Volume 8, Issue 4 (April 2011)
Isothermal Martensite Formation at Sub-Zero Temperatures
Sub-zero treatment of steels with an Mf below 0°C 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, 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 sub-zero temperature and time. Isothermal martensite formation was observed on interrupting the continuous cooling (5 K/min) at temperatures in the range of 80–233 K. The kinetics of isothermal martensite formation depends strongly on the temperature and can be described by Johnson–Mehl–Avrami–Kolmogorov kinetics. Isothermal experiments with dilatometry indicated the occurrence of a 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.