In Situ Monitoring of the Impact of Carbon Content on the Austenitization Transformation in Plain Carbon Steels

Locations within the heat affected zone of C-Mn steel welds are subjected to a range of heating and cooling cycles that produce a wide spatial variation in microstructural features.  It is well known that changes in the carbon content in these steels have a significant effect on the kinetics of the austentite decomposition transformation on cooling.  However, the impact of these changes in carbon content on the austenitization (a→g) transformation during continuous heating is not well established.  In this study, the kinetics of the austenitization transformation at carbon contents of 0.05 wt.%, 0.45 wt.%, and 0.80 wt.% are analyzed under controlled heating and cooling cycles using in situ synchrotron x-ray diffraction.  The selection of these three steels allows for the impact of carbon content across the range of hypo-eutectoid through eutectoid steels to be analyzed.  Heating rates between 1ºC/sec to 10ºC/sec were also selected in order to replicate the heating rates common to the heat affected zone of thick section welds.  In situ x-ray diffraction measurements provide direct evidence for the progress of the austentitization transformation across all carbon levels and the dissolution and reformation of cementite at high carbon levels as a function of time.  Additional insight into the mechanism of the austenitization transformation during heating is also provided for the range of carbon concentrations analyzed here.