Determination of the crack formation causes in a welded joint of a pipeline made of dissimilar steels

This article identifies the causes of crack formation in a welded joint of a pressure pipeline, which had been in operation for over 70 years and failed under normal operating conditions. The appearance of cracks was detected along a welded joint of the pipe sections. Microstructural investigations have shown that the pipeline sections were fabricated from Grade 4 ferrite-perlite with a content of 75% ferrite grains, which related to readily weldable materials, and Grade 5 steel with a content of 28% ferrite grains, which refers to limited weldable materials. The obtained ultimate tensile strength and yield strength values of the base metals of Grade 4 and Grade 5 steels correspond to strength classes K50 and K52. On the basis of conducted microstructural and fractographic investigations, it has been shown that welding of such materials negatively affects the welded joint quality. The critical defect contributing to crack formation was an extensive lack of fusion with a depth of ~3–4.5 mm located at the weld root. This defect, found in the heat-affected zone of the Grade 5 steel, acted as a stress concentrator, leading to cracks initiation and propagation, which caused the pipeline technical failure.