Martempering

Martempering or marquenching permits the transformation of austenite to martensite to take place at the same time throughout the structure of the metal part. By using interrupted quench, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the centre to cool to the same temperature as the surface. Then cooling is continued through the martensite region followed by the usual tempering.

Martempering of steel (and of cast iron) consists of quenching from the austenitizing temperature into a hot fluid medium (hot oil, molten salt, molten metal, or a fluidized particle bed) at a temperature usually above the martensite range (M_s point), holding in the quenching medium until the temperature throughout the steel is substantially uniform, and cooling (usually in air) at a moderate rate to prevent large differences in temperature between the outside and the centre of the section.

The advantage of martempering lies in the reduced thermal gradient between surface and centre as the part is quenched to the isothermal temperature and then is air cooled to room temperature. Residual stresses developed during martempering are lower than those developed during conventional quenching. Martempering also reduces or eliminates susceptibility to cracking. Another advantage of martempering in molten salt is the control of surface carburizing or decarburizing.