Additivity hypothesis and effects of stress on phase transformations in steel

Theoretical analysis shows that the fraction of pearlite formed from nucleation is additive and that from growth is not. A modified additivity model is established with two continuous cooling experiments. Calculations of Ae3 temperature for proeutectoid ferrite formation under stress and nucleation rate as well as incubation period of ferrite or pearlite transformation under stress are successfully made. Kinetics equations for ferrite and pearlite transformations under stress are expressed from modification of J–M–A equation with addition of a stress item. The acceleration effect of stress on bainite formation is mainly attributed to the increase of diffusivity of solute atoms and even iron. By consideration of the grain size effect, Patel and Cohen’s equation expressing the effect of stress on Ms is modified. Calculations of Ms for fcc → bcc(bct) and fcc → hcp under stress are introduced. An equation showing the relationship between strain and nucleation of martensite which can well explain the morphology of martensite formed under stress is mentioned. Appearance and mechanism of mechanical stabilization of austenite in martensitic transformation, i.e., the lowering of Ms, resulted from the work hardening of austenite, are different from retardation of bainite formation under stress, i.e., after Bs raising, occurring the retardation of bainite growth resulted from hindrance by defects.