P 1311 - Stress-optimised repair of welded joints in the production of components made of high-strength fine-grained structural steels

P 1310 - Corrosion mechanisms during laser welding of duplex steels The aim of the research project was the quality-compliant production of filigree structures made of duplex steel using laser beam welding without the use of filler metal and/or solution annealing. By developing a process technology adapted to the material, investigations were mainly carried out on the most industrially relevant material 1.4462 (rolled) in the sheet thickness range up to 2 mm. In a first step, comprehensive investigations were carried out using a single-beam process. In addition to the influence of the optical setup, the effect of multi-component gas mixtures and pulse modulation (temporal variation of the energy input) were also included in the experimental investigations. In parallel to the experimental test series, a numerical simulation model was developed to model the complex physical processes involved in laser beam welding. Based on the results of the simulation model, in-situ post-heating was implemented using a two-beam setup consisting of fixed and scanner optics. The minimum permissible austenite content of 30% in the weld metal according to several specifications and technical literature could not be achieved. After varying and finally optimising the laser welding process and the post-weld treatment, the corrosion resistance of the laser-welded duplex samples obtained was to be characterised using various measurement methods. In particular, a new electrochemical measuring cell was to be developed with which all the usual electrochemical tests in the micrometre range could be carried out. At the same time, the resistance of the laser-welded duplex steel specimens to corrosion was tested using iron(III) chloride solution on a laboratory scale using the standard ASTM G48 test method. After developing and setting up the new microelectrochemical measuring cell, the breakthrough potentials of the laser weld seams were determined in a 3% NaCl solution. The measurement results confirmed that there is a good correlation between chemical and electrochemical results for assessing the corrosion resistance of the laser weld seams. Despite the low austenite content, the use of the developed dual-beam process strategy resulted in high resistance to pitting corrosion on the materials analysed. In addition, the implementation of post-heating resulted in the formation of a weld seam bead which, in contrast to the tests with a laser beam, exhibited hardly any weld spatter and external seam irregularities. The requirements of assessment group B according to DIN EN ISO 13919-1:2020-03 are therefore fulfilled. Publication date: January 2023 Authors: M. Schmitz, Dr M. Aziz

P 1313 - Bimodal toughness behaviour of ultra-high toughness pipeline steels - cross-scale root cause analysis and adapted evaluation concepts For many decades now, fossil fuels have been transported through kilometres of pipelines. The need for a reliable pipeline transport network will continue to be indispensable in the future in order to make hydrogen available to end consumers as an energy source in addition to fossil fuels. The pipeline steels required for the conversion and expansion of the supply network have been continuously improved in recent years thanks to the great efforts of the steel and pipeline industry. Thanks to thermo-mechanical rolling strategies and an increase in the purity of the steels, damage-tolerant microstructures have been developed, leading to an increase in strength and toughness properties. These excellent material property profiles offer great resource-saving lightweight construction potential for future applications. However, these modern pipeline steels are subject to failure phenomena during toughness testing that have not yet been sufficiently researched. These include inverse fracture behaviour, fracture surface tearing and compressed or overlapping transition behaviour. The latter is characterised by an overlap of the low and high layer branches over a temperature range of 10-30°C in the notched impact energy-temperature diagram, which in turn leads to uncertainties in component design and associated application inhibitions. As part of this project, a bainitic pipeline material of grade X65, which exhibits overlapping toughness behaviour, was extensively characterised. Titanium nitrides (TiN) and calcium sulphides (CaS) were identified as crack-initiating structural components for both brittle and ductile material failure. Using microstructure simulations, it could be shown that the local stress and strain concentrations in the microstructure vary greatly depending on the position, size and tilting of the inclusion. The highest stress concentration was found for a 6 µm TiN inclusion with a 45° tilt. The development of a cross-scale simulation methodology subsequently made it possible to evaluate the influence of different inclusion configurations on the material and component behaviour. The microstructure-related parameter sets derived in the simulation of representative volume elements (RVE) were used in the macroscopic damage model to map the material behaviour in the notched bar impact test. Using this methodology, the compressed transition behaviour could be described numerically and the model transferred to piping applications. In the component simulations, brittle or ductile component failure occurs in the pipeline pipe depending on the selected microstructure-based parameter set. A microstructure with critical inclusion leads to brittle component failure and a higher crack propagation velocity, while a microstructure without critical inclusion causes ductile component failure. The developed cross-scale simulation approach for microstructure-based component evaluation can be transferred to other material concepts and areas of application in the future. On the one hand, this makes it possible to support the clarification of material-physical phenomena and, on the other hand, to inspire integrative material and component design for resource-saving lightweight construction. Publication: 2021 Authors: Prof. Dr.-Ing. S. Münstermann, M.Sc. Y. Sparrer

P 1309 - Extension of application limits and increase in flexibility of single-sided mechanical joining processes by process-integrated joining part heating for steel-intensive lightweight construction The aim of the research project was to extend the application limits of flow drilling screwdriving (FLS) on the basis of process-integrated joining part conditioning in order to exploit unused lightweight construction potential in the field of steel-intensive lightweight construction with one-sided accessibility to the joining point. In order to achieve this goal, a test system with modified control, regulation and adapted process monitoring was developed for thermally assisted flow punch forming. At the beginning of the project, heating tests were carried out using variations of different influencing variables to investigate the basic interactions with regard to localised joint heating. Based on the findings, constructive boundary conditions for the process-integrated conditioning process were determined. Using the developed test system, specific process parameters suitable for joining were then determined, taking into account the preceding heating of the joining part. The influence of local conditioning on the process-related reaction variables of the FLS process was determined for different joining part materials by means of controlled bolting tests in locally heated joining parts. In addition, a controlled conditioning process was implemented so that different heat treatment strategies could be investigated. Essentially, the strategy of short-time tempering and austenitising was pursued. Suitable joining process parameters were determined for all of the material combinations investigated, so that the application limits of flow punch forming bolting could be significantly extended with regard to ultra-high-strength multiphase steels and ultra-high-strength hot-forming steels. In addition, the joint formation of the screw connections of the thermally assisted FLS process was analysed metallographically. Microstructural changes were analysed on the basis of contrasted sections. In addition, hardness measurements were carried out to quantify the locally softened joint areas in terms of their hardness and geometric distribution, taking into account different heat treatment strategies. For the industrial implementation of the thermally assisted FLS process, the process robustness was analysed using a disturbance variable analysis. The knowledge gained from this was used to design an adapted process monitoring strategy, taking into account the previous results. Publication: 2020 Authors: Prof. Dr.-Ing. G. Meschut

P 1314 - Space-forming load-bearing structures made of sandwich elements In addition to their outstanding space-enclosing function and thermal insulation properties, (steel) sandwich elements have load-bearing properties that allow loads to be transferred in and out of their plane. Single-storey buildings, such as small warehouses, petrol station shops and car washes, can therefore only be built with sandwich elements without additional load-bearing frame substructures. There is a lack of verification methods, in particular for load transfer, which would enable this economical construction method. The aim of the research project was to validate existing calculation approaches for load-bearing and serviceability and the detailed solutions for load transfer and distribution by means of experimental investigations and, if necessary, to develop them further. The knowledge gained from the holistic consideration of the construction method should then serve as a basis for the acquisition of a building authority approval. To achieve these goals, a self-supporting construction made of sandwich elements was first designed as an example object. For this purpose, all necessary construction drawings and detailed solutions for the formation of the load application points and joints were worked out and defined for this application. Experimental investigations into the load-bearing behaviour of a self-supporting construction made of sandwich elements were carried out using a test matrix defined in the research proposal. The extensive local and global axial compression tests were carried out on test specimens, taking into account a practical and application-related design of the load application points. The deformations and maximum loads were measured. The measured loads were then statistically evaluated and the actual local and global load-bearing capacity as well as the failure behaviour of the self-supporting construction made of sandwich elements were analysed. The analytically determined ultimate loads in the ultimate and serviceability limit states were used as a reference. It was found that the experimentally determined loads were far higher than the analytically calculated values. This showed that the design of the structure with the existing design approaches provides conservative results and is therefore on the safe side. The design of the load introduction structure for uplifting forces could also be ensured using a presented design approach. In order to ensure a holistic view of the construction, an economic and ecological analysis of the construction method was carried out. In the economic analysis, it was shown that self-supporting constructions made of sandwich elements without a steel substructure save approx. 9% of the investment costs and approx. 10% of the heating costs. The life cycle assessment also showed that the construction method results in emission and energy savings and is more environmentally friendly. Overall, the research report provides a sound scientific and technical basis for the design, construction and dimensioning of self-supporting structures made of sandwich elements, as well as the necessary test results and analyses for obtaining building authority approval. Publication: September 2022 Authors: Univ.-Prof. Dr.-Ing. D. Ungermann, M.Sc. G. A. Eseme