Computer-aided development of ductile ferritic steels with high strengths for ultra-supercritical steam-turbine applications

The efficiency of the conventional fossil-power plant can be improved by increasing the steam temperature and pressure. The DOE is targeting an increase of the steam temperature from 593°C to 677°C by the year 2010 and to 760°C by 2020. Currently, ferritic steels are used at temperatures below 620°C for fossil-energy conversion systems, but these materials exhibit limited creep resistance at higher temperatures. Although Fe-Al-Ni-Cr-based alloys can provide the required creep resistance, these materials are limited by poor ductility and cleavage fracture at ambient temperature. This project will develop a new type of ductile ferritic steels with a tensile ductility of 10% ~ 15% at ambient temperature, without sacrificing the creep resistance. In Phase I, the ductility of the quaternary Fe-Al-Ni-Cr system will be determined as a function of the alloy composition, the amount of the B2 phase, and the elemental partitioning in the B2 and body centered cubic matrix phases. The study will employ (1) thermodynamic calculations to identify potential alloy compositions that have good ductility; (2) microstructural characterizations using transmission-electron microscopy, analytical-electron microscopy, and scanning-electron microscopy to verify the thermodynamic calculations; and (3) mechanical-property experiments to optimize the alloy composition. Commercial Applications and other Benefits as described by the awardee: The new type of aluminide-strengthened ferritic steels should find use in steam-turbine components operating at temperatures higher than 700°C, leading to a significant capital cost reduction and increased thermal efficiency for steam turbines.