There is a large volume expansion when martensite forms from austenite. As the martensite plates form during quenching, they surround and isolate small pools of austenite , which deform to accommodate the lower density martensite. However, for the remaining pools of austenite to transform, the surrounding martensite must deform. Because the strong martensite resists the trans- formation, either the existing martensite cracks or the austenite remains trapped in the structure as retained austenite. Retained austenite can be a serious problem. Martensite softens and becomes more ductile during tempering. After tempering, the retained aus- tenite cools below the Ms and Mf temperatures and transforms to martensite, since the surrounding tempered martensite can deform. But now the steel contains more of the hard, brittle martensite! A second tempering step may be needed to eliminate the mar- tensite formed from the retained austenite. Retained austenite is also more of a problem for high-carbon steels. The martensite start ðMsÞ and finish ðMf Þ temperatures are re- duced when the carbon content increases. High-carbon steels must be
refrigerated to produce all martensite.