Abstract: Leakage occurred in the stainless steel tee-junction of a low-pressure nitrogen system in a nuclear power plant. After cutting, cracking was found in the pipe fitting. The failure causes of the stainless steel tubes were analyzed by means of morphology observation, scanning electron microscopy, energy spectrum analysis, metallographic examination, chemical composition analysis, hardness test, intergranular corrosion test, and residual stress test. The results show that there was a large cold deformation and no solid solution treatment during the processing of the tee-junction, which led to a large number of slip bands in the matrix structure, resulting in high hardness of the pipeline and increased residual stress, which was the root cause of the tee-junction fracture. The C content of the tee-junction was relatively high, and there was intergranular carbide precipitation during welding. Cold deformation led to large residual tensile stress in the tee sample, which increased the electrochemical activity of the material, reduced the pitting potential of the material, and increased the corrosion sensitivity. The above factors together led to intergranular stress corrosion cracking of the tee-junction in a humid marine atmosphere.