Residual stresses in components are a central issue in almost every manufacturing process, as they influence the performance of the final part. Regarding hot forming processes, there is a great potential for defining a targeted residual stress state, as many adjustment parameters, such as deformation state or temperature profile, are available that influence residual stresses. To ensure appropriate numerical modeling of residual stresses in hot forming processes, comprehensive material characterization and suitable multiscale Finite Element (FE) simulations are required. In this paper, experimental and numerical investigations of thermo-mechanically processed steel alloy 1.3505 (DIN 100Cr6) are presented that serve as a basis for further optimization of numerically modeled residual stresses. For this purpose, cylindrical upsetting tests at high temperature with subsequently cooling of the parts in the media air or water are carried out. Additionally, the process is simulated on the macroscale and compared to the results based on the experimental investigations. Therefore, the experimentally processed specimens are examined regarding the resulting microstructure, distortions, and residual stresses. For the investigation on a smaller scale, a numerical model is set up based on the state-data of the macroscopic simulation and experiments, simulating the transformation of the microstructure using phase-field theory and FE analysis on micro- and meso-scopic level.

The aim of this work is to generate an advantageous compressive residual stress distribution in the surface area of hot-formed components by intelligent process control with tailored cooling. Adapted cooling is achieved by partial or temporal instationary exposure of the specimens to a water–air spray. In this way, macroscopic effects such as local plastification caused by inhomogeneous strains due to thermal and transformation-induced loads can be controlled in order to finally customise the surface-near residual stress distribution. Applications for hot-formed components often require special microstructural properties, which guarantee a certain hardness or ductility. For this reason, the scientific challenge of this work is to generate different residual stress distributions on components surfaces, while the geometric as well as microstructural properties of AISI 52100 alloy stay the same. The changes in the residual stresses should therefore not result from the mentioned changed component properties, but solely from the targeted process control. Within the scope of preliminary experimental studies, tensile residual stresses in a martensitic microstructure were determined on reference components, which had undergone a simple cooling in water (from the forming heat), or low compressive stresses in pearlitic microstructures were determined after simple cooling in atmospheric air. Numerical studies are used to design two tailored cooling strategies capable of generating compressive stresses in the same components. The developed processes with tailored cooling are experimentally realised, and their properties are compared to those of components manufactured involving simple cooling. Based on the numerical and experimental analyses, this work demonstrates that it is possible to influence and even invert the sign of the residual stresses within a component by controlling the macroscopic effects mentioned above.

Residual stresses resulting from hot-forming processes represent an important aspect of a component’s performance and service life. Considering the whole process chain of hot forming, the integrated heat treatment provided by a defined temperature profile during cooling offers a great potential for the targeted adjustment of the desired residual stress state. Finite element (FE) simulation is a powerful tool for virtual process design aimed at generating a beneficial residual stress profile. The validation of these FE models is typically carried out on the basis of individual surface points, as these are accessible through methods like X-ray diffraction, hole-drilling, or the nanoindentation method. However, especially in bulk forming components, it is important to evaluate the quality of the model based on residual stress data from the volume. For these reasons, in this paper, an FE model which was already validated by near surface X-ray diffraction analyses was used to explain the development of residual stresses in a reference hot forming process for different cooling scenarios. Subsequently, the reference process scenarios were experimentally performed, and the resulting residual stress distributions in the cross-section of the bulk specimens were determined by means of the contour method. These data were used to further validate the numerical simulation of the hot forming process, wherein a good agreement between the contour method and process simulation was observed.

Hot forging is a complex process involving the mutual influence of numerous thermo-mechanical-metallurgical material phenomena. In particular, the strains of transformation-induced plasticity (TRIP) have a significant influence on the distortions and residual stresses of the components. The TRIP strains refer to the anisotropic strains depending on the orientation and significance of the stress conditions during cooling superimposed to the phase transformation. With the use of numerical models, the impact of this effect can be investigated in order to ensure the production of high quality components. However, an experimental determination of the characteristic values of TRIP is challenging, which is why only few corresponding data are available in the literature. Therefore, this paper presents an experimental and numerical methodology as well as the results of studies on the interaction between stresses and phase transformations in the materials AISI 4140 and AISI 52100. The investigations of the TRIP strains are carried out using hollow specimens, which are thermo-mechanically treated in the physical forming simulator Gleeble 3800-GTC. The specimens are austenitised, quenched to test temperature and held there while diffusion controlled phase transformation takes place. The extent of TRIP as a result of different superimposed tensile or compressive loads is determined by means of dilatometry. In addition, the extent of TRIP for diffusionless martensitic phase transformations was investigated by continuous cooling tests under tensile and compressive loads. It was found that the transformation plasticity varies depending on the material, the phase type, the temperature and the tensile or compressive stresses. Subsequently, simulations of the physical experiments using the FE software Simufact.Forming verified the determined phase specific values of TRIP.