Isothermal heat treatments below the martensite start temperature are carried out in a 0.25C-1.4Mn-1.4Si-0.32Mo steel (in wt%). The microstructural observation reveals a combination of tempered lath and plate martensite, accompanied by bainite and retained austenite. The temperature variation of the austenitic carbon content under different constrained equilibrium models is calculated and compared with experimental measurements. Neither of the existing models seems to predict the behavior of the measured austenitic carbon content in the temperature range studied. The results suggest that the diffusional formation of bainite governs the carbon content of austenite at temperatures nearby the M_S.

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 well below the M_S. Neither of the existing models for predicting the equilibrium between martensite and austenite (i.e., constrained carbon equilibrium CCE and CCEθ) seems to predict the measured austenitic carbon content nearby the M_S, which might indicate that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.