Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter IV. STRAIN PARTITIONING 105
pared surfaces and then spun at 1200rpm for 1min in order to obtain a uniform film thickness.
Finally, the coated specimens were baked for 30min at 140°C, to promote the resin adhesion
and to improve the mechanical resistance of the film.
(2) The coated samples were then irradiated with the electron beam of a SEM, following the pat-
tern of a square grid. Conditions (magnification, current beam…) were selected to produce a
grid step of 5μm. The austenite grain size was around 50-60μm in the equiaxed morphology,
and the austenite lath thickness was around 30μm in the Widmanstätten morphology. A 5μm-
grid step was chosen in order to introduce as many microgrid intersections as possible in
every grain, and make further strain calculation more reliable.
(3) After the irradiation step, the irradiated polymer was dissolved using a specific solvent: 75%-
propanol and 25%-butanol for 1min and 30s.
(4) The samples were electro-etched with a 40%-aqueous nitric acid solution applying 4.5V for
about 1s, removing material only within the small areas without resin (the microgrid pattern).
This step was undoubtedly the most critical step and will be discussed later.
(5) The final step consisted in dissolving the remaining resin in ethyl acetate by means of ultra-
sonic cleaning, resulting in engraved microgrids with a step of 5μm.
a) b) c)
d) e) f)
Figure IV.7. Steps of modified microgrid technique; a) polishing and electro-etching; b) coating with
an electro-sensitive polymer; c) irradiation of the resin with the electron beam; d) dissolution of the
irradiated polymer; e) engraving by electro-etching; f) dissolution of the remaining polymer [50].