38CrMoAl钢液气弹簧活塞杆断裂失效分析

Fracture failure analysis of 38CrMoAl steel piston rod of hydro-pneumatic spring

采用光学显微镜、扫描电镜及能谱仪对38CrMoAl钢液气弹簧活塞杆断裂样品进行检测。结果发现,断口呈快速断裂的放射状条纹特征形貌,表明该失效件断裂速度快。活塞杆末端外圆存在数条圆弧状表面裂纹,推测该处受到外力作用的高速撞击,使得法兰端承受极大的弯曲应力,最终在应力集中最大的凹槽部位产生断裂。断口剖面显微组织检测发现,法兰端存在两处凹槽,由靠近杆部的第一凹槽产生断裂。实测第二凹槽倒圆角半径R极小并产生表面裂纹,表面裂纹两侧渗氮层明显加深,表明该表面裂纹形成于渗氮处理之前,属于调质处理过程形成的淬火应力集中开裂,由此推断第一处凹槽同样存在应力集中开裂倾向。断裂源区存在表面裂纹及表面剥落,表面渗氮层脆性大。杆部外圆存在表面脱碳层,并产生脆性针状氮化物。样品表层及心部的显微组织为粗大马氏体位向的回火索氏体,基体组织为粗大过热组织,材料强韧性比正常晶粒组织显著降低。

Fractured specimen of a 38CrMoAl steel piston rod of hydro-pneumat spring was detected by means of metallographic microscope,scanning electron microscope and energy spectrum.The results show that the fracture is fast fracture with radial fringe features.There are several arc-shaped surface cracks on the outer circle of the piston rod end,it is inferred that the high-speed impact from the external force causes the flange end to bear great bending stress and finally produces the fracture in the groove where the stress concentration is greatest.Microstructure examination of the fracture shows that there are two grooves at the flange end of the piston rod,and the fracture is caused by the first groove near the rod.At the second groove where the rounded corner is,the radius R is very small and the surface crack appears.The nitriding layer deepens obviously on both sides of that surface crack,indicating that the surface crack is formed before nitriding and belongs to the quenching stress concentrated crack formed in the quenching and tempering process.Through that,it is concluded that the stress concentration cracking tendency also exists in the first groove.There are surface cracks and surface spalling in the fracture source area,and the surface nitrided layer is brittle.The surface decarburization layer exists in the outer circle of the rod,and brittle acicular nitrides are produced.The microstructure is tempered sorbite with coarse martensite orientation,and the matrix is coarse superheated microstructure,which cause the strength and toughness of the material to be reduced significantly.