Abstract: Longitudinal cracks with different severity are found via non-destructive flaw detection in some drill pipe bodies from an X95 lot as having undergone joint friction welding and internal surface coating. The morphology of and causes for the said cracks are analyzed by means of such methods as metallography,SEM and EDS. Furthermore,causes for the cracks are traced based on the manufacturing process. As a result,it is identified that the said cracks are caused due to such facts that some low melting point electrode material sticks to the external surface of the pipe body which results in certain heavy metal pollution,when the pipe body contacts the electrode during the pipe-end magnetic particle examination operation,and that in the processing operations as follow,the pipe is reheated,and consequently,the heavy metal elements spread along the grain boundary,which eventually leads to the development of the cracks.