Evaluation of process-induced damage based on dynamic recrystallization during hot caliber rolling

The amount of damage induced during hot forming depends not only on the stress state but also on material softening due to dynamic recrystallization (DRX), which can be used to delay or reduce damage initiation. An extension to the Gurson–Tvergaard–Needleman (GTN) model has been proposed recently that includes a new coupled DRX-damage nucleation criterion taking DRX and the stress state into account. In the present paper, a 4-pass caliber rolling process is considered where the bar stock is rotated by 90° after each rolling pass. The extended GTN model is applied to the caliber rolling process to predict the internal damage induced during the rolling process. The simulation results show that the highest void volume fraction (VVF) occurs during the first rolling pass. However, the variations of stress state during each pass assist in reducing the damage in the subsequent passes. The material softening due to increasing DRX in subsequent passes also helps to reduce the void nucleation. The microscopic analysis of the rolled bar confirms the damage distribution predicted by the simulations.