Evaluation of T91 after 130,000 hours in service

Two superheater tubes fabricated from T91 were removed for evaluation after 130,000 hours service. Regions of interest included a hot section that replaced type 321 stainless steel, dissimilar welds to type 321 stainless steel safe ends, and cold bends. The evaluation consisted of metallography, transmission electron microscopy (TEM) and precipitate analysis, hardness, tensile properties, creep, and stress rupture. Observed microstructure and properties of the tubing were compared with results on unexposed and thermally aged materials. Overall, the properties were similar to expectations. Substructural coarsening occurred which produced reductions in the short-time tensile and creep strength. However, ductility remained high. Corrosion was minimal, although somewhat greater on the fire side of the cold bend. Short oxide wedge cracks were observed at the dissimilar metal welds, and type 4 cracking was observed in an untempered region of a stainless steel attachment weld.