W形弹条疲劳断裂分析及对策

Fatigue Fracture Analysis and Countermeasure for W⁃type Elastic Bar

W形弹条在疲劳检验过程中发生断裂,以断裂弹条为研究对象,采用金相显微镜、扫描电子显微镜(Scanning Electron Microscope,SEM)、洛氏硬度计、布氏硬度计等,对断裂弹条及同批次60Si2Mn原材料进行力学性能、金相组织、脱碳层、断口扫描、硬度等试验,分析弹条断裂原因,并提出改进措施。结果表明:该60Si2Mn原材料的力学性能、脱碳层、布氏硬度均满足标准要求;距断裂表面约100μm处存在夹杂物,夹杂物中有Ca、Si、P、O、K等元素,为疲劳断裂裂纹源;弹条断裂的根本原因是,弹簧钢浇注过程中结晶器液面发生波动,导致保护渣卷入钢液。原材料进厂增加DS夹杂物级别和非金属夹杂物位置判定后,生产弹条疲劳试验过程未出现断裂。建议TB/T 3395.5—2015《高速铁路扣件第5部分WJ-8型扣件》增加DS夹杂物级别和非金属夹杂物位置指标。

A W-type elastic bar fractured during the fatigue test.Taking fracture elastic bars as the research object,the mechanical properties,metallographic structure,decarburization layer,fracture scanning,hardness and other tests were conducted on the fracture elastic bars and the same batch of 60Si2Mn raw materials using metallographic microscopy,scanning electron microscopy(SEM),Rockwell hardness tester,Brinell hardness tester,etc.The reasons for the fracture of the bars were analyzed,and improvement measures were proposed.The results show that the mechanical properties,decarburization layer,and Brinell hardness of the 60Si2Mn raw material could meet the standard requirements.There are inclusions at point about 100μm from the fracture surface,which contain elements such as Ca,Si,P,O,K,and are the sources of fatigue fracture cracks.The fundamental reason for the fracture of the elastic bar is that the liquid level in the crystallizer fluctuates during the pouring process of the spring steel,leading to the inclusion of protective slag in the steel liquid.After adding DS inclusion level and determining the location of non-metallic inclusions to the raw materials entering the factory,there was no fracture during the fatigue test of the production elastic bar.It is suggested that adding DS inclusion level and non-metallic inclusion location indicators to TB/T 3395.5—2015 Fastening systems for high speed railways Part 5:WJ-8 fastening system.