Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
52 Chapter III. HOT CRACKING RESISTANCE
III.2.3.2 Microstructural characterizations
The specimens were ground down on abrasive paper (silicon carbide) to 1200 grit, and polished with a
diamond paste down to 3µm. The final polishing step was performed with a 1µm alumina powder.
The microstructure observations were carried out by optical and scanning electron microscopy (SEM,
Zeiss Ultra 55) after etching. Two reagents were employed: the Beraha chemical selective etchant
(100mL H2O, 30mL HCl and 1-1,5g K2S2O5) or a 40% aqueous sodium hydroxide electrolytic solution
under a tension of 3V. Volume fractions of each phase, austenite lath thickness and grain size distribu-
tions were determined by image analysis using the Aphelion ® software.
A few thin foils for TEM observations were also prepared from the DENT specimens with the following
procedure: (1) samples were machined close to the fracture surface, (2) the samples were ground
down on abrasive paper to 1200 grit up to reach a thickness of 70µm, (3) 3mm-diameter pellets were
cut from the 70μm-thin metal sheet, (4) the 3mm-diameter pellets were electro-polished with a mixture
of acetic and perchloric acids.
III.2.3.3 Damage characterization
Damage characterization consists in estimating the number of voids per unit area as a function of the
strain. Sections cut perpendicularly to the ligament were polished. Then, the section was divided into
different zones with a step of 500µm, see Figure III.8. Finally the cracks were manually counted in
each region. The true effective strain in each region was estimated as:
2 �
ln
�
�
3 � t
�
�
�
�
eq
� i �
ti 0
, eq III-6
where ti is the thickness of the sample at a position i from the fracture, and t0 (3mm) is the initial thick-
ness of the DENT specimen. The thickness ti was measured at the middle of each region. It has also
to be noted that the plane strain assumption deteriorates when the distance from the ligament in-
creases. The situation is then close to a simple tension.
500 μm
ε i
t i
Strain increases
Figure III.8. Section perpendicular to the ligament in order to quantify the density of cracks per unit area
(mm²) as a function of strain; the dotted lines correspond to the 500μm-step partition; the black arrow
shows the position where the thickness was measured to estimate the strain; and the red circled zones
highlight a few voids.
500 μm