The driving force for lath and plate martensite and the activation energy for isothermal martensite in ferrous alloys

From information on M-s for lath and lenticular martensite the driving force for the start of a formation of the two types of martensite was calculated in a number of Fe-X systems. By plotting the calculated driving force against temperature the results indicate that the driving force for formation of martensite may not be much affected by solution hardening but mainly be a function of temperature. From the kinetics of isothermal alpha martensite in ferrous alloys one can clearly distinguish between two groups of alloys, high alloy steels and carbon containing steels. High alloy steels with low M-s temperature have a temperature dependence corresponding to a very low activation energy, possibly 7 kJ/mol. It can hardly depend on any diffusion process. Carbon containing steels have a temperature dependence corresponding to an activation energy of about 80 kJ/mol. Its rate of formation can be explained by assuming that it is triggered by submicroscopic plates of bainite formed with a rate of carbon diffusion.