1. Annealing of steel
Steel is heated to a certain temperature and kept warm for a period of time, and then allowed to cool slowly, which is called annealing. The annealing of steel is a heat treatment method that heats the steel to the temperature of phase transformation or partial phase transformation and then slowly cooling after holding. The purpose of annealing is to eliminate the microstructure defects, improve the structure, homogenize the composition and refine the grain, improve the mechanical properties of steel, and reduce the residual stress; At the same time, it can reduce hardness, improve elasticity, and toughness, and improve machining performance. Therefore, annealing is not only to eliminate and improve the organizational defects and internal stress left by the previous process but also to prepare for the subsequent process, so annealing is a semi-finished heat treatment, also known as a pre-heat treatment.
2. Normalizing steel
Normalizing is a heat treatment method in which the steel is heated above the critical temperature, all the steel is transformed into uniform austenite and then cooled naturally in the air. It can eliminate the mesh cementite of hypereutectoid steel, and normalize the lattice of hypereutectoid steel to improve the comprehensive mechanical properties. It is more economical to replace the annealing process with normalizing for parts with low requirements.
3. Quenching of steel
Quenching is the heat treatment method that heats the steel above the critical temperature, keeps it warm for a period of time, and then quickly puts it into the quenching agent so that the temperature suddenly decreases, and the speed of rapid cooling is greater than the critical cooling speed and obtains the unbalanced organization mainly with martensite. Quenching increases the strength and hardness of steel but reduces its plasticity. Commonly used quenching agents are: water, oil, alkali water, and salt solution.
4. Steel tempering
The hardened steel is reheated to a certain temperature and then cooled by a certain method called tempering. Its purpose is to eliminate the internal stress caused by quenching, reduce hardness and brittleness, and achieve the desired mechanical properties. Tempering is divided into high-temperature tempering, medium-temperature tempering, and low-temperature tempering. Tempering is often used with quenching and normalizing.
(1) Quenching and tempering treatment: the heat treatment method of high-temperature tempering after quenching is called quenching and tempering treatment. High-temperature tempering refers to the tempering between 500-650 ℃. Quenching and tempering can make the performance of steel, the material can be greatly adjusted, its strength, plasticity, and toughness are better, with good comprehensive mechanical properties.
The aging treatment: in order to eliminate the precision measuring tools or molds, parts in the long-term use of size and shape changes, often in low temperature tempering (low temperature tempering temperature 150-250 ℃) before finishing, the workpiece is reheated to 100-150 ℃, keep 5-20 hours, this is stable precision parts quality treatment, known as aging. The aging treatment of steel components under low temperature or dynamic load is particularly important to eliminate residual stress and stabilize steel structure and size.
5. Surface heat treatment of steel
(1) Surface quenching: is the surface of the steel rapidly heating to the critical temperature above, but the heat has not had time to spread to the heart before rapid cooling so that the surface layer can be quenched in the martensitic organization, and the heart has not undergone phase transformation, which realizes the purpose of surface hardening and the heart unchanged. Suitable for medium carbon steel.
(2) The chemical heat treatment: refers to the chemical element of the atom, with the ability of atomic diffusion at high temperature, 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 with specific requirements of the organization and performance of a heat treatment process. According to the kinds of infiltration elements, chemical heat treatment can be divided into carburizing, nitriding, cyanidation, and metal infiltration law.
Carburizing: Carburizing is the process by which carbon atoms penetrate the surface layer of steel. It is also to make the low carbon steel workpiece with high carbon steel surface layer, and then after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, and the central part of the workpiece still maintains the toughness and plasticity of low carbon steel.
Nitriding, or nitriding, is the process by which the surface layer of steel penetrates nitrogen atoms. The purpose is to improve the hardness and wear resistance of the surface layer and improve the fatigue strength and corrosion resistance. At present, the gas nitriding method is used in production.
Cyanidation, also known as carbonitriding, is the simultaneous infiltration of carbon and nitrogen atoms into steel. It makes the surface of steel carburizing and nitriding characteristics.
Metal penetration: refers to the penetration of metal atoms into the surface layer of steel. It is to make the surface layer of steel alloying, in order to make the workpiece surface has some alloy steel, and special steel characteristics, such as heat resistance, wear resistance, oxidation resistance, corrosion resistance, etc.. Commonly used in the production of aluminizing, chromizing, boronizing, silicon, and so on.
