Effect of additional axial tension on formability of equal curvature-diameter bending without mandrel for 0Cr18Ni9 stainless steel tube

Abstract

The equal curvature-diameter (ECD; R0/D0 = 1.0; where R0 is the bending radius, and D0 is the outer diameter of tube) bending process without mandrel was regarded as the tube’s minimal bending condition. Compared with pure bending, this represented a very complex physical process with multifactor effects. The quality and forming limit of tube bending are affected by additional axial tension (AAT); therefore, in this study, the ECD bending process without mandrel for tubes was performed by combining the finite element (FE) method and experimental research. The effects of AAT on the bending formability, such as wall thickness variation, cross-section distortion, and springback, were investigated. The results showed that: (1) under a certain friction coefficient between the tube and pressure die, AAT was constant, the maximum wall thickness variation rate, the maximum cross-section minor axis variation rate, and the springback angle all increased with the increase of the bending angle; (2) under a certain bending angle, maximum wall thinning rate, maximum cross-section short axis variation rate, and springback angle were positively associated with the friction coefficient, while the wall thickening rate was negatively associated with it. These data implied that the maximum wall thinning rate, the maximum cross-section short axis variation rate, and the springback angle all increased, while the maximum wall thickening rate decreased with the increase of AAT; (3) when the friction coefficient was too small, AAT was too small. Although the degree of the wall thinning, cross-section distortion, and springback were relatively small, the degree of wall thickening was large and even wrinkled. On the contrary, when the friction coefficient was too large, AAT was too large. Although the degree of maximum wall thickening was relatively small, the degree of maximum wall thinning, maximum cross-section short axis variation rate, and the springback angle were large, and the tube’s surface was extensively scratched. When the friction coefficient was too large, it would have a negative impact on the comprehensive bending formability of tube. In conclusion, our study indicates that in order to ensure a tube’s comprehensive bending formability, AAT should be properly controlled by reducing the friction coefficient between the tube and pressure die.