某天然气公交车发动机主轴承盖螺栓发生断裂，通过宏观观察、化学成分分析、扫描电镜及能谱分析、硬度试验和金相检验等方法，分析了螺栓断裂的原因。结果表明：螺栓的失效模式为氢脆断裂；在服役过程中，螺栓在较长时间的应力作用下，其头部圆角处产生应力集中，且存在初始裂纹，螺栓、水汽和天然气中的微量氢元素向螺栓初始裂纹附近扩散、聚集，当氢含量达到临界值时，裂纹扩展，最后导致螺栓发生断裂。

The main bearing cap bolt of a natural gas bus engine was fractured. The causes of bolt fracture were analyzed through macroscopic observation, chemical composition analysis, scanning electron microscope and energy spectrum analysis, hardness test and metallographic inspection. The results showed that the failure mode of bolt was hydrogen embrittlement fracture. During service process, stress concentration occurred at fillet of bolt head under the action of a long period of stress, and there was an initial crack. The trace hydrogen in the bolt, water vapor and nature gas diffused and accumulated near the intial crack of the bolt. When the hydrogen content reached a critical value, the crack expanded, and finally the bolt was fractured.