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
2EURATOM/UKAEA
Fusion Association, Culham Science Centre, Abingdon,
3EFDA
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.
[1] R. Lindau et al., Fusion Eng. Design (2005)
75-79, 989-996
[2]
S.P. Fitzgerald et al., Proc. Royal
Soc. London A (2008) 464, 2549–2559
*Michael.Rieth@imf.fzk.de