Sub-Zero Treatment

Retained austenite is undesirable in the hardened structure because the part then stays softer and can transform later under load. Deep freezing right after quenching to room temperature pushes the transformation to martensite to completion by bringing the part to very low temperatures. It's important that you cool slowly and afterwards also warm up slowly to room temperature, otherwise you risk stresses. After that, you must temper, otherwise the part becomes brittle and cracks – this mainly affects welded tool steels, where retained austenite often remains after welding and hardening.