Failure cause assessment of steam reformer radiant tube after long‐term service

• Published in: Materials and corrosion 2022-08, Vol.73 (8), p.1265-1273
• Main Authors: Dobrotvorskaia, Anna N., Dobrotvorskii, Mstislav A., Shevyakova, Elena P., Simanov, Michail A.
• Format: Article
• Language: English
• Subjects: Carbides; centrifugally cast steel furnace tubes; Chemical composition; Creep (materials); creep rupture; Failure analysis; Grain boundaries; material degradation; Mechanical properties; microstructural observation; Microstructure; Overheating; Oxidation; phase transformation; Phase transitions; reformer furnace tube; Room temperature; Root cause analysis; Silicon dioxide; Spectrum analysis; Tubes
• ISSN: 0947-5117; 1521-4176
• DOI: 10.1002/maco.202213068

This study investigates the damage, microstructure, and mechanical evolution of service‐exposed HP40NbTi radiant tubes of a steam reformer furnace. Tube failure was reported after 6‐year of working at 525°C–830°C in the form of visible longitudinal and transverse cracks. The microstructure was evaluated using optical and scanning electron microscopy equipped with energy‐dispersive X‐ray spectrum analysis. The carbide phase transformation was determined by the method based on the matrix dissolution technique and X‐ray diffraction. The results of this study showed that the primary cause of the longitudinal crack formation is the local overheating and the oxidation of the dendritic grain boundaries. The M7C3 carbide transforms completely into the metastable M23C6 phase during high‐temperature service. Metallographic examinations revealed the shrinkage cavities and creep voids that are nucleated on both Cr‐ and Nb‐carbides. The continuous oxide layers of Cr2O3 and SiO2 formed near the inner and outer tube surfaces and the matrix's chemical composition near the outer tube surface was found to be depleted of Cr and C. Mechanical properties are irregular across the samples. A little margin of safety is noted, while the complete ductility dip is detected at room temperature. The root cause analysis of the failure under investigation showed that the tube failed due to the creep. Material degradation analysis of 6‐year service radiant tubes revealed the local overheating to be the primary cause of visible crack formation. Shrinkage cavities and numerous creep voids near the Cr‐ and Nb‐carbides were detected. A precipitate‐free region was found near the outer tube surface. The little margin of safety and complete ductility dip were noted.