbon steel?

9. Why are alloying elements added to gray cast iron, and when is die addi-

tional cost of these elements justified? What are the general effects of

(a) nickel? (b) chromium? (c) molybdenum?

10. How is malleable cast iron produced? What is it used for? Give the approxi-

mate tensile strength and elongation of American malleable cast iron. What

is the relation between the two? Compare endurance ratio, impact re-

sistance, and machinability of malleable cast iron with those of gray cast

iron.

11. What are the significant differences between pig iron and gray cast iron?

In what way are they similar? In what respect does mottled iron resemble

un-annealed, nodular iron?

12. How does the development, or formation, of graphite in gray cast iron

differ from that in malleable cast iron? Give at least one reason for using

a lower carbon content in malleable iron than in gray iron.

13. In Table 20.2, study the percentages of carbon and silicon recommended

for various types of castings and satisfy yourself why these analyses arc good

for the particular application.

14. Why is closer control of chemical composition necessary in producing high

strength gray iron?

15. Draw a section through a cupola and label the operating parts.

16. List the requirements of coke that would be suitable for use in a cupola.

17. Why are large amounts of sulfur and phosphorus undesirable in cast iron?

18. Differentiate between a graphitizing element and a carbide-forming element.

19. What roles are performed by alloying elements when added to cast irons?

20. What different types of graphite patterns are formed in gray cast iron.

CHAPTER

21

Mach inability, We ar Resistance,

and Deep - D rawing Properties

IrvincJ. Lkvinson, M.S., Professor of Mechanical En-

gineering, Lawrence Institute of Technology, Detroit,

Michigan

oINCE complex metallurgical, mechanical, andenvironmental factors are involved, a precise evaluation of machinability,wear resistance, and deep-drawing is extremely difficult. This problem ofevaluation has been attacked from many fronts, but to date only gener-

alities have been formulated—no hard and fast rules exist.

21.1. Variables Affecting Machinability

To remove metal by machining, the tool must first penetrate the

surface. The ease with which this occurs is a function of the static- anddynamic hardness of the cutter and the metal to be machined, the rate of

penetration, the depth of penetration, the frictional and shear forces

involved, and the relative rigidity of the system.

Once the tool has penetrated the surface the chip formed shouldreadily break. While spectacular, the long coiled chip is not the desired

one. Breaking the chip mechanically involves a loss of power and thus

is not the most desirable answer. The ideal method of breaking the chipis to provide a built-in breaker in the form of a weak and brittle con-

stituent. It is wrong to assume that because a metal is soft it is easy to

machine. Toughness is usually a corollary of softness, and, as a con-

sequence, the chips formed separate only with difficulty. In steel, a

completely spheroidized structure is considered the easiest to machine,although the presence of some small pearlite islands does promote chipbreakage.

21.2. Evaluation of Metallic Materials for Machinability

Experience has shown that a correlation exists between the basic

physical properties of the metals and their machinability. Figure 21.1,

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1910, p. 251)

for example, shows that machinability decreases with increasing tensile

strength. This property, together with hardness, is probably a good

criterion of machinability. It is also true that, under controlled condi-

tions, there is very little difference in the machinability of cast and hot-

rolled steels, of carbon and low-alloy steels, or of acid and basic steels,

provided tensile strength and hardness are the same.

Cast steels and nonferrous alloys may be more difficult to machine

than wrought material of the same composition and structure if the

skin of the casting has cooled rapidly enough to harden it, or if the sand

has not been removed thoroughly from the surface.

Many efforts have been made to devise a laboratory test to indicate

accurately relative machinability in service. Recent attempts have been


concerned with the work-hardening capacity of the material cut, ihe

object being to use the maximum hardness so induced as a measure of

machinability.

21.3. Free Machining Steels

Despite the impossibility of evaluating machinability accurately, ex-

perience has shown that the machinability of some materials can be im-

proved by certain treatments and, further, that there are alloys of certain

compositions which can be machined more easily at high speed than

other alloys of the same general type. Such materials are, therefore, used

widely in automatic machines for mass production of machined parts.

The machinability of those low-carbon steels which arc too soft andtough to machine easily is improved by heat treatment and, especially

by cold working. Quenching a low-carbon steel to increase the Brinell

hardness number from 90 or 100 to 170 to 210 improves its machinability

(provided, of course, that no martensite is formed by quenching) . Coldworking is even more effective in increasing the hardness and at the

same time decreasing the toughness; hence, most low-carbon steels that

must be machined economically are cold drawn or cold rolled. Thisoperation also has the advantage of producing a smooth surface.

The best and cheapest method of improving the machinability of low-

carbon steels is to add sulfur, or lead, or both. However, owing to the

Table 21.1. Relative Machinability Rating of Cold-Worked Free-Cutting Steels

Composition, per cent Machinabil-S.A.E. ity rating,

No. per centC Mn S P

X1112 0.08 to 0.16 0.60 to 0.90 0.20 to 0.30 0.09 to 0.13 1401112L* 0.08 to 0.16 0.60 to 0.90 0.10 to 0.20 0.09 to 0.13 1401112 0.08 to 0.16 0.60 to 0.90 0.10 to 0.20 0.09 to 0.13 100

X1314 0.10 to 0.20 1.00 to 1.30 0.075 to 0.150 0.045 max. 90XI 31

5

0.10 to 0.20 1.30 to 1.60 0.075 to 0.150 0.045 max. 871115 0.10 to 0.20 0.75 to 1.00 0.075 to 0.150 0.045 max. 81

1120 0.15 to 0.25 0.60 to 0.90 0.075 to 0.150 0.045 max. 78XI 330 0.25 to 0.35 1.35 to 1.65 0.075 to 0.150 0.045 max. 76X1335 0.30 to 0.40 1.35 to 1.65 0.075 to 0.150 0.045 max. 72XI 340 0.35 to 0.45 1.35 to 1.65 0.075 to 0.150 0.045 max. 701020 0.15 to 0.25 0.30 to 0.60 0.055 max. 0.045 max. 631040 0.35 to 0.45 0.60 to 0.90 0.055 max. 0.045 max. 601010 0.05 to 0.15 0.30 to 0.60 0.055 max. 0.045 max. 53

* Contains 0.20 to 0.25 per cent lead.

Machinability, Wear Resistance 441

high sulfur (plus high phosphorus for some of the steels), these "free-

machining" or "free-cutting" steels have much lower impact resistance

than low-sulfur steels of the same carbon content and should be em-

ployed, therefore, only where low impact resistance is not a handicap.

Leaded steel is a development of the past few years. The addition of

lead improves the machinability of low- as well as of high-sulphur steel.

Leaded steel is also more resistant to impact than high-sulphur material.

A somewhat similar effect is obtained in stainless steel with the addition

of Selenium. The relative machinability of the free-machining steels com-

pared with three low-sulphur steels is given in Table 21.1.*

Iron carbide particles, if favorably located and of favorable shape, act

as chip breakers; thus, unalloyed carbon steels containing 0.20 to 0.40

per cent carbon, when properly annealed, machine much more easily

than those containing carbon below 0.15 per cent (Table 21.1). The

best chip breaker is manganese sulphide (MnS) . In high-sulphur steel, in-

clusions of MnS, when well distributed as small particles, break up the

chips very effectively and, as Table 21.1 shows, greatly improve ma-

chinability. Despite their low impact resistance large tonnages of high-

sulphur steels are used.

21,4. Relative Machinability of Steel and Nonferrous Alloys

An approximate evaluation of the relative machinability of some com-

mon ferrous and nonferrous materials can be made by averaging the data

re|>orted by Bostont for drilling, milling, and planning tests. They were

reported as the horsepower required to remove one cubic inch of metal

per minute. The order is as follows:

Horsepower per cu. in.

Material per min.

Magnesium alloys 0.30

Bearing bronze 0.35

Aluminum alloy, 8% Cu 0.35

Free-cutting brass 0.38

Manganese bronze 0.60

Hard cast iron 0.60

Malleable iron 0.75

Yellow brass, unleaded 0.85

Free-cutting, steel, cold drawn 0.90

Forged carbon steel, 020% C 1.12

Nickel steel, 0.40% C 1 .20

Annealed copper 1 .35

Tool steel, high carbon 1 .60

Low-carbon, high-chromium steel 1 .70

Monel metal 1.70

• Adapted from a tabulation by H. W. Graham, Metal Progress, July, 1939, p. 53.

tO. W. Boston, Proc. Am. Soc. Testing Materials, v. 31, 1931, part II, pp. 388-421.


This is not an infallible evaluation, and the order might be changed for

some of the alloys if the conditions of the tests were changed slightly.

It does, however, show that it requires much more power to machine the

free-cutting steels than is needed for some of the magnesium and alumi-

num alloys, some of the free-cutting brasses and bronzes, or cast or

malleable iron.

Soft gray cast iron, containing no hard excess carbide or phosphide

particles, and malleable iron are the most readily machinable of all fer-

rous materials, usually ranking higher than high-sulphur steel. The ease

with which these materials are cut is due primarily to the softness of the

ferrite plus the numerous small particles of graphite which act as chip

breakers and as a lubricant.

In general, copper- and aluminum-base alloys are easy to machine.

There is, however, enough difficulty with these alloys so that special

compositions are used for material that is cut in automatic machines.

Free-cutting brass and bronze contain 1 to 4 per cent lead; their ma-

chinability is 70 to 100 as compared to 20 to 30 for the same alloys con-

taining no lead.

21.5. Types of Wear

The importance of wear resistance needs no amplification. Wear oc-

curs in machines when shafts rotate; when pistons and valves operate;

when a gear meshes and drives another; when steel or cast-iron wheels

start, run, or stop on rails; when brakes are set; when excavating, con-

veying, crushing, or mixing machines handle sand, clay, rock, coal, or

minerals; when agricultural implements arc used; and in a multitude of

other engineering applications. Some of the most destructive wear of

metals is caused by rapidly moving soft materials; for example, the wear-

ing of hardened steel parts by cotton or silk thread in textile machinery

and the wear of turbine blades by high-velocity gases.

The wear of metals may be divided broadly into metal-to-metal wear

and abrasive wear. Another division is possible: into wear under rolling

friction or under sliding friction and, further, according to whether

lubrication can or cannot be used. The latter is, of course, most important

in the metal-to-metal contact of machine parts. Lubrication may be

omitted from detailed consideration because, although it greatly influ-

ences the rate of wear, it probably has little or no effect on the type of

wear. Wear resistance is an important property of metals used in ma-

chines even if lubrication is of the best, because perfect lubrication is

more of an ideal than a reality. Moreover, even if lubrication is so

thorough that it prevents a metal-to-metal wear, it frequently happens

Machinabiiity, Wear Resistance 443

that extraneous grit in the lubricant causes severe abraisive wear. Another

important factor entering into the wear of machines is corrosion of the

wearing surfaces caused by condensed moisture or acids in gases and

lubricants.

Wear involving a single type is rare; in most machinery, both abrasive

and metal-to-metal wear occur. This is the case in such parts as shafts

revolving in bearings, where abrasive wear occurs until the shaft and

bearing are "run in" and worn to a mirror finish, after which metal-to-

metal wear predominates. Since in such service wear can rarely be

avoided completely, even with the best lubrication, it is common to use

a hard metal and a relatively soft one together, the softer material being

used (as in a bearing) for the part which is most economical to replace.

In some kinds of excavating and material-handling equipment, abrasive

wear is the principal type involved.

21.6. Variables Affecting Wear Resistance

Like machinabiiity, wear resistance depends upon two factors, the

metallurgical and the mechanical. The latter has been discussed briefly

in the previous paragraphs and may, consequently, be dismissed with

the statement that wear of metal depends as much, or even more, on

the conditions of service as upon the metal used.

From the metallurgical standpoint, wear resistance is inversely pro-

portional to machinabiiity and, in general, directly proportional to hard-

ness. Hardness, however, is not the only metallurgical factor of im-

portance. Wear by abrasion, like machinabiiity, involves tearing off small

particles of the metal. Consequently, strength and toughness are im-

portant factors in resisting wear,

The wear resistance of the various carbon and alloy steels receives

attention elsewhere; it can be dismissed here with a brief mention of

the general types of ferrous material used for wear resistance. The first

and most common, especially for metal-to-metal wear, is a cast iron or

a steel with a surface of high hardness. Usually, in an application require-

ing high surface hardness, considerable toughness, especially of the core,

is also required. This combination of properties is attained by quenching

and tempering an alloy steel or by case carburizing or nitriding a carbon

or alloy steel and heat treating to produce high surface hardness com-

bined with considerable ductility in the core.

For abrasion resistance, two kinds of materials are used: (1) an aus-

tenitic steel, which is relatively soft but which, under the severe cold

working of the abrasion, transforms to martensitic; and (2) a carbon or

alloy steel, on which a hard surface layer of some special alloy is de-

posited by electric arc or other method.


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Fig. 21.2. Effect of carbon content and heat-treatment on metal-to-metal wearof carbon steels. (Rosenberg)

21.7. Evaluation of Steel for Wear Resistance

The evaluation of the wear resistance of steels is one of the most diffi-

cult problems a metallurgist is called upon to solve. Simulating actual

service conditions by a laboratory test—the most important phase of this

problem—can be accomplished only rarely and then only if wear in

service occurs rapidly and if it is possible to design a machine which

duplicates service conditions exactly. Wear is usually a slow process.

Shidle* notes that, when a 5-ton truck has finally worn out, it has lost

only 5 lb. of metal. The useful life of the average automobile engine is

probably 100,000 miles; hence the difficulty of devising a test which will

simulate such service conditions accurately.

Several types of laboratory machines have been developed that deter-

mine the wear resistance of a material fairly rapidly and under relatively

closely controlled conditions. For determining metal-to-metal wear, the

N. G. Shidle, Automotive Ind., v. 66, Mar. 19, 1932, p. 449.


most common is the Amsler machine, which tests a specimen under pure

sliding or pure rolling friction, or a combination of the two. Pressures

can be varied at will, and wear can be determined at any temperature,

with or without lubrication. Some data on metal-to-metal wear at room

temperature of carbon steels, together with Rockwell hardness, are shown

in Fig. 21.2.*

For the determination of abrasive wear, two machines are commonly

employed: the Brinell and the Spindel (used widely in Germany) , which

use sand or emery as the abrasive. Much care is necessary in the selection

of the abrasives as these may not be uniform. The order of merit of a

series of steels and alloys with one abrasive may change completely if

another abrasive or a different lot of the same abrasive is used. It has

been said with considerable justification that machines for determining

resistance to abrasive wear are a better test of the abrasive than of the

metal.

The important fact to be recognized is that any wear-testing machine

determines the relative resistance of a series of metals or alloys under a

few carefully regulated conditions, but that the results obtained, while

valuable for comparing the materials, cannot be translated quantitatively

into probable life in service unless the test used duplicates in all respects,

including time, the conditions of service.

21.8. Importance of Deep-Drawing Properties

Practically all steels and most of the nonferrous alloys can be de-

formed cold, but how much deformation they will withstand without

failure, or without annealing to restore ductility, varies greatly, depend-

ing upon composition, structure, and other factors. In general, the higher

the ductility as shown by the tensile test, and the lower the hardness, the

more a material can be deformed without fracture. Thus, low-carbon

wire can be drawn through a die to a smaller size than high-carbon wire

without intermediate annealing to restore the original ductility and soft-

ness.

As cold forming by pressing or stamping is cheap and readily adapted

to large-scale production, this method is used widely, especially by those

industries producing automobile fenders and bodies and a variety of

small stamped utensils for domestic and other uses. With the exception

of aluminum and stainless steels, practically all deep-drawn products

are coated—galvanized, tinned, painted, enameled, lacquered, or plated.

High tensile strength is usually not important; consequently, the ma-

• S. J. Rosenberg, Iron Age, v. 128, 1931, pp. 1366, 1367.


terials most widely used are those with the least tendency to harden

when cold worked. The principal ferrous materials are, therefore, ingot

iron, unalloyed basic open-hearth steels containing 0.04 to 0.10 per cent

carbon, and the soft stainless steels, all in the form of thin-annealed or

annealed and slightly cold-worked sheets.

As the manufacture of deep-drawn products, such as of fenders and

bodies by the automotive industry, is a continuous low-cost operation,

it is common practice for fabricators of the grade of sheet to arrange

their various operations so that the material is worked until it strain

hardens almost to the breaking point before annealing is necessary. Steel

with a ductility just slightly below the standard will, therefore, show

excessive breakage.

Consumers' requirements have become more severe year by year. It is

only necessary to compare the automobile fender of the last five years

with those of fifteen years ago to prove how the steel industry has kept

up with this demand by raising the quality of its deep-drawing stock.

There is still much room for improvement, both on the part of the steel

maker and on the part of the consumer of deep-drawing sheets. What is

needed most urgently is a test giving a reliable indication of drawability

and, even more importantly, a standardization of consumers' require-

ments.

21.9. Evaluation of Steel for Deep Drawing

Drawability is a weighted average of a large number of components,

including, among others, yield point and yield ratio in tension, hardness,

elongation, reduction of area, and possibly impact. Drawing different

shapes involves a different relation of the components; hence, even if all

the components were known and could be weighted correctly, it would

be necessary to change the weighting for different drawing operations.

The difficulty of weighing even the known components accurately is

increased by the virtual impossibility of determining accurately reduction

of area—probably the most important component—on thin test speci-

mens Some users of deep-drawing stock have arrived at certain combina-

tions of tensile strength, elongation, and hardness for different thicknesses

of sheet to indicate satisfactory drawing properties, but such correlation

is worthless to others who draw their product under slightly different

conditions.

To overcome these difficulties, and in the hope that a single test could

be devised to measure drawability, much time and effort have been

devoted to the cupping test. Several varieties have been developed, chief

of which arc the Erichsen, used in Europe, and the Olsen, used in


• ';</

Fie. 21.8. Structure and Erichsen cupping tests o[ (A) fine-grained and (B)

coarse-grained low-carbon sheet steel. (F. Kbrber, Stahl u. Eisen, v. 47, 1927, p. 1158)

the United States.* Essentially, the test consists of supporting a speci-

men of the sheet between two dies, while a ball or a plunger is brought

down against the specimen forcing it into the shape of a cup. The flow

of the metal can be followed in a mirror, and the first fracture can be

observed. The depth of the cup, or the pressure necessary to cause

failure, is measured. The test gives some information on how the ma-

terial deforms but not enough invariably to predict how it will deform

in service. Metallurgical opinion on the value of cupping tests is divided;

some metallurgists consider them not only unreliable, but actually mis-

leading.

Because the structure and the hardness of steel are important factors

in deep drawing, great care is exercised in the melting, rolling, annealing,

and finishing of the sheet so that the grain size will be controlled ac-

• Cupping tests and their significance are discussed by S. Epstein and F. T. Sisco.

in The Alloys of Iron and Carbon, Vol. I, pp. 272-278, and Vol. II, pp. 646-650 re-

spectively.


Strain-

Fic. 21.4. Stress-strain curves of low-carbon deep-drawing steel showing upper yield

point A, lower yield point B, and yield-point elongation E. Slight cold rolling reduces

the amount of yield-point elongation (second curve from right), and further cold

working eliminates it entirely (right).

curately and the hardness will be within certain limits. A fine-grained

steel can be deformed to a greater extent and has a much better surface

after deforming than coarse-grained material. This is shown in Fig. 21.3.

The surface of the fine-grained steel is smooth after cupping; the coarse-

grained material, on the contrary, has a crinkled "orange peel" surface.

21.10. Yield-Point Elongation, Stretcher Strains, and Deep-DrawingProperties

Most annealed low-carbon steel used for deep drawing (0.04 to 0.10 per

cent carbon) has two yield points, an upper and a lower one. The upper

yield point (A in Fig. 21.4.) is the stress at which yield begins, and its

location is dependent to a considerable extent upon testing conditions.

The lower yield point (B in Fig. 2 1 .4) represents the lowest stress to which

the applied load sinks during stretching; this value is probably morecharacteristic of the material than the upper point.

The distance that the material stretches while passing through the yield

point is known as the yield-point elongation (E in Fig. 21.4) , and this

value is an important one in determining whether the sheet will have

satisfactory deep-drawing properties.

When annealed low-carbon sheet is deep drawn, strain lines—knownas stretcher strains—frequently appear on the surface (Fig. 21.5). This

defect is analogous to the orange-peel surface caused by large grains

(Fig. 21.3). These same stretcher strains appear on the surface of a

tensile-test specimen which has a double yield point and considerable

yield-point elongation, and the severity of the defect apparently is related

directly to the amount that the specimen stretches between the upper


Fie. 21.5. Stretcher strains in cold-drawn low-carbon sheet. (J. Winlock and G. L.

Keltey, Trans. Am. Soc. Steel Treat., v. 18, 1930, p. 241)

and lower yield points. By subjecting deep-drawing sheet to a slight

amount (0.5 to 2 per cent) of cold rolling on a "temper mill," the yield-

point elongation is eliminated and stretcher straining is prevented. The

stress-strain curve for a steel in this structural condition is shown at the

right in Fig. 21.4.

There is still much to be learned about deep-drawing steels and their

behavior under extreme cold deformation. In general, microscopic ex-

amination, Rockwell B hardness, yield ratio, and yield-point elongation,

plus some form of cupping test, give about as much information on draw-

ability as can be secured at present.

21.11. Deep-Drawing Properties of INonferrous Alloys

In general, the working of metals and alloys of suitable properties by

deep drawing is an economical operation, and a considerable tonnage

of nonfcrrous materials is fabricated into finished and semifinished arti-

cles by this process. Second only to deep-drawing low-carbon steel in

tonnage is alpha brass, containing more than 62 per cent copper and

less than about 38 per cent /inc. The variables affecting the deep-draw-

ing properties of alpha brass are very important and have, as a result,

been subjected to more study than similar properties for any other deep-

drawing material with the possible exception of steel for automotive

fender and body stock.

The principal variables that affect the cold-working characteristics of

alpha brass are composition, hardness, and grain size. Ductility is lowered


by small amounts of tin, lead, and iron, and the presence of much beta

constituent should be avoided. Beta is a hard constituent that is likely

to occur if the copper is less than 62 or 63 per cent and may be present

even if the copper is 65 or 66 per cent. Considerable brass of this com-position is deep drawn. To be readily subjected to a large amount of cold

deformation, the grain size should be small; the optimum size depends onthe nature of the drawing operation and to some extent on the degree of

smoothness desired on the surface. An average grain size of from 0.035

to 0.10 mm. is about right. Brass strain hardens readily and must be an-

nealed at intervals if the cold working is severe. The annealing tempera-

ture varies from 900 to 1100°F. (480 to 590°C.) and care is exercised to

prevent grain growth during annealing.

Deep-drawn brass will occasionally have an orange-peel surface, similar

to that found in deep-drawn steel (Fig. 21.3), caused usually by large

grain size. The principal defect to which deep-drawn alpha brass is

subject is season cracking, a variety of stress-corrosion cracking that is

encountered in a large number of nonferrous alloys and in some steels,

especially the stainless grades. Season cracking is due to a combination

of high internal stress and some forms of corrosion and has been espe-

cially troublesome in deep-drawn cartridge cases. It can be prevented byannealing to relieve residual stresses.

Most of the other soft metals, especially aluminum, copper, lead, tin,

nickel, and zinc, and some of their alloys are deep drawn commercially

into a variety of useful shapes. In most cases, special techniques andequipment—for example, impact extrusion for collapsible tubes used

widely for drugs, cosmetics, and toilet articles—must be used, and for

each metal there are special problems to be solved, many of which are

metallurgical. For example, one of the principal problems in deep draw-

ing aluminum is to prevent the abnormal grain growth which is likely to

occur in annealing, and which may cause a rough surface and premature

breakage of the deep-drawn part. In deep drawing high-purity copper,

abnormal grain growth is also a problem.

QUESTIONS1. Calculate the theoretical horsepower required to machine a hollow monel

metal shaft having an outside diameter of 3.250 inches, an inside diameter

of 1.625 inches, and a length of 36.00 inches, from a blank 36>/j inches

long, having a outside diameter of 3% inches.

2. Design a dynamometer to determine the power required to turn a given

material in a lathe. Make use of the spring characteristic s of the tool holder,

and show how the power could be calculated on the basis of the data

obtained.


3. Compare the variables affecting machineability with those affecting wear re-

sistance. What conclusions can you draw?

4. Discuss the effect of wear on fatigue life.

5. Differentiate between erosion, galling, and corrosion as a mode of wear.

List some examples of cadi.

6. Discuss the difference between rolling friction and sliding friction as a

mode of wear.

7. Discuss the possibility of using radioactive materials to study the rate of

wear.

8. List the environmental factors that effect wear resistance. Why are these

as important as the physical properties of the material itself?

9. Describe the cupping test and discuss its limitations.

10. What is meant by "double-yield point" and how does it effect the deep-

drawability?

11. What benefits are obtained in having a homogeneous structure in a material

which is to undergo deep drawing?

12. Discuss the effects of lattice configuration on deep drawing properties.

CHAPTER

Corrosion and Oxidation

22 Mars G. Fontana, Ph.D., Professor and Chairman, De-

partment of Metallurgical Engineering, The OhioState University, Columbus, Ohio

Richard Edward Grace, Ph.D., Associate Professor of

Metallurgical Engineering, Purdue University, Lafay-

ette, Indiana

L4ORROSION causes destruction or deteriora-

tion of metals and alloys because of electrochemical or chemical reaction

with the environment. Corrosion can be considered as metallurgy in

reverse. For example, iron and steel are made from iron-oxide ores, and

they revert to oxides when rusting occurs.

The annual cost of corrosion in the United States is estimated at §6

billion. Practically all environments—water, soils, acids, alkalies, gases,

oils, etc.—are corrosive to some degree. Corrosion of fuel systems in

automobiles alone costs $100 million per year. One plant spends $2

million annually for painting to prevent external rusting of steel equip-

ment. Some of the results of corrosion are poor appearance, high opera-

tion and maintenance costs, plant shutdowns, contamination of products,

loss of valuable products, and safety hazards.

Corrosion can be classified in two general ways, namely (1) wet and

dry or (2) direct combination and electrochemical. Wet corrosion occurs

when liquids are present, usually electrolytes. Dry corrosion occurs in.

vapors and gases: a liquid phase is not present. Dry corrosion is usually

associated with high temperatures.

Direct combination usually involves reaction between a metal andnonmetallic elements or compounds such as steam, oxygen, sulfur dioxide,

and chlorine at high temperatures similar to dry corrosion. This sub-

ject is discussed later under high-temperature oxidation.

22.1. Electrochemical Corrosion

The electrochemical theory of corrosion is the accepted theory. There

are two basic requirements: (1) anodes and cathodes must be present to

452

Corrosion and Oxidation 453

form a cell, and (2) direct current must flow. The anodes and cathodes

may be very close together (local cells) or they may be far apart, for

example, two sections of pipe. The current may be self-induced or it may

be impressed on the system from an outside source.

The anode, or anodic area, is where corrosion occurs and where current

leaves the metal. The cathode, or cathodic area, does not corrode; cur-

rent enters the metal here. Anodes or cathodes can form on a single

piece of metal because of local differences in the metal or in the environ-

ment. The metal at the anode dissolves, loses electrons, is oxidized, and

becomes an ion. This situation is shown schematically in Fig. 22.1. The

Fie. 22.1. Formation of Ferrous Ions in the Corrosion of Iron.

electrons (e) are left on the metal and travel through it to the cathode.

Current flows in the opposite direction. Immersion or contact with an

electrolyte is required to complete the circuit or to carry current from

the anode to the cathode.

Corrosion in pure water (or acids) occurs as follows:

Fe— 2e = Fe++ (Anode reaction)

2H+ + 2e = H2 (Reduction at cathode)

The overall reaction is:

Fe + 2H+ = Fe+++ H 2 .

The quantity of current which passes through this cell is directly pro-

portional to the amount of metal that corrodes. For example, one ampere

per year is equal to about 20 lbs. of steel. Anything that changes these

reactions or the cell circuit could reduce or increase corrosion. If the


hydrogen bubbles collect on the cathode and thus form an insulating

blanket, current flow is reduced and corrosion is practically stopped.

This is called polarization and, in this case, cathodic polarization

because it occurs on the cathode.

Most waters contain dissolved oxygen which combines with the hydro-

gen as shown in Fig. 22.2. This is cathodic depolarization and corrosion

proceeds. This is the basis for one of the earliest forms of corrosion con-

trol, namely deaeration of boiler water.

Steel Wafer

+ 2-<-2H2

Fig 22.2. Corrosion of Steel by Ordinary Water Containing Oxygen. Depolarization.

In neutral or alkaline waters the anode reaction is the same, but the

cathode reaction is:

H 2 + i/2 2 + 2e = 2 OH-.The overall reaction is

Fe + H2 + i/?

2 = Fe (OH) 2 .

Fig 22.3 is a basic diagram which summarizes the requirements for

corrosion of metals by electrolytes.

Differences in the metal surface, or lack of homogeneity, may be due to

impurities, grain boundaries, orientation of grains, differences in composi-

tion of the microstructure, differences in stress, scratches, nicks, and holes.

It might be said that nature abhors a vacuum—corrosion abhors homo-

geneity.

22.2. EMF Series and Passivity

Different metals have different tendencies to corrode or vary in re-

activity. The familiar Electromotive Force Series, shown in Table 22.1,


lists elements in order of decreasing corrosion activity. Hydrogen is

listed and shown as zero because a hydrogen electrode is used as the

reference electrode or half cell to measure the electrode potentials of the

other elements. The more negative the potential of an element is, the

greater the tendency to corrode.

Table 22.1

Electromotive Force Series Galvanic Series—Sea Water

Cation Volts Magnesium and magnesium alloys*

Zinc

Na+ -2.71* 2S Al (Comm. pure)

Mg++ -2.40 CadmiumA1 +++ -1.70 24 ST Al (4.5% Cu, 1.5% Mg, 0.6% Mn)Be++ -1.69 (Steel or iron

[Cast ironU + +++ -1.40

Mn+ + -1.10 13% Cr iron (active)

Zn++ -0.76 Ni-Resist (High Ni cast iron)

Cr ++ -0.56 (l8-8 (active)

U8-8-M0 (active)Cr+++ -0.50

Fe ++ -0.44 Lead-tin solders

Cd++ -0.40 LeadCo++ -0.28 Tin

[Nickel (active)

llnconcl(active) (80% Ni, 13% Cr, 7% Fe)Ni ++ -0.23

Sn++ -0.14

Pb+ + -0.12 fHastelloy B (60% Ni, 30% Mo, 6% Fe, 1% Mn)Ichlorimct 2 (66% Ni, 32% Mo, 1% Fe)H+ 0.00

Sb+++ +0.10 Brasses (Cu-Zn)

Bi ++ + +0.23 CopperCu++ +0.34 Bronzes (Cu-Sn)

Cu + +0.47 Cupro-nickcU (60-90% Cu, 40-10%Ni)

Ag+ +0.80 Moncl (707c Ni, 30% Cu)

Pb++ ++ +0.80 Silver solder

Pt ++++ +0.86 (Nickel (passive)

l,Inconel (passive)Au+++ +1.36Au + + 1.50t rChromium steel (passive) (11 to 30% Cr)

18-8 (passive)

18-8-Mo (passive)

fHastelloy C (62% Ni, 17% Cr, 15% Mo)ichlorimct 3 (62% Ni, 18% Cr, 18% Mo)Silver

TitaniumGraphite

GoldPlatinum t

* Anodic end

t Cathodic end


According to the EMF Series, the addition of chromium to iron should

increase corrosion; yet this mixture results in stainless steels. Aluminumis active hut it is widely used for corrosion resistance. This effective

increase in nobility or corrosion resistance is due to passivity, which can

be simply defined as "a condition that causes the metal not to corrode

when it should." Passivity is generally due to surface films which act as

barriers between the metal and its environment. These films could be

absorbed gases or adherent corrosion products. A series for commercial

metals and alloys in a given environment could show different posi-

tions than indicated by the EMF Series. Table 22.1 gives the order for

the Galvanic Series (sea water) . Chromium, aluminum, and silicon

as elements or as alloy additions to steel are the three most important

from the passivity standpoint.

WET CORROSION

The Eight Forms of Corrosion

Corrosion may be classified into the eight forms in which it manifests

itself. The basis for this classification is the appearance of the corroded

metal. Some of these forms are unique, but all are interrelated. This

classification is arbitrary, but it covers practically all corrosion failures and

problems. The eight forms are (1) uniform corrosion, (2) galvanic or

two-metal corrosion, (3) concentration cell corrosion, (4) pitting, (5)

intergranular corrosion, (6) stress corrosion, (7) dezincification, and (8)

erosion-corrosion.

22.3. Uniform Corrosion

Uniform corrosion or overall general attack occurs when anodic and

cathodic areas keep shifting, and corrosion takes place more or less uni-

formly over the entire exposed surface. The metal becomes thinner and

eventually fails. This form of corrosion accounts for most of the destruc-

tion of metals on a tonnage basis. From the technical standpoint, how-

ever, uniform attack causes the least concern largely because service

life can often be quite accurately estimated on the basis of relatively

simple corrosion tests. It is the localized form of corrosion that often

results in an unexpected failure.

22.4. Galvanic or Two-Metal Corrosion

When two dissimilar metals are in contact, or otherwise connected

electrically, and exposed to a corrosive electrolyte, coirosion of the less

resistant metal is increased and corrosion of the more corrosion-resistant

metal is decreased. All of the former usually becomes the anode, and

the latter the cathode. Current flows here similar to the situation shown

Corrosion and Oxidatio7i 457

Environment

Currentleaves ./y

Currententers

Fie. 22.3. Basic Diagram Showing Requirements for the Corrosion of Metals by

Electrolytes.

in Fig. 22.3. The driving force for the current is the potential generated

by the electrodes. This, of course, is the principle of the so-called dry

battery.

Table 22.1 shows a practical galvanic series for metals and alloys ex-

posed to actual sea water. The farther apart two metals are in this

series the greater the potential when they are coupled. The material

higher in the series becomes anodic to one below it. Magnesium and

copper would form a bad galvanic couple as far as corrosion is concerned.

In many cases, the accelerated attack occurs on the anodic metal near the

junction with the cathode. The effect spreads farther in a linear direction

as the conductivity of the environment increases.

An important effect that is often overlooked, and rapid failures result,

is the area effect, or the ratio of cathodic area to anodic area. A large

cathode and small anode accelerate further the attack on the anode.

For example, a metal by itself in a given environment may corrode at

X mils per year; the rate may be lOx when coupled to a more noble metal;

but it may be 1000X when the area ratio is large. Fig. 22.4 illustrates this

effect. Note that the steel rivets (small anodic area) are completely

corroded; whereas, the other specimen with a large anodic area still

represents a strong joint.

An excellent practical example of rapid failure because of an unfavor-

able area ratio concerns large tanks handling a solution mildly corrosive


;*V'

*&*:

-&&-

COPPER RIVETS

IN STEEL PLATE

LARGE ANODE

SMALL CATHODE

STEEL RIVETS

IN COPPER PLATE

LARGE CATHODE

SMALL ANODE

Fie. 22.4. Effect of Area Relationship on Corrosion of Rivets in Sea Water (15

Months). [Courtesy The International Nickel Co.)

to steel. The bottoms were made of 18-8 stainless steel, and they were

welded to ordinary carbon steel sides. The steel was covered with a

baked coating. Small defects in the coating exposed tiny anodic areas

which were in electrical contact through the welds with the larger

bottom area of stainless steel. The steel in the sidewalls near the welds

failed by perforation in several weeks.

22.5. Concentration Cell Corrosion

Anodes and cathodes can form because of differences in the environ-

ment. For example, a potential difference can be measured between two

identical copper electrodes immersed in different concentrations of hydro-

chloric acid. The cells formed because of differences in the environment

are called concentration cells. This form of corrosion is also designated

crevice corrosion, gasket corrosion, and deposit corrosion, and is usually

associated with stagnant conditions.


Low Metal Ion Concentration

High Metal Ion Concentration

Metal Ion Concentration Cell Riveted Lap Joint

High Oxygen Concentration

Low Oxygen Concentration

Oxygen Concentration Cell

Fig. 22.5. Typical Examples of Concentration Cell Corrosion. (Courtesy The Inter-

national Nickel Co.)

The most common forms of concentration cells are oxygen cells

and ion cells as shown in Fig. 22.5. Fig. 22.6 shows attack on the face of

the flange of a pipe because of concentration cell corrosion. The inside

surface of the pipe is in good condition.

22.6. Pitting

Pitting corrosion is readily recognized because of pits or holes. This

is one of the most vicious forms of corrosion and one of the hardest to

predict. The anodic area remains stationary and corrosion progresses in-

wardly on one spot. Pitting can be considered as the intermediate situa-

tion between no corrosion (complete passivity) and uniform corrosion

(entirely active) in that the surface breaks down and corrodes only in

relatively small areas.


Fig. 22.6. Gasket Corrosion Caused by Concentration Cells.

Pitting often starts because of concentration cell effects such as under

a permeable deposit. The environment under the deposit becomes ex-

hausted of oxygen, or increases in ion concentration; whereas, the sur-

rounding metal away from the deposit is exposed to essentially a constant

concentration of oxygen or ions. The rate of penetration often acceler-

ates because the pit acts as a crevice and thereby increases the concentra-

tion cell effect. An unfavorable ratio of cathode to anode also exists.

This is one reason why the stainless steels often show severe pitting in

some environments with most of the surface showing no attack.

22.7. Intergranular Corrosion

Intergranular corrosion consists of selective or localized attack at the

grain boundaries of a metal or alloy. In some cases, complete disintegra-

tion of the metal results even though a relatively small portion of the

total weight is dissolved. The austenitic stainless steels, such as 18-8, are

particularly susceptible when they are not properly heat treated. If this

steel is heated in the range of about 950° to 1400°F. for an appreciable

time, it becomes sensitized or susceptible to intergranular attack in


environments where high chromium content is needed for corrosion re-

sistance. The commonly accepted explanation for this phenomenon is

that chromium carbides (Cr23C„) precipitate in grain boundaries. This

removes chromium from solid solution and lowers or depletes the chro-

mium in areas adjacent to the grain boundaries. This lowers the cor-

rosion resistance of these areas, and corrosion proceeds aided by galvanic

corrosion and a large cathode area represented by the grains, which are

cathodic. Fig. 22.7 shows intergranular attack, or weld decay as it is

often called.

Fife 22.7. Intergranular Corrosion of Welded 18-8 Stainless Steel.

The plate to the left of the weld in Fig. 22.7 contains about 0.60%

titanium. These steels are stabilized with niobium (columbium),

titanium, and tantalum, which are strong carbide formers and thus

preferentially tie up the carbon so that chromium carbides cannot form.

The relatively new, very low carbon steels (0.03% max.) can stand much

more abuse from this standpoint. Another procedure to minimize inter-

granular corrosion is to finally heat the part or equipment to about

1950°F. at which point the chromium carbides dissolve, and then rapidly

cool by quenching through the sensitizing temperature zone.

Sometimes the stabilizing element fails to do its duty, as shown by

Fig. 22.8. In this case, intergranular corrosion occurs in a narrow zone

in the parent metal immediately adjacent to the weld or melted metal.


I"ic. 22.8. Knife-Line Attack of Stabilized 18-8 Stainless Steel.

The attacked zone is usually a few grains wide and hence has been

christened knife line attack. The mechanism is briefly as follows. The

metal next to the weld is heated to a very high temperature, and when

rapidly cooled, the niobium stays in solution because niobium carbide

does not have time to form. If this material is then heated in the sensitiz-

ing zone, chromium carbides form as if no stabilizing element were

present. Experiments have shown the niobium carbides dissolve at

temperatures over 2250°F. One recommended cure for this situation is to

heat the material to around 1950°F, where chromium carbides dissolve

and niobium carbides form.

Other alloys are also sometimes attacked intergranularly, as shown in

Figure 22.9, which is typical of attack on nickel and high-nickel alloys

when exposed to sulfur-bearing atmospheres at high temperatures.

22.8. Stress Corrosion

Stress corrosion could be defined in a general way as the acceleration

of corrosion by stress. In some cases, more or less over-all corrosion is

increased because of stresses in the metal. Stress corrosion is more gener-

ally interpreted as the type of attack that causes cracking, and it is this

form of failure that is under discussion here.

In most cases, the magnitude of the stresses required to cause stress

corrosion is high. These stresses could be due to applied loads or to

residual stresses, although failures because of the former are relatively rare

because most structures are over-designed. The most common of the

residual stresses that cause stress corrosion are those resulting from cold

working or cold forming and also the "locked-in" stresses resulting from

Corrosion and Oxidalion 463

Fig. 22.9. Intcrgranular Corrosion of Nickel at High Temperatures. Top SOX.

Bottom 500X.

welding. In a few cases, plant failures have occurred because of severe

thermal gradients in the metal, which cause high stresses. Tensile

stresses are required for cracking.

Stress corrosion usually occurs when over-all or uniform corrosion is

low and often negligible. It is somewhat similar to pitting in this respect

in that most of the metal is passive but active sites develop. The presence

of stress often accelerates this localized attack. A crack is initiated and

propagates to failure. Two of the earliest and classic examples of stress

corrosion arc the "season cracking" of brass cartridge cases and the

"caustic embrittlement" of riveted steel locomotive steam boilers. Am-munition became worthless during the wet seasons. Boilers exploded be-

cause of cracks starting near the rivets or stressed areas.

Statements have been made to the effect that all metals and alloys

could be made to crack under selected conditions of stress and corrosion.

Fortunately stress corrosion happens much less frequently than other

forms of corrosion but cracking failures are often serious and unexpected.

Table 22.2 lists a number of metals and alloys and the environments in

which stress corrosion may occur. This does not mean that failure wouldalways develop in these combinations, but one should be aware of the

possibilities. For example, nickel is widely used in caustic services, and

cracking is difficult to develop unless severely cold-worked metal is used.

On the other hand, brass cracks readily in ammonia, and "as welded"


2 50

.200 —>•

g150

100

10 20 30 40 50

% CONCENTRATION

Fig. 22.10. Effect! of Temperatures and Concentration on Cracking of As-WeldedSteel Equipment in Sodium Hydroxide Based on Service Experience.

steel will fail quite readily in strong sodium hydroxide at elevated tem-

peratures. Fig. 22.10 shows the relation between temperatures and con-

centration of caustic on the cracking of as-welded steel. These data are

based on actual service performance.*

A number of theories have been proposed for the mechanisms of stress

corrosion. The Electrochemical Theory calls for susceptibility to selective

corrosion along more or less continuous paths and a high stress tending to

pull the metal apart along these paths. These continuous paths could be

along grain boundaries where a precipitated anodic phase, or an anodic

depleted zone resulting from precipitation, may exist. Trenching or

pitting results causing stress raisers or notches where cracks begin to

form and then propagate because of corrosion and stress at the point of

the crack. Differences in potential between the grain boundary area and

the grains have been observed. This theory explains the intergranular

cracking observed in some systems.

The Mechanical Theory states briefly that localized corrosion forms

the notch or stress raiser, but the propagation of the crack is then due to

mechanical effects, and corrosion docs not play a continuous role.

According to the Strain Accelerated Decomposition Theory an anodic

phase is formed because of the presence of stress. In other words a site

that is not susceptible to attack becomes susceptible because of a phase

decomposition or formation caused by stress.

'Corrosion, 7, 295-302 (1951), H. Vv". Schmidt, P. J. Gegner, G. Heincman.


Tabu: 222. Environments That May Cause Stress Corrosion of Metals and Alloy's

Material

Aluminum alloys

Copper alloys

Cold alloys

Inconel

LeadMagnesium alloys

Monel

Nickel

Ordinary steels

Stainless steels

Titanium

Environment

NaCl-HjOj solutions

NaCl solutions

Sea water

Air, water vapor

Ammonia vapors and solutions

AminesWater, water vapor

FeCh solutions

Acetic acid—salt solutions

Caustic soda solutions

Lead acetate solutions

NaCl-KsCrO-i solutions

Rural and coastal atmospheres

Distilled water

Fused caustic soda

Hydrofluoric acid

Hydrofluosilicic acid

Fused caustic soda

NaOH solutions

NaOH-NaaSiO* solutions

Calcium, ammonium, and sodium nitrate solutions

Mixed acids (HsSOi-HNOs)

HCN solutions

Acidic HiS solutions

Sea water

Molten Na-PI» alloys

Acid chloride solutions such as MgClz and BaChNaCl-H202 solutions

Sea water

HjSNaOH-H2S solutions

Red fuming nitric acid

The Anodic Shift Theory explains the propagation of the crack by

stating that when a certain stress concentration is brought about by cor-

rosive attack (pitting or trenching) , some plastic deformation takes place

and the electrode potential of the metal is shifted in an anodic direction

at the crack base because of the latent energy produced by the deforma-

tion. The anodic area produced in this manner is attacked, and the crack

again produces stress concentration sufficient for plastic behavior. This

process can be visualized as a continuous shifting in potential and simul-

taneous metal removal.

The Film Theory is another form of the electrochemical theory of

stress corrosion but suggests a different driving force than composition

differences. Stress corrosion is started by the usual electrochemical


attack, and stress concentration at the base of the incipient crack is

sufficient to tear or break any protective film that may be present. Thecrack is propagated by the simultaneous corrosion of the exposed metal

and tearing of any film that may form. The fact that potential differences

do exist between filmed and film-free metal surfaces has been confirmed

many times. In one case, the potential at the base of notches in alloys

subject to tensile stretching was found to be as much as 0.7 volt anodic

to the remainder of the specimen. Attack in these bare areas becomespermanently faster than on the filmed surface, and preferential corrosion

occurs.

One of the mysteries of stress corrosion concerns an explanation of why,in some cases, intergranular cracking occurs while in others the cracks

are transgranular or across the grains.

The work of Priest, Beck, and Fontana* shows that corrosion and stress

must be present simultaneously for cracking to occur. Motion pictures of

crack propagation at actual speed show that cracking stops when cathodic

protection is applied, and propagation resumes when it is removed. Thisindicates that the mechanical theory is not correct.

Failure is called corrosion fatigue when cyclic stresses are involved.

22.9. Dezinci fixation

As this term implies, the phenomenon of dezincification was first ob-

served on brasses. The zinc is selectively leached out of the alloy leaving

a brittle, weak, porous mass consisting predominantly of copper plus

copper oxides. The obvious mechanism is solution or corrosion of the

brass; then the zinc stays in solution, and the copper plates back on.

Dezincification can be readily observed because the attacked areas showthe color of copper as compared to the yellow brass. Brasses with 15 percent or less zinc are practically immune, and additions of tin, arsenic,

phosphorous, and antimony increase the resistance of brasses to dezincifi-

cation.

De/incification can occur uniformly, as shown in Fig. 22.11, or as the

plug type, or attack in spots. Fig. 22.12 shows a section through a de-

zincified plug and also the porous nature of a dezincified area.

The selective removal of one of the constituents of any alloy falls into

the category described here. The so-called graphitization of cast iron is

a misnomer. What actually occurs is the removal or corrosion of iron

leaving the graphite network. Selective removal of cobalt in high-cobalt

alloys, and silicon in copper-silicon alloys have been observed.

•Trans. ASM. 47, 473-492 (1955).


A :-.'....

Fie. 22.12. Section Through Dezincified Plug.

Fie. 22.11. Uniform Dezincification of Brass.

5000

45 50 55DEGREES CENTIGRADE

Fie. 22.13. Inhibiting Effect of Copper Ion in a Sulfuric Acid Slurry on Erosion-

Corrosion of I8-8SM0 Stainless Steel.


22.10. Erosion-Corrosion

Many metals and alloys depend upon surface films or corrosion

products for corrosion resistance. If these surface layers are removedby mechanical wear effects, active and rapid corrosion occurs. Erosion-

corrosion is often encountered under conditions of high velocity, tur-

bulence, impingement, and solids in suspension. Valves, pumps, elbows,

centrifugals, agitators, and heat exchange tubes often fail because of

erosion-corrosion.

Fig. 22.13 shows the effect of high velocity on corrosion of I8-8SM0stainless steel by a sulfuric acid slurry. The x on the abscissa shows noattack under static conditions. The curve marked "no copper" shows the

rapid increase in corrosion with temperature under high velocity con-

ditions. The addition of cupric sulfate reduced attack under these

conditions. Fig. 22.14 shows the typical wavy appearance due to erosion-

corrosion.

Fig. 22.14. Erosion-Corrosion of a Pump Impeller.

Cavitation may be considered as one form of erosion-corrosion. Rough-ened surfaces or deep craters are often found on the trailing faces of ship

propellers and pump impellers. This is cavitation and is largely due to

the mechanical effect or pounding caused by the formation and collapse

of air or vapor bubbles in the liquid on the metal surface.

Fretting corrosion is the attack that occurs at contact areas betweenmetals under load and subject to vibration and actual slip at these sur-

faces. It is sometimes called chafing corrosion, friction oxidation, andfalse Brinelling. The latter because the attacked areas often look like

mechanical indentations. The corrosion product on steel is usually a


ferric oxide debris. Two proposed mechanisms involve tearing away of

metal particles because of seizing or galling and then oxidation of these

particles, or oxidation first and then wearing away of the oxides. Fretting

could be considered as one type of erosion-corrosion.

22.11. Methods for Combating Corrosion

Many corrosion problems can be solved by more than one method

or means. Sometimes a combination is used. In any case, the most

economical solution to the problem is usually adopted. Methods for

combating corrosion are arbitrarily classified into eight general categories

that cover essentially all corrosion problems.

Alloying, or better corrosion resistance, involves using materials with

better resistance to corrosion. If steel corrodes, copper, stainless steel,

or high-silicon iron may do the job. Proper heat treatment for optimum

corrosion resistance is also in this classification.

Cathodic protection involves making the structure to be protected

the cathode of a cell. This can be done by two general methods namely,

use of sacrificial anodes or impressed currents. The most widely known

sacrificial anode system is galvanized steel where the zinc coating is the

anode and corrodes preferentially, thereby protecting the steel. Zinc and

magnesium anodes are also widely used. The important point is to have

direct current entering the entire surface of the structure to be protected.

Impressed currents involve passing a direct current from an external

source through an anode, through the corrosive environment, and then

to the structure. In this case, a very corrosion-resistant or "permanent"

anode is needed. Carbon and 14.5% silicon iron (Duriron) are used for

impressed current anodes. Underground pipe lines, water tanks, sea-going

vessels and piers are examples of structures using cathodic protection.

Metallic and inorganic coatings are typified by nickel, chromium, and

tin electroplates, and glass and enamels. The principle here is to install

a barrier to separate the metal from the environment.

Organic coatings involve paint-type materials, which act as a barrier.

Proper metal surface preparation, such as sandblasting, is the most im-

portant factor, followed by a proper primer for good adhesion and com-

patability with over-coats, and then by good top coats themselves. The

use of an expensive paint does not guarantee that good protection will

obtain.

Metal purification is not often utilized, but it does solve some problems.

High-purity aluminum, which is available at reasonable cost, is a good

example. Contamination and catalytic effects are sometimes involved.


^.-4.3-rs--?

Fig. 22.15. Pump Casing Lined with F.puxy Resin. (Courtesy, The Duriron Co., Inc.)

Alteration of the environment often concerns addition or removal of

an ingredient in the environment. Deaeration is an example of the

former and inhibitors of the latter. An inhibitor is anything added to anenvironment to decrease corrosion. In many cases, reduction of temp-

erature or concentration, without substantially affecting the process, solves

a corrosion problem or permits the use of less expensive materials such

as steel.


Nonmetallic materials involve integral or monolithic construction or

thick coverings such as sheet linings. These materials can be classed into

five general types: (a) carbon and graphite; (b) ceramics, such as stone-

ware, porcelain, fused silica, glass, brick, mortars, and cement; (c) plastics,

such as phenolics, vinyls, polyethylenes, acrylics, epoxies, styrenes, nylon,

silicones, and fluorinatcd polymers (d) natural and synthetic rubbers;

and (e) wood. Fig. 22.15 shows a cast iron pump casing lined with a

plastic.

Design involves the shape of a structure. Equipment life can be pro-

longed or corrosion costs reduced through the use of bottom outlets de-

signed to drain completely, readily replaceable or interchangeable parts,

standard lengths of tubing, increased thickness in more vulnerable areas,

designing to prevent crevices or stagnant areas, and the use of butt-welded

instead of riveted joints.

22.12. Corrosion Testing

Corrosion testing is conducted to evaluate or select a metal or alloy for

a specific environment or definite application, to evaluate a given material

to determine environments where it is suitable or attacked, for controlling

corrosion resistance of a material or corrosiveness of an environment, or

for corrosion research purposes. These tests include laboratory, pilot

plant, large scale plant, or field tests.

The most important point in corrosion testing is to duplicate as closely

as possible the actual conditions to be encountered. Factors such as

temperature, concentration, velocity, aeration, time, galvanic effects, stress,

and intermittent or continuous operation must be considered. Chemical

and metallurgical history of the specimens must be known and the

speciments identified. A clean metal surface is preferred.

Most corrosion tests involve determination of loss in weight of the

specimen and conversion to a rate of linear penetration for prediction

of life expectancy. An expression for corrosion rates that is widely used

by engineers is "mils per year," and its formula is as follows:

534 Wmils per year =

DATW = Weight loss in milligrams

D = Density in grams per cubic centimeter

A = Area in square inches

T = Time in hours.


22.13. Liquid-Metal Corrosion*

The use of atomic energy for commercial power has increased interest

in liquid metals such as sodium, lead, and lithium, as heat-transfer media.

In addition to the usual forms of corrosion, another important factor

enters the picture. This is mass transfer or the solution of the equipmentin a hot zone and deposition of this metal or alloy in a colder area. Thisis due to marked change in solubility of one metal in another with changein temperature. In addition, a more careful selection of materials of

construction must be made because equipment is not as readily accessible

for repair as in more conventional systems.

HIGH-TEMPERATURE OXIDATION

22.14. High-Temperature Oxidation

High-temperature oxidation of metals and alloys, commonly called drycorrosion, can be described as the chemical reaction of a metal or alloy

with oxygen, sulfur, nitrogen or any halogen. Generally, however, the

agents involved are solid alloys and oxygen gas. The resulting oxide is

generally less dense than the metal from which it was formed, and it haspoor light reflectivity, electrical conductivity, and mechanical strength.

While the term oxidation can apply to many different kinds of reactions,

the common chemical change is the valence increase of the metal involved

to a higher positive value. In the subsequent paragraphs, the effects of

oxygen pressure, temperature, and substrate material are discussed to

point out the salient features of all oxidation processes. Reference to

different metals and alloys will be made, but much practical informationcan be gained from the metallurgical literature.

Little is known about the nuclcation of oxide phases on metal surfaces.

On the other hand, the growth of oxides has been studied extensively;

various theories and experimental techniques have progressed well enoughso that theoretical predictions can often be checked quite accurately.

Unlike simple diffusion studies in solid solutions, oxidation measurementscall for boundary conditions fulfilled at interphase interfaces. The gas-

solid oxide interface is the external boundary while the solid-oxide, solid-

metal interface is the usual internal boundary. In rate studies of oxidegrowth it is usually desirable to know the oxygen pressure in the gas

phase and the oxygen solubility in the metal phase. These quantities fix

the concentration gradient of cations and anions in the oxide phase, but

• Certain of the material presented in Sections 21.1 to 21.13 of this chapter appearedin a monthly column on Corrosion by M. G. Fontana for Industrial and EngineeringChemistry, who gave permission to reproduce it in this textbook.


they do not give information about the growth mechanism. Oxygen pres-

sure is known to influence the rate of oxide growth, but the effect is

rather small. Increased temperature generally increases the oxidation

rate; however, the criterion of oxidation rate may yield ambiguous re-

sults. For example, if the criterion for oxidation were simply grams of

oxide gained per unit area and unit time on a metal substrate and the

oxide had a very high vapor pressure, temperature increases could result

in vaporization or loss of oxide, yet oxidation would proceed rapidly.

An example of this behavior is the Mo, Mo() 3 ,O, system at temperatures

above 600°C. Internal oxidation occurs in many copper and silver-base

alloys and in certain steels. This kind of oxidation is characterized by the

formation of small particles of oxides of the alloying elements below the

external scale.

Since scaling or oxidation is primarily a deleterious effect, engineers

are concerned with design problems in which the part to be used may be

subject to hot gases. Size and shape retention may be important design

requirements, but frequently the electrical and optical properties of the

oxide phase are considered. In electrical contacts, for example in relays,

the choice of an oxidation-resistant alloy is imperative, because the

formation of a poorly conducting oxide scale can prevent electrical con-

tact from being made. The choice of stainless steels and aluminum alloys

for kitchen utensils which should have eye appeal is evident when it is

known that the oxides of these alloys are nearly transparent and do

not materially affect the light reflectivity of surfaces at low temperatures.

Lastly, it is obvious that through oxidation the reduction in area of any

structural member designed to carry a given load may make the part unfit

for service. This effect on strength can be magnified when parts are used

in fatigue-loaded operations where the oxide is continually cracked off and

fresh oxide forms at the expense of base metal.

22.15. Formation of Oxides

The most common method for obtaining quantitative oxidation data

on metals and alloys is by measuring the weight gain per unit time and

unit area. In vapor-deposition processes such as the condensation reaction

at the cool end of a zinc retort, the species which exists in the gas phase is

frequently the same species that exists on the substrate and the reaction

progresses because the substrate material is cold. Similarly, in metal

cladding by vapor-phase deposition, the vapor may be a different atomic:

species than the substrate, but intermetallic compound formation is not

important in determining the rate of the condensation reaction. The

driving force for these reactions is a supersaturation brought about by a


temperature distribution. Furthermore, the nucleation of new grains in

these deposition processes occurs in the same way that nucleus formationproceeds in the water-to-ice transformation, the growth of rock candy,

the freezing of metals, and the electrodeposition of metals.

The problem of nucleation of an oxide on a metal substrate is com-plicated by the fact that the gas species chemically reacts with the sub-

strate form a structure of the type MxO v . The following single steps mayoccur to a greater or lesser extent in the formation of any oxide. As is

evident from the listing, most of the steps are likely to affect the nucleation

rate and the growth rate of an oxide grain. The rate of any oxidation

reaction is governed by the single slowest step of the type listed below:

1. Surface adsorption of an O t molecule on metal.

2. Dissociation of 2 to O atoms.

3. Ionization of O to 0=.

4. Ionization of M to M+x and electrons.

5. Surface migration of O, 0—, M or M+x.

6. Diffusion of O in the metal phase until the solubility limit is reached.

7. Nucleation of the oxide phase.

Once an initial oxide layer has been formed, several other possible

steps may occur. There are:

1. Diffusion of2 gas within cracks in oxide film.

2. Surface adsorption of an 2 molecule on metal oxide.

3. Dissociation of 2 to O atoms.

4. Diffusion of 0= through oxide layer.

5. Ionization of M to M+x and electrons at the metal-metal oxide

surface.

6. Diffusion of M+x through oxide layer.

Fig. 22.16 shows a sequence of microscopic steps that may be operative

when a bare metal surface is oxidized. Since the view has been taken

that the rate of oxidation is governed by the single slowest step, anyexperimental work necessitates the reduction of a system of many factors

to a system in which a single variable can be studied to yield meaningfulresults.

22.16. Dependence of Oxide Growth on Gas Pressure

The effect of pressure on oxidation rate has been successfully investi-

gated for very few metals. Among these are iron, copper, zinc, zirconium,

titanium, vanadium, and cobalt, reacting with oxygen. At various pres-

sures the dependence of a reaction rate of a solid with oxygen is intimately

related to the mechanism of the reaction. The conditions for growth are

related to the nonstoichiometry of the oxide phases developed, and the


/ J~— LINES Of

Fic. 22.16. Gulbransen's postulation of oxidation mechanism, a-adsorbed oxygen

film, b-fine mosaic structure of crystals, c-nucleation of large crystals, d-uniform

oxide layer appears. Taken from E. Gulbransen, W. R. McMillian and K. F. Andrew,

Trans. A. I. M. E. 200, 1027 (1954).

mechanism for growth is assumed to be diffusion controlled within the

oxide phase. Two metals which have been studied extensively are copper

and zinc. The following paragraphs present the information which

predicts and supports the pressure dependence of the observed oxidation

rate.

1. The electrical conductivity of Cm2 was observed to increase with

oxygen pressure according to the relation.^

Vsa = constant PQ

•J. Gunderman, K. Hauffe. and C. Wagner, Zeit. phys. Chem., B37 (1937). 148.

Since the three variables that make up electrical conductivity are

concentration, valence, and mobility of the conducting species, varying

oxygen pressure must change one or more of the variables to account for

the pressure dependence. If the structure of the oxide is viewed as cation

deficient, the following model could represent the structure of Cu20.

Cu"

Cu*

O

Cu->

Cu^

o-

Cu<

0=

Cu*

Of

o-

Then, one oxygen molecule may form two molecules of CutO by the

following relationship.

Ot + 4«- -* 4Cu +D + 4*"D + 2 c"*°


Applying the law of mass action and realizing that the concentrations of

Cu+ vacancies and Cu++ions are identical, one finds

C8 defects— = constant

If the oxidation rate of copper is determined by the concentration of

defects operative in the oxide phase near the gas-oxide interface, then

oxidation rate a C defects cc PjJOj

Wagner and Grunewald* report that the oxidation rate of copper at

1000°C. is a linear function of the seventh root of the oxygen pressure.

This agreement supports the theory reasonably well because such measure-

ments are difficult to make when the pressure dependence is so small.

Oxides which behave similarly are FeO, NiO, CoO, and AgÔ.2. On the other hand, a class of oxides exists in which the electrical

conductivity decreases with increasing oxygen pressure. A typical ex-

ample of this type of oxide is ZnO. The following structure shows a

typical cation excess oxide.

Zn*+ 0- &++ 0- Z"*+ 0-

0- Z" ++ o- Zn*+ o- fir*

dB>Zn+* 0- Zn** 0- Zn+* 0-

CD0- Zn++ 0- Z"

+* 0- Z"* +

If the number of interstitial ion and electron defects were decreased byan increased oxygen pressure according to the reaction,

i/aO, + <^f) + 2 (P) -> ZnO

Electrical conductivity

or

Oxidation rate a C defects a ôVt

•C. Wagner, and K. Grunewald, Zeit. phys. Chem., B40 (1938), 455.


FIG. 22.17. The oxidation-time relationships. Taken from O. Kubachewski and B.

Hopkins. Oxidation of Metals and Alloys. Butterworth's Scientific Publications, London

(1953).

Various measurements have shown that electrical conductivity varies

with the negative fifth root of the oxygen pressure, and the agreement

with theory is relatively good. Other oxides which possess cation excess

structures are BeO, FetO s , MoO s and SnOt.

22.17. Dependence of Oxide Growth Upon Time

Various weight change versus time relationships exist for oxidation

rates. Fig. 22.17 shows a composite graph of the oxidation-time relation-

ships. The two most important of these empirical relations are the linear

and parabolic equations.

The linear relationship is the simplest equation that has been found

to express experimental data. The equation Am = kt

relates the change in mass per unit area with time; k, is called the linear

rate constant. No other constant is necessary in this linear equation

because at t= o, [\m = o. Another form of this relationship is

Ax = k,*

where Ax is the change in thickness of the oxide layer. These relations

have been found to apply when the molar volume of the oxide phase is

less than the molar volume of the parent metal which reacted. Physically,

these oxides cannot cover the entire metal surface; fresh metal surface is


continually exposed and the oxidation rate is quite constant. Commonmetals which conform to this type of oxidation are lithium, sodiumand magnesium.

Pilling and Bedworth expressed their criterion of linear vs parabolic

oxidation rate by the following relation:

Mi >.Dm <h

where M = molecular weight of oxide phase,

D = density of oxide phase,

m = atomic weight of metal phase,

d = density of metal phase.

When the above ratio is less than unity, the linear rate law is obeyed;

but when the ratio is greater than unity, the parabolic rate law is called

for.

That is to say, if the molar volume of the oxide is greater than the

metal which reacted (but not too great to crack off under stress duringgrowth), a protective film would be expected. A simple statement of this

situation would be that the increase in film thickness would be inversely

proportional to the actual film thickness at any time. For example, if the

following equation were integrated

d(Ax) = kp 1_

dt Axto

Ax* = 2k p t

we have a statement of the parabolic expression for rate of oxidation.

This "law" was enuciated by Pilling and Bedworth and frequently bears

their name. A similar expression for the change of mass of an oxide film

can be written by substituting Am for Ax and changing the units kp

accordingly. Fig. 22.18 shows typical oxidation data which follows the

parabolic equations. Protective films are found on many metals and alloys

and are highly desirable. Examples of these are aluminum, chromium,copper, beryllium, titanium, and iron.

Lastly, some oxidation rates have been observed that are faster than the

parabolic rate at first but tend to protect the parent metal very well.

Examples of these rates are the cubic and logarithmic expressions whichfollow:

(Ax)3 = k c t (cubic)

andAx = A'log t -+- b (logarithmic)


16-

* 6

•00'

10 12

Fig. 22.18. Oxidation of oxygen-saturated titanium. Taken from M. Simnad, Aija

Spilners and 0. Katz, Trans. A. I. M. E. 203, 645 (1955).

These expressions are commonly used to represent data on very thin

oxide film formation at relatively low temperatures. At intermediate

temperatures most metals tend to behave parabolically, and at high

temperatures the linear rate is often observed. Engineering choices of

metals for a particular operation often necessitate the gathering of data

on oxidation resistance in the temperature interval desired and under

the exact service conditions of gas composition, moisture, pressure, and

surface preparation.

22.18. Dependence of Oxide Growth Upon Temperature

The temperature dependence of oxidation rate is generally the same

form as that for diffusion and ionic conductivity. The following ex-

ponential relation applies for a single oxide in a given temperature in-

terval.

k = l-^r'lIn this expression, k is the experimental rate constant (linear, parabolic,

cubic, etc.) and C is a constant having the same units as k. A term called

/


the experimental activation energy is given by A£„, while R is the gasconstant (1.986 calories/mole degree) and T is the temperature inabsolute units (Kelvin)

. Information about the mechanism of oxidationand electrical conductivity can often be gained by comparing the experi-mental activation energy for diffusion of a known species with the activa-tion energies for oxidation or electrical conduction. If the values forA£„„ are identical, one may generally assume that a diffusing speciesaccounts for a given oxidation rate or an experimental specific con-ductivity. Many accurate data are needed in this particular method of de-ducing the mechanism of metallurgical reactions.

On the other hand, some metals and alloys tend to oxidize very rapidlyupon reaching certain temperatures. It has been observed that thetemperature at which "catastrophic" oxidation occurs frequently coin-cides with the formation of liquid oxide phase, generally a binary orternary eutectic. Severe attack of this type may actually result in aweight loss of the oxidized specimen, because the oxide may volatilizeappreciably.

22.19. Oxidation Prevention

Gold and platinum are the only metals which will not form oxides inair at room temperature. Silver, palladium and rhodium oxides thermallydecompose to the metals and oxygen gas at 190, 800 and" 1 100°C, respec-tively. In some instances it is possible to use one of the noble metals at atemperature where its oxide simply cannot form. However, high costs

prohibit such applications. Cladding base metals with precious metalshas been attempted, but oxygen diffusion through the precious metalcoating is fast enough at elevated temperatures to oxidize the base metalinternally. This kind of oxidation results in an oxide layer formed be-tween the base and the precious metal.

One way to retard elevated-temperature oxidation of carbon steels andlow-alloy steels is to heat them in a neutral or reducing gas. Such pre-cautions, although expensive, are used in annealing low-carbon sheetand wire to prevent discoloration or the formation of an oxide coating ofappreciable thickness. If it is necessary to prevent oxidation of medium-and high-carbon steels by heating in a neutral or reducing atmosphere—as is sometimes the case in heating treating tools—precautions must betaken to prevent decarburization of the surface layers. Removal of carbonfrom the surface with the production of a soft outer layer is sometimesmore of an evil than scale formation.

The common method of protection of a metal or alloy is by the for-

mation of a protective oxide film which inhibits further oxidation. In


designing a part for a particular operation it may be possible to select

a particular material that will protect itself after a thin film of oxide has

formed. The classic example of this is the Delhi pillar of India. This

mass of wrought iron, some 24 ft. high and weighing about 6.5 tons, was

made by hand welding small blooms. It was erected about A.D. 300.

Owing to a "tight" oxide coating and a pure, dry atmosphere, it had

resisted corrosion remarkably well for about 1600 years, but when Had-

field brought a piece of the pillar to England and moistened the surface,

it rusted badly overnight.

Oxides formed on pure metals are relatively simple in structure and

composition when compared with the oxides formed on alloys which have

many elements in solid solution or have several phases in their micro-

structure. When several oxide phases form, as on iron, the oxide closest

to the metal, has the highest per cent of the metal while the oxide which

forms at the external surface has the lowest per cent of metal. Thus,

when iron oxidizes at elevated temperatures, an innermost layer of FeO,

and intermediate layer of Fe3 4, and an outermost layer of Fe2 3 appear.

When alloying elements are present, their oxides may appear separately

or as special oxides of the type A„B,0, where A and B denote metals and

x, y and z are small whole numbers. Oxidation resistance of a pure metal

can be increased or decreased by the addition of alloying elements.

Additions of chromium, nickel, aluminum, and silicon to iron and steel

serve to increase the oxidation resistance, because the oxides protect the

base metal. In these cases the oxidation rate tends to be parabolic.

Boron, molybdenum, and vanadium markely decrease oxidation resistance

of steels because the resulting oxides are nonprotective. In dilute copper

alloys beryllium, aluminum, magnesium, and silicon increase resistance to

oxidation while titanium, cobalt, nickel, and manganese seem to have

little effect.

High chromium, usually together with high nickel or high cobalt, is

necessary for resistance to oxidation at elevated temperatures. Typical

costly alloys include low-carbon 18 per cent chromium, 8 per cent nickel

steel; 10 to 35 per cent chromium steel; 35 per cent nickel, 15 per cent

chromium steel; an 80 per cent nickel, 20 per cent chromium alloy; a 62

per cent nickel, 15 per cent chromium, 23 per cent iron alloy; the

nickel-molybdenum alloys of the Hastelloy type; the cobalt-chromium

alloys of the Stellite type, and a number of others. All have long life at

temperatures of 1300 to 1800°F. (700 to 980°C.) or even higher and

are used in oil refineries, power plants, furnaces, and chemical plants.

A typical oxidation study yielded the results shown in Fig. 22.19. Type

410and 430 stainless steels, having 11 per cent and 16 per cent Cr, were


tooTME M HOURS

Fig. 22.19. Oxidation of types 410 and 430 stainless (electropolishcd) at 1600"F inmoist and dry air. Taken from D. Caplan and M. Cohen, Trans. A. I. M. E. 194 1057(1952).

oxidized at 1600°F. in wet and dry air. The wet air caused rapid attack

on both steels in comparison with the dry air, and in both cases the oxideformed on the 16 per cent Cr steel was more protective than that on the1 1 per cent Cr steel.

One general feature in tailor-making an oxidation-resistant alloy is toadd an alloying element which has a very high affinity for oxygen andwhose oxide has very low electrical conductivity. Such oxides either havefew ions free or able to migrate or have structures which hinder diffusion.

In either case, the oxides formed tend to be protective.

Another way of protecting a base metal is by cladding it with anothermetal which oxidizes very slowly. Cladding techniques include:

1. Metal spraying.

2. Hot dipping.

3. Roll cladding.

4. Casting.

5. Electroplating.

6. Cementation.


While all of these methods are not generally suitable for the application

of a single metal on a base metal substrate, frequently two or more are

used with success. The use of nickel clad steel results in almost mirror-

like surface finishes after a year or longer of operation near room tem-

perature. Studies on the oxidation of nickel-clad steel showed that the

oxide formed was invisible to the naked eye. Chromium plating is as

effective as nickel, which is generally used as an undercoat for chromium

plated objects. Aluminum-coated irons have been shown to be 30

times more resistant to oxidation than noncoated irons, and aluminum is

one of the most versatile metals to clad with. Every clading technique is

used to a greater or lesser extent with aluminum-clad products. Copper,

zinc, lead, brass, bronze, palladium, and rhodium are all used extensively

for cladding to hinder surface reactions.

QUESTIONS

1. Why do metals corrode in electrolytes?

2. What is meant by anodic polarization?

3. Describe an example where two of the eight methods of combating corrosion

arc used simultaneously.

4. A common piece of equipment used for heating water in a plant consists

of a shell-and-tube exchanger with water inside the tubes and steam in the

shell. In many cases copper tubes with thick cast iron tube sheets are used.

Why?

5. Describe in detail procedures and equipment needed to prevent inter-

granular corrosion of a large tank (for a railroad tank car) fabricated by

welding 18% Cr-8% Ni stainless steel (not stabilized).

6. Describe a test you would devise to determine whether or not a given

material would be susceptible to stress-corrosion (cracking) in a given en-

vironment.

7. Sketch a heat exchange system using liquid sodium indicating a difficulty

that may arise because of "mass transfer."

8. Why are the stainless steels often used for applications involving heat and

oxidation resistance?

9. Based on the electrochemical theory of corrosion, why docs "graphitiation"

of grey cast iron occur?

10. Describe the mechanism of ••knife-line" attack of the stabilized stainless

steels.

11. Describe a defect lattice which is a (a) cation deficient structure, (b) cation

excess structure.

12. The following data arc given for several metals and their oxides. Do these

metals oxidize linearly or parabolically?


(A) Metal Atomic Weight Density, g/rm3, 25°C

Al 26.97 2.70

Be 9.02 4.SOCu 63.57 8.92Pl> 207.21 11.34

Mg 24.32 1.75

Na 22.99 0.97Zn 65.38 7.H

(B) Metal Oxide Molecular Weight Density, g/cmS, 25°CA1L,03 101.94 3.7

BeO 25.02 3.0CiigO 143.14 6.0CuO 79.57 6.4PbO 223.21 9.5MgO 40.32 3.7Na,0 61.99 2.3ZnO 81.38 5.5

13. Calculate the mobility of electrons in a mole of pure solid copper at 25°C,if the specific conductivity a = cze^, where a = 10» ohm-' cm-*, c =concentration of carriers, z = valence of carriers, e = electronic charge and/i = electron mobility.

14. Test the following data for "fit" to the parabolic, linear, or cubic ratelaws for the oxidation of metal A.

Oxide Thickness, A time, minutes

500 10

1000 402000 160

15. If the experimental activation energy for oxidation of Zn to ZnO is 28.5Kcal/mole and the frequency factor, C, is 2.8 x 10-» cmVsec, what is thevalue of the oxidation rate constant at 400°C?

16. Describe in your own words the Pilling and Bedworth rule of oxidation.

17. Are metal oxides better or poorer electrical conductors than metals? Explainyour reasoning.

18. What is catastrophic oxidation?

19. If a first order interference color of yellow is seen on a slightly oxidizedtool steel, estimate the oxide thickness. Assume a representative blueradiation of 4700 A, and that first order extinction of blue occurs when

A" = -.— . when A is the wave length being destroyed and v is the index

of refraction of the oxide film (^ ~ 3).

CHAPTER

Effect of Temperature on

MechanicalProperties of Metals

23 James R. Macdonai.d, Ph.D., Chairman, Department of

Mechanical Engineering, School of Engineering, The

University of Mississippi, University, Mississippi

George V. Smith, Ph.D., Assistant Director for Metal-

lurgical Engineering, School of Chemical and Metal-

lurgical Engineering, College of Engineering, Cornell

University, Ithaca, New York

FOR centuries man's only interest in the reac-

tions of metals to stresses at elevated temperatures was related to the

reactions of the metals to stresses used to form them into usable shapes.

He learned that at elevated temperatures metals could be subjected to

repeated instantaneous stresses of forging, rolling, or drawing without

fracturing easily.

Interest in metals for extended service at elevated temperatures had its

beginning in 1924. Prior to that time most studies were confined to short-

time high-temperature tests, for at that time there was no need to study

the effects of long-time stress at high temperatures. But with the increased

temperatures and pressures being employed in the steam-power generating

industry, problems that were not understood began to develop. Equip-

ment operating at high temperatures and presumably designed conserva-

tively with an ample factor of safety warped and even failed; the carbon

and low-alloy steels being used, instead of exhibiting only elastic deforma-

tion as well-behaved steels should, deformed plastically under surprisingly

low loads.

At normal temperatures, most high-melting point metals and alloys

deform elastically under working loads. There is no appreciable plastic

deformation (permanent set) so long as the applied load does not exceed

the elastic limit. Furthermore, most metals and alloys when subjected at

normal temperature to stresses above the elastic limit—as in cold working

—strain harden; in other words, permanent deformation distorts the

grains and increases strength and hardness. This also holds true at ele-

vated temperatures so long as the temperature is below that at which

rapid recrystallization takes place.

485


As is evident from this introduction, the problem facing the metallurgist

and engineer was a phenomenon involving small amounts of plastic de-

formation over long periods of time at elevated temperatures. Thisphenomenon is called creep. Entering importantly into the problem ofcreep is strain hardening, which is opposed to the softening that occursduring recovery and recrystallization. Recovery and recrystallization de-

pend upon time, temperature, prior plastic strain and the previous struc-

tural condition of the material. For steel, the recrystallization tempera-ture ranges from about 700°F. to the lower critical temperature.

23.1, The Importance of Creep

Dickinson demonstrated in 1922 that steel deformed slowly at a redheat under stresses which were much lower than the values used in whatwas then considered good design. The connection between this slowdeformation at high temperature and the failure of certain power-generating equipment was established, and the importance of creep wasrecognized. As a result, the American Society of Mechanical Engineers

and the American Society for Testing Materials set up a joint committeein 1924 to study the phenomenon, to devise a method for determiningslow deformation, and to accumulate useful data on slow deformation of

industrially important metallic materials. This work has spread frommetals to ceramics and has develoj>ed considerable interest in a newseries of materials which are produced from mixtures of metals andceramics known as cermets.

Laboratories are continuing to study creep, and although many datahave been accumulated, much more needs to be done.

23.2. The Engineering Significance of Creep

In the selection of a steel or other alloy for high-temperature use the

mechanical properties of the material as determined at room temperature

are of little importance. The requirements for metallic material whichis to be used in various kinds of equipment operating at high tem[>eratures

vary widely, but in general the following are important: (1) The metal

should be resistant to oxidation or other attack by the hot gases (including

air) which are present; (2) it should have a stable structure which will

remain relatively unchanged under the stress and temperature conditions

imposed; (3) it should withstand the stress imposed with a negligible or

at least with a harmless amount of distortion for the useful life of the

equipment; and (4) it should withstand momentary overloads at the usual

operating temperature, or short periods of fluctuating temperature at

Effect of Temperature 487

normal loads. There are few metallic materials which meet all these

requirements. It is, therefore, not easy for the metallurgist to supply

the engineer with materials suitable for equipment operating at high

temperatures and fairly high loads. This difficulty is not lessened by

the fact that engineers are usually uncertain about the exact loads and

the exact temperatures in the equipment they design and use.

The steam-power generating industry is an excellent example of the

trend to higher temperatures during the past years. In 1905, the initial

temperature of commercially used steam turbines was only 500°F. In

a two-year period from 1906 to 1908 it was raised 100°F. and it remained

at 600°F. for nine years. In the next three years (1917 to 1920) it was

raised another 100°F., but it was not until 1926 that the temperature

attained 750°F. Making use of the accumulated high-temperature data

manufacturers increased the steam temperature to 950° by 1938. During

World War II there was no change because of the scarcity of critical

materials, but shortly thereafter the temperature was increased to 1 100°F.

Certain units in the petroleum industry have always been operated

at metal temperatures in excess of those employed by the steampower

industry. The ability to do this is based on shorter expected metal life,

shorter operating cycles, and more frequent inspection, with opportunities

of replacement prior to fracture of the parts operating at the higher

temperatures. In fact, prior to 1930, operation of the thermal cracking

units was generally of the intermittent-coking type, so that the cycles werevery short, often only 40 to 50 hours. Today the length of the operating

cycle is often measured in months, or years, rather than in hours. Develop-

ments in high-temperature alloys have contributed to this ability to use

longer operating-time periods, which in turn have greatly increased pro-

duction because of less down-time of the units.

General conclusions with respect to temperature trends within the

chemical industry cannot be drawn because of the wide variety of

products manufactured by the different companies, and often within

the same organization. The requirements of this industry do extend from

moderate temperatures and extremely high pressures to very high metal

temperatures (1800 to 2000°F. (980 to 1095°C.)) with low pressures.

During World War II there was an ever-increasing demand for stronger

and stronger alloys that could be used at higher and higher temperatures.

The exhaust turbosupercharger for aircraft engines involved rotor tem-

pertures up to 1350°F. (730°C.) and blade temperatures to 1500°F.

(815°C.) or even higher. The requirements for engines in present-day jet

planes are somewhat lower. The expected life in these applications is


currently relatively short, a fact which helps to simplify the metallurgical

asjjects of the problem. The temperatures in gas turbines for central sta-

tion power generation are somewhat lower and the expected life very

much longer.

Another application, just beginning to acquire importance, that

may require alloys that can be employed at very high temperatures is the

commercial use of nuclear energy for generating power.

c

/\,s

z ELEVATEMPERJ

ED

D r TURE

< J_/

//

Uip TE^Xi

( 1

A0-B ELASTIC DEFORMATION AT ROOM EM 'ERATURE '

A-C PLASTIC DEFORMATION. OR CREEPA-0+ CREEP CURVEA-D FIRST-STAGE CREEP F FRACTURE

- D-E SECOND-STAG-E CREEPE-F THIRD-STAGE CREEP

i i i i . i i 1 1 1 1 1 1

uTIME OF STRESS APPLICATION

Fic. 23.1. Diagrammatic sketch of elastic versus plastic properties.

23.3. The Creep to Rupture Curve

From Fig. 23.1 the complete creep curve under constant temperature

and constant load is seen to consist of three stages. The portion of the

curve AD, known as the first stage, represents a condition of continually

decreasing creep rate; section DE, the second stage, is one of substantially

constant creep rate; and EF, called the third stage, is one of continually

increasing creep rate and ends in fracture (rupture) . The total deforma-

tion to fracture in this case would be represented by OA, the elastic de-

formation, plus AC, the plastic deformation.

When tests are made at other loads (holding temperature constant)

or at other temperatures (holding load constant) , a family of curves, de-

picted schematically in Fig. 23.2, is obtained.

At relatively high temperature and load, the period of first-stage pri-

mary creep may be virtually absent; whereas, at relatively low tempera-

ture and load, creep may cease, for practical purposes.

The characteristic form of the creep curve (Fig. 23.1) led early inves-

tigators to suggest an interplay between the forces of strain hardening


Increasing temperature (constant load)

Increasing load (constant temperature)

Time

Fic. 235. Schematic illustration of creep at low and high temperatures and loads.

and strain softening, the latter arising from crystal recovery or in oc-

casional instances from recrystallization. According to this view, harden-

ing predominates during first stage or primary creep, and the creep rate

diminishes with time; whereas, during second stage creep, the forces of

hardening and softening balance one another, and the rate of creep is

essentially constant.

This conception of the creep curve is, in principle, still currently ac-

cepted, with refinements, however, as regards the nature of the plastic

flow processes and crystal recovery.

The tertiary period of accelerating creep (third stage) is clearly as-

sociated with impending fracture. The mode of fracture at elevated tem-

peratures may be either transgranular, i.e., through the grains, or inter-

granular, i.e., between the grains. Transgranular fracture, which is the

predominant mode of fracture of metals at ordinary temperatures, is less

likely to occur, the higher the test temperature and the slower the rate

of deformation (or the longer the time for rupture) . The intergranular

mode of fracture is characterized by equiaxed rather than elongated

grains, by intensive intergranular voiding, by a minimization of local

"necking" deformation, and often by relatively low over-all strain at frac-

ture. Hence, little or no advance warning of impending fracture is

afforded.

23.4. Determination of Creep

The knowledge of what a metal might do over a short period of time

cannot be projected to cover a long period of time. Much effort was

wasted in the early work on determining the reactions of alloys to stresses

at elevated temperatures for longer periods by attempts to find a rapid

method. It took considerable time and effort to discover there was no

easy short cut. However, determination of short-time, high-tempcrature


Kir.. 23.3. Modern creep-testing equipment. The electric furnaces for heating the

specimens arc shown at the left (white): the lever system for loading the specimens

is above the furnaces. (Batlelle Memorial Institute)

tensile strength, which is a simpler test than the creep test, and of the

time to rupture in relatively short times, as determined in creep to rup-

ture tests at high stresses are usually undertaken, for the data are useful

in deciding whether an alloy is suitable for parts, as in rockets, that

would be subjected to high temperatures for short periods, or for parts

that would be under maximum stress or maximum temperature only dur-

ing the event of failure of other parts. Such data also point out which

alloys justify further investigation where the maintaining of stresses for

longer periods at elevated temperatures is required.

The American Society for Testing Materials has adopted recommended

practices for conducting creep tests, E22-41, and time for rupture tests,

E85-50T. These standards place restrictions on temperature distribution

and variation, on the sensitivity of the extensometer system, and on the

accuracy of the loading mechanism. Specimens are usually maintained un-

der constant load for at least 1000 hours, and the amount of creep is deter-

mined periodically during the test.

The difference in deformation between any two points divided by

the difference in time of the two points represents the creep rate at the


average of the periods of time. The stress that will result in an average

creep rate equal to an assigned creep rate is known as the creep strength

for the alloy at the specified temperature. The stress causing rupture at a

given temperature in a specific time is known as the rupture strength.

The equipment necessary consists of an electric furnace capable of

close temperature control and a suitable device for holding the specimen

in exact alignment in the furnace. The load is applied by dead weights

through a system of levers. A battery of modern creep-testing furnaces

and their auxiliary equipment is shown in Fig. 23.3. Test specimens

are usually 0.505 in. in diameter (cross section 0.2 sq. in.) and have a

gage length of at least 2 in.

From the data accumlated in a series of creep and rupture tests at

different stresses and temperatures, the information necessary for selec-

tion of working stresses is obtained. Fig. 23.4 shows the temperature

variation of the stresses necessary for 1 per cent creep at several time inter-

vals in comparison with the temperature variation of the yield and tensile

strengths determined in short-time tensile tests.

23.5. Effect of Variables on Creep

Creep is one of the most structure-sensitive properties of metals and

is, therefore, greatly influenced by many factors that have little or no in-

fluence on the other characteristics. For example, a series of steels having

practically the same tensile properties and hardness at room tempera-

ture may shoiv wide differences in their creep resistance at higher

temperatures.

Owing to the complexity of the subject, information is not avail-

able concerning all the possible variables and it is probable that, as

yet, not all of these are known. Certain information is available regarding

many of them and among those to be considered are chemical composition,

heat treatment, grain size, method of manufacture, previous deformation,

and structural instability.

Chemical composition. In general, creep and rupture strengths of

metals increase with increase of the melting point. Thus, lead creeps

significantly at room temperature, iron at perhaps 800°F., and molyb-

denum at perhaps 1500°F. In any base, alloying additions are effective in

further strengthening a metal. The specific effect of an alloying addition

depends on whether it enters into solid solution or results in the appear-

ance of an additional phase. In general, greater strengthening can be

effected by dispersed secondary phases than by formation of a solid

solution.


Temperature, deg. C.

POO 800 900 1000 1100Temperature, deg. F.

isoo

Fig. 23.1. Effect of temperature on short-time tensile properties and on creep

stress of cast carbon steel. (Kanter)

Low alloy steels, when appropriately heat treated, are in general most

effectively strengthened by alloying additions of so-called strong carbide-

forming elements, like molybdenum, vanadium, and tungsten, the first

two of which are extensively used for this purpose. Chromium is widely

employed to improve scaling resistance, but it is deleterious to creep

strength beyond about 1.5 per cent. Molybdenum is also an effective

strengthener of the austenitic chromium-nickel stainless steels. The ultra-

high strength alloys currently in use for high-temperature applications

often comprise a solid-solution base composition, containing, for example,

cobalt, nickel, chromium, and perhaps iron, with a dispersal phase or

phases resulting from alloying additions of such elements as columbium,

vanadium, molybdenum, and tungsten.

Heat Treatment and Microstructure. The effects of alloying additions

cannot be considered without reference to heat treatment, whereby con-

trol is exercised over the grain size and the nature and dispersion of the

precipitate phases. At the more elevated temperatures, creep and rupture

strengths are greater, the coarser the structural grain size. However, there

is some evidence that an optimum grain size exists for each temperature

and strain rate or rupture time, and it seems unwise to generalize too

broadly. Similarly, fine dispersions generally appear stronger at relatively

low temperature, and coarser dispersions stronger at higher teni]>eratures,

but exceptions exist.


70 000

60 000

10' 10"

Stress Cycles -

Fig. 23.5. Effect of temperature on fatigue of 12 Cr steel. (Welch and Wilson)

Other Variables. The methods employed in manufacturing a metal

may exercise indirect but important effects. Included in this category are

melting process, deoxidation practice, casting temperatures, etc., varia-

tions of which result in variations of chemical composition and micro-

structure. For example, a higher casting temperature is conducive to

coarser grain size.

Strain hardening by prior cold working or by warm working may be

useful for increasing creep and rupture strengths for service temperatures

below the temperature range of recrystalli/ation.

The microstructure of metals and alloys used at elevated temperatures

often changes during service with consequent effect on properties and be-

havior (see Section 23.7) .

23.6. Fatigue and Combined Fatigue and Creep

In general, as temperature increases, fatigue strength progressively

decreases. At the same time, the slope of the curve of stress vs. number of

cycles becomes steeper, and, even for metals like steel, the curve does not

exhibit a horizontal portion corresponding to the "endurance limit," Fig.

23.5. Moreover, all environments, including air, become corrosive for

the usual structural metals and lower the fatigue strength accordingly.

An exception may occur to the general rule of decrease in fatigue

strength with increasing temperature when the temperature range is one

in which metallurgical changes occur that strengthen the metal. Such

may be the case for certain steels which undergo strain-aging at tempera-


Sites* to Mean Stress

5 10 15 20 25Mean Stress. 1000 psi

Fie. 23.6. Stress combinations Tor 0.2% and .5% creep and for rupture of 24S-T4aluminum at 500°F.; cycle stress applied at 3600 times per minute. (ASM Metals Hand-bonk, 1955 Supplement, p. 184.)

tures up to 600-700°F., so that the fatigue strength of such steels in this

temperature range is not much different than at room temperature.

At temperatures sufficiently high that creep occurs under static loads,

superimposed cyclic stressing results in complex behavior involving

both creep and fatigue phenomena. Whereas there is an apparent

absence of plastic deformation in fatigue fractures at ordinary tempera-

tures, fracture at elevated temperatures under cyclic stress may showboth creep deformation and the conchoidal markings that characterize

fatigue.

Behavior under combined fatigue and creep conditions cannot be

predicted other than approximately from separately obtained creep andfatigue data. Moreover, since creep and fatigue properties vary differently

with temperature, behavior under combined conditions cannot be

predicted for other temperatures from data obtained at one temperature.

Hence, it is necessary to conduct tests involving combined fatigue andcreep). Owing to the elevated temperatures of testing, certain variables

become more important than in fatigue at low temperatures. These in-

clude the effect of corrosive agents in the environment, the changes in

microstructure that may occur {see Sect. 23.8) , and since creep is time-

dependent, the frequency of cycling.

Typical results of combined fatigue-creep tests, are illustrated in Fig.

23.6 for the aluminum alloy 24S-T4 tested at 500°F. The ordinate scale


charts the alternating stress in completely reversed loading, that is for

zero mean stress, whereas the abscissae axis corresponds to static loading,

that is, for zero alternating stress. Within the quadrant, there are both

a steady stress and a superimposed cycling or fatigue stress. At relatively

low mean stress, fatigue conditions predominate and fatigue failure will

tend to determine working stress, whereas at low alternating stress, be-

havior corresponds more to creep conditions, and working stress will be

prescribed by excessive creep or creep-rupture. Fig. 28.6 indicates by the

perpendicular approach of the data curves to the axes that the super-

imposition of a small alternating stress on a given mean stress or of a

small steady stress on an alternating stress does not appreciably lower

the stress that can be supported.

23.7. Structural Changes During Creep

Most metals and alloys undergo changes in microstructure during heat-

ing at elevated temperatures. The changes cause changes in properties,

¥9

AS POLLED

^ ' " \ •

'

-• v • v * •

AFTER JQOOOHR. SERVICE

C-A40 STEEL IOOOX

Fig. 23.7. Carbide spheroidizalion in steel containing 05% molybdenum, (U.S. Steel

Corp. Research Laboratory.)


ORIGINAL HOOF' m?m

XIOOO

L •

ORIGINAL HOOF

mjgOOF I500F

'i

'

i: 1 ty\I300F I500F

Fig. 23.8. Microstrucniral changes in Cr-Ni-Mo austenitic siainless steel during 3000

hours creep tcsi. (U. S. Steel Corp.)

and hence, the material has different properties at different times after

being put in service. The rate at which the changes develop is principally

dependent upon the temperature and the time, but may also be influenced

by the imposed stress and the resulting strain. Large stresses and plastic

strains may greatly accelerate microstructural changes, whereas small

stresses and strains may not cause detectable differences from unstressed

material.

The kinds of readily observable changes in microstructurc which mayoccur are varied, and include carbide spheroidization or graphiti/ation in

steels, recrystallization, grain growth and transformation or precipitation

processes of various kinds. Other changes such as strain-aging, temper

embrittlement of steels, 885°F. embrittlement of high-chromium ferritic

steels are not readily, if at all, detectable in the microstructure.

In ferritic steels, carbide spherodization during service is widespread

as the temperature exceeds perhaps 900° F. A well-advanced example is

shown in Fig. 23.7 representing the microstructures of i/£ per cent

molybdenum—0.15 per cent carbon steel before and after 30,000 hours

of service at about 1 100°F. in a petroleum refinery. Clearly the properties

of the steel before and after service would be different. In some steels

the carbide phase may break down to elemental carbon (graphite) and

iron.

Microstructural changes during creep test of an austenitic Cr-Ni-Mo

stainless steel are shown in Fig. 23.8. In this material carbide and chi

phases, not differentiated in the illustration, precipitate. In other


Fie. 23.9. Thermal conductivity of various metals—temperature dependence. (Hay-

thorne)

. .000012

i

1 .00000;

0O0OOT

.000004

.000005

COEFFICIENTS OFTHERMAL EXPANSION

COEFFICIENTS ARE MEANVALUES FROM 70T TOINOICATEO TEMPERATURE

JOO 400 800 MO «O0 BOO 1400

TEwpepaTum - -f

Fie. 23.10. Coefficients of thermal expansion-temperature dependence. (Michel)

austenitic stainless steels, or in high chromium ferritic steels, sigma phase

may precipitate.

The microstructural changes that occur during exposure at elevated

temperatures generally, but not necessarily, cause a deterioration in

strength as measured at elevated temperatures. Often the most profound


effects are observed in loss of toughness at room or lower temperature,

and as a result the material may fail during shut-down periods rather

than at elevated temperatures. In a particularly severe instance the

notched-bar impact energy absorption of austenitic stainless steel exposed

at 1500°F. (Fig. 23.8) , was lowered to 10-15 foot lbs.

23.8. Variation of Other Properties with Temperature

A number of properties of metals other than strength may be of interest

in the design and operation of equipment for service at elevated tempera-

tures. Among such properties are thermal conductivity, thermal ex-

pansivity, clastic modulus and thermal shock or thermal fatigue.

AUSTENITIC STEELS

leCi-BNI 13041l8Cr-8Ni-Mo<3l6>iec«-aNi.Ti (321)I8C'-8N1-CM347]25Cr-l2NI 1309125O-20NU3I0)

FERRITIC STEELS0.10 CARBON1, 00 CARBON0.SM00.8C'-0.3MOICr-0.5Me2Cr-0.»M02 25Cr-IMo

3CY-0.5Mo-l.5S3Cf-0.5Mp-l.5Si5Cr-0.5Mo-Cb5C'-0.SMO-TI9Cr -IMol2Cf (410)ITCr (430)

AOSTEMTiCSTEELS"

600 800 1000 1200TEMPERATURE - F

Fig. 23.11. Young's modulus E of various materials—temperature dependence.

(Michel)

Although thermal conductivity and expansivity vary with variation in

chemical composition, they are relatively unaffected by other metallurgical

variables such as heat treatment and microstructure. The variation with

temperature of thermal conductivity of a representative group of metals

and alloys is shown in Fig. 23.9. Fig. 23.10 shows a corresponding chart

for thermal expansivity. Of particular interest from the practical point

of view is the disparity between the expansion coefficients of carbon steel

and austenitic stainless steel, which leads to "thermal" stresses when the

materials are joined and heated, as they sometimes are. The austenitic

steels also have significantly poorer thermal conductivity at relatively low


temperature; at relatively high temperatures different steels possess ap-

proximately the same conductivity.

The elastic moduli at elevated temperatures are sometimes needed for

computing elastic strains. These constants may change gradually with

composition, but are relatively insensitive to metallurgical structure.

With increasing temperature, both the tensile modulus (Young's) and

shear modulus decrease. The temperature variation of Young's modulus

for a representative group of metals is shown in Fig. 23.11. The rate of

decrease is approximately linear up to a certain temperature level which

depends upon the material, beyond which a more rapid decrease occurs.

Poisson's ratio does not change significantly with temperature.

Thermal shock and thermal fatigue represent a common mode of failure

in applications involving relatively rapid changes in temperature.

Thermal shock and thermal fatigue may be differentiated depending

on whether failure occurs after relatively few or many cycles, respectively,

of heating and cooling. There are no standardized tests by which to

measure these properties, and resort is had to special tests which attempt

to simulate actual service conditions. The stresses giving rise to thermal

shock or fatigue are influenced by such properties as thermal expansivity,

thermal conductivity, strength and toughness and by the rate of change

of temperature and the geometrical shape of the part.

23.9. Design for Elevated Temperature Service

Although a number of properties will be of interest in the design of

equipment for successful operation at elevated temperatures, the govern-

ing criteria, at least where corrosive conditions are not severe, will

generally be the strength. Owing to creep phenomena, however, strength

at elevated temperatures is not a simple property. From the very fact

that creep is occurring, the design engineer recognizes that in time, equip-

ment will either distort beyond a tolerable limit or will fracture or both.

It is necessary therefore to choose working stresses such that neither of

these events occurs. The amount of deformation that may be tolerated

depends on the specific application; in a steam turbine it may be only a

fraction of one per cent in 100,000 hours, whereas in steam piping, several

per cent deformation in the same interval may be tolerated. In all appli-

cations, of course, the working stress must not be large enough to cause

fracture.

In all equipment designed to operate under creep conditions it is

necessary to design for a specific life, not for unlimited life. The basic

information relating to the amount of deformation as a function of lime

comes from creep tests, which must be made under dynamic conditions if


l*S 2 * « 2*6 Z * 6

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Fic. 23.12. Design data for alloy containing 20 Cr-20 Ni-20 Co-3 Mo-2W-l Cb-Bal. le,

1200°F. (ASM Handbook, 1955 Supplement)

service involves superimposed cyclic stresses. The basic data for design

against fracture comes from creep to rupture tests, from which it is pos-

sible to chart time for rupture as a function of stress. Although there is

a general relation between the stress for a specific amount of creep in a

certain time and the stress for rupture in a specific time, it is not always

possible to insure against fracture by insuring against excessive deforma-

tion, because the amount of deformation at fracture at elevated tempera-

tures may be less than that which the designer may be willing to accept in

a specific application. Moreover, design of equipment for long-life service

necessarily involves extrapolations, with attendant uncertainties as to

when creep will enter the third or accelerating-rate state.

A useful form in which to summarize creep and rupture data for the

design engineer is in the form of "design charts," an example of which is

given in Fig. 23.12. Such charts show the variation with stress of the time

for several specific amounts of creep, the time for transition to accelerat-

ing-rate creep and the time for fracture, and from them it should be

possible to select the working stress for specific applications.

For certain general classes of construction, permissable working stresses

may be set by Code bodies, an important one of which is the Boiler and

Pressure Vessel Code Committee of the American Society for Mechanical

Engineers.

23.10. Variation in Mechanical Properties at Reduced Temperatures

In addition to knowing the properties of metallic materials at elevated

temperatures, the metallurgist and engineer should know something of the


behavior o£ metals and alloys at reduced temperatures. In northern

latitudes, the metals used for rails and railroad equipment, automobiles

and other machines are subject at times to temperatures as low as 50 to

60°F below zero. During World War II a number of merchant ships

split into two sections while subject to low sea or air temperatures.

Aircraft or guided missies operating at high altitudes also encounter low

temperatures, and low temperatures are of increasing importance also

in the chemical process industry.

In a number of fields, therefore, it is necessary to know the properties

of metals at reduced temperatures. The testing of metals at these tempera-

tures involves experimental difficulties, but equipment and techniques

have been developed for obtaining reliable test data, using refrigerants

such as ice, dry ice (solid carbon dioxide) , liquid nitrogen, liquid air or

liquid helium.

Tensile test properties of a number of metals and alloys at room temp-

erature and at -300°F are given on page 504. These data show that

tensile and yield strengths increase as the temperature decreases. The

effect of temperature on ductility, however, depends upon the nature

of the metal. For some, ductility decreases, for others it increases. The

decrease in ductility of the carbon and nickel steels and the 16% Cr iron

are noteworthy and a manifestation of a general characteristic of ferritic

steels to undergo, with decreasing temperature, a transition in the

character of the fracture from one that is relatively ductile to one that is

relatively brittle.

The temperature range over which the ductile to brittle transition

occurs for a specific ferritic steel depends upon the state of stress and

the speed of testing. The greater the normal stress (acting across a refer-

ence plane) relative to the shear stress (acting in the reference plane),

the

higher the temperature at which the transition occurs. Consequently, in

notched bar specimens which, compared to the tension test, have a high

ratio of normal to shear stress, increasing with the sharpness of the notch,

low ductility will be experienced at appreciably higher temperature than

in the tension test and, in fact, transition from ductile to brittle behavior

may occur at room or higher temperature. With increased speed of test-

ing, the transition from ductile to brittle behavior occurs at higher tem-

perature.

In line with the effects of stress state and speed of testing, "impact-

testing of specimens having "keyhole" or "vee" notches has come into

widespread use for evaluating the "toughness" of metals. Fig. 23.13

shows the variation with temperature of notched-bar impact energy to

fracture for a number of ferritic steels and for an austenitic chromium-


nickel steel which does not exhibit a transition from ductile to brittle

fracture.

23.11. Effects of Metallurgical Variables

The transition from ductile to brittle fracture depends upon a numberof metallurgical variables. The effects of these variables moreover maynot be revealed by the simple tension test unless made at extremely

low temperatures. It is possible therefore to have two heats of steel whichare nominally identical as judged by chemical analysis or by tensile

properties, in tests at room temperature, but one of which is tough in

notch-bar impact test at room temperature, the other brittle. It is also

possible to have two heats which exhibit similar notched-bar toughness at

room temperature, one of which undergoes transition to brittle behavior

at 50°F, the other at —50°F. It should be clear therefore that this charac-

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Fir. 23.13. Effect of low temperatures on the impact resistance of (A) water-quenchedlow-carbon 8 per cent chromium, 8 per cent nickel steel; (B) annealed low-carbon 3.5

per cent nickel; (C) annealed low-carbon 13 per cent chromium steel; (D) normalized

0.35 per cent carbon, 1.5 per cent nickel, 0.0 per cent chromium steel; (K) annealed0.35 per cent carbon steel; and (!•') normalized 0.35 per cent carbon steel. Specimensfor steels B and E had keyhole notch; others had V notch. (The International Nickel

Company)


teristic of metals and the effects of metallurgical variables thereon can be

properly studied only if tests are made over a range of temperatures.

The specific metallurgical variables that affect the ductile to brittle

transition in ferritic steels include chemical composition, deoxidation

practice, finishing temperature of hot working, heat treatment and the

resulting grain size and microstructurc, degree of cold work, and strain-

aging or teni|>er embrittlement, if any. Other factors being the same,

transition temperature of steels increases with carbon content and de-

creases with manganese up to one per cent or more. Nickel is generally

credited with being beneficial to low-temperature toughness of low-carbon

steel; commercial steels are available containing 3.5% (Fig. 23.13), 5%and 9% of nickel with transition temperatures progressively lower the

greater the nickel.

The transition temperature of deoxidized ("killed") steels is lower

than for undeoxidized (rimmed or capped). Aluminum is the most

effective of the elements usually used for deoxidation, but silicon is

beneficial. The lower the finishing temperature of hot working, the lower

the transition temperature, and normalizing after hot working is bene-

ficial. Transition temperature decreases with decrease in grain size. Cold

work is detrimental in itself and may also induce strain-aging which may

raise the transition temperature substantially. Of the various micro-

structures which may be imparted to ferritic steel, that of tempered

martensite exhibits optimum toughness, provided the tempering treat-

ment is such as to avoid temper embrittlement, which has a deleterious

effect on toughness. Since alloying elements play a prominent role in

determining to what extent the fully martensitic structure can be ob-

'tained, they thus may exercise an indirect effect on toughness.

Transition from ductile to brittle fracture with lowered temperature is

by no means a characteristic solely of ferritic steels. Although the an-

nealed, wrought austenitic steels exhibit toughness at -300°F sub-

stantially equivalent to that at room temperature, as do a number of other

metals having the face-centered-cubic crystal structure, it is now recog-

nized that most and perhaps all metals not having this crystal structure,

including zinc, titanium, molybdenum, and tungsten do exhibit transition

behavior.

23.12. Design for Low-Temperature Service

The problem of design for service at reduced temperatures, except for

applications in which corrosive conditions are severe, is essentially one

of insuring against brittle fracture. This is not to say that other prop-

erties, such as thermal expansivity, are not of interest, and in some cases

controlling factors.

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Since metals are stronger at reduced temperatures than at room tem-

perature, working stresses based on room temperature test data are

readily supported at low temperatures, if the material possesses adequate

toughness to preclude brittle behavior in the presence of stress concen-

trations and fast speeds of loading.

Bi iuie behavior in metals depends on the presence of tensile stresses,

and if these are negligible, the designer need not be concerned with the

problem of toughness. If, however, the tensile stresses are significant,

and the material of construction is one that undergoes transition from

ductile to brittle behavior, the equipment should be designed so that its

transition temperature under service loads is below the lowest service

temperature. This is more easily specified than effected since the transi-

tion behavior is not a simple property of a metal, and the range of

temperature over which it occurs depends upon the geometry and size of

the test specimen and speed of loading. Consequently, data obtained on

small, laboratory test specimens are of questionable application to engi-

neering structures and the problem of design against brittle fracture has

not been satisfactorily resolved insofar as choosing the most economical

material for a specific application. If cost is not an important considera-

tion, brittle fracture can be easily guarded against by selecting high-nickel,

ferritic steel or chromium-nickel austenitic steel in the ferrous group, or

one of the face-centered cubic non-ferrous metals, assuming adequate

strength.

QUESTIONS

1. What is creep, and why is it important to engineers? What are the principal

requirements for a metallic material to be used satisfactorily in equipment

exposed to high temperatures?

2. How is creep determined? What are the most important variables in creep

testing? How much time is necessary for a creep test?

3. How is the creep stress, which produces no more than the maximum allow-

able distortion of the material over the expected life, determined? How

accurate is such a procedure? What is the relation of creep to other mechani-

cal properties?

4. Describe what happens during the three stages of creep. What is the effect

of strain hardening and recrystallization on- second-stage creep?

5. What is the effect on creep of (a) grain size? (b) deoxidation? (c) precipita-

tion hardening? (d) condition of the iron carbide?

6. What is the relative creep strength (at constant temperature) of carbon,

plain molybdenum, chromium-molybdenum, plain high-chromium, and aus-

tenitic chromium-nickel steels? How does the creep strength of these steels

compare with the creep strength of the supcrstainless steeb?

7. Compare the suitability of carbon steel, brass, admiralty metal, bronze, Monel

metal, duralumin, and the magnesium-base alloys for use at temperatures o£


500 to 800°F. (260 to 425°C), Compare the creep strength of nickel and

Monel with Inconel and with the Hastclloy- and Stellite-type alloys.

8. What is the general effect of elevated temperature on the tensile strength,

yield strength, elongation, reduction of area, and endurance limit of metallic

materials? If a steel has a tensile strength of 120,000 lb. per sq. in. at roomtemperature, what will be the approximate strength at I200°F. (G50°C.)?

at I100°F. (760°C.)?

9. What are stress-rupture tests? What is the significance of these tests? Howdoes the rupture strength at I200°F. (fi50°C.) of austenitic chromium-nickel

steel compare with that of superstainless steel and of the cobalt-rich non-

ferrous alloys?

10. Why is it important to know the effect of low temperatures on the properties

of metallic materials? What is the usual effect of these temperatures on

tensile and impact properties? What is primarily responsible for low-tem-

perature brittleness in carbon steels, and what alloys reduce this brittleness?

Index

Acid process, 216, 217, 225

Activation energy for oxidation, 480, 484

Afterblow, 219

Ag-Cu system, 106

Age hardening, 51, 52, 111

Aging, 107

Air cooling, 254

Air furnace, 424

Allotropic transformation, 29, 51-52

Alloy steels,

classification, 241

composition, 242

high-quality production of, 226-227

low-alloy composition, 319-326

low-alloy production, 319

low-alloy S. A. E., 328-332

low-alloy similarity of properties, 332-339

low-alloy structural, 326-328

precipitation hardening, 126-127

Alloys, controlled expansion, 385-386

Alloys, for high temperatures, 198-202

Alloying effects, 47-51

Alloying elements, effect on carbon steel.

320-322

Alnico, 201

Annealing, alter cold work, 42-44

Alpha Stabilizers, 256

Aluminum.chemical properties. 134

deoxidation agent, 135

economics. 130

history, 129

manufacture, 132

nitriding agent, 135

ores, 131

outstanding properties, 131

physical properties, 133

purification, 133

thermite welding, 135

vs. steel, 130, 132

Aluminum alloys, 1 17-123,

alloying elements, 135

annealing, 1-18

brazing, 152

casting, 140

cold and hot working, 147

cold working vs. heat treatment, 147

composition, 138, 142

corrosion resistance, 150

free-machining, 136

function of alloy, 143-147

joining, 152

number code, 136, 137

precipitation heat treatment, 149

protection against corrosion, 155

riveting, 155

sand, mold and die castings, 141

solution heat treatment, 149

tensile properties, 138

welding, 153

wrought, 136

American Iron and Steel Institute steel

specifications, 329-331

Amslcr test, 445

Annealing of cast iron, 431

Anodes and anodic areas, 453

Anode reactions, 453-454

Anodization, 151

Area effect. 457

A.S.M.E. boiler and pressure vessel code

committee, 500

A.S.T.M. standards, 299, 300,

carbon steel castings, 299, 300

Atomic imperfections, 14, 15

Atomic structure, 5

Aiistempering,

commercial limitations, 289

critical cooling rate curve. 286

defined. 285

hardness and strength of austempercd

steel, 285

process. 285

to obtain greater ductility, 285

Austenite, defined, 246

Austenitic manganese steel, 385

Austcnilic stainless steel. 256

Austenitic steel, 374,

expansion of, 377

mechanical properties. 377

sensitization, 376

507

508 Index

Babbit. 192

Bainitc, 253. 289,

hardcnability, 257

Bars, carbon steel cold worked, 306

Basic process, 216, 219

Bayer process, 131

Bearing metals, 191

composition, 193

mechanical properties, 192, 194

Beginning of transformation, 252

Bend tests, low alloy steels, 77

Beryllium,

copper, 117, 124, 125, 127

manufacture. 157


toxicity, 159

use, 159

Bessemer process, 217

Binary equilibrium diagrams, 102

Binary system, 101

Blast furnace,

chemistry, 214

description, 212

efficiency, 214products of, 213

Blow holes, in steel, 237-238

Boron, effect on hardenabilily, 361

Brasses, 160, 168, 178, 179,

additions to. 178-180

cartridge, 170, 178

high, 168, 178

leaded, 178-179

low, 170

Muni/, metal, 169, 178

naval, 180

phase diagram. 165

red. 170, 178

special 180-181

yellow, 169, 178

Brinel] hardness, 70,

conversion tables 75, 76

Brittle fracture, 32, 82

Building-up principle, 9

Calculated hardenabilily, 356,

accuracy, 360

table, S59Campbell process, 230

Carbide formers, 257

Carbide, precipitation, 37(5, 461

Carbide stabilizing, 257Carbide tools, 405, 406

Carbon,

amount and importance in steel, 210-211

Carlion (Cont.)

in cast iron, 408. 109

solubility in iron, 216

Carbon monoxide, in steel, 237-238

Carbon steel,

castings, 298, 302

composition, 233-242

effect of alloying elements, 318-326

clfect of carbon, 247-250

elFect of chromium and nickel, 323

elfect of copper, 325

effect of molybdenum. 325

effect of manganese and silicon, 322

elfect of phosphorous, 322

effect of section si/c on, 315

effect of tempering, 338

cutectoid, 247

heat treated, 312

hypercutectoid, 247

hypocutcctoid, 247

silicon in, 239

tensile properties, 322, 324

tonnage production, 297, 306

Carbontriding, 292

Carburizing, defined, 289

Cast iron,

definition. -108

effect of section size, 415-417

machinabiliiy, 409, 412, 418, 420

relation of properties to cooling rate,

416-417

Cast tool alloy, 404, 405

Casting process, 25-28

Catastrophic oxidation, 480Cathodes and cathodic areas, 453

Calhodic protection, 409

Cathode reactions, 453, 454

Cavitation, 468

Cemented carbide tools, 405, 406

Ccmentitc, defined, 245

Ccmcntitc. stability in cast iron, 408

Cermets. 198

Chapmani/ing, 292

Charging. 222

Charpy test, 83

Chemical interaction in metal solutions, 18

Chilled iron, 408, 410, 420Chromium,

elfccl on tensile properties, 371

ferritcs, 369

in stainless steels, 364

in tool steels, 395, 398, 403

Chromium steel,

low-chromium, effect of chromium,

323, 325

Index 509

Chromium steel (Cont.)

engineering importance, 326-329

tensile properties, 371

heat treatment, 367

mechanical properties, 369

resistance to oxidation and corrosion,

370-372

Cladding techniques, 482

Cobalt. 190-191,

alloys of. 201

uses', 201, 201

Coefficient of mass diffnsivity, 99

Coherent precipitate, 21

Cold treatment, 274

Cold-worked carbon steel, 306

Cold-worked wire, 311

Cold-worked bars, sheet. 306

Cold working. 28, 40-42, 259, 306,

effects on strength and ductility, 308

effect on dynamic properties, 312

purpose, 232

Columnar grains, 27

Combating corrosion, 469

Completion of transformation, 252

Composition, effect of, 303-305

Compression tests, 77

Concentration cell corrosion, 458

Continuous cooling, 254

Cooling curve, 98

Cooling rate, 344. 350

Cupola- furnace, 422

Copper, 162-164, 178-9,

arsenical, 178

lake, 163

leaded, 178

OFHC, 163

phosphorized, 178

production, 162

properties and uses, 164

silver-bearing, 178

tellurium, 178

Copper alloys. 165-189,

constitution, 165-178 to 181

nomenclature, 167

precipitation hardening, 117, 124-125

127

constitution. 165, 178-182

phase diagram, 165

Corrosion,

cost, 452

definition. 452

dry, 452

electrochemical, 452

fatigue, 92

rate, 471

Corrosion (Cont.)

testing, 471

wet, 452, 456

Covalent Bond, 10

Creep,

curve, 488-490

definition, 486

determination of, 489

effect of chemical composition, 491

effect of Cr, Mo, 492

effect of micro-structure, 492

effect of variables, 491

importance of, 486

intergranular, 489

rale, 490

significance of, 486

stages, 488

strength, 491-493

transgranular, 489

Critical cooling rate, 255

Crucible process, 230

Crystal systems, 11

Crystallographic fibering, 38, 39

Cubic system, 12

Cu-Al system, 108

Cu-Mo, '379-380

Cu-N'i system, 102

Cupola, use, 215

Cupping test, 447

Curie temperature, 59

Cutlery steel. 365, 368

Cyaniding, 292

Cvclic stresses, at elevated temperatures,

494

Damping capacity, 94,

of cast iron, 420. 122

DeBtoglie'a equation, 6

Decarburi/ation, 91

Deep drawing.

evaluation of steel for, 446

properties of nonferrous alloys, 449

Defect lattice, 475

Defects in crystals, 14,

atomic imperfections, 14, 15

importance of defects, 15

line imperfections, 14

surface imperfections, 14

Deformation, 32

elastic, 488

of polycrystallinc metal, 37-42

plastic, 488

Degrees of freedom. 100,

Dendrites, 26

510 Index

Design. 471

chan, 500

for elevated temperatures service,

499-500

for low temperature service, 504-505

Dczincification, 406

Die steels, 396, 397

Differential hardening, 294

Diffusion. 99

Dislocations 14, 15, 23

Dispersion hardening. 111

Domain, 58

Double yield point, 448

Ductile cast iron, 409, 420, 421

Duplex process, 229

Duplex Talbot process, 230

Dynamic Properties, 81

Effect,

carbon in cast iron, 434

Chromium in cast iron, 435

composition, 302, 303

copper in cast iron, 436gTain size, 301

molybdenum in cast iron, 435

nickel in cast iron, 430phosphorus in cast iron, 435

silicon in cast iron, 434

sulphur in cast iron, 435

vanadium in cast iron, 436

Elastic limit 63, 64, 485

Elastic Modulus, 498, 499

Electrical conductivity of oxides, 475

Electrical resistivity, 58Electromotive force series, 454, 455

Electric furnace, 226-227

Electron cloud diagrams, 6Electron compounds, 21

Elongation 67, 68,

method of measuring, 67

yield-point elongation, 63

Energy level diagrams, 7End-quench test, 349, 350, 352Endurance limit, 87

Endurance strength, 88Engineering, relation to metallurgy, 3, 4Epoxy resin, 470

Equi-axed grains, 27

Equilibrium, 97

Erichscn test, 446

Erosion-corrosion, 468

Eutetic, 105

Eutcctoid, iron-carbon, defined, 247

Fatigue, 493,

combined with creep, 493-495

failure, 86

Eerrite,

defined, 246strengthen, 256

Kick's law, 99

Flame hardening, 293,

of cast iron, 434

Fleming, Sir Ambrose, 3

Free-machining steels, 440Fretting, 91

Fretting corrosion, 468

Ftigacity, 98

Full annealing of steels. 280Furnace cooling, 254

Galvanic corrosion, 456Galvanic series, 455

Gamma loop, 256, 366Gamma rays, source materials, 204

Gamma stabilizers, 256

Gas carburizing, 289Gibbs, J. Willard, 100

Gold, 190-191, 202

Grain boundaries, 17

Grain growth. 44,

and hot working, 267control of, 265

in steel, above critical temperature, 264soaking time and grain growth, 264tendency for, 265

Grain refining, 257Grain size, 15,

and properties, 29-31

control, 24-29

In relation to hardenability, 270

in relation to toughness and macliin-

ability, 266measurement of, 266process annealing and grain size, 264

Grains in metals, 15

Graphite,

comparison of forms in cast irons,

408-409

control in gray cast iron, 418in malleable cast iron, 410in gray cast iron, 412, 417in ductile or nodular cast iron, 420

Graphite flake size, 429

Graphite precipitation, 257Graphitizing agents in cast iron, 408Gray cast iron, 409, 412-420,

properties, 412-415, 417-421

structure, 409, 414, 415. 418

Index 511

Grossman, 356

Hadfielcl steel, 256

Hall process, 132

Hansgirg process, 156

Hardenabilily,

and alloying elements, 269

and cooling rate, 344

and engineering properties, 354

and tempering, 356

and time delay, 346

bands, 353

calculated, 356

determining, 348

effect of boron on, 361

factors affecting, 346

in carbon steels, 343

in low-alloy steels, 320. 344

in relation to grain size, 269

in relation to quenching medium, 270

Hardness,

conversion scales, 75, 76

conversion to tensile strength, 75

definition, 69

determination, 69-77

nonferrous alloys, 76

numbers. 75

tests compared, 72

tool steels, 392, 399

Hardening of cast iron, 432

Hardening spheroidized steel, 281

Hastelloy, 199-200

Heat treatment, 300-302

Heat treating carburized steel, 289

Heisenberg's uncertainly principle, 6

Hexagonal system, 12

High manganese steel, 381

High nickel steels, 384

High speed steels,

classification, 399

composition, 399

heat treatment, 400

structure, 400

Homogcnizalion of auslcnite, 279

Hoopes process, 133

Hot metal mixer, 222

Hot rolled, 303

Hot worked carbon steels, 302, 303

Hot working, 28, 38-40,

purpose, 231

Ideal diameter, 357

Ideal quench, 357

Impact specimens, 83

Impact strength. 81,

low-alloy steels, 328. 336, 340

Inconel, 199-200

Indices of direction, 13

Induction hardening, 293

of cast iron, 434

Ingot iron, definition, 211

Inhibitors, 470

Inoculants in cast iron, 429

Inoculation agents, 28

Insulators, II

Interference colors, 484

Intergranular corrosion, 151, 376, 460

Intermediate phases in alloy systems, 20,

electron compounds, 21

intermediate solid solutions, 20

interstitial compounds, 21

laves phase, 21

normal intermetallic compounds, 21

sigma phases, 21

Intermediate solid solution, 20

Intermetallic compound, 107

Internal stresses, 283

Interstitial compound, 21

Interstitial solid solution, 20, 101

Interstitial solutes, 20

Ionic bond, 10

Iron,

allotropy, 244-45

alpha, 244

cooling curve, 244, 245

Iron-carbon alloys,

critical temperature, 247

phase changes, slowly cooled alloys,

247-250

phase diagram, 245-247

Iron-chromium phase diagram, 366

Iron-nickel alloys, 384

Ironal, 201

Irreversibility. 99

Isothermal heat treatment of cast iron,

433Isothermal transformation, 251

lzod test, 83

Jominy test, 349

Killed steel, 237-238

Knife-line attack, 462

l.aves phases, 21

Lead, 190-191,

alloys of. 193-195

in type metals, 194

uses, 197

Lever principle, 104

Lime boil, 223

512 Index

Line imperfections, 14

Liquid metal corrosion, 472

Liquid solutions, 19

Liquids, 102

Low alloy steels, 257,

A.I.S.I. steels, 329-331

chromium as an alloying element, 324,

325

copper in, 325

engineering importance, 318-340general effects of alloying elements

320-322

manganese and molybdenum in, 322,

326

nickel and silicon in, 323, 324prosphorus as an alloying clement, 322properties, heat treated, 332, 340S.A.E. steels, 328-332

structural, composition and properties,

326-328

tungsten and vanadium in, 325, 326Low carbon steel, 125-126,

strain aging, 126

Low Temperature, 84

Machinability,

evaluation of metallic materials for,

138, 441

ratings for free-cutting steels, 440relation of cutting speed to tensile

strength, 438variables affecting, 438

Malleable cast iron, 409-412, 421

Magnesium,manufacture, 156

properties, 155

vs. aluminum, 156

Magnesium alloys,

casting, 158

corrosion resistance, 158

heat treatment, 157

precipitation hardening, 121-124

Magnetic properties, ferromagnetic ma-terials, 58, 59

Mallcablizing cycle, 430Malleablizing white cast iron, 410Manufacture of iron, 212-214

Manufacture of steel,

Campbell process, 230crucible process, 231

duplex process, 229

duplex Talbot process, 230

electric-furnace process, 226-227

Monell process, 230

super-refining process, 230

Manufacture of steel (Cont.)

Talbot process, 230triplex process, 230

Martcmpering compared to austempering,287,

commercial limitations, 289described. 286

underlying principle, 287Martens. 2Marlensite. 255

Mass transfer, 99

Mechanical fibering, 39Mechanical properties, 299-300

in cast irons, 41 1, 421

Mechanical working, 28-29,

and engineering properties 32-47

Melting the charge, 223Metal purification, 469Metallic bond, 1

1

Metallurgy,

as a science, 2as an art, 1

importance to engineers, 3, 4Metals,

static properties, significance, 57-79effect of micro-structure, 59

Metals in atomic reactors,

control materials, 20-1

fuel elements, 203gamma-ray source, 204structural materials, 203

Mg-Ca system, 107

Mg-I.i system, 109

Miller indices, 13

Miscibility of metals, 18

Modulus of elasticity, 63-65

Modulus of rigidity, 64

Modulus of rupture, in cast iron, 419

Molybdenum in tool and die steels, 396,

399, 403

Molybdenum in steel, effect on properties,

325, 326

Moncl, 199-200,

K-Monel, 125

Metal, 102

Process. 230S-Monel, 125

Mottled cast iron, 409

Newton's law of cooling, 345

Nickel, 190-191, 199,

alloys of, 199-200

in cermets, 198


Index 513

Nickel in steel,

effect on properties, 323, 324

Nimonic, 199

Nitriding,

advantages, 291

defined, 291

limited to alloy steels, 291

process, 291

Ni-Vee Bronze, 127

Nodular cast iron, 409, 420, 421

Nodular graphite formation, 430

Nodular pearlite, 253

Normalizing of steels, 280, 281

Nose of the curve, 254

Notch sensitivity in cast iron, 413-420

Nondcforming tool and die steel, 396

Normal intermetallic compounds, 21

Normalizing of cast iron, 431

Notched-bar impact, 502

Notched-bar test, 85

Notch brittlencss, 81

Notch constraint, 85

Notch sensitivity, 89

Objectives of annealing, 280

Olsen test, 446

Open-hearth processes, 220

Orange peel, 30

Ore boil, 223

Overaging, 52, 116-117,

precipitation hardening, 116

Oxidation,

high-temperature, 472

internal, 473

of copper, 475

of stainless steel, 482

of titanium, 479

of zinc, 476

prevention, 480

rate,

linear. 477

parabolic, 478, 484

cubic, 478

logarithmic, 478

resistant alloys, 481

Oxide formation, 473

Oxygen pressure, 475

Pack carburizing, 289

Passivity, 454, 455

Patenting, defined, 287,

use of patented wire, 288

Paul! exclusion principle, 7

Pearlite, 253,

defined. 247

hardcnability, 257

malleable cast iron, 412

Perfect crystal, 23

Pcritectic reaction, 108

Pitting. 459

Pewter, 191

Phase, 97

Phase diagrams in metal systems, 19

Phase rule, 100

Phosphorus,

in gray cast iron, 413-415

in steel elfect on properties, 322

Physical testing, 59

Pidgeon process, 156

Pig iron,

classed, 409

composition, 211

Pilling and Bedworth rule, 478, 484

Planck's constant, 6

Plastic constraint, 85

Plastic design, background for, 63

Platinum, 190-191, 202,

metals, 203

Polarization, 454

Polycrystalline metals, 16

Polyphase structures, 49-51

Powder metallurgy, 29

Precious metals, 190, 202

Precipitates, 21,

coherent, 21

non-coherent, 21

Precipitation hardening, 52, 53, 107,

alloy requirements. 111

alloy steel, 126-127

aluminum alloys, 117-123

carbon steel, 125-126

copper base alloys, 117, 124, 125, 127

definition, 111

delayed precipitation, 113

dislocations, 115-120

elfect of temperature, 117

effect of time, 117

Guinier- Preston zones. 115

lattice conformity, 112

magnesium base alloys. 120-124

overaging, 116

process for, 1 12

theory of, 114

time and temperature effects, 114,

119-120

Prior history elfect in metals, 15

Process annealing of steels, 280

Proeulecloid cementite, 254

514 Index

Procutectoid fcrritc, 254

Properties of a spheroidized steel, 281

Properties of solid solutions, 20

Proportional limit, 63, 64

Protective atmospheres, 280Protective oxidation, 478

Pumps, 470

Purpose of hardening, 282

Quality factors, 23, 24Quantum numbers. 6

Quench cracks, 284Quenching,

mechanism of, 270

media, 282stages, 282, 283

Rate of growth, 27-28

Rate of nucleation, 27-28

Radiation damage, 53-54

Recovery, 42

Recrystallization, 28, 43, 45-46

Reduced temperatures, 500-505

Residual stresses, 93

Retained austenite, 274

Reverberatory furnace, 424

Rockwell hardness, described, 70, 71,

relation to other hardness values, 75

Rotating-beam test, 87

Rupture strength, 491

S.A.E. steels,

low-alloy, 328-332

properties, 333-340

specifications for, 329, 331

Scaling, 473

Sensitization of stainless steel, 460

Service factors, 54

Shear modulus, 64

Shear tests, 77

Shot pecning, 92

Sigma phase, 21

Silchrome, 365

Silicon in steel,

effect on properties, 239, 323

Silver, 190-191, 202

Size factor, in forming metal solutions.

18

Slip. 33-36

S-N diagram, 88

Solid solutions, 20,

alloys, 102

effects, 47-49

Solidus, 102

Solute atoms, 18,

clfcct of solute atoms on properties, 20

formation of intermediate phases, 20

interstitial solid solutions, 20substitutional solid solutions, 20

Solution heat treatment, 111, 112,

alloy requirements, 112

elfect of quenching, 113

elfect of temperature, 113

Solutions in metal systems, 18,

factors controlling extent of solubility,

18

liquid solutions, 19

solid solutions, 20

Sonic testing, 78, 79

Sorby, 2

Space group, 12

Space lattice, 12

Specific gravity, of the heavy metals, 191

Spheroidite, 289

Sphcroidization. 495

Spheroidizing steels, 281

Stabilizing, 123-124

Stainless iron, 365

Stainless steels, classes of, 364

Stainless W. 379-380

Static mechanical properties, 60

Steel,

hardening of, early study. 2

mechanical treatment, 231-232

precipitation hardeningalloy steels, 126-127

low carbon steels, 125-126

stainless steel, 127

properties, charts, value, 333-334

strain aging, 126

world production, 208Steel making processes, classification, 208Stellite, 404, 405

Strain aging, 126

Strain hardening, 35-36, 493

Strain rate, 82

Strain softening. 489

Strengthening nonferrous metals,

precipitation hardening. 111

Stress concentration, 89,

locating and magnitude, 60

Stress corrosion, 462, 464, 465

Stress raisers, 60

Stress relief in cast iron, 430

Stress-strain curves. 307

Stress-strain curves, cast iron, 65,

low carbon, cold-worked steel, 62common metals, 64

Index 515

Stretcher strains, 30, 448

Structural changes, 495-498

Structure of carburized steel, 290

Structure of steel,

effect of cold working, 259

Subatmospheric cooling, 258

Substitutional solid solutions, 101

Substitutional solutes, 20

Sulphur in cast iron, 413-415, 420

Super refining process, 230

Super saturated solid solution, 101

Super stainless steels, 382-383

Surface finish, 89, 91

Surface imperfections, 14

Surface protection during heating, 279,

280

Surface-rolling, 92

System, 97

Talbot process, 230

Temper embrittlement, 305

Temper graphite, 430

Temperature, effects of, 485-506

Tempered martensile, 285

Tempering,

and hardenability, 356

change in toughness, 275

effect on physical properties of steel, 284

method, 284

of cast iron, 132

purpose of, 284

retained austenite and quenching, 274

Tensile strength, metals 61-69,

standard specimens, 67, 68

standardization, 61

Ternary-system iron, carbon, silicon, 425,

427

Terne plate, 191, 196

Thermal conductivity, 497, 498

Thermal expansivity, 497, 498

Thermal fatigue, 499

Thermal shock, 499

Thermodynamics, second law of, 98

Thermostatics, 97

Time delay, 346Tin, 190, 191,

alloys of, 192-193

in type metals, 194-195

plating, 197

Tin bronze, 125

Titanium carbide, 405,

manufacture, 159

properties, 159

use. 159, 160

Tool material,

cast alloys, 404, 405

Cemented carbides, 405

tool steels, 389-404

Tool steels,

characteristics, 389, 391, 392

chromium in, 395, 398, 403

classification, 391

composition, 391

high alloy. 397

high carbon, 390

high speed, 398-401

low alloy, 395

manganese in, 396

medium alloy, 396

molybdenum in, 396, 399, 403

properties, 392

silicon in, 397

structure, 394, 398, 401

tempering, 393, 402, 403

tungsten in. 399, 403

vanadium in, 399

Torsion tests, 77

Toughness, 502

Transformation on cooling of cast iron,

428

Transition elements, 9

Transition temperature, 83, 502

Transverse test, cast iron, 77Triaxial stress, 85

Triple point, 101

Triplex process, 230

'ITT curve. 252

Tungsten carbide, 405, 406

Tungsten, in tool and die steels, 399, 403

Twinning, 36-37

Two-metal corrosion, 456

Type metals, 191-194

Types of graphite flakes, 429

Understressing, 93

Uniform corrosion, 456

Unit cell, 12

Uses of carbon steels. 302

Vanadium,In steel, 325

in tool steel, 399Van dcr Waals' bond, 10

Vicalloy, 201

Wave mechanics, 6

Wear resistance,

elfect of carbon content and heat treat-

ment, 444

516 Index

Wear resistance (Conl.)

evaluation of, 444

variables affecting, 443

Wear-types, 442

White cast iron, 408, 410

White metals, 190-191

Wire, carbon steel cold rolled, 30G

Working stresses, 499

Wrough t iron, 2 1

1

Wrought iron, manufacture, 227, 229

Yield point, 06, 67

Yield strength, 66

Zinc, 191, 197

crystal structure, 198

die castings, 197

fabricating methods, 198

mechanical properties, 198

plating, 197

uses, 198

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Engineering Metallurgy