Failure analysis of cracked 2.25Cr–1.0Mo steel tubes of an oil refinery boiler

•Failure analysis of cracked tubes in the superheater section of an oil refinery boiler has been investigated.•The cracks initiated from oxygen corrosion pits and propagated intergranular until final fracture.•Based on finite element analysis, residual stress developed at tube weldment.•The major mechanism of failure was stress corrosion cracking due to a combination of residual stress and oxygen corrosion pits. After failure of a refinery boiler, some cracks were detected in its tubes made of 2.25Cr-1Mo steel. The tubes were in the boiler superheater section. To determine causes of the cracking, samples were taken from the tubes, and different microscopical and analytical techniques such as scanning electron microscopy (SEM) and mechanical tests were used. Furthermore, finite element analysis (FEA) was used to evaluate the stress conditions in one of the cracked tubes. The results of analysis showed that the tube material was in accordance with the ASTM A213 chemical composition requirement however based on the calculated J-factor, the material was susceptible to temper embrittlement. The FEA results showed tensile residual stress formed at welding zone due to non-alignment before tubes welding. Considering the intergranular propagation of the cracks, the hardness of HAZ and other findings, it was concluded that stress corrosion cracking was the major damage mechanism.