Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 91
III.7.3 Discussion about the nature, size and density of the inclusions
III.7.3.1 Nature of the inclusions
The chemical analysis performed by EDS (Electron Dispersive Spectroscopy) on polished samples
has shown that the majority of the inclusions were oxides involving undesirable chemical elements
such as aluminium, calcium, or magnesium. In Figure III.52, an example of a back scattered micro-
graph reveals the presence of several inclusions. Their chemical composition was determined using
EDS maps. The nature of the inclusions does not differ between the D1_W microstructure and the
D1bis_W microstructure.
These elements were present during the elaboration of the alloy at the steel factory. Therefore, it is not
surprising to find these elements in the inclusions. Aluminium is added during the steel making in order
to reduce the oxides. Calcium is used to trap the sulfur and hence to reduce its content in the matrix.
Magnesium is one of the main constituents of the refractory ceramic materials used in the steel facto-
ry.
20 μm
a) b)
Al
20 μm
c) d)
O
γ
δ
20 μm
γ
20 μm
Figure III.52. Nature of the inclusions present in the D1_W and D1bis_W microstructures; a) SEM micrograph
showing a string of particles; b) chemical composition map in oxygen; c) chemical composition
map in aluminium; d) chemical composition map superimposing chromium, manganese, aluminium and
calcium.