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汽车发动机用的一批35CrMo钢缸盖螺栓使用不久后即发生大比例的断裂,断裂位置都位于螺纹尾部磨削加工段,且断口形貌都比较接近。化学成分、显微组织、硬度、表面和断口形貌的检查和分析结果表明,断裂螺栓属于氢脆引起的延迟断裂。断裂批次螺栓的酸洗工序安排在了磨削和滚丝之后,螺纹尾部在酸洗后即出现了大量表面沿晶微裂纹,服役过程中沿晶裂纹进一步扩展并最终断裂。存在过酸洗和酸洗工序安排不合理是引起螺栓断裂的主要原因。
A large number of 35CrMo steel cylinder head bolts used in automobile engines break down immediately after they are used. The fracture locations are all located at the tail grinding section of the thread, and the fracture appearances are relatively close. Inspection and analysis of chemical composition, microstructure, hardness, surface and fracture morphology show that the fracture bolt belongs to the delayed fracture caused by hydrogen embrittlement. The pickling process of the broken batch bolts was arranged after grinding and thread rolling. A large amount of surface intergranular microcracks appeared after the pickling at the tail of the thread, and the intergranular cracks further expanded and eventually fractured during service. The existence of acid pickling and pickling process is unreasonable arrangement is the main cause of bolt breakage.