Abstract: The cracks were found during machining the external threads of some fillet of tool joint pins.Microstructure observation was used to analyze the cracking cause.The results show that the main cracking reason was that the precipitation of carbides caused microstructure overheating in the precipitation during forging.The cracks generated in the bonding position between tops of carbides and matrix resulting from the stress concentration of heat stress and machining stress,and then formed cracks during deformation.