Abstract: Although cold treatments have been used to reduce the retained austenite in the cases of carburised steels for many years, there are little data on deep cold temperatures below -70oC or treatment times longer than an hour or two. This study set out to determine the effects of such deep cold treatments at temperatures down to -150oC for up to 24 hours. The study investigated the effects of deep cold on microstructure, hardness profile, residual stress and internal oxidation on two typical carburising steels, 16MnCr5 and 21NiCrMo2. The study found that for both 16MnCr5 and 21NiCrMo2 carburised to a case depth of approximately 0.8mm, the longer and colder the deep cold treatment, the more of the austenite retained in the case was converted to martensite and the harder it became. After low temperature tempering the hardness difference was smaller, but still significant. In both steels the case appeared more refined and homogeneous after deep cold treatment. Deep cold treatment had a negligible effect on the core properties of either steel.

Key words: Carburised steel, Deep cold, Retained austenite