Knowledge of heat treatment of steel

Knowledge of heat treatment of steel
1. Annealing of steel

Heating steel to a certain temperature and holding it for a period of time, and then allowing it to slowly cool down, is called annealing. Annealing of steel is a heat treatment method in which steel is heated to a temperature at which phase change or partial phase change occurs, and then slowly cooled after heat preservation. The purpose of annealing is to eliminate structural defects, improve the structure, homogenize the composition and refine the grain, improve the mechanical properties of the steel, reduce residual stress; at the same time, it can reduce the hardness, increase the plasticity and toughness, and improve the cutting performance. Therefore, annealing is not only to eliminate and improve the structural defects and internal stress left by 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.

2. Normalizing of steel

Normalizing is a heat treatment method in which the steel is heated to above the critical temperature, so that the steel is transformed into uniform austenite, and then cooled naturally in the air. It can eliminate the network cementite of hypereutectoid steel, and can refine the crystal lattice for normalizing of hypoeutectoid steel and improve the comprehensive mechanical properties. It is more economical to replace the annealing process with normalizing for less demanding parts.

3. Quenching of steel

Quenching is to heat the steel to above the critical temperature, keep it for a period of time, and then quickly put it in the quenching agent to make its temperature suddenly drop, and then rapidly cool it at a rate greater than the critical cooling rate to obtain an imbalance dominated by martensite The heat treatment method of the organization. Quenching can increase the strength and hardness of steel, but reduce its plasticity. Quenching agents commonly used in quenching are: water, oil, alkaline water and salt solutions.

4. Tempering of steel

Reheating the quenched steel to a certain temperature and then cooling it by a certain method is called tempering. Its purpose is to eliminate the internal stress caused by quenching, reduce the hardness and brittleness, in order to obtain the expected mechanical properties. There are three types of tempering: high temperature tempering, medium temperature tempering and low temperature tempering. Tempering is mostly used in conjunction with quenching and normalizing.

⑴ 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 tempering between 500-650℃. Quenching and tempering can make the performance and material of steel be adjusted to a large extent, and its strength, plasticity and toughness are good, and it has good comprehensive mechanical properties.

⑵Aging treatment: In order to eliminate the size and shape changes of precision measuring tools or molds and parts during long-term use, the workpiece is often reheated to 100-150 after low temperature tempering (low temperature tempering temperature 150-250℃) before finishing. ℃, keep for 5-20 hours, this kind of treatment to stabilize the quality of precision parts is called aging. It is especially important to perform aging treatment on steel components under low temperature or dynamic load conditions to eliminate residual stress and stabilize steel structure and size.

5. Surface heat treatment of steel

⑴Surface quenching: the surface of the steel is rapidly heated to above the critical temperature, but the heat is quickly cooled before the heat can be transmitted to the core, so that the surface layer can be quenched in the martensite structure, and the core does not occur Phase change, which achieves the purpose of surface hardening and the core unchanged. Suitable for medium carbon steel.

⑵Chemical heat treatment: refers to the atom of chemical elements, with the help of the ability of atom diffusion at high temperature, to infiltrate the surface layer of the workpiece to change the chemical composition and structure of the surface layer of the workpiece, so as to achieve a specific surface layer of steel A heat treatment process with required organization and performance. According to the different types of infiltrating elements, chemical heat treatment can be divided into four types: carburizing, nitriding, cyaniding and metalizing.

Carburizing: Carburizing refers to the process of infiltrating carbon atoms into the surface layer of steel. It also makes the low-carbon steel workpiece have a surface layer of high-carbon steel. After quenching and low-temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the center part of the workpiece still maintains the toughness and toughness of low-carbon steel. Plasticity.

Nitriding: Also known as nitriding, it is the process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve the hardness and wear resistance of the surface layer, and to improve the fatigue strength and corrosion resistance. At present, gas nitriding is mostly used in production.

Cyanation: Also known as carbonitriding, it refers to the process of simultaneously infiltrating carbon and nitrogen atoms into 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 alloys the surface layer of steel so that the surface of the workpiece has the characteristics of certain alloy steels and special steels, such as heat resistance, wear resistance, oxidation resistance, and corrosion resistance. Commonly used in production are aluminizing, chromizing, boronizing, siliconizing, etc.