Microscopic Simulation and Life Prediction of High Cr Steel Welds Subject to Type IV Creep Damage

概要

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The micro-macro combined creep damage simulation based on the grain-boundary-fracture-resistance model was applied to the evaluation of small defects distribution in fine-grained heat-affected zones of longitudinal welds in the actual size elbow of modified 9Cr-1Mo (9Cr-1MoVNb) steel subject to internal pressure at 923K. On the basis of simulation results, a prediction scheme for final rupture life of welds was discussed using the damage mechanics concept and effective stress. The applicability of nonlinear fracture mechanics was also discussed assuming the crack length from the microscopic simulation results. Obtained results are summarized as follows : As results of the simulation showed, peaks of the number density of small defects in the subsurface could be predicted, showing good correspondence with the observed results. Final failure life prediction based on the damage mechanics concept was found to be applicable, by assuming both the final failure surface connecting the weakest grain boundaries, and effective stress against this surface. Fracture mechanics approach was also found to be applicable when assuming the crack length from the high peaks of the simulated small defect in the last stage of creep life.