Influence of a P92 typical post weld heat treatment on the long-term properties of Alloy 617 at 600°C power plant operation

In several state of the art coal-fired power plants, Alloy 617 is used as connector material between the austenitic boiler tubes and the ferritic martensitic header nozzles. During header fabrication, Alloy 617 is repeatedly heat treated jointly with the martensitic steel at temperatures around 760 °C, i.e. during post-weld heat treatment or annealing after cold working. In this paper, the influence of a typical annealing after welding and cold working with P92 typical parameters on the long-term properties of Alloy 617 at 600°C power plant operation is investigated by means of slow strain rate tests and complementary microscopic investigations on different material conditions. Due to the header fabrication annealing with P92 typical parameters and subsequent in-service exposure of Alloy 617 at around 600 °C, pronounced increase of strength values can be observed while fracture values slightly decrease. This can be attributed to the formation of very fine M23C6 carbides and precipitates of ??-phase. However with increasing exposure time, the slow strain rate fracture strain increases again, indicating the beginning of a recovery of the materials deformability due to a further evolution of the precipitation microstructure to a more favorable size and distribution of precipitates.