Metal heat treatment technology can be roughly 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 technologies. Different heat treatment techniques are used for the same metal to obtain different arrangements and then have different functions. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are various types of steel heat treatment technologies.

1. Overall heat treatment

Overall heat treatment is a metal heat treatment technology that heats the entire workpiece and then cools it at an appropriate rate to change its overall mechanical properties. The overall heat treatment of steel generally has four basic techniques: annealing, normalizing, quenching and tempering.

Annealing is to heat the workpiece to an appropriate temperature, select different holding times according to the material and the size of the workpiece, and then slowly cool it, the purpose is to make the internal structure of the metal reach or approach the equilibrium state, achieve excellent technical performance and application performance, or to further Quenching for arrangement preparation. Normalizing 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, but the resulting arrangement is finer. It is often used to improve the cutting performance of the material, and sometimes it is used for some parts with low demand. Heat treated as a finish. Quenching is to rapidly cool the workpiece in a quenching medium such as water, oil or other inorganic salts, organic aqueous solutions, etc. after heating and maintaining the workpiece. After quenching, the steel becomes hard, but also becomes brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature but lower than 650 ℃, and then cooled. This technique is called tempering. Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment. The quenching and tempering are closely related, and they are often used together and are indispensable. The "four fires" have evolved different heat treatment technologies with different heating temperatures and cooling methods. In order to obtain a certain strength and resistance, the technology 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 kept 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 technique is called aging treatment. The method of effectively and closely combining pressure deformation and heat treatment to make the workpiece achieve excellent strength and tolerance is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only makes The workpiece is not oxidized or decarburized, and the surface of the workpiece is kept clean after treatment, which improves the performance of the workpiece, and can also be chemically heat treated with a penetration agent.

2. Surface heat treatment

Surface heat treatment is a metal heat treatment technology that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to only heat the surface layer of the workpiece without allowing too much heat to be transferred into the workpiece, the heat source used must have a high energy density, that is, a larger amount of heat energy is given to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous. reach high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. Commonly used heat sources are flames such as oxyacetylene or oxypropane, induced current, laser and electron beam.

3. Chemical heat treatment

Chemical heat treatment is a metal heat treatment technology that changes the chemical composition, structure and function of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements, keep it for a long time, and then make the surface of the workpiece enter the elements such as carbon, nitrogen, boron and chromium. After entering the elements, other heat treatment techniques such as quenching and tempering are sometimes performed. The main methods of chemical heat treatment are carburizing, nitriding, and metalizing.

Heat treatment is one of the important processes in the production of mechanical parts and tools. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It is also possible to improve the arrangement and stress conditions of the blank to facilitate various cold and hot processes. For example, white cast iron can obtain malleable cast iron through long-term annealing treatment to improve plasticity; gears adopt the correct heat treatment technology, and the service life can be doubled or dozens of times longer than that of gears without heat treatment; Entering certain alloying elements has the properties of some expensive alloy steels, which can replace some heat-resistant steels and stainless steels; almost all tools and dies need to be heat treated before they can be used.