铁路货车钩缓装置中的钩尾销螺栓在螺杆近方头一侧根部位置发生断裂。采用微观分析方法分析了该螺柱断裂原因。综合分析表明：该断裂螺栓方头一侧可能仅局部加热后锻打成四方头,并发生过热,造成在方头一侧的螺柱根部形成马氏体组织和魏氏体组织过渡的U型区域。不良组织的存在增大了钢的脆性,冷加工性能大大降低,在U型区域和螺柱表面交界处萌生了微细裂纹。当钩尾扁销螺栓在列车运行过程中受到过大的冲击时,该螺栓上的微细裂纹在螺栓靠近方头一侧存在的沿轴向的拉应力和螺栓正常紧固力的合力联合作用下张开并迅速扩展,造成螺栓断裂。

The bolts of coupler yoke fractured in the head of screw near the location of the roots.The reasons for the bolts fracture were analyzed by microcosmic analysis tools. Comprehensive analysis showed that the side of the head of the fractured bolts were likely to be heated locally then forged into the square,and be overheated simultaneously. There were larger difference between microstructure from the side of the head to that of the screw thread. The transition zone of the martensite structure and the widmanstaten structure was formed ahead of the U shape near the fracture,and there existed larger tensile stress than other zone. Since non-performing organization existed,the brittleness of steel increased,cold working performance decreased greatly and microcrack initiated. When the bolts of coupler yoke were too large impact in the course of train operation,the microcrack on bolts splayed and expanded rapidly,then the bolts fractured finally under the combination of tensile stress along axes near the side of head and composition of forces of normal fastening bolts.