Creep behavior and microstructure evaluation of welded joint in dissimilar modified 9Cr-1Mo steels

In the present work, the creep behavior and microstructure before and after creep test of the dissimilar modified 9Cr-1Mo steels welded joint were investigated in detail. Firstly, creep tests were carried out at different stress values/levels under the temperature of 538 °C and 500 °C, and then the power law equation of steady-state stage were achieved based on the results. The microstructure of welded joint were characterized in detail, the tempered martensite structure is the typical microstructure of whole welded joint, and the δ-ferrite phase is observed in heated affect zone (HAZ) and the base metal without Co and B elements (BM1). A soft zone called over-tempering zone (OTZ) occurred in both HAZs, which was determined by micro-hardness measurement, due to the lower dislocation density and coarsened M23C6 carbides of this zone. The fracture location is in OTZ adjacent to BM1 side determined by microstructure, micro-hardness and geometric calculation. Lots of voids are observed along the triple grain boundaries associated with large second phase particles after creep rupture. The stress concentration caused by the coarsened carbides at the triple boundaries and softening matrix are considered as the main factor of creep voids occurring in the OTZ of welded joint.