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Kalpakjian SchmidManufacturing Engineering and Technology 2001 Prentice-Hall Page 4-1

CHAPTER 4

Metal Alloys: Their Structure andStrengthening by Heat Treatment


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Induction-Hardened Surface

Figure 4.1 Cross-section of

gear teeth showinginduction-hardenedsurfaces. Source: TOCCODiv., Park-Ohio Industries,Inc.


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Chapter 4 Outline

Figure 4.2 Outline of topics described in Chapter 4.


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Two-Phase System

Figure 4.3 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughoutthe structure of a two-phase system, such as a lead-copper alloy. The grains represent lead in solidsolution in copper, and the particles are lead as a second phase. (b) Schematic illustration of a two-

phase system consisting of two sets of grains: dark, and light. The dark and the light grains haveseparate compositions and properties.


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Cooling Curve

Figure 4.4 Cooling curve forthe solidification of puremetals. Note that freezing takes

place at a constant temperature;during freezing the latent heatof solidification is given off.


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Nickel-Copper Alloy Phase Diagram

Figure 4.5 Phasediagram for nickel-copper alloy systemobtained at a slowrate of solidification.

Note that pure nickeland pure copper eachhas one freezing ormelting temperature.The top circle on theright depicts thenucleation of

crystals. The secondcircle shows theformation ofdendrites (seeSection 10.2). Thebottom circle showsthe solidified alloy,

with grainboundaries.


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Mechanical Properties of Copper-Nickel and

Copper-Zinc AlloysFigure 4.6 Mechanicalproperties of copper-nickeland copper-zinc alloys as afunction of theircomposition. The curves

for zinc are short, becausezinc has a maximum solidsolubility of 40% in copper.Source: L. H. Van Vlack;Materials for Engineering.Addison-WesleyPublishing Co., Inc., 1982.


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Lead-Tin Phase DiagramFigure 4.7 Thelead-tin phasediagram. Note thatthe composition ofthe eutectic point for

this alloy is 61.9%Sn-38.1% Pb. Acomposition eitherlower or higher thanthis ratio will have ahigher liquidustemperature.


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Iron-Iron Carbide Phase DiagramFigure 4.8 The iron-ironcarbide phase diagram.Because of theimportance of steel as an

engineering material, thisdiagram is one of themost important of allphase diagrams.


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Austenite, Ferrite, and Martensite

Figure 4.9 The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of

carbon (by weight) on the lattice dimensions for martensite is shown in (d). Note the interstitial positionof the carbon atoms (see Fig. 1.9). Note, also, the increase in dimension c with increasing carbon content;this effect causes the unit cell of martensite to be in the shape of a rectangular prism.


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Iron-Carbon Alloy Above and Below Eutectoid

Temperature

Figure 4.10 Schematic illustrationof the microstructures for an iron-carbon alloy of eutectoidcomposition (0.77% carbon), aboveand below the eutectoid temperatureof 727 C (1341 F).


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Pearlite Microstructure

Figure 4.11 Microstructure ofpearlite in 1080 steel, formedfrom austenite of eutectoid

composition. In this lamellarstructure, the lighter regions areferrite, and the darker regions arecarbide. Magnification: 2500X.Source: Courtesy of USXCorporation.


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Extended Iron-Carbon Phase Diagram

Figure 4.12 Phase diagram for the iron-carbon system with graphite (insteadof cementite) as the stable phase. Note that this figure is an extended versionof Fig. 4.8.


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Microstructures for Cast Irons(a) (b) (c)

Figure 4.13 Microstructure for cast irons. Magnification: 100X. (a) Ferritic gray iron with graphite flakes. (b)Ferritic Ductile iron (nodular iron), with graphite in nodular form. (c) Ferritic malleable iron; this cast ironsolidified as white cast iron, with the carbon present as cementite, and was heat treated to graphitize the carbon.Source: ASM International.


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Austenite toPearlite

Transformation

Figure 4.14 (a) Austenite-to-pearlite transformationof iron-carbon alloy as afunctionof time andtemperature. (b)Isothermal transformationdiagram obtained from (a)for a transformation

temperature of 675 C(1247 F). (continued)


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Austenite to Pearlite Transformation (cont.)

Figure 4.14 (c) Microstructuresobtained for a eutectoid iron-carbonalloy as a function of cooling rate.

Source: ASM International.


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Hardness and Toughness of Annealed Steels

Figure 4.15 (a) and (b) Hardness and (c) toughness for annealed plain-carbon steels, as a function of carbideshape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. Thespheroidite structure has spherelike carbide particles. Note htat the percentage of pearlite begins to decrease

after 0.77% carbon. Source: L. H. Van Vlack;Materials for Engineering. Addison-Wesley Publishing Co.,Inc., 1982.


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Mechanical Properties of Annealed Steels

Figure 4.16 Mechanical properties of annealed steels, as a function of composition and microstructure. Note(in (a)) the increase in hardness and strength and (in (b)) the decrease in ductility and toughness, withincreasing amounts of pearlite and iron carbide. Source: L. H. Van Vlack;Materials for Engineering.Addison-Wesley Publishing Co., Inc., 1982.


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Eutectoid Steel Microstructure

Figure 4.17 Microstructureof eutectoid steel.Spheroidite is formed bytempering the steel at 700 C(1292 F). Magnification:1000X. Source: Courtesy ofUSX Corporation.


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Martensite

(b)

Figure 4.18 (a) Hardness of martensite, as a function of carbon content. (b) Micrograph of martensitecontaining 0.8% carbon. The gray platelike regions are martensite; they have the same composition as theoriginal austenite (white regions). Magnification: 1000X. Source: Courtesy of USX Corporation.


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Hardness of Tempered Martensite

Figure 4.19 Hardnessof temperedmartensite, as afunction of temperingtime, for 1080 steelquenched to 65 HRC.Hardness decreasesbecause the carbideparticles coalesce andgrow in size, therebyincreasing theinterparticle distanceof the softer ferrite.


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End-QuenchHardenability

Test

Figure 4.20 (a)End-quench testand cooling rate.(b) Hardenabilitycurves for fivedifferent steels, asobtained from theend-quench test.Small variations incomposition canchange the shape of

these curves. Eachcurve is actually aband, and its exactdetermination isimportant in theheat treatment ofmetals, for better

control ofproperties. Source:L. H. Van Vlack;Materials forEngineering.Addison-WesleyPublishing Co.,

Inc., 1982.


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Aluminum-Copper Phase Diagram

Figure 4.21 (a) Phase diagram for the aluminum-copper alloy system. (b) Various micro-structures obtained during the age-hardening process. Source: L. H. Van Vlack;Materials forEngineering. Addison-Wesley Publishing Co., Inc., 1982.


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Age Hardening

Figure 4.22 The effect of aging

time and temperature on the yieldstress of 2014-T4 aluminum alloy.Note that, for each temperature,there is an optimal aging time formaximum strength.


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Outline of Heat Treatment Processes for

Surface HardeningTABLE 4.1Process Metals hardened Element added to

surface

Procedure General Characteristics Typical applications

Carburizing Low-carbon steel

(0.2% C), alloy

steels (0.080.2%

C)

C Heat steel at 870950 C (16001750

F) in an atmosphere of carbonaceous

gases (gas carburizing) or carbon-

containing solids

(pack carburizing). Then quench.

A hard, high-carbon surface is

produced. Hardness 55 to 65

HRC. Case depth < 0.51.5 mm

( < 0.020 to 0.060 in.). Some

distortion of part during heat

treatment.

Gears, cams, shafts,

bearings, piston pins,

sprockets, clutch plates

Carbonitriding Low-carbon steel C and N Heat steel at 700800 C (13001600

F) in an atmosphere of carbonaceous

gas and ammonia. Then quench in oil.

Surface hardness 55 to 62 HRC.

Case depth 0.07 to 0.5 mm

(0.003 to 0.020 in.). Less

distortion than in

carburizing.

Bolts, nuts, gears

Cyaniding Low-carbon steel

(0.2% C), alloy

steels (0.080.2%

C)

C and N Heat steel at 760845 C (14001550

F) in a molten bath of solutions of

cyanide (e.g., 30% sodium cyanide) and

other salts.

Surface hardness up to 65 HRC.

Case depth 0.025 to 0.25 mm

(0.001 to 0.010 in.). Some

distortion.

Bolts, nuts, screws, small

gears

Nitriding Steels (1% Al,

1.5% Cr, 0.3%

Mo), alloy steels

(Cr, Mo), stainless

steels, high-speed

tool steels

N Heat steel at 500600 C (9251100 F)

in an atmosphere of ammonia gas or

mixtures of molten cyanide salts. No

further treatment.

Surface hardness up to 1100

HV. Case depth 0.1 to 0.6 mm

(0.005 to 0.030 in.) and 0.02 to

0.07 mm (0.001

to 0.003 in.) for high speed

steel.

Gears, shafts, sprockets,

valves, cutters, boring

bars, fuel-injection pump

parts

Boronizing Steels B Part is heated using boron-containing

gas or solid in contact with part.

Extremely hard and wear

resistant surface. Case depth

0.025 0.075 mm (0.0010.003 in.).

Tool and die steels

Flame hardening Medium-carbon

steels, cast irons

None Surface is heated with an oxyacetylene

torch, then quenched with water spray or

other quenching methods.

Surface hardness 50 to 60 HRC.

Case depth 0.7 to 6 mm (0.030

to 0.25 in.). Little distortion.

Gear and sprocket teeth,

axles, crankshafts, piston

rods, lathe beds and

centers

Induction

hardening

Same as above None Metal part is placed in copper induction

coils and is heated by high frequency

current, then quenched.

Same as above Same as above


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Heat Treatment Processes

Figure 4.23 Heat-treating temperature ranges forplain-carbon steels, as indicated on the iron-ironcarbide phase diagram. Source: ASMInternational.

Figure 4.24 Hardness of steels in the quenched andnormalized conditions, as a function of carbon content.


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Properties of Oil-Quenched SteelFigure 4.25 Mechanical properties ofoil-quenched 4340 steel, as a functionof tempering temperature. Source:Courtesy of LTV Steel Company


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Induction Heating

Figure 4.26 Types of coils used in induction heating of various surfaces of parts.

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