Dissimilar weld region located at the cooling tube of pump was damaged and failed. We performed a root cause analysis using the Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy for the fractured surface. Many internal defects were shown in the fracture section. Root cause analysis shows that the failure is due to welding at higher temperature over 1000. This was confirmed by high-temperature cracks observed with SEM. In this study Failure scenario including initial crack generation, propagation of corrosion cracks and final failure, has been constructed and verified.