Abstract: Through the macroscopic inspection,metallurgical structure inspection,energy spectrum analysis,and on-site tracking,crack formation reason of NC40 joint of drilling pipe was analyzed.The results show that the crack on the outer wall of the female joint was considered as the forging folding defect.Relatively too large length-diameter ratio of the joint round bar would be prone to the depression and fold in the middle of the tong,which was the main cause of the transverse crack on the joint surface.In addition,partial over heating caused alloy elements melting and segregating along the grain boundary.The folding defect extended along the weakened grain boundary.Finally,the corresponding improvement measures and suggestions were proposed combined with practical production.