What is quenching? The quenching of steel is to heat the steel to a temperature above the critical temperature Ac3 (hypo-eutectoid steel) or Ac1 (hypereutectoid steel), keep it for a period of time, make it fully or partially austenitized, and then cool it at a temperature greater than the critical cooling rate. The heat treatment process of rapid cooling to below Ms (or isothermal near Ms) for martensite (or bainite) transformation. The solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with rapid cooling process is also called quenching. The purpose of quenching: 1) Improve the mechanical properties of metal finished products or parts. For example: improve the hardness and wear resistance of tools, bearings, etc., improve the elastic limit of springs, improve the comprehensive mechanical properties of shaft parts, etc. 2) Improve the material properties or chemical properties of some special steels. Such as improving the corrosion resistance of stainless steel, increasing the permanent magnetism of magnetic steel, etc. When quenching and cooling, in addition to the reasonable selection of quenching medium, there must be a correct quenching method. The commonly used quenching methods mainly include single-liquid quenching, double-liquid quenching, graded quenching, isothermal quenching, local quenching, etc. The steel workpiece has the following characteristics after quenching: ① Unbalanced (that is, unstable) structures such as martensite, bainite, and retained austenite are obtained. ② There is a large internal stress. ③ The mechanical properties cannot meet the requirements. Therefore, steel workpieces are generally tempered after quenching. What is tempering? Tempering is a heat treatment process in which a quenched metal product or part is heated to a certain temperature, and after a certain period of time, it is cooled in a certain way. A process, so the combined process of quenching and tempering is called final treatment. The main purposes of quenching and tempering are: 1) Reduce internal stress and reduce brittleness. The quenched parts have great stress and brittleness. If they are not tempered in time, they will often cause deformation or even cracking. 2) Adjust the mechanical properties of the workpiece. After the workpiece is quenched, it has high hardness and brittleness. In order to meet the different performance requirements of various workpieces, it can be adjusted by tempering, hardness, strength, plasticity and toughness. 3) Stable workpiece size. The metallographic structure can be stabilized by tempering to ensure that no deformation will occur in the subsequent use process. 4) Improve the cutting performance of some alloy steels. The function of tempering is: ‍① Improve the stability of the structure, so that the workpiece will no longer undergo structural transformation during use, so that the geometric size and performance of the workpiece remain stable. ② Eliminate internal stress in order to improve the performance of the workpiece and stabilize the geometry of the workpiece. ③ Adjust the mechanical properties of steel to meet the requirements of use. The reason why tempering has these effects is that when the temperature increases, the atomic activity increases, and the atoms of iron, carbon and other alloying elements in the steel can diffuse more quickly, realizing the rearrangement and combination of the atoms, thus making it unstable. The unbalanced organization gradually transforms into a stable balanced organization. Relief of internal stress is also associated with a reduction in metal strength as temperature increases. Generally, when steel is tempered, the hardness and strength decrease and the plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with high content of alloying elements will precipitate some fine-grained metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening. Tempering requirements: Workpieces with different uses should be tempered at different temperatures to meet the requirements in use. ①Tools, bearings, carburizing and quenching parts, and surface quenching parts are usually tempered at a low temperature below 250 °C. After low temperature tempering, the hardness does not change much, the internal stress decreases, and the toughness increases slightly. ② The spring is tempered at medium temperature at 350～500℃ to obtain high elasticity and necessary toughness. ③ The parts made of medium carbon structural steel are usually tempered at high temperature at 500-600 ℃ to obtain a good combination of suitable strength and toughness. When steel is tempered at around 300 °C, its brittleness is often increased, and this phenomenon is called the first type of temper brittleness. Generally, tempering should not be performed in this temperature range. Some medium carbon alloy structural steels are also easy to become brittle if they are slowly cooled to room temperature after being tempered at high temperature.