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The Atlas Steels Technical Handbook of Stainless Steels Copyright © Atlas Steels Revised : July 2010 Produced by Atlas Steels Technical Department

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Page 1: The Atlas Steels Technical Handbook of Stainless Steelsbjhowes.com.au/Stainless Steel Technical Handbook.pdf · ATLAS STEELS Technical Handbook of Stainless Steels

The

Atlas Steels

Technical Handbook

of

Stainless Steels

Copyright © Atlas SteelsRevised : July 2010

Produced byAtlas Steels

Technical Department

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ATLAS STEELS Technical Handbook of Stainless Steels

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www.atlassteels.com.au

FOREWORDThis Technical Handbook has been produced as an aid to all personnel of Atlas Steels, their customers and theengineering community generally. It is intended to be both background reading for technical seminars conductedby Atlas Steels Technical Department, and also as a source of ongoing reference data. Any suggestions forimprovements, additions or corrections would be very welcome; these should be directed to:

Technical Manager, Atlas SteelsTelephone +61 3 9272 9999, E-mail [email protected]

Copies of this handbook can be downloaded from the Atlas Steels web site at www.atlassteels.com.au.

Details of specific products are given in the Atlas Steels “Product Reference Manual”, the series of Atlas GradeData Sheets and in Atlas Technotes, as listed below; copies of all these can be viewed or downloaded from theAtlas Steels Website.

Atlas Steels Technotes1. Qualitative Sorting Tests for Steels2. Pitting & Crevice Corrosion of Stainless Steels3. Stainless Steels - Properties & Equivalent Grades4. Machining of Stainless Steels5. Cleaning, Care & Maintenance of Stainless Steels6. Life Cycle Costing7. Galvanic Corrosion8. "L", "H" and Standard Grades of Stainless Steels9. Stainless Steel Tube for the Food Industry10. Restrictions of Hazardous Substances (RoHS)11. Magnetic Response of Stainless Steels12. Pipe Dimensions

Atlas Grade DatasheetsConcise datasheets, covering all the common stainless steels, include chemical composition, mechanical andphysical properties, fabrication and application of each grade. Again these are available from the AtlasSteels Website.

Information from any Atlas publications can be freely copied, but it is requested that the source beacknowledged.

Atlas Steels Technical DepartmentAtlas Steels Technical Department comprises experienced metallurgists, based at our Melbourne Head Office.We offer a free information service, including: Metal grade selection Fabrication information Special steels applications Specification assistance (such as equivalents of foreign specifications and trade names) Metallurgical properties of metals Supply of technical literature published by Atlas Steels and other metals institutions and bodies.We also have our own metallurgical laboratory providing facilities to assist in investigation of products andapplications.

This assistance is provided as a free service to Atlas Steels' valued customers, and to all members of theAustralian and New Zealand engineering community.

Freecall 1800 818 599 (from within Australia) or +61 3 9272 9999E-mail [email protected]

Limitation of LiabilityThe information contained in this Technical Handbook is not an exhaustive statement of all relevant information. Itis a general guide for customers to the products and services available from Atlas Steels and no representation ismade or warranty given in relation to this information or the products or processes it describes.

Published by Atlas Steels Technical DepartmentCopyright © Atlas Steels

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TABLE OF CONTENTS

FOREWORD 2

Atlas Steels Technotes 2Atlas Grade Datasheets 2Atlas Steels Technical Department 2Limitation of Liability 2

TABLE OF CONTENTS 3

THE FAMILY OF MATERIALS 4

Steel Grade Designations 4

STAINLESS STEELS - INTRODUCTION TOTHE GRADES AND FAMILIES 6

The Families of Stainless Steels 6Austenitic Stainless Steels 6Ferritic Stainless Steels 7Martensitic Stainless Steels 7Duplex Stainless Steels 7Precipitation Hardening Stainless Steels 7

Characteristics of Stainless Steels 7Standard Classifications 7

Austenitic and Duplex Stainless Steels 8Ferritic and Martensitic Stainless Steels 9Comparative Properties of the Stainless SteelAlloy Families 10

CORROSION RESISTANCE 11

General Corrosion 11Pitting corrosion 11Crevice corrosion 12Stress corrosion cracking (SCC) 12Sulphide Stress Corrosion Cracking (SSC) 13Intergranular corrosion 13Galvanic corrosion 14Contact corrosion 14

HIGH TEMPERATURE RESISTANCE 16

Scaling Resistance 16Creep Strength 16Structural Stability 17Environmental Factors 17Thermal Expansion 17

CRYOGENIC PROPERTIES 19

MAGNETIC PROPERTIES 20

Magnetically Soft Stainless Steels 20

MECHANICAL PROPERTIES 21

Mechanical Properties of Wire 22Mechanical Properties of Bar 22

FABRICATION 23

Forming Operations 23Machining 25Welding 26Soft Soldering 27Brazing ("Silver Soldering") 27

HEAT TREATMENT 29

Annealing 29

Hardening 30Stress Relieving 30Surface Hardening 31

SURFACE FINISHING 32

Passivation 32Pickling 32Degreasing 33Electropolishing 33Grinding and Polishing 33Mechanical Cleaning 34Blackening 34

SURFACE CONTAMINATION INFABRICATION 35

Contamination by Carbon Steel 35Contamination by Chlorides 35Contamination by Carbon 35

DESIGN CONSIDERATIONS INFABRICATION OF STAINLESS STEELS 36

Grade Selection for Fabrication 36Design to Avoid Corrosion 36Specific Design Points - To Retain CorrosionResistance 37

GUIDELINES FOR STAINLESS STEEL GRADESELECTION 39

APPENDICES 41

Grade Summary - Stainless & Heat ResistingSteels 41Comparison of Grade Specifications ofStainless Steels 43Typical Physical Properties - AnnealedCondition 44Hardness Conversions 45Unit Conversions 46Dimensional Tolerances for Bar 47Further Information 48Internet Sites of Interest 49

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THE FAMILY OF MATERIALSMaterials can be divided into metals and non-metals; the history of civilisation has largely beencategorised by the ability to work metals - hence"bronze age" and "iron age" - but until quiterecently most large-scale construction was still innon-metals, mostly stone or masonry and wood.

Today a vast number of materials compete fortheir share of the market, with more newmaterials being added every year. Someparticularly exciting developments are nowoccurring in the fields of ceramics, plastics andglasses and composites of these materials. Theday of the ceramic car engine is probably not allthat far off - already there are some hightemperature components made from the newgeneration of tougher ceramics, and the modernmotor vehicle also offers many examples of theuse of engineering plastics. Recent developmentsin metals have re-asserted their competitiveposition in auto engineering, in particular the useof aluminium and magnesium alloys. A majorrevolution under way at present is thereplacement of much copper telecommunicationscabling with glass optical fibre. For metals tocompete they must be able to demonstratesuperior properties to their competitors.

In a similar fashion each of the metals has tocompete for its market share, based ondemonstrated superiority of properties oreconomics. It is therefore worth identifying thevarious metals available and indicating just whattheir most important features are. A basicdifferentiation is to divide metals into "ferrous"and "non-ferrous", ie those iron-based and all theothers.

Amongst the non-ferrous metals the mostimportant for engineering applications are thefamilies of aluminium alloys (with very lowdensities, high electrical and thermal conductivity,good formability and good corrosion resistancethese find applications in aircraft, high tensionelectricity conductors, yacht masts etc) and ofcopper alloys (with very high electrical andthermal conductivities and ready formability thesefind their principal applications in electricalwiring). Other important non-ferrous alloys (analloy is simply a mixture of two or more metals)are the brasses and bronzes.

The family of ferrous metals incorporates a vastnumber of alloys. Those alloys containing a veryhigh proportion of carbon (over about 2%) arecalled cast irons. Virtually all of the remainder aretermed steels and these can be found in eithercast form (produced by pouring molten metal intoa mould of the shape of the finished part) orwrought form (cast as ingots or continuous castbillets or slabs, but then hot rolled or forged toproduce bars, plates or complex shapes such asrail sections and beams). They can also be formedto finished shape by sintering powdered metal athigh temperature. Steels are categorised by theirmajor alloying elements (carbon, manganese,chromium, nickel and molybdenum) and by thepresence or absence of minor elements (silicon,sulphur, phosphorus, nitrogen and titanium), asshown in the table in Figure 1.

"Micro" additions of alloys are also present insome grades.

Atlas Steels distributes product from all fourcategories (plain carbon, low alloy, stainless andtool steels).

Steel Grade DesignationsDesignation systems for metals vary widely. In thepast every producer had their own name for eachgrade they produced - some examples were"Duraflex" (BHP's name for 1045) and "Sixix"(Atlas Steels Canada's name for M2 high speedsteel).

Thankfully this practice is now reducing, withbenefits to all users. In some instances there isjustification for the use of a specific trade name,for instance where a manufacturer has made a

Type TypicalGrade

AlloyContent

TypicalUses

plaincarbonsteels

1020 0.2% C bridges,buildingframes,machineryshafts

lowalloysteels

4140 0.4% C1.0% Cr0.2% Mo

highlystressedshafts,forgedmachinecomponents

stainless& highalloysteels

304 0.05% C18% Cr9% Ni

corrosionresistanttanks, bolts,springs

toolsteels

H13 0.4% C1.05% Si5.2% Cr1.3% Mo1.0% V

tools forcasting andhot forging

Figure 1 Typical grades in each steel group

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grade significantly different from other similarproducts. This is particularly appropriate in newproduct areas such as duplex stainless steels,where national standards lag behind commercialalloy development, and where grades are stillevolving. Some producers, however, cling to theuse of trade names for quite standard grades inthe hope of generating sales on the basis ofperceived rather than actual product superiority.

Apart from trade designations a variety of namingsystems exist, supported by one or otherstandards body. In Australia metals designationstend to more or less follow those of the USA -principally the American Iron and Steel Institute(AISI) and American Society for Testing andMaterials (ASTM). These bodies many years agodeveloped three-digit designations for stainlesssteels, four-digit designations for carbon and lowalloy steels and one letter plus one or two digitdesignations for tool steels. All three systemshave proven inadequate for coping with an on-going series of new alloy developments, so a newUnified Numbering System (UNS) has beenimplemented by ASTM and the Society ofAutomotive Engineers (SAE). The UNSdesignations have been allocated to all metals incommercial production, throughout the world; asingle letter indicates the alloy family (N = nickelbase alloys, S = stainless steels, etc) and fivedigits denote the grade. This system is nowincorporated in all ASTM standards, and also somestandards from other countries such as Australia.

Japanese grade designations are based on theAISI/ASTM designations as far as stainless steelsare concerned, with some modifications andexceptions. They follow their own system for othersteels.

Until recent years British Standards had adesignation system for steel grades based uponthe AISI/ASTM system, but with extra digits tospecify slight variants of grades, eg 316S31 was aparticular variant of Grade 316 stainless steel.

Other European national standards were quitedifferent, and also differed among themselves.The most commonly encountered system was theGerman Werkstoff (Workshop) Number giving allsteels a single digit plus four digit designation, eg"1.4301" for Grade 304. A second identifierassociated with each grade is the DIN designation,eg "X 5 CrNi 18 9" for Grade 304.

In addition to these national specifications thereare International Standards (ISO) which tend tofollow various European systems, and a newlydeveloping set of "Euronorm" (EN) Europeanspecifications from the European Union. TheseEuronorms have now largely replaced nationalspecifications from Britain, Germany, France andother member nations. For stainless steels themost common grade designation system inAustralia and New Zealand is that of the AmericanASTM. There is however increased awareness ofthe Euronorm system.

An interesting development in 2006 was the minorrevision of composition limits for grade 304 flatrolled product in ASTM A240 with the specific aimof harmonizing it with the European equivalentgrade 1.4301. There is a long way to go, butperhaps this is the beginning of a truly uniformworld-wide agreement on specifications.

Numerous cross references between grades arepublished; a very comprehensive publication is theGerman "Stahlschlüssel" (Key to Steel). This isparticularly good for German specifications butdoes cover all significant steel specifyingcountries, including Australia, and lists tradenames in addition to national specifications.Several web sites also give specificationcomparisons.

A summary of these grade equivalents (or nearalternatives) is shown in Appendix 2.

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STAINLESS STEELS - INTRODUCTION TO THE GRADES AND FAMILIES

The group of alloys which today make up thefamily of stainless steels had their beginning in theyears 1900 to 1915. Over this period researchwork was published by metallurgists in France andGermany on ferritic, martensitic and austeniticstainless steels. Commercialisation began in theyears 1910 to 1915, with the Americans Dansitzenand Becket’s work on ferritic steels and Maurerand Strauss developing austenitic grades inGermany. The often-related development ofmartensitic steels was in 1913 in Sheffield,England; Harry Brearley was trying a number ofalloys as possible gun barrel steels, and noticedthat samples cut from one of these trial Heats didnot rust and were in fact difficult to etch. When heinvestigated this curious material - it containedabout 13% chromium - it lead to the developmentof the stainless cutlery steels for which Sheffieldbecame famous.

Although the consumption of stainless steels isgrowing very rapidly around the world (average of5.8% per annum in the Western world over theperiod 1950 to 2001) average per capitaconsumption in Australia is very low bycomparison with other developed, and manydeveloping countries. In 1999 Australians eachconsumed about 5kg, compared with about 8kgper head in France, 13kg in Japan, 16kg inGermany, 26kg in Singapore and 38kg in Taiwan.On average each Chinese consumed about 1.3kg,but this figure is rapidly rising.

The Families of Stainless SteelsStainless steels are iron based alloys containing aminimum of about 10.5% chromium; this forms aprotective self-healing oxide film, which is thereason why this group of steels have theircharacteristic "stainlessness" or corrosionresistance. The ability of the oxide layer to healitself means that the steel is corrosion resistant,no matter how much of the surface is removed;this is not the case when carbon or low alloy steelsare protected from corrosion by metallic coatingssuch as zinc or cadmium or by organic coatingssuch as paint.

Although all stainless steels depend on thepresence of chromium, other alloying elementsare often added to enhance their properties. Thecategorisation of stainless steels is unusualamongst metals in that it is based upon the natureof their metallurgical structure - the terms useddenote the arrangement of the atoms which makeup the grains of the steel, and which can beobserved when a polished section through a pieceof the material is viewed at high magnificationthrough a microscope. Depending upon the exactchemical composition of the steel themicrostructure may be made up of the stablephases austenite or ferrite, a "duplex" mix of

these two, the phase martensite created whensome steels are rapidly quenched from a hightemperature, or a structure hardened byprecipitated micro-constituents.

The relationship between the different families isas shown in Figure 2. A broad brush comparison ofthe properties of the different families is given inFigure 5.

Austenitic Stainless Steels

This group contain at least 16% chromium and6% nickel (the basic grade 304 is sometimesreferred to as 18/8) and range through to the highalloy or "super austenitics" such as 904L and 6%molybdenum grades.

Additional elements can be added such asmolybdenum, titanium or copper, to modify orimprove their properties, making them suitable formany critical applications involving hightemperature as well as corrosion resistance. Thisgroup of steels is also suitable for cryogenicapplications because the effect of the nickelcontent in making the steel austenitic avoids theproblems of brittleness at low temperatures, whichis a characteristic of other types of steel.

The relationship between the various austeniticgrades is shown in Figures 3.

Figure 2 Families of stainless steels

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Ferritic Stainless Steels

These are plain chromium (10½ to 29%) gradessuch as Grades 430 and 409. Their moderatecorrosion resistance and poor fabricationproperties are improved in the higher alloyed andstabilised grades such as 439 and 444 and in theproprietary grade AtlasCR12.

The relationship between the various ferriticgrades is shown in Figure 4.

Martensitic Stainless Steels

Martensitic stainless steels are also based on theaddition of chromium as the major alloyingelement but with a higher carbon and generallylower chromium content (eg 12% in Grades 410and 416) than the ferritic types; Grade 431 has achromium content of about 16%, but themicrostructure is still martensite despite this highchromium level because this grade also contains2% nickel.

The relationship between the various martensiticgrades is shown in Figure 4.

Duplex Stainless Steels

Duplex stainless steels such as 2205 and 2507(these designations indicate compositions of 22%chromium, 5% nickel and 25% chromium, 7%nickel but both grades contain further minoralloying additions) have microstructurescomprising a mixture of austenite and ferrite.Duplex ferritic - austenitic steels combine some ofthe features of each class: they are resistant tostress corrosion cracking, albeit not quite asresistant as the ferritic steels; their toughness issuperior to that of the ferritic steels but inferior tothat of the austenitic steels, and their strength isgreater than that of the (annealed) austeniticsteels, by a factor of about two. In addition theduplex steels have general corrosion resistancesequal to or better than 304 and 316, and ingeneral their pitting corrosion resistances aresuperior to 316. They suffer reduced toughnessbelow about -50oC and after exposure above300oC, so are only used between thesetemperatures.

The relationship between the various duplexgrades is shown in Figures 3.

Precipitation Hardening Stainless Steels

These are chromium and nickel containing steelswhich can develop very high tensile strengths. Themost common grade in this group is "17-4 PH";also known as Grade 630, with the composition of17% chromium, 4% nickel, 4% copper and 0.3%niobium. The great advantage of these steels isthat they can be supplied in the "solution treated"condition; in this condition the steel is justmachinable. Following machining, forming etc. thesteel can be hardened by a single, fairly lowtemperature "ageing" heat treatment which

causes no distortion of the component.

Characteristics of StainlessSteelsThe characteristics of the broad group of stainlesssteels can be viewed as compared to the morefamiliar plain carbon "mild" steels. As ageneralisation the stainless steels have:- Higher work hardening rate Higher ductility Higher strength and hardness Higher hot strength Higher corrosion resistance Higher cryogenic toughness Lower magnetic response (austenitic only)

These properties apply particularly to theaustenitic family and to varying degrees to othergrades and families.

These properties have implications for the likelyfields of application for stainless steels, but alsoinfluence the choice of fabrication methods andequipment.

Standard Classifications

There are many different varieties of stainlesssteel and the American Iron and Steel Institute(AISI) in the past designated some as standardcompositions, resulting in the commonly usedthree digit numbering system. This role has nowbeen taken over by the SAE and ASTM, whoallocate 1-letter + 5-digit UNS numbers to newgrades. The full range of these standard stainlesssteels is contained in the Iron and Steel Society(ISS) "Steel Products Manual for Stainless Steels",and in the SAE/ASTM handbook of UnifiedNumbering System. Certain other grades do nothave standard numbers, but are instead coveredby other national or international specifications, orby specifications for specialised products such asstandards for welding wire. The following diagramsshow most of the grades of stainless steelsdistributed by Atlas Steels and also some otherimportant grades, identified by their gradenumbers or common designations, illustratingsome of the important properties of the variousfamilies of grades.

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Austenitic and Duplex Stainless Steels

Austenitic Stainless Steel

304Basic Grade

310

316 317

316L

304L

308L

303

302HQ

253MAS30815

904L6Mo

S31254

321

347

Free Machining grade

Low work hardening rate for cold heading

Welding consumable grades

Weld stabilized grades

Weld stabilized grades

Increasing corrosion resistance >>>>

Duplex Stainless Steels

2304S32304

2250S31803

super duplexgrades

Increasing high temperature resistance >>>

Figure 3 The families of Austenitic and Duplex Stainless Steels

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Ferritic and Martensitic Stainless Steels

Ferritic Stainless Steels

430basic grade

444

409 3CR12

430F

420

431

440A

Free machining grade

Utility grades with increasing

toughness >>>

410basic grade

Higher corrosion resisting weldable grade.

440B 440C

Martensitic Stainless Steels

Higher hardness grade

Higher corrosion resistanceand higher toughness grade

Increasing hardness after heat treatment>>>

416 Free machining grade

Figure 4 The families of Ferritic and Martensitic Stainless Steels

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Comparative Properties of the Stainless Steel Alloy Families

Alloy Group MagneticResponse(note 1)

WorkHardening

Rate

CorrosionResistance

(note2)

Hardenable Ductility HighTemperatureResistance

LowTemperatureResistance

(note 3)

Weldability

Austenitic Generally No Very High High By Cold Work Very High Very High Very High Very High

Duplex Yes Medium Very High No Medium Low Medium High

Ferritic Yes Medium Medium No Medium High Low Low to High

Martensitic Yes Medium Medium Quench &Temper

Low Low Low Low

PrecipitationHardening

Yes Medium Medium Age Hardening Medium Low Low High

Notes1. Attraction of the steel to a magnet. Note some austenitic grades can be attracted to a magnet if cold worked, cast or welded.2. Varies significantly between grades within each group. e.g. free machining grades have lower corrosion resistances, those grades higher in chromium,

molybdenum and nitrogen have higher resistances. Corrosion resistance is not primary related to the alloy group (structure) but more by the composition.3. Measured by toughness or ductility at sub-zero temperatures. Austenitic grades retain ductility to cryogenic temperatures.

Figure 5 Comparative properties

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CORROSION RESISTANCE

Although the main reasons why stainless steelsare used is corrosion resistance, they do in factsuffer from certain types of corrosion in someenvironments and care must be taken to select agrade which will be suitable for the application.Corrosion can cause a variety of problems,depending on the applications:

Perforation such as of tanks and pipes, whichallows leakage of fluids or gases,

Loss of strength where the cross section ofstructural members is reduced by corrosion,leading to a loss of strength of the structureand subsequent failure,

Degradation of appearance, where corrosionproducts or pitting can detract from adecorative surface finish,

Finally, corrosion can produce scale or rustwhich can contaminate the material beinghandled; this particularly applies in the case offood processing equipment.

Corrosion of stainless steels can be categorisedas:-General CorrosionPitting CorrosionCrevice CorrosionStress Corrosion CrackingSulphide Stress Corrosion CrackingIntergranular CorrosionGalvanic CorrosionContact Corrosion

General CorrosionCorrosion whereby there is a general uniformremoval of material, by dissolution, eg whenstainless steel is used in chemical plant forcontaining strong acids. Design in this instance isbased on published data to predict the life of thecomponent.

Published data list the removal of metal over ayear - a typical example is shown in Figure 6.

Tables of resistance to various chemicals arepublished by various organisations and a verylarge collection of charts, lists, recommendationsand technical papers is available through the AtlasSteels Technical Services Department.

Pitting corrosionUnder certain conditions, particularly involvinghigh concentrations of chlorides (such as sodiumchloride in sea water), moderately hightemperatures and exacerbated by low pH (ie acidicconditions), very localised corrosion can occurleading to perforation of pipes and fittings etc.This is not related to published corrosion data as itis an extremely localised and severe corrosionwhich can penetrate right through the crosssection of the component. Grades high inchromium, and particularly molybdenum andnitrogen, are more resistant to pitting corrosion.

The Pitting Resistance Equivalent number (PRE)has been found to give a good indication of thepitting resistance of stainless steels. The PRE canbe calculated as:

PRE = %Cr + 3.3 x %Mo + 16 x %NOne reason why pitting corrosion is so serious isthat once a pit is initiated there is a strongtendency for it to continue to grow, even althoughthe majority of the surrounding steel is stilluntouched.

The tendency for a particular steel to be attackedby pitting corrosion can be evaluated in thelaboratory. A number of standard tests have beendevised, the most common of which is that givenin ASTM G48. A graph can be drawn giving thetemperature at which pitting corrosion is likely tooccur, as shown in Figure 7.

Figure 7 Critical pitting temperaturesfor different alloys, rated by ASTMG48A test

The graph is based on a standard ferric chloridelaboratory test, but does predict outcomes in

Figure 6 Iso-corrosion curves forvarious stainless steels in sulphuricacid

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many service conditions.

A very common corrosive environment in whichstainless steels are used is marine, generally up toa few hundred metres from quiet (eg: bay) water,or up to a few kilometres from a shore withbreaking waves. Corrosion in this environment issometimes called “tea staining”… a term used byASSDA (Australian Stainless Steel DevelopmentAssociation) to describe light surface rusting,usually visible as a fairly wide-spread brown-reddiscolouration. A very full description of thecauses and prevention of tea staining is given inthe ASSDA Technical Bulletin on the topic,available at the ASSDA website atwww.assda.asn.au

For many years 316 has been regarded as the“marine grade” of stainless steel. It must berecognised however, that in the more aggressivemarine environments 316 will not fully resistpitting corrosion or tea staining. It has beenfound that finer surface finishes – generally withan Ra value of no coarser than 0.5µm will assist inrestricting tea staining. It is also important thatthe surface is free of any contaminants.

Crevice corrosionThe corrosion resistance of a stainless steel isdependent on the presence of a protective oxidelayer on its surface, but it is possible under certainconditions for this oxide layer to break down, forexample in reducing acids, or in some types ofcombustion where the atmosphere is reducing.Areas where the oxide layer can break down canalso sometimes be the result of the waycomponents are designed, for example undergaskets, in sharp re-entrant corners or associatedwith incomplete weld penetration or overlappingsurfaces. These can all form crevices which canpromote corrosion.

To function as a corrosion site, a crevice has to beof sufficient width to permit entry of thecorrodent, but sufficiently narrow to ensure thatthe corrodent remains stagnant. Accordinglycrevice corrosion usually occurs in gaps a fewmicrometres wide, and is not found in grooves orslots in which circulation of the corrodent ispossible. This problem can often be overcome bypaying attention to the design of the component,in particular to avoiding formation of crevices or atleast keeping them as open as possible.

Crevice corrosion is a very similar mechanism topitting corrosion; alloys resistant to one aregenerally resistant to both. Crevice corrosion canbe viewed as a more severe form of pittingcorrosion as it will occur at significantly lowertemperatures than does pitting. Further details ofpitting and crevice corrosion are given in AtlasTechnote 2.

Stress corrosion cracking (SCC)Under the combined effects of stress and certaincorrosive environments stainless steels can besubject to this very rapid and severe form ofcorrosion. The stresses must be tensile and canresult from loads applied in service, or stresses setup by the type of assembly e.g. interference fits ofpins in holes, or from residual stresses resultingfrom the method of fabrication such as coldworking. The most damaging environment is asolution of chlorides in water such as sea water,particularly at elevated temperatures. As aconsequence many stainless steels are limited intheir application for holding hot waters (aboveabout 50°C) containing even trace amounts ofchlorides (more than a few parts per million). Thisform of corrosion is only applicable to theaustenitic group of steels and is related to thenickel content. Grade 316 is not significantly moreresistant to SCC than is 304. The duplex stainlesssteels are much more resistant to SCC than arethe austenitic grades, with grade 2205 beingvirtually immune at temperatures up to about150°C, and the super duplex grades are moreresistant again. The ferritic grades do notgenerally suffer from this problem at all.

In some instances it has been found possible toimprove resistance to SCC by applying acompressive stress to the component at risk; thiscan be done by shot peening the surface forinstance. Another alternative is to ensure theproduct is free of tensile stresses by annealing asa final operation. These solutions to the problemhave been successful in some cases, but need tobe very carefully evaluated, as it may be verydifficult to guarantee the absence of residual orapplied tensile stresses.

From a practical standpoint, Grade 304 may beadequate under certain conditions. For instance,Grade 304 is being used in water containing 100 -300 parts per million (ppm) chlorides at moderatetemperatures. Trying to establish limits can berisky because wet/dry conditions can concentratechlorides and increase the probability of stresscorrosion cracking. The chloride content ofseawater is about 2% (20,000 ppm). Seawaterabove 50oC is encountered in applications such asheat exchangers for coastal power stations.

Recently there has been a small number ofinstances of chloride stress corrosion failures atlower temperatures than previously thoughtpossible. These have occurred in the warm, moistatmosphere above indoor chlorinated swimmingpools where stainless steel (generally Grade 316)fixtures are often used to suspend items such asventilation ducting. Temperatures as low as 30 to40°C have been involved. There have also beenfailures due to stress corrosion at highertemperatures with chloride levels as low as 10ppm. This very serious problem is not yet fullyunderstood.

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Sulphide Stress CorrosionCracking (SSC)Of greatest importance to many users in the oiland gas industry is the material's resistance tosulphide stress corrosion cracking. The mechanismof SSC has not been defined unambiguously butinvolves the conjoint action of chloride andhydrogen sulphide, requires the presence of atensile stress and has a non-linear relationshipwith temperature.

The three main factors are:a) Stress level

A threshold stress can sometimes beidentified for each material - environmentcombination. Some published data show acontinuous fall of threshold stress withincreasing H2S levels. To guard againstSSC NACE specification MR0175 forsulphide environments limits the commonaustenitic grades to 22HRC maximumhardness.

b) EnvironmentThe principal agents being chloride,hydrogen sulphide and pH. There issynergism between these effects, with anapparently inhibiting effect of sulphide athigh H2S levels.

c) TemperatureWith increasing temperature, thecontribution of chloride increases but theeffect of hydrogen decreases due to itsincreased mobility in the ferrite matrix.The net result is a maximum susceptibilityin the region 60-100°C.

A number of secondary factors have also beenidentified, including amount of ferrite, surfacecondition, presence of cold work and heat tint atwelds.

Intergranular corrosionIntergranular corrosion is a form of relatively rapidand localised corrosion associated with a defectivemicrostructure known as carbide precipitation.When austenitic steels have been exposed for aperiod of time in the range of approximately 425to 850°C, or when the steel has been heated tohigher temperatures and allowed to cool throughthat temperature range at a relatively slow rate(such as occurs after welding or air cooling afterannealing), the chromium and carbon in the steelcombine to form chromium carbide particles alongthe grain boundaries throughout the steel.Formation of these carbide particles in the grainboundaries depletes the surrounding metal ofchromium and reduces its corrosion resistance,allowing the steel to corrode preferentially alongthe grain boundaries. Steel in this condition issaid to be "sensitised".

It should be noted that carbide precipitationdepends upon carbon content, temperature and

time at temperature, as shown in Figure 8. Themost critical temperature range is around 700°C,at which 0.06% carbon steels will precipitatecarbides in about 2 minutes, whereas 0.02%carbon steels are effectively immune from thisproblem.

It is possible to reclaim steel which suffers fromcarbide precipitation by heating it above1000°C, followed by water quenching to retainthe carbon and chromium in solution and soprevent the formation of carbides. Moststructures which are welded or heated cannot begiven this heat treatment and therefore specialgrades of steel have been designed to avoid thisproblem. These are the stabilised grades 321(stabilised with titanium) and 347 (stabilisedwith niobium). Titanium and niobium each havemuch higher affinities for carbon than chromiumand therefore titanium carbides, niobiumcarbides and tantalum carbides form instead ofchromium carbides, leaving the chromium insolution and ensuring full corrosion resistance.

Figure 8 Susceptibility to intergranularcorrosion of grade 304 with variouscarbon contents (ref AWRA Technote 16)

Another method used to overcome intergranularcorrosion is to use the extra low carbon gradessuch as Grades 316L and 304L; these haveextremely low carbon levels (generally less than0.03%) and are therefore considerably moreresistant to the precipitation of carbide.

Many environments do not cause intergranularcorrosion in sensitised austenitic stainless steels,for example, glacial acetic acid at roomtemperature, alkaline salt solution such as sodiumcarbonate, potable water and most inland bodiesof fresh water. For such environments, it wouldnot be necessary to be concerned aboutsensitisation. There is also generally no problem inlight gauge steel since it usually cools very quicklyfollowing welding or other exposure to hightemperatures. For this reason thin gauge sheet isoften only available in standard carbon content,but heavy plate is often only available in lowcarbon “L” grades. More information on L, H andstandard grades is given in Atlas Technote 8.

It is also the case that the presence of grain

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boundary carbides is not harmful to the hightemperature strength of stainless steels. Gradeswhich are specifically intended for theseapplications often intentionally have high carboncontents as this increases their high temperaturestrength and creep resistance. These are the "H"variants such as grades 304H, 316H, 321H and347H, and also 310. All of these have carboncontents deliberately in the range in whichprecipitation will occur.

Sensitised steels have also been found to sufferfrom intergranular stress corrosion cracking(IGSCC). This problem is rare, but can affectunstabilised ferritic and austenitic grades if theyare treated or held in appropriate temperatureranges.

Galvanic corrosionBecause corrosion is an electrochemical processinvolving the flow of electric current, corrosion canbe generated by a galvanic effect which arisesfrom the contact of dissimilar metals in anelectrolyte (an electrolyte is an electricallyconductive liquid). In fact three conditions arerequired for galvanic corrosion to proceed ... thetwo metals must be widely separated on thegalvanic series (see Figure 9), they must be inelectrical contact, and their surfaces must bebridged by an electrically conducting fluid.Removal of any of these three conditions willprevent galvanic corrosion.

The obvious means of prevention is therefore toavoid mixed metal fabrications or only use thoseclose together in the galvanic series; copper alloysand stainless steels can generally be mixedwithout problem for instance. Frequently this isnot practical, but prevention can also be byremoving the electrical contact - this can beachieved by the use of plastic or rubber washersor sleeves, or by ensuring the absence of theelectrolyte such as by improvement to draining orby the use of protective hoods. This effect is alsodependent upon the relative areas of the dissimilarmetals. If the area of the less noble material (theanodic material, further towards the right in Figure9) is large compared to that of the more noble(cathodic) the corrosive effect is greatly reduced,and may in fact become negligible. Conversely alarge area of noble metal in contact with a smallarea of less noble will accelerate the galvaniccorrosion rate. For example it is common practiceto fasten aluminium sheets with stainless steelscrews, but aluminium screws in a large area ofstainless steel are likely to rapidly corrode.

The most common instance of galvanic corrosionis probably the use of zinc plated carbon steelfasteners in stainless steel sheet. Further details ofgalvanic corrosion are given in Atlas Technote 7.

Contact corrosionThis combines elements of pitting, crevice and

galvanic corrosion, and occurs where smallparticles of foreign matter, in particular carbonsteel, are left on a stainless steel surface. Theattack starts as a galvanic cell - the particle offoreign matter is anodic and hence likely to bequickly corroded away, but in severe cases a pitmay also form in the stainless steel, and pittingcorrosion can continue from this point. The mostprevalent cause is debris from nearby grinding ofcarbon steel, or use of tools contaminated withcarbon steel. For this reason some fabricatorshave dedicated stainless steel workshops wherecontact with carbon steel is totally avoided.

Figure 9 Galvanic series for metals inflowing sea water. More negativevalues for stainless steels are for activeconditions, such as in crevices

All workshops and warehouses handling or storingstainless steels must also be aware of thispotential problem, and take precautions to preventit. Protective plastic, wood or carpet strips can beused to prevent contact between stainless steelproducts and carbon steel storage racks. Otherhandling equipment to be protected includes forklift tynes and crane lifting fixtures. Clean fabricslings have often been found to be a usefulalternative.

If stainless steel does become contaminated bycarbon steel debris this can be removed bypassivation with dilute nitric acid or pickling with amix of hydrofluoric and nitric acids. See the latersection on pickling and passivation for furtherdetails.

Contamination by carbon steel (also referred to as

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“free iron”) can be detected by:

A “ferroxyl” test is very sensitive, but requiresa freshly made up test solution. Refer toASTM A380.

Copper sulphate will plate out copper on freeiron. Again details in ASTM A380.

The simplest test is to wet the surfaceintermittently for about 24 hours; anycontamination will be revealed as rust spots.

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HIGH TEMPERATURE RESISTANCE

The second most common reason stainless steelsare used is for their high temperature properties;stainless steels can be found in applications wherehigh temperature oxidation resistance isnecessary, and in other applications where hightemperature strength is required. The highchromium content which is so beneficial to the wetcorrosion resistance of stainless steels is alsohighly beneficial to their high temperaturestrength and resistance to scaling at elevatedtemperatures, as shown in the graph of Figure 10.

Scaling ResistanceResistance to oxidation, or scaling, is dependenton the chromium content in the same way as thecorrosion resistance is, as shown in the graph ofFigure 10. Most austenitic steels, with chromiumcontents of at least 18%, can be used attemperatures up to 870°C and Grades 309, 310and S30815 (253MA) even higher. Mostmartensitic and ferritic steels have lowerresistance to oxidation and hence lower usefuloperating temperatures. An exception to this isthe ferritic grade 446 - this has approximately24% chromium, and can be used to resist scalingat temperatures up to 1100°C.

The table in Figure 11 shows the approximatemaximum service temperatures at which thevarious grades of stainless steels can be used toresist oxidation in dry air. Note that thesetemperatures depend very much on the actual

environmental conditions, and in some instancessubstantially lower temperatures will result in

destructive scaling.

Creep StrengthThe high temperature strength of materials isgenerally expressed in terms of their "creepstrength" - the ability of the material to resistdistortion over a long term exposure to a hightemperature. In this regard the austenitic stainlesssteels are particularly good. Design codes such asAustralian Standard AS1210 "Pressure Vessels"and AS4041 "Pressure Piping" (and correspondingcodes from ASME and other bodies) also stipulateallowable working stresses of each grade at arange of temperatures. The low carbon versions ofthe standard austenitic grades (Grades 304L and316L) have reduced strength at high temperatureso are not generally used for structuralapplications at elevated temperatures. "H"versions of each grade (eg 304H) have highercarbon contents for these applications, whichresults in significantly higher creep strengths. "H"grades are specified for some elevatedtemperature applications. A more completedescription of the application of L, H and standardaustenitic grades is given in Atlas Technote 8.

The scaling resistances of the ferritic stainlesssteels are generally as suggested by the graph ofFigure 10. So 11% chromium grades (409 orAtlasCR12) have moderate sealing resistances,17% grades (430) have good sealing resistanceand 25% Cr grades (446) have excellent sealingresistance. The ferritic structure however, doesnot have the high creep strength of the austeniticgrades, so the use of ferritics at very hightemperatures is often limited to low stressapplication. Addition of niobium to ferritic gradesdoes improve the creep strength substantially, andsome niobium-stabilised grades find application inhigh temperature auto exhaust components forinstance.

Figure 10 Effect of chromium content onscaling resistance

Grade Intermittent(°C)

Continuous(°C)

304309310316321410416420430

S30815

87098010358708708157607358701150

925109511509259257056756208151150

Figure 11 Maximum service temperaturesin dry air, based on scaling resistance (ref:ASM Metals Handbook)

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Although the duplex stainless steels have goodoxidation resistance due to their high chromiumcontents, they suffer from embrittlement ifexposed to temperatures above about 350°C, sothey are restricted to applications below this.

Both martensitic and precipitation hardeningfamilies of stainless steels have high strengthsachieved by thermal treatments; exposure ofthese grades at temperatures exceeding their heattreatment temperatures will result in permanentsoftening, so again these grades are seldom usedat elevated temperatures.

Structural StabilityThe problem of grain boundary carbideprecipitation was discussed under intergranularcorrosion. This same phenomenon occurs whensome stainless steels are exposed in service totemperatures of 425 to 815°C, resulting in areduction of corrosion resistance which may besignificant. If this problem is to be avoided the useof stabilised grades such as Grade 321 or lowcarbon "L" grades should be considered. It mustbe understood that a high carbon content, as inthe “H” grades, such as 304H, is beneficial toelevated temperature strength. Such steels donot have good aqueous corrosion resistance, butthis is often not a problem. Refer to AtlasTechnote 8 for further details.

A further problem that some stainless steels havein high temperature applications is the formationof sigma phase. The formation of sigma phase inaustenitic steels is dependent on both time andtemperature and is different for each type of steel.In general Grade 304 stainless steel is practicallyimmune to sigma phase formation, but not sothose grades with higher chromium contents(Grade 310) with molybdenum (Grades 316 and317) or with higher silicon contents (Grade 314).These grades are all prone to sigma phaseformation if exposed for long periods to atemperature of about 590 to 870°C. Sigma phaseembrittlement refers to the formation of aprecipitate in the steel microstructure over a longperiod of time within this particular temperaturerange. The effect of the formation of this phase isto make the steel extremely brittle and failure canoccur because of brittle fracture. Once the steelhas become embrittled with sigma it is possible toreclaim it by heating the steel to a temperatureabove the sigma formation temperature range,however this is not always practical. Becausesigma phase embrittlement is a serious problemwith the high silicon grade 314, this is nowunpopular and largely replaced by high nickelalloys or by stainless steels resistant to sigmaphase embrittlement, particularly S30815(253MA). Grade 310 is also fairly susceptible tosigma phase formation in the temperature range590 to 870°C, so this "heat resistant" grade may

not be suitable for exposure at this comparativelylow temperature range and Grade 321 is often abetter choice.

Environmental FactorsOther factors which can be important in the use ofsteels for high temperature applications arecarburisation and sulphidation resistance. Sulphurbearing gases under reducing conditions greatlyaccelerate the attack on stainless alloys with highnickel contents. In some instances Grade 310 hasgiven reasonable service, in others grade S30815,with a lower nickel content is better, but in othersa totally nickel-free alloy is superior. If sulphurbearing gases are present under reducingconditions it is suggested that pilot test specimensbe first run under similar conditions to determinethe best alloy.

Thermal ExpansionA further property that can be relevant in hightemperature applications is the thermal expansionof the particular material. The coefficient ofthermal expansion is expressed in units ofproportional change of length for each degreeincrease in temperature, usually x10-6/°C,μm/m/°C, or x10-6cm/cm/°C, all of which areidentical units. The increase in length (ordiameter, thickness, etc) can be readily calculatedby multiplying the original dimension by thetemperature change by the coefficient of thermalexpansion. For example, if a three metre longGrade 304 bar (coefficient of expansion 17.2μm/m/°C) is heated from 20°C to 200°C, thelength increases by:

3.00 x 180 x 17.2 = 9288 μm = 9.3 mm

The coefficient of thermal expansion of theaustenitic stainless steels is higher than for mostother grades of steel, as shown in the followingtable.

This expansion coefficient not only varies betweensteel grades, it also increases slightly withtemperature. Grade 304 has a coefficient of 17.2 x10-6/°C over the temperature range 0 to 100°C,but increases above this temperature; details ofthese values are given in the table of physical

Coefficient ofThermal Expansion

(x10-6/°C)

Carbon SteelsAustenitic SteelsDuplex SteelsFerritic SteelsMartensitic Steels

1017141111

* or micrometres/metre/°C

Figure 12 Coefficient of thermal expansion- average values over the range 0-100°C

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properties in Appendix 3 of this Handbook, and inthe individual Atlas Grade Data Sheets.

The effect of thermal expansion is most noticeablewhere components are restrained, as theexpansion results in buckling and bending. Aproblem can also arise if two dissimilar metals arefabricated together and then heated; dissimilarcoefficients will again result in buckling orbending. In general the quite high thermalexpansion rates of the austenitic stainless steelsmean that fabrications in these alloys may havemore dimensional problems than similarfabrications in carbon or low alloy steels, inferritic, martensitic or duplex stainless steels.

The non-austenitic stainless steels also havesomewhat higher thermal conductivities than theaustenitic grades, which may be an advantage incertain applications. Thermal conductivities ofstainless steels are again listed in the table ofphysical properties in Appendix 3 of thisHandbook, and in the individual Atlas Grade DataSheets.

Localised stresses from expansion during heatingand cooling can contribute to stress corrosioncracking in an environment which would notnormally attack the metal. These applicationsrequire design to minimise the adverse effects oftemperature differentials such as the use ofexpansion joints to permit movement withoutdistortion and the avoidance of notches and abruptchanges of section.

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CRYOGENIC PROPERTIES

The austenitic stainless steels possess a uniquecombination of properties which makes themuseful at cryogenic (very low) temperatures, suchas are encountered in plants handling liquefiedgases. These steels at cryogenic temperatureshave tensile strengths substantially higher than atambient temperatures while their toughness isonly slightly degraded. Typical impact strengthsare as shown in Figure 13.

Note: There is substantial variation in cryogenicimpact properties, depending on testmethods and steel condition.

Considerable austenitic stainless steel hastherefore been used for handling liquefied naturalgas at a temperature of -161°C, and also in plantsfor production of liquefied gases. Liquid oxygenhas a boiling temperature of -183°C and that ofliquid nitrogen is -196°C.

The ferritic, martensitic and precipitationhardening steels are not recommended for use atsub-zero temperatures as they exhibit a significantdrop in toughness, even at only moderately lowtemperatures, in some cases not much belowroom temperature.

The duplex stainless steels have a better lowtemperature ductility than the ferritic andmartensitic grades; they are generally quiteuseable down to at least -50°C but have reducedductility below this temperature. This thereforeusually places a lower temperature limit on theirusefulness. The "ductile to brittle transition" fromwhich these grades suffer is also a commonfeature of carbon and low alloy steels, some ofwhich have Ductile to Brittle TransitionTemperatures (DBTT) close to 0°C.

These limitations are incorporated into designcodes, such as for instance the “material designminimum temperature (MDMT)” quoted in theAustralian pressure vessel code AS1210.

The ductile to brittle transformation is not apermanent change; a steel that is brittle at lowtemperatures returns to its ductile condition whenit is brought back to higher temperatures.

Figure 13 Typical impact energies of stainlesssteels down to cryogenic temperatures.

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MAGNETIC PROPERTIES

Magnetic Permeability is the ability of a material tocarry magnetism, indicated by the degree to whichit is attracted to a magnet. All stainless steels,with the exception of the austenitic group, arestrongly attracted to a magnet. All austeniticgrades have very low magnetic permeabilities andhence show almost no response to a magnet whenin the annealed condition; the situation is,however, far less clear when these steels havebeen cold worked by wire drawing, rolling or evencentreless grinding, shot blasting or heavypolishing. After substantial cold working Grade304 may exhibit quite strong response to amagnet, whereas Grades 310 and 316 will in mostinstances still be almost totally non-responsive, asshown in Figure 14.

The change in magnetic response is due to atomiclattice straining and formation of martensite. Ingeneral, the higher the nickel to chromium ratiothe more stable is the austenitic structure and theless magnetic response that will be induced bycold work. Magnetic response can in some casesbe used as a method for sorting grades ofstainless steel, but considerable caution needs tobe exercised.

Any austenitic (300 series) stainless steel whichhas developed magnetic response due to coldwork can be returned to a non-magnetic conditionby stress relieving. In general this can be readilyachieved by briefly heating to approximately 700– 800°C (this can be conveniently carried out bycareful use of an oxy-acetylene torch). Note,however, comments elsewhere in this publicationabout sensitisation (carbide precipitation) unlessthe steel is a stabilised grade. Full solutiontreatment at 1000 – 1150°C will remove allmagnetic response without danger of reducedcorrosion resistance due to carbides.

Many cold drawn and/or polished bars have anoticeable amount of magnetism as a result of theprevious cold work. This is particularly the casewith grades 304 and 303, and much less so for thehigher nickel grades such as 310 and 316, asshown in the graph of Figure 14. Even within thechemical limitations of a single standard analysisrange there can be a pronounced variation in therate of inducement of magnetic response fromcold work.

Austenitic stainless steel castings and welds(which could be viewed very small costings) areusually deliberately designed to have a minorproportion of ferrite. Approximately 5-12% offerrite assists in preventing hot cracking. Thismicrostructure responds slightly to a magnet, andin fact ferrite meters based on measuring thisresponse can be used to quantify the proportion offerrite.

If magnetic permeability is a factor of design or isincorporated into a specification, this should beclearly indicated when purchasing the stainlesssteel from a supplier.

Magnetically Soft StainlessSteelsIn some applications there is a requirement for asteel to be "magnetically soft". This is oftenrequired for solenoid shafts, where it is necessaryfor the plunger to respond efficiently to themagnetic field from the surrounding coil when thecurrent is switched on, but when the current isswitched off the magnetic field induced in the steelmust quickly collapse, allowing the plunger toreturn to its original position. Steels which behavein this way are said to be magnetically soft. Forcorrosion resisting applications there are ferriticstainless steels which are magnetically soft,usually variants of a grade "18/2" (18% chromiumand 2% molybdenum) but with very tightlycontrolled additions of silicon and often withsulphur added to make them free machining.Special mill processing guarantees the magneticproperties of the steels.

Figure 14 Magnetic response of austeniticstainless steels after cold work.

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MECHANICAL PROPERTIESThe mechanical properties of stainless steels arealmost always requirements of the productspecifications used to purchase the productsdistributed by Atlas Steels.

For flat rolled products the properties usuallyspecified are tensile strength, yield stress (almostalways measured as 0.2% proof stress),elongation and sometimes Brinell or Rockwellhardness. Much less frequently there arerequirements for impact resistance, either Charpyor Izod. Bar, tube, pipe, fittings etc. also usuallyrequire at least tensile strength and yield stress.Hardness is commonly measured for annealedtube. These properties give a guarantee that thematerial in question has been correctly produced,and are also used by engineers to calculate theworking loads or pressures that the product cansafely carry in service.

Typical mechanical properties of annealedmaterials are as in the graph of Figure 15. Notethat the high cold work hardening rate of theaustenitic grades in particular results in actualproperties of some commercial products beingsignificantly higher than these values. The yieldstress (usually measured as the 0.2% proofstress) is particularly increased by even quiteminor amounts of cold work. More details of thework hardening of stainless steels are given in thesection of this handbook on fabrication.

An unusual feature of annealed austenitic stainlesssteels is that the yield strength is a very lowproportion of the tensile strength, typically only40-45%. The comparable figure for a mild steel isabout 65-70%. As indicated above a small amountof cold work greatly increases the yield (muchmore so than the tensile strength), so the yieldalso increases to a higher proportion of tensile.Only a few % of cold work will increase the yieldby 200 or 300MPa, and in severely cold workedmaterial like spring temper wire or strip, the yield

is usually about 80-95% of the tensile strength.

As engineering design calculations are frequentlymade on yield criteria the low yield strength ofaustenitic stainless steels may well mean thattheir design load cannot be higher than that ofmild steel, despite the tensile strength beingsubstantially higher. Design stresses for variousgrades and temperatures are given in AustralianStandards AS1210 "Pressure Vessels" and AS4041“Pressure Piping”.

The other mechanical property of note is theductility, usually measured by % elongation duringa tensile test. This shows the amount ofdeformation a piece of metal will withstand beforeit fractures. Austenitic stainless steels haveexceptionally high elongations, usually about 60-70% for annealed products, as shown in Figure16. It is the combination of high work hardeningrate and high elongation that permits the severefabrication operations which are routinely carriedout, such as deep drawing of kitchen sinks andlaundry troughs.

Hardness (usually measured by Brinell, Rockwellor Vickers machines) is another value for thestrength of a material. Hardness is usually definedas resistance to penetration, so these testmachines measure the depth to which a very hardindenter is forced into a material under the actionof a known force. Each machine has a differentshaped indenter and a different force applicationsystem, so conversion between hardness scalesmay not be very accurate. Standard tables havebeen produced, and a summary of these forstainless steels, is given in Appendix 4 of thishandbook. These conversions are onlyapproximate, and should not be used to determineconformance to standards. Conversions for carbonand low alloy steels are more reliable than thosefor austenitic stainless steels.

0

100

200

300

400

500

600

700

800

Aust

eniti

c

Duple

x

Ferritic

Mild

Steel

Alu

min

ium

Brass

Ten

sile

an

dP

roo

fS

tres

s(M

Pa

)

Tensile Strength Proof Strength

Figure 15 Typical tensile properties of annealedmaterials

0

10

20

30

40

50

60

70

Aust

eniti

c

Duple

x

Ferri

tic

Mild

Steel

Alu

min

ium

Brass

Elo

ng

ati

on

(%)

Figure 16 Typical elongations of annealedmaterials

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It is also sometimes convenient to do a hardnesstest and then convert the result to tensilestrength. Although the conversions for carbon andlow alloy steels are fairly reliable, those foraustenitic stainless steels are much less so. Nostandard conversion tables for hardness to tensilestrength are published for austenitic alloys, andany conversions should be regarded withsuspicion.

Mechanical Properties of WireThe mechanical properties of the majority of thestainless steel wire and bar products stocked byAtlas Steels are generally sufficiently described bythe tensile strength. Each of the wire productsrequire mechanical properties which are carefullychosen to enable the product to be fabricated intothe finished component and also to withstand theloads which will be applied during service. Springwire has the highest tensile strength; it must besuitable for coiling into tension or compressionsprings without breaking during forming. However,such high tensile strengths would be completelyunsuitable for forming or weaving applicationsbecause the wire would break on forming.

Weaving wires are supplied in a variety of tensilestrengths carefully chosen so that the finishedwoven screen will have adequate strength towithstand the service loads, and yet soft enoughto be crimped and to be formed into the screensatisfactorily.

Mechanical properties of “cold Heading” wire forfasteners are another example where a carefulbalance in mechanical properties is required. Inthis type of product the wire must be ductileenough to form a quite complex head but the wiremust be hard enough so that the threads will notdeform when the screw or bolt is assembled intothe component. Good examples are roofing bolts,wood screws and self-tapping screws; to achievethe mechanical properties required for suchcomponents requires careful consideration of thecomposition of the steel so that the workhardening rate will be sufficiently high to formhard threads on thread rolling and yet not so highas to prevent the head from being formed.

Mechanical Properties of BarFor bar products a compromise must also bemade; a large proportion of bar will be machined,so it is important that the hardness be not toohigh, but better load carrying capacity is achievedif the strength is high, and for drawn bar a goodbright finish is achieved only by a reduction whichsignificantly increases strength levels.

In practice round austenitic stainless steel barfrom about 5mm up to about 26mm in diameter ismost commonly manufactured by cold drawing toits final size, typically on a “Schumag” or similar

machine. The resulting finish is called BrightDrawn or “BD”. The approximately 10% reductionin area of this draw gives a significant increase intensile strength and much more so in proof stress.By contrast, bars with diameters of about 26mmand over are most commonly manufactured bytaking hot rolled annealed bar and simply cuttingoff the rough and uneven outer surface to achievefinal size. Such a finish is called Smooth Turned or“ST”. Final straightening does slightly increase thestrength (and more so the surface hardness) butthese large sized bars have properties quite closeto annealed.

Typical mechanical properties for these productsare shown in Figure 17.

Finish Sizeapprox(mm)

TensileStrength

(MPa)

YieldStrength

(MPa)

Elongation(%)

Drawn 5-26 700 600 35

Turned >26 600 300 55

There are other implications of these twomanufacturing routes -

1. Large diameter ST bars will have bettermachinabilities than the same grade in smallerBD sizes, because a lower hardness, lowerstrength steel is easier to cut.

2. Large diameter ST bars will be free of surfacelaps or seams because the hot rolled surface hasbeen cut off in turning. The smaller BD bars mayhave minor surface imperfections because nometal has been removed; the original hot rolledsurface has merely been burnished smooth bydrawing, thus covering minor blemishes.

3. All drawn wire and bar has a higher surfacehardness because that is where most of the coldwork is concentrated. This makes little differencein the small diameter wires and bars, but for barapproaching 25mm in diameter the surface willbe significantly higher hardness than its centre.Even ST bar has a higher surface hardnessbecause of cold working imparted by finalmachining and bar straightening.

Figure 17 Typical mechanicalproperties of 304/316 round bars

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FABRICATION

Forming OperationsOne of the major advantages of the stainlesssteels, and the austenitic grades in particular, istheir ability to be fabricated by all the standardfabrication techniques, in some cases even moreseverely than the more well-known carbon steels.The common austenitic grades can be folded,bent, cold and hot forged, deep drawn, spun androll formed. Because of the materials' highstrength and very high work hardening rate all ofthese operations require more force than forcarbon steels, so a heavier machine may beneeded, and more allowance may need to bemade for spring-back.

Austenitic stainless steels also have very highductility, so are in fact capable of being veryheavily cold formed, despite their high strengthsand high work hardening rates, into items such asdeep drawn kitchen sinks and laundry troughs.Few other metals are capable of achieving thisdegree of deformation without splitting.

All metals work harden when cold worked and theextent of work hardening depends upon the gradeselected. Austenitic stainless steels work hardenvery rapidly, but the ferritic grades work hardenonly a little higher than that of the plain carbonsteels. The rapid cold working characteristics ofaustenitic stainless alloys makes them particularlyuseful where the combination of high strength andcorrosion resistance are required, such as formanufacture of springs for corrosiveenvironments. The relationship between theamount of cold work (expressed as "% reductionof area") and the resulting mechanical propertiesis shown in the chart in Figure 18.

Figure 18 Effect of Cold work on tensilestrength of stainless steels. Values aretypical for cold drawn wire.

It is important to realise that work hardening isthe only way in which austenitic stainless steels

can be hardened. By contrast the martensiticstainless steels (eg. 410, 416, 420 and 431) canbe hardened by a quench-and-temper thermaltreatment in the same way as carbon and lowalloy steels. Ferritic stainless steels (such asGrades 430, 439 and 444) are similar to austeniticgrades in that they can only be hardened by coldworking, but their work hardening rates are low,and a substantial lift in strength cannot beachieved.

In cold working such as cold drawing, tensileproperties over 2000 MPa may be obtained withGrades 301, 302 and 304. However, these veryhigh tensile properties are limited to thin sectionsand to fine wire sizes.

As the size increases, the amount of cold worknecessary to produce the higher tensile propertiescannot be practically applied. This is due to thefact that the surface of larger sections rapidlywork hardens to the extent that further work isnot practical, while the centre of the section is stillcomparatively soft. To illustrate this point, Grade304 6mm round, cold drawn with 15 per centreduction in area will show an ultimate strength ofabout 800 MPa. A 60mm round, drawn with thesame reduction will have about the same tensile infull section. However, if sections are machinedfrom the centre of each bar, the 60mm round willshow much lower tensile properties, whereas the6mm round will test about the same as in fullsection. Also the 6mm round can be cold workedto much higher tensile properties, whereas the60mm round must be annealed for further coldwork, because of excessive skin hardness.

In general, the austenitic grades showing thegreatest work hardening rate will also have thehighest magnetic permeability for a given amountof cold work.

Ferritic and martensitic alloys work harden atrates similar to low carbon steel and are magneticin all conditions at room temperatures. Wire sizesmay be cold worked to tensile properties as highas approximately 1000 MPa. However, bar sizesare seldom cold worked higher than 850 MPa.Although the ferritic grades (e.g. 430, 409 andAtlasCR12) cannot be heat treated, themartensitic grades (e.g. 410, 416 and 431) areusually heat treated by hardening and temperingto develop mechanical properties and maximumcorrosion resistance. Cold working, therefore, ismore of a sizing operation than a method ofproducing mechanical properties with thesegrades.

The rate of work hardening, while relativelyconsistent for a single analysis, will show amarked decrease in the rate as the temperatureincreases. This difference is noticeable at

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temperatures as low as 80°. At slightly increasedtemperature there is also a reduction in strengthand increase in ductility. Advantage is taken ofthis in some deep drawing applications as well asin "warm" heading of some difficult fasteners.

Another feature of cold forming of stainless steelsis that more severe deformation is possible atslower forming speeds - this is quite different fromcarbon steels which have formabilities virtually thesame no matter what the forming rate. So theadvice given to those attempting difficult coldheading (or other high speed forming operations)is to slow down; stainless steel is almost alwaysheaded slower than is carbon steel.

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MachiningAustenitic stainless steels are generally regardedas being difficult to machine, and this has led tothe development of the free-machining Grade 303.There are also free-machining versions of thestandard ferritic (Grade 430) and martensitic(Grade 410) grades - Grades 430F and 416respectively - these grades have improvedmachinability because of the inclusions ofManganese Sulphide (formed from the sulphuradded to the steel) which act as chip breakers.

The free-machining grades have significantly lowercorrosion resistances than their non-freemachining equivalents because of the presence ofthese non-metallic inclusions; these grades areparticularly prone to pitting corrosion attack andmust not be used in aggressive environments suchas for marine exposure. The free-machininggrades containing high sulphur levels also havereduced ductility, so cannot be bent around a tightradius nor cold forged. Because of the sulphuradditions these grades are very difficult to weld,so again would not be chosen for weldedfabrication.

Ugima Improved Machinability GradesRecently a number of manufacturers have offered"Improved Machinability" versions of the standardaustenitic Grades 304 and 316. These steels areproduced by proprietary steel melting techniqueswhich provide enough of a chip-breaking effect tosignificantly improve the machinability, but theystill remain within the standard grade compositionspecifications and still retain mechanicalproperties, weldability, formability and corrosionresistance of their standard grade equivalents.These materials are marketed under trade namessuch as "Ugima". For "Ugima" the improvement inachievable machining speed is about 20% overthe equivalent standard grades; in addition it iscommonly experienced that greatly enhanced toollife is obtained, which considerably reduces thecost of machining. In many instances this is ofeven greater benefit than is the potentialimprovement in cutting speed.

A "Ugima 303" is available as a "super-machineable" grade; like other 303 stainlesssteels weldability, formability and corrosionresistance are compromised in order to achievemaximum machinability. Relative machinabilitiesof various stainless steels, expressed ascomparison of achievable cutting speeds, areshown in the graph of Figure 19. Moreinformation on machining of stainless steels is inAtlas Technote 4.

Rules for Machining Stainless SteelsSome general rules apply to most machining ofaustenitic stainless steels:

1. The machine tool must be sturdy, havesufficient power and be free fromvibration.

2. The cutting edge must be kept sharp at alltimes by re-sharpening or replacement.Dull tools cause glazing and workhardening of the surface. Sharpeningmust be carried out as soon as the qualityof the cut deteriorates. Sharpeningshould be by machine grinding usingsuitable fixtures, as free-hand sharpeningdoes not give consistent and long-lastingedges. Grinding wheels must be dressedand not contaminated.

3. Light cuts should be taken, but the depthof the cut should be substantial enough toprevent the tool from riding the surface ofthe work - a condition which promoteswork hardening.

4. All clearances should be sufficient toprevent the tool from rubbing on thework.

5. Tools should be as large as possible tohelp to dissipate the heat.

6. Chip breakers or chip curlers prevent thechips from being directed into the work.

7. Constant feeds are most important toprevent the tool from riding on the work.

8. Proper coolants and lubricants areessential. The low thermal conductivity ofaustenitic stainless alloys causes a largepercentage of the generated heat to beconcentrated at the cutting edges of thetools. Fluids must be used in sufficientquantities and directed so as to flood boththe tool and the work.

0 20 40 60 80 100

Relative Machinability (%)

303

Ugima 303304

Ugima 304316

Ugima 316

410416

430430F

431

2205

Figure 19 Relative machinabilities ofstainless steels

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WeldingThe weldabilities of the various grades of stainlesssteels vary considerably. Nearly all can be welded,and the austenitic grades are some of the mostreadily welded of all metals. In general thestainless steels have weldabilities which dependupon the family to which they belong.Recommendations for welding the common gradesare given in Figure 20, and in the individual Atlasgrade data sheets available on the Atlas Steelswebsite. Australian Standard AS 1554.6 coversstructural welding of stainless steels, and gives anumber of pre-qualified conditions for welding.Pre-qualified welding consumables for welding ofsame-metal and mixed-metal welding are alsogiven in AS 1554.6. This excellent standard(available from Standards Australia) also enablesspecification of welding procedures appropriate toeach particular application. It is also highlyrelevant to non-structural welding.

Austenitic Stainless SteelsThe austenitic grades are all very readily welded(with the exception of the free-machining grade303 noted elsewhere). All the usual electricwelding processes can be used - Manual Metal Arc(MMAW or "stick"), Gas Tungsten Arc (GTAW orTIG), Gas Metal Arc (GMAW or MIG), Flux Cored(FCAW), Submerged Arc (SAW) and laser. A fullrange of welding consumables is readily availableand standard equipment can be used.

The use of low carbon content grades (304L and316L) or stabilised grades (321 or 347) needs tobe considered for heavy section product which isto be welded. This overcomes the problem of"sensitisation" discussed in the previous section onintergranular corrosion. As the sensitisationproblem is time/temperature dependent, so thinmaterials, which are welded quickly, are notusually a problem. It should be noted that if afabrication has become sensitised during weldingthe effect can be reversed and the materialrestored to full corrosion resistance by a fullsolution heat treatment.

The free-machining grade 303 is notrecommended for welded applications as it issubject to hot cracking; the Ugima improvedmachinability grades, Ugima 304 and Ugima 316,offer a much better combination of reasonablemachinability with excellent weldability.

Duplex Stainless SteelsDuplex stainless steels also have good weldability,albeit not quite as good as that of the austenitics.Again all the usual processes can be used, and arange of consumables is available. For the mostcommon duplex grade 2205 the standardconsumable is a 2209 - the higher nickel contentensures the correct 50/50 ferrite/austenitestructure in the weld deposit, thus maintainingstrength, ductility and corrosion resistance. One of

the advantages of duplex stainless steels overaustenitics is their comparatively low coefficient ofthermal expansion. This is only a little higher thanthat of carbon steels, as shown in the table in thesection of this handbook on high temperatureproperties of stainless steels.

Ferritic Stainless SteelsThe ferritic grades are weldable but not as readilyas the austenitic grades. The three majorproblems encountered are sensitisation, excessivegrain growth and lack of ductility, particularly atlow temperatures. Sensitisation can be avoided byuse of stabilised grades. Filler metal can be of asimilar composition or more commonly anaustenitic grade (e.g. Grades 308L, 309, 316L or310) which is helpful in improving weld toughness.The excessive grain growth problem is difficult toovercome, so most grades are only welded in thingauges, usually up to 2 or 3mm. Heat input mustbe kept low for the same reason. Stabilised ferriticgrades include 409 and 430Ti and the moremodern 444 (Durinox F18MS), 439 (Durinox F18S)and Durinox F20S. These possess considerablybetter weldability compared to the unstabilisedalternatives such as 430. Such grades can bewelded, even in structural or pressure vesselapplications, but only in thin gauges.

AtlasCR12 and AtlasCR12Ti are proprietary ferriticgrades which have a very low carbon content andthe remaining composition and the mill processingroute balanced to enable welding. The CR12 typesare the only ferritic grades weldable in heavysection plate. As for other ferritic grades it isnormal to use austenitic stainless steel fillers.

Martensitic Stainless SteelsMartensitic stainless steels can if necessary bewelded (again with the high sulphur freemachining grade 416 being not recommended) butcaution needs to be exercised as they will producea very hard and brittle zone adjacent to the weld.Cracking in this zone can occur unless much careis taken with pre-heating and with post weld heattreatment. These steels are often welded withaustenitic filler rods to increase the ductility of thedeposit.

Welding Dissimilar MetalsWelding together of different metals, such as ofGrade 316 to Grade 444 or a stainless steel to amild steel, can be carried out, although someextra precautions need to be taken. It is usuallyrecommended that over-alloyed austenitic weldingrods, such as Grade 309, be used to minimisedilution effects on the stainless steel component.The composition of the weld deposit resulting fromdissimilar grade welding is shown in the Schaefflerdiagram or its successors by De Long and morerecently the WRC. Australian Standard AS 1554.6contains a table giving the pre-qualifiedconsumables for each combination of dissimilarmetal welds.

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Further Information on WeldingSpecific recommendations are given in Figure 19.Further details on welding of stainless steels aregiven in the booklet "Guidelines for the WeldedFabrication of Nickel-containing Stainless Steelsfor Corrosion Resistant Services" (Nickel InstituteReference Book, No. 11 007) available from theNickel Institute. Specific details on applicationscan be provided by the welding electrode, gas andequipment manufacturers. Excellent information isalso included in WTIA Technote 16 on "WeldingStainless Steels ". This is available from theWelding Technology Institute of Australia.

Soft SolderingAll grades of stainless steel can be soldered withlead-tin soft solder. Leaded solders should not beused when the product being soldered is used forfood processing, serving or transport. Solderedjoints are relatively weak compared to thestrength of the steel, so this method should not beused where the mechanical strength is dependentupon the soldered joint. Strength can be added ifthe edges are first lock-seamed, spot welded orriveted. In general welding is preferable tosoldering.

Recommended procedure for soldering:-

1. The steel surfaces must be clean and freeof oxidation.

2. A rough surface improves adherence ofthe solder, so roughening with grindingwheel, file or coarse abrasive paper isrecommended.

3. Use a phosphoric acid based flux.Hydrochloric acid based fluxes requireneutralising after soldering as anyremnant traces will be highly corrosive tothe steel. Hydrochloric acid based fluxesare not recommended for soldering ofstainless steels.

4. Flux should be applied with a brush, toonly the area being soldered.

5. A large, hot iron is recommended. Use thesame temperature as for carbon steel, buta longer time will be required because ofstainless steel's low thermal conductivity.

6. Any type of solder can be used, but atleast 50% tin is recommended. Solderwith 60-70% tin and 30-40% lead has abetter colour match and greater strength.

Brazing ("Silver Soldering")When welding is impractical and a stronger jointthan soft soldering is required, brazing may beemployed. This method is particularly useful forjoining copper, bronze, nickel and other non-ferrous metals to stainless steel. The corrosionresistance of the joint will be somewhat lower thanthat of the stainless steel, but in normal

atmospheric and mildly corrosive conditionsbrazed joints are satisfactory. Because mostbrazing operations involve temperatures at whichcarbide precipitation (sensitisation) can occur inthe austenitic grades, low carbon or stabilisedgrades (304L, 316L or 321) should be used.Ferritic grades such as 430 can be quenched fromthe brazing temperatures, but hardenablemartensitic grades (410, 420, 431) should not beheated above 760°C when brazing. The freemachining grades 303, 416 and 430F shouldgenerally not be used as a dark scum forms on thesurface when fluxing and heating, which adverselyaffects the appearance of the steel.

Recommended procedure for brazing:-

1. Use silver brazing alloys with meltingpoints from 590-870°C. Select the alloyfor best colour match.

2. Remove dirt and oxides from the steelsurfaces and apply flux immediately.

3. A slightly reducing flame should be playedacross the joint to heat uniformly.

4. For high production work use inductionheating or controlled atmosphere furnaces(argon, helium, vacuum or dissociatedammonia with dew point of about -50°C).

5. After brazing remove all remaining fluxwith high pressure steam or hot water.

6. When brazing grade 430 use a silversolder with 3% nickel. This alloy alsohelps to minimise crevice corrosion whenused with austenitic grades.

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Grade Pre-heat Post Weld Heat Treatment Filler

304 (a) Cool rapidly from 1010-1090°C only if welding over

about 3 – 5mm thickness.

308L

304L (a) Not required 308L

309 (a) Usually unnecessary as this grade is generally used at

high temperatures (b).

309

310 (a) As for 309 310

316 (a) Cool rapidly from 1060-1150°C only if welding over

about 3 – 5mm thickness.

316L

316L (a) Not required 316L

321 (a) Not required 347

253MA (a) Not required S30815(h)

410 (c) Air cool from 650-760°C 410 (d)

430 (c) Air cool from 650-760°C 430 (d)

444 (a) Not required 316L

AtlasCR12 (g) Not required 309 (e)

2205 (f) Not generally required 2209

This table gives broad over-view recommendations. Further details are available from welding consumablesuppliers. For critical application, welding procedures should be qualified in accordance with AS1554.6 orother applicable standards.

Notes:

a. Unnecessary when the steel is above 15°C.b. Where corrosion is a factor, 309S and 310S (0.08% Carbon maximum) are used, with a post

weld heat treatment of cooling rapidly from 1120-1180°C.c. Pre-heat at 200-320°C; light gauge sheet is frequently welded without pre-heat.d. May be welded with 308L, 309 or 310 electrodes without pre-heat if the steel is above 15°C.e. May be welded with 309, 309L, 309Mo, 309MoL, 316L or 308L, depending on the application.f. If temperature is below 10°C then a 50°C pre-heat is recommended.g. In case of critical structural welding of AtlasCR12 destined for corrosive environments, please

refer to Atlas Steels Technical Department.h. 309 Consumables can be used if a reduced creep strength and oxidation resistance can be

tolerated.

Figure 20 Recommendations for welding of stainless steels.

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HEAT TREATMENT

Stainless steels are often heat treated; the natureof this treatment depends on the type of stainlesssteel and the reason for the treatment. Thesetreatments, which include annealing, hardeningand stress relieving, restore desirable propertiessuch as corrosion resistance and ductility to metalaltered by prior fabrication operations or producehard structures able to withstand high stresses orabrasion in service. Heat treatment is oftenperformed in controlled atmospheres to preventsurface scaling, or less commonly to preventcarburisation or decarburisation.

AnnealingAustenitic Stainless SteelsThe austenitic stainless steels cannot be hardenedby thermal treatments (but they do harden rapidlyby cold work). Annealing (often referred to as“solution treatment”) not only recrystallises thework hardened grains but also takes chromiumcarbides (precipitated at grain boundaries insensitised steels) back into solution in theaustenite. The treatment also homogenisesdendritic weld metal structures, and relieves allremnant stresses from cold working. Annealingtemperatures usually are above 1040°C, althoughsome types may be annealed at closely controlledtemperatures as low as 1010°C when fine grainsize is important. Time at temperature is oftenkept short to hold surface scaling to a minimum orto control grain growth, which can lead to "orangepeel" in forming.

Annealing of austenitic stainless steel isoccasionally called quench annealing because themetal must be cooled rapidly, usually by waterquenching, to prevent sensitisation (except forstabilised and low carbon grades).

Before annealing or other heat treating operationsare performed on stainless steels, the surfacemust be cleaned to remove oil, grease and othercarbonaceous residues. Such residues lead tocarburisation during heat treating, which degradescorrosion resistance.

Duplex Stainless SteelsThe duplex grades are generally solution treatedin a very similar way to the austenitics, and atvery similar temperatures. Duplex grades aremore susceptible than are austenitics toprecipitation of sigma phase in the temperaturerange 600 – 950°C; a full solution treatmentfollowed by rapid cooling will correct this.

Martensitic and Ferritic Stainless SteelsAll martensitic and most ferritic stainless steelscan be subcritical annealed (process annealed) byheating into the upper part of the ferrite

temperature range, or full annealed by heatingabove the critical temperature into the austeniterange, followed by slow cooling. Usualtemperatures are 760 to 830°C for sub-criticalannealing, but this is different for each grade.

When material has been previously heated abovethe critical temperature, such as in hot working, atleast some martensite is present even in ferriticstainless steels such as grade 430. Relatively slowcooling at about 25°C/hour from full annealingtemperature, or holding for one hour or more atsubcritical annealing temperature, is required toproduce the desired soft structure of ferrite andspheroidised carbides. However, parts that haveundergone only cold working after full annealingcan be sub-critically annealed satisfactorily in lessthan 30 minutes.

The ferritic types that retain predominantly single-phase structures throughout the workingtemperature range (grades 409, 442, 446 and26Cr-1Mo) require only short recrystallisationannealing in the range 760 to 955°C.

Atmosphere ControlStainless steels are usually annealed in controlledatmospheres to prevent or at least reduce scaling.Treatment can be in a salt bath, or in a vacuumannealing furnace. Steel mills anneal moststainless steel in open (oxidising) furnaces, so asubsequent pickle is usual to remove the resultanthigh temperature oxide. Another option is "brightannealing" in a highly reducing atmosphere.Products such as flat rolled coil, tube and wire areregularly bright annealed by their producers,usually in an atmosphere of nitrogen andhydrogen. The result is a surface requiring nosubsequent scale removal; the product is as brightafter as before annealing. These products areoften referred to as "BA".

Stabilising AnnealingA stabilising anneal is sometimes performed afterconventional annealing for grades 321 and 347.Most of the carbon content is combined withtitanium in grade 321 or with niobium in grade347 when these are annealed in the usual manner.A further anneal at 870 to 900°C for 2 to 4 hoursfollowed by rapid cooling precipitates all possiblecarbon as a titanium or niobium carbide andprevents subsequent precipitation of chromiumcarbide. This special protective treatment issometimes useful when service conditions arerigorously corrosive, especially when service alsoinvolves temperatures from about 400 to 870°C,and some specifications enable this treatment tobe specified for the product – in some ASTMspecifications it is an optional “SupplementaryRequirement”.

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HardeningMartensitic Stainless SteelsMartensitic stainless steels are hardened byaustenitising, quenching and tempering much likelow alloy steels. Austenitising temperaturesnormally are 980 to 1010°C, well above thecritical temperature. As-quenched hardnessincreases with austenitising temperature to about980°C and then decreases due to retention ofaustenite. For some grades the optimumaustenitising temperature may depend on thesubsequent tempering temperature.

Preheating before austenitising is recommended toprevent cracking in high-carbon types and inintricate sections of low-carbon types. Preheatingat 790°C, and then heating to the austenitisingtemperature is the most common practice.

Martensitic stainless steels have high hardenabilitybecause of their high alloy content. Air coolingfrom the austenitising temperature is usuallyadequate to produce full hardness, but oilquenching is sometimes used, particularly forlarger sections. Parts should be tempered as soonas they have cooled to room temperature,particularly if oil quenching has been used, toavoid delayed cracking. Parts sometimes arerefrigerated to approximately -75°C beforetempering to transform retained austenite,particularly where dimensional stability isimportant, such as in gauge blocks made of grade440C. Tempering at temperatures above 510°Cshould be followed by relatively rapid cooling tobelow 400°C to avoid "475°C" embrittlement.

Precipitation Hardening SteelsPrecipitation hardening stainless steels are usuallysupplied in the “solution treated” condition, oftenreferred to as “Condition A”. The intention is thatany fabrication will be done while the steel is inthis condition, and then the final hardening heattreatment carried out.

Some precipitation-hardening stainless steelsrequire more complicated heat treatments thanstandard martensitic types. For instance, a semi-austenitic precipitation-hardening type mayrequire annealing, trigger annealing (to conditionaustenite for transformation on cooling to roomtemperature), sub-zero cooling (to complete thetransformation of austenite) and aging (to fullyharden the alloy). On the other hand, martensiticprecipitation-hardening types (such as 17-4PH orGrade 630) often require nothing more than asimple aging treatment.

Heat treatment procedures are summarised in theindividual Atlas grade data sheets.

Stress Relieving

Stress relieving of austenitic stainless steels attemperatures below 400°C is an acceptablepractice but results in only modest stress relief.Stress relieving at 425 to 925°C significantlyreduces residual stresses that otherwise mightlead to stress corrosion cracking or dimensionalinstability in service. One hour at 870°C typicallyrelieves about 85% of the residual stresses.However, stress relieving in this temperaturerange can also precipitate grain boundarycarbides, resulting in sensitisation that severelyimpairs corrosion resistance in many media. Toavoid these effects, it is strongly recommendedthat a stabilised stainless steel (grade 321) or alow-carbon type (304L or 316L) be used,particularly when lengthy stress relieving isrequired. See also the section of this Handbookdealing with intergranular corrosion.

Full solution treatment (annealing), generally byheating to about 1080°C followed by rapid cooling,removes all residual stresses in austenitic grades,but is not a practical treatment for most large orcomplex fabrications.

When austenitic stainless steels have been coldworked to develop high strength, low temperaturestress relieving will increase the proportional limitand yield strength (particularly compressive yieldstrength). This is a common practice for austeniticstainless steel spring wire. A two hour treatmentat 345 to 400°C is normally used; temperaturesup to 425°C may be used if resistance tointergranular corrosion is not required for theapplication. Higher temperatures will reducestrength and sensitise the metal, and generallyare not used for stress relieving cold workedproducts.

Stainless steel weldments can be heated totemperatures below the usual annealingtemperature to decrease high residual stresseswhen full annealing after welding is impossible.Most often, stress relieving is performed onweldments that are too large or intricate for fullannealing or on dissimilar metal weldmentsconsisting of austenitic stainless steel welded tolow alloy steel.

Stress relieving of martensitic or ferritic stainlesssteel weldments will simultaneously temper weldand heat affected zones, and for most types willrestore corrosion resistance to some degree.However, annealing temperatures are relativelylow for these grades, and normal subcriticalannealing is the heat treatment usually selected ifthe weldment is to be heat treated at all.

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Surface Hardening

Only limited surface hardening treatments areapplicable to the stainless steels. In mostinstances hardening of carbon and low alloy steelsis due to the martensitic transformation, in whichthe achievable hardness is related to the carboncontent - as most martensitic stainless steels havecarbon contents ranging from fairly low toextremely low, this hardening mechanism is oflittle use.

It is possible to surface harden austenitic stainlesssteels by nitriding. As in nitriding of other steelsthe hard layer is very hard and very thin; thismakes the process of limited use as the underlyingstainless steel core is relatively soft andunsupportive in heavily loaded applications. Afurther drawback is that the nitrided case has asignificantly lower corrosion resistance than theoriginal stainless steel.

A number of alternative, proprietary surfacehardening processes for austenitic stainless steelshave been developed but these have not as yetbecome commercially available in Australia.

An interesting alternative is the PVD (PhysicalVapour Deposition) process. This enables very thinbut hard layers to be deposited on manymaterials, including stainless steels. The mostcommonly applied coating is Titanium Nitride"TiN", which in addition to being very hard is alsoan aesthetically pleasing gold colour. Because ofits appearance this coating has been applied,generally on No8 mirror polished surface, toproduce gold mirror finished architectural panels.Again it must be noted that this hard and very thinsurface layer rests on a core of relatively softaustenite, so engineering applications are limited.

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SURFACE FINISHINGTo a very large extent stainless steels are usedbecause of the corrosion resistance of theirsurfaces. This excellent corrosion resistance canonly be achieved if proper cleaning and finishingoperations are carried out after any fabricationprocess which has impaired the surfacecondition.

PassivationThis process is recommended where the surfacehas been contaminated by “free iron”. Thepresence of any iron, cast iron, mild steel, carbonsteel or low alloy steel particles on the surface ofstainless steel will promote pitting corrosion at thecells set up between the "free" iron and stainlesssteel. This potentially very serious (and certainlyunsightly) problem most often occurs due tocontamination by scraping with carbon steel toolsor fixtures, or from grinding swarf. “Passivation” isa chemical process of removal of thiscontamination. Passivation also aids in the rapiddevelopment of the passive surface layer on thesteel and removes manganese sulphide inclusionsfrom the surface; all these actions result inincreased resistance to pitting corrosion.

The removal of the iron can be readily carried outby the procedures in the table of Figure 21.

Grades with at least 16% Chromium (exceptfree machining grades such as 303):

20-50% nitric acid, at room temperatureto 40°C for 30-60 minutes.

Grades with less than 16% Chromium (exceptfree machining grades such as 416):

20-50% nitric acid, at room temperatureto 40°C for 60 minutes.

Free machining grades such as 303, 416 and430F:

20-50% nitric acid + 2-6% sodiumdichromate, at room temperature to 50°Cfor 25-40 minutes.

Notes:1. If no dulling of the metal surface can be

tolerated a trial treatment should first becarried out.

2. All passivation treatments must befollowed by thorough rinsing.

3. Observe all precautions for handing acids– nitric acid is highly corrosive anddangerous to exposed skin.

Figure 21 Passivation procedures.Refer ASTM A380

PicklingPickling is an acid treatment to remove hightemperature scale produced in welding, heattreatment or hot working. It also removes red rustfrom corrosion of the steel or from corrosion ofcontaminant iron or steel particles. Note thatpassivation is not sufficiently aggressive toremove this corrosion product after the free ironhas begun to rust. High temperature dark scale isnot only undesirable for aesthetic reasons - it alsoresults in a reduced corrosion resistance of theunderlying steel surface layer.

All stainless steels (except free machininggrades):

8-11% sulphuric acid, at 65-80°C for5-45 minutes.

This treatment is useful to loosen heavy heattreatment scale prior to other treatments.

Grades with at least 16% Chromium (exceptfree machining grades).

15-25% nitric acid + 1-8% hydrofluoricacid at 20-60°C for 5-30 minutes.

Free machining grades and grades with lessthan 16% Chromium:

10-15% nitric acid + 0.5-1.5%hydrofluoric acid at 20-60°C for 5-30minutes.

Notes:1. Trial treatments should be carried out

first to confirm that dulling is acceptable.2. Pickling should preferably be carried out

on fully annealed stainless steels due torisk of grain boundary attack. Thisproblem is especially relevant to steelssensitised in welding.

3. All pickling treatments must be followedby thorough rinsing.

4. Observe all precautions for handingacids – sulphuric, nitric and especiallyhydrofluoric acid are highly corrosiveand dangerous to exposed skin.

Figure 22 Picking procedures.Refer ASTM A380

The type of scale and hence the methods toremove it will depend upon the steel grade andthe heating conditions involved. The straight-chromium grades such as 410, 416, 430 444 scalemore readily and unfortunately the resulting scaleis also more tenacious.

All pickling operations result in metal removal, andthe outcome is therefore to some degree a dullingof the visual brightness and perhaps also asignificant reduction in dimensions.

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The best solution to the scale problem is not tocreate it in the first place! Heat treatment in avacuum or a good controlled atmosphere, such asbright annealing, eliminates the need for pickling,and generally results in a better final surfacefinish.

If pickling does need to be carried out thetreatments given in Figure 22 can be used. Aninitial pickle in sulphuric acid is often beneficial forheat treated components as this softens the scaleso that it can more readily be removed bysubsequent pickling in hydrofluoric and nitricacids.

Pickling is often referred to as “Pickling andPassivating” because this treatment also achievesall the aims of a straight passivation.

A very convenient method for pickling is use of"Pickling Paste". This is a prepared mix of strongacids in a stiff paste which enables it to be appliedto small areas and to vertical or even overhangingsurfaces. It is especially useful for pickling toremove heat tint following welding. Againprecautions for handling acids must be followedand the residue flushed thoroughly to a suitablewaste stream after completion. Most commercialpickling paste is formulated for the austeniticgrades, so if it is used to clean lower alloyedgrades such as AtlasCR12 the process must beclosely monitored to ensure the paste is quicklyremoved and very thoroughly rinsed offafterwards.

Less aggressive pickling paste and pickling liquidis available, especially formulated for the low alloystainless steels. These products are alsoadvocated as being safer to use and lessenvironmentally damaging.

Degreasing

Grease, oil, cutting fluids, drawing compounds andother lubricants must be removed from thesurface of stainless steel components before heattreatment (to prevent carbon pick-up) or finalpassivating treatments (to enable full access bythe treatment). Parts must also be degreasedprior to further assembly by welding, again toprevent pick-up of carbon at high temperature.

Both liquid and vapour degreasers are used. Liquidcleaning is often by hot alkaline detergents;proprietary mixes may also contain variousadditives. The parts should be thoroughly rinsedafterwards.

Organic solvents can be applied by spraying,swabbing or vapour degreasing. These treatmentsshould again be followed by thorough hot waterrinsing.

As with cleaning operations on other metals, therate of cleaning can be increased by the use ofbrushing, jetting or stirring etc. during theoperation.

Electropolishing

Electropolishing is an electrochemical processwhich brightens the steel surface by selectivedissolution of the high points - it is the opposite ofelectroplating, and is carried out with broadlysimilar equipment. Apart from the obviousoutcome of a visual brightening, electropolishing isa very effective way of improving pitting corrosionresistance.

The process is able to produce a very attractivecorrosion resisting and hygienic finish, but trialsshould first be conducted to determine theoptimum prior surface condition and polishingparameters. Electropolishing of some surfacesresults in a frosted rather than smooth finishparticularly if they are in the sensitised (ie grainboundary carbides) condition.

Grinding and Polishing

Stainless steels can be readily ground, polishedand buffed, but certain characteristics of thesematerials require some modification of standardtechniques for best results. Most notably, the highstrength, tendency to "load up" abrasive media,and low thermal conductivity of stainless steels alllead to build-up of surface heat. This in turn canproduce heat tinting (surface oxidation) or surfacesmearing, and in extreme cases even sensitisationof austenitic stainless steels or "burning" (re-hardening) of heat treated martensitic grades.Techniques that help prevent build-up of surfaceheat include (a) use of lower speeds and feeds,and (b) careful selection of lubricants, and ofproper grit size and type, so as to minimiseloading of the abrasive.

Corrosion resistance of stainless steels may beadversely affected by polishing with coarseabrasives. Corrosion resistance is often adequatefollowing polishing to a No.4 (approx 180 – 240grit) finish. Polishing with fine alumina orchromium oxide to obtain still higher finishes -such as buffed finishes No.7 and No.8 - removesfine pits and surface imperfections and generallyimproves corrosion resistance. Buffing can also becarried out by using a "Scotch-brite" buffingwheel.

There is evidence that surfaces with roughnessvalues (Ra) coarser than 0.5µm have impairedresistance to corrosion. Information on “teastaining” corrosion of these coarse finishes is

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given in ASSDA Technical Bulletin No 2. (availableat www.assda.asn.au).

Iron contamination must be avoided or removed ifpolished stainless steel surfaces are to have goodcorrosion resistance. Abrasives and polishingcompounds must be essentially iron-free (less that0.01% for best results), and equipment used forprocessing stainless steels must not be used forother metals. If these conditions cannot be met, acleaning/passivation treatment (after pre-cleaningto remove polishing compounds and lubricants)will be required to restore good corrosionresistance. Even if iron-free abrasives have beenused, a final passivation or electropolish willimprove corrosion resistance by removingmanganese sulphide inclusions from the cutsurface.

Mechanical Cleaning

Problems associated with chemical cleaningprocesses can be avoided by using mechanicalcleaning. With all mechanical cleaning processesgreat care must be taken to prevent the stainlesssteel surface from becoming contaminated by iron,steel or iron oxide particles.

Sand blasting is often used - this must be withclean silica or garnet sand. Garnet is readilyavailable with a “free of iron” guarantee, but thisis less certain for silica. Shot, grit and cut wireblasting must be done with stainless steel mediaof a grade, equal in corrosion resistance to themetal being cleaned, and scale particles must becontinually separated from the shot.

Wire brushing is useful to remove light heat tint,but again brushes must be of stainless steel, andthese must never be used on materials other thanstainless steels.

Barrel finishing and vibratory finishing both useabrasive media to mechanically polish small partsand are widely used on fasteners such as screwsand bolts and on pipe fittings.

Mechanically cleaned parts are not quite ascorrosion resistant as acid pickled materialbecause mechanical cleaning leaves some scaleresidues and may not remove the chromiumdepleted layer of steel from beneath the scale. Itcan be used as a preparatory step to speed upsubsequent acid pickling.

Blackening

Blackening to produce a non-reflective or lowreflective black oxide surface can be accomplishedby several methods. Immersion in sulphuricacid/potassium dichromate solutions at 80 to 99°Cor immersion in a molten salt bath of sodium

dichromate at 400°C are two methods. Blackchromium plating is another.

Most treatments are performed by specialist firms,who should be consulted regarding a specificapplication. All these processes have sufferedreduced Australian usage in recent years.

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SURFACE CONTAMINATION IN FABRICATION

Stainless steels must have clean surfaces, freefrom contamination, to achieve their optimumcorrosion resistance. During fabrication care mustbe taken to either protect the metal surface fromcontamination or to restore the surface afterfabrication is complete.

Three common and undesirable contaminants thatmay be encountered during fabrication andshipping are carbon (mild) steel, salt (sodiumchloride) and carbon.

Contamination by Carbon SteelAs well as carbon steel the same effect can comeabout by contact with any low alloy steel, toolsteel or cast iron: any non-stainless ferrousmaterial able to be deposited onto the stainless.

The problem is visually very apparent as red rustspots on the stainless steel. The pattern of therust often indicates the source of thecontamination – rows of intermittent or continuousrust suggest a scrape while distributed spotssuggest air-borne dust or particles.

The problem may not become apparent until thedeposited carbon steel starts to rust; immediatelyfollowing deposition the only evidence is often aminor scratch or some small innocuous lookingspot. The extent of the damage is often onlyrevealed when the contaminated area becomeswet.

It is possible to test a stainless steel surface forcontamination. ASTM A380 lists several options;the most sensitive is a “ferroxyl test” but a verypractical, easy and relatively quick test is usuallyto intermittently wet the surface with potablewater for about 24 – 48 hours. This will start therusting of any contamination that is present.

Corrosion initiation from mild steel contaminationmay occur in two ways:-

1. The protective oxide film on the stainless steelmay be broken when the mild steel is broughtinto contact with the stainless steel. Thiscontact creates a mild steel/stainless steelinterface which is a corrosion cell.

2. The mild steel contamination may be at theouter surface of the oxide film on the stainlesssteel. Then, under moist conditions, the mildsteel will corrode and cause both oxygendepletion and ionic concentration beneath theferrous deposit.

Typical causes of contamination by carbon steelinclude:

Using carbon steel hooks, chains, and wireropes for lifting unprotected stainless steelmaterials (Suitably placed dunnage, or coverthe lifting tackle to avoid damage).

Lifting unprotected stainless steel with liftinggear contaminated from previously liftingunprotected carbon steel.

Dragging stainless steel over mild steel, ofhitting the stainless with carbon steel.

Falling particles of mild steel welding andflame cutting dross from higher workings.

Grinding dust thrown up by power tools usedon mild steel.

Contamination from tools that have previouslybeen used on mild steel … commonly polishingtools, drills, files, and screwdrivers.

Contamination by ChloridesWhen contamination by common salt (sodiumchloride) occurs the dissolved chloride ions enterthe structure of the protective oxide film on thesurface of the stainless steel. When moistureevaporates at the surface of the metal, thechloride ions concentrate and can break down theprotective film.

Contamination by common salt can occur from: Sea spray and seawater. Environmental contamination, such as in

butter and cheese plants where air blastingdrives salt into the product.

Sweaty hands. Salt applied to avert or melt road ice (not in

Australia). Salts added to concrete to aid setting. Hydrochloric acid used to clean brickwork and

chemically etch concrete floors.

Contamination by CarbonCarbon contamination usually occurs when themetal is heated (e.g. during welding), under whichconditions any organic materials may break downon the surface of the metal and contaminate boththe solid surface and any molten metal; atelevated temperatures carbon can be absorbed bythe stainless steel – this carburised region is thenliable to become sensitised.

Contamination by carbon can occur from:- Pencil, paint or marking pen markings. Combustion of oil films, paper, organic matter,

backing strip materials, bonding agents orsooty gas flames in gas welding or heattreating.

By welding to carbon steel.

Solid carbon (eg gasket material) can also act as asource of galvanic corrosion of the stainless, oneof very few materials more noble than stainless.

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DESIGN CONSIDERATIONS IN FABRICATION OF STAINLESS STEELS

Whatever the intended function of a component itis essential that the design also consider the wayin which it will be constructed. Some of theseconsiderations such as allowing sufficient accessfor welders and for the tightening of bolts, arecommon to other metals, but in other respects theunique properties of stainless steels need to beconsidered.

Because of austenitic stainless steels' highexpansion rate and low thermal conductivity it isimportant to not unduly restrain componentsduring welding, and to not constrain carbon steeland stainless steel which behave differently whenheated.

The machinability of most stainless steels issomewhat lower than that of many other metals,so there is extra incentive to reduce the amount ofmachining required. One way in which this can beachieved is to use bright (ie cold finished) bar of asize such that no machining is required over thelargest component size; this same result may beachieved by using bright finished hexagonal orother shaped bar.

If machining is to be carried out it is importantthat sufficient clean-up be allowed so that areasonable cut is achieved; very light cuts canresult in the tool skidding across a very heavilycold worked surface. Good machining practice forstainless steels, and the austenitic (300 series)grades in particular, is to use heavier feeds andlower speeds than for carbon steels.

Grade Selection for FabricationRefer also to the section on Guidelines forStainless Steel Grade Selection.

If extensive machining is to be carried out the useof free machining grades 303 or 416 should beconsidered, but consideration must also be givento the relatively low corrosion resistance,weldability and formability of these grades.

Improved Machinability grades such as the"Ugima" range (eg Ugima 303, 304 and 316) areavailable - these offer better machinability thanstandard stainless steels but still retain theexcellent corrosion resistance, weldability andformability of their standard grade equivalents.

Stainless steel grades can also be selected forease of cold forming; Grade 302HQ (UNS S30430)is a low work hardening rate grade available inwire form specifically for the cold forming offasteners such as bolts and screws. By contrastgrades 301 and 304 have a very high workhardening rate and can be supplied in a heavilycold worked condition suitable for the manufacture

of springs; these require no hardening treatmentafter forming.

Components to be welded must be fabricated froma grade selected on that basis; to avoid problemsassociated with "sensitisation" - caused by holdingin the temperature range of about 450 to 850°C -it may be necessary to use a low carbon "L" gradeor a stabilised grade such as Grade 321. In thecase of all welding it is essential that weldingconsumables are selected to match the gradebeing welded. See also the sections of thisHandbook on intergranular corrosion, welding, andon grade selection.

Design to Avoid CorrosionWhen designing for stainless steel fabrication it isnecessary to be aware of the factors which cancause premature corrosion failures. The principalproblems are:

General corrosion - a widespread wall thinningcaused typically by exposure to strongreducing acids particularly at highconcentration or temperature

Pitting corrosion - related to chlorides, even inlow concentrations and particularly at slightlyelevated temperatures

Crevice corrosion - also related to chloridesbut made worse by small crevices in whichliquid is trapped

Intergranular corrosion due to prolongedheating in either welding or in application inconjunction with incorrect grade selection

Stress corrosion cracking due to appliedtensile stress, again in conjunction withchlorides and raised temperature

Galvanic corrosion due to proximity of metalswidely spaced in the electrochemical series

Contact corrosion due to contamination of thestainless steel by a material such as mild steelparticles.

More detailed descriptions of each of thesecorrosion mechanisms are given in the section ofthis Handbook on Corrosion Resistance.

Often measures to prevent several of theseproblems are similar. Design of stainless steelcomponents must be made to prevent build-up ofstagnant water, to encourage circulation of liquids,to discourage evaporation-concentration and tokeep stresses and temperatures as low aspossible.

Good design alone is not sufficient to preventproblems; fabricators must also be aware of theseproblems and may need to modify their practicesaccordingly. It is strongly recommended thatspecialist stainless steel fabricators be used.

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Specific Design Points - To Retain Corrosion Resistance

1. Invert Structural MembersAvoid entrapment of moisture within membersand within attachments, as shown in Figure 23.Stagnant liquid remnants are likely toconcentrate and to lead to pitting corrosion.

Figure 23

2. Ensure Tanks & Pipes Drain Fully WhenIdleTanks and pipelines left with small residualfluid quantities also encourage pittingcorrosion, as shown in Figure 24. The problemis made worse if the fluid is spread to a thinfilm.

Figure 24

3. Raise Tanks Off the FloorTank bottoms placed directly on concrete floorswill create crevices; ideal sites for corrosion inthe event of liquid spillage. Sealing the gapimproves the position, but is subject to mis-application and deterioration. A drip skirtprevents liquid collecting beneath the tank,while raising the tank on legs removes the

crevice entirely. These options are shown inFigure 25.

4. Smooth, Rounded Corners InsideTanksEfficient maintenance cleaning of tanks isoften important to remove built-up debrisor stains; this reduces the likelihood ofcrevice corrosion under sediments andalso may be important to retain hygienicconditions or prevent productcontamination. All internal tank cornersshould if possible be well rounded andsmooth. Welds should be in tank sides,not at corners. Welds should also beground smooth (much easier if the weld isin the side, not corner) and fullpenetration or from both sides. All ofthese measures improve the fatigueresistance of the structure, as well asremoving crevices.

5. Insulation or Lagging of Pipelines andVesselsThermal insulation of tanks and pipesshould be free of chlorides. The insulationshould be clad to totally prevent entry ofwater, as pitting corrosion or stresscorrosion cracking can occur in the warm,moist environment. The outside of a hottank or pipe can be a highly corrosiveenvironment because of evaporation ofliquid resulting in very high localisedchloride contents. The outside may in factbe a more corrosive environment than theinside!

6. Incomplete Filling ProblemsVapour phases given off by some fluidscan be quite corrosive; in these cases ifthe tank cannot be filled completely thevapour space should be well ventilated toremove the vapour.

7. Inlet LocationWhen dosing or making up a tank a highlycorrosive chemical may be added. Inthese instances it is important that theinlet be located away from side walls andin a moving liquid stream, so that theaddition is quickly diluted.

8. Reduce Splashing Within TanksA further problem may be caused bysplashing during filling or mixing - splashdrops on the inside tank walls will undergoevaporation and hence concentration ofthe corrosive species. Splashing shouldtherefore be avoided, perhaps by ensuringthat the inlet pipe terminates beneath theliquid level or by running mixingpropellers slowly and fully submerged.

9. Heater LocationCorrosion of all types proceeds more rapidly athigher temperatures. It is therefore importantthat immersion heaters in vessels are placed so

Figure 25

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as not to locally heat any section of the vesselwall, and processes should be run at the lowestconstant temperature possible.

10. Avoid Settling in Pipes and VesselsCrevice corrosion can occur beneath debriswhich settles out of stagnant or slow movingliquid, and in some environments low liquidvelocity also permits marine organisms to growon the steel, with similar increase in crevicecorrosion initiation. Designs should bothmaintain a reasonable flow rate (about 1 m/sechas been shown to substantially decrease thepitting rate in sea water) and result in totaldraining when the operation ceases. Dosing thefluid with a biocide may be a solution to thefouling problem, but chlorinated biocides suchas hypochlorites can themselves be highlycorrosive to stainless steels. Over-dosing mustbe avoided.

11. Pipe WeldsWhen joining pipe runs or using butt welding orsocket weld fittings it is highly desirable toachieve full penetration welds. Incomplete weldpenetration will result in a joint which appearsgood on the outside but has severe crevices atthe root of the weld. Full penetration can befacilitated by the use of consumable inserts orby GTAW with hand fed filler metal.

12. Structural AttachmentsCrevices are readily created when supportingrings, attachment pads etc are welded tostainless steel vessels. Intermittent weld runsmay give adequate mechanical strength butonly fully sealed welds give freedom fromcrevices.

13. Welding Mild Steel to Stainless SteelMixed-metal welding can be satisfactory,generally using an over-alloyed weldingconsumable such as Grade 309L, but cautionmust be exercised to prevent carbon migrationinto the part of the stainless steel exposed tothe corrosive environment. Differentialexpansion rates can also lead to excessivestresses, particularly at tack welds. Stainlesssteel attachment pads between a stainlesssteel vessel and its mild steel support canassist in reducing these problems.

14. Weld Repair to Avoid CrevicesIf a leaking tank is patched this should be donein a manner which avoids creating furthercrevices, on either inside or outside, as inFigure 26.

Figure 26

15. Preparation for WeldingMinimum amounts of energy shouldalways be put into stainless steel welds.The material should be carefully preparedand shaped before welding commences.The welder should avoid chill casting thefirst metal laid down and shrinkage cracksin the final weld pools.

16. De-scaling of Welds and SurfaceCleaningTo achieve maximum corrosion resistanceall weld scale must be removed and thesurface ground, polished and possiblybuffed to the specified finish. Bestresistance to corrosion is achieved whenthe steel surface is mirror smooth andtotally free of scale or other contaminants.Pickling or passivating will assist in thisprocess, and "pickling paste" is availableto easily carry it out. See the section ofthis Handbook on pickling and passivation.

17. Avoid Sharp Machined CornersMachining should avoid sharp internalcorners at sites such as changes of shaftdiameter and the corners of keyways, asthese may act as crevices both by theirown geometry and by the tendency duringservice for material to build up in thesesites (and perhaps be difficult to cleanout). Radiusing of corners will alsoimprove the fatigue resistance of anycomponent subject to fluctuating stresses,as it does also for other materials.

18. Avoid Sleeving of ShaftsFitting of sleeves to shafts createspossible crevice sites between the innerand outer components. A better solutionfrom this point of view is to machine froma solid bar.

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GUIDELINES FOR STAINLESS STEEL GRADE SELECTIONFundamental Properties for SelectionWhen considering the choice of a stainless steelfor a particular application, the first considerationneeds to be on the basis of which of thefundamental "competitive advantage" propertiesneeds to be exploited, as tabulated in Figure 26.These basic properties for selection can be initiallylooked at from the point of view of the five basicalloy groups - austenitic, duplex, ferritic,martensitic andprecipitation hardening and then the selectionrefined by considering individual grades.

Selection for Corrosion Resistance

The selection of the most cost-effective grade fora particular corrosive environment can be acomplex task. Often the most revealing guide tomaterial selection is the simple consideration ofwhat has been used before (here or in a similarenvironment), what was the service life and howand when did it corrode.

For resistance to environments such as strongacids, where uniform general corrosion is thecontrolling mechanism, there are published tablesof recommended grades, and iso-corrosion curvesthat indicate the rate at which the steel can beexpected to corrode. These are usuallyconstructed so that several grades can becompared, and the applicable one selected for theexpected environment. Although this approach isuseful, some care needs to be taken as there areoften minor differences between apparently similarenvironments that can make a large difference tothe corrosion rates in practice. Traces of chloridefor instance can be harmful.

Local corrosion is very frequently the mechanismby which stainless steels are likely to corrode. Therelated mechanisms of pitting and crevicecorrosion are very largely controlled by thepresence of chlorides in the environment,exacerbated by elevated temperature. Theresistance of a particular grade of stainless steel

to pitting and crevice corrosion is indicated by itsPitting Resistance Equivalent number, or PRE, asshown in the table of Figure 28.

Figure28

The PRE can be calculated from the composition asPRE = %Cr + 3.3 %Mo + 16 %N

For grades containing tungsten the formula issometimes written as

PRE = %Cr + 3.3 (%Mo + 0.5 %W) + 16 %NClearly grades high in the alloying elementschromium and especially molybdenum andnitrogen are more resistant. This is the reason forthe use of grade 316 (2%Mo) as the standard formarine fittings, and also explains the selection ofduplex grade 2205 (S32205 or S31803) with22%Cr, 3%Mo and a deliberate addition of0.15%N for resistance to higher chlorides athigher temperatures. More severe chloride-containing environments can be resisted by the"super austenitic" grades (e.g. N08904 andS31254) with up to 6%Mo and by the "superduplex" grades (e.g. S32750, S32550 andS32520) with very high chromium, molybdenumand nitrogen additions. The use of these gradescan extend the useful resistance in high chlorideenvironments up to close to boiling point.

Required Property Alloy Groups and Grades Likely to be Selected

Corrosion Resistance Selection depends upon environment. Corrosion resistance depends mainly oncomposition, not on alloy group. PRE gives a first guide to resistance. See text above.

Heat Resistance Austenitic grades, particularly those high in chromium, often also with high silicon,nitrogen and rare earth elements (eg: grade 310 and particularly 253MA). Highchromium ferritic grades can be useful (eg: 446). Ferritic grades with niobium haveimproved creep strength over other ferritic grades.

Cryogenic (lowtemperature)Resistance

Austenitic grades have excellent toughness at very low temperatures. All othergrades have a transition from ductile to brittle behaviour at some temperature.

Magnetic Response Austenitic grades have low magnetic permeability; higher nickel grades (eg 316 or310) are guaranteed non-magnetic even if cold worked. All other grades always showmagnetic response.

High Strengths Martensitic and precipitation hardening grades. Duplex grades are higher strengththan austenitics.

Grade Alloy Group PRE

AtlasCR12 Ferritic 11

430 Ferritic 17

439 Ferritic 18

303 Austenitic 18*

304/L Austenitic 18

316/L Austenitic 24

444 (F18MS) Ferritic 25

2304 Duplex 26

2205 Duplex 34

904L Austenitic 34

S31254 Austenitic 43

S32750 Duplex 43

S32520 Duplex 43

*See comments in the text regarding grade 303

Figure 27 Competitive Advantage

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A particular problem for the common austeniticgrades (e.g. 304 and 316) is stress corrosioncracking (SCC). Like pitting corrosion this occursin chloride environments, but it is possible for SCCto take place with only traces of chlorides, so longas the temperature is over about 60°C, and solong as a tensile stress is present in the steel,which is very common. The ferritic grades arevirtually immune from this form of attack, and theduplex grades are highly resistant. If SCC is likelyto be a problem it would be prudent to specify agrade from these branches of the stainless familytree.

Selection for Mechanical and PhysicalPropertiesHigh strength martensitic (e.g. 431) andprecipitation hardening (e.g. 630 / 17-4PH)grades are often the material of choice for shaftsand valve spindles - here the high strength is asfundamental to the selection process as is thecorrosion resistance. These grades have strengthsup to twice that of grades 304 and 316.

More commonly, however, the grade is selectedfor required corrosion resistance (or resistance tohigh or low temperature or because of requiredmagnetic response), and then the structure orcomponent is designed around the mechanical andphysical properties of the grade selected. Thesesecondary aspects should be consideredas early as possible in the selection process. Theselection of a high strength duplex grade such as2205 may not only solve the corrosion problembut also contribute to the cost effectiveness of theproduct because of its high strength. The selectionof a ferritic grade such as AtlasCR12 may result inadequate corrosion resistance for a non-decorativeapplication, and its low coefficient of thermalexpansion could be desirable because of lessdistortion from temperature changes. The thermalexpansion rates of the ferritic grades are similar tothat of mild steel, and only 2/3 that of austeniticgrades such as 304.

Selection for Fabrication

Again it is usually the case that grades areselected for corrosion resistance and thenconsideration is given to how the product can befabricated. Fabrication should be considered asearly as possible in the grade selection process, asit greatly influences the economics of the product.The table in Figure 29 lists some common gradesand compares their relative fabricationcharacteristics. These comparisons are onarbitrary 1 to 10 scales, with 10 indicatingexcellent fabrication by the particular method.

It is important to realise that there may be atrade-off between desirable properties. Anexample is grade 303. This has excellentmachinability, but the high sulphur content whichincreases the cutting speed so dramatically alsosubstantially reduces the grade's weldability,formability and corrosion resistance. With thisgrade the calculated PRE is wrong, as it does notfactor in the negative effect of the sulphur. Thisgrade must not be used in any marine or otherchloride environments.

Selection CriteriaBefore selecting a grade of stainless steel it isessential to consider the required properties suchas corrosion resistance, but it is also important toconsider the secondary properties such as thephysical and mechanical properties and the easeof fabrication of any candidate grades. The correctchoice will be rewarded not just by long, trouble-free life, but also by cost-effective fabrication andinstallation.

Grade Form-ability

Machin-ability

Weld-ability

303 1 8 * 1

304 8 5* 8

316 8 5* 8

416 1 10 1

430 4 6 2

444 6 5 6

2205 5 4 5

AtlasCR12 5 6 7

* Improved Machinability versions of thesegrades offer higher machinabilities in someproducts.

Figure 29 Grade fabrication ratings.

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APPENDICES

Appendix 1

Grade Summary - Stainless & Heat Resisting Steels

Typical Composition (%)GradeDesign.

UNSNo. C Mn Cr Mo Ni others

Description and Applications

Austenitic Stainless Steels

301 S30100 0.10 1 17 7 Primarily for deep drawn components andhigh strength springs and roll-formedpanelling.

302HQ S30430 0.03 0.6 18 9 Cu 3.5 Wire for severe cold heading applicationssuch as cross-recess screws.

303 S30300 0.06 1.8 18 9 S 0.3 Free machining grade for high speedrepetition machining. Also available as"Ugima" 303 improved mach inability barfor even higher machinability.

304 S30400 0.05 1.5 18.5 9 Standard austenitic grade - excellentfabrication characteristics with goodcorrosion resistance. Also available as"Ugima" 304 improved machinability bar.

304L S30403 0.02 1.5 18.5 9 Low carbon version of 304 gives resistanceto intergranular corrosion for heavy sectionwelding and high temperature applications.

308L S30803 0.02 1 19.5 10.5 Filler wire for welding 304 and similargrades.

309 S30900 0.05 1.5 23 13.5 Good corrosion resistance and goodresistance to attack by hot sulphurcompounds in oxidising gases. Filler forwelding dissimilar metals.

310 S31000 0.08 1.5 25 20 Good resistance to oxidation andcarburising atmospheres in temperatures850-1100°C.

316 S31600 0.05 1 17 2 11 Higher resistance than 304 to many media,particularly those containing chlorides. Alsoavailable as "Ugima" 316 improvedmachinability bar.

316L S31603 0.02 1 17 2 11 Low carbon version of 316 gives resistanceto intergranular corrosion for heavy sectionwelding and high temperature applications.

321 S32100 0.04 1 18 9 Ti 0.5 Titanium stabilised grade resistsintergranular corrosion during exposure at425-850°C. High strength in thistemperature range.

347 S34700 0.04 1 18 9 Nb 0.7 Niobium stabilised grade resistsintergranular corrosion as for 321, butmore commonly used as a filler for welding321.

904L N08904 0.02 1 20 4.5 24 Cu 1.5 Super austenitic grade with very highcorrosion resistance, particularly tosulphuric acid and warm chlorides.

253MA S30815 0.08 0.6 21 11 N 0.16Ce 0.06

Excellent scaling and creep resistance attemperatures up to 1150°C.

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Stainless & Heat Resisting Steels

Typical Composition (%)GradeDesign

UNSNo. C Mn Cr Mo Ni others

Description and Applications

Ferritic Stainless Steels

AtlasCR12 S41003 0.025 0.5 11.5 0.35 "Utility" stainless steel with usefulresistance to wet abrasion, and goodformability and weldability.

409 S40900 0.06 1 11 Ti 0.4 Resists atmospheric and automotiveexhaust gas corrosion. Usedextensively in auto exhaust systems.

430 S43000 0.03 0.4 16.5 Good combination of corrosionresistance and formability. Used forinterior panelling and cold headedfasteners.

430F S43020 0.07 1 16.5 S 0.25 Free machining version of 430. Notcommonly available.

F18S(439)

S43932 0.02 0.8 17.5 Ti/Nb0.4

Weldable general-purpose chromiumferritic grade with corrosion resistanceapproaching that of 304.

F18MS(444)

S44400 0.02 0.8 18 2 Ti/Nb0.4

Weldable chromium-molybdenumferritic grade has excellent corrosionresistance to hot water containingminor amounts of chlorides.

Duplex Stainless Steels

2304 S32304 0.02 0.8 23 0.3 3 N 0.15 High strength and with corrosionresistance matching that of 316.

2205 S32205S31803

0.02 0.8 23 3 5 N 0.17 High strength and good resistance topitting corrosion and stress corrosioncracking.

UR52N+2507Cu

S32520S32550

0.02 0.8 25 3.5 7 N 0.25Cu 0.7

Super duplex grade exhibitingexceptional resistance to hot chloridesand strong acids, and with highstrength.

2507 S32750 0.02 0.8 25 3 6 N 0.27 Super duplex grade exhibitingexceptional resistance to hot chloridesand strong acids, and with highstrength.

Martensitic Stainless Steels

410 S41000 0.1 0.5 12 Resists dry atmospheres, fresh waterand other mild environments. Hardenedand tempered to achieve best strengthand corrosion resistance.

416 S41600 0.12 1.0 12 Free machining hardenable grade.Corrosion resistance not as good as for410.

420 S42000 0.3 0.5 13 Higher carbon content than 410 giveshigher hardness for cutlery, knifeblades, dies and surgical instruments.

431 S43100 0.2 0.6 15 2 High strength, excellent toughness andcorrosion resistance similar to that of304. Used for pump shafts, bolts andvalve components.

440C S44004 1.0 0.5 17 Very high hardness grade excellent forcorrosion resistant knives and cuttingtools.

Precipitation Hardening Stainless Steels

630

(17-4PH)

S17400 0.05 0.6 16 4 Cu 4Nb

0.25

Precipitation hardening ("aging")treatment after machining gives highstrength without distortion. Corrosionresistance similar to 304.

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Appendix 2

Comparison of Grade Specifications of Stainless Steels

British Euronorm Swedish JapaneseGrade UNS No

BS No Name SS JIS

301301LN302303303Se304304L304H304N(302HQ)305309S310310S314316316L316H316LN316Ti317L321329347403405409410416416Se420420F430430F431434440C444630631904L(253MA)220523042507(AtlasCR12)(4565S)(Zeron100)(2507Cu)

S30100S30153S30200S30300S30323S30400S30403S30409S30451S30430S30500S30908S31000S31008S31400S31600S31603S31609S31653S31635S31703S32100S32900S34700S40300S40500S40900S41000S41600S41623S42000S42020S43000S43020S43100S43400S44004S44400S17400S17700N08904S30815S32205S32304S32750S41003S34565S32760S32520

301S21-

302S25303S31303S42304S31304S11

--

394S17305S19309S24310S24310S16314S25316S31316S11316S51316S61320S31317S12321S31

-347S31403S17405S17409S19410S21416S21416S41420S37416S29430S17

-431S29434S17

---

460S52904S13

-318S13

------

1.43101.43181.43191.4305

-1.43011.43061.4948

-1.45671.43031.48331.48401.48451.48411.44011.4404

-1.44061.45711.44381.45411.44601.45501.40001.40021.45121.40061.4005

-1.4021

-1.40161.41041.40571.41131.41251.45211.45421.45681.45391.48351.44621.43621.44101.40031.45651.45011.4507

X10CrNi18-8X2CrNiN18-7

-X8CrNiS18-9

-X5CrNi18-10X2CrNi19-11X6CrNi18-11

-X3CrNiCu18-9-4X4CrNi18-12X12CrNi23-13X15CrNi25-20X8CrNi25-21X15CrNiSi25-21X5CrNiMo17-12-2X2CrNiMo17-12-2

-X2CrNiMoN17-11-2X6CrNiMoTi17-12-2X2CrNiMo18-15-4X6CrNiTi18-10X3CrNiMoN27-5-2X6CrNiNb18-10X6Cr13X6CrAl13X2CrTi12X12Cr13X12CrS13

-X20Cr13

-X6Cr17X14CrMoS17X17CrNi16-2X6CrMo17-1X105CrMo17X2CrMoTi18-2X5CrNiCuNb16-4X7CrNiAl17-7X1NiCrMoCuN25-20-5X9CrNiSiNCe21-11-2X2CrNiMoN22-5-3X2CrNiN23-4X2CrNiMoN25-7-4X2CrNi12X2CrNiMnMoN24-17-6-4X2CrNiMoCuWN25-7-4X2CrNiMoCuN25-6-3

2331--

2346-

23322352

-2371

----

2361-

23472348

--

235023672337232423382301

--

23022380

-2303

-2320238323212325

-2326

-238825622368237723272328

----

SUS 301-

SUS 302SUS 303SUS 303SeSUS 304SUS 304L

-SUS 304N1SUS XM7SUS 305SUS 309SSUH 310SUS 310S

-SUS 316SUS 316L

-SUS 316LNSUS 316TiSUS 317LSUS 321SUS 329J1SUS 347SUS 403SUS 405SUH 409SUS 410SUS 416

-SUS 420J1SUS 420FSUS 430SUS 430FSUS 431SUS 434SUS 440CSUS 444SUS 630SUS 631

--

SUS 329J3L------

Designations in (parentheses) not recognised by ASTM; some are registered trade marks.The above comparisons are approximate only - in some instances they are very close, in others much less so.The list is intended as a comparison of functionally similar materials not as a schedule of contractualequivalents. If exact equivalents are needed original specifications must be consulted.

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Appendix 3

Typical Physical Properties - Annealed Condition

Mean Coefficient ofThermal Expansion (b)

ThermalConductivity

Grade UNSNo

Densitykg/m³

ElasticModulus

(a)GPa

0-100°Cμm/m/°C

0-315°Cμm/m/°C

0-538°Cμm/m/°C

at100°CW/m.K

at500°CW/m.K

SpecificHeat

0-100°CJ/kg.K

ElectricalResistivity

nΩ.m

201202301302302B303304304L302HQ304N305308309310314316316L316N317317L321329330347384409410416420430430F431434436440C444446630631904L253MA220523042507CR124565S2507CuZeron100

S20100S20200S30100S30200S30215S30300S30400S30403S30430S30451S30500S30800S30900S31000S31400S31600S31603S31651S31700S31703S32100S32900N08330S34700S38400S40900S41000S41600S42000S43000S43020S43100S43400S43600S44004S44400S44600S17400S17700N08904S30815S32205S32304S32750S41003S34565S32520S32760

780078007900790079007900790079007900790079007900790079007900800080008000800080007900780080007900790077007700770077007700770077007700770076507750775077507750800078007800780078007740800078007800

207-

193193193193193193193196193193200200200193193196193200193186196193193208200200200200200200200200200200200196204200200200200200200190205190

16.617.517.017.216.217.317.217.017.217.217.217.215.015.9

-15.915.915.915.916.516.610.114.416.617.211.09.99.910.310.410.410.210.49.310.111.011.010.811.015.017.013.713.013.010.814.513.512.6

18.018.417.217.818.017.817.817.217.817.817.817.816.616.215.116.216.216.216.2

-17.211.516.017.217.811.711.411.010.811.011.012.111.0

-10.310.6

-11.611.6

-17.214.7

-14.011.316.314.013.9

19.619.218.218.419.418.418.418.218.818.418.418.417.217.0

-17.517.517.517.518.118.6

-16.718.618.412.411.611.611.711.411.4

-11.4

-11.711.4

----

18.2---

12.517.214.5

-

16.316.316.316.3

-16.316.316.316.316.316.315.215.614.217.516.316.314.416.214.416.1

--

16.116.225.824.924.924.923.926.120.2

-23.924.226.820.918.416.413.015.519.016.014.230.514.517.014.4

--

21.521.5

-21.521.521.521.521.521.521.618.718.720.921.521.5

-21.5

-22.2

--

22.221.527.528.728.7

-26.026.3

-26.326.0

--

24.422.721.8

-----

40.0---

500500500500500500500500500500500500500500500500500500500500500460460500500460460460460460460460460460460420500460460500500450470460480450450480

6706907207207207207207207207207207207807207707407407407407907207501020720790600570570550600600720600600600620670800830850850850850850570920850850

Notes: (a) 1 GPa = 1000 MPa (b) μm/m/°C = x10-6/°CMagnetic Permeability of all 300 series austenitic steels in the annealed condition is approximately 1.02.

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Appendix 4

Hardness ConversionsApproximate conversions for Austenitic Stainless Steels and High Nickel Alloys

Brinell(HB)

Vickers(HV)

Rockwell C(HR C)

Rockwell B(HR B)

479513 50

450481 48

425452 46

403427 44

382404 42

363382 40

346362 38

330344 36

315326 34

301309 32 106

286285 28 104

271266 26 102

256248 23 100

240234 20 98

226220 17 96

213209 14 94

202198 12 92

192188 90

183179 88

174171 86

167164 84

160157 82

153151 80

147144 78

142140 76

137137 74

132133 72

128 129 70

Note:Conversions between hardness scales are approximate only and should not be used to determineconformance with specifications. Data from ASTM A370 and ASTM E140. Actual values obtained for hardnesswill depend very much upon product type. Cold worked products may have significantly higher hardnessesclose to the surface.

Conversions between hardness and tensile strength are not standardised for stainless steels, and no reliableconversions are possible.

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Appendix 5

Unit Conversions

To Convert to Multiply By

megapascal (MPa) pound-force/in² (psi) 145.03

pound-force/in² (psi) megapascal (MPa) 0.006895

megapascal (MPa) ton-force/in² (tsi) 0.064749

ton-force/in² (tsi) megapascal (MPa) 15.444

megapascal (MPa) kilogram-force/mm² (kgf/mm²) 0.10197

kilogram-force/mm² (kgf/mm²) megapascal (MPa) 9.8066

megapascal (MPa) newton/mm² (N/mm²) 1.0000

pound (lb) kilogram (kg) 0.4536

kilogram (kg) pound (lb) 2.2046

newton (N) kilogram (kg) 0.10197

kilogram (kg) newton (N) 9.8066

ounce (oz) gram (g) 28.35

gram (g) ounce (oz) 0.035274

inch (in) millimetre (mm) 25.40

millimetre (mm) inch (in) 0.039370

foot (ft) metre (m) 0.3048

metre (m) foot (ft) 3.2808

gallon (gal) litre (l or L) 4.546

litre (l or L) gallon (gal) 0.2200

Formulae

Temperature

9

532)x-F(=C 32+)

5

9Cx(=F

Mass of plate or sheet Mass (kg) = T x W x L x Density

Billing weight (kg) = T x W x L x Billing Factor

T is thickness in mm

W is width in metres

L is length in metres

Austenitic Ferritic

Density 7.9 7.7

Billing Factor 8.177 8.000

Mass of round wire or bar Austenitic

Mass (kg/m) = 0.006205 x d²

Ferritic

Mass (kg/m) = 0.006063 x d²

More complete unit conversion factors are given in the table on the Atlas Website.

A spreadsheet for mass calculations of common products is also available on the Atlas website.

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Appendix 6

Dimensional Tolerances for Barh and k Limit Deviations

Tolerance NumberNominal Bar Size(mm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Up to 3 0.8 1.2 2 3 4 6 10 14 25 40 60 100 140 250 400 600

Over 3 to 6 1 1.5 2.5 4 5 8 12 18 30 48 75 120 180 300 480 750

Over 6 to 10 1 1.5 2.5 4 6 9 15 22 36 58 90 150 220 360 580 900

Over 10 to 18 1.2 2 3 5 8 11 18 27 43 70 110 180 270 430 700 1100

Over 18 to 30 1.5 2.5 4 6 9 13 21 33 52 84 130 210 330 520 840 1300

Over 30 to 50 1.5 2.5 4 7 11 16 25 39 62 100 160 250 390 620 1000 1600

Over 50 to 80 2 3 5 8 13 19 30 46 74 120 190 300 460 740 1200 1900

Over 80 to 120 2.5 4 6 10 15 22 35 54 87 140 220 350 540 870 1400 2200

Over 120 to 180 3.5 5 8 12 18 25 40 63 100 160 250 400 630 1000 1600 2500

Over 180 to 250 4.5 7 10 14 20 29 46 72 115 185 290 460 720 1150 1850 2900

Over 250 to 315 6 8 12 16 23 32 52 81 130 210 320 520 810 1300 2100 3200

Over 315 to 400 7 9 13 18 25 36 57 89 140 230 360 570 890 1400 2300 3600

Over 400 to 500 8 10 15 20 27 40 63 97 155 250 400 630 970 1550 2500 4000

Tolerance values given in µm (microns) = X0.001mmh = all minus Examples: 25.40mm diameter bar to h9 = +Nil, -0.052mmk = all plus 160mm hot rolled bar to k14 = +1.000mm, - Nil

Notes:

1. Tolerances are as given for shafts in ISO 286.2 and AS 1654.2. These references should be consulted for full details and for other tolerances.2. Shaft k tolerances are according to the above table between k8 and k13 only.

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Appendix 7

Further InformationFurther information on the topics covered in this Technical Handbook can be found in the followingreferences, among others:-

1. Stahlschlüssel "Key to Steel". Anextremely comprehensive cross-reference of compositionalspecifications. Particularly good forEuropean designations.

2. "Woldman's Engineering Alloys", ASM.Another comprehensive cross-referenceof alloys, but with an American slant.

3. Iron and Steel Society (ISS) "SteelProducts Manual - Stainless Steels"March 1999. Background data onproducts and on material specifications.This book was published in previouseditions by the AISI - American Iron andSteel Institute.

4. ASTM (American Society for Testing andMaterials) Standards. An Index volumelists all ASTM standards, and individualvolumes cover flat product, tube/pipeand bars etc. Individual ASTM standardscan be down-loaded from internet sitewww.astm.org.

5. UNS Numbering System book, publishedjointly by ASTM and SAE. Lists allcommercially produced metals by UNSnumber.

6. AS (Australian Standards) cover some ofthe products handled by Atlas Steels,but in general, because we are sourcingon the world market, AustralianStandards are less relevant than themajor overseas standards. In stainlesssteel products ASTM standards are oftenuniversally recognised.

7. ASM "Specialty Handbook of StainlessSteels". A very good source ofinformation on almost any topic relatedto stainless steels. This is compiled fromthe various ASM Metals Handbooks.

8. "Design Guidelines for the Selection andUse of Stainless Steel", American Ironand Steel Institute, Distributed by NickelInstitute (NiDI publication No. 9014).

9. "Australian Stainless Reference Manual"published by the Australian StainlessSteel Development Association (ASSDA).ASSDA has been established to assist allstakeholders in the Australian stainlesssteel industry, and membership is opento any company.

10. "Guidelines for the Welded Fabrication ofNickel-Containing Stainless Steels forCorrosion Resistant Services". NickelInstitute Reference Book, Series No.11007.

11. “Specification for fabrication &installation of stainless steel plant &equipment in the food and beverageindustry”. ASSDA. An industryspecification for fabrication, withapplicability beyond food processingplants.

12. Moore, P.J., Good Design in StainlessSteel, Metal Working Australia, Vol 8,No. 3, Aug/Sep 1993.

13. ASM Handbook Vol 13, "Corrosion". Anexcellent resource on its topic, for allmetals.

14. “Corrosion Handbook”, OutokumpuStainless AB. An excellent reference ongrade selection for many environments.

15. Lai, G.Y., "High Temperature Corrosionof Engineering Alloys", ASM, 1990. Agood source for elevated temperatureperformance of all metals.

16. Sedriks, A.J. "Corrosion of StainlessSteels". Rigorous and in-depthdiscussion. 2nd edition 1996.

17. Dillon, C.P., “Corrosion Control in theChemical Process Industries”. Anexcellent practical guide to selection ofcorrosion resistant materials.

18. “The Machining of Stainless Steels”,Stainless Steel Specialist Course, Module9. ISSF.

19. WTIA Technical Note 16 "WeldingStainless Steels". Published by WeldingTechnology Institute of Australia.

20. Lacombe, P., Baroux, B., Beranger, G.(editors) – Stainless Steels, Les Editionsde Physique, 1993.

Many other textbooks and articles from thetechnical literature are held by the Atlas SteelsTechnical Department. Please feel free to contactus.

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Internet Sites of Interest

Organisation Internet Address

Atlas Steels www.atlassteels.com.au

Australian Stainless Steel Development Association – ASSDA www.assda.asn.au

British Stainless Steel Association – BSSA www.bssa.org.uk

New Zealand Stainless Steel Development Association – NZSSDA www.hera.org.nz/nzssda/

Specialty Steel Industry of North America – SSINA www.ssina.com

Southern Africa Stainless Steel Development Association – SASSDA www.sassda.co.za

Nickel Institute www.nickelinstitute.org

International Stainless Steel Forum – ISSF www.worldstainless.org

European Stainless Steel Development Association – Euro Inox www.euro-inox.org

American Society for Testing and Materials – ASTM www.astm.org

Standards Australia – for standards writing www.standards.com.au

SAI Global – to purchase Australian and many foreign standards www.saiglobal.com

Standards New Zealand www.standards.co.nz

Australasian Corrosion Association – ACA www.corrosion.com.au/

Welding Technology Institute of Australia – WTIA www.wtia.com.au

National Association of Testing Authorities – NATA www.nata.asn.au