nal structure in the steel and somewhat higher strength than annealing produces

Nal structure in the steel and somewhat higher

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nal structure in the steel and somewhat higher strength than annealing produces. Machinability and toughness are usually improved over the as-rolled condition. Through-Hardening and Quenching and Tempering. Through-hardening [Figure 2–13(d)] is accomplished by heating the steel to above the transformation range where austenite forms and then rapidly cooling it in a quenching medium. The rapid cooling causes the formation of mar- tensite, the hard, strong form of steel. The degree to which martensite forms depends on the alloy’s composition. An alloy containing a minimum of 80% of its structure in the martensite form over the entire cross section has high hard- enability . This is an important property to look for when selecting a steel requiring high strength and hardness. The and temper), and case hardening. (See References 6 and 16–18.) Figure 2–13 shows the temperature–time cycles for these heat-treatment processes. The symbol RT indicates normal room temperature, and LC refers to the lower criti- cal temperature at which the transformation of ferrite to austenite begins during the heating of the steel. At the upper critical temperature (UC), the transformation is complete. These temperatures vary with the composition of the steel. For most medium-carbon (0.30–0.50% carbon) steels, UC is approximately 1500°F (822°C). References giving detailed heat-treatment process data should be consulted. Annealing. Full annealing [Figure 2–13(a)] is performed by heating the steel above the upper critical temperature and holding it until the composition is uniform. Then the steel is cooled very slowly in the furnace to below the lower criti- cal temperature. Slow cooling to room temperature outside the furnace completes the process. This treatment produces a soft, low-strength form of the material, free of significant internal stresses. Parts are frequently cold-formed or ma- chined in the annealed condition. Stress relief annealing [Figure 2–13(b)] is often used following welding, machining, or cold-forming to relieve UC LC RT Time Temperature UC LC RT Time Temperature Very slow cooling in furnace Slow cooling Slow cooling ( a ) Full annealing ( b ) Stress relief annealing UC LC RT Time Temperature UC LC RT Time Temperature Slow cooling Slow cooling ( c ) Normalizing ( d ) Quenching and tempering (through-hardening) Tempering temperature Quenching Note: RT = room temperature LC = lower critical temperature UC = upper critical temperature FIGURE 2–13 Heat treatments for steel ( Note : RT = room temperature, LC = lower critical temperature, UC = upper critical temperature)
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