Comparison of transformation behavior of the peritectic steels with different carbon content and its correlation to the slab quality during solidification

In order to investigate the phase transformation behavior of peritectic steels, we devise an integrated facility for X-ray transmission imaging combined with Laue diffraction analysis and precise temperature control. This integrated facility can measure the temperature at which the delta phase transforms into the gamma phase during solidification with continuous cooling. The investigation of four alloys with different carbon content reveals the dependence of carbon content on the minimum undercooling required for gamma phase formation. As carbon content increases, the amount of undercooling for the peritectic transformation decreases, and the gamma phase onset temperature necessarily increases. X-ray transmission images, which present the growing dendrites and moving delta-to-gamma interface, confirm that the peritectic reaction is likely to occur with high carbon content, leading to the gamma phase formation before the end of solidification.

On the contrary, the low carbon content suppresses the gamma phase formation, and the solid delta and liquid phase exist under equilibrium peritectic temperature. Once after the gamma phase nucleation, the phase change occurs rapidly with increasing undercooling. The delay in gamma phase formation results in massive phase transition and rapid change of interface velocity with a slight variation of undercooling. The fast kinetic of solid phase transformation is associated with a corresponding increase of internal stress, leading to the degradation of slab quality.