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Fracture behaviors of neutron-irradiated ferritic steels studied by the instrumented charpy impact test
217
Journal of Nuclear Materiats 169 (1989) 217-224 Norm-Holland
FRACTURE BE~VIORS BY THE INS~UM~N~D H. YOSHIDA’,
OF NEURON-I~DIA~D CHARPY IMPACT TEST
K. MIYATA’,
M. NARUI
FERRIS
2 and H. KAYANO
STEELS STUDIED
*
’ Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan 2 The Oarai Branch, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-13, Japan Received 9 February 1989; accepted 11 September 1989
The instrumented Charpy impact test for quarter-size specimens was developed and applied to study fracture behavior of ferritic steels and a fer~tic-martensiti~ steel (JFMS) before and after neutron irradiation. The load-deflection curves obtained for U- and V-notched specimens showed typical characteristics of fracture properties of these steels. The temperature dependence of the fracture energy (E,) and the failure deflection (Dt) clearly indicates ductile-brittle transition and the DBTT can be determined from the Et and I+ versus temperature curves. The V-notched specimens showed sharper transition at higher temperatures for the JFMS than the U-notched ones, where the former were sensitive to brittle fracture and the latter well demonstrated the behavior of crack propagation. For the ferritic steels the DBTTs showed low values at compositions containing approximate 8-10413Cr and the increase of the DBTT (A DBT’T) due to irradiation also showed a similar tendency. The ADBTT appeared to be relatively larger for the JFMS than the ferritic steels.
1. Introduction In order to detect the radiation-embrittlement of reactor pressure vessel, the Charpy impact test has been applied to measure the change of the DBTT. The instrumented Charpy testing has recently been developed as a useful technique which enables us to get the load-time and load-deflection curves associated with fracture behavior as an important information [l-7]. The standardization of the test procedure has been discussed for the evaluation of the pressure vessel steels for nuclear reactors [l]. Recently much effort has been done to investigate the dynamic fracture toughness by introducing the elastic-plastic fracture mechanics approach, and several methods to analyze the load-deflection curves in the instrumented Charpy test has been proposed 1%101. Using the instrumented impact test machine several kinds of specimens with different size and different notch-shape have been examined for ferritic and austenitic steels, as well as aluminum alloys [4,17,12]. The feature of the fracture behavior associated with the load-deflation curves was also confirmed by the seanning electron microscope observation of the fracture surface and lateral expansion of the tested specimens [7]. In the present paper the experimental results on
0022-3115/89/$03.50 0 Elsevier Science Publishers B.V. (Norm-Holland)
several commercial ferritic steels and a reference ferritic-martensitic steel, which were irradiated by the Japan Material Testing Reactor (JMTR), will be presented and the feature of their fracture behavior will be discussed based on analysis of the experimental results.
2. Experimental procedures Quarter-size Charpy specimens with U- and V-notch were prepared from several ferritic and ferriticmartensitic steels. The chemical compositions are shown in table. 1. The fabrication conditions and the specimen size (5 X 5 X 45 mm; 4 X 4 X 45 mm) are also indicated. Neutron irradiations up to 2.0 x 1019 n/cm* (E > 0.1 MeV) were carried out using the hydraulic rabbit and for the irradiations with higher fluence than 1 x 10” n/cm2 using the irradiation capsules of JMTR at < 573 K. The instrumented Charpy testing machine in a hot cell at the Oarai Laboratory of Institute of Materials Research, Tohoku University, was used for the present experiments [S]. Charpy impact tests were performed at temperatures between 77 and 523 K. The test specimens were kept in a bath at the test temperature, which was controlled by a heater and/or by a flow of liquid
218
H. Yoshida et al. / Fracture behaviors of neutron-irradiatedferritic
steels
Table 1 Chemical composition of the steels used (wt%) No.
State
C
Si
Mn
P
S
Ni
Cr
MO
V
Nb
Co
Cu
M9 9M JFMS MS13 430 SMnC
9Cr-1Mo a) 9Cr-2Mob’ f-m’) 12Cr-lMod’ sus d, Low Mn d,
0.09 0.07 0.05 0.20 0.09 0.44
0.51 0.31 0.67 0.35 0.31 0.27
0.45 0.48 0.58 0.55 0.34 1.57
0.021 0.021 0.009 0.027 0.023 0.02
0.001 0.013 0.006 0.003 0.311 0.023
0.08 0.08 0.94 0.50 0.30 0.03
8.08 8.18 9.85 11.80 16.35 0.63
0.96 1.93 2.31 0.88 0.05 -
0.12 0.30 _
_ 0.06 _ _
_ _ _ _
_ _ _ _ 0.13
‘) b, ‘) d,
N _ 0.01 _ -
Sumitomo Metals Ind., Co.; pipe, 920°C normalized and 725 ’ C annealed; 5 x 5 x 45 mm. Sumitomo Metals Ind., Co.; pipe, 950°C normalized and tempered; 4 x 4 x 45 mm. University reference sample; plate, 1100 ’ C annealed; 5 x 5 x 45 mm. Sanyo Special Steels, Co.; plate, 11OW’Cannealed; 5 x 5 x45 mm.
nitrogen. After the test specimen was quickly set from the bath to the set point by a manipulator, it then automatically transferred to the test position of the machine. The machine was operated from outside of the hot cell and the signal was recorded by the computer, The load-deflection curves were printed out on an X-Y recorder and the values of main parameters were calculated by the computer program [6,7]. The load-deflection curves obtained by the instrumented Charpy impact tests can be analyzed in three regions as illustrated schematically in fig. 1, i.e., of the initial rapid load increase until Py due to elastic deformation, of the load increase up to the maximum Pm due to plastic deformation until crack initiation, and of the following load decrease due to ductile fracture and crack propagation until complete failure. In fig. 1 Pr is the load corresponding to brittle fracture. The energy required to fracture initiation is given by E,, and E, mainly corresponds to the energy for the process of crack propagation. In case of fracture without complete
z
Y D
B
Et=El+E2+E3
1
DeflectIon
(mm1
Fig. 1. Schematic load-deflection curve obtained from the instrumented Charpy impact test.
failure, the extra tail appears as unstable load caused by interaction between the testing bent specimen and the machine edge, which gives an extrinsic energy ( E3). The fracture strength and failure deflection are identified by the maximum load (Pm) and the deflection at complete failure (D,), respectively [4,11]. For the fracture without complete failure, the value of D, includes an extrinsic deflection due to the extra tail. The fracture energy (E,) is in principle the sum of El and E,. For brittle materials, the load-deflection curves show a sharp drop after the P,,, corresponding to a small fracture energy where E, = E,, while ductile materials usually show broad curves with large deflection corresponding to large fracture energy where E, = E, + E, + ( E3) [6,7,12]. The values of Pm, D,, and E,, E,, E, and E, were obtained by the computer calculation in the present work [6,7].
3. Results and discussions Figs. 2 and 3 show the typical load-deflection curves for the U-notched specimens of 12Cr-1Mo (MS13) and 9Cr-1Mo (M9) ferritic steels, respectively. These curves indicate characteristics of the fracture behavior of ferritic steels. For the unirradiated MS13 steel the curve shows a load increase until the Pm followed by a narrow region of rapid load decrease at 173 K, and the load decrease appeared slowly showing a slight expansion in width at 300 K. The Py reduces at 300 K, but the Pm slightly rises with an increase of ductility, as seen in fig. 2. A sharp peak appeared at 113 K, suggesting brittle fracture occurred before P,,. After the neutron irradiation of 2.0 X 10” n/cm2 (E > 0.1 MeV) the sharp brittle fracture appeared at 133 K, and the width of the curve at 173 K became narrower than that before the irradiation. The change in width of the curve indicates
H. Yoshida et al. / Fracture behaviors
Unirradiated
2 x10” n lcm2
Fig. 2. Typical load-deflection curves obtained for the U-notched specimens of 12Cr-1Mo (MS13) steel before and after a neutron irradiation of 2.0 x lOI n/cm* (E > 0.1 MeV).
the change of fracture energy corresponding to embrittlement induced by neutron irradiation. For the 9Cr-1Mo (M9) ferritic steel a different feature of the load-deflection curves can be seen in fig. 3. The values of loads are slightly less than in fig. 2 and the width of the curves is relatively wide. The brittle fracture appeared at 98 K in the mm-radiated specimen, and at 143 and 173 K in the specimen irradiated with fast neutrons to 6.5 and 20 X 10” n/cm’, respectively. This is a typical result by increase of DBTT due to neutron irradiations. At higher temperatures than the DBTT the load after P,,, showed gradual decreases with zigzag shape which corresponds to irregular crack growth, where crack propagation is retarded by small precipitates dispersed in the matrix [7]. The P,,, increased with decreasing temperatures in general. At the same temperature above DBTT the P, increased by radiation hardening and the D, decreased by radiation embrittlement with an increase of the neutron fluence. Figs. 4(a) and (b) show the typical load-deflection curves for the U- and V-notched specimens of a ferritic-martensitic steel (JFMS), respectively. For the unirradiated U-notched specimens the curves showed a rapid load increase until P,,, followed by a gradual load decrease resulting in the wide shape of large E, at all temperatures except 88 K. In the period of crack propa-
ofneutron-irradiated ferritic
219
steels
gation the load rose sometimes showing a second peak at high temperatures. The fact suggests that the crack growth is retarded by fine second phases of martensite and the strength of the martensite phases is larger than that of ferritic phases especially at high temperatures. The curve of the unirradiated V-notched specimens showed relatively narrow shape compared with that of the corresponding U-notched specimen at the same temperature. The sharp drop due to brittle fracture appeared at 123 K suggesting the DBTT appearing at higher temperature for the V-notched specimen than for the U-notched one. The former may be more sensitive for brittle fracture than the latter. The result might be caused by the fact that stress concentration at the tip of V-notch is larger than that of U-notch. As the recording system of instrumented machine can well follow dynamic fracture behavior during Charpy impact test, the U-notched specimen is good enough to examine the behavior of ductile-brittle transition in the U- and Vnotched specimens as can be seen in figs. 5(c) and (b) for the temperature dependences of fracture energy (E,) and failure deflection (Of), respectively. The DBTTs for
lL3K
1KN
I
98K
300 K
m
Uninad. I;!I*-,
173K
300K
lL3K 1KN I
m
6.5x10'* n/cm2 I;‘;h
1KN
2.0~10’9
I
nJcm2
Fig. 3. Typical load-deflection curves obtained for the U-notched specimens of 9Cr-1Mo (M9) steel before and after neutron irradiations of 6.5 and 20 X 10” n/cm*.
220
H. Yoshida et al. / Frucmre behaviors
of neurrun-irradjatedferri~ic se&
link-radiated 123K
173K
Unlrradiated
250K
280K
‘93K
233K
280K
280K
333 K
3.5 x10’qn/cm2
333K
,~~~‘n,~rn’ 173 K
b
a Fig. 4. Typical load-deflection
curves for JFMS before and after JMTR irradiations of 3. 5 and 46 x lOI n/cm’. (a, left) U-notch; (b, right) V-notch (depth 1 mm).
the unirradiated JFMS obtained from the E, versus temperature curves of the U- and V-notched specimens are approximately 120 and 170 K, respectively. The difference of about 50 K may be caused by the difference in stress concentration at the notch tip. After the neutron irradiations the shape of load-deflection curves for the JFMS became narrow showing small E,, if they are compared with those for the uni~adiat~ ones at the same temperatures. as seen in figs. 4(a) and (b). The slight increase of P,,, by irradiation is observed at 3.5 X 10” n/cm’, but little additional increase occurs at 4.6 X 10” n/cm’ which may be due to saturation of embrittlement for the high fluence irwdiation. Brittle fracture appeared at higher temperatures for the irradiated specimens than for the unirradiated ones. The DBTT increased from 120 to 180 and 200 K for the U-notched specimens irradiated to 3.5 and 46 X lOI9 n/cm2 (E > 0.1 MeV), respectively. For the V-notched specimens the DBTTs appeared at 250 and 330 K for fluences up to 3.5 and 46 x lOI The increase of DBTT due to n/cm2, respectively. irradiations (A DBTT) appeared to be considerably large for the V-notched specimens, as seen in fig. 5(c). Above the DBTTs the load-deflection curves of the U-notched
specimens showed a wider shape indicating a larger E, than those of the V-notched ones. The former well demonstrates the characteristics of crack propagation during ductile fracture even in the irradiated specimens. The fracture strength (P,) slightly increased after the irradiation due to radiation hardening, however it decreased at lower temperatures than the DBTT because of its heavy brittleness, as seen in fig. 5(a). The clear transition also appeared in the D, versus temperature curves (fig. 5(b)), in which the temperatures approximately correspond to the transition in the E, versus temperature curves. Fig. 6 shows, as an example, the temperature dependence of the failure deflection (I+) and fracture energy (Ef) for the commercial 9Cr-1Mo (M9) steel. The ductile-brittle transition clearly appeared in both the D, and E, versus temperature curves for the unirradiated and irradiated specimens. For the 9Cr-2Mo (9M) and 12Cr-1Mo (MS13) steels the increases of DBTT also appeared in the Et and D, versus temperature curves after neutron irradiation [7]. At a fluences of 6.5 and 20 X 10” n/cm’ (E > 0.1 MeV) the DBTT increased from 120 to 160 and 200 K, respectively, for the 9Cr-1Mo (M9) steel. For the 9Cr-2Mo (9M) steel the
221
H. Yashida et al. / Fracture behaviors of neutron-irradiated ferritic steels
51
JFMS(U-1)
0
Unirradioted
JFMStU-11
i
15
T
,i+ -
A Unirradioted A Irradiated1 . Irrodlated(H1
JFMSiV-1)
IRl
-
I
i:l , ,“‘“r::,
1
i
JFMSiV-1)
i?i
2 0
loo
200 Temperature
LOO
300 (K
15
9
500
10
1
I
5 50 -
JFMSfU-II
0
Unirrodmted 0 Irrodmtedf l IrrodiatedlHI
10 -
A
JFMS (V-1)
A
A
01 0
’
’
r/s 4
100
d1kc
200 300 Temperature (K
IR
Unrradiated IrradiatedI Irrodlated(H1
’
’
LOO
IR
’
i i
-
A Unlrrodsated A Irradtatedl A IrrodiatedlHI
IR)
1
_
n
-0
100
200 300 Temperature (K)
LOO
500
1 I
’ ’
500
1
Fig. 5. Temperature dependence of (a) the maximum strength (P,), (b) failure deflection (Of) and (c) fracture energy (E,) for JFMS. U-l: U-notch; V-l: V-notch (depth 1 mm); IR: 3.5 x 1019 n/cm$ HI: 4.6 x 10” n/cm’ (E > 0.1 MeV).
DBTT increased from 90 to 120 and 160 K for the same fluences as above for the U-notched specimen (depth = 2 mm), and also from 150 to 180 and 200 K for the V-notched specimen (depth = 1 mm). For I2Cr-1Mo (MS13) steef the values of ADB’IT were 75 and 90 K at 6.5 and 20 x 10’s n/cm’, respectively. These values of A DBTT are rather small, if they are compared with the values of ADBTT = 140 and 128 K at 2.0 X 10”’ n/cm’ for the SMnC and 430 steels, respectively [12]. Fig. 7 shows dependence of the DBTT and ADBTI on the Cr concentration for the steeIs listed in table 1,
except the 9M steel because of different specimen size. In the same figure the DBTTs and ADBTTs obtained in the previous works are also plotted, being examined under the same conditions for the V-notched specimens of binary Fe-3, 5, 9, 15% Cr and ternary Fe-3, 5, 9% Cr-1% MO alloys and SCr-ZMo, 7Cr-1.5Mo and 9Cr-1Mo steels [13]. It is shown that before irradiation the DB’ITs in ternary alloys and in the ferritic steels are almost at the same low level, although those in the binary alloys are slightly at higher level. For the Unotched specimens the DBITs appear slightly lower
222
H. Yoshida et al. / Fructure behaviors of neutron-irradiated ferritzc steels
than the V-notched ones. After the neutron irradiation the DBTTs increase to much higher levels for the binary alloys than for the ternary alloys and ferritic steels. For the ADBTTs a similar dependence on Cr concentration is observed, but the binary and ternary alloys show relatively higher values than the steels at the same neutron fluence. It is shown in fig. 7 that the ADBTTs for the U-notched specimens were higher than those for the V-noched ones for the ferritic steels, which was different from the JFMS. In general the DBTT should vary depending on many factors, i.e., microstructure, existence of precipitates and second phases, notch shape etc., however the result summarized in fig. 7 for the whole ferritic type steels examined indicates as rough tendency that the DBTT in the ferritic steels varies mainly depending on their Cr concentration 112,131. A similar tendency of the Cr concentration dependence for ADBTTs is also seen in fig. 7. In fig. 8 the ADBTTs obtained in the present investigation, together with those in the previous works on binary and ternary alloys and ferritic steels [13,14], are plotted against the fluence of fast neutron (E > 0.1 MeV) in a logarithm scale. In the figure the dotted lines correspond to the curves for the data obtained previously for A533 type steels irradiated at various temperatures. The full line is the fitting curve presented by the
I
I
I
Notch ___.__ Jn,rr,d 65~10’8”/cm
? x 10’9n/cm2
I\ B
3 5 x lOI’ “!Crn’
\ L6 ~1020”fcm2
h \\
LOO"
'ti 1' 0
R
\\ \ \ I \ \ 1
I3
t-
_ 300 Y b -)_
m a 200
I03 i
I
00
t
t 00
.
\
\ .-I
I
5
IO
Cr content
t
15
JO
(%I
Fig. 7. Dependence of the DBTTs of several ferritic steels in table 1 on the Cr concentration before and after neutron irradiations, in comparison with the previous data for binary (Fe-Cr) and ternary (Fe-Cr-Mo) alloys and steels (5Cr-2Mo. 7Cr-1.5Mo, 9Cr-1Mo) obtained by H. Kayano et al. [13]. (v) Fe-Cr; (A) Fe-Cr-Mo; (0.0) ferritic steels and JFMS.
following equation for the ADBTTs of SUS410 ferritic steels irradiated at 573 K [14], AT=A+B
@t is the neutron
fluence,
- 140.5 and C = 4.14. The data obtained
LO
2 30 z 20 10
i
_I
0
@pt)’
and A = 1192, B = for SMnC and SIX430 steels, as well as 3 and 15% Cr binary alloys, are scattered around the former curve at > 563 K showing early increase of ADBm. These steels are sensitive to radiation embrittlement. The data for the other ferritic steels, which include the 9%Cr ternary alloy and the BCr-lMo, 7Cr-1.5Mo and 410 steels, are close to the full curve indicating a very slow rate of increase of DBTT due to irradiation. Among the data of JFMS the ADBTTs of U-notched specimens are close to the full curve, while the V-notched ones showed much larger ADBTTs, as seen in fig. 8. The reason where
-
log Ot+C(log
100
200 Temperature
300
Loo
(Kl
Fig. 6. Temperature dependence of failure deflection fracture energy (E,) for 9Cr-1Mo (M9) steel before irradiations of 6.5 and 20X 10” n/cm2.
(Dt ) and and after
and
H. Yoshidu et al. / Fracture ~haviors
of n~tr~n-irradiated
223
ferritic steels
I
I 300 _
i
e0
530 SMnC m MS13
v. Xr-1Mo 9Cr-1Mo v SCr-1Mo
0 I49 . 9t.i
A 3cr
c503/~/
‘503K
.l./
Id8
Id9
lozo
Neutron
102’
dose In/cm21
1oz2
1o23
bO.lMeV 1
Fig. 8. Dependence of ADBTT on neutron fluence obtained for several ferritic and ferritic-martensitic
steels ((0, 0, 0) U-notched
specimen; others : V-notched specimens). The dotted curves correspond to AS33 type steels irradiated at various temperatures and the full curve to SW410 and ferritic steels irradiated at 573 K [13,14].
might be related to microstructure of JFMS. Coexistence of ferrite and martensite phases effectively gives large resistance for crack propagation showing relatively large E,. Also the existence of fine particles of hard martensite phase induces a high strength showing a considerably large P,. However, the martensite phase is not insensitive for radiation embrittlement rather than the ferrite phase, and the brittleness is emphasized under the condition of a large stress concentration, i.e., at the root of the V-notch. As a result the ferritic-martensitic steel might show a larger ADBTT than the ferritic steels, especially for the V-notched specimens.
it is shown that the DBTI’s are mainly dependent
on the Cr concentration and low values of DBTT appeared at a concentration of about 8-10s Cr. The ADBTIs due to neutron irradiation also show a similar tendency.
Acknowledgements
The authors would like to express their thanks for Sumitomo Metal Industries, Co. and Sanyo Special Steel, Co. for their supply of the commercial steels. This work was partly supported by Grand-in-Aid for Scientific Research from the Japanese Ministry of Education.
4. Conclusion
The load-defection curves obtained from the instrumented Charpy impact tests showed typical characteristics of fracture behavior for the ferritic and ferritic-martensitic steels. The curves gave the fracture strength (P,,,), failure deflection (Of) and fracture energy (E,). For brittle materials the curves showed a sharp drop after P, in~cat~g a small I), and E,. For ductile materials the curves showed wide shape indicating a large D, and E,. For the steels containing fine precipitates or martensite phases a relatively large P,,, and E, were observed. The DB’IT can be determined by the temperature dependence of D, and E,. The increases of DBTT (A DBTTs) induced by neutron irradiation were clearly observed depending on the neutron fluence for every steel tested in the present work. From the present work together with the previous works
References [l] W.L. Server, J. Test. Eval. 6 (1978) 29. [2] P.T. Lum and C.H. Curll, ASTM STP 626 (1977) p. 21. [3] L.R. Rice, P.C. Paris and J.G. Merkle, ASTM STP 536 (1973). [4] H. Yoshida, T. Kozuka, K. Miyata, H. Kodaka and Y. Hayashi, Proc. ICMC, Kobe, 1982, p. 158; H. Yoshida, T. Kozuka, K. Miyata and H. Kodaka, Austenitic Steels at Low Temperatures (Plenum, Press, New York, 1982) p. 349. [5] R.G. Odette and G.E. Lucas, J. Nucl. Mater. 117 (1983) 264. 161 H. Kayano, M. Narui, S. Ohta and S. Morozumi, J. Nucl. Mater. 133 & 134 (1985) 649. [7] H. Yoshida, K. Miyata, Y. Hayashi, M. Narui and H. Kayano, J. Nucl. Mater. 133 & 134 (1985) 317.
224
H. Yoshida et al. / Fraciure behaviors of neutron-Irradiated ferntic steels
[8] F.J. Loss, US Nuclear Regulatory Commission, Report NUREG/CR, Naval Research Laboratory, NRL Report 4064 (1979). [9] T. Kobayashi, I. Yamamoto and M. Niinomi, Eng. Fracture Mech. 24 (1986) 773; 26 (1987) 83. [lo] T. Kobayashi and M. Niinomi, Nucl. Engrg. Des. 111 (1989) 27. [ll] T. Kozuka and H. Yoshida, Kyoto University Research
Reactor Institute, Techical Report KURRI-TR-309 (1988). [12] H. Yoshida, K. Miyata, H. Kodaka and S. Nishikawa, Proc. 14th Int. Symp. on Radiation Effects on Material, ASTM, Andover, 1988 (ASTM) in press. [13] H. Kayano, A. Kimura, M. Narui, Y. Sasaki, Y. Suzuki and S. Ohta, J. Nucl. Materials 155-157 (1988) 978. [14] H. Kayano. to be published.
Journal of Nuclear Materiats 169 (1989) 217-224 Norm-Holland
FRACTURE BE~VIORS BY THE INS~UM~N~D H. YOSHIDA’,
OF NEURON-I~DIA~D CHARPY IMPACT TEST
K. MIYATA’,
M. NARUI
FERRIS
2 and H. KAYANO
STEELS STUDIED
*
’ Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan 2 The Oarai Branch, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-13, Japan Received 9 February 1989; accepted 11 September 1989
The instrumented Charpy impact test for quarter-size specimens was developed and applied to study fracture behavior of ferritic steels and a fer~tic-martensiti~ steel (JFMS) before and after neutron irradiation. The load-deflection curves obtained for U- and V-notched specimens showed typical characteristics of fracture properties of these steels. The temperature dependence of the fracture energy (E,) and the failure deflection (Dt) clearly indicates ductile-brittle transition and the DBTT can be determined from the Et and I+ versus temperature curves. The V-notched specimens showed sharper transition at higher temperatures for the JFMS than the U-notched ones, where the former were sensitive to brittle fracture and the latter well demonstrated the behavior of crack propagation. For the ferritic steels the DBTTs showed low values at compositions containing approximate 8-10413Cr and the increase of the DBTT (A DBT’T) due to irradiation also showed a similar tendency. The ADBTT appeared to be relatively larger for the JFMS than the ferritic steels.
1. Introduction In order to detect the radiation-embrittlement of reactor pressure vessel, the Charpy impact test has been applied to measure the change of the DBTT. The instrumented Charpy testing has recently been developed as a useful technique which enables us to get the load-time and load-deflection curves associated with fracture behavior as an important information [l-7]. The standardization of the test procedure has been discussed for the evaluation of the pressure vessel steels for nuclear reactors [l]. Recently much effort has been done to investigate the dynamic fracture toughness by introducing the elastic-plastic fracture mechanics approach, and several methods to analyze the load-deflection curves in the instrumented Charpy test has been proposed 1%101. Using the instrumented impact test machine several kinds of specimens with different size and different notch-shape have been examined for ferritic and austenitic steels, as well as aluminum alloys [4,17,12]. The feature of the fracture behavior associated with the load-deflation curves was also confirmed by the seanning electron microscope observation of the fracture surface and lateral expansion of the tested specimens [7]. In the present paper the experimental results on
0022-3115/89/$03.50 0 Elsevier Science Publishers B.V. (Norm-Holland)
several commercial ferritic steels and a reference ferritic-martensitic steel, which were irradiated by the Japan Material Testing Reactor (JMTR), will be presented and the feature of their fracture behavior will be discussed based on analysis of the experimental results.
2. Experimental procedures Quarter-size Charpy specimens with U- and V-notch were prepared from several ferritic and ferriticmartensitic steels. The chemical compositions are shown in table. 1. The fabrication conditions and the specimen size (5 X 5 X 45 mm; 4 X 4 X 45 mm) are also indicated. Neutron irradiations up to 2.0 x 1019 n/cm* (E > 0.1 MeV) were carried out using the hydraulic rabbit and for the irradiations with higher fluence than 1 x 10” n/cm2 using the irradiation capsules of JMTR at < 573 K. The instrumented Charpy testing machine in a hot cell at the Oarai Laboratory of Institute of Materials Research, Tohoku University, was used for the present experiments [S]. Charpy impact tests were performed at temperatures between 77 and 523 K. The test specimens were kept in a bath at the test temperature, which was controlled by a heater and/or by a flow of liquid
218
H. Yoshida et al. / Fracture behaviors of neutron-irradiatedferritic
steels
Table 1 Chemical composition of the steels used (wt%) No.
State
C
Si
Mn
P
S
Ni
Cr
MO
V
Nb
Co
Cu
M9 9M JFMS MS13 430 SMnC
9Cr-1Mo a) 9Cr-2Mob’ f-m’) 12Cr-lMod’ sus d, Low Mn d,
0.09 0.07 0.05 0.20 0.09 0.44
0.51 0.31 0.67 0.35 0.31 0.27
0.45 0.48 0.58 0.55 0.34 1.57
0.021 0.021 0.009 0.027 0.023 0.02
0.001 0.013 0.006 0.003 0.311 0.023
0.08 0.08 0.94 0.50 0.30 0.03
8.08 8.18 9.85 11.80 16.35 0.63
0.96 1.93 2.31 0.88 0.05 -
0.12 0.30 _
_ 0.06 _ _
_ _ _ _
_ _ _ _ 0.13
‘) b, ‘) d,
N _ 0.01 _ -
Sumitomo Metals Ind., Co.; pipe, 920°C normalized and 725 ’ C annealed; 5 x 5 x 45 mm. Sumitomo Metals Ind., Co.; pipe, 950°C normalized and tempered; 4 x 4 x 45 mm. University reference sample; plate, 1100 ’ C annealed; 5 x 5 x 45 mm. Sanyo Special Steels, Co.; plate, 11OW’Cannealed; 5 x 5 x45 mm.
nitrogen. After the test specimen was quickly set from the bath to the set point by a manipulator, it then automatically transferred to the test position of the machine. The machine was operated from outside of the hot cell and the signal was recorded by the computer, The load-deflection curves were printed out on an X-Y recorder and the values of main parameters were calculated by the computer program [6,7]. The load-deflection curves obtained by the instrumented Charpy impact tests can be analyzed in three regions as illustrated schematically in fig. 1, i.e., of the initial rapid load increase until Py due to elastic deformation, of the load increase up to the maximum Pm due to plastic deformation until crack initiation, and of the following load decrease due to ductile fracture and crack propagation until complete failure. In fig. 1 Pr is the load corresponding to brittle fracture. The energy required to fracture initiation is given by E,, and E, mainly corresponds to the energy for the process of crack propagation. In case of fracture without complete
z
Y D
B
Et=El+E2+E3
1
DeflectIon
(mm1
Fig. 1. Schematic load-deflection curve obtained from the instrumented Charpy impact test.
failure, the extra tail appears as unstable load caused by interaction between the testing bent specimen and the machine edge, which gives an extrinsic energy ( E3). The fracture strength and failure deflection are identified by the maximum load (Pm) and the deflection at complete failure (D,), respectively [4,11]. For the fracture without complete failure, the value of D, includes an extrinsic deflection due to the extra tail. The fracture energy (E,) is in principle the sum of El and E,. For brittle materials, the load-deflection curves show a sharp drop after the P,,, corresponding to a small fracture energy where E, = E,, while ductile materials usually show broad curves with large deflection corresponding to large fracture energy where E, = E, + E, + ( E3) [6,7,12]. The values of Pm, D,, and E,, E,, E, and E, were obtained by the computer calculation in the present work [6,7].
3. Results and discussions Figs. 2 and 3 show the typical load-deflection curves for the U-notched specimens of 12Cr-1Mo (MS13) and 9Cr-1Mo (M9) ferritic steels, respectively. These curves indicate characteristics of the fracture behavior of ferritic steels. For the unirradiated MS13 steel the curve shows a load increase until the Pm followed by a narrow region of rapid load decrease at 173 K, and the load decrease appeared slowly showing a slight expansion in width at 300 K. The Py reduces at 300 K, but the Pm slightly rises with an increase of ductility, as seen in fig. 2. A sharp peak appeared at 113 K, suggesting brittle fracture occurred before P,,. After the neutron irradiation of 2.0 X 10” n/cm2 (E > 0.1 MeV) the sharp brittle fracture appeared at 133 K, and the width of the curve at 173 K became narrower than that before the irradiation. The change in width of the curve indicates
H. Yoshida et al. / Fracture behaviors
Unirradiated
2 x10” n lcm2
Fig. 2. Typical load-deflection curves obtained for the U-notched specimens of 12Cr-1Mo (MS13) steel before and after a neutron irradiation of 2.0 x lOI n/cm* (E > 0.1 MeV).
the change of fracture energy corresponding to embrittlement induced by neutron irradiation. For the 9Cr-1Mo (M9) ferritic steel a different feature of the load-deflection curves can be seen in fig. 3. The values of loads are slightly less than in fig. 2 and the width of the curves is relatively wide. The brittle fracture appeared at 98 K in the mm-radiated specimen, and at 143 and 173 K in the specimen irradiated with fast neutrons to 6.5 and 20 X 10” n/cm’, respectively. This is a typical result by increase of DBTT due to neutron irradiations. At higher temperatures than the DBTT the load after P,,, showed gradual decreases with zigzag shape which corresponds to irregular crack growth, where crack propagation is retarded by small precipitates dispersed in the matrix [7]. The P,,, increased with decreasing temperatures in general. At the same temperature above DBTT the P, increased by radiation hardening and the D, decreased by radiation embrittlement with an increase of the neutron fluence. Figs. 4(a) and (b) show the typical load-deflection curves for the U- and V-notched specimens of a ferritic-martensitic steel (JFMS), respectively. For the unirradiated U-notched specimens the curves showed a rapid load increase until P,,, followed by a gradual load decrease resulting in the wide shape of large E, at all temperatures except 88 K. In the period of crack propa-
ofneutron-irradiated ferritic
219
steels
gation the load rose sometimes showing a second peak at high temperatures. The fact suggests that the crack growth is retarded by fine second phases of martensite and the strength of the martensite phases is larger than that of ferritic phases especially at high temperatures. The curve of the unirradiated V-notched specimens showed relatively narrow shape compared with that of the corresponding U-notched specimen at the same temperature. The sharp drop due to brittle fracture appeared at 123 K suggesting the DBTT appearing at higher temperature for the V-notched specimen than for the U-notched one. The former may be more sensitive for brittle fracture than the latter. The result might be caused by the fact that stress concentration at the tip of V-notch is larger than that of U-notch. As the recording system of instrumented machine can well follow dynamic fracture behavior during Charpy impact test, the U-notched specimen is good enough to examine the behavior of ductile-brittle transition in the U- and Vnotched specimens as can be seen in figs. 5(c) and (b) for the temperature dependences of fracture energy (E,) and failure deflection (Of), respectively. The DBTTs for
lL3K
1KN
I
98K
300 K
m
Uninad. I;!I*-,
173K
300K
lL3K 1KN I
m
6.5x10'* n/cm2 I;‘;h
1KN
2.0~10’9
I
nJcm2
Fig. 3. Typical load-deflection curves obtained for the U-notched specimens of 9Cr-1Mo (M9) steel before and after neutron irradiations of 6.5 and 20 X 10” n/cm*.
220
H. Yoshida et al. / Frucmre behaviors
of neurrun-irradjatedferri~ic se&
link-radiated 123K
173K
Unlrradiated
250K
280K
‘93K
233K
280K
280K
333 K
3.5 x10’qn/cm2
333K
,~~~‘n,~rn’ 173 K
b
a Fig. 4. Typical load-deflection
curves for JFMS before and after JMTR irradiations of 3. 5 and 46 x lOI n/cm’. (a, left) U-notch; (b, right) V-notch (depth 1 mm).
the unirradiated JFMS obtained from the E, versus temperature curves of the U- and V-notched specimens are approximately 120 and 170 K, respectively. The difference of about 50 K may be caused by the difference in stress concentration at the notch tip. After the neutron irradiations the shape of load-deflection curves for the JFMS became narrow showing small E,, if they are compared with those for the uni~adiat~ ones at the same temperatures. as seen in figs. 4(a) and (b). The slight increase of P,,, by irradiation is observed at 3.5 X 10” n/cm’, but little additional increase occurs at 4.6 X 10” n/cm’ which may be due to saturation of embrittlement for the high fluence irwdiation. Brittle fracture appeared at higher temperatures for the irradiated specimens than for the unirradiated ones. The DBTT increased from 120 to 180 and 200 K for the U-notched specimens irradiated to 3.5 and 46 X lOI9 n/cm2 (E > 0.1 MeV), respectively. For the V-notched specimens the DBTTs appeared at 250 and 330 K for fluences up to 3.5 and 46 x lOI The increase of DBTT due to n/cm2, respectively. irradiations (A DBTT) appeared to be considerably large for the V-notched specimens, as seen in fig. 5(c). Above the DBTTs the load-deflection curves of the U-notched
specimens showed a wider shape indicating a larger E, than those of the V-notched ones. The former well demonstrates the characteristics of crack propagation during ductile fracture even in the irradiated specimens. The fracture strength (P,) slightly increased after the irradiation due to radiation hardening, however it decreased at lower temperatures than the DBTT because of its heavy brittleness, as seen in fig. 5(a). The clear transition also appeared in the D, versus temperature curves (fig. 5(b)), in which the temperatures approximately correspond to the transition in the E, versus temperature curves. Fig. 6 shows, as an example, the temperature dependence of the failure deflection (I+) and fracture energy (Ef) for the commercial 9Cr-1Mo (M9) steel. The ductile-brittle transition clearly appeared in both the D, and E, versus temperature curves for the unirradiated and irradiated specimens. For the 9Cr-2Mo (9M) and 12Cr-1Mo (MS13) steels the increases of DBTT also appeared in the Et and D, versus temperature curves after neutron irradiation [7]. At a fluences of 6.5 and 20 X 10” n/cm’ (E > 0.1 MeV) the DBTT increased from 120 to 160 and 200 K, respectively, for the 9Cr-1Mo (M9) steel. For the 9Cr-2Mo (9M) steel the
221
H. Yashida et al. / Fracture behaviors of neutron-irradiated ferritic steels
51
JFMS(U-1)
0
Unirradioted
JFMStU-11
i
15
T
,i+ -
A Unirradioted A Irradiated1 . Irrodlated(H1
JFMSiV-1)
IRl
-
I
i:l , ,“‘“r::,
1
i
JFMSiV-1)
i?i
2 0
loo
200 Temperature
LOO
300 (K
15
9
500
10
1
I
5 50 -
JFMSfU-II
0
Unirrodmted 0 Irrodmtedf l IrrodiatedlHI
10 -
A
JFMS (V-1)
A
A
01 0
’
’
r/s 4
100
d1kc
200 300 Temperature (K
IR
Unrradiated IrradiatedI Irrodlated(H1
’
’
LOO
IR
’
i i
-
A Unlrrodsated A Irradtatedl A IrrodiatedlHI
IR)
1
_
n
-0
100
200 300 Temperature (K)
LOO
500
1 I
’ ’
500
1
Fig. 5. Temperature dependence of (a) the maximum strength (P,), (b) failure deflection (Of) and (c) fracture energy (E,) for JFMS. U-l: U-notch; V-l: V-notch (depth 1 mm); IR: 3.5 x 1019 n/cm$ HI: 4.6 x 10” n/cm’ (E > 0.1 MeV).
DBTT increased from 90 to 120 and 160 K for the same fluences as above for the U-notched specimen (depth = 2 mm), and also from 150 to 180 and 200 K for the V-notched specimen (depth = 1 mm). For I2Cr-1Mo (MS13) steef the values of ADB’IT were 75 and 90 K at 6.5 and 20 x 10’s n/cm’, respectively. These values of A DBTT are rather small, if they are compared with the values of ADBTT = 140 and 128 K at 2.0 X 10”’ n/cm’ for the SMnC and 430 steels, respectively [12]. Fig. 7 shows dependence of the DBTT and ADBTI on the Cr concentration for the steeIs listed in table 1,
except the 9M steel because of different specimen size. In the same figure the DBTTs and ADBTTs obtained in the previous works are also plotted, being examined under the same conditions for the V-notched specimens of binary Fe-3, 5, 9, 15% Cr and ternary Fe-3, 5, 9% Cr-1% MO alloys and SCr-ZMo, 7Cr-1.5Mo and 9Cr-1Mo steels [13]. It is shown that before irradiation the DB’ITs in ternary alloys and in the ferritic steels are almost at the same low level, although those in the binary alloys are slightly at higher level. For the Unotched specimens the DBITs appear slightly lower
222
H. Yoshida et al. / Fructure behaviors of neutron-irradiated ferritzc steels
than the V-notched ones. After the neutron irradiation the DBTTs increase to much higher levels for the binary alloys than for the ternary alloys and ferritic steels. For the ADBTTs a similar dependence on Cr concentration is observed, but the binary and ternary alloys show relatively higher values than the steels at the same neutron fluence. It is shown in fig. 7 that the ADBTTs for the U-notched specimens were higher than those for the V-noched ones for the ferritic steels, which was different from the JFMS. In general the DBTT should vary depending on many factors, i.e., microstructure, existence of precipitates and second phases, notch shape etc., however the result summarized in fig. 7 for the whole ferritic type steels examined indicates as rough tendency that the DBTT in the ferritic steels varies mainly depending on their Cr concentration 112,131. A similar tendency of the Cr concentration dependence for ADBTTs is also seen in fig. 7. In fig. 8 the ADBTTs obtained in the present investigation, together with those in the previous works on binary and ternary alloys and ferritic steels [13,14], are plotted against the fluence of fast neutron (E > 0.1 MeV) in a logarithm scale. In the figure the dotted lines correspond to the curves for the data obtained previously for A533 type steels irradiated at various temperatures. The full line is the fitting curve presented by the
I
I
I
Notch ___.__ Jn,rr,d 65~10’8”/cm
? x 10’9n/cm2
I\ B
3 5 x lOI’ “!Crn’
\ L6 ~1020”fcm2
h \\
LOO"
'ti 1' 0
R
\\ \ \ I \ \ 1
I3
t-
_ 300 Y b -)_
m a 200
I03 i
I
00
t
t 00
.
\
\ .-I
I
5
IO
Cr content
t
15
JO
(%I
Fig. 7. Dependence of the DBTTs of several ferritic steels in table 1 on the Cr concentration before and after neutron irradiations, in comparison with the previous data for binary (Fe-Cr) and ternary (Fe-Cr-Mo) alloys and steels (5Cr-2Mo. 7Cr-1.5Mo, 9Cr-1Mo) obtained by H. Kayano et al. [13]. (v) Fe-Cr; (A) Fe-Cr-Mo; (0.0) ferritic steels and JFMS.
following equation for the ADBTTs of SUS410 ferritic steels irradiated at 573 K [14], AT=A+B
@t is the neutron
fluence,
- 140.5 and C = 4.14. The data obtained
LO
2 30 z 20 10
i
_I
0
@pt)’
and A = 1192, B = for SMnC and SIX430 steels, as well as 3 and 15% Cr binary alloys, are scattered around the former curve at > 563 K showing early increase of ADBm. These steels are sensitive to radiation embrittlement. The data for the other ferritic steels, which include the 9%Cr ternary alloy and the BCr-lMo, 7Cr-1.5Mo and 410 steels, are close to the full curve indicating a very slow rate of increase of DBTT due to irradiation. Among the data of JFMS the ADBTTs of U-notched specimens are close to the full curve, while the V-notched ones showed much larger ADBTTs, as seen in fig. 8. The reason where
-
log Ot+C(log
100
200 Temperature
300
Loo
(Kl
Fig. 6. Temperature dependence of failure deflection fracture energy (E,) for 9Cr-1Mo (M9) steel before irradiations of 6.5 and 20X 10” n/cm2.
(Dt ) and and after
and
H. Yoshidu et al. / Fracture ~haviors
of n~tr~n-irradiated
223
ferritic steels
I
I 300 _
i
e0
530 SMnC m MS13
v. Xr-1Mo 9Cr-1Mo v SCr-1Mo
0 I49 . 9t.i
A 3cr
c503/~/
‘503K
.l./
Id8
Id9
lozo
Neutron
102’
dose In/cm21
1oz2
1o23
bO.lMeV 1
Fig. 8. Dependence of ADBTT on neutron fluence obtained for several ferritic and ferritic-martensitic
steels ((0, 0, 0) U-notched
specimen; others : V-notched specimens). The dotted curves correspond to AS33 type steels irradiated at various temperatures and the full curve to SW410 and ferritic steels irradiated at 573 K [13,14].
might be related to microstructure of JFMS. Coexistence of ferrite and martensite phases effectively gives large resistance for crack propagation showing relatively large E,. Also the existence of fine particles of hard martensite phase induces a high strength showing a considerably large P,. However, the martensite phase is not insensitive for radiation embrittlement rather than the ferrite phase, and the brittleness is emphasized under the condition of a large stress concentration, i.e., at the root of the V-notch. As a result the ferritic-martensitic steel might show a larger ADBTT than the ferritic steels, especially for the V-notched specimens.
it is shown that the DBTI’s are mainly dependent
on the Cr concentration and low values of DBTT appeared at a concentration of about 8-10s Cr. The ADBTIs due to neutron irradiation also show a similar tendency.
Acknowledgements
The authors would like to express their thanks for Sumitomo Metal Industries, Co. and Sanyo Special Steel, Co. for their supply of the commercial steels. This work was partly supported by Grand-in-Aid for Scientific Research from the Japanese Ministry of Education.
4. Conclusion
The load-defection curves obtained from the instrumented Charpy impact tests showed typical characteristics of fracture behavior for the ferritic and ferritic-martensitic steels. The curves gave the fracture strength (P,,,), failure deflection (Of) and fracture energy (E,). For brittle materials the curves showed a sharp drop after P, in~cat~g a small I), and E,. For ductile materials the curves showed wide shape indicating a large D, and E,. For the steels containing fine precipitates or martensite phases a relatively large P,,, and E, were observed. The DB’IT can be determined by the temperature dependence of D, and E,. The increases of DBTT (A DBTTs) induced by neutron irradiation were clearly observed depending on the neutron fluence for every steel tested in the present work. From the present work together with the previous works
References [l] W.L. Server, J. Test. Eval. 6 (1978) 29. [2] P.T. Lum and C.H. Curll, ASTM STP 626 (1977) p. 21. [3] L.R. Rice, P.C. Paris and J.G. Merkle, ASTM STP 536 (1973). [4] H. Yoshida, T. Kozuka, K. Miyata, H. Kodaka and Y. Hayashi, Proc. ICMC, Kobe, 1982, p. 158; H. Yoshida, T. Kozuka, K. Miyata and H. Kodaka, Austenitic Steels at Low Temperatures (Plenum, Press, New York, 1982) p. 349. [5] R.G. Odette and G.E. Lucas, J. Nucl. Mater. 117 (1983) 264. 161 H. Kayano, M. Narui, S. Ohta and S. Morozumi, J. Nucl. Mater. 133 & 134 (1985) 649. [7] H. Yoshida, K. Miyata, Y. Hayashi, M. Narui and H. Kayano, J. Nucl. Mater. 133 & 134 (1985) 317.
224
H. Yoshida et al. / Fraciure behaviors of neutron-Irradiated ferntic steels
[8] F.J. Loss, US Nuclear Regulatory Commission, Report NUREG/CR, Naval Research Laboratory, NRL Report 4064 (1979). [9] T. Kobayashi, I. Yamamoto and M. Niinomi, Eng. Fracture Mech. 24 (1986) 773; 26 (1987) 83. [lo] T. Kobayashi and M. Niinomi, Nucl. Engrg. Des. 111 (1989) 27. [ll] T. Kozuka and H. Yoshida, Kyoto University Research
Reactor Institute, Techical Report KURRI-TR-309 (1988). [12] H. Yoshida, K. Miyata, H. Kodaka and S. Nishikawa, Proc. 14th Int. Symp. on Radiation Effects on Material, ASTM, Andover, 1988 (ASTM) in press. [13] H. Kayano, A. Kimura, M. Narui, Y. Sasaki, Y. Suzuki and S. Ohta, J. Nucl. Materials 155-157 (1988) 978. [14] H. Kayano. to be published.