Austempering / Bainitic hardening
Austempering consists of rapidly cooling the metal part from the austenitizing furnace temperature to about 230 to 400 °C (depending on the transformation characteristics of the particular steel involved), holding at a constant temperature to allow isothermal transformation, followed by air cooling. Austempering is applicable to most medium-carbon steels and alloy steels. Low-alloy steels are usually restricted to 9.5mm or thinner sections, while more hardenable steels can be austempered in sections up to 50 mm thick.
Molten salt baths are usually the most practical for austempering applications. Oils have been developed that suffice in some cases, but molten salts possess better heat-transfer properties and eliminate the fire hazard.