P 1291 - Workpiece-side influence and modelling of tribological conditions during press hardening

P 1290 - Determination of influencing factors and development of effusion parameters for the safe use of high-strength steel components endangered by hydrogen-induced damage In the metalworking industry, particularly in the automotive industry, the use of high-strength or ultra-high-strength steels has steadily increased in recent years. However, these steels have an increased risk of hydrogen embrittlement. Heat treatment steps for the effusion of hydrogen introduced during production are often longer than necessary due to a lack of knowledge of the underlying effusion mechanisms. Possible recourse claims in the event of damage should thus be avoided. In this research project, the influence of material and microstructure on hydrogen diffusion, absorption and effusion is being investigated. The aim is to calculate effusion times at different effusion temperatures that allow the hydrogen content to be reduced to a non-critical level. Calculated effusion diagrams are intended to provide a simple aid for heat treatment in the production plants of SMEs. The press hardening steel 22MnB5, the cold forming steel DP 800 and the 42CrMo4 as a classic engineering steel with different degrees of deformation and microstructures are analysed. These include two martensitic microstructures (22MnB5, 42CrMo4), the dual-phase microstructure of DP 800 with forming degrees ?=0, 0.4 and 0.7 and the normalised microstructure of 42CrMo4 with forming degrees ?=0, 0.4 and 0.7. The influence of the microstructure and temperature on hydrogen diffusion and effusion is to be determined by means of permeation measurements. With the help of permeation measurements at RT and 70 °C, diffusion and effusion coefficients can be extrapolated for other temperatures. Effusion times for possible heat treatment temperatures are calculated numerically on the basis of the diffusion and effusion coefficients determined. The numerical simulations are validated by means of permeation measurements and ageing tests in a thermal desorption spectroscopy (TDS) furnace. Using electrochemical hydrogen loading with variable loading times, saturation curves are to be determined and the hydrogen uptake as a function of the microstructure investigated. The influence of mechanically applied stresses on hydrogen fusion will also be investigated in a thermally controlled sample chamber of a tensile testing machine. The permeation measurements and ageing curves show that the diffusion and effusion rate of the hydrogen decreases significantly, particularly as a result of plastic deformation. The plastic deformation also results in a drastic increase in hydrogen absorption. In contrast, the differences in diffusion behaviour and hydrogen absorption between the predominantly ferritic and martensitic microstructures are comparatively small. The comparison of simulated effusion curves with real measurement data shows that hydrogen diffusion can be modelled comparatively well using numerical calculations. Effusion diagrams are calculated which enable a simple estimation of heat treatment times for the materials analysed. The effusion diagrams are tested for their applicability with the aid of deep-drawn cups and Rastegajev immersion specimens, which were used as sample components. The effusion diagrams depict the overall effusion comparatively conservatively, so that excessively short and therefore insufficient effusion times are avoided. It is also shown that mechanically applied stresses significantly slow down hydrogen effusion. Publication: 2021 Authors: Prof. Dr.-Ing. M. Pohl

P 1294 - Novel joining technology for the production of hybrid component structures with a short flange made of high-strength steel and aluminium High-strength steels are increasingly being used in lightweight vehicle construction and combined with components made of aluminium alloys. Mechanical joining processes have been further developed for this combination of materials, but these have fundamental technological limitations and are not as widely used as resistance welding technology, especially in the SME sector. Especially when joining mould-hardened components with thin aluminium sheets, high joining forces and small tolerance fields are required for a stable joining process. In order to realise a cost-attractive lightweight construction, a process for mixed joints of high-strength and ultra-high-strength steels with aluminium sheets is to be developed in which simple, cost-effective joining elements and short flanges can be implemented. In addition, this process should also be flexibly applicable under different production boundary conditions, such as those often required by SMEs and where short flanges < 10 mm are possible. The technology is intended to enable SMEs to use their expertise in resistance welding technology to produce steel-aluminium joints without having to switch to riveting or screwing technology, which would involve considerable investment in plant and staff. The joining elements were made from welding wire and form the material transition from steel to aluminium at the joint level without intermetallic phases impairing the joint properties. The connection can be made by a very short welding process < 30 ms, which introduces so little heat that additional bonding is possible. The joint properties were determined both on suitable test specimens with a wire element and on special multi-point specimens that simulate the complex stresses in the production process and in operation. In addition to users from the component and supplier industry, manufacturers of welding systems, most of which are small and medium-sized, should benefit from the results. Publication: 2021 Authors: Prof. Dr.-Ing. S. Jüttner

P 1296 - Adhesive strengthening of fatigue damage for steel structures In steel construction, unforeseen fatigue damage can occur in cyclically stressed structures, making repair measures necessary. Adhesive bonding technology represents a promising alternative to conventional strengthening methods. The aim of the research project was therefore to develop and analyse concepts for the adhesive repair of fatigue damage by using bonded steel patches for steel structures, taking into account defined site-specific operating conditions. As part of the completed StressPatches research project, the strengthening of typical steel construction notch details using bonded steel patches was investigated. For this purpose, the boundary conditions and strain rates from real bridge loads, component tolerances and loads on the bond from the environment were first determined. Based on a specification sheet, more than ten adhesives were identified and provided by the industry, which were analysed in initial tests with regard to workability and final properties. Based on these results, two cold-curing epoxy resin adhesives and a polyurethane adhesive were selected for further investigation and validated in tests on pre-treatment options, curing behaviour under ambient conditions and accelerated curing. The refurbished notch details with patches were then tested experimentally using quasi-static and cyclic small component, component and large component tests. Analytical and numerical investigations were used to identify parameters, recalculate and optimise the tests and to derive a (preliminary) design concept. During the production of the samples, it became clear that assembly aids will be required to support the joining of the components, which will need to be developed in the future. Furthermore, the potential of using fibre optic sensors for structural monitoring was demonstrated. Based on the adhesives in focus, various manufacturing concepts and process sequences were then considered and discussed, and the options were also taken into account in the economic feasibility study. In view of all the results, a pre-dimensioning concept was drawn up. It was shown that the bonding of steel sheets to a bridge structure has the potential to significantly limit the development of possible damage in the form of cracks. These results are now available to SMEs in the engineering services and planning sectors. At the same time, there is a need for further research to enable implementation on real structures. To this end, a further research application has been submitted and already approved, as well as a ZIM project. Publication: 2022 Authors: Prof. Dr.-Ing. Ch. Schuler, Prof. Dr.-Ing. M. Feldmann, Prof. Dr.-Ing. K. Dilger

P 1297 - Investigation of linear polarisation as an electrochemical method for the evaluation of new thin-film pretreatments for corrosion protection in the industrial sector One aim of the investigations in this project was to find out what contributions linear polarisation can make to the evaluation of the corrosion protection effect of pre-treatments. To this end, four thin-film pre-treatments and, as a benchmark, zinc phosphating (trication process) and untreated substrates were characterised using linear polarisation and the results compared with classic corrosion protection tests carried out on KTL-coated sheet metal. Zinc phosphating offers the best corrosion protection on cold-rolled steel (CRS). Here, the current density in the cathodic branch correlates well with the cathodic infiltration at the scribe after the salt spray test. No such correlation was found for the galvanised substrates (electrogalvanised and zinc-magnesium coating). Zinc phosphating is rated worst on ZE in the salt spray test. The ranking of thin-film pre-treatments is also partially reversed on ZE. On ZM, some thin-film pre-treatments are at least on a par with classic zinc phosphating. The salt spray test and the VDA alternating test VDA 233-102 show clear differences in the corrosion mechanism, especially on galvanised substrates. The results of the tested systems (substrate-VBH-KTL) can only be transferred to other systems to a limited extent. The user must therefore check the requirements of each system before use and decide whether the use of thin-film VBH is suitable in his case. Publication: 2018 Authors: Prof. Dr.-Ing. T. Bauernhansl