Quenching

Quenching is the rapid cooling of a workpiece, in water, oil, gas, polymer or air, to obtain a desired material property. As a form of heat treating it works by shrinking the window of time during which an undesired reaction, such as an unwanted phase transformation, remains both thermodynamically favorable and kinetically possible, and in doing so it can reduce the crystal grain size of a metallic or plastic material and increase its hardness. In metallurgy it is most often used to harden steel by inducing a martensite transformation: the steel is cooled rapidly through its eutectoid point, the temperature at which austenite becomes unstable, which prevents the formation of cementite and instead forces carbon atoms to stay dissolved in the ferrite lattice. Alloying steel with metals such as nickel and manganese lowers the eutectoid temperature so quenching can begin lower and proceed more easily, while extremely rapid cooling can suppress the formation of any crystal structure at all, producing an amorphous metal known as metallic glass.