Precipitate phases in type 316 austenitic stainless steel resulting from long-term high temperature service

The precipitate phases in a type 316 steel superheater tube which failed by creep were studied by using a newly developed X-ray internal standard technique and scanning transmission electron microscopy. The precipitate phases identified were sigma, M23C6 and eta; the eta phase was the predominant precipitate in the sample. The major effect of the formation of large amounts of eta phase is the removal of molybdenum from solid solution, causing a decline in long-term creep strength. By comparing the precipitate phases of the failed tube with laboratory-tested specimens it was possible to estimate the mean exposure temperature of the failed tube. This was found to be approximately 923 K (6500C) and was about 80 K above the design temperature. (Auth.)