船用柴油机针阀体失效分析及改进措施

针阀体是柴油机喷油器的关键零件。某船用柴油机仅运行了5h,其喷油器针阀体即开裂。开裂的针阀体材料为18CrNi8钢,经过930℃×4h乙炔气真空渗碳、820℃×2.5h氮气淬火、-100℃×2h深冷处理和180℃×4h回火处理。对开裂的针阀体进行了化学成分分析、断口分析、金相检验、硬度测试和有效硬化层测试。结果表明:针阀体肩胛倒角处组织过热,且存在易成为裂纹起始点的机加工缺陷,导致其在发动机运行过程中在交变应力的作用下发生疲劳开裂。根据针阀体开裂的原因提出了改进措施,取得了良好的效果。

Analysis on Failed Needle Valve Body of Marine Diesel and Improvement

Needle valve body is a type of key component of diesel injector.Needle valve body of a certain marine diesel injector cracked at the time when the diesel operated for as little as 5 h.The needle valve body was made of 18CrNi8 steel,and subjected to vacuum carburizing at 930℃for 4 h with acetylene gas,soaking at 820 for 2.5 h then quenching in nitrogen,cold treating at-100℃for 2 h,followed by tempering at 180℃for 4 h.The failed needle valve body was tested for chemical composition,fracture feature,microstructure,hardness,and effective hardening depth.The results demonstrated that the shoulder chamfer of the needle valve body exhibited overheated structure and machining defect from which crack emanated,thus resulting in its fatigue cracking by the action of alternating stress during the operating of engine.Some improvements were put forward on the basis of the cause of the needle valve body cracking,and good result has been acquired.