Numerical and experimental investigations on the fatigue life of hot work tool steel X38CRMOV5-3 under forging process conditions

Hot forging dies experience during service excessive cyclic thermo-mechanical, tribological as well as chemical loads. These loads occur in a repeated manner and may cause premature fatigue failure of the forming tools and thus lead to an interruption of the production process. Die failures due to fatigue crack initiation constitute about 25 % of all failure types. The initiation and propagation of fatigue cracks can mainly be ascribed to high cyclic thermal and mechanical loads exerted on the tool material. Thus the hot work tool steel in service should combine a high red hardness with the ability to withstand heat checking at a high abrasion resistance. One of the most commonly used hot work tool steels for manufacturing high quality tools for hot forming operations like forging and casting is AISI H13 (X38CrMoV5-3), which also provides these properties. The numerical simulation based on the finite element method (FEM) has so far become an indispensable tool for the design and optimisation of hot forging processes. So far FE based process simulations are limited to obtaining accurate results related to the formability of the workpiece material and the necessary press force. Tool related aspects like the prediction of the tool life quantity and the estimation of abrasive wear is so far limited to cold forming tools. Due to the complex thermo-mechanical phenomena occurring in the interface layer between workpiece and forming tool it was so far not possible to give a reliable estimation on the tool life as current modelling approaches do not capture relevant influences in order to describe forging die fatigue and damage mechanisms in a realistic manner. For the prediction of the maximum cycles until fatigue failure it is still a common approach to resort to strain amplitude based models which neither take into account the transient temperature evolution nor the triaxiality of the local stress state. It is obvious that hot work tool steel materials need a more sophisticated modelling as severe thermo-mechanical loads are prevailing. In order to make a reliable estimation on the tool life quantity of forging dies it is therefore necessary to use advanced and sophisticated material models.