12Cr2Ni4A滑油附件主动轴螺纹根部裂纹分析

Analysis of Cracks in Thread Root of Drive Shaft of 12Cr2Ni4A Lubricating Oil Accessories

航空发动机在大修时发现12Cr2Ni4A滑油附件主动轴轴头螺纹根部有裂纹显示。采用化学成分、金相、断口分析以及硬度检测等手段对裂纹产生原因进行分析。结果表明:裂纹为过大螺母预紧力产生的应力裂纹,但螺纹部位的加工设计不当加剧了裂纹的产生,例如螺纹根部存在氰化层降低了材料的断裂韧度;止动槽与螺纹交界处未倒圆易形成应力集中;止动槽上部宽底部窄,导致垫圈内锁片挤压U型槽底部产生挤压变形。为避免此类故障的发生,提出相应的改进措施:使用限力扳手控制螺母不要过度预紧,对不需要氰化的螺纹部位采取遮蔽的方式,对止动槽与螺纹转接处处进行适当倒圆。

In this paper cracks were found in the root of the spindle head of the 12Cr2Ni4A lubricating oil accessories when the engines were overhauled in the factory.Different means such as chemical composition analysis,metallographic analysis,fracture analysis and hardness testing were used to study the causes of cracks.The results show that stress cracks are caused by the nuts excessive pre-tightening force,but the improper processing design of the thread area promotes the formation of the crack.For example,existence of cyanide layer on the thread root area reduces the fracture toughness of the material;stress concentration produced by the geometric transition of the edge of lock groove;the upper area of the lock groove is wider and the bottom is narrower,which leads to the extrusion deformation at the bottom of the U groove.Through the above analysis,the corrective actions are put forward to avoid such failures that pre-tightening force of the nut produced by using a limited force plate hand should not be overly large,and also a shaded way should be taken for the thread area that do not need cyanide carburizing,at the same time,proper roundness is carried out at the geometric transition edge between the lock groove and the thread.