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. •Paper investigates the effect of post-weld heat treatment on creep rupture of Grade 91 cross-weld specimens.•Type IV failure and Type I failure reproduced as a function of heat treating temperature.•Numerical model of cross-weld creep developed.•Stress-relaxation test used as test method for input creep rates for the model.