Reason Analysis of Elbow Cracking of a Gas Pipe

The reasons for the elbow leakage of a gas pipe in a gas production plant were analyzed by means of macro analysis, physical and chemical property analysis, microscopic analysis and fracture analysis. The results show that there was no obvious abnormality in the wall thickness of elbow pipe, the inner wall of the elbow was corroded seriously, and the inner arc side was the most serious corrosion part. The chemical composition and mechanical properties of the failure pipe were in line with the requirements of 20 steel. The hardness test showed that the hardness of the area near the crack was higher than that of other areas. The microstructure of the elbow was tempered martensite, but some of the martensite was relatively coarse and contained non-tempered martensite. The surface of the crack growth area was covered by a thick layer of corrosion products, and the crack had obvious characteristics of multiple sources. The elbow cracked because of sulfide stress corrosion. The existence of wet H₂S gas in pipe, non-tempered martensite and corase martensite in the microstructure were the primary reasons for stress corrosion cracking.