Transformation from Ferrite to Austenite during/after Solidification in Peritectic Steel Systems: an X-ray Imaging Study

X-ray transmission imaging with X-ray diffractometry and time-resolved tomography with three-dimensional X-ray diffraction microscopy have been used to observe solidification and transformation in carbon steel and other Fe-based alloys. The imaging techniques showed a massive-like transformation, in which multiple austenite grains were produced in a single δ grain through a solid–solid transformation. The critical velocity from the diffusion-controlled growth of the γ phase to the massive-like transformation was as low as 5 µm/s. X-ray imaging indicated that the δ phase transforms massively to the γ phase in the conventional solidification processes, such as continuous casting. The massive-like transformation and multiple γ grains that were produced in the transformation were related to the subsequent microstructure evolution and casting defect formation. Solidification model including the massive-like transformation is expected to improve our understanding the solidification and the related phenomena in the peritectic steel systems.