Role of Microstructure on Creep Rupture Behaviour of Similar and Dissimilar Joints of Modified 9Cr-1Mo Steel

Creep tests carried out on the joints of ferritic steels at 823 K over a stress range of 100 - 380 MPa indicate that the similar joints possessed lower creep rupture strength than its ferritic base metal. On the other hand, failure of the dissimilar joint occurred at relatively lower stresses at the ferritic/austenitic weld interface. Microhardness measurements on the two different joints reveal that a hardness peak at the ferritic/austenitic weld interface and a hardness trough at the intercritical heat affected zone (HAZ) in ferritic base metal, which suggest the role of distinct microstructural consequence. Un- tempered martensite was found at the ferritic/austenitic weld interface imparting high hardness, resulting in inhomogeneity in the strength at the interface. On the other hand, annealing of martensitic structure of modified 9Cr-1Mo steel by intercritical heating during welding thermal cycle resulted in hardness trough in the intercritical HAZ. On creep exposure, while secondary phase, Z-phase that formed in the HAZ regions of similar joints resulted in extensive cavity nucleation leading to premature failure of the joint, the inhomogeneity in strength that resulted from the formation of martensite in the dissimilar joint is found to play a major role in the failure.