An assessment of creep deformation and rupture behaviour of 9Cr-1.8W-0.5Mo-VNb (ASME grade 92) steel

Creep deformation and rupture behaviour of 9Cr-1.8W-0.5Mo-VNb steel have been investigated at 873 K, 923 K and 973 K over a stress range of 80-220 MPa. The absence of clear primary creep regime and prolonged secondary stage of creep deformation have been noticed under lower stress level at 973 K. The variation of minimum creep rate with applied stress obeyed Norton's power law of creep. The apparent stress exponents of 15.2, 12.3 and 5.8, and apparent activation energy of 619 kJ/mole have been estimated for creep deformation of the steel. The apparent stress exponents and activation energy have been rationalised on the basis of threshold stress. The threshold stress values of 137.5 MPa, 83.3 MPa and 29.7 MPa were obtained at 873 K, 923 K and 973 K respectively. The threshold stress compensated true stress exponent of 4 and true activation energy of 244 kJ/mole, and threshold stress normalised by Orowan stress confirms that the lattice diffusion assisted localised climb of dislocation is the rate controlling of creep deformation in the steel. The steel obeyed Monkman and modified Monkman-Grant relationships. Damage tolerance factor of 6 in the steel demonstrates that the microstructural degradation such as coarsening of precipitates and subgrain structure is the dominant creep damaging mechanism in the steel.