Anomalies in segregation kinetics of solutes under tensile stress in heat resistant steel

In heat resistant steels, tensile stress accelerates the decomposition of preexisting smaller carbides and the growth of larger carbides. Consequently, the dissolved carbon is enriched in the matrix during the decomposition. Under the tensile stress, the massive diffusion of the enriched carbon results in more active segregation to the grain boundary/carbide interfaces for the growth of the larger carbides than the carbide-free grain boundaries. The segregation kinetics of carbon forms a convex profile during the growth process of carbides. A concave segregation profile of chromium corresponds to the convex profile of carbon. This is attributed to the diffusivity of chromium much lower than that of carbon which makes the chromium as the rate-controlling factor for the carbide growth. Due to the repulsive segregation behavior between carbon and phosphorus, the concave segregation profile of phosphorus results from the convex segregation profile of carbon formed during the active carbide growth. A convex segregation profile of sulfur is also formed owing to the formation reaction of MnS particles, but it is independent of the segregation behaviors of the other solutes.