Dual approach in modelling TMF life in hot forging dies

Hot forging dies are affected by different failure mechanisms, which can mutually influence the total damage and are caused by the application of cyclic mechanical and thermal load; wear, plastic deformation, creep and thermo-mechanical fatigue (TMF) are the principal actors in generating the damage and therefore determining the service life of the dies. Focusing the attention on the thermo-mechanical fatigue phenomena it can be recognized that it involves complex experimentation and long testing runs. A survey of the literature clarifies that testing techniques consider a constant or slowly varying temperature of the specimen and temperature distribution in the cross section is usually uniform. These conditions are really far from those present in the hot forging dies, where temperature changes reach some hundreds of °C in few seconds and a thermal gradient is present. Aimed to investigate thermo-mechanical fatigue in hot forging process a new test has been proposed suitable to include the whole set of variables present in the forging process. The paper presents a dual approach to investigate thermo-mechanical fatigue, leading to a simple interpretation of the experimentation (heuristic model) and to a more advanced theoretical model based on the concept of the damage per cycle. In the former part Design of Experiments (DoE) techniques have been adopted in order to determine the influence of process parameters on TMF life of a typical hot-working steel. A heuristic model, able to estimate TMF life as a function of maximum temperature cycle, minimum temperature cycle and the ratio between equivalent stress and yield strength at corresponding temperature, has been obtained using Surface Modelling (RSM). In the latter part a theoretical model is proposed as a generalization of the Woehler-Miner law. This generalized model is based on the theory of damage formulated by Chaboche and Lemaitre and is tuned and validated using a subset of the experimental plan.