Failure Analysis on Fracture of a Water Supply Three-way Valve Stem

Macro examination, chemical composition analysis, tensile test, impact test, metallographic examination and hardness test were performed on a fractured water supply three-way valve stem to find out its fracture failure reasons. The results show that the fracture of the valve stem was fatigue fracture. The main reason for the fracture was that the nitriding process increased the brittlement of the changeable cross-section position of stem and stem core, and the structure change of stem and stem core caused stress concentration, thus forming a crack source when the stem was extruded and sheared with fluid. Under the action of alternating stress during service of the valve stem, the formed crack gradually extended from the nitriding layer to the valve stem base material, which led to the eventual fracture of the valve stem.