Annealing, normalizing, quenching and tempering are the four basic processes in overall heat treatment, which are called the "four fires". Among them, quenching and tempering are closely related and are often used together and are indispensable. Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties.

Annealing: It is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and workpiece size, and then slowly cool it. The purpose is to make the internal structure of the metal reach or approach an equilibrium state and obtain good process performance and usability performance, or prepare the tissue for further quenching.

Purpose of annealing:

(1) Reduce hardness and improve cutting processability.

(2) Eliminate residual stress, stabilize dimensions, and reduce deformation and crack tendencies;

(3) Refine the grains, adjust the structure, and eliminate structural defects.

(4) Uniform material structure and composition, improve material properties or prepare the structure for subsequent heat treatment.

Normalizing: it is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, except that the obtained structure is finer. It is often used to improve the cutting performance of materials, it is also sometimes used as the final heat treatment for some parts with lower requirements.

The purpose of normalizing:

(1) Remove the internal stress of the material;

(2) Reduce the hardness of the material and improve the plasticity.

Quenching: after the workpiece is heated and kept warm, it is quickly cooled in a quenching medium such as water, oil or other inorganic salts, organic aqueous solutions. After quenching, the steel becomes hard but at the same time becomes brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept for a long time at an appropriate temperature above room temperature and below 710°C, and then cooled. This process is called tempering.

The purpose of quenching:

(1) Increase its hardness and wear resistance;

(2) Adjust the organizational structure of steel to give it better overall performance;

(3) Improve the corrosion resistance of steel;

Tempering: After the steel parts are hardened, they are heated to a certain temperature below the critical temperature, kept for a certain period of time, and then cooled to room temperature. The heat treatment process is called tempering. Generally, quenched steel parts cannot be used directly and must be tempered before they can be used. Because quenched steel has high hardness and brittleness, brittle fracture often occurs when used directly. Tempering can eliminate or reduce internal stress, reduce brittleness, and improve toughness; on the other hand, the mechanical properties of quenched steel can be adjusted to achieve the performance of the steel. According to the different tempering temperatures, tempering can be divided into three types: low temperature tempering, medium temperature tempering and high temperature tempering.

Purpose of tempering:

(1) Eliminate the residual stress generated during quenching of the workpiece and prevent deformation and cracking;

(2) Adjust the hardness, strength, plasticity and toughness of the workpiece to meet performance requirements;

(3) Stabilize the structure and size and ensure accuracy;

(4) Improve and enhance processing performance.