Visual analysis of solidification and δ – γ transformations in steels

The theory of solidification of steels and the kinetics of austenite to α-ferrite phase transformation were extensively studied; however, comparatively, little information is available concerning the kinetics of the δ-ferrite to austenite transformation due to the difficulty of making in situ observations. In the present study, a laser scanning confocal microscopy with an infrared image furnace was implemented with which the in situ observations at the high temperature of the dynamic behaviour of the δ/γ grain nucleation and growth and interphase boundaries of the steels are made possible. The solidification mode of the carbon steel and the austenitic stainless steel during welding can be directly observed, and the definitive sequence of phase transformation that led to the final microstructure was detected in real time. Finally, new experimental results will be presented and compared with previous studies.