is the immersion of a heated manufactured steel part, such as a gear or axle, in a fluid to achieve rapid and uniform cooling. Petroleum oils are often used for this purpose. Quenching provides hardness superior to that possible if the heat-treated part were allowed to cool slowly in air. Marquenching is a slower cooling process that minimizes distortion and cracking. There are two types of marquenching: martempering and austempering; the latter is the slower process and helps improve ductility.

Also see quenching oil.