Investigation of Early Cracking of Dissimilar Metal Welds in a 1000-MW Ultra-supercritical Power Plant Close to the Sea

The early cracking of dissimilar metal welds (DMWs) between T91 martensitic steel and austenitic steel in a 1000-MW ultra-supercritical power plant was investigated in this article. Optical microscopy and scanning electron microscopy were used to characterize the microstructures of DMWs. The result shows that the breakdown of DMWs was brittle rupture along the prior austenite crystal boundary. The crack extended from T91 heat-affected zone to T91 base metal. Compared with the spare DMWs, its tensile strength and bending properties met the standard for DMWs. Hardness of T91 HAZ from failed DMWs was much higher than that of spare DMWs. Some pitting was observed at the T91 HAZ. Associating the high hardness of DMWs and particular service environment, it is concluded that the failure of DMWs was caused by the stress-corrosion crack.