Abstract: A large number of cracks were found in the gasket slot of high pressure flange in a hydrocracking unit. The maximum depth of the cracks was 130 mm and the length was close to the whole flange circumference. Some of them had extended into the parent metal from the weld overlay. In order to find the reasons for the formation and expansion of these cracks, the high pressure flange was tested by macro inspection, non-destructive testing, metallographic examination, hardness and mechanical testing, chemical composition analysis, fracture scanning electron microscopy and corrosion products test of fracture. It was found that the reasons for the high-pressure flange cracking include the incomplete solution treatment which gives rise to the sigma phase in the material, the corrosive elements such as Cl and S in the medium, the production of polythionic acid, the long-term contact with hydrogen and the stress concentration environment caused by multifaceted stress

Key words: high pressure flange, weld overlay, crack, gasket slot, stress concentration