Abstract: A quenching apparatus and a method for hardening steel parts is disclosed. The method optimizes cooling conditions preventing the occurrence of film boiling and the “self-regulated thermal process” while optimizing the depth of the hardened layer and providing increased strength for the steel part. The optimum depth of hardening is considered to be when the surface compressive stresses on the part are at their maximum value and depth. In addition, “maximum” steel strength is achieved when the cooling rate of the part is above a certain minimum level. However, additional strengthening of the part occurs when the rate of cooling avoids both film boiling and subsequent nucleate boiling, and cooling of the part proceeds directly into the convection cooling process. At the end of the quenching process, some steel parts may benefit from isothermal cooling in air, i.e., self tempering.