Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
164 Chapter V. CONCLUSIONS and PERSPECTIVES
V.2 Recommendations for an optimized microstructure
The results provided by the present work lead to formulating several recommendations to design a
duplex microstructure highly resistant to edge-cracking. Only some of the recommendations can be
industrially applied, that is why the recommendations are divided into two different categories. The first
category involves the recommendations which can be relatively easily applied industrially; this catego-
ry is called “realistic recommendations”. The second category gives the solutions which cannot be
changed on the industrial process: “unrealistic recommendations”. For example generate an equiaxed
morphology before the hot rolling process constitutes an unrealistic solution to improve the hot worka-
bility of duplex steels. The main recommendations are summarized in Table I.1.
Realistic recommendations Unrealistic recommendations
� A well-controlled temperature of the material
during the hot rolling operations,
especially for the edge parts.
� A high quality refining stage to maintain
the sulphur and the oxygen contents as
low as possible.
� Addition of alloying elements in order to
reduce as much as possible the rheological
contrast between ferrite and austenite.
� Generation of an equiaxed austenite
morphology.
� Generation of very fine Widmanstätten
austenite.
Table V.1. Summary of the recommendations for a perfect microstructure, i.e. a high-resistant
edge-crack microstructure.
V.3 Perspectives
Technical and conceptual limitations were found during the present investigation, but could not be
addressed due to time constraints. Therefore, some future tasks are suggested to complement the
work presented in this report.
V.3.1 Essential work of fracture concept
Some perspectives for the EWF concept are given below.
� The Cotterell test turns out to be an interesting tool to mechanically characterize materials ex-
hibiting some defects. Indeed, performing tensile tests on specimens with defects do not give
quantitative data due to a strong dispersion of the results: the tensile specimen breaks where
the more critical defect is located. In other words, the initial defects of the specimen govern
the final fracture. When the EWF is applied, DENT specimens are used, that permits to local-
ize the deformation and the fracture along the ligament. The DENT specimen is broken taking
into account the plasticity in the total specific work of fracture.
� The essential work of fracture concept applied at high temperature with different specimen
thicknesses would allow determining the high-temperature toughness of the materials.