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

Abstract

During a maintenance shutdown, a localized bulging was observed in a section of the outermost tube of the superheater, an ASTM A213 grade T12 (UNS K11562), Cr-0.5Mo ferritic steel. The objective of this work was to investigate the failure of the steel tube, part of a L-shape superheater of a CFB boiler. The investigation was performed through microstructural characterization, chemical analysis, thermodynamic calculations, hardness measurements, tensile and creep tests, measurements of the oxide scale thickness on selected regions of the failed tube for determination of the remaining life assessment. It was concluded that the failure was due to a conjunction of long time overheating and creep. A welding defect (root dropout) caused an alteration of the steam flow and, therefore, impaired the proper heat transfer between the tube and the steam. In addition, the uneven flow of the flue gas induced a heterogeneous heat distribution along the tube length, particularly in the area facing the flue gas, evidenced by the erosion marks. The high local temperature resulted in intense oxidation, decarburization and degradation of the microstructure. As a consequence, the steel strength was reduced, with consequent bulging and failure. These points were reinforced by the results of the RUL assessment, where the value was negative at the referred region, meaning the end of its operational life.