Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 83
a)
b)
D2_E
D2_W
γ
δ
δ
γ
γ
δ
γ
Crack path propagation
δ
γ
δ
Crack path propagation
Figure III.44. Illustration of the difference of cracking propagation between two different morphologies;
a) an equiaxed morphology D2_E; b) a Widmanstätten morphology D2_W.
Cotterell and Reddel [70] have also proposed the following relationship:
w � �
e � k c c , eq III-11
where k is a factor depending on the shape of the load-displacement curve, σc is a critical stress which
can be considered as the mean value of the maximum stress averaged over the ligament, and δc is a
critical displacement which corresponds to the elongation that must be imposed on the elementary
volume representative of the material at the blunted crack tip (fracture process zone) to completely
break the material. This critical displacement can be written as,
� c�
X c � X 0 , eq III-12
where Xc is the height of the fracture process zone at fracture and X0 is the initial height of the band of
material in which, at some point in the deformation history, all the deformation and damage will local-
ize. In the Widmanstätten microstructure, this zone is equal to the mean ferrite channel thickness,
X0_W ≈ 30μm. The crack is forced to propagate between two austenite laths and then the crack path
depends on the orientation of the austenite laths, see Figure III.45.a.
In the equiaxed microstructure, it is a bit more difficult to define, but it should approximately be equal
to the spacing between austenite particles, i.e. X0_E ≈ 65μm.
In others words, it means that in the Widmanstätten morphology the crack can select a preferential
path such that X0 is smaller compared to the equiaxed morphology, see Figure III.45.
δ
δ