Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment

Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating media, heating temperature and cooling methods, each category can be divided into several different heat treatment processes. The same metal can obtain different structures and thus have different properties by using different heat treatment processes. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many types of steel heat treatment processes.

1 Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to change its overall mechanical properties. There are roughly four basic processes for overall heat treatment of steel: annealing, normalizing, quenching and tempering. Annealing, normalizing, quenching and tempering are the “four fires” in overall heat treatment, among which quenching and tempering are closely related and are often used in combination, and neither is indispensable.

Annealing: heating the workpiece to an appropriate temperature, using different holding times according to the material and workpiece size, and then slowly cooling, the purpose is to make the internal structure of the metal reach or approach a state of equilibrium, obtain good process performance and performance, or prepare for further quenching.

Normalizing: heating the workpiece to an appropriate temperature and cooling it in air. The effect of normalizing is similar to annealing, but the resulting structure is finer. It is often used to improve the cutting performance of low-carbon materials, and is sometimes used as the final heat treatment for some parts with low requirements.

Quenching is to heat and keep the workpiece warm, and then quickly cool it in a quenching medium such as water, oil or other inorganic salts, organic aqueous solutions. After quenching, the steel becomes hard, but also brittle.

Tempering: In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature above room temperature and below 650℃, and then cooled.

“Four fires” have evolved into different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering.
After some alloys are quenched to form a supersaturated solid solution, they are placed at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical and magnetic properties of the alloy. Such a heat treatment process is called aging treatment.
The method of effectively and closely combining pressure processing deformation with heat treatment to obtain a good combination of strength and toughness of the workpiece is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only prevents the workpiece from oxidation and decarburization, keeps the surface of the workpiece smooth after treatment, and improves the performance of the workpiece, but also allows the infiltrant to be passed through for chemical heat treatment.

2 Surface heat treatment
A metal heat treatment process that only heats the surface of the workpiece to change its surface mechanical properties. In order to heat only the surface of the workpiece without transferring too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a large amount of heat energy per unit area of ​​the workpiece 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 heating heat treatment. Commonly used heat sources include flames such as oxyacetylene or oxypropane, induction current, laser and electron beam.

3 Chemical heat treatment
A metal heat treatment process that changes the chemical composition, structure and properties of the workpiece surface. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the workpiece surface. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloy elements and keep it warm for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes required. The main methods of chemical heat treatment are carburizing, nitriding and metallizing.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. It can control various properties of the workpiece, such as wear resistance, corrosion resistance, magnetic properties, etc. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing. For example, white cast iron can be annealed for a long time to obtain malleable cast iron, which improves its plasticity. The service life of gears can be increased by several times or even dozens of times compared with gears that have not been heat treated by using the correct heat treatment process. In addition, cheap carbon steel can have some of the properties of expensive alloy steels by infiltrating certain alloy elements, and can replace some heat-resistant steels and stainless steels. Almost all tools and dies need to be heat treated before they can be used.