Failure Investigation of Superheater Tubes in Utility Boiler

Abstract The failure investigation of superheater tubes (SA213 Gr T22 grade materials) after 28 years of service at one of the Saudi Basic Industries Corporation (SABIC) affiliates is presented in this paper. The superheater tubes operate within the design conditions at a pressure of 50.8 kg/cm2 (0.722 Ksi) and temperature of 551 °C. The failed tubes exhibit fish-mouth rupture. The microstructure of the superheater tubes was analyzed by optical microscope and scanning electron microscope (SEM) to examine microstructural evolution over the service period. Tube sample composition was determined by X-Ray Fluorescence Spectroscopy (XRF). Deposits and scale phases and chemistry were examined by X-Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS), respectively. Micro-hardness test was conducted on the samples to identify the hardness variation at and remote to the fracture. Deformed grains with elongated creep voids were observed at the grain boundaries, while significant scale deposits about 2.3 and 1.2 mm thick, mostly iron oxides, were observed on the outer and internal surface of the tube, respectively. Extensive spheroidization of the microstructure lead to carbides and sulfides precipitation due to extended exposure at high temperature. The failure mechanisms are related aging of the tubes due to long-term overheating, creep, and the degradation of microstructure.