P 1361 - Quantitative evaluation and efficient characterisation of intrinsic and extrinsic influences on the cyclic properties of high-strength steels

P 1362 - Welding under operational stress in compliance with standardised requirements for weld quality and fatigue strength The maintenance of steel bridges is an acute challenge in Germany, as many bridges have suffered considerable damage in some cases, jeopardising the load-bearing capacity of the structure. Repair work is currently carried out by partially or fully closing the bridge in order to prevent gap opening movements at the welded joints. However, the repairs should be carried out without (full) closure of the bridge if possible in order to minimise traffic restrictions. Welding under operational stress has not yet been standardised and is therefore only possible with approval in individual cases. The aim of this project was to provide a method that allows welding work to be carried out on old and new structures under operational stress and to achieve weld seam qualities and fatigue strengths in accordance with DIN EN ISO 5817 [1] and DIN EN 1993-1-9 [2]. In the first step, a cracked detail (connection between cover plate and cross girder) was selected on a structure and the real local deformation was recorded and classified for typical traffic situations. A suitable measuring system was developed for this purpose, which records the gap opening movements. The recorded deformation time histories were used to determine the amplitude of the displacement and the frequency as a function of the traffic load. The gap opening parameters determined in the process were modelled in a test rig specially designed for the project. In order to investigate the influence of operational stress on the fatigue strength, reference samples were produced without gap opening movements and samples were welded in the test rig under simulated operational stress. A gap opening amplitude of 0.1 mm at a frequency of 2 Hz was realised. By using a repair method in which the first welded root layer is completely removed from the opposite side, crack-free joints could be produced with these gap opening parameters. All vibration specimens produced were characterised with regard to their weld quality and divided into evaluation groups in accordance with EN ISO 5817 [1]. These were then tested in Wöhler tests and compared with fatigue strength values from the regulations. The fatigue strengths of joints welded under operational stress match the fatigue strengths of samples welded at rest. Overall, the fatigue strengths are higher than those of DIN EN 1993-1-9 [2]. The findings are currently being prepared for practical application in the form of a DVS technical bulletin. The aim of the research project has been achieved. Publication: March 2023 Authors: M. Sc. J. Müller, M. Sc. T. Tóth, Univ.-Prof. Dr.-Ing. K. Dilger, M. Sc. F. Begemann, Dr.-Ing. J. Unglaub, Univ.-Prof. Dr. sc. techn. K. Thiele

P 1365 - Modelling the wear behaviour of coated forming tools for cold forging by means of experimental and numerical methods using the example of flat rolling Hot and cold forming leads to high tool wear due to thermal and mechanical alternating stresses. One way of reducing wear is to coat the forming tools. However, the effort involved in the experimental development of new, wear-protective coatings is immense for SMEs, as there is currently no known method for quantifying the wear and adhesive strength of metal coatings on forming tools. The aim of the research project was to develop an experimental and numerical methodology for the evaluation of wear protection coatings on forming tools under the specific process boundary conditions of cold rolling steel strip. To this end, process models were set up at macro and meso level to calculate the load spectra and methods were developed to determine the adhesive strength and fatigue of the coating on the forming tool. The frequently used hard chrome with layer thicknesses of 20 µm and 50 µm and the potentially also usable electroless nickel with a layer thickness of 20 µm were selected as roller coatings. The adhesive shear strength of the electroless nickel coating was determined for the first time using a newly developed micro-adhesive shear test by shearing off deposited electroless nickel microstructures with a nanoindeter at 315 MPa. The influence of cyclic loading on coating failure was also investigated using the nanoindenter. A significant influence of cyclic loading in the normal direction is not recognisable. However, the risk of delamination of the Ni coating increases significantly with the number of lateral loading cycles. As the adhesive strength of the Cr coating could not be determined experimentally, adhesive strength values from the literature were used for the simulation. The two-stand industrial cold rolling process was simulated using the macro FE model. The rolling force determined during operation could be precisely reproduced with the macro model. Based on a parameter study with the meso model, the delamination risk under the calculated loads in the rolling gap can be assessed as relatively high for the nickel layer and low for the chromium layer. Furthermore, a semi-empirical wear model for the nickel coating was created using simulation. According to this model, delamination should occur after 165,000 work cycles. To validate the method, a 3-roller wear test rig was set up and operated under loads that were as similar as possible to those in the cold rolling process. Delamination of the electroless nickel or chromium layers from the work rolls could not be detected even after a high number of 500,000 load cycles. In the case of the 20 µm and 50 µm chromium layer, this is in agreement with the simulation results. However, the simulation results, which predict delamination of the nickel layer after approx. 165,000 load cycles, cannot be confirmed experimentally. One reason for this could be that the tensile and shear bond strengths as input parameters for the simulation could not yet be determined accurately enough experimentally. Publication: June 2022 Authors: Dr A. Bán, A. Tacke, Z. Liu, A. Quadfasel, Prof. Dr.-Ing. G. Hirt

P 1315 - Control of heat input during GMAW thick-wire welding using the example of fine-grain structural steels GMAW thick-wire welding with wire diameters up to d = 4.0 mm is a promising high-performance welding process which, however, has not yet enabled reproducible and economical joint welding of long weld seams. The applicability of the high-performance welding process in the area of temperature-sensitive, high-strength fine-grain structural steels was also previously unknown. In the research project, the new welding process was analysed and validated with regard to its suitability for the material-specific processing of fine-grain structural steels in the thick plate range (t ? 10 mm). The base material S690Q (1.8931) was used as the reference material for the group of high-strength fine-grain structural steels within the process qualification. In preliminary tests, a thermophysical material characterisation was carried out on a dilatometer, whereby the critical limit ranges of the cooling times t8/5 were defined and confirmed (5 s ? t8/5 ? 15 s). Within the project, systematic welding tests were carried out to analyse the influence of different wire diameters on the welding behaviour and the welding result at the upper technical test limit for MSG thick-wire welding. Solid wires of type G69 with diameters of d = 2.4; 3.0; 3.2 and 4.0 mm were used for this purpose. The solutions for controlling the energy input into the base material were aimed at extending the determined technical test limit in the upper performance range. For example, upstream wire heating, which was achieved by increasing the contact tube spacing (?s = 30 mm), resulted in a reduction in the absolute welding power requirement of up to 20 %. With the solid wire, deposition rates of approximately MR = 20 kg/h could be achieved. This corresponded to an increase in deposition rate of approx. 30 %. Furthermore, the use of a basic cored wire with a diameter of d = 3.0 mm had a positive effect on the welding behaviour and allowed deposition rates of up to approx. MR = 30 kg/h to be achieved. A separate cold wire feed into the melt during the welding process made it possible to increase the deposition rate of the thick wire process by up to 60 %. Alternatively, significantly reduced cooling times t8/5 could be realised with the help of targeted process adaptation: - Use of cored wire - 35 % - Increased contact tube spacing - 25 - Cold wire feed - 10 % In addition to the direct control of the energy and heat input into the base material, this made it possible to increase economic efficiency by increasing the deposition rate during GMAW thick-wire welding. With the process combination of cored wire, increased contact tube spacing and cold wire feed, maximum deposition rates of approx. MR = 45 kg/h were achieved in the project, which represents the current maximum value for GMAW thick wire welding. The pulsed and alternating current technologies were not considered to be effective in terms of reducing the energy input for the GMAW thick-wire welding process. In comparison with submerged arc welding, the potential of GMAW thick-wire welding was determined with regard to welding parameters and economic parameters, as well as mechanical-technological quality values for joint welds of fine-grain structural steels. Here, t = 20 mm thick sheets of grade S690Q were welded together in only 70 % of the time and with 60 % of the total costs of the submerged arc welding process using GMAW thick wire welding, while saving on powder costs and powder handling. In addition, MSG thick-wire welding enables joint welds up to t = 25 mm with just two welding beads. The process qualification (tensile tests, hardness measurements, notch impact tests and bending tests) did not result in any negative impairment of the mechanical-technological quality values of the high-strength fine-grain structural steel (S690Q) used in the case under consideration with regard to the cooling times t8/5 and processing using GMAW thick-wire welding. It was reproducibly and economically possible to place the failure location of the welded high-strength component outside the weld seam and the heat-affected zone. Publication: 2021 Authors: Prof. Dr.-Ing. Hensel, Dr.-Ing. M. Kusch, Dr.-Ing. A. Hälsig, ]}, M. Sc. J. Kimme

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