Creep rates of heat-affected zone of grade 91 pipe welds as determined by stress-relaxation test

Cross-weld creep testing conducted at 650 °C under 69 MPa stress has reproduced either the Type I or Type IV failure mode in Grade 91 welds, depending on the post-weld heat treatment procedures. Welds post-weld heat treated below the AC1 temperature have ruptured in the Type VI failure mode, while welds heat-treated above the AC1 temperature of the alloy have ruptured in the Type I failure mode. Heat-treatments at lower temperatures and shorter durations have produced a reduced creep rate. The accelerated short term stress-relaxation test has been conducted to obtain the input creep rates for a finite element model for the cross-weld creep testing. The model predicted secondary creep rates are in good agreement with the results from the conventional cross-weld creep tests. From the finite element model, the creep damage by cavitation is believed to start at regions where the first principal stress and stress triaxiality concentrate.