P 811 - Material and joining analysis and optimisation of a reformer for fuel cell applications

The investigations clearly showed that the atmosphere on the burner side in particular has a very aggressive effect on the combustion chamber material. In the further course of the project, numerous tensile test specimens were prepared according to a matrix of different base materials, joining methods and coating types. Half of these samples were also subjected to a 1,000-hour thermocyclic ageing test under corrosion conditions. Subsequently, a tensile test for each sample provided information on the remaining material strength. In this way, suitable and unsuitable combinations could be determined and possible damage caused by the joining and coating itself could also be analysed. In order to investigate the influence of geometry, the thermal stresses in the combustion chamber were visualised in a computer simulation using real temperature measurement data. The weak points in the original geometry known from experiments were confirmed by high thermal stress concentrations. By varying the shape of the combustion chamber accordingly, these stresses could be significantly minimised. The final stage of the project was the construction of the new reformer reactor, taking into account the knowledge gained during the course of the project. The material 1.4958 with a specially developed aluminium diffusion layer was a suitable material coating combination. The laser welding process proved to be the most suitable joining method. For an optimised component geometry, a curvature and a transition radius to the shell surface proved to be essential design modifications for the combustion chamber cover. Finally, the new reactor was subjected to a 1,000-hour thermocyclic test under real operating conditions and later analysed for damage. The aim of the project was achieved. The basic specific damage mechanisms were determined and effective countermeasures were identified. The follow-up examinations of the optimised combustion chamber after the endurance test showed both the general durability and effectiveness of the diffusion layer, as well as the protective effect that still needs to be improved, particularly in the highly stressed centre of the combustion chamber lid. Despite this potential for improvement, no deeper damage was detected within the base material and in the area of influence of the weld seam, which can be considered a great success. It was not possible to estimate the service life beyond the 1,000-hour cycle test in this project. In future, this would require the transient simulation of the thermal stresses that occur during heating and cooling processes. Furthermore, additional findings on the long-term strength behaviour of the joined material coating systems under the specific load combination must be developed. The economic analysis of these coating systems in comparison to nickel-based materials, for example, is a further task. The IGF project 16118 BG of FOSTA -Forschungsvereinigung Stahlanwendung e.V., Düsseldorf, was funded by the Federal Ministry of Economics and Technology via the AiF as part of the programme for the promotion of joint industrial research and development (IGF) on the basis of a resolution of the German Bundestag. The project was carried out at the Zentrum für Brennstoffzellen Technik gGmbH, Duisburg, at the Günther-Köhler-Institut für Fügetechnik und Werkstoffprüfung GmbH, Jena, and by the DECHEMA Gesellschaft für Chemische Technik und Biotechnologie e.V., Frankfurt/Main.