When the metal is heated to a certain temperature, the atomic vibration will increase, the lattice spacing will increase and the free energy will increase. At this time, if the metal cools rapidly, the atoms will stop vibrating and tend to a stable state. At this time, the lattice spacing of metal decreases and the free energy decreases. This process of realizing the stable state of atoms through rapid cooling is the essence of quenching.

Quenching is a heat treatment process. Quenching of steel is to heat the steel to a temperature above the critical temperature of Ac3 (hypoeutectoid steel) or Ac 1 (hypereutectoid steel), and keep this temperature for a period of time to make it completely or partially austenitized.

And then rapidly cooled to below Ms (or isothermal close to Ms) at a cooling rate greater than the critical cooling rate, so as to carry out the heat treatment process of martensite (or bainite) transformation. Usually, the solution treatment of aluminum alloy, copper alloy, titanium alloy, tempered glass and other materials or the heat treatment process with quenching process is also called quenching.

The purpose of quenching is to make undercooled austenite undergo martensite or bainite transformation to obtain martensite or bainite structure, and then temper at different temperatures.

In order to greatly improve the rigidity, hardness, wear resistance, fatigue strength and toughness of steel, so as to meet the different use requirements of various mechanical parts and tools. It can also meet the special physical and chemical properties of some special steels such as ferromagnetism and corrosion resistance through quenching.

A metal heat treatment process in which a metal workpiece is heated to an appropriate temperature 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. 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.