Overall activation energy of isothermal transformation in metal alloy and its mechanism I. Medium temperature (bainite) isothermal transformation in steels

Upper bainite, lower bainite and granular structure in isothermally treated steels have all themselves C curves. A single microstructure is always produced in the primary stage, whereas the nearby two-microstructure complex is usually produced in the medium or final stage of the transformation. Only a single microstructure and Arrhenius equation must be used to deduce the overall activation energy for the transformation product. A combination of overall activation energy, morphology and free energy curve can explain the bainitic transformation mechanism and granular structure formation mechanism. The former follows military atom diffusionless martensite-like shear, which occurred in carbon-depleted region controlled by carbon atom diffusion in austenite, and the latter results from civilian atom diffusionless interface control transformation, which occurred in the most carbon-depleted region controlled by carbon atom diffusion in austenite.