Interrupted quenching

At first, the cast steel part is quenched in water for so long until red heat has disappeared.

Subsequently, quenching is continued to the end using a milder quenching agent, such as oil. Water bath quenching suppresses pearlite or bainite formation and makes it possible for further cooling to be performed at lower rates since martensite and thus the obtainable degree of hardness is not dependent on the cooling rate.

However, hot-bath hardening is a better solution (q. v.).

Additional references:
Flame hardening
Case hardening
Torch hardening
Induction hardening
Immersion hardening
TTT diagram