Forging of Steam Turbine Blades with an Fe 3 Al-based Alloy
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Forging of Steam Turbine Blades with an Fe3Al-based Alloy P. Janschek1, K. Bauer-Partenheimer1, R. Krein2, 3, P. Hanus2, 4 and M. Palm2 1 Leistritz Turbinenkomponenten Remscheid GmbH, Lempstrasse 24, D-42859 Remscheid, Germany 2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, D-40237 Düsseldorf, Germany 3 now at: Salzgitter Mannesmann Forschung GmbH, Ehinger Str. 200, D-47259 Duisburg, Germany 4 Technical University of Liberec, Studentska 2, Liberec 461 17, Czech Republic ABSTRACT The forging capabilities of two high-strength Fe3Al-based alloys have been evaluated. Based on these results one alloy has been used for forging steam turbine blades. Blades of about 600 mm length were successfully forged by a standard procedure otherwise used for forging of 9-12 wt.% Cr steels. The forged steam turbine blades showed very good form filling, no pores and smooth surfaces. The blades were finished by cutting and grinding by standard procedures. The microstructure consists out of a Fe3Al matrix with additional Laves phase which predominantly precipitated on grain boundaries. The large Fe3Al grains in the cast precursors did only partially recrystallise during forging. This may be partially due to pinning of the grain boundaries by Laves phase precipitates. Cracks may form at those grain boundaries which are decorated with these brittle precipitates. INTRODUCTION Fe-Al-based alloys are light weight materials with favourable wear resistance and outstanding corrosion behaviour [1]. Recent advances in strengthening these alloys at high temperatures now make them suitable for applications in extreme environments such as steam turbines for power generation [2]. Forging has been applied to iron aluminides variously to consolidate powders and to produce billets and rods for subsequent rolling, see e.g. [3-5]. Though, little detailed information exists about the forgeability of these materials [6-8]. From the extensive work on iron aluminides carried out at Oak Ridge National Laboratory (ORNL) it is known that forging of Fe3Al-based alloys is possible in the temperature range 850 to 1100 °C, however these forgings were conducted at a very slow speed [9]. Hot forging of FeAl-based alloys is also possible but necessitates heating of the dies [10]. For these FeAl-based alloys it was found that the forging window could be considerably enlarged if the microstructure was refined [10].
EXPERIMENTAL Two alloys of nominal compositions Fe-25Al-20Ti-4Cr [11] and Fe-25Al-2Ta [12, 13] (all compositions in at.%) were produced by vacuum induction melting (VIM). In order to evaluate their forging behaviour these two alloys were forged on a 900 ton screw press. Eleven samples of 28 mm height and 25 mm diameter were forged at temperatures between 900 and 1150 °C. For forging the steam turbine blades, blanks of 660 mm length each weighing about 12 kg have been produced by investment casting. The blanks were heated up to 1100 °C and were then forged on a 630 kJ counterblow hammer with 10 blows each. The dies were standard
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