Cause Analysis of Superheater Tube Cracking in Medium Temperature Separated Circulating Fluidized Bed Boiler

After running 41808 hours, the superheater tube of a medium temperature separated circulating fluidized bed boiler in a plant cracked. The tube is swollen and thick-lip-shape fracture is covered with oxides. By utilizing energy dispersive spectrometer(EDS), transmission electron microscopy(TEM), optical microscopy(OM), scanning electron microscopy(SEM), ANSYS, and other methods to detect the tube, it is found that the element content of the tube conforms to the standard. In terms of the shape of the fracture morphology, it is transcrystalline rupture, along with dimple and pores in the grain boundary. The carbides gather and grow at the grain boundaries, and the morphology of tempered sorbite is not obvious. By thermal field and stress analysis, the tube stressed runs in super-temperature environment. The tensile strength of the failed tube is not up to the standard. In all, the failure reason of superheater tube is high temperature creep cracking which is caused by the tube is not up to the T91 steel standard and long-term overheating resulted from the oxide layer.