Creep deformation and fracture behaviour of modified 9Cr-1Mo steel in flowing liquid sodium environment

The creep deformation and rupture behaviour of modified 9Cr-1Mo ferritic steel in flowing liquid sodium has been studied at 873 K over a stress range of 130-160 MPa and compared with those tested in air environment. In the flowing sodium environment, the steel exhibited better creep deformation and rupture strength than those in air environment. The onset of tertiary stage of creep deformation has delayed in sodium environment over that in air environment, which led to the decrease in minimum creep rate. Also the rate of tertiary creep strain accumulation was lower in sodium environment than that in air, leading to the increase in rupture life. The steel failed in ductile dimple fracture mode in both the testing environments. The steel exhibits relatively higher creep damage tolerance in sodium environment than in the air environment. Minimal oxidation was observed in the specimens creep tested in sodium environment. The near absence of oxidation of the steel in sodium environment delayed on onset of tertiary stage of creep deformation and decrease the rate of tertiary creep strain accumulation, leading to the increase in creep deformation and rupture strength.