Austempering

This is the method that can be used to overcome the restrictions of conventional quench and tempering. The quench is interrupted at a higher temperature than for martempering to allow the metal at the centre of the part to reach the same temperature as the surface. By maintaining that temperature, both the centre and the surface are allowed to transform to bainite and are then cooled to room temperature.

The austempering heat treatment consists of three steps: austenitization in the temperature range of 840–950°C for a time sufficient to produce fully austenitic matrix, rapid cooling of the entire part to an austempering temperature in the range of 230–450°C without any transformations, and isothermal treatment at the austempering temperature, at which during the transformation only bainitic ferrite forms in a favourable case.

Advantages of austempering are less distortion and cracking than martempering, no need for final tempering, improvement of toughness, and improved ductility. Limitations of austempering are that the austempering can be applied to parts where the transformation to pearlite can be avoided. This means that the section must be cooled fast enough to avoid the formation of pearlite. Thin sections can be cooled faster than the bulky sections.