Analysis of high temperature phase transformation behavior of ultra-low-carbon steel using X-ray topography, diffraction, and synchronized temperature measurement

Despite the technical importance of delta / gamma phase transition in steel alloys, conventional thermal analysis techniques like calorimetry lack precise kinetics measurement. This study introduces an integrated approach combining Xray topographic imaging, diffraction, and precise temperature measurement to overcome these limitations. This methodology allows for the accurate quantification of recalescence due to release of latent heats from solidification and solid-state phase transformation. It also enables tracking of the fractional volume changes of the delta phase during continuous cooling. We demonstrate that in Ultra -Low Carbon steels, the delta to gamma transformation occurs rapidly in the single gamma phase region far below the solidus temperature in the phase diagram, and this transformation accelerates further in the higher cooling rate. Furthermore, it can be inferred that in phase transformation, its rate becomes a critical factor in the temperature increase resulting from the released latent heat, overshadowing the influence of undercooling during continuous cooling.