Tempering

Tempering is a postquench thermal treatment designed to reduce the stress imposed on a part by rapid quenching and martensite transformation. This stress, if not relieved, can increase the brittleness and notch sensitivity of the part, making it more prone to fracture. This stress is increased with increasing density level in the part. All parts with density levels above 6.7 g/cm3 should be tempered after hardening. Recommended tempering temperatures for P/M parts range from 150 to 200 °C (300 to 400 °F). Above this range, entrained quench oil can ignite, creating a hazardous condition in the furnace. Tempering above 200 °C (390 °F) results in improved toughness and fatigue properties in the hardened part, at the expense of tensile strength and impact resistance; however, the tempering furnace would need to be specially adapted to handle the high volume of smoke generated by the volatilization of the quench oil. Tempering is also used to reduce the effects of retained austenite that can occur in alloy steels when rapidly quenched. This constituent has been shown to influence dimensional change and hardness of the material. Freezing by cryogenic treatment (below -100 °C, or -148 °F) for several hours transforms the retained austenite to martensite. This is usually followed by a second temper at 200 °C (390 °F) to stress relieve the newly formed martensite. Figure 10 shows the influence of tempering temperature on impact resistance of FL4205 alloy after hardening.

Tempering temperature, °C

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7.25 £

3/cm3 y

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g/cm3

0.75 (

j/cm3

100 200 300 400 500 600 Tempering temperature, °F

100 200 300 400 500 600 Tempering temperature, °F

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Fig. 10 Effect of density and tempering on impact energy

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