Numerical Simulation of hot forging with an integrated heat treatment considering the impact of unsteady stress state on the transformation induced plasticity

在热块体金属成形工艺之后的后续冷却过程中，发生奥氏体到第二相（例如马氏体）的结构相变。除了弹性，塑性和热膨胀应变，相变诱导的体积变化以及相变诱导塑性（TRIP）的出现。相变体积应变是由奥氏体的晶格结构向另一相的变化引起的，这导致材料体积的伴随变化。相变塑性应变产生于在奥氏体基体中形成例如马氏体颗粒期间相边界处的微塑性现象。它们对最终的残余应力状态有决定性的影响，并且可能是热锻部件中不期望的变形的原因。该项目的目的是进一步发展的残余应力状态和相变相关的变形预测的既定的数值方法。在此基础上，考虑了相变塑性（TRIP的回流效应）对载荷的影响。基于基本的实验研究，确定了两种典型热锻钢的载荷相关TRIP行为所需的材料数据。这些数据将用于扩展在第一个应用阶段开发的模型，随后将在商业FE系统中使用用户定义的子程序实现。最后，扩展的材料模型将进行测试和验证的基础上，一个演示组件的背景下，一个封闭的模锻工艺链与集成热处理。成形过程将进一步延长e. G.去毛边或修整阶段。在本研究建议的范围内，调查的重点是扩散控制的转变类型。因此，在中等或缓慢冷却速率下的集成冷却将在平静的空气中或在砂浴中进行。为了实施去毛刺阶段，将重新设计和制造相应的工具系统。作为最后一点，开发的数值方法的验证是通过比较计算的变形和残余应力与实验金相和XRD研究中测量的真实的组件。

In the course of a subsequent cooling after a hot bulk metal forming process, a structural phase transformation of austenite into secondary phases (e.g. martensite) occurs. In addition to elastic, plastic and thermal expansion strains, transformation induced volumetric changes as well as transformation induced plasticity (TRIP) arises. Transformation volumetric strain results from the change in the lattice structure of austenite to another phase, which leads to an accompanying change in the material volume. Transformation plasticity strains arise from the micro-plasticity phenomena at the phase boundary during the formation of e.g. martensite particles in an austenitic matrix. They have a decisive impact on the resulting residual stress state and can be the reason for undesirable distortion in the hot forged component. The aim of this project is a further development of the established numerical approaches for the prediction of residual stress state and transformation related distortions. Hereby, the load-dependent influence of transformation plasticity (back flow effect of TRIP) has been taken into account. Based on the fundamental experimental investigations, required material data regarding the load-dependent TRIP-behavior for two typical hot forging steels is to be determined. This data will be used to extend the models developed in the first application period, which will subsequently be implemented in a commercial FE system using user-defined subroutines. Finally, the extended material model will be tested and validated on the basis of a demonstrator component in the context of a closed die forging process chain with an integrated heat treatment. The forming process will be furthermore extended by e. g. a deflashing or a trimming stage. Within the scope of this research proposal, the investigations are focused on the diffusion-controlled transformation types. Therefore, the integrated cooling under moderate or slow cooling rates will be carried out in calm air or in sand bath. To implement the deflashing stage, the corresponding tool system will be redesigned and manufactured. As a final point, the validation of the developed numerical methods is performed by the comparison of calculated distortions and residual stresses with the ones measured on real components within experimental metallographic and XRD investigations.