某油田用抽油泵柱塞在服役两个月内即发生凡尔罩断裂失效，利用光学显微镜、扫描电镜和显微硬度计等仪器从断口的宏观及微观形貌特征、显微组织、硬度和化学成分等方面对其进行了分析。结果表明：该凡尔罩不合理的结构设计和较差的表面加工质量导致了表面应力的集中和疲劳裂纹源的过早产生；同时大量大尺寸含硅夹杂物冶金缺陷的存在导致了沿晶裂纹的迅速扩展，最终导致凡尔罩发生了疲劳断裂失效。

The valve cover of a oil-well pump plunger fractured after use for less than two months in an oil field. The macro and micro fracture appearance characteristic, microstructure, hardness and chemical compositions of the valve cover were analyzed by optical microscope, scanning electron microscope, microhardness tester, etc. The results show that the unreasonable structure design and poor surface machining quality of the valve cover caused its surface stress concentration and premature appearance of fatigue crack source. At the same time, the existence of lots of inclusions containing silicon in the material resulted in rapid propagation of the intergranular cracks, which led to the final fatigue fracture of the valve cover.