Kinetic transitions and substititional solute (Mn) fields associated with later stages of ferrite growth in Fe-C-Mn-Si

A low-carbon alloy steel with relatively high Mn and Si concentrations (0.04 wt pct C-3 wt pct Mn-1.9 wt pct Si) has been used to explore the effects of alloy chemistry and austenite grain size on ferrite growth. Even at high levels of supersaturation, the volume fraction of ferrite is found to increase slowly relative to the relaxation time for carbon diffusion. A series of scanning transmission electron microscopy (STEM) analyses for Mn indicates that initial unpartitioned ferrite growth is replaced by partitioned growth, accompanied by a dramatic drop in growth rate, and a persistent level of residual supersaturation in the remaining austenite. The results are interpreted in terms of a transition from an initial paraequilibrium interfacial condition to partitioned ferrite growth.