The role of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metal. According to different purposes of heat treatment, heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment. The purpose of preliminary heat treatment is to improve machining properties, eliminate internal stress and prepare the microstructure for final heat treatment. The difference between normalizing, tempering, annealing and quenching is that the process is different, the material structure changes are different, and the material properties change results are different.

Normalizing is heating the workpiece to Ac3 (Ac refers to the heating free ferrite into austenite terminal temperature, generally from 727℃ to 912℃) or Acm (Acm is the actual heating eutectoid steel complete austenitizing critical temperature line) above 30~50℃, after a period of heat, Heat treatment of metals taken from a furnace and cooled in air or by water, spray, or blow.

Tempering is the cooling of hardened or normalized steel at a rate after immersion below the critical temperature for a period of time.

Annealing is a heat treatment process in which the metal is heated to a certain temperature, held for a sufficient time, and then cooled at a suitable rate (usually slow cooling, sometimes controlled cooling).

Quenching is the heat treatment process of heating steel to the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel) above temperature, holding for a period of time, so that all or part of the austenitizing, and then with a cooling rate greater than the critical cooling rate of fast cooling to below Ms (or near Ms isothermal) martensite (or bainite) transformation.

The properties of heat-treated materials change differently:

When normalizing, the steel crystal grain can be refined during slightly faster cooling, which can not only obtain satisfactory strength, but also significantly improve the toughness (AKV value) and reduce the cracking tendency of members. Some low alloy hot rolled steel plates, low alloy steel forgings and castings are normalized, the comprehensive mechanical properties of the materials can be greatly improved, and the cutting performance can also be improved.

After tempering, some carbides can be precipitated and some residual stress caused by rapid cooling can be eliminated. Therefore, the toughness and flexibility of the material can be improved.

Annealing can reduce hardness and improve machinability. Eliminate residual stress, stabilize size, reduce deformation and crack tendency; Refine grain, adjust microstructure, eliminate microstructure defects. Uniform material structure and composition, improve material properties or prepare for future heat treatment.

Quenching can greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel, so as to meet the different requirements of various mechanical parts and tools. It can also meet the special physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels by quenching.