Correlation between carbide reaction and anomalous interfacial segregation of solutes under tensile stress in heat-resistant steels

Depending on the phosphorus bulk content, the phosphorus segregation profile at the grain boundary/carbide interface of heat-resistant ferritic steels is composed of a convex profile or two maximum peaks and one minimum peak during rupture test at 650 °C. In the higher phosphorus bulk content steel, the smaller pre-formed non-equilibrium M3C carbides, which arise from the higher phosphorus segregation concentration at the carbide interface and thus its carbide growth retardation, are massively dissolved into the matrix after the first maximum peak. While the dissolved carbon is consumed in forming the equilibrium M7C3 carbides, the carbon segregates at the same time to the grain boundary/formed M7C3 carbide interface and the matrix/formed M7C3 carbide interface, resulting in a strong repulsive segregation between carbon and phosphorus. However, because the transformation of the thermally unstable M3C carbides to the M7C3 carbides is very early finished in the lower phosphorus bulk content steel, such a repulsive segregation is not observed. Meanwhile, the decrease in phosphorus segregation concentration after the second maximum segregation peak in the former steel or a plateau in segregation concentration of the latter steel is due to the formation of equilibrium MX carbides which results in the increase in total interface area. Due to the finer and denser MX particles formed on the M7C3 carbide surface of the former steel, the phosphorus segregation concentration decreases abruptly with increasing rupture time after the second maximum. The difference in rupture behavior of the steels is understood from the viewpoint of such a correlation.