Kinetics of pearlite transformation: The effect of grain boundary engineering

The present study investigates the effect of grain boundary engineering (GBE) on the pearlite transformation in high-Mn steel. Towards this, as-forged (AF) specimens were thermo-mechanically processed (TMP) to realize GBE microstructure. The electron backscatter diffraction analyses revealed that the GBE specimen exhibit higher fraction of Σ3 and its variant boundaries, larger twin related domain (TRD) size and greater number of grains per TRD than AF and other TMP specimens. Further, isothermal treatment was performed on AF and GBE specimens at 823 K for different time intervals (1 h to 48 h) to understand the kinetics of pearlite transformation. The scanning electron microscope observations revealed that, unlike the AF specimen, the GBE specimen exhibits a significantly lower fraction of grain boundary pearlite after isothermal annealing at 823 K for 3 h. The aforesaid findings are validated using grain boundary nucleation rate calculation by considering the presence of random and Σ3 boundaries. Further, the Johnson-Mehl-Avrami-Kolmogrov model demonstrate that the presence of significant proportion of Σ3 boundaries leads to delay in the overall transformation kinetics of pearlite transformation. Overall, this work highlights that pearlite does not form uniformly on the austenite grain boundaries since it is reluctant to nucleate on low energy special boundaries like the Σ3 boundaries. This retards the overall transformation kinetics in grain boundary engineered steels having a high proportion of special low energy boundaries.