阀杆螺母断裂原因分析

CAUSE ANALYSIS ON THE SCREW NUT FRACTURE OF VALVE STEM

通过化学成分分析、微区成分分析等方法,对阀杆螺母断裂原因进行了分析。断裂螺母的主要合金元素符合铝黄铜化学成分标准规定,表明螺母材质经长期使用未发生材质化学组成改变。由EDS分析可见,选择点为富铁相组成。螺母拉伸试验结果表明,屈服强度有较大的提高,说明材料产生了明显的加工硬化效应,致使断裂前的塑性储备降低。断口分析显示,近螺母内壁断口平齐、表面覆盖油质物质,证实螺母存在初始裂纹,初始裂纹的存在也将产生很大的应力集中,容易诱发裂纹和使裂纹扩展。螺母在阀门操作或密封时承受很大载荷时,在螺母变径处应力突变发生断裂。

Through chemical composition analysis and EDS analysis,this paper analyzes the fracture cause of the screw nut of valve stem.Primary alloy element compositions of the screw nut are within the standard of aluminum brass,which indicates that the chemical composition of the screw nut hasn't been changed.Through EDS analysis,composition of the selected area is Fe-rich phase.Through tensile test of the screw nut,yield strength increased obviously.This indicates that obvious work hardening effect of the materials took place,which led to the decrease of the ductility allowance.Analysis of fracture morphology shows that fracture near inner wall of screw nut is flat and smooth and there is oily substance covered on the surface,which proves that there are original cracks on the screw nut.The existence of original cracks makes stress concentration larger,which results in cracks and crack propagation easily.Fracture occurs because of sudden increase of stress on the variable diameter section of the screw nut while subjected to large load during the operation or sealing of the valve.