Abstract: During the course of piercing medium/heavy-wall shells with the cone type piercer(CTP),lots of longitudinally parallel crack-like folds are formed on the inner surface of the shell due to failure of the plug with folded surface of its rolling section.Taking the actual piercing operation situation and the cause for the inner fold as identified via the investigation into consideration,preventative actions are taken,including adopting the plug made in the H13 steel,improving the piercing process parameters,and strictly controlling the reduction ratio of the shell,etc.As a result,the said defective fold is eliminated,thus qualified blanks are ensured to be provided to the following cold-working process,and the primary yield greatly is considerably enhanced.