Mechanische Eigenschaften von Höchstleistungswerkstoffen für Hochtemperaturwärmeübertrager und Pkw-Abgasanlagen

摘要

This work was performed at Forschungszentrum Jülich, IEK-2, within the joint research project “Development of high performance materials for high temperature heat exchangers and automobile exhaust systems – Ferrit 950“, funded by the German federal ministry of education and research. The production of compact, powerful and efficient internal combustion engines leads to an increase of temperatures and mechanical loads in the exhaust system. Currently used chromia forming materials don’t show sufficient strength in the temperature range of 900-1000°C. In Solid Oxide Fuel Cell (SOFC) systems high temperature heat exchangers are used to pre-heat the process gases. Chromia forming materials show high chromium evaporation, which could lead to accelerated ageing of the SOFC cathode. On the other hand, current alumina forming materials show relatively low high temperature strength. The aim of this work was the development of both, a new chromia forming ferritic material with increased strength at 900-1000°C for the use in automotive exhaust systems and an alumina forming ferrite for the use in high temperature heat exchangers on the basis of the material Crofer 22 H. The increase of strength ought to be achieved by solid solution and precipitation strengthening by Laves phases with enhanced dissolution temperature. Main focus of the subproject of Forschungszentrum Jülich - IEK2 was the experimental investigation of the high temperature material properties as well as accompanying microstructural investigation. High temperature strength, short time creep strength up to 1000 hours, the evolution of precipitates at temperatures of 900-1000°C as well as the thermomechanical fatigue behavior were investigated. By increasing the dissolution temperature of the Laves phase an increase in short time stability of precipitation structure and long term stability of the grain structure could be achieved at temperatures up to 950°C. Due to the addition of aluminum the materials for the use in high temperature heat exchangers show a reduction of the number of precipitates of around 50% in comparison to Crofer 22 H. In materials with high carbon and nitrogen as well as low titanium and aluminum contents, small amounts of the Fe3Nb3X phase have been detected. In comparison to the Laves phase this phase exhibited better high temperature stability. Because of dynamic strain ageing, caused by Al atoms, the Al-added materials show superior high temperature strength and creep strength as well as a very strong increase of the thermomechanical fatigue strength compared to the standard material X6CrAl18-4 at maximum temperature - in spite of the lower amount of Laves phase. Because of a higher number of precipitates the new chromia forming materials for the use in automotive exhaust systems show an increase in high temperature strength, short time creep strength and thermomechanical fatigue strength in comparison to the standard material 1.4509.