Effect of temperature, carbon content and crystallography on the lengthening kinetics of bainitic ferrite laths

The lengthening of bainitic ferrite laths in a model alloy containing a carbon gradient was observed in-situ using a Laser Scanning Confocal Microscope (LSCM). Results show that the observed lengthening rates increase with increasing temperature and decreasing carbon content. Crystallographic analysis was conducted by ex-situ electron backscattered diffraction to understand the relationship between growth kinetics and orientation relationships (OR). The OR of bainitic ferrite with respect to austenite deviates from the exact K-S and N-W models. The deviation between the close parallel planes, Delta theta CPP, increased with increasing transformation temperature and carbon content. Angle deviations between the close parallel directions,Delta theta CPD, also increased with carbon content, while changes with transformation temperature were very small. The classical Zener-Hillert approach provided a reasonable first-order description of the global kinetics, but could not account for the observed effects of crystallography. A better fit of the experimental results was obtained by introducing finite mobility that varied with OR of the interface. It suggests that the lengthening kinetics of an individual bainitic lath may be dependent on its crystallographic orientation with respect to the parent austenite.