Quenching is a kind of heat treatment, including tempering, normalizing and quenching. The purpose of quenching is to improve the surface hardness of materials.

Quenching of steel is a heat treatment process, in which the steel is heated to a temperature above the critical temperature of Ac3 (sub-* * * steel precipitation) or Ac 1 (over * * * steel precipitation) for a period of time, and then cooled to below Ms (or isothermal near Ms) at a cooling rate greater than the critical cooling rate 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.

Heat treatment refers to the metal hot working process in which materials are heated, insulated and cooled in solid state to obtain the expected structure and properties. In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment was gradually recognized by people.

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In the 6th century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, quenching technology has been developed rapidly. Two swords and a halberd unearthed in Yanxiadu, Yixian County, Hebei Province, China, have martensite in the microstructure, indicating that they have been quenched. With the development of quenching technology, people have gradually discovered the influence of quenching agent on quenching quality.

In the Three Kingdoms period, Shuman Puyuan once made 3,000 knives for Zhuge Liang in this oblique valley in Shaanxi. According to legend, he sent people to Chengdu to get water for quenching. This shows that ancient China attached importance to the cooling capacity of different water quality, and also attached importance to the cooling capacity of oil and water. The sword unearthed in the tomb of Wang Jing in Zhongshan in the Western Han Dynasty (206 BC-24 AD) in China has a carbon content of 0. 15-0.4% and a surface carbon content of more than 0.6%, indicating that carburizing technology has been applied.

But at that time, as a personal "craft" secret, it refused to spread, so it developed slowly. 1863, British metallographers and geologists showed six different metallographic structures of steel under a microscope, which proved that the internal structure of steel would change when it was heated and cooled, and the high-temperature phase in steel would change into a harder phase when it was rapidly cooled.

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