Martempering

Martempering, also known as interrupted quenching, is a heat-treatment process for steel in which the material is austenitized and then quenched in a medium such as oil or molten salt that is maintained at a temperature just above the martensite start (Ms) temperature. The objective is to reduce thermal gradients and transform-induced stresses that cause distortion, cracking, or warping during rapid quenching.

In practice, the steel is heated to the austenitizing temperature and immersed in the quench bath kept slightly above Ms. The part is held there until the temperature throughout its cross-section becomes uniform. Once thermal equilibrium is achieved, the workpiece is removed and allowed to cool in air (continuing through the martensitic transformation as the temperature falls below Ms). A tempering treatment is commonly applied afterward to achieve the desired hardness and toughness.

The intended microstructure after martempering is martensite with minimized internal stresses and distortion compared with conventional quenching. The method is particularly useful for medium and large sections and for steels sensitive to cracking or distortion when quenched rapidly. It may require careful control of bath temperature and cycle time and is more equipment-intensive than simple oil or water quenching. Applications include automotive, tooling, and structural steels where dimensional stability and surface integrity are important.