Abstract: The cracks were detected in a SAF2507 duplex stainless steel pipelines for a nuclear power plant. The causes of the cracking were analyzed using chemical composition analysis, metallographic examination, scanning electron microscope observation, energy spectrum analysis. The results show that the cracks type is SCC, the cracks originates form the inner surface of pipeline. The root cause of the defects is that there is excessive repair welding in the weld, which increases the sensitivity of the material to intergranular corrosion, and SCC occurs under the combined action of residual stress and chloride ion in seawater.