Carbide changes and sulphur segregation in erMa steel during creep

An investigation was carried out into microstructural changes at the grain boundary of interrupted creep-tested and long-term heated 2sCr-1Mo steel by means of transmission electron microscopy with energy dispersive X-ray spectrometry. Grain boundary precipitates alternated their types of M₇C3 carbides with M₆C carbides, and were spheroidized during creep or long-term heating. These carbide changes and spheroidization of precipitates were found to be accelerated by creep stress. Sulphur migrated to segregate at the interface between M₆ C and the matrix, while this phenomenon was not observed at the Ms3 carbide or precipitate-free grain boundary. Consequently it was considered that the intergranular creep damage in 2sCr-Mo steel was caused by a reduction in interfacial energy due to the sulphur segregation at the interface between the matrix and M₆C carbides.