Abstract:A large number of fractures or cracks occurs in brass three way valve flange of GIS equipment in a substation, which is easy to cause SF6 gas leakage and has serious safety hazards. Through the analysis and research on macroscopic appearance, chemical composition, metallographic structure, structure of the failed three way valve, the reasons for the cracking of three way valve are clarified and the improvement measures are put forward. Results show that the flange of three way valve is lead brass and subjected to large circumferential tensile stress due to improper structural design and assembly method. Under the action of surrounding corrosive environment containing oxygen and sulfur elements, stress corrosion cracking occurs along the stress concentration area of the flange.