Search

In this work, tensile samples of a Yttrium-alloyed nickel-base superalloy 718 were eletrochemically charged with hydrogen and compared with reference samples with no Y addition. It was shown that Y promoted an increase in the ductility of the material for both the hydrogenated and the reference samples, especially for the homogeneous deformation. This effect was attributed to the refinement of the microstructural features and the control of deleterious elements like sulphur and oxygen, mitigating the embrittling effect of hydrogen. Furthermore, the Y-alloying hampered the H ingress into the material and increased the critical concentration needed for fracture along the grain boundaries. [Display omitted] • Y-alloying increased the overall ductility of the alloy. • Loss of ductility due to H was lower for the Y-alloyed samples. • Y can provide more H trapping and lower the H ingress. • Y lowers the content of harmful elements and increases grain boundary cohesion. [ABSTRACT FROM AUTHOR]

[Display omitted] • 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. 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. [ABSTRACT FROM AUTHOR]