Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter II. STATE OF THE ART 27
II.1.3 Plastic deformation
The microstructural evolution taking place during hot deformation within both phases in a duplex mi-
crostructure can significantly differ from the evolution observed in single-phase materials. This is be-
cause, in addition to their respective high (ferrite) and low (austenite) stacking fault energies, others
factors, such as relative strength and morphology play a crucial role on strain partitioning.
Ferritic stainless steels undergo dynamic recovery, hence developing a well-defined subgrain
microstructure that remains equiaxed and with a constant size once steady state is reached. In duplex
microstructures, dynamic recovery is the primary softening mechanism in ferrite, see subgrains in
Figure II.9. The ferrite substructure becomes more polygonized at higher deformations and low strain
rates. However, the interphase boundary imposes some restrictions. As the strain increases, ferrite
becomes partially enclosed between austenite stringers. The thickness of ferrite subregions decreases
with increasing strain, until it becomes comparable with the ferrite subgrain size. This is quite an hete-
rogeneous process that leads to a bamboo-type structure. Narrow bands of ferrite are limited laterally
by the interphase boundaries and are subdivided by a succession of mixed low and high-angle ferrite-
ferrite boundaries. The mechanism responsible for the formation of high-angle boundaries in the ferrite
has been attributed to continuous dynamic recrystallization or extended recovery [21-24].
Due to their low stacking fault energy, austenitic stainless steels undergo significant work har-
dening before the onset of dynamic recrystallization at hot working temperatures. In duplex stainless
steels, microstructural observations have shown that, even at high strains and after very long anneal-
ing, recrystallization is rarely observed. In addition, when it is observed, it involves negligible volume
fractions [5, 23, 24]. In fact, when deforming a duplex microstructure around 1000°C, the austenite
remains structureless.
γ
δ
γ
20 μm
Figure II.9. SEM backscattered electron picture; as-cast 2304 duplex stainless steel deformed to
0.4 at 1000°C and 1s -1 [9].