某公司一正常使用的气瓶瓶阀突然发生断裂，采用宏观分析、化学成分分析、金相检验、硬度测试、扫描电镜分析等方法对瓶阀的断裂原因进行了分析。结果表明：组织中局部有铅成球粒状沿晶界分布，造成晶界弱化；且瓶内存在低温介质，在低温环境下瓶阀的力学性能下降。内表面大、小孔过渡区为应力集中部位，裂纹在应力集中部位由内向外扩展，最终导致该气瓶瓶阀发生脆性断裂。

A gas cylinder valve suddenly fractured in normal use. The fracture causes of the cylinder valve were analyzed by means of macroscopic analysis, chemical composition analysis, metallographic examination, hardness test and scanning electron microscope analysis. The results show that there were spherical particles of lead distributed along the grain boundary in the microstructure, which weakened the grain boundary. In addition, there was low temperature medium in the bottle, and the mechanical properties of the bottle valve decreased under the low temperature environment. The transition zone between the large and small holes on the inner surface was the stress concentration part, and the crack propagated from the inside to the outside, which led to brittle fracture of the cylinder valve.