Martempering is the process of interrupted quenching from austenitizing temperature in stainless steels, alloys, cast and tools. This is done to delay cooling just above the transformation for a length of time to equalize the temperature throughout the piece. Thus by reducing the cracking, distortion and residual stress. The microstructure is usually martensitic that is untampered and brittle.
Martempering of steel is generally made by
- Quench from a suitable austenitizing temperature through a liquid medium (oil, molten salt, molten metal) at a temperature above the martensite range.
- Holding in quenching medium until the temperature is substantially uniform.
- Cooling through atmospheric air at a moderate rate to prevent large differences in temperature from outside and center of the section.
The martensite occurs uniformly throughout the work piece at the time of cooling at room temperature by avoiding formation of excessive amounts of residual stress. With the help of marquenching bath, straightening or forming is accomplished. The material retains its shape on subsequent cooling in fixture or on air cooling after removal of forming die.
- There is less thermal gradient between the center part and surface due to quenching in isothermal temperature and cooled to room temperature.
- The residual stress is lower than the conventional quenching due to thermal variations of steel in plastic austenitic condition and final transformation and thermal changes occurring on the complete part at the same time.
- Martempering reduces distortion, residual stress and also eliminates cracking.