Microstructure and Properties and Remaining Life Prediction of T91 Heat Resistant Steel/TP347H Stainless Steel Dissimilar Steel Welded Joint after Service

The microstructure and properties of the T91 heat resistant steel/TP347H stainless steel dissimilar steel welded joint of a certain supercritical unit after 760 000 h of service were studied. According to the tensile test and creep test, the remaining life of the joint was predicted by the Larson-Miller parameter method. The results show that coarse M₂₃C₆ carbides and Laves phase precipitated at grain boundaries and plate boundaries of T91 heat-resistant steel base metal, and the degradation characteristics during high-temperature service were significant. The fusion zone at T91 heat-resistant steel side had a transition layer enriched with M₂₃C₆ carbides and Laves phase, and a decarburization layer formed by carbon elements migration. The microstructure in the weld zone was dense. The MX particles were uniformly and evenly dispersed. Spherical and chain-like M₂₃C₆ carbides precipitated at grain boundaries and plate boundaries of TP347H stainless steel base metal and its fusion zone, and no σ phase or ζ phase appeared in the grains. The hardness of the joint showed gradient distribution characteristics. The hardness of T91 heat-resistant steel base metal exceeded the required upper limit, and the performance was deteriorated. The hardness of the weld zone fluctuated around the specified lower limit, and performance in partial areas was deteriorated. The hardness of the TP347H stainless steel base metal still met the requirements. Compared with room temperature, the tensile fracture position of the joint decreased at 575 ℃ high temperature shifted from the T91 martensitic heat-resistant steel base material to the fusion zone. The remaining life of the joint was 130 000 h by predication.