某船用柴油机曲轴运行不到3 a发生断裂，采用力学性能测试、化学成分分析、金相分析、断口观察以及有限元分析等方法对其进行了失效分析。结果表明：该曲轴的断裂性质为低应力下的疲劳断裂；电渣熔铸过程中卷入的保护渣是曲轴发生断裂的直接原因，未达标的调质和渗氮处理则进一步降低了曲轴的疲劳强度，加速了疲劳裂纹的扩展，最终导致曲轴断裂。

A marine diesel engine crankshaft fractured after running for less than 3 a. The failure analysis was conducted on the crankshaft by mechanical property test, chemical composition analysis, metallographic analysis, fracture observation and finite element analysis. The results show that the fracture property of the crankshaft was fatigue fracture under low stresses. The protective slag involved in the electroslag casting process was the direct reason for fracture of the crankshaft. The substandard tempering and nitriding treatment further reduced the fatigue strength of the crankshaft, and accelerated the propagation of the fatigue cracks, eventually leading to the fracture of the crankshaft.