Numerical investigations on the fatigue failure of forging tools due to thermo-mechanical cyclic loading

During service hot forging dies are exposed to a combination of cyclic thermo-mechanical, tribological and chemical loads. Besides abrasive and adhesive wear on the die surface, fatigue crack initiation with subsequent fracture is one of the most frequent cause of failure. Fatigue cracks in forging dies are caused by local elasto-plastic strains due to cyclic mechanical and thermal loads. Aiming at a reduction of tool and production costs of forgings, tool life extension is necessary. In order to reach this goal, process design and process optimisation are currently done with the help of the finite element analysis (FEA). An integral part of this procedure is the tool design based upon fatigue life maximisation of forging dies in the Low-Cycle-Fatigue (LCF) region. Within the framework of this research the material fatigue damage of a forging die due to cyclic thermo-mechanical loads is analysed based on FEA process simulations. Besides this a material model which considers cyclic elasto-plastic material behaviour was used to describe the constitutive behaviour of the forging dies. The results of a fatigue life damage analysis applied to an industrial forging process in which the lower die shows a clear fatigue crack initiation. The crack initiation sites predicted with the FEA process simulations agree with the ones observed on the real die component.