Microstructure evolution of austenite-to-martensite transformation in low-alloy steel via thermodynamically assisted phase-field method

The austenite-to-martensite phase transformation plays a crucial part in the microstructure evolution of steel, with the morphology and grain size of martensite being key factors that influence the mechanical properties. This study explores the influence of cooling rate, prior austenite grain size (PAGS), and carbon content on the transformation process. Integrating thermodynamic calculation and phase-field modeling, it is observed that increasing the cooling rate from 20.0 °C/s to 40.0 °C/s elevates the martensite start temperature (Ms) from 320 °C to 348 °C, while simultaneously reducing the average martensite grain size from 8.36 μm to 7.79 μm. Reducing the PAGS from 63.08 μm to 31.54 μm hinders martensite nucleation, resulting in a decrease in Ms from 340 °C to 328 °C and a decrease in the average martensite grain size from 8.92 μm to 7.91 μm. Increasing the carbon content from 0.22 wt% to 0.36 wt% causes a reduction in Ms from 380 °C to 348 °C, with the average martensite grain size decreasing from 8.02 μm to 7.79 μm. These findings contribute to a deeper understanding of the martensitic transformation mechanisms and offer valuable guidance for the optimization of steel microstructures in practical applications.