发动机缸盖螺栓疲劳断裂失效分析

通过断口(宏观)形貌分析、微观组织分析、化学成分检测以及硬度测试等手段对SCM435钢发动机缸盖螺栓的疲劳断裂进行失效分析。结果表明，失效SCM435钢缸盖螺栓的化学成分和显微组织合格，硬度符合技术要求。失效样件1的断裂原因为表面脱碳引起的脆性断裂，螺栓断裂部位的表面脱碳层宽为120μm;失效样件2主要是非金属夹杂物所引起的疲劳断裂，近球状的CaS-CaO-Al₂O₃-MgO复合型冶金夹杂物造成了沿晶裂纹。对材料的塑韧性、表面脱碳及非金属夹杂物等指标，提出了改善建议。

Failure analysis of fatigue fracture of engine cylinder head bolts

Failure analysis of fatigue fracture of SCM435 steel engine cylinder head bolts was carried out by means of fracture (macroscopic) analysis, microstructure analysis, chemical composition detection and hardness test. The results show that the chemical composition and microstructure of the failed SCM435 steel cylinder head bolts are qualified, and the hardness meets the technical requirement. The brittle fracture of the failed specimen 1 is caused by surface decarburization, the width of surface decarburization layer at the fractured part of the bolt is 120 μm. The fracture of the failed specimen 2 is mainly fatigue fracture caused by nonmetallic inclusions, and the near spherical CaS-CaO-Al₂O₃-MgO composite metallurgical inclusions cause intergranular crack. Suggestions for improving the indicators such as plasticity and toughness, surface decarburization and nonmetallic inclusions of the material are put forward.