tempering

is the heat treatment technique for metals and alloys. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or ferrite. Precipitation hardened alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal.  In metallurgy, there is always a tradeoff between ductiliety and brittleness. This delicate balance highlights many of the subtleties inherent to the tempering process. Precise control of time and temperature during the tempering process are critical to achieve a metal with well balanced mechanical properties.

Typically steel is heat treated in a multi-step process. First it is heated to create a solid solution of iron and carbon in a process called austenizing. Austenizing is followed by quenching to produce a martensitic microstructure. The steel is then tempered by heating between the ranges of 300°F - 500°F (150°C - 260°C) and 700°F - 1200°F (370°C - 650°C). Tempering in the range of 500°F-700°F (260°C - 370°C) is sometimes avoided to reduce temper brittling.

 

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