Abstract: The welded joint of dissimilar steels connecting the inlet pipeline and flange of a raw material cooler experienced failure and cracking. The causes of the failure and cracking were comprehensively investigated through methods such as macroscopic inspection, non-destructive testing, chemical composition analysis, hardness testing, metallographic analysis, and macroscopic morphology of the fracture surface. The chemical composition, hardness testing, and non-metallic inclusions of the dissimilar steel welded joint all met the relevant requirements. The metallographic structure demonstrated that the crack was located in the fusion line area between the weld and the base metal on the pipeline side, initiating from the inner wall of the pipe and propagating towards the outer wall. The crack surface of the fracture was brown, which was the original morphology of the weld surface. The weld bead was clear, and the weld surface was smooth, being the free surface left by welding. Comprehensive analysis ultimately indicated that the failure and cracking occurred due to the poor welding quality of the weld. The medium was likely to remain and accumulate at the defect sites. Meanwhile, there was a condensation phenomenon in the medium near the failure section. Due to the different materials of the base metal and the weld metal, in the presence of an electrolyte solution, galvanic corrosion occurred at the fusion line between the weld and the base metal on the pipeline side.