Transformation of retained austenite during freezing

Quenched and tempered ferrous metallurgy components frequently retain some of the austenite phase at room temperature. In small amounts, it is not considered detrimental and can even be beneficial for some applications. The amount of retained austenite is usually a function of the carbon concentration and can be minimized, when necessary, by a deep freeze immediately after quenching. Heat treated components with a significant amount of retained austenite which have been exposed to freezing temperatures sometimes transform a portion of the austenite to untempered martensite, resulting in a size change. At other times there is no transformation because the austenite has stabilized. This report calcualtes the martesite start and finsh temperatures for P/M alloys as a function of carbon content, discusses the room temperature stabilization of austenite, and gives examples of size change on transformation at freezing temperatures for FC 0208 and FL 4605.