What are the types of heat treatment? What does it do?
Heat treatment refers to a metal hot working process in which the material is in the solid state, through heating, heat preservation and cooling to obtain the desired structure and performance.
Heat treatment category:
1. Surface heat treatment:
A metal heat treatment process that changes the mechanical properties of the surface layer by heating and cooling the steel surface. Surface quenching is the main content of surface heat treatment. Its purpose is to obtain a high hardness surface layer and favorable internal stress distribution to improve the wear resistance and fatigue resistance of the workpiece.
2. Overall heat treatment:
The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties.
3.Chemical heat treatment:
Chemical heat treatment is a metal heat treatment process that uses chemical reactions, sometimes combined with physical methods to change the chemical composition and organizational structure of the steel surface, so as to obtain better technical and economic benefits than homogeneous materials.
Heating the steel to a certain temperature and holding it for a period of time, and then slowly cooling it is called annealing. Steel annealing is a heat treatment method that heats the steel to a temperature where a phase change or a partial phase change occurs, and then slowly cools down after heat preservation. The purpose of annealing is to eliminate the defects of the structure, improve the structure to make the composition uniform and refine the grain, improve the mechanical properties of the steel and reduce the residual stress; at the same time, it can reduce the hardness, improve the plasticity and toughness, and improve the cutting performance. Therefore, annealing is not only to eliminate and improve the tissue defects and internal stress left in the previous process, but also to prepare for the subsequent process, so annealing is a semi-finished product heat treatment, also known as pre-heat treatment.
Normalizing is a heat treatment method that heats the steel above the critical temperature, so that all the steel is transformed into uniform austenite, and then naturally cooled in the air. The purpose of normalizing: it can eliminate the network cementite of the eutectoid steel. For the normalizing of the subeutectoid steel, the lattice can be refined and the comprehensive mechanical properties can be improved. For the parts that are not demanding, the normalizing process is used instead of the annealing process. More economical.
Quenching is to heat the steel above the critical temperature, keep it warm for a period of time, and then quickly put it in the quenching agent to make its temperature suddenly decrease, and cool down rapidly at a speed greater than the critical cooling rate, and obtain an imbalance mainly dominated by martensite Organization heat treatment method. The purpose of quenching: to increase the strength and hardness of steel, but to reduce its plasticity. Quenching agents commonly used in quenching are: water, oil, alkaline water and salt solutions.
Reheating the quenched steel to a certain temperature and then cooling it by a certain method is called tempering. The purpose of tempering is to eliminate the internal stress generated by quenching, reduce the hardness and brittleness, and obtain the expected mechanical properties. Tempering is divided into three types: high temperature tempering, medium temperature tempering and low temperature tempering. Tempering is mostly used in combination with quenching and normalizing.
5. Tempering treatment: The heat treatment method of high temperature tempering after quenching is called quenching and tempering treatment. High temperature tempering refers to tempering between 500-650 ° C. The purpose of quenching and tempering: it can make the performance and material of steel be adjusted to a great extent, its strength, plasticity and toughness are better, and it has good comprehensive mechanical properties.
6. Aging treatment: In order to eliminate the change in size and shape of long-term use of precision measuring tools, molds and parts, it is often necessary to reheat the workpiece to 100- before finishing after low temperature tempering (low temperature tempering temperature 150-250 ° C) It is kept at 150 ℃ for 5-20 hours. This kind of treatment to stabilize the quality of precision parts is called aging. Aging treatment is carried out on steel components under low temperature or dynamic load conditions. The purpose of aging is to eliminate residual stress and stabilize the structure and size of the steel, which is particularly important.
7. Surface heat treatment
(1) Surface quenching: the surface of the steel part is quickly heated to above the critical temperature, but the heat can be quickly cooled before it reaches the core in the future, so that the surface layer can be quenched in the martensite structure, but the core does not A phase change occurs, which achieves the purpose of hardening the surface without changing the core. Suitable for medium carbon steel.
⑵, chemical heat treatment: refers to the atom of chemical elements, with the ability of atoms to diffuse at high temperature, to penetrate it into the surface layer of the workpiece to change the chemical composition and structure of the surface layer of the workpiece, so as to achieve the surface layer of steel A heat treatment process with specific requirements for organization and performance. According to different types of infiltrating elements, chemical heat treatment can be divided into four types: carburizing, nitriding, cyaniding and metalizing methods.
- Carburization: Carburization refers to the process of infiltrating carbon atoms into the steel surface layer. It is also to make the low-carbon steel workpiece have a high-carbon steel surface layer, which is then quenched and tempered at a low temperature, so that the surface layer of the workpiece has high hardness and wear resistance, and the central part of the workpiece still maintains the toughness and low carbon steel Plasticity.
- Nitriding: also known as nitriding, which refers to the process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to increase the hardness and wear resistance of the surface layer as well as to improve fatigue strength and corrosion resistance. At present, the gas nitriding method is mostly used in production.
- Cyanidation: also known as carbonitriding, which refers to the process of simultaneously infiltrating carbon atoms and nitrogen atoms in steel. It makes the steel surface have the characteristics of carburizing and nitriding.
- Metal infiltration: refers to the process of infiltrating the surface layer of steel with metal atoms. It is to alloy the surface layer of steel so that the surface of the workpiece has certain alloy steel and special steel characteristics, such as heat resistance, wear resistance, oxidation resistance and corrosion resistance. Commonly used in production are aluminizing, chromizing, boronizing, siliconizing, etc.
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