Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
22 Chapter II. STATE OF THE ART
II.1 Microstructural evolution during hot working
II.1.1 As-cast microstructure
At the millimeter scale, the typical solidification macrostructure consists of three different zones
through the slab thickness: a refined equiaxed grain region close to the chill surface, a columnar zone
and a coarse equiaxed grain zone in the centre [6]. In general, DSS solidify to δ-ferrite. The as-cast
microstructure consists of a ferritic matrix with two types of austenite inclusions: allotriomorphic auste-
nite on the grain boundaries and Widmanstätten austenite laths inside the grains, see Figure II.1. This
kind of solid-state phase transformation leads to a Kurdjumov-Sachs (K-S) or a Nishiyama-
Wasserman (N-W) crystallographic orientation relationship between the new austenite and the parent
ferrite [7-10]. These orientation relationships lead to semi-coherent interphase boundaries due to lat-
tice plane correspondences:
� K-S: {110}δ // {111}γ and δ // γ;
� N-W: {110}δ // {111}γ and δ // γ.
The allotriomorphic austenite at the ferrite grain boundaries corresponds to one of these orientation
relationships with one of the adjacent ferrite grains, whereas the other interphase boundaries are in-
coherent [7].
The amount, morphology and size of the austenite depend on the thermal history: initial temperature
and cooling rate. For instance, the thickness of Widmanstätten austenite laths depends on the cooling
rate, the higher the cooling rate is, the thinner the austenite laths.
Figure II.1. As-cast microstructure in a conventional 2304 duplex stainless steel; the dotted lines indicate
the primary δ-ferrite grain, allotriomorphic austenite decorates the primary δ-ferrite grain boundaries
and Widmanstätten austenite appears inside the primary δ-ferrite grain [12].
When the as-cast material is reheated at high temperature before hot working, it is the as-cast micro-
structure which is relevant for the analysis of damage. This as-cast microstructure will evolve through-
out the process (Figure II.2). For instance, as the rolling reduction increases, the austenite grains be-
come more and more elongated in the rolling direction.