Morphology and Crystallography of Bainite Transformation in a Single Prior-Austenite Grain of Low-Carbon Steel

To elucidate the difference in the microstructure development of the bainite and the martensite-austenite (M-A) constituents between blocks (groups with bainitic ferrite having a similar crystal orientation), in a single prior-austenite grain, the microstructural development and morphology of bainite in one coarse grain formed in a simulated heat-affected zone of low-carbon steel were analyzed by laser-scanning confocal microscopy and an electron back-scattering diffraction method. The correlation between the morphologies of bainitic ferrite and the M-A constituent was elucidated by analyzing blocks in a coarse and single prior-austenite grain. The possibility of M-A constituent formation was enhanced when the misorientation between bainite ferrite was large. Furthermore, it was found via in situ observations that bainitic ferrite units belonging to the same Bain group were formed synchronously, although they were apparently separated from each other by another Bain group.