Rapid cooling is carried out in order to obtain martensite in hardened steel. Austenite to pearlite or bainite transformation is suppressed in this process. This means that higher cooling rates will be required only in that temperature range within which unstable austenite decomposes to ferrite-cementite aggregate. Once this range is crossed, austenite will transform to martensite even on slow cooling. Therefore, it is not essential to rapidly cool the steel component throughout the complete range of temperature from hardening temperature to quenching medium temperature. Minimum possible cooling rate will develop the lowest internal stresses in the hardened steel. Large magnitude of internal stresses may either lead to distortion of piece or to the formation of internal or external cracks.