Long-term overheating cracking analysis of 12Cr1MoV boiler superheater tube

After running 39840 h, a boiler superheater tube made by 12Cr1MoV steel in a plant cracked. The tube is blocked and the thick-lip-shape fracture is covered with oxides. By utilizing optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and other methods to detect the tube, it is found that the compositions of blocking substances are Na₃PO₄, Fe₂O₃ and NaFeO₂. In the unblocking position of the inner wall oxide is Fe₃O₄, however, in the blocking position, it is Fe₂O₃. In terms of the shape of the fracture morphology, it is inter granular, along with secondary cracks and pores in the grain boundary. The research results illustrate that the metallographic microstructures in samples of near crack and away from the crack are the same. With the lamellar pearlite of the initial state dissolves, the carbides gather and grow at the grain boundaries. The organization spherifies seriously, and the carbide is M₂₃C₆. According to the electric power industry standard, the spherical degree is identified as level 5. By utilizing spheroidization level and run-time to estimate the wall temperature, it can be know the tube run in over-temperature environment. To be specific, the hardness in all the samples is about 135 HV, which is relatively low. In all, the failure mode of superheater tube is high temperature creep cracking which is caused by long-term overheating resulted from the oxide layer and the blocking substances.