Direct observation of thermal stability of M23C6 carbides during reheating using in situ synchrotron X-ray diffraction

Precipitation reactions of heat-resistant steel occurring during welding, both growth and coarsening of existing phases and the formation of new phases, can be detrimental for creep strength. However, understanding of the thermal stability of precipitates during the heat cycle of welding is insufficient. In this study, an attempt has been made to investigate the possibility of analyzing M23C6 carbides stability occurring in the heat-affected zone based on in situ time-resolved X-ray diffraction using a synchrotron. The d-spacing of M23C6 carbide exhibits a discontinuous change accompanied by coarsening. Similarly, the volume fraction curve of M23C6 carbide shows a slight increase before it decreases dramatically as the temperature increases. The mean crystallite size of fine M23C6 particles can be characterized by the integral breadth method. Compared to as-received state, there are no M23C6 carbides in the reheated microstructure.