Failure of an ASTM A213 T12 steel tube of a circulating fluidized bed boiler

•Localized bulging was observed in a section of the outermost tube of the superheater.•Severe degradation of the microstructure was evidenced in the bulging region.•A weld defect affected the steam flow and local heat transfer.•The heterogeneous flue gas flow induced localized overheating of the tube.•The failure was related to a conjunction of long time overheating and creep.