Creep damage evaluation of 9Cr–1Mo–V–Nb steel welded joints showing Type IV fracture

By conducting long-term creep rupture tests for 9Cr–1Mo–V–Nb (P91) steel welded joints, creep rupture properties and microstructures were examined. Creep rupture tests were conducted at three temperatures of 823, 873 and 923 K, under applied stresses of 160–230, 80–130, and 40–80 MPa, respectively. The rupture locations were found to shift from the weld metal at the higher stress condition to the fine-grained HAZ adjacent to the base metal at lower stress conditions at 873 and 923 K. The relationship between microstructural changes and crack nucleation site and propagation path was clarified. A remarkable decrease of dislocation density and growth of precipitates of M23C6 and Laves phase during creep was observed in the vicinity of the fine-grained HAZ adjacent to the base metal for the Type IV fractured welded joint specimen. The stress–strain distribution in the welded joint was investigated by the finite element method (FEM) using creep data of the simulated HAZ specimen. It was found that the observed crack initiation site and crack growth path coincided better with the distribution of the stress triaxiality factor than that of the equivalent creep strain.