Hardening is the collective name for the thermal methods used to create a martensitic or bainitic structure in order to increase the hardness or strength of a steel. The thermal process entails heating the steel to hardening temperature (austenitizing) followed by quenching in a suitable quenchant, determined by the hardenability of the steel and the dimensions of the workpiece. Quenching can be conducted either as direct hardening, martempering or austempering. Direct hardening is the most common form. Hardening to a martensitic condition is normally followed by tempering.

Ref. Steel and its Heat Treatment – A Handbook (8.3)