Microstructural change of 9% Cr-welded joints after long-term creep

Creep tests were performed at 600 °C to investigate the microstructural changes in welded joints of 9% Cr-tempered martensite ferritic steel (grade E911), focusing on the influence of the multiaxiality of stress on the microstructure of a fine-grained heat-affected zone (FGHAZ). A higher creep cavity density was observed in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. This difference in the creep cavity density was related to a high degree of stress multiaxiality in the central part of the specimen. It was observed that VX particles were stable during long-term creep in both the base metal and the FGHAZ. M23C6 carbides coarsened faster in the FGHAZ than in the base metal; however, the effect of stress multiaxiality on the coarsening could not be detected. A Z-phase formed during creep, and the number density of the Z-phase particles was higher in the center of the specimen than in the surface region. The subgrain sizes increased and the dislocation densities decreased during long-term creep. However, the evolution of the dislocation substructure was not significantly different in the inner part of the specimen compared to the surface regions.