A process carried out on fully hardened martensitic steel, which is very hard but also brittle and liable to cracking, in order to relieve the internal stresses caused by quenching and to toughen the steel. This is carried out by warming the steel for a time between approximately 100 and 300̊ C. A temperature of 100-200̊ C is sufficient to relieve internal stresses while leaving the martensite in an acicular and slightly less hard form, called tempered martensite. Although it resembles lower bainite, it is much harder. Between 200 and 300̊ C further decomposition to bainite takes place and above this and up to about 450̊ C decomposition into a precipitation of fine iron carbide particles in ferrite takes place. This structure is tougher but less hard than martensite. At even higher temperatures, 450-650̊ C, annealing takes place, when the carbide particles coalesce to produce fewer, larger particles in a spheroidal form often known as sorbite. In this condition the steel is yet tougher but less hard. A similar sub-critical annealing treatment is sometimes found to have been used on steels which have been air-cooled. The temperature ranges used to be gauged by colours.

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