Abstract: A 40Cr steel ball core fails twice in a row during the fatigue test of an automotive thrust rod. To determine the cause of the failure, this article conducts a detailed analysis using methods such as macroscopic observation, microscopic observation, metallographic structure analysis, chemical composition analysis, and hardness testing. The results show that the fracture failure modes are all fatigue fracture, and the chemical composition of the ball core and Rockwell hardness meet the design requirements. The micro-cracks existing inside the surface decarburization layer and the loose surface structure and other surface defects cause the stress concentration coefficient at the structural mutation location to increase, forming stress concentration points. Under the continuous cyclic loading force, the cracks start to initiate from the defect and continuously expand inward until fracture. This research has certain reference value for the failure modes of the same type of 40Cr steel ball cores.

Key words: ball core; fatigue fracture; crack; loose; stress concentration

摘要： 某 40Cr 钢球芯在进行汽车推力杆疲劳试验时连续两次发生断裂失效，为确定失效原 因，文章采用宏观观察、微观观察、金相组织分析、化学成分分析、硬度检测等方法进行了 详尽的分析。结果表明，断裂失效模式均为疲劳断裂，球芯化学成分和洛氏硬度符合设计要 求；表面脱碳层内部存在的微小裂纹及表层组织疏松等表面缺陷导致结构突变位置应力集中 系数提升，形成应力集中点，在持续的循环载荷力作用下裂纹由缺陷处开始萌生并不断向内 扩展，直至断裂。该研究对 40Cr 钢球芯同类型的失效形式具有一定参考价值。

关键词: 球芯；疲劳断裂；裂纹；疏松；应力集中