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

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.