Forgeabilité des aciers inoxydables austéno-ferritiques
Forgeabilité des aciers inoxydables austéno-ferritiques
Forgeabilité des aciers inoxydables austéno-ferritiques
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tel-00672279, version 1 - 21 Feb 2012<br />
106 Chapter IV. STRAIN PARTITIONING<br />
After the fabrication of the microgrid, a low magnification-high resolution picture (4096x4096 pixels)<br />
was obtained by SEM, covering large areas before deformation (0.25mm²). Secondary electron con-<br />
trast was used. The best matching of the homologous points by image correlation requires generating<br />
an optimal contrast on the SEM pictures. In other words, the picture must involve the entire 256-grey<br />
level histogram and must not be centered on a few grey levels. Furthermore, the grey values do not<br />
exceed the 0 and 255 boundaries in order to avoid saturation in the white or black field. A testing pro-<br />
cedure consists in plotting the grey level histogram and to check its spreading. If the spreading is not<br />
acceptable, it will be changed by adjusting the brightness and contrast. Some examples of acceptable<br />
engraved microgrids are given in Figure IV.8.<br />
a) b)<br />
Figure IV.8. Examples of good microgrid high resolution pictures before deformation; a) equiaxed<br />
morphology; b) Widmanstätten morphology.<br />
As highlighted previously, the most critical step in the modified microgrid method is the engraving step.<br />
The main difficulties are summarized below.<br />
(i) Duplex stainless steels are two-phase material, and in general, the micro-etching prin-<br />
ciple consists in dissolving one phase and not the other one. To engrave microgrids in<br />
both phases requires finding a solution which could etch ferrite as well as austenite.<br />
The 40%-aqueous nitric acid electro-etching solution was that which gave the best re-<br />
sults.<br />
(ii) As can be seen in Figure IV.9.a, the microgrid appears to be more defined in the aus-<br />
tenite compared to the ferrite. This was due to the etching used to reveal the phases<br />
prior the application of the modified microgrid technique. The etchant used for this<br />
purpose affects the ferrite and not the austenite. As a consequence when a uniform<br />
layer of resin was deposited on the surface, the resin on the top of the ferrite is<br />
thicker. Figure IV.9.b shows a schematic cross section of the material close to the sur-<br />
face and the layer of resin on it. When an electron beam of constant intensity and di-<br />
ameter scans the surface, the resin on top of the austenite is burnt all the way through
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