P 1308 - Improved toughness requirements for high-strength steels in pressure vessel construction

P 1307 - Hybrid grout connections for steel structures Joints in steel construction are currently produced by welding, bolting and riveting. However, these joining methods have disadvantages. With adhesive joints, components can be joined without these disadvantages. However, if adhesive gap thicknesses in the millimetre to centimetre range are required between the components to be bonded, as is the case in the construction industry due to unavoidable dimensional tolerances, conventional adhesive bonds are unsuitable. Another joining method for hollow profiles, particularly in the construction of foundation structures for offshore wind turbines, is the so-called grout connection. In these pipe-in-pipe plug-in connections on a large scale, the cylindrical joint gap, whose gap dimensions are specifically defined to compensate for component tolerances, is grouted with a high-strength fine-grain mortar (grout). Due to the low adhesion of the grout to the pipe wall, mechanical shear keys are welded onto the surfaces of the parts to be joined. However, these weld seams can in turn be the starting point for damage in the grout and in the steel components. This research project focuses on an innovative, hybrid joining process for steel components that utilises grout and adhesive layers and combines them advantageously, taking into account the respective material properties. The hybrid grout joining technology has a novel, multi-layered structure and is produced in several process steps. The blasted joining surfaces of the structural components are first coated with a thin organic adhesive layer (e.g. epoxy resin) under controlled conditions in the manufacturing plant, into which an inorganic granulate (e.g. corundum, quartz sand) is then sprinkled. The actual form fit between the opposing parts to be joined is created by pouring and setting a commercially available, high-strength grouting mortar into the remaining joint gap and interlocking the mortar with the inorganic granulate over the entire surface. The bond between the adhesive layer, which is adhesively bonded to the steel surface of the parts to be joined, and the grouting mortar is created by the strong surface interlocking with the inorganic granules that are embedded and bonded. Extensive analytical, experimental and numerical investigations were carried out to systematically analyse the hybrid grout joint. First of all, the steel, adhesive and grout materials to be used were selected and investigated on the basis of a catalogue of requirements drawn up in the project. The most efficient combinations of adhesive, grout material and granulate are determined in extensive tensile tests on hybrid grouted pipe plug connections (small scale) and prioritised for further investigations. Extensive tests are then carried out to determine the influence of the parameters joint gap thickness, pipe cross-sections, overlap length, imperfections and stress direction on the load-bearing behaviour under axial stress for the two prioritised adhesive-grout combinations. All test series show a high load-bearing capacity of the hybrid grout connection (bond strengths of up to 30 MPa). The investigated geometric imperfections do not result in a reduced load-bearing capacity. In bending tests on hybrid grouted pipe joints, the bending load-bearing behaviour is investigated as a function of joint gap thickness, overlap length and joint materials. In all the bending tests carried out as part of the project, the plastic load-bearing capacity of the pipe cross-sections was achieved without any failure of the joint being detected. Investigating the durability of the hybrid grout joint is another research focus. Tensile tests on pipe joints at -20 °C and +60 °C represent construction-relevant operating temperatures. At low temperatures, both adhesive materials show an increase in load-bearing capacity. At a test temperature of +60 °C, the load-bearing capacity of the joint decreases by a maximum of 20 %. The work package is supplemented by the quantification of the axial tensile load-bearing capacity after ageing the pipe samples for 1000 hours in artificial seawater and after carrying out the temperature cycling test in accordance with VW PV1200. No reduction in the load-bearing capacity was observed for either exposure scenario. Against the background of the intended use of hybrid grouted joints under non-stationary loading, fatigue tests were carried out. were carried out. The load-bearing and failure behaviour of the hybrid grouted pipe joint is investigated under axial compressive swelling stress under non-static loading. Both adhesive-grout combinations show a high fatigue load capacity. Based on the tests carried out, the fatigue strength of the hybrid grout connection is approx. 50 % of the quasi-static load-bearing capacity. Numerical simulations accompany the experimental investigations and form the basis for the development of design approaches. Based on the results of diagonal tension tests on bonded and hybrid grouted steel specimens, a fracture criterion is developed for the hybrid grouted joint that takes into account the interaction of shear stresses in the bonded joint and normal stresses orthogonal to the bonded joint. The fracture criterion is statistically analysed and successfully applied to determine the load-bearing capacity of the hybrid grouted joint in a probabilistic design method. All the knowledge gained in the research project is then combined in the production and experimental testing of large components. For this purpose, compression tests are carried out on circular hollow profiles with external diameters of approx. 270 mm and a nominal joint gap of 20 mm. With an overlap length of 150 mm, maximum load-bearing capacities of approx. 1700 kN on average were achieved. The results of the research project clearly demonstrate the performance of the new, hybrid grout joint. Due to the high load-bearing capacities determined in this project under quasi-static and non-static loading as well as after ageing, the hybrid grout connection has great potential for use in steel construction. Publication: 2021 Authors: Prof. Dr.-Ing. T. Ummenhofer, Prof. Dr. B. Mayer

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 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 1305 - Development of a learning transformation and dilatometer model for the virtual process design of press hardening processes Press hardening of ultra-high-strength steel alloys is an established process for the production of safety-relevant car body components such as B-pillars or bumpers. In addition to the conventional process, there are other variants of press hardening to produce components with customised properties. What all these process variants have in common is that the mechanical properties are essentially determined by the phase transformations during the process chain. The design of conventional and so-called partial press hardening processes therefore requires precise knowledge of the underlying transformation kinetics. For this reason, complex experimental investigations, for example on a forming dilatometer, usually have to be carried out. In this research project, a physically-based model is therefore being developed in order to reduce the effort involved in designing press hardening processes. To this end, a comprehensive database was first created. In this context, the two process routes of continuous quenching with a variable cooling rate and isothermal holding at different temperatures were analysed. The transformation kinetics of the steel material 22MnB5 and the resulting mechanical properties form the basis for modelling. Based on the experimental results, existing transformation models were then extended by parameterising the model variables. Furthermore, a learning function was developed, which is based on an artificial neural network. The learning function enables iterative test planning in order to calibrate the extended material model for new alloys with as little testing effort as possible. In combination with a graphical user interface, the parameterised material model and the learning function form the virtual dilatometer. Using other industrially relevant process routes, it was also possible to demonstrate the basic transferability of the functionality of the virtual dilatometer. Furthermore, by pressing a sample component, it was demonstrated that the extended material model can predict the mechanical properties of partial press hardening well. For SMEs in particular, the stand-alone tool offers a good opportunity to expand their own product portfolio and ensure good competitiveness, as both the experimental effort and potential financial hurdles are reduced. Publication date: March 2022 Authors: Dr.-Ing. J. Buhl, M.Sc. T. Hart-Rawung, Prof. Dr.-Ing. habil. M. Bambach, M.Sc. A. Horn, Prof. Dr.-Ing. habil. M. Merklein