Cracking Failure Analysis of a 2205 Duplex Stainless Steel Elbow in a Natural Gas Treatment Station

A leakage accident occurred in an elbow of a natural gas processing station in western China. In order to analyze the cause of the accident, non-destructive testing, chemical composition analysis, hardness testing, metallographic examination, scanning electron microscopy analysis and other experimental analyses were carried out on the elbow. The obtained results showed that the present leakage is mainly attributed to the cracks. In fact, the microstructure around the cracks was decarburized which fully indicates that the cracks have experienced a high temperature environment. In other words, the elbow had formed initial cracks before the heat treatment process of manufacturing. Then, the cracks grew until they penetrated the wall thickness under the action of working pressure. In addition, considering that the material properties test results of the elbow meet the standard requirements, and there are no H2S, H2 and other media that cause SSC and HIC in the service environment, it is once again confirmed that the failure of the elbow is caused by defects in the original manufacturing stage. It is recommended that the same batch of elbows in service to carry out internal and external defects, and strengthen the quality inspection of the new elbows, to avoid defective elbows in the field to put into use.