某10.9级螺栓经淬火、回火处理后装配到汽车座椅上，经历颠簸疲劳试验2万次后发生断裂。从化学成分、断口形貌、硬度、显微组织方面，对螺栓断裂的原因进行了分析。结果表明：螺纹牙底过渡部位粗糙不平且有缺口，易造成应力集中产生裂纹；螺纹表层产生脱碳，增加了开裂倾向；在疲劳试验中，由于交变应力的作用形成微裂纹，造成螺栓早期疲劳断裂。

After undergoing the bumpy fatigue experiment for 2×10⁴ times, the fracture was happened to a 10.9 grade bolt that was assembled on the car seat after quenching and tempering treatment. The fracture reasons of the bolt were analyzed from the aspects of chemical composition, fracture morphology, hardness and microstructure. The results show that:the transition of the thread root was rough with gap, and cracks would be easily generated by the stress concentration; the decarburization in the thread surface layer increased the cracking tendency; micro-cracks formed as a result of the action of alternating stress in the fatigue test, causing the bolt early fatigue fracture.