P 1310 - Corrosion mechanisms during laser welding of duplex steels

P 1311 - Stress-optimised repair of welded joints in the production of components made of high-strength fine-grained structural steels During the assembly of steel structures, unacceptable irregularities are occasionally detected in the weld area, despite the fact that the welds have been produced in accordance with the requirements. The processing regulations recommend localised thermal gouging of affected areas and rewelding, but provide little information on adequate repair concepts due to a lack of scientifically sound investigations. This applies in particular to the consideration and optimisation of resulting welding-related stresses due to high shrinkage restraints in the gouging grooves and the structural degradation of adjacent areas due to gouging and rewelding. Especially with high-strength steel grades, this often results in reduced mechanical properties and additional welding-related stresses as well as recurring weld defects. For this reason, systematic component-relevant investigations of the welding-related stresses and structural changes in repaired weld seams were carried out for the research project as a function of the shrinkage restraint and heat conduction during welding and gouging as well as the number of repair cycles. The results of the investigation show which factors are suitable for reducing stresses even under high clamping conditions and how degradation of the microstructure and properties of the weld seam and repeated weld seam defects in the repair seam can be avoided. In particular, adaptive heat conduction can be used to reduce welding-related stresses in the repair weld seams. Component tests also ensure the transferability of the welding experiments into practice. Based on the findings, recommendations for stress and material-appropriate repair concepts could be derived and made. The analyses were carried out on the high-strength steel grades S500MLO for the offshore sector and S960QL for mobile crane construction. This takes particular account of the economic production of highly efficient structures for wind turbines and the high-strength structures required for their erection. The research results provide an essential basis for the further development of corresponding standards and regulations. This can ultimately prevent damage and often expensive reworking and improve the utilisation of the high strength potential of high-strength steels. SMEs in particular can also benefit from the findings on the use of high-strength steels, which are necessary for the efficient realisation of the energy transition in Germany, with regard to the costs of production, welding work and materials. Publication: July 2022 Authors: M. Sc. A. Becker, Dr.-Ing. D. Schröpfer, Univ.-Prof. Dr.-Ing. habil T. Kannengießer

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 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 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