Martempering is a heat treatment for steel involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the martensite start (Ms) point. Soaking must be long enough to avoid the formation of bainite. The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimises distortion.
Martempering is used to produce martensite without developing the high stresses that usually accompany its formation. It is similar to conventional hardening except that distortion is minimized. Nevertheless, the characteristic brittleness of the martensite remains in a gray iron casting after martempering, and martempered castings are almost always tempered. The casting is quenched from above the transformation range in a salt, oil, or lead bath: held in the bath at a temperature slightly above the range at which martensite forms (200 to 260°C or 400 to 500°F. for unalloyed irons) only until the casting has reached the bath temperature; and then cooled to room temperature.
If a wholly martensitic structure is desired, the casting must be held in the hot quench bath only long enough to permit it to reach the temperature of the bath. Thus, the size and shape of the casting dictate the duration of martempering.