Quenching is the rapid cooling of a heated material. How rapid is the cooling depends on the medium used for quenching. Water, brine solution, polymer solutions, molten salts, oils and sometimes even molten metals are used for quenching.

The rate of cooling in a quenching medium is defined with respect to water; arbitrarily, (non-agitated) water is rated at 1.0. Agitated water could reach a ranking of about 4.0. Non-agitated and agitated brine have ratings of 2 to 5, while non-agitated and agitated oil rank between 0.25 and 1.0. Thus, by choosing the appropriate media, the severity of quench can be controlled, which, in turn will decide the microstructures and hence the properties.