Characterization of carbides at different boundaries of 9Cr-steel

A combination of structural and analytical investigation by transmission electron microscopy was carried out to characterize carbides formed at two types of boundaries in strain-relieved 9Cr-steel, prior-austenite grain boundary (PAGB) and small-angle grain boundary of subgrain boundary (martensite-lath boundary). Structural characterization by selected-area electron diffraction pattern revealed the presence of fine V-rich MC- and coarse Cr-base M23C6-type carbides at prior-austenite grain boundaries, while fine Mo-base M2C- and coarse M23C6-type carbides at small-angle grain boundaries. Analytical investigation, spectral and mapping analysis, by energy dispersive X-ray spectroscopy (EDS) showed the elemental composition and distribution of carbides at both boundaries. The composition of M23C6-type carbides at both boundaries were almost the same. Even though the processes of M23C6-type carbide formation are expected to be different from each other, the origin of similarities in compositions is ascribed to being the stable phase of M23C6-type carbides. The formation mechanisms of these carbides at different boundaries are also proposed.