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

锻造工具的氮化被认为是锻造应用的最新技术，并正被用于工业应用。由于氮的扩散，在靠近表面的区域发生了相变，从而提高了刀具的耐热性、耐磨性、疲劳强度和耐腐蚀性。虽然已知通过渗氮可以普遍提高刀具水平，但还没有建模技术来执行准确的氮化锻造刀具磨损的数值计算。在该项目的框架内，这种用于氮化锻造刀具磨损数值计算的建模技术正在开发中。为了描述硬度的变化，需要开发在循环热应力和循环热机械应力下表征硬度发展的试验方法。在锻造过程中，工具不仅暴露在热载荷下，而且还暴露在复杂的机械应力集中。文献中指出，机械载荷降低了热锻模材料的奥氏体初生温度AC1。因此，基于时间-奥氏体化(ZTA)图的时间-温度奥氏体化行为的研究是有必要的。硬度作为温度和机械载荷的函数进行调整，通过硬度测量实验确定。随后，在商业有限元程序中实现了所得到的硬度曲线。最后，以反挤压工艺(NRFP工艺)为例对磨损模型进行了验证。因此，可利用的结果是一种新的建模技术，利用该技术可以预测热块金属成形中真实的刀具磨损。

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.