某电厂调速器步进电机轴在开机调负荷过程中发生断裂，对断裂电机轴进行了宏观检验、化学成分分析、硬度测试、金相检验和断口分析，并对步进电机轴材料进行了切应力校核。结果表明：该调速器步进电机轴断裂失效为低应力高周旋转/弯曲疲劳断裂。电机轴断裂失效的主要原因一方面是因为变径部位退刀槽位置容易造成应力集中现象，从而促使步进电机轴表面产生疲劳裂纹；另一方面是因为硫化物、碳化物等夹杂物的存在会降低材料的塑性、韧性和疲劳强度，进一步造成应力叠加，材料力学性能降低，加速疲劳裂纹的形成和扩展。

The governor step motor shaft of a power plant fractured during the startup load adjustment process. The fractured motor shaft was analyzed by macroscopic examination, chemical composition analysis, hardness test, metallographic examination and fracture analysis. And the shear stress of the step motor shaft material was checked. The results show that the failure of the governor step motor shaft was low stress and high cycle rotation/bending fatigue fracture. The main reasons for the fracture failure of the step motor shaft were as follows:on the one hand, the stress concentration phenomenon always happened at the variable diameter position of receding groove, which caused fatigue cracks on the surface of the step motor shaft; on the other hand, the existence of inclusions such as sulphide and carbides would reduce the plasticity, toughness and fatigue strength of the material, and further more, it caused stress superposition, reduced mechanical properties, and accelerated fatigue crack formation and expansion.