Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterruptible. Metal heat treatment process can be roughly divided into integral heat treatment, surface heat treatment and chemical heat treatment three categories.

Bulk heat treatment

Integral 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. The overall heat treatment of steel is roughly annealing, normalizing, quenching and tempering four basic processes.

Annealing treatment:

Annealing is a metal heat treatment process in which metal parts are heated to a certain high temperature, maintained for a period of time, and then allowed to cool naturally.

Normalizing treatment

Normalizing treatment is to heat the metal parts to a certain high temperature, keep it for a period of time, and then let it cool in the air by spraying water, spraying, blowing, etc., which is different from annealing treatment, its cooling speed is faster, so the obtained material organization is finer, and the mechanical properties are also improved.

Quenching treatment:

Quenching treatment is a heat treatment process in which metal parts are heated to a temperature above the critical temperature Ac3 or Ac1, maintained for a period of time, so that all or part of it is austenitized, and then cooled to the following martensitic transformation at a cooling rate greater than the critical cooling rate.

Tempering treatment:

Tempering treatment is a heat treatment method that the steel after quenching hardening or normalization treatment is then dipped below the critical temperature for a period of time, and cooled down at a certain speed to increase the toughness of the material.

Annealing, normalizing, quenching and tempering are the "four fires" in the overall heat treatment, of which quenching and tempering are closely related and often used together.

Surface heat treatment

Surface heat treatment is a metal heat treatment process that only heats the surface layer of the workpiece to change the mechanical properties of the surface layer.

In order to heat only the surface of the workpiece without too much heat passing into the interior of the workpiece, the heat source used must have a high energy density, that is, give a large amount of heat energy on the workpiece per unit area, so that the surface or local part of the workpiece can reach a high temperature for a short time or instantaneously.

The main methods of surface heat treatment are flame quenching and induction heat treatment, commonly used heat sources such as aerobic acetylene or oxypropane flame, induced current, laser and electron beam.

Chemical heat treatment

Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and properties of the workpiece surface. Chemical heat treatment differs from surface heat treatment in that the latter changes the chemical composition of the surface layer of the workpiece.

Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements, and hold heat for a long time, so that the surface of the workpiece penetrates carbon, nitrogen, boron and chromium and other elements.

After infiltrating elements, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding and metallizing.

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