On the origin of carbon supersaturation in bainitic ferrite

Growing evidence in the literature highlights the presence of bainitic ferrite with partial carbon supersaturation at low temperatures. Diffusional growth models correctly predict the lengthening rate of plate-like ferrite but often underestimate carbon content due to the assumption of carbon local equilibrium at the interface, valid at elevated temperatures but less so as temperature decreases. Carbon transfer across the interface becomes increasingly important during bainite formation at lower temperatures, potentially resulting in carbon supersaturation within ferrite. This study proposes a more realistic approach for the carbon interfacial conditions. By relaxing the local equilibrium assumption, a potential carbon activity difference arises between austenite and growing bainite, inducing carbon supersaturation. Comparative analysis with literature data demonstrates that the revised Zener-Hillert model can predict carbon supersaturation trends across diverse alloys and temperatures. This extended model enhances our understanding of bainite growth at lower temperatures, with implications for alloy design and heat treatment processes.