A transition from local equilibrium to paraequilibrium kinetics for ferrite growth in Fe–C–Mn: A possible role of interfacial segregation

Ferrite growth kinetics was measured under highly controlled decarburization conditions in an Fe–0.94%Mn–0.57%C (mass%) alloy. The observed growth kinetics closely followed the predictions of the local equilibrium non-partitioning (LE-NP) model over the temperature range 725–775°C. A transition from LE-NP to paraequilibrium (PE) kinetics was observed, for the first time, as the temperature was increased from 775°C to 825°C. Long-lived parabolic kinetic states that are intermediate between LE-NP and PE were observed at these intermediate temperatures. Above 825°C, ferrite growth appeared to follow the predictions of the PE model up to the T0 temperature at which ferrite growth stopped. These novel results are attributed to the segregation of Mn to the moving interface. It is thought that this process has the effect of decreasing the Mn concentration on the austenite side of the interface and, consequently, extending the PE state.