Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point and then quenched in a salt bath kept at a temperature of 150-300° C. The workpiece is held at this temperature above (Ms) until the temperature becomes uniform throughout the cross-section of workpiece. After that it is cooled in air or oil to room temperature. The steel is then tempered. 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.
In the martempering process, austenitized metal part is immersed in a bath at a temperature just above the martensite start temperature (Ms). By using interrupted quenching, the cooling is stopped at a point above the martensite transformation region to ensure sufficient time for the center to cool to the same temperature as the surface. The metal part is then removed from the bath and cooled in air to room temperature to permit the austenite to transform to martensite. Martempering is a method by which the stresses and strains generated during the quenching of a steel component can be controlled.
Martempering is used to produce martensite without developing the high stresses that usually accompany its formation. 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 only until the casting has reached the bath temperature; and then cooled to room temperature.