Heat treatment is one of the important processes in the manufacture of mechanical parts and molds.It is an important industrial process used to change the physical and mechanical properties of materials, primarily metals and alloys. This process involves controlling heating and cooling to achieve specific changes in material properties such as hardness, strength, toughness and durability. Heat treatment is widely used and is an integral part of various industries such as automotive, aerospace, manufacturing and electronics.In general, it can ensure and improve the various properties of the workpiece, such as wear resistance, corrosion resistance and so on. It can also improve the structure and stress state of the blank to facilitate a variety of cold and hot processing.
Heat treatment is a metal material placed in a certain medium heating, insulation, cooling, by changing the surface of the material or the internal crystal phase structure, to change its performance of a metal hot working process.The process can be roughly divided into three categories: integral heat treatment, surface heat treatment and chemical heat treatment. According to the heating medium, heating temperature and cooling method, each category can be divided into a number of different heat treatment processes.Heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment, the earliest is the use of charcoal and coal as heat sources, and then the application of liquid and gas fuel. The application of electricity makes heating easy to control, and no environmental pollution. These heat sources can be used for direct heating, or indirect heating through molten salts or metals, and even floating particles.When the metal is heated, the workpiece is exposed to the air, and oxidation and decarbonization often occur (that is, the surface carbon content of steel parts is reduced), which has a very adverse effect on the surface properties of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.
The heating temperature is one of the important parameters of the heat treatment process, and the selection and control of the heating temperature is the main problem to ensure the quality of the heat treatment. The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but it is generally heated above the phase transition temperature to obtain the desired organization. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must also be maintained at this temperature for a certain time, so that the internal and external temperature is consistent, so that the microstructure is completely transformed, this period of time is called the holding time. When using high energy density heating and surface heat treatment, the heating speed is very fast, generally there is no holding time or holding time is very short, and the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process, and the cooling method varies according to the process, mainly to control the cooling speed. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are different requirements due to different types of steel, for example, air-hard steel can be hardened with the same cooling rate as normalizing.
Metal heat treatment process can be roughly divided into integral heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to the heating medium, heating temperature and cooling method, each category can be divided into a number of different heat treatment processes. The same metal using different heat treatment processes, can obtain different structures, and thus have 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 steel heat treatment processes.
The overall heat treatment is divided into normalizing, annealing, quenching, tempering, tempering, stabilization treatment, solid solution treatment, water toughening treatment, failure treatment. Among them, normalizing, annealing, quenching and tempering are called the "four fires" in heat treatment. The main methods of surface heat treatment are flame quenching and induction heat treatment. Chemical heat treatment is mainly divided into carburizing, nitriding, carbonitriding and so on.