Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
48 Chapter III. HOT CRACKING RESISTANCE
The lower limit is directly related to the specimen thickness: l0 must be greater than 3 to 5 times the
thickness of the specimens in order to allow necking to develop fully before cracking, in such a way
that the same plane stress conditions prevail whatever the size of the ligament (Figure III.3).
To summarize, if eq III-2 is applied, this method should provide reliable values of the plane stress frac-
ture toughness of a specimen plate on the condition that the ligament length lies between the limits
given by eq III-3.
3 0
t0 ,..., 5t0
� l � min( L / 3,
rp
) , eq III-3
rp is the size of the plastic zone, which can be estimated as
1 Ew
rp � , eq III-4
� �
e
2
0
where E is the Young’s modulus and σ0 the yield stress [73].
The EWF concept was widely used to characterize the fracture toughness of several ductile metallic
materials (brass, bronze, zinc alloys, aluminium alloys and steels) [72, 74-77] or polymer thin sheets or
layers at room temperature [73, 77-82]. Barany et al. [83] have recently reviewed the results provided
by the EWF concept applied to polymers and composites. When the conditions of validity of the
method are fulfilled, it is relatively simple to determine the specific essential work of fracture which
quantifies the tearing resistance. In previous investigations, concerning polymers, the EWF concept
was used at intermediate temperatures (100-300°C) [84-91]. In this temperature range, the tensile
tests on the DENT specimens were carried out inside a furnace.
For higher temperatures (>1000°C), performing EWF test is an experimental challenge. In addition to
the difficulties related to the control of the microstructures at high-temperature in materials affected by
phase transformations, carrying out the test in a homogeneous temperature furnace is difficult. Such
challenge was recently taken up by Chehab et al. [92, 93] using a Gleeble-3500 thermo-mechanical
simulator in order to characterize the high-temperature tearing resistance of dual-phase ferritic
stainless steels. Chehab et al. [92] also pointed out that the application of the EWF concept at high
temperature presents several advantages:
(i) The EWF method quantifies the resistance of the material to the propagation of a
crack which is not true in usual hot ductility measurements (tensile or shear tests);
(ii) The conditions of validity of the method will almost always be fulfilled owing to the
large ductility at high temperature;
(iii) The method does not require any crack detection systems which are complex to implement
at high temperature;
(iv) The measurement of the overall displacement is accurate enough for calculating the
energy spent during the test.
To summarize, the results from Chehab et al. [92, 93] have demonstrated the effectiveness of the
essential work of fracture method to quantify the hot cracking resistance of metallic materials. As ex-
plained in the previous chapter, duplex stainless steels present in general a poor hot workability lead-
ing to edge cracks during the hot rolling process. This method is expected to be very useful to discri-
minate different grades or different parameters such as the microstructure: phase morphology, inclu-
sion content.