Quenching and tempering refer to a heat treatment process used to improve the comprehensive mechanical properties of steel materials. That is, after quenching, it is subjected to high-temperature tempering treatment. The purpose is to make the steel parts obtain a good combination of strength and toughness, which not only has high strength, but also has excellent toughness, plasticity, and cutting performance. Some alloy steels or low alloy steels must be quenched and tempered to obtain good comprehensive properties, and these steels can also be called quenched and tempered steels. After quenching and tempering treatment of ductile iron, it can not only improve the strength but also improve the hardness and wear resistance.
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Introduction
Quenching and tempering treatment refers to the double heat treatment method of quenching and high-temperature tempering, and its purpose is to make the workpiece have good comprehensive mechanical properties. High-temperature tempering refers to tempering between 500-650 °C. Most of the quenched and tempered parts work under relatively large dynamic loads. They are subjected to tension, compression, bending, torsion, or shearing. Some surfaces also have friction, requiring certain wear resistance and so on.
In summary, parts operate under various compound stresses. Such parts are mainly structural parts of various machines and mechanisms, such as shafts, connecting rods, studs, gears, etc., which are widely used in the manufacturing industries such as machine tools, automobiles, and tractors. Especially for large components in heavy machine building, the tempering treatment is used more. Therefore, the quenching and tempering treatment occupies a very important position in the heat treatment.
The quenched and tempered parts in mechanical products have different performance requirements because of their different stress conditions. Generally speaking, all kinds of quenched and tempered parts should have excellent comprehensive mechanical properties, that is, the appropriate combination of high strength and high toughness, to ensure the long-term smooth operation of the parts.
Steps and Principles
Quenching is the first step, the heating temperature depends on the composition of the steel, and the quenching medium is selected according to the hardenability of the steel and the size of the steel parts. After quenching, the steel has large internal stress and is very brittle, and must be tempered to eliminate stress, increase the toughness and adjust strength.
Tempering is the most important process to finalize the mechanical properties of quenched and tempered steel. The curve of the mechanical properties of various steels changing with the tempering temperature, also known as the tempering curve of the steel, can be used as the basis for selecting the tempering temperature. For the high-temperature tempering of some alloy quenched and tempered steels, care should be taken to prevent the occurrence of the second type of temper brittleness to ensure the performance of the steel.
Application
Quenching and tempering treatment is widely used in structural parts that require excellent comprehensive performance, especially under alternating loads, such as automobile shafts, gears, turbine shafts of aero-engines, compressor discs, etc. Structural steel parts that need induction heating quenching are usually quenched and tempered before surface quenching to obtain fine and uniform sorbite, which is beneficial to the surface hardened layer and also enables the core to obtain good comprehensive mechanical properties.
Nitriding parts are quenched and tempered before nitriding, which can improve the processing properties of steel and prepare the structure for nitriding. To make the measuring tool get a higher finish before quenching, eliminate the stress caused by rough machining, reduce the quenching deformation, and make the hardness after quenching high and uniform, it can be quenched and tempered before finishing.
For tool steels with reticulated carbides or coarse grains after forging, quenching and tempering can be used to eliminate the carbide network and refine the grains, and spheroidize the carbides to improve the machinability and prepare the structure for the final heat treatment.
Processing technology
The heat treatment process of steel includes methods such as annealing, normalizing, quenching, tempering, and surface heat treatment. Among them, tempering includes high-temperature tempering, medium-temperature tempering, and low-temperature tempering.
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 generated by quenching, reduce hardness and brittleness, and achieve the expected mechanical properties.
Quenching and tempering usually refer to the heat treatment process of quenching + high-temperature tempering to obtain tempered sorbite. The method is to quench first, and the quenching temperature: hypoeutectoid steel is Ac3+30~50℃; hypereutectoid steel is Ac1+30~50℃; alloy steel can be slightly higher than carbon steel. After quenching, it can be tempered at 500~650℃.
Quenched and tempered steel
Generally, the so-called quenched and tempered steel refers to medium carbon steel with a carbon content of 0.3-0.6%. Parts made of this type of steel require good comprehensive mechanical properties, that is, good plasticity and toughness while maintaining high strength. People often use modulation treatment to achieve this purpose, so people are accustomed to This type of steel called quenched and tempered steel. It should be noted that sometimes quenched and tempered steel also refers to the process of adding manganese and silicon when smelting steel, so pay attention to the difference.
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