某型船用柴油机在运行中自动停机,检查发现机体上固定凸轮轴的止推板螺钉、止推板以及凸轮轴自由端均断裂;断裂凸轮轴材料为16MnCr5钢,凸轮轴使用时间约8000h。通过宏观分析、化学成分分析、力学性能试验、断口分析以及金相检验等方法对凸轮轴断裂原因进行了分析。结果表明:凸轮轴在加工时采用传统的退刀加工工艺,过渡圆角半径过小,且加工刀痕明显,导致倒角处应力集中过大;加之凸轮轴材料硬度偏低,铁素体条带组织严重,凸轮轴表面未经任何强化处理,因而疲劳强度严重降低;在交变应力作用下,凸轮轴于应力集中严重的过渡圆角处萌生裂纹并以疲劳方式扩展。凸轮轴疲劳断裂后进而引起止推板及其固定螺钉断裂,最终造成柴油机自动停机故障。最后从制造、修理、使用管理和故障分析角度提出了一些可行性建议。

Automatic shutdown fault happened to a certaint ype of marine diesel engine during operation. By dissembling inspection, it was found that both the thrust plate screws and the thrust plate that fixed on the camshaft fractured, and the camshaft also fractured at the free end. Fracture reasons of the camshaft were analyzed by means of macro examination, chemical compositions analysis, mechanical properties test, fracture analysis and metallographic examination. The results show that traditional retracting processing made the transition radius of the camshaft was too small and the machining marks were obvious, so serious stress concentration formed at the chamfering position. At the same time, fatigue strength of the camshaf tdecreased because of lower hardness, serious banded ferrite structure and no surface treatment. Under the action of alternating stress, cracks formed from the stress concentration position and propagated in fatigue form. And then the fracture of the camshaft caused fracture of the thrust plate and its fixing screws, and finally resulted in shutdown of the diesel engine. Some feasible suggestions were put forward from the aspects of manufacture, repair and using management, fault analysis.