某动车机组用304奥氏体不锈钢螺栓在使用约2个月后发生断裂，采用宏观观察、化学成分分析、微观分析等方法分析了其断裂原因。结果表明：螺栓的原材料为轧制态，显微组织为奥氏体+铁素体+碳化物，导致螺栓原材料的力学性能产生方向性，使材料径向塑性和韧性明显降低，在螺纹搓丝过程中，螺纹根部附近产生了较多的加工裂纹；织构组织使这些裂纹在低周径向应力的作用下更易发生扩展，最终导致螺栓发生疲劳断裂。

The 304 austenitic stainless steel bolt for a motor train unit fractured after about 2 months of use. The fracture causes were analyzed by means of macroscopic observation, chemical composition analysis and microscopic analysis. The results show that the raw material of the bolt was rolled, and the microstructure was austenite + ferrite + carbide, which led to the directionality of the mechanical properties of the raw material of the bolt, and the radial plasticity and toughness of the material were obviously reduced. In the process of thread rubbing, there were many processing cracks near the root of the thread. The textured structure made these source cracks more likely to expand under the action of low-cycle radial stress, which eventually led to fatigue fracture of bolts.