Heat treatment completely changes the properties of steel, whose structure is changed by heating and quenching. Both pencil leads and diamonds are composed of the same carbon, but look quite different. This is because the molecular structure of the former is random, but that of the latter is regular, as a result of the high temperatures and compression caused by as a result of changes in the earth's crust. This is an example of the discovery by pioneering researchers that selecting specific heating and cooling temperatures produce various material structures.
Metal heat treatment is a kind of "modern alchemy". The media used to quench heated metals are water, oil and air cooling. In recent years, polymer quenching agents, gas cooling, etc. have also been. As water-soluble media, these media have no flash point. Desired hardness can be obtained by controlling the water concentration.