Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
102 Chapter IV. STRAIN PARTITIONING
In the paper published by Grant et al. [113], a method is presented for obtaining good images of sam-
ple surfaces at high temperature, suitable for strain measurement by digital image correlation (DIC)
without the use of surface markers or speckles. This is accomplished by suppressing black-body rad-
iation through the use of filters and blue illumination. Using only relatively low levels of illumination the
method is demonstrated to be capable of providing accurate DIC measurements up to 1100 °C.
A novel modification of the classical microgrid technique has been proposed by Pinna et al. [14], allow-
ing the measurement of deformation at the microstructural scale in laboratory hot-worked steels: aus-
tenitic and duplex stainless steels [50, 97, 125, 126]. Square microgrids were engraved by chemical
etching on the surface of a small rectangular steel sample, see Figure IV.5.a and Figure IV.5.b. The
sample was then inserted in a plane strain compression specimen to be deformed at high temperature
(850-1050°C) and 1s -1 strain rate. After the hot compression, the sample was extracted from the plane
strain compression specimen and analyzed in the SEM, see Figure IV.5.c and Figure IV.5.d. The dis-
tortion of the microgrid revealed deformation features such as sliding at the interphase boundaries in
the case of duplex stainless steel. The displacement of the microgrid provided also the strain maps
showing the heterogeneous strain distribution within the duplex microstructure as well as in the auste-
nitic stainless steel microstructure, see Figure IV.5.e and Figure IV.5.f.
IV.3 Selection of the experimental technique for measuring
the micro-scale strain distribution
The purpose of this section is to establish all the requirements for the technique which will be selected
to measure the high temperature micro-scale strain distribution in duplex stainless steels.
(1) Deformation conditions have to be representative of the industrial hot rolling, i.e. large strain,
temperature range (850°C-1250°C), strain rate between 0.1 and 100s -1 and plane strain com-
pression.
(2) Markers must be high-temperature resistant.
(3) As the objectives are to determine the average deformation per phase and to map the strain,
the markers must not hide the microstructure.
(4) The technique has to permit to avoid problems related to oxidation at the surface of samples
which would destroy classical markers.
All these specifications lead to select the modified microgrid method developed by Pinna et al. [14].