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Small punch tests for evaluating ductile-brittle transition behavior of irradiated ferritic steels
194
Journal
of Nuclear
SMALL PUNCH TESTS FOR EVALUATING DUCTILE-BRITTLE OF IRRADIATED FERRITIC STEELS T. MISAWA, Department
Received
T. ADACHI,
of Metallurgical
21 October
M. SAITO
Engineering,
1986; accepted
Muroran
150 (19X7) 194 -702 North-Holland. Amsterdam
Materials
TRANSITION BEHAVIOR
and Y. HAMAGUCHI lnstrtute of Technology,
Mizumoto-rho,
Muroran
050, Japan
22 May 1987
Small punch (SP) tests using small and TEM disk miniaturized specimens were developed to evaluate the ductile-brittle transition temperature (DBTT) of HT-9 and JFMS ferritic steels as candidates for the structural materials of a fusion reactor. It was shown that the temperature dependence of SP fracture energies with scatter in miniaturized testing can give the reliable information on the DBT’T by use of the statistical analysis based on the Weibull distribution. An empirical correlation between the DBTT measured by the Charpy-impact test and that by the SP test was obtained. It became possible to estimate the reliable DBTT and lower-bound SP fracture energy curve using the data partitioning method when there are few data points
and/or
scatter
1. Introduction Ferritic
steels
are recently
recognized
[1,2] to be next
candidate alloys for fusion reactor structural materials. In contrast to austenitic alloy steels, the ferritic steels have the advantage of a larger resistance to void swelling. However, a rise of the ductile-brittle transition temperature (DBTT) due to fast neutron irradiation is a problem, which may preclude the usage of the ferritic steels. Therefore, it is particularly important to evaluate the effect of radiation damage on the shift of the DBTT of the ferritic steels. Usually the DBTT shift is determined by means of the Charpy V-notch impact test, which requires relatively large size specimens. However, since there are severe limitations on the specimen capacity at material irradiation testing facilities, it is necessary to develop special techniques using miniaturized specimens, called a small specimen technology [3]. In recent years, miniaturized punch tests have been developed to obtain strength and/or ductility information from transmission electron microscopy (TEM) disk specimens. Tests using a spherical punch have been referred as bulge [4-61 and disk bend [7,8] tests and those using a cylindrical punch as shear punch tests [9]. Baik et al. [lO,ll] and Kameda [12] evaluated a total plastic work done to the fracture in small punch (SP) tests at various test temperatures. They demonstrated that SP test results in temper embrittled Ni-Cr low alloy steel exhibit a clear ductilebrittle transition
0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
behavior. In the previous papers [13,16], we have reported the preliminary results of the ductile-brittle transition behavior using the SP test in HT-9 type ferritic steel, possessing excellent swelling resistance. The present work shows a possible extension of previous small (10 X 10 X 0.25 mm3) SP specimens to a further-miniaturized small specimen as small as TEM disk (3 mm diameter with 0.25 mm thickness) SP specimens, having two objectives. The first one is to confirm an empirical correlation between the DBTT measured by the standard Charpy-V-notch impact test and that on the SP test in HT-9 and JFMS ferritic steels. The second one is to evaluate large SP-energy scatter caused by unavoidable microstructural heterogeneity to cracking in a miniature specimen of engineering alloys and to estimate the lower-bound energy curve from the large scatter points and/or few data because of smaller irradiation volumes, using the statistical analysis for Weibull distribution. Moreover, the effect of the irradiation of ll-MeV protons [13,14,16] and the cathodic hydrogen charging on the DBTI of HT-9 steel is also measured by using the SP test method to obtain information of hydrogen- and irradiation-induced embrittlement on ferritic steels.
2. Experimental The materials (12Cr-lMo-V-W)
B.V.
used in the present study were HT-9 ferritic steel and JFMS (low
T. ~isawu
195
et al. / Small punch fests for evaluat~~~ ductile - b~iff~e frunsifio~
Table 1 Chemical compositions (mass%) Steel
c
Si
Mn
Ni
Cr
MO
V
w
Nb
N
HT-9 JFMS
0.20 0.05
0.21 0.67
0.52 0.58
0.52 0.94
12.24 9.85
1.03 2.31
0.29 0.12
0.48 -
0.06
0.006 0.010
C-9Cr-2Mo-lNi-V-Nb) ferritic/ martensitic dualphase steel. The chemical composition of the steels is shown in table 1. The plate stock of HT-9 steel was austenitized for 2 h at 1273 K and air cooled, then tempered for 2 h at temperature of 1043 K, 973 K, 873 K and 773 K. The plate of JFMS steel was subjected to normalizing heat treatment for 0.5 h at 1323 K, and tempered for I h at IO48 K followed by air cooling. In addition, long-term aging was performed for 1OOOh at 773 K on the previous as-tempered condition of JFMS steel. The standard Charpy V-notched (CVN) specimens, 10 x 10 X 55 mm3 in size, were machined after heat treatment. Two kinds of specimens for SP tests were fabricated. (1) Small specimens in the form of plates, 10 x 10 X 0.25 mm3 in size, were prepared from the undamaged portion of broken CVN bars. The thin plates with 1 mm thickness were extracted by slicing and polished up to 0.25 mm in thickness on 600 emery paper [13]. (2) TEM disk specimens of 3 mm diameter with 0.25 mm thickness, as a further-~niatu~zed SP specimen, were prepared from rod form steels of 3 mm in diameter. A disk material sliced at every 0.3 mm thickness by strain-free cutting was polished on WOO emery paper and fi~shed by a buffing operation. The deviation of finishing thickness in each specimen was accurate within +0.005 mm. The SP (small punch) test is similar to that described by Baik et al. [lO,ll] and the experimental configuration consists of a clamped centrally loaded specimen. The SP specimen holder consisted of an upper and lower dies, and four clamping screws. Using this specimen holder, it is possible to prevent specimens from cupping upward during punching and therefore the plastic deformation was concentrated in the region below the punch (steel ball). Each clamping screw was tightened at 0.5 Nm moment by torque wrench at all tests. The punch deformation was achieved by pushing a 2.4 mm hard steel ball against the small specimens and a 1 mm hard steel ball against the TEM disk specimens, respectively. The lower-die bore diameters of the small and TEM disk specimens were 4 and I.5 mm, respectively. All tests were performed at a crosshead moving speed of 0.02 mm/s over a range of test temperatures from liquid nitrogen to 473 K. In all cases the load/deflection
curves on SP specimens were recorded and the SP fracture energies were calculated. The effects of 1LMeV proton irradiation and hydrogen charging on the SP fracture energy were also observed. The expe~mental conditions of proton irradiation and cathodic hydrogen charging are described in previous papers [13,14]. The irradiation condition of the protons was as follows; proton energy: I1 MeV, proton current: 500 mA/m2, irradiation area: 8 mm 0, irradiation time: 1 h, and irradiation temperature is room temperature, but actual irradiation temperature should be 500-600 K. The specimen was covered with a 250 pm thick soft iron plate to avoid heavy induced activity in the specimen. Since the penetrating range of Il-MeV protons in iron is about 350 pm, the maximum damage position in the specimen is estimated to be about 100 pm in depth.
3. Results Fig. 1 shows examples of the load versus deflection curves of the HT-9 steel obtained for SP tests of both small and TEM disk specimens, respectively, at various test temperatures. As the test temperat~e increases, a similar behavior was observed on a set of load/ 1.0 ---
SMALL
-
TEM
SPECIMEN DISK
SPECIMEN
J DEFLECTION
Fig. 1. Examples of load versus deflection curves for SP tests of small (10X 10 X0.25 mm3) and TEM disk (3 mm in dia. and 0.25 mm thick) specimens on HT-9 steel tempered for 2 h at 973 K, at various test temperatures.
deflection curves for different size specimens. The macroscopically detectable cracking occured at the maximum load. The SP fracture energy necessary for fracture initiation was evaluated by the area under the load/deflection curve up to the sudden drop of load. Hereafter, we will use this definition of SP energy to characterize the ductile-brittle transition behavior. The SP energy values obtained from these SP tests were plotted against the test temperatures. The influences of torque for tightened screws and crosshead speed on the SP energies were measured in advance for the small specimens of the 973 K tempering HT-9 steel at the test temperature of 293 K. The values of SP energy were little affected in the torque range of 0.3 to 0.7 N m and also unaffected by the crosshead speed in the range from 8 X 10e4 to 3 X 10-l mm/s. 3.1. DBTT
0
100
200
300
TEMPERATURE
400
500
( K )
Fig. 3. SP fracture energy transition behavior obtained from the small specimens of HT-9 steel, tempered for 2 h at 773 K.
evaluation using small specimens
Figs. 2 and 3 show the temperature dependence of SP energies obtained from small specimens of the HT-9 steel, tempered for 2 h at 973 K and 773 K, respectively. The same behavior in SP energy transition was observed on the HT-9 steel, tempered at 1043 K and 873 K, and as-tempered and long-term aged JFMS steels. In general, the ductile-brittle transition temperature of the HT-9 steel shifted to higher temperature with lowering tempering temperature. The SP energy data for the HT-9 specimens with the tempered martensite structure containing carbide precipitates show more scatter than the data obtained by Baik et al. [lO,ll] for homoge-
-200
TEMPERATURE
( “C )
-100
0
100
I
I
I
I
200
I
00
8 o_ 0
neons, fine grained, temper-embrittled low alloy steels of 0.5 mm thick SP specimens. As shown in figs. 2 and 3, the scatter of data in the region of ductile-brittle transition is especially large. There was a remarkable scatter of the SP data in HT-9 steel tempered at 773 K, which showed the temper embrittlement. It might be practially impossible to avoid the influence of metallurgical heterogeneity in the quite miniaturized specimens of engineering alloy steels which have the dimension of on inhomogeneous phase, comparable to that of specimens with a thickness of 0.25 mm. Therefore, these SP test data with scatter are better to be analysed statistically. It has been shown that the Weibull distribution is suitable for dealing with variability in fracture data obtained by the Charpy V-notch impact test in low alloy (HSLA) steel [17]. In the case of SP fracture energy. the data scattering should be expressed by the Weibull distribution. The distribution function F(E), mean value p and dispersion u are written in the following form: F(E)=l-exp(-E/E,)m,
(1)
p= E,[r(l
+ l/m>],
(2)
0 = E,[ r(l
+ 2/m)
0
Q0 0
00
0
100
I
I
I
200
300
TEMPERATURE
I
LOO
I
500
( K )
Fig. 2. SP fracture energy transition behavior obtained from the small specimens of HT-9 steel, tempered for 2 h at 973 K. Effects of proton irradiation (0) and cathodic hydrogen charging (A) on SP energies are also shown.
- r*(l
-t l/m)]“*,
(3)
where E is the SP energy and E,, and m are the Weibull distribution parameters that depend on the test temperature. The data plotted on the Weibull probability paper, i.e., in ln(l/(l - F(E))] versus In E plots lied on straight lines, as shown in fig. 4. A single distribution was observed on the HT-9 and JFMS steel specimens tempered at different temperatures, except for the 773 K tempered HT-9 steel data at and around the
T. Misawa et al. / Small punch
0.005
0.01
Sf’
0.05 0.1
ENERGY
0.5
197
tests for evaluating ductile- brittle transition
1
(J)
Fig. 4. Weibull plots of SP energy obtained from the small specimens of HT-9 steel tested at various temperatures, tempered for 2 h at (a) 973 K and (b) 773 K.
DBTT which fell on the composite distribution line, as shown in fig. 4(b). The parameters m (shape parameter) and E, are determined from the gradient and the intersection of lines by using the least mean square method, respectively. Calculated values of the mean energy p on the 973 K tempered HT-9 steel are shown in fig. 5 as a solid line, with the estimated lower-bound
curve described after. Fig. 5 is also a plot of the Charpy-energy data versus temperature. In order to confirm a correlation between the DB’IT of the SP tests and that of the CVN tests in ferritic steels, the results obtained from small specimens in the mono-phase HT-9 and dual-phase JFMS steels have been plotted in fig. 6. The DB’M can be determined as the temperature corresponding to the middle point between upper and lower shelf energies. The DBTT (Tsp) obtained from the SP test on the fracture initiation energy is much lower than the DBTT (‘I&.,) obtained from the CVN impact test. Nevertheless, there is a correlation between the transition temperatures as shown in fig. 6. A kinetic model for ductile-brittle fracture has been developed by Kameda [12]. The comparison of the theoretical relationship between T&., and Tsp with the experimental results was attempted by Kameda [12]. The linear relationship of the experimental results is in agreement with the theoretical prediction. The proportionality coefficient (a) for the HT-9 and JFMS steels is a little different from that for the temper-embrittled Ni-Cr low alloy steels. In the case of impurity-doped Ni-Cr steels, the fracture mode is intergranular, but in this case of ferritic/martensitic steels, the fracture mode is transgranular. The difference of fracture mode seems to have effect on the value of a. The influences of ll-MeV proton irradiation (500 mA/m2 - 1 h) at room temperature and cathodic hydrogen charging on the SP energies in HT-9 steel are also shown in fig. 2. The proton irradiation did not cause a large shift of the DBTT. However, a remarkable
4001
z
w I-
a ul
I
I
. 200
-
I
1048K/ 1 h/AC:(I V (1)+773K/lOOOh/AC
JFMS
I
I
) o(
q0.41
-
0 0
100
200 TEMPERATURE
300
400
500
( K )
Fig. 5. Small specimen SP-energy measurements and two curves of mean value p (solid line) and 5% lower-bound energy estimate (dotted line) expressed as a Weibull distribution versus test temperature for HT-9 steel, tempered for 2 h at 973 K. Charpy V-notch (CVN) energy measurement curve is also shown.
0
100
200
DBTT
BY CVN
300 TEST,
400 TCVN
500
( K)
Fig. 6. Correlation of the DBTT between CVN and SP tests for tempered HT-9 and JFMS fertitic steels.
T. Misawa et al. / Small punch tests for eualuaiing duct& - hrrrtle transltlm
198
3
99F_
0.8
I
I
I
=
5
90 70 2 50 z 30 Q 20 $ 10
/
!
235
1i
0.5 -0.3
1TEM DISK SPECIMEN
m =9.80
0.2 I
1,
I,,,
0.001
1
I I
III,
I
I,
0.1
0.0 I SP
ENERGY
( J
0.5
)
I
Fig. 7. Weibull plots of SP energy obtained from the TEM disk specimens of HT-9 steel tempered for 2 h at 973 K, at various test temperatures.
decrease
in SP fracture
perature
was observed
in
‘\.,
I
Y
I ”
“.\.\
I!’
0.6
E
5 2>
-.\
HaSO,
initiation energy at room temon the samples hydrogen-charged
+ CN(NH,),
A/m2
for
5 h.
center
part
of
Fig. 9. Scanning
(thiourea)
Many the
small
electron
solution
microcracking specimens
micrographs specimens
at
initiated and
-100 at
the
0
100
200
300
TEMPERATURE
400
500
( K 1
Fig. 8. TEM disk specimen SP-energy measurements and both curves of mean value p (solid line) and 5th percentile curve associated with the Weibull distribution (dotted line) for HT-9 steel, tempered for 2 h at 973 K. A dot-dash-line shows the mean value p obtained from the small specimen tests displayed in fig. 5. Arrows indicate the DBTT.
the fracture
of cracking mode change (top view) and fracture surface of the 973 K tempering tested at 83 K ((a), (d)), 118 K (DBTT) ((b), (e)) and 293 K ((c), (f)).
TEM disk
T. Misawa et al. / Small punch tests for evaluating ductile-brittle transition surface with the intergranular and cleavage facets were observed on the internal hydrogen charged specimens by a scanning electron microscope. 3.2. DBTT
by TEA4 disk specimens
The SP test using a further-miniaturized SP specimen as small as the TEM disk specimens with a 3 mm diameter was developed to obtain the DBTI’ data and to confirm the size dependence of &,. Fig. 7 shows the Weibull plots of SP fracture energy obtained from the TEM disk specimens of HT-9 steel tempered for 2 h at 973 K, at various test temperatures, and the data follow a straight line. The temperature dependence of SP fracture energy values of the TEM disk specimens is illustrated in fig. 8. The evaluation of specimen-independent DBTT by SP test was confirmed by comparing the DBTT value of 116 K for the TEM disk specimens with that of 118 K for the small specimens, as shown by the arrows above the mean energy curves in fig. 8. This result suggests that the SP test by a “significant” miniaturized specimen makes feasible to evaluate the DBTT of irradiated ferritic structural alloy steels. Fig. 9 shows the fracture appearance and fracture surface at the various test temperatures. As can be seen, the cracking mode change was observed in the SP specimens fractured below or above the DBTT. In the range of brittle lower shelf below the DBTT, cracking initiated at the center part of the small punch specimens propagating straight along the radial directions. The failure in the ductile upper shelf range above the DBTT occurred along the circumferential edge of the buldge caused by the punch extrusion. The mixed mode of failure was observed in the ductile-brittle transition range. The fracture surface of the SP specimens of HT-9 steel tempered for 2 h at 973 K showed a transgranular cracking over the whole test temperature. However, the ductile fibrous fracture surface showing void formation around carbide particles increased with increasing the testing temperatures.
4. Discussion The SP test, which has been developed by Baik et al. [lO,ll] and Kameda [12], makes it possible to estimate the values of DBTT shift by CVN impact tests in ferritic steels after irradiation on the basis of the empirical correlation between T,,, and TsP, as shown in fig. 6. A remarkable rise in the DBTT by the CVN specimens (10 X 10 X 55 mm3) has been observed in the HT-9 steel tempered for 2.5 h at 1053 K by the
199
high-fluence neutron irradiation in the EBR-II reactor [18]. The DBTT by the CVN tests was elevated by 108-113 K after irradiation [18]. This change in the DBTT for HT-9 is expected to rise the DBTT by 44-46 K in SP tests from the proportionary coefficient of fig. 6. The measuring sensitivity of DBTT in the SP test is less than that in the CVN test, but enough for the measurement of such large shifts. There were no large shifts of the DBTI and the SP fracture energy in our specimens of the HT-9 after the ll-MeV proton irradiation, as shown in fig. 2, which is because of the low damage in the case of proton irradiation. Another origin for these results is that the damaged region is localized in a small volume of the SP specimen, as confirmed by the experimental results on hardness increase in the previous paper [16]. In contrast the SP energy is a measure of the total volume fracture energy. The use of thinner specimens might be necessary to obtain clear information for the effect of proton irradiation on the DBTT. 4.1. Fracture properties indicated in the Weibull distribution parameters Probability distribution of SP fracture initiation energies on the HT-9 and JFMS ferritic steels was found to be described as the Weibull distribution which is based on weakest-link theory and widely used in reliability engineering. Fig. 10 shows the normalized temperature dependence of the shape parameter and the dispersion of HT-9 steels tempered at 973 K and 773 K. The magnitude of the shape parameter, i.e., m 5 1 or m > 1, gives the valuable information on fracture mode on the basis of reliability engineering. The condition which gives the shape parameter of unity is likely to correspond to a chance failure or random occurence of crack initiation, and the shape parameter smaller than unity indicates an early-stage-failure. On the other hand, the shape parameter larger than unity means a wearout-failure. As shown in fig. 10(a), the temperature dependence of the m values in the 973 K tempering HT-9 steel of the small and TEM disk specimens varies from the crack initiation of a wear-out-type (m > 1) within the ductile temperature range T/T,, > 1 to the crack initiation of a chance or initial stage occurences (m s 1) within the brittle temperature range T/T,, < 1. The dispersions corresponding to the 973 K tempering HT-9 specimens show a sharp maximum around the DBTT, which is reasonable because of the mixing mode of ductile and brittle fracture at that temperature as shown in fig. 9(b). This temperature dependence of the shape parameter and the dispersion, which might be a
T. Misawa et al. / Small punch tesis for evaluating ductrle - brittle transition
200
12
(a) 973 K
o’a SMALL S&MEN 0 A TEM DISK SPECIMEN
this work that the temperature dependence Weibull parameter is useful for the evaluation DBTT and ductile-brittle transition behaviour ritic steels by the SP test.
of the of the in fer-
4.2. Estimation of the DBTT and the lower-bound SP fracture energy curve
I
773
I
10 A SMALL
(b)
It is important for the screening of candidate alloy steels to estimate the data of the DBTT and fracturetoughness and their change with the irradiation and heat treatment. For the irradiation experiments of candidate materials of the fusion reactor first wall, the small amounts of miniature size specimens must be used SPECIMEN]
K
T’
from the economical and practical viewpoint. The ferritic/martensitic steels like HT-9 and JFMS possess
TSP
Fig. 10. Temperature dependence of Weibull modulus (shape parameter) m and dispersion o in HT-9 steel, tempered for 2 h at (a) 973 K and (b) 773 K. Test temperatures are normalized with the corresponding DB’lT (TsP) obtained by the SP tests for small and TEM disk specimens.
TEST
normal material property to crack initiation, was also observed on the SP test results of the 1043 K tempering HT-9 and JFMS small specimens. In contrast the m values of the 773 K tempering HT-9 specimens were almost a constant value of unity except for both sides of the temperature and showed a pan-shape temperature dependence, as shown in fig. 10(b). There is a large dispersion at and above the DBTT. On the 773 K tempering HT-9 the composite distribution consisting of mode 1 (m, = 0.87) and mode 2 (M z = 2.06) distributions was also found at the test temperature of the DBTT, as shown in fig. 4(b). The HT-9 steels tempered at temperature between 773 K and 823 K result in secondary hardening with the precipitation of M& carbide particles in the tempered martensite matrix and the rise in the DBTT caused by temper brittleness. This remarkably different temperature dependence of the shape parameter of HT-9 steel tempered at 773 K may reflect the change of microstructure and fracture mechanism in connection with the temper hardening and temper brittleness. Thus, it has been demonstrated in
TEMPERATURE
I
TEST
I
I
(K)
I
TEMPERATURE
I
I
I
(K)
Fig 11. Examples of temperature dependence of SP fracture energy estimates based on the data partitioning method for small specimens of HT-9 steel tempered for 2 h at 973 K: (a) all the measured data, (b) two data in each measured temperature. The middle solid lines show the estimated 15, in the separated two regions. A set of parallel thin lines indicates the upper- and lower-bound
fracture
energies.
T. Misawa et al. / Small punch tests for evaluating ductile - brittle transition
201
ture curve and the DBTT. The difference between the DBTT obtained by both method on the 773 K tempered HT-9 specimen is about 19 K, which is a little larger than that for the other specimens. The less fitness of the data partitioning method to that specimen should be due to the anomalous behavior of the Weibull shape parameter.
essentially heterogeneous microstructures, which might cause the large scatter of data on the fracture related parameters. In addition to this nature, the temperature dependence of the Weibull parameter shows the drastic change with the microstructure and fracture mode of materials, as demonstrated in this work. Bishop et al. [17] have applied a recently developed statistical analysis to the estimation of lower-bound Charpy V-notch impact energies when there are few data points and/or large scatter. Mar&an et al. [19] have improved the method of statistical analysis to be applicable for the full temperature range of data, that is the data partitioning method for dealing separately with upper-shelf and transition-region data. This data partitioning method has been applied to the determination of the DB’M and the estimation of the lower-bound SP fracture energy curve on the HT-9 and JFMS steels. In the first case, all the measured data are used for the statistical analysis. An example of the results is shown in fig. 11(a), which is for the small specimen of HT-9 steel tempered at 973 K. The lower-bound curve in the figure is defined as a 5% failure probability and the upper-bound curve corresponds to 95% failure probability, respectively. The DBTI is defined as the temperature corresponding to the middle energy between that at the cross-point of two &,-curves and at 50 K. The DBTT obtained by this definition is quite similar with that obtained by the standard method described in section 3.1. on every specimen used for the SP test, except that on the 773 K tempered HT-9 specimens. This means that the data partitioning method is reliable for the estimation of the fracture energy versus tempera-
Secondly, three data in each measured temperature are selected randomly for estimating the number of specimens to obtain reliable results by using the statistical analysis. The results of the analysis show similar behavior and DBTT on every specimen. As for the extreme case two data are used to the fitting procedure, the results of which also show the same behavior, as shown in fig. 11(b). Obtained results of the DBTT are presented in table 2. From these results, it is quite useful and practical to the irradiation experiments in the fusion program that two or three data measured at the test temperatures with appropriate interval are enough for the estimation of the DBIT.
5. Conclusions The SP tests using small and TEM disk specimens were performed for HT-9 and JFMS ferritic steels under consideration for evaluating ductile-brittle transition behavior of the irradiated fusion materials. The results obtained are summarized as follows: (1) The SP tests demonstrated ductile-brittle transition behavior and the DBTT was located at a lower temperature than that in the CVN tests.
Table 2 Estimates of the DBTT by small and TEM disk specimen SP tests for tempered HT-9 and JFMS steels Steel
HT-9
Tempering and aging
1043K/2h 973K/2h
Specimen size
Small Small TEM
873K/2h 773K/2h JFMS
1048K/lh 1048K/lh + 773K/lOOOh
Small Small
DBTT (K) Standard method (IL line) 93 118 118 116 116 126 166 166
Data partitioning method Iteration
1 2 1 2 1 2
All data
Three data
Two data
115 124 124 123 123 126 185 185
120 126 123 124 123 126 192 185
113 126 127 126 123 126 180 183
120
123
Small
123
120
Small
125
133
132
T. Misawa et al. / Small punch tests
202 (2) The temperature
dependence of SP fracture energies with scatter in miniaturized testing could give reliable information on the DBTT by use of the statistical analysis based on the Weibull distribution. An empirical correlation of the DBTT was obtained between the standard CVN and SP tests in the tempered steels. Weibull shape parame(3) The temperature-dependent ter and dispersion in SP energies was illustrated to give valuable information on the ductile-brittle fracture mode and properties in various tempered steels. (4) It has become possible to estimate the reliable DBTT and the lower-bound SP fracture energy curve using the statistical analysis of the data partitioning method, when there are few data points and/or scatter.
Acknowledgements The authors are grateful to Messrs. T. Yamada, Y. Toda and the members of the Cyclotron Group in Japan Steel Works Ltd. for performing experiments and Mr. R. Miura of JSW for his helpful cooperation. We would also like to thank Professor H. Takahashi of the Tohoku University and Dr. T. Kodaira of the Japan Atomic Energy Research Institute for valuable discussions. This research is supported partly by the Grant-inAid for Fusion Research by the Ministry of Education, Science
and
Culture
of Japan.
References [l] Proc. Topical Conf. on Ferritic Alloys for Use in Nuclear Energy Technologies, Snowbird, Utah, June 19-23, 1983, Eds. J.W. Davis and D.J. Michel (AIME, Pennsylvania, 1984). [2] T. Lechtenberg, J. Nucl. Mater. 133 & 134 (1985) 149.
for evaluating ductile-brittle
transitron
[3] G.E. Lucas and G.R. Odette, Nucl. Engrg. Des./Fusion 2 (1985) 145. [4] M. Dooley, G.E. Lucas and J.W. Sheckherd. J. Nucl. Mater. 103 & 104 (1981) 1583. [5] A. Okada, T. Yoshie, S. Kojima, K. Abe and M. Kiritani, J. Nucl. Mater. 133 & 134 (1985) 321. [6] A. Kohyama, K. Asakura, Y. Kohno, K. Komamura. K. Suzuki, M. Kiritani, T. Fujita and N. Igata, J. Nucl. Mater. 133 & 134 (1985) 628. [7] M.P. Manahan, AS. Argon and O.K. Harling. J. Nucl. Mater. 103 & 104 (1981) 1545. [8] F.H. Huang, M.L. Hamilton and G.L. Wire, Nucl. Technol. 57 (1982) 234. [9] G.E. Lucas, J.W. Sheckherd, G.R. Odette and S. Panchanadeeswaran, J. Nucl. Mater. 122 & 123 (1984) 429. PO1 J.M. Baik, J. Kameda and 0. Buck, Scripta Met. 17 (1983) 1443. 1111 J.M. Baik, J. Kameda and 0. Buck, The Use of Small-Scale Specimens for Testing Irradiated Materials, ASTM STP 888, Eds. W.R. Corwin and G.E. Lucas (ASTM. Philadelphia, 1986) p. 92. U-A J. Kameda, Acta Met. 34 (1986) 2391. J. u31 T. Misawa, H. Sugawara, R. Miura and Y. Hamaguchi. Nucl. Mater. 133 & 134 (1985) 313. H. Kuwano, H. Kamide, R. Miura and T. u41 Y. Hamaguchi. Yamada, J. Nucl. Mater. 133 & 134 (1985) 636. The mechanism of fracu51 T. Misawa and Y. Hamaguchi, ture, in: Proc. Int. Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, Salt Lake City, Utah, Dec. 2-6, 1985. Ed. V.S. Goel (ASM, Ohio. 1986) p. 521. H. Kuwano, T. Misawa, R. Miura and T. U61 Y. Hamaguchi, Yamada, J. Nucl. Mater. 141 & 143 (1986) 781. and A.R. Rosenfield, Met. [771T.A. Bishop, A.J. Markworth Trans. 14A (1983) 687. and V. Provenzano, Ef1181 F.A. Smidt, Jr., J.R. Hawthorne fects of Radiation on Materials, ASTM STP 725. Eds. D. Kramer, H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1981) p. 269. S. Quayle, A.R. Rosenfield and D.K. P91 M.P. Manahan, Shetty, The mechanism of fracture, in: Proc. Int. Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, Salt Lake City, Utah, Dec. 2-6. 1985, Ed. V.S. Goel (ASM, Ohio. 1986) p. 495.
Journal
of Nuclear
SMALL PUNCH TESTS FOR EVALUATING DUCTILE-BRITTLE OF IRRADIATED FERRITIC STEELS T. MISAWA, Department
Received
T. ADACHI,
of Metallurgical
21 October
M. SAITO
Engineering,
1986; accepted
Muroran
150 (19X7) 194 -702 North-Holland. Amsterdam
Materials
TRANSITION BEHAVIOR
and Y. HAMAGUCHI lnstrtute of Technology,
Mizumoto-rho,
Muroran
050, Japan
22 May 1987
Small punch (SP) tests using small and TEM disk miniaturized specimens were developed to evaluate the ductile-brittle transition temperature (DBTT) of HT-9 and JFMS ferritic steels as candidates for the structural materials of a fusion reactor. It was shown that the temperature dependence of SP fracture energies with scatter in miniaturized testing can give the reliable information on the DBT’T by use of the statistical analysis based on the Weibull distribution. An empirical correlation between the DBTT measured by the Charpy-impact test and that by the SP test was obtained. It became possible to estimate the reliable DBTT and lower-bound SP fracture energy curve using the data partitioning method when there are few data points
and/or
scatter
1. Introduction Ferritic
steels
are recently
recognized
[1,2] to be next
candidate alloys for fusion reactor structural materials. In contrast to austenitic alloy steels, the ferritic steels have the advantage of a larger resistance to void swelling. However, a rise of the ductile-brittle transition temperature (DBTT) due to fast neutron irradiation is a problem, which may preclude the usage of the ferritic steels. Therefore, it is particularly important to evaluate the effect of radiation damage on the shift of the DBTT of the ferritic steels. Usually the DBTT shift is determined by means of the Charpy V-notch impact test, which requires relatively large size specimens. However, since there are severe limitations on the specimen capacity at material irradiation testing facilities, it is necessary to develop special techniques using miniaturized specimens, called a small specimen technology [3]. In recent years, miniaturized punch tests have been developed to obtain strength and/or ductility information from transmission electron microscopy (TEM) disk specimens. Tests using a spherical punch have been referred as bulge [4-61 and disk bend [7,8] tests and those using a cylindrical punch as shear punch tests [9]. Baik et al. [lO,ll] and Kameda [12] evaluated a total plastic work done to the fracture in small punch (SP) tests at various test temperatures. They demonstrated that SP test results in temper embrittled Ni-Cr low alloy steel exhibit a clear ductilebrittle transition
0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
behavior. In the previous papers [13,16], we have reported the preliminary results of the ductile-brittle transition behavior using the SP test in HT-9 type ferritic steel, possessing excellent swelling resistance. The present work shows a possible extension of previous small (10 X 10 X 0.25 mm3) SP specimens to a further-miniaturized small specimen as small as TEM disk (3 mm diameter with 0.25 mm thickness) SP specimens, having two objectives. The first one is to confirm an empirical correlation between the DBTT measured by the standard Charpy-V-notch impact test and that on the SP test in HT-9 and JFMS ferritic steels. The second one is to evaluate large SP-energy scatter caused by unavoidable microstructural heterogeneity to cracking in a miniature specimen of engineering alloys and to estimate the lower-bound energy curve from the large scatter points and/or few data because of smaller irradiation volumes, using the statistical analysis for Weibull distribution. Moreover, the effect of the irradiation of ll-MeV protons [13,14,16] and the cathodic hydrogen charging on the DBTI of HT-9 steel is also measured by using the SP test method to obtain information of hydrogen- and irradiation-induced embrittlement on ferritic steels.
2. Experimental The materials (12Cr-lMo-V-W)
B.V.
used in the present study were HT-9 ferritic steel and JFMS (low
T. ~isawu
195
et al. / Small punch fests for evaluat~~~ ductile - b~iff~e frunsifio~
Table 1 Chemical compositions (mass%) Steel
c
Si
Mn
Ni
Cr
MO
V
w
Nb
N
HT-9 JFMS
0.20 0.05
0.21 0.67
0.52 0.58
0.52 0.94
12.24 9.85
1.03 2.31
0.29 0.12
0.48 -
0.06
0.006 0.010
C-9Cr-2Mo-lNi-V-Nb) ferritic/ martensitic dualphase steel. The chemical composition of the steels is shown in table 1. The plate stock of HT-9 steel was austenitized for 2 h at 1273 K and air cooled, then tempered for 2 h at temperature of 1043 K, 973 K, 873 K and 773 K. The plate of JFMS steel was subjected to normalizing heat treatment for 0.5 h at 1323 K, and tempered for I h at IO48 K followed by air cooling. In addition, long-term aging was performed for 1OOOh at 773 K on the previous as-tempered condition of JFMS steel. The standard Charpy V-notched (CVN) specimens, 10 x 10 X 55 mm3 in size, were machined after heat treatment. Two kinds of specimens for SP tests were fabricated. (1) Small specimens in the form of plates, 10 x 10 X 0.25 mm3 in size, were prepared from the undamaged portion of broken CVN bars. The thin plates with 1 mm thickness were extracted by slicing and polished up to 0.25 mm in thickness on 600 emery paper [13]. (2) TEM disk specimens of 3 mm diameter with 0.25 mm thickness, as a further-~niatu~zed SP specimen, were prepared from rod form steels of 3 mm in diameter. A disk material sliced at every 0.3 mm thickness by strain-free cutting was polished on WOO emery paper and fi~shed by a buffing operation. The deviation of finishing thickness in each specimen was accurate within +0.005 mm. The SP (small punch) test is similar to that described by Baik et al. [lO,ll] and the experimental configuration consists of a clamped centrally loaded specimen. The SP specimen holder consisted of an upper and lower dies, and four clamping screws. Using this specimen holder, it is possible to prevent specimens from cupping upward during punching and therefore the plastic deformation was concentrated in the region below the punch (steel ball). Each clamping screw was tightened at 0.5 Nm moment by torque wrench at all tests. The punch deformation was achieved by pushing a 2.4 mm hard steel ball against the small specimens and a 1 mm hard steel ball against the TEM disk specimens, respectively. The lower-die bore diameters of the small and TEM disk specimens were 4 and I.5 mm, respectively. All tests were performed at a crosshead moving speed of 0.02 mm/s over a range of test temperatures from liquid nitrogen to 473 K. In all cases the load/deflection
curves on SP specimens were recorded and the SP fracture energies were calculated. The effects of 1LMeV proton irradiation and hydrogen charging on the SP fracture energy were also observed. The expe~mental conditions of proton irradiation and cathodic hydrogen charging are described in previous papers [13,14]. The irradiation condition of the protons was as follows; proton energy: I1 MeV, proton current: 500 mA/m2, irradiation area: 8 mm 0, irradiation time: 1 h, and irradiation temperature is room temperature, but actual irradiation temperature should be 500-600 K. The specimen was covered with a 250 pm thick soft iron plate to avoid heavy induced activity in the specimen. Since the penetrating range of Il-MeV protons in iron is about 350 pm, the maximum damage position in the specimen is estimated to be about 100 pm in depth.
3. Results Fig. 1 shows examples of the load versus deflection curves of the HT-9 steel obtained for SP tests of both small and TEM disk specimens, respectively, at various test temperatures. As the test temperat~e increases, a similar behavior was observed on a set of load/ 1.0 ---
SMALL
-
TEM
SPECIMEN DISK
SPECIMEN
J DEFLECTION
Fig. 1. Examples of load versus deflection curves for SP tests of small (10X 10 X0.25 mm3) and TEM disk (3 mm in dia. and 0.25 mm thick) specimens on HT-9 steel tempered for 2 h at 973 K, at various test temperatures.
deflection curves for different size specimens. The macroscopically detectable cracking occured at the maximum load. The SP fracture energy necessary for fracture initiation was evaluated by the area under the load/deflection curve up to the sudden drop of load. Hereafter, we will use this definition of SP energy to characterize the ductile-brittle transition behavior. The SP energy values obtained from these SP tests were plotted against the test temperatures. The influences of torque for tightened screws and crosshead speed on the SP energies were measured in advance for the small specimens of the 973 K tempering HT-9 steel at the test temperature of 293 K. The values of SP energy were little affected in the torque range of 0.3 to 0.7 N m and also unaffected by the crosshead speed in the range from 8 X 10e4 to 3 X 10-l mm/s. 3.1. DBTT
0
100
200
300
TEMPERATURE
400
500
( K )
Fig. 3. SP fracture energy transition behavior obtained from the small specimens of HT-9 steel, tempered for 2 h at 773 K.
evaluation using small specimens
Figs. 2 and 3 show the temperature dependence of SP energies obtained from small specimens of the HT-9 steel, tempered for 2 h at 973 K and 773 K, respectively. The same behavior in SP energy transition was observed on the HT-9 steel, tempered at 1043 K and 873 K, and as-tempered and long-term aged JFMS steels. In general, the ductile-brittle transition temperature of the HT-9 steel shifted to higher temperature with lowering tempering temperature. The SP energy data for the HT-9 specimens with the tempered martensite structure containing carbide precipitates show more scatter than the data obtained by Baik et al. [lO,ll] for homoge-
-200
TEMPERATURE
( “C )
-100
0
100
I
I
I
I
200
I
00
8 o_ 0
neons, fine grained, temper-embrittled low alloy steels of 0.5 mm thick SP specimens. As shown in figs. 2 and 3, the scatter of data in the region of ductile-brittle transition is especially large. There was a remarkable scatter of the SP data in HT-9 steel tempered at 773 K, which showed the temper embrittlement. It might be practially impossible to avoid the influence of metallurgical heterogeneity in the quite miniaturized specimens of engineering alloy steels which have the dimension of on inhomogeneous phase, comparable to that of specimens with a thickness of 0.25 mm. Therefore, these SP test data with scatter are better to be analysed statistically. It has been shown that the Weibull distribution is suitable for dealing with variability in fracture data obtained by the Charpy V-notch impact test in low alloy (HSLA) steel [17]. In the case of SP fracture energy. the data scattering should be expressed by the Weibull distribution. The distribution function F(E), mean value p and dispersion u are written in the following form: F(E)=l-exp(-E/E,)m,
(1)
p= E,[r(l
+ l/m>],
(2)
0 = E,[ r(l
+ 2/m)
0
Q0 0
00
0
100
I
I
I
200
300
TEMPERATURE
I
LOO
I
500
( K )
Fig. 2. SP fracture energy transition behavior obtained from the small specimens of HT-9 steel, tempered for 2 h at 973 K. Effects of proton irradiation (0) and cathodic hydrogen charging (A) on SP energies are also shown.
- r*(l
-t l/m)]“*,
(3)
where E is the SP energy and E,, and m are the Weibull distribution parameters that depend on the test temperature. The data plotted on the Weibull probability paper, i.e., in ln(l/(l - F(E))] versus In E plots lied on straight lines, as shown in fig. 4. A single distribution was observed on the HT-9 and JFMS steel specimens tempered at different temperatures, except for the 773 K tempered HT-9 steel data at and around the
T. Misawa et al. / Small punch
0.005
0.01
Sf’
0.05 0.1
ENERGY
0.5
197
tests for evaluating ductile- brittle transition
1
(J)
Fig. 4. Weibull plots of SP energy obtained from the small specimens of HT-9 steel tested at various temperatures, tempered for 2 h at (a) 973 K and (b) 773 K.
DBTT which fell on the composite distribution line, as shown in fig. 4(b). The parameters m (shape parameter) and E, are determined from the gradient and the intersection of lines by using the least mean square method, respectively. Calculated values of the mean energy p on the 973 K tempered HT-9 steel are shown in fig. 5 as a solid line, with the estimated lower-bound
curve described after. Fig. 5 is also a plot of the Charpy-energy data versus temperature. In order to confirm a correlation between the DB’IT of the SP tests and that of the CVN tests in ferritic steels, the results obtained from small specimens in the mono-phase HT-9 and dual-phase JFMS steels have been plotted in fig. 6. The DB’M can be determined as the temperature corresponding to the middle point between upper and lower shelf energies. The DBTT (Tsp) obtained from the SP test on the fracture initiation energy is much lower than the DBTT (‘I&.,) obtained from the CVN impact test. Nevertheless, there is a correlation between the transition temperatures as shown in fig. 6. A kinetic model for ductile-brittle fracture has been developed by Kameda [12]. The comparison of the theoretical relationship between T&., and Tsp with the experimental results was attempted by Kameda [12]. The linear relationship of the experimental results is in agreement with the theoretical prediction. The proportionality coefficient (a) for the HT-9 and JFMS steels is a little different from that for the temper-embrittled Ni-Cr low alloy steels. In the case of impurity-doped Ni-Cr steels, the fracture mode is intergranular, but in this case of ferritic/martensitic steels, the fracture mode is transgranular. The difference of fracture mode seems to have effect on the value of a. The influences of ll-MeV proton irradiation (500 mA/m2 - 1 h) at room temperature and cathodic hydrogen charging on the SP energies in HT-9 steel are also shown in fig. 2. The proton irradiation did not cause a large shift of the DBTT. However, a remarkable
4001
z
w I-
a ul
I
I
. 200
-
I
1048K/ 1 h/AC:(I V (1)+773K/lOOOh/AC
JFMS
I
I
) o(
q0.41
-
0 0
100
200 TEMPERATURE
300
400
500
( K )
Fig. 5. Small specimen SP-energy measurements and two curves of mean value p (solid line) and 5% lower-bound energy estimate (dotted line) expressed as a Weibull distribution versus test temperature for HT-9 steel, tempered for 2 h at 973 K. Charpy V-notch (CVN) energy measurement curve is also shown.
0
100
200
DBTT
BY CVN
300 TEST,
400 TCVN
500
( K)
Fig. 6. Correlation of the DBTT between CVN and SP tests for tempered HT-9 and JFMS fertitic steels.
T. Misawa et al. / Small punch tests for eualuaiing duct& - hrrrtle transltlm
198
3
99F_
0.8
I
I
I
=
5
90 70 2 50 z 30 Q 20 $ 10
/
!
235
1i
0.5 -0.3
1TEM DISK SPECIMEN
m =9.80
0.2 I
1,
I,,,
0.001
1
I I
III,
I
I,
0.1
0.0 I SP
ENERGY
( J
0.5
)
I
Fig. 7. Weibull plots of SP energy obtained from the TEM disk specimens of HT-9 steel tempered for 2 h at 973 K, at various test temperatures.
decrease
in SP fracture
perature
was observed
in
‘\.,
I
Y
I ”
“.\.\
I!’
0.6
E
5 2>
-.\
HaSO,
initiation energy at room temon the samples hydrogen-charged
+ CN(NH,),
A/m2
for
5 h.
center
part
of
Fig. 9. Scanning
(thiourea)
Many the
small
electron
solution
microcracking specimens
micrographs specimens
at
initiated and
-100 at
the
0
100
200
300
TEMPERATURE
400
500
( K 1
Fig. 8. TEM disk specimen SP-energy measurements and both curves of mean value p (solid line) and 5th percentile curve associated with the Weibull distribution (dotted line) for HT-9 steel, tempered for 2 h at 973 K. A dot-dash-line shows the mean value p obtained from the small specimen tests displayed in fig. 5. Arrows indicate the DBTT.
the fracture
of cracking mode change (top view) and fracture surface of the 973 K tempering tested at 83 K ((a), (d)), 118 K (DBTT) ((b), (e)) and 293 K ((c), (f)).
TEM disk
T. Misawa et al. / Small punch tests for evaluating ductile-brittle transition surface with the intergranular and cleavage facets were observed on the internal hydrogen charged specimens by a scanning electron microscope. 3.2. DBTT
by TEA4 disk specimens
The SP test using a further-miniaturized SP specimen as small as the TEM disk specimens with a 3 mm diameter was developed to obtain the DBTI’ data and to confirm the size dependence of &,. Fig. 7 shows the Weibull plots of SP fracture energy obtained from the TEM disk specimens of HT-9 steel tempered for 2 h at 973 K, at various test temperatures, and the data follow a straight line. The temperature dependence of SP fracture energy values of the TEM disk specimens is illustrated in fig. 8. The evaluation of specimen-independent DBTT by SP test was confirmed by comparing the DBTT value of 116 K for the TEM disk specimens with that of 118 K for the small specimens, as shown by the arrows above the mean energy curves in fig. 8. This result suggests that the SP test by a “significant” miniaturized specimen makes feasible to evaluate the DBTT of irradiated ferritic structural alloy steels. Fig. 9 shows the fracture appearance and fracture surface at the various test temperatures. As can be seen, the cracking mode change was observed in the SP specimens fractured below or above the DBTT. In the range of brittle lower shelf below the DBTT, cracking initiated at the center part of the small punch specimens propagating straight along the radial directions. The failure in the ductile upper shelf range above the DBTT occurred along the circumferential edge of the buldge caused by the punch extrusion. The mixed mode of failure was observed in the ductile-brittle transition range. The fracture surface of the SP specimens of HT-9 steel tempered for 2 h at 973 K showed a transgranular cracking over the whole test temperature. However, the ductile fibrous fracture surface showing void formation around carbide particles increased with increasing the testing temperatures.
4. Discussion The SP test, which has been developed by Baik et al. [lO,ll] and Kameda [12], makes it possible to estimate the values of DBTT shift by CVN impact tests in ferritic steels after irradiation on the basis of the empirical correlation between T,,, and TsP, as shown in fig. 6. A remarkable rise in the DBTT by the CVN specimens (10 X 10 X 55 mm3) has been observed in the HT-9 steel tempered for 2.5 h at 1053 K by the
199
high-fluence neutron irradiation in the EBR-II reactor [18]. The DBTT by the CVN tests was elevated by 108-113 K after irradiation [18]. This change in the DBTT for HT-9 is expected to rise the DBTT by 44-46 K in SP tests from the proportionary coefficient of fig. 6. The measuring sensitivity of DBTT in the SP test is less than that in the CVN test, but enough for the measurement of such large shifts. There were no large shifts of the DBTI and the SP fracture energy in our specimens of the HT-9 after the ll-MeV proton irradiation, as shown in fig. 2, which is because of the low damage in the case of proton irradiation. Another origin for these results is that the damaged region is localized in a small volume of the SP specimen, as confirmed by the experimental results on hardness increase in the previous paper [16]. In contrast the SP energy is a measure of the total volume fracture energy. The use of thinner specimens might be necessary to obtain clear information for the effect of proton irradiation on the DBTT. 4.1. Fracture properties indicated in the Weibull distribution parameters Probability distribution of SP fracture initiation energies on the HT-9 and JFMS ferritic steels was found to be described as the Weibull distribution which is based on weakest-link theory and widely used in reliability engineering. Fig. 10 shows the normalized temperature dependence of the shape parameter and the dispersion of HT-9 steels tempered at 973 K and 773 K. The magnitude of the shape parameter, i.e., m 5 1 or m > 1, gives the valuable information on fracture mode on the basis of reliability engineering. The condition which gives the shape parameter of unity is likely to correspond to a chance failure or random occurence of crack initiation, and the shape parameter smaller than unity indicates an early-stage-failure. On the other hand, the shape parameter larger than unity means a wearout-failure. As shown in fig. 10(a), the temperature dependence of the m values in the 973 K tempering HT-9 steel of the small and TEM disk specimens varies from the crack initiation of a wear-out-type (m > 1) within the ductile temperature range T/T,, > 1 to the crack initiation of a chance or initial stage occurences (m s 1) within the brittle temperature range T/T,, < 1. The dispersions corresponding to the 973 K tempering HT-9 specimens show a sharp maximum around the DBTT, which is reasonable because of the mixing mode of ductile and brittle fracture at that temperature as shown in fig. 9(b). This temperature dependence of the shape parameter and the dispersion, which might be a
T. Misawa et al. / Small punch tesis for evaluating ductrle - brittle transition
200
12
(a) 973 K
o’a SMALL S&MEN 0 A TEM DISK SPECIMEN
this work that the temperature dependence Weibull parameter is useful for the evaluation DBTT and ductile-brittle transition behaviour ritic steels by the SP test.
of the of the in fer-
4.2. Estimation of the DBTT and the lower-bound SP fracture energy curve
I
773
I
10 A SMALL
(b)
It is important for the screening of candidate alloy steels to estimate the data of the DBTT and fracturetoughness and their change with the irradiation and heat treatment. For the irradiation experiments of candidate materials of the fusion reactor first wall, the small amounts of miniature size specimens must be used SPECIMEN]
K
T’
from the economical and practical viewpoint. The ferritic/martensitic steels like HT-9 and JFMS possess
TSP
Fig. 10. Temperature dependence of Weibull modulus (shape parameter) m and dispersion o in HT-9 steel, tempered for 2 h at (a) 973 K and (b) 773 K. Test temperatures are normalized with the corresponding DB’lT (TsP) obtained by the SP tests for small and TEM disk specimens.
TEST
normal material property to crack initiation, was also observed on the SP test results of the 1043 K tempering HT-9 and JFMS small specimens. In contrast the m values of the 773 K tempering HT-9 specimens were almost a constant value of unity except for both sides of the temperature and showed a pan-shape temperature dependence, as shown in fig. 10(b). There is a large dispersion at and above the DBTT. On the 773 K tempering HT-9 the composite distribution consisting of mode 1 (m, = 0.87) and mode 2 (M z = 2.06) distributions was also found at the test temperature of the DBTT, as shown in fig. 4(b). The HT-9 steels tempered at temperature between 773 K and 823 K result in secondary hardening with the precipitation of M& carbide particles in the tempered martensite matrix and the rise in the DBTT caused by temper brittleness. This remarkably different temperature dependence of the shape parameter of HT-9 steel tempered at 773 K may reflect the change of microstructure and fracture mechanism in connection with the temper hardening and temper brittleness. Thus, it has been demonstrated in
TEMPERATURE
I
TEST
I
I
(K)
I
TEMPERATURE
I
I
I
(K)
Fig 11. Examples of temperature dependence of SP fracture energy estimates based on the data partitioning method for small specimens of HT-9 steel tempered for 2 h at 973 K: (a) all the measured data, (b) two data in each measured temperature. The middle solid lines show the estimated 15, in the separated two regions. A set of parallel thin lines indicates the upper- and lower-bound
fracture
energies.
T. Misawa et al. / Small punch tests for evaluating ductile - brittle transition
201
ture curve and the DBTT. The difference between the DBTT obtained by both method on the 773 K tempered HT-9 specimen is about 19 K, which is a little larger than that for the other specimens. The less fitness of the data partitioning method to that specimen should be due to the anomalous behavior of the Weibull shape parameter.
essentially heterogeneous microstructures, which might cause the large scatter of data on the fracture related parameters. In addition to this nature, the temperature dependence of the Weibull parameter shows the drastic change with the microstructure and fracture mode of materials, as demonstrated in this work. Bishop et al. [17] have applied a recently developed statistical analysis to the estimation of lower-bound Charpy V-notch impact energies when there are few data points and/or large scatter. Mar&an et al. [19] have improved the method of statistical analysis to be applicable for the full temperature range of data, that is the data partitioning method for dealing separately with upper-shelf and transition-region data. This data partitioning method has been applied to the determination of the DB’M and the estimation of the lower-bound SP fracture energy curve on the HT-9 and JFMS steels. In the first case, all the measured data are used for the statistical analysis. An example of the results is shown in fig. 11(a), which is for the small specimen of HT-9 steel tempered at 973 K. The lower-bound curve in the figure is defined as a 5% failure probability and the upper-bound curve corresponds to 95% failure probability, respectively. The DBTI is defined as the temperature corresponding to the middle energy between that at the cross-point of two &,-curves and at 50 K. The DBTT obtained by this definition is quite similar with that obtained by the standard method described in section 3.1. on every specimen used for the SP test, except that on the 773 K tempered HT-9 specimens. This means that the data partitioning method is reliable for the estimation of the fracture energy versus tempera-
Secondly, three data in each measured temperature are selected randomly for estimating the number of specimens to obtain reliable results by using the statistical analysis. The results of the analysis show similar behavior and DBTT on every specimen. As for the extreme case two data are used to the fitting procedure, the results of which also show the same behavior, as shown in fig. 11(b). Obtained results of the DBTT are presented in table 2. From these results, it is quite useful and practical to the irradiation experiments in the fusion program that two or three data measured at the test temperatures with appropriate interval are enough for the estimation of the DBIT.
5. Conclusions The SP tests using small and TEM disk specimens were performed for HT-9 and JFMS ferritic steels under consideration for evaluating ductile-brittle transition behavior of the irradiated fusion materials. The results obtained are summarized as follows: (1) The SP tests demonstrated ductile-brittle transition behavior and the DBTT was located at a lower temperature than that in the CVN tests.
Table 2 Estimates of the DBTT by small and TEM disk specimen SP tests for tempered HT-9 and JFMS steels Steel
HT-9
Tempering and aging
1043K/2h 973K/2h
Specimen size
Small Small TEM
873K/2h 773K/2h JFMS
1048K/lh 1048K/lh + 773K/lOOOh
Small Small
DBTT (K) Standard method (IL line) 93 118 118 116 116 126 166 166
Data partitioning method Iteration
1 2 1 2 1 2
All data
Three data
Two data
115 124 124 123 123 126 185 185
120 126 123 124 123 126 192 185
113 126 127 126 123 126 180 183
120
123
Small
123
120
Small
125
133
132
T. Misawa et al. / Small punch tests
202 (2) The temperature
dependence of SP fracture energies with scatter in miniaturized testing could give reliable information on the DBTT by use of the statistical analysis based on the Weibull distribution. An empirical correlation of the DBTT was obtained between the standard CVN and SP tests in the tempered steels. Weibull shape parame(3) The temperature-dependent ter and dispersion in SP energies was illustrated to give valuable information on the ductile-brittle fracture mode and properties in various tempered steels. (4) It has become possible to estimate the reliable DBTT and the lower-bound SP fracture energy curve using the statistical analysis of the data partitioning method, when there are few data points and/or scatter.
Acknowledgements The authors are grateful to Messrs. T. Yamada, Y. Toda and the members of the Cyclotron Group in Japan Steel Works Ltd. for performing experiments and Mr. R. Miura of JSW for his helpful cooperation. We would also like to thank Professor H. Takahashi of the Tohoku University and Dr. T. Kodaira of the Japan Atomic Energy Research Institute for valuable discussions. This research is supported partly by the Grant-inAid for Fusion Research by the Ministry of Education, Science
and
Culture
of Japan.
References [l] Proc. Topical Conf. on Ferritic Alloys for Use in Nuclear Energy Technologies, Snowbird, Utah, June 19-23, 1983, Eds. J.W. Davis and D.J. Michel (AIME, Pennsylvania, 1984). [2] T. Lechtenberg, J. Nucl. Mater. 133 & 134 (1985) 149.
for evaluating ductile-brittle
transitron
[3] G.E. Lucas and G.R. Odette, Nucl. Engrg. Des./Fusion 2 (1985) 145. [4] M. Dooley, G.E. Lucas and J.W. Sheckherd. J. Nucl. Mater. 103 & 104 (1981) 1583. [5] A. Okada, T. Yoshie, S. Kojima, K. Abe and M. Kiritani, J. Nucl. Mater. 133 & 134 (1985) 321. [6] A. Kohyama, K. Asakura, Y. Kohno, K. Komamura. K. Suzuki, M. Kiritani, T. Fujita and N. Igata, J. Nucl. Mater. 133 & 134 (1985) 628. [7] M.P. Manahan, AS. Argon and O.K. Harling. J. Nucl. Mater. 103 & 104 (1981) 1545. [8] F.H. Huang, M.L. Hamilton and G.L. Wire, Nucl. Technol. 57 (1982) 234. [9] G.E. Lucas, J.W. Sheckherd, G.R. Odette and S. Panchanadeeswaran, J. Nucl. Mater. 122 & 123 (1984) 429. PO1 J.M. Baik, J. Kameda and 0. Buck, Scripta Met. 17 (1983) 1443. 1111 J.M. Baik, J. Kameda and 0. Buck, The Use of Small-Scale Specimens for Testing Irradiated Materials, ASTM STP 888, Eds. W.R. Corwin and G.E. Lucas (ASTM. Philadelphia, 1986) p. 92. U-A J. Kameda, Acta Met. 34 (1986) 2391. J. u31 T. Misawa, H. Sugawara, R. Miura and Y. Hamaguchi. Nucl. Mater. 133 & 134 (1985) 313. H. Kuwano, H. Kamide, R. Miura and T. u41 Y. Hamaguchi. Yamada, J. Nucl. Mater. 133 & 134 (1985) 636. The mechanism of fracu51 T. Misawa and Y. Hamaguchi, ture, in: Proc. Int. Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, Salt Lake City, Utah, Dec. 2-6, 1985. Ed. V.S. Goel (ASM, Ohio. 1986) p. 521. H. Kuwano, T. Misawa, R. Miura and T. U61 Y. Hamaguchi, Yamada, J. Nucl. Mater. 141 & 143 (1986) 781. and A.R. Rosenfield, Met. [771T.A. Bishop, A.J. Markworth Trans. 14A (1983) 687. and V. Provenzano, Ef1181 F.A. Smidt, Jr., J.R. Hawthorne fects of Radiation on Materials, ASTM STP 725. Eds. D. Kramer, H.R. Brager and J.S. Perrin (ASTM, Philadelphia, 1981) p. 269. S. Quayle, A.R. Rosenfield and D.K. P91 M.P. Manahan, Shetty, The mechanism of fracture, in: Proc. Int. Conf. on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Analysis, Salt Lake City, Utah, Dec. 2-6. 1985, Ed. V.S. Goel (ASM, Ohio. 1986) p. 495.