Sub-zero treating

Holding steel components at a temperature below zero degrees Centigrade to obtain the required structure. The temperature used is usually between -70ºC and -196ºC and the process is always followed by tempering.

Sub-zero treatment is carried out in order to complete the transformation of retained austenite to martensite after hardening and before tempering. It is usually applied to high carbon, high alloy steels such as tool steels but is more-widely applied by aerospace companies to guarantee complete transformation.

In the early days of sub-zero treating, when large, low-temperature refrigerators were not available, the problem was how to get reproducible low temperature processing equipment. The answer was to add dry ice to a bath containing a suitable liquid such as industrial alcohol or trichloroethylene. With sufficient dry ice, the temperature of the liquid could be maintained at a temperature of -78.5ºC. Accordingly, most specifications require a temperature between -70ºC and -80ºC. Nowadays, with the ready availability of liquid nitrogen‘ at -196ºC, many companies have based their sub-zero treating requirements on that lower temperature.

An unwelcome outcome of the hardening of some steels, which becomes more likely as the carbon and alloy content increases, is incomplete transformation to martensite during quenching. The resulting crystal structure contains retained austenite which renders the steel unstable as this austenite is able to transform over time leading to component distortion, as well as an increased risk of cracking. Cryogenic, or sub-zero treatments at temperatures down to -150°C are necessary, following hardening and tempering to cause the retained austenite to transform to martensite. A further tempering treatment at a temperature of 150- 180°C is then required to provide complete stabilisation. Cryogenic treatment is cost effective and regularly employed in the manufacturing cycle of critical components required for demanding applications.