Failure mechanism analysis of the crack located at the neck of stainless steel flange

Abstract During new pipeline installation supervision inspections in a large petrochemical project, several dozen flanges, made of S30403, failed due to lateral cracks in the neck areas. A detailed analysis of the failure modes and mechanisms of the flanges was carried out. The chemical composition, hardness, metallographic, microstructure structure and EDS analysis in crack zones were tested by spark discharge optical emission spectrometer, Brinell hardness tester, metalloscope, and scanning electron microscope, respectively. It was proved that in the metallographic structures, excessive wire- and network-like ferrites and a large amount of carbides were observed. The crack originated from the outer surface of the neck of the flange and extended to the center of the flanges along the wire- and network-like ferrites. Meanwhile, a large amount of impurities were found inside the cracks as well, which were confirmed as chromium oxides. It was indicated that the flange may have cracked during the forging process, and then oxidation occurred at a high temperature. Some micro-cracks were formed at the aggregating areas of non-metallic inclusions during the forging of the flange, and masses of ferrites provided the path for rapid crack growth. Therefore, the failure of the flanges should be caused by improper heat treatment processes and excessive non-metallic inclusions in the materials.