Approach for the numerical determination of the hardness evolution in the tool surface layer due to thermal loads during press hardening

Tailored tempering is a process for manufacturing components with locally adapted strength. By using partially preheated tools, the critical cooling rate of the blank material 22MnB5 is selectively undercut locally in order to suppress the formation of martensite in the component. However, the increased tool temperature and the occurring press forces lead to a high thermal load on the tool engraving and ultimately to wear phenomena that are dependent on the hardness evolution and occurring tempering effects. This affects the dimensional accuracy of the components and the tools have to be reworked or even renewed in a cost- and time-intensive manner. In this context, the heat transfer between the blank and the tool is another important aspect that is needed to calculate the resulting surface temperatures of the tool. In order to measure the heat transfer from the blank to the tool during a press hardening process, a realistic test was set up in the first phase of the project. Thermocouples were placed in the test tools close to the edge layer in order to be able to record temperature curves in the process by varying the surface pressure. The temperature curves recorded were used to determine heat transfer coefficients, which are used in the course of the project to increase the calculation accuracy of the boundary layer temperatures in the context of an FE simulation.

In the next stage, thermal test profiles were defined on the basis of the experimentally recorded temperature curves, with which four tool steels (1.2367, 1.3343, CP2M and CR7V-L) were cyclically loaded using a dilatometer. The profiles were defined in such a way that not only conventional press hardening, but also tailored tempering could be considered by varying the cycle start temperature up to 600 °C. Especially for the tailored tempering load cases, a clear decrease in hardness could be observed, which explains the increased wear already observed in real industrial applications. After metallographic evaluation, the recorded hardness values were used to develop an analytical approach and implement it in a commercial FE application. This allows the local calculation of the tool hardness as a function of the forming cycles and the existing tool temperature within the framework of forming simulations.

In the last stage of the project, an experimental validation was carried out by means of a model test designed and set up at the research centre for the production of components with locally controlled hardness distribution. With the software implementation also developed in the project, the hardness decrease in the tool could be successfully predicted realistically.

The IGF project "Approach for the numerical determination of the hardness evolution in the tool edge layer due to thermal loads during mould hardening", IGF project no. 19518 of the Forschungsvereinigung Stahlanwendung e. V. (FOSTA), Sohnstraße 65, 40237 Düsseldorf, Germany, was funded by the German Federal Ministry for Economic Affairs and Energy via the Alliance for Industrial Research (AiF) within the framework of the programme for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag. The final report can be requested from the Forschungsvereinigung Stahlanwendung e. V. (FOSTA).