NEW CONCEPTS OF BAINITIC AND MARTENSITIC TRANSFORMATIONS IN STEELS BASED ON MULTISTEP GAMMA-ALPHA TRANSFORMATION

The features of the gamma-alpha transformation under isothermal condit ions and during continuous cooling for a wide range of rates (0.1-500 000 K/s) were studied on Fe-C alloys, on chromium steels containing up to 0.4% C and 4.3; 6.6 and 9.4% Cr and on some low-carbon alloyed ste els. It is established that there are discrete, staged kinetics of the gamma-alpha transformation in Fe-Cr alloys and alloyed steels. The tr ansition from a diffusionless transformation to a bainite transformati on in low-carbon chromium steels is examined. All the results on the k inetics of the gamma-alpha transformation are obtained for iron alloys which show that, there are four stages of the transformation in steel s and suggest a connection between the formation of upper and lower ba inite and the transformation at stages II and III observed in pure Fe, respectively. A theory is developed which explains the two-stage depe ndence of the starting point of the martensitic transformation in stru ctural steels on the cooling rate by the change in the interaction of the moving gamma-alpha boundary with carbon atoms. The upper temperatu re stage of the transformation is realized in conditions where growth is blocked by carbon atoms, the lower temperature stage-when the inter phase boundary separates from the impurity atmosphere.