The properties of steel depend not only on the chemical composition but also on the metallographic structure. Through the process of heating, holding and cooling to get different metallographic structure and desired property is heat treatment.

Common heat treatments include normalizing, quenching, tempering, and annealing.

Normalizing treatment: The steel is heated to 30 ° C ~ 50 ° C above the AC3 line, after the heat preservation, cooling in the air.
Quenching: The steel is heated to 30 ° C ~ 50 ° C above the AC3 line. After heat preservation, it is cooled quickly (in water or oil) to obtain high strength and hardness.
Tempering treatment: heating the steel below AC1, after heat preservation, cooling down (cooling in the air).
Quenching (Q) and normalizing (N) are used to increase the strength of the steel. After quenching or normalizing, the tempering (T) is used to eliminate stress, improve the structure, and improve the toughness. Annealing treatment: heating the steel to AC3, or 30 ° C ~ 50 ° C above the AC1 line, after heat preservation, slowly (usually with the furnace) cooling. Annealing is usually used to eliminate residual relief stress or cold work hardening and so on.