Isothermal Martensite Formation at Sub-Zero Temperatures
Journal of ASTM International
Sub-zero treatment of steels with an M f below zero degrees Celsius relies (partly) on a continuation of the martensite formation. The present work reports on the observation of isothermal martensite formation in the sub-zero temperature regime for two steels: AISI 1070 and AISI 52100. Samples were austenitized and quenched in oil and thereafter investigated with vibrating sample magnetometry, which allows a quantitative assessment of the fraction of retained austenite as a function of the
... unction of the subzero temperature and time. Isothermal martensite formation was observed on interrupting the continuous cooling (5 K/min) with vibrating sample magnetometry at several temperatures in the range 80-233 K. The kinetics of isothermal martensite formation depends strongly on the temperature. Similar isothermal experiments with dilatometry indicated the occurrence of volume increase on isothermal holding, consistent with a continuation of the martensitic transformation. On prolonged isothermal holding a volume reduction was observed for AISI 52100, but not for AISI 1070. A mechanism is proposed that explains the occurrence of isothermal martensite formation.