Automatic welding technology has been widely used in the construction of long-distance oil and gas pipelines due to its reliable welding quality and efficiency.To analyze the cracking cause during the L555M steel welding in an ongoing domestic natural gas pipeline project,various methods are applied,including chemical composition analysis,non-destructive testing,macroscopic analysis,microscopic analysis,and scanning electron microscopy along with energy spectrum analysis.It is found that the crack results from copper enrichment,which occurs when the copper conductive nozzle of the fully automatic welding equipment collides with the weld groove during the welding process and gets short-circuited.The heat generated by the short circuit current melts a part of the copper conductive nozzle in contact with the weld groove.As the welding is completed,the melted copper aggregates at the grain boundary between the base material and the weld seam.Under the residual tensile stress of the weld seam,the copper enrichment area at the grain boundary undergoes embrittlement and then the crack forms.
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