Quenching and tempering is a heat treatment process that is mainly used to improve the mechanical properties of steel. It consists of two main steps: quenching and tempering. During quenching, the austenite in the steel is transformed into martensite, which increases hardness and strength. During the tempering process, the quenched martensitic is transformed into a tempered structure, which further improves the toughness and plasticity of the steel. 

Quenching: The steel parts are heated to the austenite forming zone, usually 800-900 degrees Celsius, and after holding for a period of time, they are quickly cooled to room temperature. During the quenching process, austenite gradually forms and replaces the original structure.

Tempering: Reheat the quenched steel to the appropriate temperature, usually 500-700 degrees Celsius, keep it warm for a period of time, and then slowly cool it to room temperature. During the tempering process, the martensitic decomposes to form a tempering structure that improves the toughness and plasticity of the steel.