Explain the Purposes of tempering
Tempering is a heat treatment approach for alloys and metals. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of cementite and ferrite. Precipitation hardening alloys, such as numerous grades of aluminum and super alloys, are tempered to precipitate intermetallic particles that strengthen metal.
Brittle martensite becomes strong and ductile after it is tempered. Carbon atoms were trapped in the austenite when it was rapidly cooled, typically by water or oil quenching, forming martensite. Martensite becomes strong after being tempered as when reheated, microstructure can rearrange and the carbon atoms can diffuse out of distorted BCT structure. After carbon diffuses, result is nearly pure ferrite.
In metallurgy, there is always a trade-off between brittleness and ductility. This delicate balance highlights many of the subtleties inherent to tempering process. Precise control of time and temperature during the tempering process are critical to achieve a metal with well-balanced mechanical properties.