Examination and Simulation of Failure Analysis in 90-Degree Elbow Joints

In this study, the elbow joint of a natural gas pipeline at a 90-degree angle was investigated at the pressure reduction station. Despite not being in operational conditions, a longitudinal crack appeared in the joint after welding operations. To identify the factors leading to joint degradation, tests such as fracture testing, examination of the macroscopic crack structure, chemical composition analysis using quantum testing, optical microscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, macrohardness testing, and tensile testing at ambient temperature were conducted. Based on the analysis of the results and modeling performed on the joint, the primary factor contributing to crack formation is attributed to issues in the construction stages and residual stresses resulting from welding operations. The increase in hardness near the crack indicates the influence of residual stresses on the microstructure of the joint during welding operations. This also results in localized reduction in mechanical strength. Consequently, flaws, weaknesses in the microstructure, and microscopic cracks were observed in the images obtained from optical and electron microscopy. Therefore, it can be concluded that the examined joint has become sensitive due to the welding process and is subjected to residual stresses.