Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 65
III.3.3 Conclusions
� In this part, the EWF concept was applied at high temperature to different grades of duplex
stainless steels in the as-cast conditions. The results have shown that this method is a discri-
minating tool to characterize the hot tearing resistance of such materials.
� At 1050°C, the grade D1 is two times more resistant to crack propagation than the grade D2.
� At 1200°C, the grade D1 remains more ductile than the grade D2 but the difference in term of
essential specific work of fracture is not significant considering the dispersion of the results.
� Fracture observations and damage quantification match very well with the EWF results.
� Hardness characterizations have permitted to highlight the softening mechanisms involved or
not in each phase: dynamic recovery in the ferrite and no sign of recrystallization but a high
dislocation density in the austenite. This last comment suggests the occurrence of stress and
strain partitioning between ferrite and austenite at high temperature with a more significant ef-
fect in the D2-grade.
� The dispersion of the results is significant, attributed to the microstructure heterogeneity
through the slab thickness: phase ratio and austenite lath size. This last comment suggests
that better results, i.e. with less scattered and more accurate values of the essential specific
work of fracture, could be obtained if homogeneous microstructures are considered.
III.4 Generation of model microstructures
The goal is to develop homogeneous microstructures of either the lath (Widmanstätten microstructure:
‘W’) or the equiaxed γ-austenite (equiaxed microstructure: ‘E’) in a δ-ferrite matrix which is stable at
1050°C, which is thus the temperature selected to measure the hot ductility. This effort to engineer
microstructures must result in (1) a reduction of the dispersion, hence improving the accuracy of the
results and (2) a more accurate determination of the influence of phase morphology on the hot worka-
bility comparing two different austenite morphologies with the same phase ratio. This last point has to
be emphasized because, as pointed out in the bibliographical review (Chapter II), the phase ratio can
significantly affect the hot ductility of duplex stainless steels.
Particular attention is paid to microstructural evolutions leading to different austenite morphologies
starting from the same metallurgical state. In depth understanding of the nucleation and growth during
the δ → γ phase transformation allows the identification of the appropriate heat treatments. Some
details concerning the different heat treatments are given for the D2 grade. For the alloy D1, as the
results are very close to those of the D2 grade, only the resulting microstructures obtained with the
appropriate heat treatments are presented.