Martempering:
This process also involves heating the steel up to austenitizing region followed by quenching in a bath maintained above the MS point. Normally between 180 and 250— C. steel is kept in the bath till the temperature through out the section equals the temperature of the bath. As soon as this temperature is achieved the steel part is taken out of the bath and cooled in air. This cooing rate should be sufficiently high and holding time considerably low to prevent pearlite to bainite transformation. Hence martensite is formed in the second stage of cooling. Process results in minimum internal stresses reduced tendency toward distortion and cracking and improved mechanical properties, compared with conventional quenching and tempering method. In order to improve the properties, martempered pieces are generally subjected to tempering.
Alloy steels are best suited for this process. Any steel that can be oil quenched can be martempered successfully. A large number of steels satisfy this requirement. Presence of alloying elements increase the incubation period and this provide sufficient holding time.
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