Investigations of gradient-dependent nitrided forging tools in hot bulk metal forming under cyclic thermomechanical loading

用于锻造应用的工具的氮化被认为是锻造应用的最先进技术，并且正在工业应用中使用。由于氮的扩散，在靠近表面的区域中存在相变，这导致工具的耐热性、耐磨性、疲劳强度以及耐腐蚀性的改善。虽然氮化可以提高工具的使用水平，但是目前还没有一种建模技术来精确计算氮化锻造工具的磨损数值。在本项目的框架内，将开发一种用于氮化锻造工具磨损数值计算的建模技术。为了描述硬度的变化，将开发用于表征循环热应力和循环热机械应力下硬度发展的试验方法。在锻造过程中，工具不仅要承受热载荷，还要承受复杂的机械应力。在文献中描述了机械载荷降低热锻造工具材料的奥氏体起始温度Ac 1。因此，时间-温度结晶化行为的基础上的时间结晶化（ZTA）图与叠加的机械负荷进行了研究。硬度作为温度和机械载荷的函数进行调节，通过硬度测量实验确定。随后，所得的硬度曲线在商业FE程序中实施。最后，以杯形件反挤压工艺为例，对磨损模型进行了验证。因此，可用的结果是一种新的建模技术，与现实的工具磨损在热大块金属成形可以预测。

The nitriding of tools for forging applications is considered to be state of the art for forging applications and is being used in industrial applications. Due to the diffusion of nitrogen, there is a phase change in the areas near the surface which leads to an improvement in heat resistance, wear resistance, fatigue strength as well as corrosion resistance of the tools. Although a general improvement in tooling levels through nitriding is known, there is no modeling technique to perform accurate numerical wear calculations of nitrided forging tools.Within the framework of the project, such a modeling technique for the numerical wear calculation of nitrited forging tools is to be developed. In order to describe the change in hardness, test methods for the characterization of hardness development under cyclic-thermal and cyclic-thermomechanical stress are to be developed. In the forging process, the tools are exposed not only to thermal loading, but also to a complex mechanical stress collective. It is described in the literature that the mechanical load lowers the austenite starting temperature Ac1 of hot forging tool materials. Therefore, the time-temperature austenitization behavior based on time-austenitization (ZTA) diagrams with an overlaid mechanical load is to be investigated. The hardness, adjusted as a function of temperature and mechanical load, is determined experimentally by means of hardness measurements. Subsequently, the resulting hardness curves are implemented in a commercial FE program. Finally, the validation of the wear model is carried out on a backward cup extrusion process (NRFP process). The available result is thus a new modeling technique with which a realistic tool wear in the hot bulk metal forming can be predicted.