Metal heat treatment processes of classification

Metal heat treatment process can be generally divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different heat treatment processes can be used for the same metal to obtain different structures and different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of heat treatment process.

The whole heat treatment is a metal heat treatment process that heats the whole workpiece and cools it at a proper speed to obtain the required metallographic structure and change its whole mechanical properties. There are four basic heat treatment processes of iron and steel: annealing, normalizing, quenching and tempering.

Process means:

Annealing is to heat the workpiece to a proper temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly, so as to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and use performance, or make organization preparation for further quenching.

Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the microstructure obtained is finer, which is often used to improve the cutting performance of materials, and sometimes used for the final heat treatment of parts with low requirements.

Quenching is the rapid cooling of workpiece in water, oil or other inorganic salt, organic water solution and other quenching media after heating and heat preservation. After quenching, the steel becomes hard but brittle at the same time. In order to eliminate the brittleness in time, it is generally necessary to temper in time.

In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at a proper temperature higher than room temperature but lower than 650 ℃, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the "four fires" in the whole heat treatment, in which quenching and tempering are closely related and often used together. With the different heating temperature and cooling mode, the "four fire" has evolved different heat treatment processes. In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called tempering. After some alloys are quenched to form supersaturated solid solution, they are kept at room temperature or a higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. Such heat treatment process is called aging treatment.

The effective and close combination of pressure processing deformation and heat treatment is called deformation heat treatment, which makes the workpiece obtain good strength and toughness matching. The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only prevent the workpiece from oxidation and decarburization, maintain the surface of the workpiece after treatment, improve the performance of the workpiece, but also penetrate the penetrant Conduct chemical heat treatment.

Surface heat treatment is a metal heat treatment process that only heats the surface of workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece without too much heat entering the interior of the workpiece, the heat source used must have a high energy density, that is, to give a larger heat energy to the workpiece per unit area, so that the surface layer or part of the workpiece can reach a high temperature in a short time or instantaneously. 

The main methods of surface heat treatment are flame quenching and induction heating heat treatment. The commonly used heat sources are flame, induction current, laser and electron beam, etc.

Chemical heat treatment is a metal heat treatment process by changing the chemical composition, structure and performance of the workpiece surface. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of workpiece surface. Chemical heat treatment is to heat the workpiece in the medium (gas, liquid, solid) containing carbon, salt or other alloy elements for a long time, so that the surface of the workpiece infiltrates carbon, nitrogen, boron, chromium and other elements. After infiltration of 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.

Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools. Generally speaking, it can guarantee and improve various properties of workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank, so as to facilitate all kinds of cold and hot processing.

For example: white cast iron can be forged and plasticity can be improved after long-time annealing treatment; gear with correct heat treatment process can have service life times or dozens of times longer than gear without heat treatment; in addition, low-cost carbon steel can replace some heat-resistant steel and stainless steel by infiltrating some alloying elements to have some valuable alloy steel properties; mold Almost all of them need heat treatment before they can be used.