Microstructure evolution of laves phase strengthened ferritic steels for high temperature applications

The present investigation focuses on a new concept of high strength, high chromium (18-23 wt.%), fully ferritic steels on the technical basis of Crofer® 22 H for the application in high temperature energy conversion systems. Fully ferritic means, that these steels possess a ferritic matrix at any temperature below the melting point, i.e. no martensitic transformation occurs. During Crofer® 22 APU and Crofer® 22 H development, over 50 trial alloys with slight changes in chemical composition were designed. Both steels are used as interconnect materials for solid oxide fuel cells (SOFCs) and were developed by the Institute for Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich GmbH in cooperation with VDM Metals GmbH. Such steels possess potentially sufficient steam oxidation resistance up to 650 °C because of their high chromium content. In contrast the steam oxidation resistance of state of the art 9-12%Cr advanced ferritic martensitic (AFM) steels is limited to temperatures of approximately 620 °C. To ensure sufficient steam oxidation resistance of AFM steels above 620 °C a higher chromium content is needed. However, this promotes Z-phase formation on the expense of the strengthening MX (M=V,Nb; X=C,N) particles, what causes a drop in long-term creep strength.Strengthening of the new fully ferritic steels is achieved by solid-solution hardening and in case of Crofer® 22 H by supplemental intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particles. The 22 H trial alloys possess superior creep behaviour in the temperature range from 600 °C to 650 °C and therefore may potentially provide a basis for tackling the future requirements of power plant operation, e.g. higher operational flexibility, higher conversion efficiency and thus lower CO2 emission.In order to further optimisation of these fully ferritic alloys the investigation was performed on three various 22 H trial alloys. The investigations aimed on the identification and classification of Laves phase particles as well as on the influence of chemical composition on the presence of different Laves phases and particle size evolution in the temperature range from 600 °C to 650 °C after different annealing time utilising electron microscopy techniques. Concurrently the suitability of a commercially available thermodynamic modelling tool was checked and rated as doubtful for further in detail alloy development of such ferritic steels. Particle evolution results explain the different creep behaviour of the trial alloys and show promising thermodynamic stability of particle over the whole covered time range (e.g. approximately 40,000 h at 600 °C and approximately 10,000 h at 650 °C). Furthermore, investigation of microstructure evolution at 650 °C focused on sub-grain formation, the formation of particle free zones and associated dislocation density in these. Due to missing particle strengthening in these zones and consequently a drop in creep strength, the particle free zones are suspected to be a reason of premature material failure