A model for the estimation of TMF life in hot forging

Thermo-mechanical fatigue (TMF) can be recognized as one of the three phenomena causing the failure of the die utilized in hot forging. The other ones (wear and plastic deformation) increase the damage continuously during the production and can be directly monitored by the loss of precision of the formed component. TMF damage also grows gradually, but its effects on production is evident at the catastrophic failure of the die, without any previous alert. For this reason a possible estimation of life of the die can be recognized as an extremely useful information, assisting in the choice of the material of the die, as well as in fine tuning the forming process. A previous experimental investigation resulted in an heuristic model, based on Response Surface Modelling (RSM), able to estimate TMF life as a function of maximum temperature cycle, minimum temperature cycle and the ratio between equivalent stress and material yield strength, the last term being function of temperature. These experimental results were the basis for a further investigation on theoretical models present in the literature, leading to a new one proposed by the authors as a generalization of W\uf6hler-Miner law [1]