P 1226 - Holistic modelling of short-term tempering in the process of inductive surface hardening

P 1228 - Tool-side influence on tribological conditions during press hardening The service life of press hardening tools, which is largely determined by adhesive and abrasive wear, plays a central role in terms of the economic efficiency of the press hardening process. The tribological conditions in the effective joint between the semi-finished product and the tool elements therefore represent a critical factor that has a decisive influence on the quality of press-hardened components. In view of the research work to date, it is clear that the existing knowledge of the tribological conditions is not sufficient to achieve a relevant improvement in the process design or to derive effective tool-side measures for a significant increase in service life. The objective of the research project is to determine the functional interactions between the topography of the tool surface and the tool material and the tribological behaviour of tools during press hardening. By understanding the functional relationships, measures can be derived that increase the service life of press hardening tools and thus the economic efficiency of the process. In order to achieve the research objective, three tool materials were selected for the investigations in consultation with the industrial partners. In addition, different methods for processing the tool surface and tool coatings were analysed on the basis of the strip drawing test. The investigations have shown that the roughness of the tool surface has no effect on the resulting friction behaviour in the laboratory tests. However, both the choice of tool material and its heat treatment can have an influence on the tribological behaviour. In addition, both friction and wear were significantly reduced by the use of PVD tool coatings. Finally, the results were successfully verified using a B-pillar geometry on a laboratory scale. This showed good transferability of the findings from the strip drawing tests to the press hardening process with a practice-relevant geometry. Publication: 2019 Authors: Prof. Dr.-Ing. M. Merklein

P 1222 - Hot cutting of carbon-martensitic chromium steels in multi-stage processes The use of hot cutting for the production of structural components made of carbon-martensitic chromium steels in the automotive industry offers economic advantages over the currently predominant laser cutting. For example, the elimination of downstream laser trimming leads to a considerable reduction in process time and contributes to an increase in productivity. With hot cutting, no material deposits occur on the surface as a result of spatter and there is potential for savings in tool costs, as cooling channels close to the surface, as required when processing the heat-treatable steel 22MnB5, are not necessary. Chromium steels are currently hardly used in the production of car body components. One reason for this is the lack of experience with regard to the process parameters for hot cutting. As part of this research project, the influence of the process parameters on the cut surface geometry, dimensional accuracy and expansion capacity was therefore scientifically investigated. The focus was on the change in the individual cut surface parameters as a result of different parameter combinations. In addition, fundamental knowledge was gained about the effect of upstream and downstream forming operations on the quality and static properties of the cut parts. The result of the research is the provision of optimal process parameters and thus an increase in the operational readiness of hot cutting processes for the production of mould-hardened components made of chromium steel. Publication: 2021 Authors: Prof. Dr.-Ing. B.-A. Behrens

P 1221 - Design method for cyclically stressed steel/CFRP bonded joints with special consideration of crack propagation Due to its specific properties and low weight, the lightweight material CFRP (carbon fibre reinforced plastic) is an established material that enables innovative constructions to meet high demands on design, safety and comfort and, not least, realises more efficient construction methods in the transport sector by reducing weight. The relatively high costs of this material require the economical use of CFRP where its specific lightweight construction potential can be effectively utilised. In combination with high-strength steels, lightweight and safe structures can be produced economically. Bonding technology represents a versatile and at the same time material-compatible joining technology for fibre-reinforced plastics in mixed construction. The design of steel/CFRP bonded joints requires precise knowledge of the joint and failure behaviour, particularly under changing types and speeds of loading. The aim of this research project is to qualify a test concept for the design of steel/CFRP bonded joints in various branches of industry. The test concept used is based on rigid steel joining parts with bonded CFRP coupons, which enables the most homogeneous and, above all, uniaxial loading of the bonded layer possible. This allows the reciprocal influences of the CFRP and the adhesive on the joint to be analysed. The basic tests are carried out quasi-statically. Building on this, strain rate-dependent tests and tests under vibration stress are carried out on steel/steel and steel/CFRP bonded joints and compared with the reference tests. The aim is to gain a comprehensive understanding of the mechanical properties and failure behaviour. In addition to continuum mechanical tests under the main types of loading, tensile and shear, fracture mechanical test specimens are developed which are used to investigate the crack propagation behaviour and the detection of interphase-specific effects. The testing of a component-like specimen rounds off the experimental investigations. Finally, the knowledge gained is summarised in a procedure model for testing and designing steel/CFRP bonded joints. In addition to sample production, testing and test interpretation, the process model also contains information on the design of the bonded joint, which can be derived depending on the epoxy resin and polyurethane adhesives used and the underlying stresses. Publication: 2020 Authors: Prof. Dr.-Ing. G. Meschut