Effect of internal pressure on hydro bending of double-layered tube

Abstract

The double-layered tube hydro bending method is proposed to solve the difficulties encountered in manufacturing ultra-thin-walled elbow tubes by traditional bending processes. The effect of internal pressure on the section flattening, the wall thickness distribution, and the bending defects are investigated. The specimen consists of an outer tube made of carbon steel and an inner tube made of stainless steel, with the thickness ratio of outer to inner layers equal to 10. The expression of the yield pressure of the double-layered tube is derived from theoretical analysis. In addition, the mechanism of the wrinkling and stabilizing of the inner tube is clarified with the help of finite element analysis. It is shown that the stability of the inner layer during bending is improved with internal pressure increasing. The wrinkling could be avoided when the internal pressure exceeds 0.2 times the yield pressure. The maximum non-circularities of the inner and outer layers under the yield pressure are only 3.3% and 2.5%, respectively. Besides, the thickness distributions of two layers along the bending line are very similar, the positions of maximum thinning of the outer and inner layers are all located at an angle from the symmetrical section. The wall thickness of two layers decreases with increasing internal pressure. It is proved by the experiments that the double-layered tube hydro bending method is feasible for manufacturing the ultra-thin elbows.