Evaluation of Low Cycle Fatigue Damage in Grade 91 Steel Weld Joints for High Temperature Applications

Grade 91 steel is a heat treatable steel and hence its microstructure is very sensitive to temperature. The weld joint consisting of a heterogeneous microstructure exhibits a lower fatigue life than that of the base metal. This is due to the presence of soft zone in the heat affected zone (HAZ). The failure location in the weld joint is very sensitive to the test parameters such as temperature and strain amplitude and application of hold. At high temperatures and under hold application, strain localization occurs in the soft intercritical HAZ (ICHAZ) which results in the failure in the HAZ. Sub- surface creep cavity formation in the soft region and their linkage cause enhanced crack propagation and that translates into lower fatigue life of the weld joint at high temperatures. Occurrence of compression dwell sensitivity in the material is attributed to the presence of surface oxides.