大型船用发动机针阀体失效分析与制造工艺改进

Analysis on Failure of Needle Valve Body of Large-sized Marine Engine and Improvement in Its Manufactruring Processes

某大型船用发动机在可靠性试验60h后针阀体外表面开裂。针阀体材料为18CrNi8钢,经过960℃真空渗碳、820℃气淬、-100℃冷处理和150℃回火。对开裂的针阀体进行了化学成分分析、断口分析、金相检验和硬度及有效硬化层深度测定,以确定其开裂的原因。结果表明:针阀体为脆性断口,有效硬化层深度超标,且有大块状碳化物,外圆面还存在机加工缺陷,这些因素导致其开裂。采取相应的预防措施后,针阀体开裂的问题已经解决。

Needle valve body of a large-sized marine engine,made from 18CrNi8 steel and vacuum carburized at 960℃,gas quenched from 820℃,cold treated at-100℃,and tempered at 150℃,cracked at external surface after 60 h of test for reliability.The cracked needle valve body was subjected to chemical composition analysis,fracture testing,metallographic examination,as well as to hardness and effective hardening depth measurements,in order to discover the reason why it cracked.The results showed that the needle valve body manifested brittle fracture,and had overproofed effective hardening depth,massive carbide and machining defect at external surface,wich resulted in its cracking.After the relative preventive measures being adopted,the problem of needle valve body cracking has been removed.