Strengthening by Heat TreatmentMetal Alloys: Their ...dl.metallurgyeng.com/files/Metal Alloys.pdfIron-Iron Carbide Phase Diagram Figure 4.8 The iron-iron carbide phase diagram. Because

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

Metal Alloys: Their Structure andStrengthening by Heat Treatment


Induction-Hardened Surface

Figure 4.1 Cross-section ofgear teeth showinginduction-hardenedsurfaces. Source: TOCCODiv., Park-Ohio Industries,Inc.


Chapter 4 Outline

Figure 4.2 Outline of topics described in Chapter 4.


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.


Cooling Curve

Figure 4.4 Cooling curve forthe solidification of puremetals. Note that freezing takesplace at a constant temperature;during freezing the latent heatof solidification is given off.


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


Mechanical Properties of Copper-Nickel andCopper-Zinc Alloys

Figure 4.6 Mechanicalproperties of copper-nickeland copper-zinc alloys as afunction of theircomposition. The curvesfor 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.


Lead-Tin Phase Diagram

Figure 4.7 Thelead-tin phasediagram. Note thatthe composition ofthe eutectic point forthis alloy is 61.9%Sn-38.1% Pb. Acomposition eitherlower or higher thanthis ratio will have ahigher liquidustemperature.


Iron-Iron Carbide Phase Diagram

Figure 4.8 The iron-ironcarbide phase diagram.Because of theimportance of steel as anengineering material, thisdiagram is one of themost important of allphase diagrams.


Austenite, Ferrite, and Martensite

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


Iron-Carbon Alloy Above and Below EutectoidTemperature

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).


Pearlite Microstructure

Figure 4.11 Microstructure ofpearlite in 1080 steel, formedfrom austenite of eutectoidcomposition. In this lamellarstructure, the lighter regions areferrite, and the darker regions arecarbide. Magnification: 2500X.Source: Courtesy of USXCorporation.


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.


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.


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 transformationtemperature of 675 °C(1247 °F). (continued)


Austenite to Pearlite Transformation (cont.)

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


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


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.


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.


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.


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.


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 ofthese curves. Eachcurve is actually aband, and its exactdetermination isimportant in theheat treatment ofmetals, for bettercontrol ofproperties. Source:L. H. Van Vlack;Materials forEngineering.Addison-WesleyPublishing Co.,Inc., 1982.


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.


Age Hardening

Figure 4.22 The effect of agingtime and temperature on the yieldstress of 2014-T4 aluminum alloy.Note that, for each temperature,there is an optimal aging time formaximum strength.


Outline of Heat Treatment Processes forSurface Hardening

TABLE 4.1Process Metals hardened Element added to

surfaceProcedure General Characteristics Typical applications

Carburizing Low-carbon steel(0.2% C), alloysteels (0.08–0.2%C)

C Heat steel at 870–950 °C (1600–1750°F) in an atmosphere of carbonaceousgases (gas carburizing) or carbon-containing solids(pack carburizing). Then quench.

A hard, high-carbon surface isproduced. Hardness 55 to 65HRC. Case depth < 0.5–1.5 mm( < 0.020 to 0.060 in.). Somedistortion of part during heattreatment.

Gears, cams, shafts,bearings, piston pins,sprockets, clutch plates

Carbonitriding Low-carbon steel C and N Heat steel at 700–800 °C (1300–1600°F) in an atmosphere of carbonaceousgas 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.). Lessdistortion than incarburizing.

Bolts, nuts, gears

Cyaniding Low-carbon steel(0.2% C), alloysteels (0.08–0.2%C)

C and N Heat steel at 760–845 °C (1400–1550°F) in a molten bath of solutions ofcyanide (e.g., 30% sodium cyanide) andother salts.

Surface hardness up to 65 HRC.Case depth 0.025 to 0.25 mm(0.001 to 0.010 in.). Somedistortion.

Bolts, nuts, screws, smallgears

Nitriding Steels (1% Al,1.5% Cr, 0.3%Mo), alloy steels(Cr, Mo), stainlesssteels, high-speedtool steels

N Heat steel at 500–600 °C (925–1100 °F)in an atmosphere of ammonia gas ormixtures of molten cyanide salts. Nofurther treatment.

Surface hardness up to 1100HV. Case depth 0.1 to 0.6 mm(0.005 to 0.030 in.) and 0.02 to0.07 mm (0.001to 0.003 in.) for high speedsteel.

Gears, shafts, sprockets,valves, cutters, boringbars, fuel-injection pumpparts

Boronizing Steels B Part is heated using boron-containinggas or solid in contact with part.

Extremely hard and wearresistant surface. Case depth0.025– 0.075 mm (0.001–0.003 in.).

Tool and die steels

Flame hardening Medium-carbonsteels, cast irons

None Surface is heated with an oxyacetylenetorch, then quenched with water spray orother quenching methods.

Surface hardness 50 to 60 HRC.Case depth 0.7 to 6 mm (0.030to 0.25 in.). Little distortion.

Gear and sprocket teeth,axles, crankshafts, pistonrods, lathe beds andcenters

Inductionhardening

Same as above None Metal part is placed in copper inductioncoils and is heated by high frequencycurrent, then quenched.

Same as above Same as above


Heat Treatment ProcessesFigure 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.


Properties of Oil-Quenched Steel

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


Induction Heating

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

Documents

Strengthening by Heat TreatmentMetal Alloys: Their ...dl.metallurgyeng.com/files/Metal Alloys.pdfIron-Iron Carbide Phase Diagram Figure 4.8 The iron-iron carbide phase diagram. Because