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

Transcription

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

2 Gear Teeth Cross-section Figure 4.1 Cross-section of gear teeth showing induction-hardened surfaces. Source: Courtesy of TOCCO Div., Park-Ohio Industries, Inc.

3 Chapter 4 Topics Figure 4.2 Outline of topics described in Chapter 4.

4 Two Phase Systems 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 dark and the light grains have separate compositions and properties.

5 Cooling of Metals Figure 4.4 (a) Cooling curve for the solidification of pure metals. Note that freezing takes place at a constant temperature; during freezing, the latent heat of solidification is given off. (b) Change in density during the cooling of pure metals.

6 Phase Diagram for Nickel-copper Alloy System Figure 4.5 Phase diagram for nickel-copper alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each has 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). The bottom circle shows the solidified alloy with grain boundaries.

7 Mechanical Properties of Copper Alloys Figure 4.6 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.

8 Lead-tin Phase Diagram Figure 4.7 The lead-tin phase diagram. Note that the composition of eutectic point for this alloy is 61.9% Sn 38.1% Pb. A composition either lower or higher than this ratio will have a higher liquidus temperature.

9 Iron-iron Carbide Phase Diagram Figure 4.8 The iron-iron carbide phase diagram. Because of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams.

10 Unit Cells 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 position of the carbon atoms (see Fig. 1.9). Also note, 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.

11 Microstructures for an Iron-Carbon Alloy Figure 4.10 Schematic illustration of the microstructures for an ironcarbon alloy of eutectoid composition (0.77% carbon) above and below the eutectoid temperature of 727 C (1341 F).

12 Microstructure of Steel Formed from Eutectoid Composition Figure 4.11 Microstructure of pearlite in 1080 steel formed from austenite of a eutectoid composition. In this lamellar structure, the lighter regions are ferrite, and the darker regions are carbide. Magnification: 2500x.

13 Iron-Carbon Phase Diagram with Graphite Figure 4.12 Phase diagram for the iron-carbon system with graphite (instead of cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.8.

14 Microstructure for Cast Irons 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 iron solidified as white cast iron with the carbon present as cementite and was heat treated to graphitize the carbon.

15 Microstructure of Eutectoid Steel Figure 4.14 Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700 C (1292 F). Magnification: 1000x.

16 Martensite Figure 4.15 (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray plate-like regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000x.

17 Hardness of Tempered Martensite Figure 4.16 Hardness of tempered martensite as a function of tempering time for the 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.

18 Timetemperaturetransformation diagrams Figure 4.17 (a) Austeniteto-pearlite transformation of iron-carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675 C (1274 F). (c) Microstructures obtained for a eutectoid iron-carbon alloy as a function of cooling rate.

19 Hardness and Toughness in Steel as a Function of Carbide Shape Figure 4.18 (a) and (b) Hardness and (c) toughness for annealed plain-carbon steel as a function of a carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has sphere-like carbide particles.

20 Mechanical Properties of Steel as a Function of Composition and Microstructure Figure 4.19 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.

21 End-Quench Hardenability Test Figure 4.20 (a) Endquench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals for better control of properties.

22 Phase Diagram for Aluminum-copper Alloy and Obtained Microstructures Figure 4.21 (a) Phase diagram for the aluminum-copper alloy system. (b) Various microstructures obtained during the age-hardening process.

23 Effect of Time and Temperature on Yield Stress Figure 4.22 The effect of again time and temperature on the yield stress of 2014-T4 aluminum alloy. Note that, for each temperature, there is an optimal aging time for maximum strength.

24 Outline of Heat Treatment Processes for Surface Hardening

25 Outline of Heat Treatment Processes for Surface Hardening, con t.

26 Heat-treating Temperature Ranges for Plain-Carbon Steels Figure 4.23 Heat-treating temperature ranges for plain-carbon steels, as indicated on the iron-iron carbide phase diagram.

27 Hardness of Steel as a Function of Carbon Content Figure 4.24 Hardness of steels in the quenched and normalized conditions as a function of carbon content.

28 Mechanical Properties of Steel as a Function of Tempering Temperature Figure 4.25 Mechanical properties of oilquenched 4340 steel as a function of tempering temperature.