The mechanical stability of austenite in maraging steels

Two different types of retained austenite were encountered in maraging steels: one obtained by the usual overaging/reversion process, and was found to be mechanically unstable at room temperature; the other, obtained in a sequence of isothermal heat treatments leading to the formation of microsegregational content zones of molybdenum and cobalt in the lath-martensite, and was found to be mechanically stable at room temperature (RT). In the unstable case the austenite transformed to martensite upon cold working at RT. In the stable case, as was shown by careful Mössbauer-effect spectroscopy and X-ray diffraction studies, the amount of retained austenite was not affected by the cold-working at RT, whereas some amount of the martensite was transformed into a ferromagnetic-type of austenite. Complementary studies by electron diffraction have shown that both Kordjumov-Sachs and Nishiyama crystallographic orientation relationships may exist between austenite and martensite, depending on the local molybdenum and cobalt segregational contents in the lath martensite. The appearance of ferromagnetic austenite, as well as other segregational effects observed by the Mössbauer-effect spectroscopy are discussed.