Complementary heat treatment to quenching that is performed to increase the toughness in detriment of reducing the hardness, resistance and yield strength after quenching. The internal tensions produced in the quenching are also diminished, the structure is homogenized and the final properties required by the piece in service are achieved.
It consists of a heating process at a subcritical temperature and a subsequent controlled cooling generally to the air. The higher the tempering temperature, the higher the toughness and the lower the hardness.
In high alloy steel it may occur that after tempering the hardness increases due to the transformation of residual austenite into martensite.