Modelling the Forming Limit during Cross-Rolling of Seamless Pipes Using a Modified Continuum Damage Mechanics Approach

For the production of seamless pipes it is necessary to pierce a round solid billet in the first forming operation. In the subsequent processing steps the hollow is elongated at the push bench and annealed. The seamless pipe gets its final dimension in the stretch reducing mill. For a realistic description of material failure during hot forming using Forge (R) 2008, the validation of the finite element model was the first step in this study. Since the used damage variable is of dimensionless type, special focus was laid on the correct calculation of the deformation path during piercing. It could be shown that a correct calculation of the geometry and the resulting forces is possible using the developed model. After a summary of previous studies this work focuses on the piercing operation in order to picture a real piercing process using damage mechanics. A modified continuum damage mechanics approach is presented, which enables the reduction of the damage parameter at negative stress triaxialities. This is obvious since a decrease of pores and cavities at hot forming conditions and negative triaxialities, e.g. cogging, is generally observed.