P 1307 - Hybrid grout connections for steel structures

P 1308 - Improved toughness requirements for high-strength steels in pressure vessel construction The aim of this work was to define new toughness requirements for HSLA steels, based on damage-mechanical methods, in order to create the possibility of using high-strength steels more intensively and to utilise their resources. In order to achieve this goal, it was first necessary to specify and extend existing models. This work focussed in particular on the aspects of temperature-dependent material parameters and feasibility in relation to nominal parameters. The model adjustments for the simulation of the nominal parameters relate primarily to the flow curve. An existing method was chosen to derive yield curves based on yield strength, tensile strength and uniform elongation. In this way, important and easily determined parameters, which are largely given by standards, are used as a basis. The method is therefore as simple and efficient as possible. The second important aspect for the damage mechanics model is the temperature dependence of the model parameters. Firstly, an efficient and precise concept was developed based on the choice of nominal flow curves, which ensures an adjustment of the flow function with the temperature. The results achieved are significantly better than those obtained with the model previously available at the IEHK. In addition, this continuity allowed the ductile material parameters to be kept constant with changing temperature. In addition, an innovative method based on the stress state was developed in this work in order to be able to precisely represent the temperature transition in the notched bar impact test, which is essential for toughness and is based on the change in the fracture mechanism. The combination of these steps allows the simulation of nominal toughness requirements in the KSBV and their transferability to components. Based on this methodology, toughness requirements were derived on the material side and applied to different model pressure vessels. The results made it possible to derive maximum loads for steels of strength class 690 MPa from critical pressure vessel details. The project objective of defining a simplified toughness requirement derived from damage-mechanical material simulations was thus achieved. Publication date: April 2022 Authors: Prof. Dr.-Ing. S. Münstermann, J. Langenberg, M.Sc., Prof. Dr.-Ing. M. Feldmann, R. Kühne, M. Sc., SFI/IWE

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

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