P 1259 - Influences on delayed cold cracking and determination of the minimum waiting time before testing welded joints made of fine-grained structural steel

P 1256 - Extension of the processing spectrum of laser-welded components in the powertrain through the use of dynamic beam deflection and reduced ambient pressure Modern solid-state lasers offer numerous advantages for joining tasks in vehicle construction and particularly in transmission technology due to their high beam quality, energy efficiency, availability and more flexible beam guidance. At the same time, however, the high beam qualities and small focus diameters place higher demands on the production and positioning accuracy of the components to be joined. The overall aim of the research project is therefore to extend the application limits of laser beam welding with solid-state lasers by increasing the process stability to compensate for production-related tolerances for use in gear manufacturing. The research project is based on the working hypothesis that the use of beam oscillation in combination with reduced ambient pressure in laser beam welding with solid-state lasers leads to a significant improvement in gap-bridging capability while simultaneously achieving high seam quality. Using the case-hardening steel 1.7131 as an example, the influence of reduced ambient pressure on the resulting gap-bridging properties is first analysed. Furthermore, the evaluation of suitable oscillation figures and their influence on the process limits with regard to an improvement in gap bridging is carried out. Based on the fundamental investigations, the effects of the gap bridging properties on the joint strength, the process transfer to axial welds and the welding processing of quenched and tempered steel are analysed. The main influencing factor on gap bridging in laser beam welding has turned out to be the weld seam width resulting from the process. A reduced ambient pressure, due to the generally narrower seam geometry, leads on the one hand to a decrease in gap bridging. On the other hand, the reduced ambient pressure causes a decrease in weld spatter formation and thus an increase in process quality, especially for the production of transmission components. To extend the process limits for bridging joining gaps, it is advisable to widen the weld seam geometry so that more material is melted on the seam flanks and is available for gap filling. This can be achieved by positive defocussing of the laser beam and line oscillation transverse to the welding direction. As a result of the wider molten zone, a lower cover layer undercamber with a flatter seam flank angle is formed. The transfer of results to the welding processing of heat-treatable steel also revealed a positive influence of the reduced ambient pressure on the weld-induced crack formation, which thus contributes to increased process reliability and an expansion of the processing spectrum for laser beam welding of difficult-to-weld materials. Publication: 2021 Authors: Prof. Dr.-Ing. K. Dilger

P 1255 - EcoWeldSafe- Required weld seam qualities for compliance with the safety standard for steel structures subject to alternating loads The project Required weld seam quality for compliance with the safety standard for steel structures subject to alternating stresses deals with the evaluation of optimised weld seam designs. The aim of the research project is to comply with and verify the existing safety standard of welded, alternately stressed steel structures using simplified manufacturing processes and types of execution and to develop and verify design approaches for the associated types of execution and execution qualities. In this project, the occurring irregularities of weld seam designs, which are sought by the industry for technical and economic reasons, are analysed and their influence on the fatigue strength of the steel structures is determined. The focus is on root notch, insufficient weld penetration, penetration notch and misalignment. Based on the analyses, new notch classes are defined depending on the irregularities that occur. In addition, the achievable weld seam quality and the associated fatigue strength are determined for specific execution methods. Based on the fatigue strength values for various production imperfections determined in the project, the application limits of the applicable standard values are to be checked and the mathematical methods adapted if necessary. In addition to the efficient nominal and structural stress concepts, findings from the local notch stress concept are also taken into account. The result is a proposal for a verification procedure based on nominal and notch stress concepts. This will be presented to the committees for discussion. These project results will enable steel construction companies to design in a resource-saving manner with lower production costs than before and demonstrably comply with the required safety standard. Labour-intensive work can be reduced, as reworking is no longer necessary due to the high demands on the quality of the weld seams. Steel construction companies (primarily SMEs) and related branches of industry achieve clear competitive advantages both nationally and internationally. Publication: 2021 Authors: Prof. Dr.-Ing. Engelhardt, Prof. Dr.-Ing. K. Rother

P 1254 - Investigation into the correct representation of the heat transmission of lightweight steel constructions in relevant standards for the construction industry Transmission heat losses (HT) and ventilation heat losses (HV) are essential for calculating the heating energy demand of a building. In calculations with constant boundary conditions, the transmission heat loss is made up of the heat losses through flat components (U-values and associated component areas) of the entire heat-exchanging enveloping surface (Ages) and through thermal bridges. Depending on the boundary conditions, correction values (Fx) are also introduced, which serve to apply suitable reductions in heat loss via an annual balance for those components that are not adjacent to the outside air. These are, for example, basement rooms or top storey ceilings that adjoin an unheated roof space. In addition, reductions in heat loss due to solar irradiation are also usually applied to opaque building components using annual or monthly balancing. This is based on the familiar formulae for heat transport through building components, which apply to static, i.e. unchanging, boundary conditions. This also includes the unchanging internal and external thermal resistance (Rsi and Rse). For today's standard verifications based on static boundary conditions, the transmission heat loss for the entire building HT is calculated using the U-value equation and a flat-rate or detailed method for the thermal bridge allowance ΔUWB. From this, a heat quantity QT is calculated for each month - with monthly constant approximate temperature differences between indoor and outdoor air (inside - outside). The sum of all monthly QT is used to calculate the amount of heat emitted by a building through transmission for the entire year, which then has to be balanced out by a heating system. In addition to the heating energy requirement, the cooling energy requirement is also calculated in a similar way with constant monthly boundary conditions. In both cases, solar gains through transparent building components, heat gains through internal loads and ventilation heat losses are included in the balance. In this case, the excessive energy input is compensated for by a system to dissipate the heat. This brief study examines the way in which buildings in lightweight metal constructions can suffer certain disadvantages with regard to transmission heat losses in the static calculation methods due to the fact that higher heat losses may occur in these largely approximate calculation methods compared to more precise transient calculation methods. The usefulness of this approach is also emphasised by high-resolution measurements from previous studies (FOSTA research project P779 [FOS14a]). Publication: 2017 Authors: R. Koschade, Dipl.-Vw. D. Greiner, Dipl.-Ing. R. Koschade

P 1253 - Building-integrated solar steel sandwich elements with mineral wool core for industrial and commercial construction Steel sandwich elements are firmly established components in industrial and commercial construction. In addition to their outstanding cost-effectiveness, they are characterised by excellent building physics properties. Future energy saving regulations will also place increasing demands on wall and roof components. The current strategy of realising this by increasing the component thickness represents a passive measure and can be viewed critically in terms of resource and energy consumption. For this reason, the active, solar thermal upgrading of steel sandwich elements is being investigated in the project as an alternative solution. The solar radiation hitting the lightweight steel façade is absorbed by the outer shell, converted into heat and transported away by a fluid that flows through the pipe registers applied to the rear. In order to maintain the architectural appeal, the components are concealed within the structure of the sandwich module so that active and non-active modules cannot be distinguished. Initial positive results in the area of elements with a polyurethane foam core have already been demonstrated in the FOSTA project P880, where the simpler feasibility in the context of elements with a mineral fibre core is being demonstrated. Based on techniques from heating and cooling ceiling construction, various possibilities for solar thermal activation are demonstrated. The further development of heat conducting sheets made of steel fibre sheets, which represent an important static, structural and physical alternative to known products, should be emphasised. In addition, it is demonstrated that economical solutions can be achieved with both copper and steel-based systems. In the evaluation, design, energy and economic aspects are considered and the investigations are carried out on both a simulative and experimental level. Among other things, the structural strengthening of the new component by the solar technology will be demonstrated. The thermal performance is successfully tested on large-format demonstrators. While the analysis of coatings available on the market shows that modules with different colours can be used, so that a high degree of design freedom can be guaranteed. Concrete solutions for the industrial production of solar-active sandwich elements are also being developed. The above findings are incorporated into typical utilisation scenarios that take into account both the direct use and the storage of thermal energy via various technical scenarios, with the focus on heat pump-based, low-temperature heat supply systems. For this purpose, typical demand profiles for industrial and commercial buildings are included and dynamic building simulations are carried out using an exemplary model hall and determined parameters of the panels. The central result of the research is the addition of the steel sandwich panel construction system, which can provide an immediate solution for saving energy and resources. Publication: 2020 Authors: M.Sc. Y. Tekinbas, M.Sc. D. Ridder, Prof. Dr.-Ing. H. Hachul, Dipl.-Ing. (FH) M. Kirchner, Dr.-Ing. F. Giovannetti