Fe-Cr-V ternary alloy-based ferritic steels for high- and low-temperature applications
M. Rieth1*, S.L. Dudarev2, J.-L. Boutard3
1Forschungszentrum Karlsruhe, IMF I, D-76021 Karlsruhe, Germany
Fusion Association, Culham Science Centre, Abingdon,
Close Support Unit,
Boltzmannstrasse 2, D-85748
The phase stability of alloys and steels developed for application in nuclear fission and fusion technology is one of the decisive factors determining the potential range of operating temperatures and radiation conditions that the core elements of a power plant can tolerate. In the case of ferritic and ferritic-martensitic steels, the choice of the chemical composition is dictated by the phase diagram for binary FeCr alloys where in the 0-9% range of Cr composition the alloy remains in the solid solution phase at and below the room temperature. For Cr concentrations exceeding 9% the steels operating at relatively low temperatures are therefore expected to exhibit the formation of a´ Cr-rich precipitates. These precipitates form obstacles for the propagation of dislocations, impeding plastic deformation and embrittling the material. This sets the low temperature limit for the use of of high (14% to 20%) Cr steels, which for the 20% Cr steels is at approximately 600°C. On the other hand, steels containing 12% or less Cr cannot be used at temperatures exceeding ~600°C due to the occurrence of the a-g transition (912°C in pure iron and 830°C in 7% Cr alloy ), which weakens the steel in the high temperature limit [1,2].
In this study, we investigate the physical properties of a concentrated ternary alloy system that attracted relatively little attention so far. The phase diagram of ternary Fe-Cr-V alloy shows no phase boundaries within a certain broad range of Cr and V concentrations. This makes the alloy sufficiently resistant to corrosion and suggests that steels and dispersion strengthened materials based on this alloy composition may have better strength and stability at high temperatures. Experimental heats were produced on a laboratory scale by arc melting the material components to pellets, then by melting the pellets in an induction furnace and casting the melt into copper moulds. The compositions in weight percent (iron base) are 10Cr5V, 10Cr10V, 10Cr15V, 10Cr10V0.2C, 10Cr10V0.4C. Tensile specimens have been fabricated, heat treated at 1100°C for 2 hours for normalization, and tested at temperatures up to 700°C. The investigations were completed by hardness tests, metallographic imaging, and microstructure analysis. The content of intermetallic (Laves) phases increases with the vanadium content and the addition of carbon leads to carbide (VC) precipitation at the grain boundaries. In general, typical ferritic microstructures are recognizable with huge grain sizes (several 100 µm) for the 10Cr5-15V alloys and with smaller grain sizes (about 50 µm) for the 10Cr10V0.2-0.4C alloys. However, the tensile tests performed so far have indicated about the same strength level at 700 °C.
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