Failure transition mechanism in creep rupture of modified casting 9Cr-1.5Mo-1Co welded joint

Highlights: • Creep rupture behavior and microstructure evolution were investigated. • Failure location shifted from OTZ to WM with the decreasing stress. • Less grain boundaries and lower hardness in OTZ dominated short-term rupture. • Laves phase particles in WM were responsible for long-term failure. Creep rupture behavior and microstructure evolution of modified casting 9Cr-1.5Mo-1Co steel welded joint were systematically investigated in this paper. Based on a series of creep rupture tests at 620 °C under 110–170 MPa, a curve of stress versus rupture time was achieved to evaluate the service life of welded joint. The results indicated that creep rupture is considered to occur in over tempered zone (OTZ) under relatively high stress above 130 MPa while failure transferred to weld metal (WM) under lower stress below 130 MPa. The ruptured specimens showed less ductility and necking with the decrease of stress. Microstructure characterization revealed OTZ with less grain boundaries than WM and relatively lower hardness than BM was more possible to deform in short-term creep rupture accompanied by the coarsening of M₂₃C₆. Besides, formation of Laves phase and deterioration of martensite lath structure were detected during long-term creep exposure. Mo segregation near M₂₃C₆ provided the condition for Laves phase to form adjacent to M₂₃C₆. The difference of plastic deformation resistance in each zones of welded joint and nucleation and coarsening of Laves phase in WM are considered as the main factors to result in the failure transition.