Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 51
using specimens with the dimensions corresponding to the conditions of validity leads to a ligament
located in the transition zone between the columnar zone and the slab heart equiaxed zone. It re-
quires reducing the width of the DENT specimens in order to locate the ligament in the columnar zone.
In addition, reducing the specimen width reduces the amount of material needed for the test.
Edge
ND
TD
10 cm
Columnar Zone
Heart of the slab
Coarse-Equiaxed grains
Columnar Zone
120 mm (standard DENT width)
85 mm (DENT width used in this study)
Position of a 40mm-ligament
Position of a 10mm-ligament
Position of a 15mm-ligament
Figure III.7. Illustration of the specimen or ligament position as a function of the dimensions.
Chehab et al. [92] suggested that the restrictions concerning the validity conditions are less severe at
high temperature than at room temperature. This difference was attributed to a better ductility and a
strong temperature gradient between the centre of the ligament and the head of the specimen which
helps for confining the plastic deformation in the ligament. As a result, it seems that the DENT width
can be reduced. A feasibility study was carried out by the Aperam Research Center [94]. Several
DENT specimens were tested with different width (44, 50, 60, 70, 80, 100, 120mm) and a constant
ligament length (40mm). The results demonstrated that the force-displacement curves were un-
changed, and consequently the area under the curves did not change when reducing the width of the
DENT to 80mm. It means that the total specific work of fracture was unaffected as long as the DENT
width exceeded 80mm. These results confirmed that it is possible to reduce the dimensions of the
DENT specimens when the method was applied at high temperature. It explains why the DENT spe-
cimens used throughout this study, have a width of 85 mm instead of the 120 mm as recommended by
the theory when tests are performed at room temperature.
III.2.3 Characterization tools
III.2.3.1 Observation of the fracture surfaces
After the high temperature tensile test, the specimens were systematically characterized by SEM to
observe the fracture surfaces and to accurately measure the thickness of the DENT specimen at frac-
ture, tf. Neglecting the strain component in the direction of crack propagation and assuming plastic
incompressibility, the average true equivalent fracture strain was estimated as:
eq
� fracture �
t f
0
, eq III-5
2 �
ln
�
�
3 � t
�
�
�
�
where t0 is the initial thickness of the DENT specimens and tf the thickness at fracture.