某厂汽车张紧轮用40Cr钢螺栓在装机试验时断裂, 为查明断裂的原因, 对其进行了失效分析。结果表明: 该螺栓断裂方式为氢致延迟断裂, 断裂原因是由于其显微组织中存在较多回火马氏体导致螺栓硬度偏高, 同时螺栓中存在微裂纹,在酸洗镀锌工序中引入的氢向裂纹处扩散, 从而诱发了断裂; 建议在加工过程中将螺栓硬度控制在32~34 HRC, 并在适当的温度烘烤足够长的时间以去除氢, 这样有利于抑制其氢致延迟断裂。

In a factory, the 40Cr steel bolt for tension wheel of vehicle fractured during trial installation. In order to confirm the fracture reason, the failure analysis was conducted on the bolt. The results show that the fracture mode of the bolt was hydrogen induced delayed fracture. The fracture reasons were that the hardness of the bolt was relatively high due to too many tempered martensite in the microstructure and that the micro-cracks existed in the bolt; The hydrogen introduced during the pickling and galvanizing process diffused into the cracks, resulting in the fracture. To control the bolt hardness in 32-34 HRC during machining process and eliminate the hydrogen by baking at an appropriate temperature for a sufficiently long time will be in favor of preventing the hydrogen induced delayed fracture of bolt.