Determine the purpose of Quenching

Quenching is most generally used to harden steel by introducing martensite, in which case steel should be rapidly cooled through its eutectoid point, temperature at which austenite becomes unstable. In steel alloyed with metals like nickel and manganese, eutectoid temperature becomes much lower, however kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.