In-situ observation on the δ to γ transformation behavior of carbon steels

In recent in-situ observations on the solidification of thin carbon steel experiments, the primary delta-phase appears to be highly undercooled into the single gamma phase region of the phase diagram, where the massive delta to gamma transformation occurred without the intervention of the liquid phase following a typical peritectic reaction. Whether such a massive transformation is the main mode, even in bulk steel alloys, is of prime importance because it would drastically change our view on the defect generation mechanism during steel solidification in continuous casting. In this research, the in-situ imaging technique using synchrotron white X-ray beam was conducted to investigate the effects of carbon content and the specimen thickness on the delta to gamma transformation behaviors in the carbon steel. The moment of delta to gamma transformation is identified through synchronized recording of Laue diffraction patterns and in-situ images. The relative average brightness (RAB) change during solidification was evaluated to analyze the transformation. The results revealed that hypo-peritectic steel grades predominantly underwent a massive transformation, while hyper-peritectic steel grades showed a combination of massive and diffusional transformations. The sudden drop in liquid fraction after massive transformation was detected, while diffusional transformation exhibited a gradual decrease in the present study. The findings demonstrated that specimen thickness influenced the transformation mode in hyper-peritectic steel grades. The study provides insights into the behavior of delta to gamma transformation and its implications for carbon steel solidification processes.