Failure analysis of a bellows expansion joint in flue gas pipeline of catalytic cracking unit

Failure analysis of a bellows expansion joint in flue gas pipeline of catalytic cracking unit is presented in this study. In order to identify the causes of the failure, a series of investigations including fracture morphology analysis, metallographic examination, mechanical property testing and finite element stress analysis were conducted. The results show that the fracture morphology has cleavage characteristics, which indicates the failure mode is brittle fracture. It is further proved by the tensile test results, i.e., the measured elongation is about 10%, which is far lower than the requirement of standard performance ≥ 30%. There is obvious difference between surface microstructure and internal microstructure of the bellows expansion joint, and mixed microstructures are also observed. This suggests that improper heat treatment is the key reason caused material embrittlement and poor plasticity. Moreover, FEM simulation results show that elliptical deformation occurred in the end tube section of the expansion joint due to the effect of the bending moment of the pipe system. It makes the bellows bear additional bending moment. In this situation, the maximum stress is located at the outer wall of the bellows wave peak. The maximum stress is as high as 368 MPa, which is higher than the permissible stress 84 MPa. Hence, the final failure occurred at the bellows wave peaks.