Fundamental investigations of gradient-dependent nitrided forging tools in hot forging under cyclic thermomechanical loads

Nitriding of tools is now state of the art in forging applications and is used industrially. This Process uses a diffusion of nitrogen into the tool material resulting in a transformation of the near-surface regions. This leads to an increase in hardness and an improvement in hot strength and thus resulting in a higher wear resistance, fatigue strength and corrosion resistance. Even though a general improvement of tool life by nitriding is known, there is still no numerical modelling technique to perform an accurate wear calculation of nitrided forging tools considering thermal and mechanical cyclic loading.

During the forging process, the tools are not only exposed to thermal, but to a complex thermo-mechanical load. It is described in the literature that the mechanical component of the loading lowers the austenite starting temperature Ac1 of hot work steels. Austenitisation of the material in combination with high cooling rates due to cooling lubrication can lead to partial hardening of the microstructure. Therefore, the time-temperature austenitising behaviour will be investigated using time-austenitising (ZTA) diagrams to be recorded with a superimposed mechanical load. The hardness that develops as a function of temperature and mechanical load is determined experimentally by hardness measurements. For this purpose, a test methodology is being developed that enables the targeted application of superimposed cyclic thermal and mechanical loads. Based on the experimental findings on the development of hardness under cyclic thermal and mechanical loading, an extended description of the Archard wear model is implemented in a commercial FE software. By taking into account the local process-related hardness development in the forging tool, the localised wear can be calculated realistically. Finally, the wear model is validated on different hot forging processes. The available result is a new test method and modelling technique that can be used to realistically predict tool wear in hot forging.