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, Δ θCPP, increased with increasing transformation temperature and carbon content. Angle deviations between the close parallel directions, Δ θCPD, 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. The Zener-Hillert approach was therefore modified to include finite mobility that varied with OR of the interface.