Failure Analysis of Inlet Branch Pipes of a Reformer

The inlet branch pipes connected the aggregate pipes and furnace tubes of a reformer, the outlines of them extended as “S” shapes, and they were made of austenitic steel 1Cr18Ni9Ti. After about 60,000 hours service, a lot of cracks were found near the elbows of inlet branch pipe and joints with the furnace tubes, many of them propagated through the pipe walls, and lead to leakage of raw material. In order to explore the failure causes, the microstructure of cracks and corrosion ingredients remained on fracture surface were analyzed by scanning electron microscopy (SEM), chemical composition, metallographic and mechanical properties of the branch-pipe steels were examined. The test result showed that the cracks propagated from the outside to inside of the pipe wall, the fracture surfaces were almost flat, there was no obvious deformation existed at the fracture section. Certain percent of chlorine and sulphur elements could be found and the secondary cracks could be observed on the microstructure fracture surfaces. The inlet branch pipes were elongated when they were connected with the aggregate pipes and furnace tubes, the stress were analyzed by finite element method (FEM) under the tensile deformation, the results showed that the stresses near the elbow were high. It could be concluded that the crack of the inlet branch pipe were stress corrosion cracks accelerated by the aggressive medium rich of CI and S ion and high stresses. Based on the causes analysis results, the corresponding countermeasures and improvements were suggested.