Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
18 Chapter I. Introduction
I.2.3.3 Microstructures in the vicinity of the edges
As shown in Figure I.4 and in Figure I.9.a, the cracks are always observed along the edges of the hot
rolled material hence the name “edge-cracks”. The first characterizations have revealed that damage
nucleation always takes place at the δ/γ interphase boundaries, see Figure I.9.b. In addition, the
cracks seem to always propagate within the ferrite. No damage nucleation site is visible in the austen-
ite and the cracks never propagate through an austenite grain.
Edge-cracks
Bulging
at the edges
Edge-cracks
Original top
surface of the plate
Bulged material
a) b)
ND
Figure I.9. a) Evolution of the transverse dimensions of the hot rolling blocks with increasing reduction;
a clear transverse strain develops and cracks have been observed only in the bulged region;
b) microstructure in the edge part illustrating the damage; red arrows emphasize the voids.
I.3 Guideline
The poor hot workability of duplex stainless steels increases dramatically the manufacturing cost and
explains partly why the duplex stainless steel market is still limited. Thus, the purpose of this work is to
understand the origin of the poor hot workability of these alloys and to suggest solutions to avoid or
control the edge-cracking phenomenon.
First of all, the main factors affecting the hot workability of duplex stainless steels are reviewed in or-
der to identify the key parameters and to point out what are the missing pieces of the “hot workability
puzzle”. The review reveals that two “pieces” are missing in the “puzzle”. On the one hand, it is neces-
sary to develop a new hot ductility test which allows discriminating microstructures in terms of high
temperature tearing resistance, and on the other hand, quantitative data about the strain partitioning
between ferrite and austenite during the hot working operations are needed. These two parts must
allow suggesting possible remedies for the edge cracking phenomenon. Figure I.10 gives a summary
of the approach followed throughout this work.
δ
TD
γ