Presentation Chapter 13

8/12/2019 Presentation Chapter 13

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Chapter 13

12.1 Designations and Classificationof Steels

12.2 Simple Heat Treatments

12.3 Isothermal Heat Treatments

12.4 Quench and Temper HeatTreatments

12.5 Effect of Alloying Elements

12.6 Application of Hardenability


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12.7 Specialty Steels

12.8 Surface Treatments

12.9 Weldability of Steel

12.10 Stainless Steels

12.11 Cast Irons

Chapter Outline (Continued)


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Figure 12.1 (a) In a blast furnace,

iron ore is reduced using coke(carbon) and air to produce liquidpig iron. The high-carbon contentin the pig iron is reduce byintroducing oxygen into the basicoxygen furnace to produce liquid

steel. An electric arc furnace canbe used to produce liquid steel bymelting scrap. (b) Schematic of ablast furnace operation. (Source:www.steel.org. Used with

permission of the American Ironand Steel Institute.)


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Designations- The AISI (American Iron and SteelInstitute) and SAE (Society of Automotive Engineers)provide designation systems for steels that use a four- or

five-digit number. Classifications- Steels can be classified based on their

composition or the way they have been processed.

Section 12.1Designations and Classification

of Steels


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Figure 12.2 (a) Theeutectoid portion ofthe Fe-Fe3C phasediagram. (b) Anexpanded version ofthe Fe-C diagram,adapted fromseveral sources.


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Figure 12.3 Electron micrographs of (a) pearlite, (b)bainite, and (c) tempered martensite, illustrating thedifferences in cementite size and shape among thesethree microconstituents ( 7500). (FromThe Making,Shaping, and Treating of Steel, 10th Ed. Courtesy ofthe Association of Iron and Steel Engineers.)


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An unalloyed steel tool used for machining aluminumautomobile wheels has been found to work well, but thepurchase records have been lost and you do not know thesteels composition. The microstructure of the steel is

tempered martensite, and assume that you cannot estimatethe composition of the steel from the structure. Design atreatment that may help determine the steels carboncontent.

Example 12.1Design of a Method to Determine

AISI Number


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Example 12.1 SOLUTION

The first way is to heat the steel to a temperature just belowthe A1temperature and hold for a long time. The steelovertempers and large Fe

3

C spheres form in a ferrite matrix.We then estimate the amount of ferrite and cementite andcalculate the carbon content using the lever law. If we measure16% Fe3C using this method, the carbon content is:

%086.1or16100)0218.067.6(

)0218.0(

CFe%3

x

x

A better approach, however, is to heat the steel abovethe Acmto produce all austenite. If the steel then cools slowly,it transforms to pearlite and a primary microconstituent. If,

when we do this, we estimate that the structure contains 95%pearlite and 5% primary Fe3C, then:

%065.1or9510077.067.6

-6.67Pearlite%

x

x


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Process Annealing Eliminating Cold Work: A low-temperature heat treatment used to eliminate all or partof the effect of cold working in steels.

Annealing and Normalizing Dispersion Strengthening:Annealing- A heat treatment used to produce a soft,

coarse pearlite in steel by austenitizing, then furnacecooling. Normalizing- A simple heat treatment obtainedby austenitizing and air cooling to produce a fine pearliticstructure.

Spheroidizing Improving Machinability: Spheroidite- A

microconstituent containing coarse spheroidal cementiteparticles in a matrix of ferrite, permitting excellentmachining characteristics in high-carbon steels.

Section 12.2Simple Heat Treatments


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Figure 12.4 Schematic summary of the simple heat treatmentsfor (a) hypoeutectoid steels and (b) hypereutectoid steels.


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Figure 12.5 The effect ofcarbon and heattreatment on theproperties of plain-carbonsteels.


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Figure 12.6 The microstructureof spheroidite, with Fe3Cparticles dispersed in a ferrite

matrix ( 850). (FromASMHandbook, Vol. 7, (1972), ASM

International, Materials Park,OH 44073.)


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Recommend temperatures for the process annealing,annealing, normalizing, and spheroidizing of 1020,1077, and 10120 steels.

Example 12.2Determination of Heat Treating

Temperatures


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Figure 12.2 (a) Theeutectoid portion ofthe Fe-Fe3C phasediagram. (b) An

expanded version ofthe Fe-C diagram,adapted fromseveral sources.


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From Figure 12.2, we find the critical A1, A3, or Acm,

temperatures for each steel. We can then specify the heattreatment based on these temperatures.


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Austempering- The isothermal heat treatment by whichaustenite transforms to bainite.

Isothermal annealing- Heat treatment of a steel byaustenitizing, cooling rapidly to a temperature between

the A1and the nose of the TTT curve, and holding untilthe austenite transforms to pearlite.

Section 12.3Isothermal Heat Treatments


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Figure 12.7 The austempering and isothermal annealheat treatments in a 1080 steel.


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Figure 12.8 The TTT

diagrams for (a) a 1050 and(b) a 10110 steel.

E l 12 3


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A heat treatment is needed to produce a uniformmicrostructure and hardness of HRC 23 in a 1050 steel axle.

Example 12.3Design of a Heat Treatment for an Axle

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Figure 12.8 The TTTdiagrams for (a) a 1050and (b) a 10110 steel.


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Figure 12.2 (a) Theeutectoid portion ofthe Fe-Fe3C phasediagram. (b) An

expanded version ofthe Fe-C diagram,adapted fromseveral sources.


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1. Austenitize the steel at 770 + (30 to 55) = 805oC to825oC, holding for 1 h and obtaining 100% .

2. Quench the steel to 600oC and hold for a minimum of 10 s.Primary ferrite begins to precipitate from the unstableaustenite after about 1.0 s. After 1.5 s, pearlite begins togrow, and the austenite is completely transformed toferrite and pearlite after about 10 s. After this treatment,the microconstituents present are:

%64100)0218.077.0(

0.0218)(0.5Pearlite

%36100)0218.077.0(

0.5)(0.77Primary

3. Cool in air-to-room temperature, preserving theequilibrium amounts of primary ferrite and pearlite. Themicrostructure and hardness are uniform because of theisothermal anneal.


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Figure 12.9 Producingcomplicated structures

by interrupting theisothermal heattreatment of a 1050steel.


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Figure 12.10 Dark feathers ofbainite surrounded by lightmartensite, obtained byinterrupting the isothermaltransformation process ( 1500).(ASM Handbook, Vol. 9Metallography and Microstructure(1985), ASM International,Materials Park, OH 44073.)


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Retained austenite- Austenite that is unable totransform into martensite during quenching because ofthe volume expansion associated with the reaction.

Tempered martensite- The microconstituent of ferrite

and cementite formed when martensite is tempered. Quench cracks- Cracks that form at the surface of a

steel during quenching due to tensile residual stressesthat are produced because of the volume change thataccompanies the austenite-to-martensite transformation.

Marquenching- Quenching austenite to a temperaturejust above the MSand holding until the temperature isequalized throughout the steel before further cooling toproduce martensite.

Section 12.4Quench and Temper Heat Treatments


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Figure 12.13Increasing carbon

reduces the MsandMftemperatures inplain-carbon steels.


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Figure 12.14 Formation of quench cracks caused by residualstresses produced during quenching. The figure illustratesthe development of stresses as the austenite transforms tomartensite during cooling.


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Figure 12.16 The CCT diagram (solid lines) for a 1080 steelcompared with the TTT diagram (dashed lines).


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Hardenability- Alloy steels have high hardenability.

Effect on the Phase Stability- When alloying elementsare added to steel, the binary Fe-Fe3C stability isaffected and the phase diagram is altered.

Shape of the TTT Diagram- Ausformingis athermomechanical heat treatment in which austenite isplastically deformed below the A1 temperature, thenpermitted to transform to bainite or martensite.

Tempering- Alloying elements reduce the rate of

tempering compared with that of a plain-carbon steel.

Section 12.5Effect of Alloying Elements

nse.


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Figure 12.18 (a) TTT and (b)

CCT curves for a 4340 steel.


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Figure 12.19 Theeffect of 6%

manganese on thestability ranges ofthe phases in theeutectoid portion ofthe Fe-Fe3C phasediagram.


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Figure 12.20 When alloying elements introduce a bayregion into the TTT diagram, the steel can be ausformed.


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Figure 12.21 The effect of alloying elements on the phasesformed during the tempering of steels. The air-hardenablesteel shows a secondary hardening peak.


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Jominy test- The test used to evaluate hardenability. Anaustenitized steel bar is quenched at one end only, thusproducing a range of cooling rates along the bar.

Hardenability curves- Graphs showing the effect of the

cooling rate on the hardness of as-quenched steel. Jominy distance- The distance from the quenched end of

a Jominy bar. The Jominy distance is related to thecooling rate.

Section 12.6Application of Hardenability


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Figure 12.22 The set-up for the Jominy test used fordetermining the hardenability of a steel.


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Figure 12.23 Thehardenability curvesfor several steels.


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A gear made from 9310 steel, which has an as-quenched

hardness at a critical location of HRC 40, wears at an excessiverate. Tests have shown that an as-quenched hardness of atleast HRC 50 is required at that critical location. Design a steelthat would be appropriate.

Example 12.5Design of a Wear-Resistant Gear

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Figure 12.23 Thehardenability curves forseveral steels.


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From Figure 12.23, a hardness of HRC 40 in a 9310 steelcorresponds to a Jominy distance of 10/16 in. (10oC/s). If we

assume the same Jominy distance, the other steels shown inFigure 12.23 have the following hardnesses at the criticallocation:

1050 HRC 28 1080 HRC 36 4320 HRC 31

8640 HRC 52 4340 HRC 60In Table 12-1, we find that the 86xx steels contain less alloyingelements than the 43xx steels; thus the 8640 steel is probablyless expensive than the 4340 steel and might be our bestchoice. We must also consider other factors such as durability.


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Figure 12.24 The Grossman chart used to determine thehardenability at the center of a steel bar for differentquenchants.


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Design a quenching process to produce a minimum hardness ofHRC 40 at the center of a 1.5-in. diameter 4320 steel bar.

Example 12.6Design of a Quenching Process


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Figure 12.24 The Grossman chart used to determine thehardenability at the center of a steel bar for differentquenchants.


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Figure 12.23 Thehardenability curvesfor several steels.

E l 12 6 SOLUTION


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Several quenching media are listed in Table 12-2. We can findan approximate H coefficient for each of the quenching media,then use Figure 12.24 to estimate the Jominy distance in a 1.5-

in. diameter bar for each media. Finally, we can use thehardenability curve (Figure 12.23) to find the hardness in the4320 steel. The results are listed below.

The last three methods, based on brine or agitated water, aresatisfactory. Using an unagitated brine quenchant might be leastexpensive, since no extra equipment is needed to agitate thequenching bath. However, H2O is less corrosive than the brinequenchant.


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Tool steels- High-carbon steels with high hardness,toughness, or resistance to high temperatures.

Secondary hardening peakUnusually high hardnessin a steel tempered at a high temperature caused by

the precipitation of alloy carbides. HSLA Low-carbon steels containing the least amount of

alloying element.

Dual-phase steels- Special steels treated to producemartensite dispersed in a ferrite matrix.

Maraging steels- A special class of alloy steels thatobtain high strengths by a combination of themartensitic and age-hardening reactions.

Section 12.7Specialty Steels


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Selectively Heating the Surface - Rapidly heat thesurface of a medium-carbon steel above the certain (A3)temperature and then quench the steel.

Carburizing- A group of surface-hardening techniquesby which carbon diffuses into steel.

Cyaniding- Hardening the surface of steel with carbonand nitrogen obtained from a bath of liquid cyanidesolution.

Carbonitriding- Hardening the surface of steel withcarbon and nitrogen obtained from a special gas

atmosphere.

Section 12.8Surface Treatments


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(a) Surface hardening by localized heating. (b) Only the surface

heats above the A1temperature and is quenched to martensite.

Case depth- The depth below the surface of a steel atwhich hardening occurs by surface hardening processes.


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Figure 12.27 Carburizing of a low-carbon steel to produce ahigh-carbon, wear-resistant surface.


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Section 12.9 Weldability of Steel

Weldability is a term used to describe the

relative ease or difficulty with which a metal oralloy can be welded.

The main problem when welding steel ishardenability.

As long as the steel contains sufficientcarbon when it is cooled rapidly from hightemperature,

a phase transformation takes place.

The phase transformation from austenite tomartensite causes the material to hardenand become brittle.


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LOW-CARBON STEEL Steel in this classification is

tough and ductile, easily welded.

It does not respond to any form of heat treating,except case hardening.

MEDIUM-CARBON STEEL

These steels arestrongand hard but cannot be welded or worked as easily asthe low-carbon steels

HIGH-CARBON STEEL/VERY HIGH-CAR-BON

STEEL

Steel in these classes respond well to heattreatment

S ti 12 9 W ld bilit f St l


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The development ofthe heat-affectedzone in a weld: (a)the structure at themaximumtemperature, (b)the structure aftercooling in a steel ofLOW HARDENABILITY,and (c) thestructure after

cooling in a steel ofHIGHHARDENABILITY.

E l 12 8


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Compare the structures in the heat-affected zones ofwelds in 1080 and 4340 steels if the cooling rate in theheat-affected zone is 5oC/s.


The cooling rate in the weld produces the followingstructures:

1080: 100% pearlite

4340: Bainite and martensite

The high hardenability of the alloy steel reducesthe weldability, permitting martensite to form andembrittle the weld.

Example 12.8Structures of Heat-Affected Zones


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Stainless steels- A group of ferrous alloys that containat least 11% Cr, providing extraordinary corrosionresistance.

Categories of stainless steels:

Ferritic Stainless Steels Martensitic Stainless Steels

Austenitic Stainless Steels

Precipitation-Hardening (PH) Stainless Steels

Duplex Stainless Steels

Section 12.10Stainless Steels

nse

.


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Figure 12.30 (a) Theeffect of 17% chromiumon the iron-carbonphase diagram. At low-carbon contents, ferriteis stable at all

temperatures. (b) Asection of the iron-chromium-nickel-carbonphase diagram at aconstant 18% Cr-8% Ni.At low-carbon contents,austenite is stable atroom temperatures.


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Figure 12.31 (a) Martensitic stainless steel containinglarge primary carbides and small carbides formedduring tempering ( 350). (b) Austenitic stainlesssteel ( 500). (FromASM Handbook, Vols. 7 and 8,

(1972, 1973), ASM International, Materials Park, OH44073.)

Example 12.9


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In order to efficiently recycle stainless steel scrap, we wish to

separate the high-nickel stainless steel from the low-nickelstainless steel. Design a method for doing this.


Performing a chemical analysis on each piece of scrap is tediousand expensive. Sorting based on hardness might be less

expensive; however, because of the different types oftreatmentssuch as annealing, cold working, or quench andtemperingthe hardness may not be related to the steelcomposition.

The high-nickel stainless steels are ordinarily austenitic,

whereas the low-nickel alloys are ferritic or martensitic. Anordinary magnet will be attracted to the low-nickel ferritic andmartensitic steels, but will not be attracted to the high-nickelaustenitic steel. We might specify this simple and inexpensivemagnetic test for our separation process.

Design of a Test to SeparateStainless Steels


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Cast iron- Ferrous alloys containing sufficient carbon sothat the eutectic reaction occurs during solidification.

Eutectic and Eutectoid reaction in Cast Irons

Types of cast irons:

Gray cast iron White cast iron

Malleable cast iron

Ductile or nodular, cast iron

Compacted graphite cast iron

Section 12.11Cast Irons


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Figure 12.32 Schematic drawings of the five types of castiron: (a) gray iron, (b) white iron, (c) malleable iron, (d)ductile iron, and (e) compacted graphite iron.


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Figure 12.33 The iron-carbon phase diagram showing therelationship between the stable iron-graphite equilibria (solidlines) and the metastable iron-cementite reactions (dashedlines).


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Figure 12.34 The transformation diagram for austenite in acast iron.


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Figure 12.35 (a) Sketch and (b) photomicrograph of theflake graphite in gray cast iron (x 100).

.


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Figure 12.36 Theeffect of the cooling

rate or casting sizeon the tensileproperties of twogray cast irons.


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Figure 12.37 The heat treatments for ferritic andpearlitic malleable irons.


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Figure 12.38 (a) White cast iron prior to heat treatment ( 100). (b) Ferritic malleableiron with graphite nodules and small MnS inclusions in a ferrite matrix ( 200). (c)Pearlitic malleable iron drawn to produce a tempered martensite matrix ( 500).(Images (b) and (c) are from Metals Handbook, Vols. 7 and 8, (1972, 1973), ASMInternational, Materials Park, OH 44073.) (d) Annealed ductile iron with a ferrite matrix( 250). (e) As-cast ductile iron with a matrix of ferrite (white) and pearlite ( 250). (f)Normalized ductile iron with a pearlite matrix ( 250).


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Figure 12.17 (Repeated for Problem 12.20) The CCTdiagram for a low-alloy, 0.2% C steel.


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Figure 12.23(Repeated forProblem 12.54)The hardenabilitycurves for severalsteels.


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Figure 12.30b(Repeated for Problem12.48) (b) A section ofthe iron-chromium-

nickel-carbon phasediagram at a constant18% Cr-8% Ni. Atlow-carbon contents,austenite is stable atroom temperature.

Documents

Presentation Chapter 13