Isothermal Phase Transformations in a Low Carbon Steel During Single and Two-Step Partitioning

Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCEθ model fits the experimental data for the isothermal heat treatment at a temperature slightly below the M_S, despite the fraction of bainite in the microstructure. The existing models for predicting the equilibrium between martensite and austenite (i.e., constrained carbon equilibrium CCE and CCEθ) seems to overestimate the austenitic carbon content well below the M_S. Equilibrium calculations between tempered martensite and austenite showed fair agreement with the experimental values for large martensite fractions. For small fractions, it is proposed that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.