Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
34 Chapter II. STATE OF THE ART
An example [19] of a strain map obtained at 950°C with the modified microgrid method is given Figure
II.14. The strain map showed that deformation is localized at the vicinity of the δ/γ interfaces.
δ
γ
a) b) c)
δ
γ
Figure II.14. Highlighting of strain partitioning via the microgrid technique in a laboratory 29.6%Cr-
11.6%Ni-0.27%Mn duplex steel deformed at 950°C and 1s -1 ; a) undeformed state of a relatively
small area; b) deformed configuration of the same area; c) distribution of the Green-Lagrange
equivalent Von Mises strain [19].
The development of the modified microgrid technique could be a good way to provide quantitative data
about strain partitioning at high temperature. However, up to now, only a few results are available and
only small areas were analyzed (3-4 grains). In addition, the available results were most of the time
obtained on model ferrite-austenite microstructures. The lack of quantitative data concerning more
realistic alloys and the influence of different parameters on the strain partitioning (temperature, phase
morphology…) is clear.
In order to fully understand the mechanical behaviour of duplex stainless steels, an essential piece of
the puzzle is missing: the rheology of both ferrite and austenite, i.e. the stress-strain behaviour of the
single phases. Duprez et al. [48] have intended to determine the rheology of each phase in a duplex
steel. The authors have measured accurately the composition of the ferrite and the austenite in a
commercial 2205 duplex stainless steel. Then, they have elaborated single-phase alloys with, respec-
tively, the composition of the austenite and the ferrite. Finally, specimens from each alloy were de-
formed under hot torsion at 1200°C. The stress strain curve revealed a large difference of yield stress
and flow stress between the ferrite and the austenite, which resulted in an austenite-ferrite steady
stress ratio (σ γ /σ δ )steady state ≈ 3.5 (Figure II.15). Nevertheless, this method presents a few disadvantag-
es. First of all, this method is very time consuming. Indeed, as the composition and the phase ratio
evolve with temperature, a different casting is required for each temperature and for each phase. In
addition, such method gives the rheology of each phase at different temperatures but it does not take
into account the possible interaction between both phases.
δ
γ