Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 81
Number of voids per unit area mm²
30
20
10
D2_W
D2_E
Close to fracture
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4
εeq i
Figure III.42. Evolution of the damage as a function of equivalent strain for both austenite morphologies.
The arrows indicate the approximate strain at which damage nucleation occurs.
III.6.2 Discussion: D2_E vs. D2_W
As the equiaxed morphology and the Widmanstätten morphology were generated in order to obtain a
similar phase ratio, this argument cannot be invoked to explain the difference of hot tearing resistance
between D2_E and D2_W. The origin of the difference between D2_E and D2_W is not yet clearly
identified. Some possible reasons of this difference are:
� The rheology;
� The nature of the interphase boundaries;
� The effect of the morphology on the crack path.
III.6.2.1 Rheology
Both morphologies have been generated using different heat treatments. The partitioning of the alloy-
ing elements between ferrite and austenite can be different from one morphology to another. Such
difference can, on the one hand, change the rheology contrast between the two phases, and on the
other hand affect the softening behaviour of the ferrite and austenite. The micro-hardness profiles
performed on both phases for each morphology were exactly the same (Figure III.43). As a conse-
quence, the argument of the difference of softening behaviour between both microstructures was not
valid, see also further in section III.6.3.2.2.
HV(0.025Kg)
340
320
300
280
260
240
220
D2_E γ
D2_W γ
Close to fracture
200
0 0.2 0.4 0.6 0.8 1 1.2 1.4
εeq i
HV(0.025Kg)
340
320
300
280
260
240
220
D2_E δ
D2_W δ
Close to fracture
200
0 0.2 0.4 0.6 0.8 1 1.2 1.4
εeq i
a) b)
Figure III.43. Comparison between the micro-hardness profiles along the homogeneous zone of DENT
specimens in both morphologies: D2_E and D2_W; a) Profile in the austenite, T=1050°C; b) Profile in
the ferrite, T=1050°C.