Abstract: Exceeding standard defects were found in a friction butt-welding joint of 139.7 mm drill pipe during examination before use.By means of macro and micro fracture analysis,the defects were the cracks originated from the internal surface of the tool head on the welding joint,which were welding cold cracks. It was caused by delayed stress relieving annealing after friction butt-welding.The cracks developed again during the subsequent quenching process and oxidized in the processes of stress relieving annealing,quenching and tempering.Oxide film with different oxidation degrees formed on the crack surface.