Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
56 Chapter III. HOT CRACKING RESISTANCE
For the values of αwp, the accuracy is also defined as plus or minus the standard deviations sa of the
slope, which, for a linear regression y=ax + b, is given by the following relation:
n
1/
2
�
2 �
� �(
y ˆ i � yi
) �
i�1
s � �
a �
,
n
eq III-7
�
2
( 2)
( )
�
�
n�
� xi
� x
�
i�1
�
�
where n is the number of (xi, yi) data, x is the mean of the xi values, and yˆ i are the values of yi on the
regression line at the corresponding value of xi (i.e. fitted yi values).
For the values of we, the accuracy is defined as plus or minus the standard deviation sb of the intercept
for x = 0, which, for a linear regression y=ax + b, is calculated from least-squares principle as:
n
n
1/
2
�
2 2 �
��
( y ˆ i � yi
) ��
xi
�
i�1
i�1
s ��
�
b
.
n
eq III-8
�
2
( 2)
( )
�
�
n n�
��
xi
� x
�
i�1
�
Table III.2 summarizes the results obtained from Figure III.12.
�
we (kJ.m - ²) αwp (kJ.m -3 )
As-cast D1 1050°C 320 ± 41 9.5 ± 1.4
1200°C 143 ± 26 6.1 ± 0.9
As-cast D2 1050°C 140 ± 38 9.3 ± 1.3
1200°C 117 ± 27 5.8 ± 0.9
Table III.2. Results of the EWF method applied to as-cast alloys D1 and D2 at 1050°C and 1200°C.
The specific work of fracture at 1050°C (also given by the intercept at the origin, see Figure III.12) is
equal to 320 kJ.m-² and 140 kJ.m-² for alloys D1 and D2, respectively. At 1050°C, the alloy D1 is two
times more resistant to ductile tearing than the alloy D2.
When increasing the temperature to 1200°C, both alloys D2 and D1 become more ductile and the
difference in terms of the essential work of fracture decreases. The work performed in the plastic zone
is similar for both alloys at 1050°C as well as at 1200°C.
III.3.1.2 Fracture observations
The fracture surfaces of broken specimens were characterized by scanning electron microscopy
(SEM). An estimation of the nominal strain to fracture for each experimental condition is provided in
Table III.3.
eq
� fracture
As-cast D1 1050°C 2.6 ± 0.2
1200°C 3.4 ± 0.2
As-cast D2 1050°C 1.1 ± 0.2
1200°C 2.2 ± 0.2
Table III.3. Estimated values of the strain to fracture by measuring the thickness of the specimen at
fracture