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

Cross-weld creep testing conducted at 650 degrees 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 An temperature have ruptured in the Type VI failure mode, while welds heat-treated above the An 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. (C) 2016 Elsevier Ltd. All rights reserved.