Tempering is the heat treatment process immediately after quenching. The tempering of quenched steel at different temperatures has different microstructure, so its mechanical properties vary greatly. The general trend is that with the tempering temperature increasing, its strength and hardness decrease, while plasticity and toughness are improved. Martensite and retained austenite in quenched steel are unstable structures, and the heating will change. With the increase of temperature, the carbon atoms gradually precipitate in the form of cementite, which leads to the transformation of structure. Finally, cementite polymerizes and disperses on ferrite matrix to form various tempering structures.