A hardening heat treatment is necessary to bring out the maximum hardness and wear resistance. Since these materials absorb heat very slowly, they should be heated gradually and allowed to remain at temperature long enough to ensure uniform temperature in thick sections. For maximum strength, hardness, and corrosion resistance, slowly heat the alloy to 1800° F (982° C) and quench to room temperature in oil. For thin sections, air cooling can be substituted for the oil quench. If retained austenite is known to be present after the austenitizing and quench to room temperature, additional hardening response may be achieved by sub-zero cooling to about –100° F (-73° C). The as-quenched structure of fresh martensite is quite brittle and should be stress-relieved or tempered at approximately 400 to 500° F (204 to 260° C) to restore some ductility. During tempering between approximately 300° F (149° C) and 600° F (316° C), a relaxation of the martensite structure occurs whereby the volumetric stresses associated with the formation of martensite upon quenching are relieved. As a result, the martensite still exhibits its high hardness and wear resistance properties but some ductility is introduced at the loss of a few pints of hardness.