Failure Analysis of Welded Joint in Superheater Tube in a Waste Incineration Boiler

The premature failure of welded joints of superheater in the waste incineration boiler was investigated. The chemical composition and microstructure the welded joints met the requirements. The hardness of the base metals met the requirements of the manufacturing standards, with a higher than nominal hardness in the heat-affected zone. The fracture surface showed a classic rock candy intergranular brittle fracture. The cracks initiated from the fusion line on the inside surfaces of 12Cr1MoVG side and propagated with branching to the outer wall. The finite element simulation confirmed that the cracks initiated in the high thermal stress area. After an initial, unrelated leak had occurred, and improper measures, the corrosive alkaline boil-out solution entered the superheater and concentrated, the crack was induced by the simultaneous interaction of a corrosive environment and high tensile stress. The failure mechanism was identified as stress corrosion cracking. It might be avoided by ensuring free thermal expansion and prevention of corrosive alkaline boil-out solution intrusion.