Creep Damage Analysis of Large-Size Uniaxial Cross Weld Specimen and Circumferentially Welded Pipe of Mod.9Cr-1Mo Steel

Creep damage simulation combining a void nucleation model and damage mechanics was applied to large-size uniaxial cross weld specimen and circumferentially welded pipe of mod.9Cr-1Mo steel subject to internal pressure and bending at 650°C．The void nucleation model depending on both creep strain rate and triaxiality factor considers the increase of number density of creep void with a size of FGHAZ (fine-grained heat affected zone) grain of the welded joint. Damage mechanics model considers both the strain softening of the FGHAZ material and damage due to the increase of creep voids. The critical value of the damage parameter was defined by the number density of creep void corresponding to the formation of micro cracks. In the case of large-size uniaxial cross weld specimens with X- and U-shaped weld configuration, damage progress in the FGHAZ was successfully expressed by considering the hardness distribution in the FGHAZ in the form of distribution of creep strain rate. In the case of circumferentially welded pipe, the damage simulation using the same material data as the large-size uniaxial cross weld specimen gave the damage initiation at the middle of thickness and the following damage progress towards inside surface of the pipe. This tendency well corresponded to the experimental results, and the damage area in the circumferential direction at the same rupture life consumption was almost the same in both simulation and experiment.