Development of method of pipe expansion with drawing for improving the fatigue property under internal pressure

The fatigue property under internal pressure is the most important property for high-pressure cylinders. Generally, this property is nearly proportion al to the tensile strength of the tube so far as the tube is free from mate rial defects and welding defects. However, this property is affected by the residual stress. In the production for high-pressure cylinders, drawing is the most widely employed practice as it ensures high dimensional accuracy and permits the production of tubes of intermediate sizes. In this method, however, the fatigue property is decreased because of the generation of ten sile residual stress. To eliminate this tensile residual stress, a method o f pipe expansion with drawing by the use of two-stage plug has been develop ed. This method ensures the compressive residual stress on a stable basis a nd improves the fatigue limit under internal pressure. As a result, the fat igue limit improved 17% under the conditions of reducing the outside diamet er by 10% and the wall thickness by 32.7%, respectively, with 0.5% pipe exp ansion.