Process Development for Passive Granular Media-Based Tube Press Hardening

To produce tubular components with high-strength and high-stiffness but without length restriction, a new process based on a combination of press hardening and granular media-based tube forming is presented. Compared to conventional tube hydroforming using liquid media or gas, the use of granular forming media offers the advantages of being stable at relatively high temperatures and avoiding leakage problems. The previously developed granular media-based tube press hardening, where the forming force is directly induced through the granular medium, is severely limited in the length of the tubes. The proposed solution is a passive granular media-based tube press hardening process. Granular material is used as a passive forming medium inside the tube, while the forming force is applied on the tube’s circumference by a rigid punch. The tube material is 22MnB5. Process defects for press hardening of tubes without forming media are discussed and the process chain for a passive granular media-based tube press hardening is presented. The impact of various levels of granulate filling is investigated with FEM simulations showing friction causes asymmetric deformation, which can be avoided by a double-acting process. A lower limit is analytically derived for the filling factor. An analytical approach to predict the punch force underestimates the required forces, as the inhomogeneous cap hardening, and the complex stress state of the granulate are not considered.