Austempering

Austempering:

This is a special heat treatment process in which austenite is transformed to bainite. Austempering consists of heating the steel above the austenitizing temperature. It is then quenched in a bath maintained at a temperature above the martensite forming point, within the bainitic range- between 200-400— C. the steel is quenched in and maintained the constant temperature in the bath itself till all the austenite is transformed to bainite. Once transformation is completed the steel part is taken out of the bath and cooled in air at any desired rate.  Since the transformation takes place at constant temperature, the process is referred as isothermal quenching or isothermal hardening. In order to form lower bainite, which has superior mechanical properties compared with tempered martensite, the temperature preferred is generally on the lower side of the bainitic region.

The process results in a better ductility at high hardness levels, improved impact and fatigue strength and freedom from distortion.  Since the transformation takes place at

                   

constant temperature through out the section the structure formed is homogenized through out the section, and so are the properties. Cooling rate and holding time are two important factors in this process. It is essential that the steel have to be cooled at such a high rate so that austenite to pearlite transformation does not take place. So the cooling rate is more than the upper critical cooling rate and this imposes restriction on chemical composition and size. Hence only comparatively thinner sections can be austempered successfully