Cracks found in the weld groove parts of valve nozzle of the shutdown cooling system of a reactor in repairing is analyzed. The reasons of the cracks, the crack number and orientations are confirmed by visual inspection and penetranting testing. According to the results of nondestructive inspection, typical cracks regions are cut into some samples including crack transect samples，longitudinal crack samples and transverse crack samples. Scanning electron microscopy is used to investigate the microstructure and micro zone composition of the crack transect. Optical microscopy is used to analyze the microstructures of the longitudinal crack and the transverse crack. The results show that cracks may extend from the inside to outside wall and that grain deformation around the crack is not found. There are no obvious grain differences between crack and non-crack regions. Brittle fracture characteristic exists in crack cross section, and the internal corrosion of crack is found. The main component of corrosion products is oxygen, and the corrosion is caused by oxidation of stainless steel. Combined with the valve nozzle operating conditions, the reason for the failure of valve nozzle is believed to be stress corrosion cracking.

valve nozzle  /  crack  /  stress corrosion cracking