Quenching

Rapid cooling, as by immersion in oil or water, of a metal object from the high temperature at which it is shaped. Quenching is usually done to maintain mechanical properties that would be lost with slow cooling. It is commonly applied to steel objects, to which it gives hardness. The quenching media and the type of agitation during quenching are selected to obtain specified physical properties with minimum internal stresses and distortions.

Oil is the mildest medium, and salt brine has the strongest quenching effect. In special cases, steel is cooled and held for some time in a molten salt bath, which is kept at a temperature either just above or just below the temperature where martensite begins to form. These two heat treatments, called martempering and austempering, both result in even less distortion of the metal. Copper objects hardened by hammering or other deformation at ordinary temperatures can be restored to malleability by heating and quenching. &Seealso; tempering.