The process of rapidly cooling a re-heated metal object by plunging it in water or another liquid, such as oil. Steels can be hardened to varying extents depending on their carbon content and the rate or speed at which the quench occurs, which varies with the thermal capacity of the quenching liquid as well as the mass of shape of the object being quenched. In an air-cooled steel the cooling rate is sufficiently slow to allow the structure of the steel to transform to one at least partly made up of pearlite. When this critical cooling rate is exceeded, the transformation to pearlite no longer has time to take place and, depending on the rate of cooling, one of several less stable structures is the result. The more efficient the quench the less stable and correspondingly harder and more brittle is the resultant structure. When the rate of cooling is rapid enough a fully quenched structure of martensite is the result.
The protection of part of a steel object from the effects of a quenching medium, and thus from rapid cooling, usually by covering it with clay. Medieval and later Japanese sword-smiths, for example, might coat a blade with a mixture of wet clay and charcoal, then scrape most of this away along the edge which was to be hardened. The blade was then heated above the upper critical temperature, and quenched in water, producing a blade with a hardened edge and a tough unhardened back.
Slack- or slow-quenching
Quenching carried out using a cooling liquid, such as oil, with a lower thermal capacity than a fast-cooling liquid like water. Depending on the carbon content of a steel quenched in this way, an intermediate transformation product such as bainite will be formed, rather than the fully quenched, much harder and more highly stressed transformation product, martensite. More gentle quenching can avoid excessive distortion or even breakage, although an insufficiently well-quenched steel might be too soft and perhaps too easily blunted or bent.