Quenching is a metal heat treatment process in which a metal workpiece is heated to an appropriate temperature and kept for a period of time, and then immersed in a quenching medium for rapid cooling. Commonly used quenching media are salt water, water, mineral oil, air, etc. Quenching can improve the hardness and wear resistance of metal workpieces, so it is widely used in various tools, molds, measuring tools and parts (such as gears, rollers, carburized parts, etc.). ) requires surface wear resistance. Through quenching and tempering at different temperatures, the strength, toughness and fatigue strength of metals can be greatly improved, and the combination of these properties (comprehensive mechanical properties) can be obtained to meet different application requirements. In addition, quenching can also make some steels with special properties obtain certain physical and chemical properties, such as enhancing the ferromagnetism of permanent magnet steel and improving the corrosion resistance of stainless steel. Quenching process is mainly used for steel parts. When common steel is heated above the critical temperature, all or most of the original structure at room temperature will be transformed into austenite. Then the steel is immersed in water or oil for rapid cooling, and austenite is transformed into martensite. Compared with other structures in steel, martensite has the highest hardness. The purpose of steel quenching is to transform all or most of its structure into martensite to obtain high hardness, and then temper at a proper temperature to make the workpiece have expected properties. Rapid cooling during quenching will cause internal stress in the workpiece, and when the internal stress is large enough, the workpiece will be distorted or even cracked. Therefore, it is necessary to choose a suitable cooling method. According to the cooling mode, the quenching process can be divided into four categories: single liquid quenching, double medium quenching, martensite graded quenching and bainite isothermal quenching.
Tempering is a metal heat treatment in which the quenched workpiece is reheated to an appropriate temperature lower than the lower critical temperature, and then cooled in air or water, oil and other media after heat preservation for a period of time. After quenching, the steel workpiece has the following characteristics: ① Unbalanced (i.e. unstable) structures such as martensite, bainite and retained austenite are obtained. ② There is great internal stress. ③ The mechanical properties can't meet the requirements. Therefore, steel workpieces are generally tempered after quenching.
The functions of action tempering are: ① to improve the structural stability, so that the structural transformation of the workpiece will no longer occur during use, thus maintaining the geometric size and performance stability of the workpiece. ② Eliminate internal stress, thus improving the working performance of the workpiece and stabilizing the geometric dimension of the workpiece. ③ Adjust the mechanical properties of steel to meet the requirements. Tempering has these effects because when the temperature rises, the atomic activity is enhanced, and the atomic energy of iron, carbon and other alloy elements in steel diffuses rapidly, realizing the rearrangement and combination of atoms, thus gradually transforming the unstable unbalanced structure into a stable balanced structure. The elimination of internal stress is also related to the decrease of metal strength when the temperature increases. When general steel is tempered, hardness and strength decrease and plasticity increases. The higher the tempering temperature, the greater the change of these mechanical properties. Some alloy steels with high alloying elements will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening. Workpieces with different uses are required to be tempered at different temperatures to meet the use requirements. ① Cutting tools, bearings, carburized and quenched parts and surface quenched parts are generally tempered at a low temperature below 250℃. After tempering at low temperature, the hardness changes little, the internal stress decreases and the toughness increases slightly. ② High elasticity and necessary toughness can be obtained when the spring is tempered at a moderate temperature of 350 ~ 500℃. ③ Parts made of medium carbon structural steel are usually tempered at a high temperature of 500 ~ 600℃ to obtain a good match of appropriate strength and toughness. The heat treatment process of quenching and tempering at high temperature is always called quenching and tempering.