Fracture Cause of Electric Gate Valve Stem in a 1 000 MW Ultra-Supercritical Unit

A electric gate valve stem at the outlet of feedwater pump in a 1 000 MW ultra-supercritical unit fractured during valve opening. The fracture cause of the stem was analyzed by macromorpholgy observation, chemical composition analysis, mechanical performance test and microstructure observation. The results show that the microstructure of the stem was nonuniform, and there was local porosity in the center area. A lot of striped or massive ferrite existed in the microstructure resulting in insufficient plasticity and uneven comprehensive mechanical properties of the stem. The serious jam occurred between the valve disc and the slide rail on the outlet side during valve opening. The machining quality of the annular platform surface of the stem was poor, and the transition arc angle of the variable section root of the annular platform at the bottom of the stem was small. The cracks initiated at the stress concentration area of the variable cross-section root of the annular platform when opening the valve, and then expanded quickly, resulting in the brittle fracture of the stem finally.