某0Cr16Ni6热轧退火钢螺纹定位销在服役过程中其变截面过渡区小端发生断裂。对该螺纹定位销进行了宏观观察、扫描电镜分析、化学成分分析、硬度测试等综合分析，并结合开展了疲劳试验、扭转试验、热曝露试验、磁粉检测、仿真模拟等相关试验，分析和验证了螺纹定位销的断裂过程及原因。结果表明：可以排除材料本身缺陷及机械加工原始缺陷的可能性；螺纹定位销的装配过程操作不当造成零件表面划伤，在划伤部位产生应力集中，使螺纹定位销的疲劳强度降低，最终导致疲劳断裂。

The small end of variable cross section transition zone of thread positioning pin of 0Cr16Ni6 hot rolled annealed steel fractured during service. The macro observation, scanning electron microscope analysis, chemical composition analysis, hardness test and other comprehensive analysis were carried out. Combined with the fatigue test, torsion test, thermal exposure test, magnetic particle testing, simulation and other related tests, the fracture process and cause of the screw positioning pin were analyzed and verified. The results show that the possibility of material defects and machining original defects could be excluded; the improper assembly process of the screw positioning pin caused the surface scratch of the parts, and the stress concentration occured at the scratch site, which reduced the fatigue strength of the screw positioning pin and eventually led to fatigue fracture.