Experimental investigation on the thermo-mechanical fatigue in hot- and warm- forging

In hot and warm forging processes, widely adopted in the manufacturing industry when complex shape or large components should be obtained in medium-high production runs, coupled thermal and mechanical cycles are the main causes of damage for dies: wear, plastic deformation and TMF cracking are the possible failure mechanisms. Differently from wear and plastic deformation where the damage is gradually incremented during the service life of the dies and their effects can be monitored on the forged components, TMF failure arises suddenly without any previous notice on the forged part. Reduction of the risk of these failures, or an improvement of tool service life, can be obtained by a proper choice of die material, as well as by tuning the forging cycle parameters. To this aim industrial practice and experience can be helpful, but at the moment estimation of tool life can not be achieved neither by experimental technique, nor by simulation software. For the investigation of thermo-mechanical fatigue (TMF) phenomena that arises in dies for hot and warm forging operations a new simulative laboratory test has been developed. The experimental apparatus can apply thermo-mechanical cycles on the material specimen generating thermal gradients in the cross-section and reproducing on a small portion of material conditions not so far from those of industrial die. TMF behaviour of a die steel (DIN X37CrMoV5-1 vacuum remelted) has been investigated on this system considering different working conditions and taking into account the effects of i) maximum temperature cycle, ii) minimum temperature cycle, and iii) eq /Y (T ) ratio (cyclic mechanical load and temperature are varied in phase) on specimen life. Design of Experiments (DoE) techniques have been used to define the experimental campaign consisting of a screening design followed up with a response surface design. Same experiments have been used to tune and validate a theoretical model proposed by the authors as a generalization of Woehler-Miner law following Chaboche approach.