Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

HEAT TREATMENT Improves properties of materials as it modifies the microstructure. Service performance of gears, cams, shafts, tools, dies and molds are enhanced by heat treatment

FIGURE 4.1 Cross-section of gear teeth showing induction-hardened surfaces. Source: TOCCO Div., Park-Ohio Industries, Inc.

FIGURE 4.2 Outline of topics described in this chapter.

ALLOYS Consists of two or more chemical elements, at least one of them is a metal Solute : minor element Solvent: major element

Alloying consists of two basic forms: 1 Alloying consists of two basic forms: 1. Solid solutions: crystal structure of solvent is maintained A. Substitutional solid solution (solute atom replaced solvent atom e.g. brass: zinc + copper B. Interstitial solid solution (solute atom occupies an interstitial position, e.g. steel: iron+carbon 2. Intermetallic components: consists of two metals; aluminides of iron or nickel

TWO PHASE SYSTEMS e.g. sand and water, water and ice, lead and copper Copper lead alloy has a property different from that of copper or lead A phase is a physically distinct and homogeneous portion in a material

FIGURE 4.3 (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead–copper alloy. The grains represent lead in solid solution 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 colored and white grains have different compositions and properties.

PHASE DIAGRAM OR EQUILIBRIUM DIAGRAM Relation between temperature, composition and phases

FIGURE 4.4 Phase diagram for copper–nickel alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each have one freezing or melting temperature. The top circle on the right depicts the nucleation of crystals. The second circle shows the formation of dendrites (see Section 10.2.2). The bottom circle shows the solidified alloy, with grain boundaries.

FIGURE 4.5 Mechanical properties of copper–nickel and copper–zinc alloys as a function of their composition. The curves for zinc are short, because zinc has a maximum solid solubility of 40% in copper.

THE IRON-IRON CARBIDE DIAGRAM Pure iron: 0. 008% Carbon Cast iron: 6 THE IRON-IRON CARBIDE DIAGRAM Pure iron: 0.008% Carbon Cast iron: 6.67% Carbon

FIGURE 4.6 The iron–iron-carbide phase diagram.

FIGURE 4.9 Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite and the darker regions are carbide. Magnification: 2500x.

FIGURE 4. 11 Microstructure for cast irons. Magnification: 100x FIGURE 4.11 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 iron solidified as white cast iron, with the carbon present as cementite, and was heat treated to graphitize the carbon.

RELATION BETWEEN HARDNESS AND % OF CARBON HARDNESS INCREASES WITH INCREASE OF % OF CARBON EFFECT OF TEMPERATURE ON HARDNESS HARDNESS DECREASES WITH INCREASE IN TEMPERATURE

FIGURE 4.13 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000x.

FIGURE 4.14 Hardness of tempered martensite as a function of tempering time for 1080 steel quenched to 65 HRC. Hardness decreases because the carbide particles coalesce and grow in size, thereby increasing the interparticle distance of the softer ferrite.

EFFECT OF INCREASE OF % OF CARBON ON HARDNESS :INCREASES ULTIMATE STRENGTH :INCREASES YIELD STRENGTH :INCREASES

EFFECT OF INCREASE OF % OF CARBON ON TOUGHNESS: DECREASES REDUCTION OF AREA : DECREASES ELONGATION : DECREASES

FIGURE 4.17 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, with increasing amounts of pearlite and iron carbide.