Small specimen test techniques for the evaluation of toughness degradation

Journal of Nuclear Materials 191-194 (1992) 1023-1027 North-Holland

Small specimen test techniques for the evaluation of toughness degradation M. Suzuki 3, M. Eto d, y . Nishiyama a, K. Fukaya 3, M. Saito c and T. Misawa Japan Atom:c Energy Research lnstttute. Tokm-mura, lbarakl-ken 319-11, Japan t, Muroran lnstuute of Technology, Mizumoto-cho. Muroran-sh: 050, Japan ' Mlyagt Prefectural Instuute of Technology, Tmhaku-ku, Sendm-sht 982. Japan a

The small punch (SP) test was apphed to neutron lrradmted TEM disks to obtain fracture related mechanical properties (DBTT, Jic) of low acttvatmn ferntlc steels of 8Cr-2WV and 8Cr-2WVTa and conventional 12Cr-IMoVW steel. The SP test using 3 mm dmmeter TEM disks is confirmed to be useful, especmlly for JIc evaluatmn DBTT determmatmn is also thought to be possible with a sufficient number of specimens

1. Introduction Small specimen techniques are necessary in fusion reactor materials development. Although several kinds of methods using miniaturized specimens have been proposed, certain kinds of mechanical properties, such as fracture toughness and ductile-to-brittle transmon behavior in ferrmc steels, are very difficult to measure with small a n d / o r hmlted numbers of specime is. As to the number or s~ze of speomens, the avadabfllty of space for the irradiation tests ~s not definitely determined yet, because it depends on the type of facditles selected for irradmtion tests to simulate a fusion environment Th~s can vary from an intense neutron source to an ion accelerator To cope with various situations, the authors have tentatively set a specxmen volume to transmission electron m~croscopy (TEM) disk size. A bulge or a mlcrohardness test has potentml for the purposes mentioned above. In a bulge (small punch, SP) test, DBTI" can be estimated by repeated testing and measuring of the energy consumed up to the onset of macrocracking as a functmn of test temperature [1,2]. The biaxlal stress state in this test also makes fracture toughness est~matmn possible [3] In the hardness test, recording and analyzing the load versus indentation depth curve during hardness testing can make the rehabdity of extracted mechanical data higher even with applied loads down to several gf It was also found that not only strength-related properties but also ductility could be estimated [4] However, further study is reqmred for obtaining fracture-related properhes In this paper, the state of the present art for extracting fiacture-related mechanical properties from

TEM disks by the SP test and especiafly results on neutron irradmted low activatmn ferntic steels are described and discussed.

2. Experimental The materials used in the present experiment were three kinds of femtlc steels: two low actwation ferntic steels, 8 Cr - 2 W V (F-82) and 8Cr-2WVTa (F-82H), and the conventional 12Cr-IMoWV (HT-9) steel. Chemical compesitions and heat treatment conditions are summarized m table I. The microstructures of these steels were composed of tempered martensite and the HT-9 also contained about 1% ~-ferrite. Transm~ssion electron microscopy (TEM) disks with diameter of 3 mm and thickness of 0.27 mm were prepared from steel rods and were electropohshed to adjust the final thickness to 0 25 + 0_13 ram. Neutron irradlaUon was performed in the Japan Research Reactor-2 (JRR-2) to a neutron fluence of 1 2 × 1024 n / m 2 at 600 K. The SP tests on irradiated specimens were conducted m the Tokai l-!,~t Laboratory using an Instron-1361 creep-fatigue testing machine in the temperature range 113 to 300 K. A hardened gteel ball of 1 mm diameter was used for punchmg the disk specimen Crosshead speed during the tests was 83 × 10 -3 m m / s . Other conventional tests for comparison w~th the SP test results adopted standard or common specimens The tensile specimen ~s a 4 mm dmmeter round type with a gauge length of 22 mm J~c was determined using a disk-shaped compact (DCT) specimen w~th a thickness of 10 mm. The general test procedure basically followed ASTM E813-81

0022-3115/92/$05.00 © 1992 - Elsevier Science Pubhshers B V All rights reserved

M Suzukt et al / Small spectmen test techmques

1024

3. Results and discussion 3.1 Irradtatton charactensncs o f the low activauon ferrittc steels Results on tensile tests before and after irradiation are shown in fig. 1. Radmuon-mduced changes in the low activation steels are less than that of HT-9, and F-82H m particular showed fairly good performance under trradlation. Elastic-plastic fracture toughness (Jic) evaluated by the DCT specimens also revealed excellent properties of F-82 and F-82H steels as shown in table 2. All tests were done at room temperature. J,c of F-82 and F-82H even increased a little after irradiation. On the other hand, Jtc of HT-9 after irradiation could not be evaluated because elastic fracture occurred. This fact shows that the ductile to brittle transmon temperature of this steel was increased significantly by ~rradlatlon. Unfortunately, we have no Charpy data after lrradmtton at the present.

phfied determination can be made from the load-deflection curve with eq. (2). However, whether the constants/3 and n remain constant after irradiation is not clear. Some indication in this respect is shown in fig. 2, where Jic measured by the DCT test versus ~qf determined using eqs. (1) or (2) in the present experiment Is shown. The value of ~:qf after irradiation was an average for two to three test runs. Most of the data on irradiated specimens satisfy the relationship of Takahashi et al. between Jtc and ~:qf. However, as is seen in the case of HT-9, in which a certain degradation was expected by the irradiation, ~-qf calculated by eq. (2) showed an even higher value, whale that calculated by eq. (1) became lower. (Change caused by irradiation is indicated by arrows in the figure. Unfortunately, J=c could not be determined for HT-9 as stated m the previous section.) The reason for this is not clear at the moment, but might be related to a change m the constants # and n caused by lrradmtlon. Thickness measurements after each test are considered necessary for reliable evaluation.

3.2. SP test ~esults 3.2.1. Fracture toughness The basic idea to extract J=c values from specimens as small as TEM disks lies in the fact tt=e Jic should have a clear relationship with the biaxial equivalent fracture strain, gqr- Map et al. [5,6] and Misawa et al. [7] assumed the following equations to calculate ~qr. (1)

~qf = I n ( t 0 / t * )

=/3(8"/to)",

(2)

where t o is the initial thmkaess, t* Is the mlmmum thickness at the fracture poruon, fl is ~ 0.09, n ~ 2, and ~* is the deflection at which a sudden load drop

3.2.2. Temperature dependence o f SP energy and D B T T evaluatton SP energy was defined as the energy consumed up to the onset of macroerackmg, which corresponds to the area under the load versus deflection curve up to a deflection 8*. The maximum load gradually increases with decreasing test temperature until a sudden decrease occurs. Hence, as the temperature decreases, SP energy falls drastically at a certain temperature. This transit,on temperature (SPDBT'D has an approximately hnear relation with the DBTI" measured by Charpy test (CVN-DB'I~I'), that is

OCCUrs.

The calculations of ~-qf using eq. (1) needs measurement of the specimen thickness after fracture. A sire-

lal

u) m

Ibl F-82H

6OO

v ~

F- 82

I i-82 [ I-7

J F- e2H

/ HT- 9 /

=o 50C d

•

(3)

SPDBTT = a X CVN-DBTT

.

101LILLI_LU

Neutron fluence 1 2xlO'4n/m 2 Irrad temperatu;e (E>IMeV)6OOK

Test temgerature

/~F~o/4~a°/

/ J /

RT

F~g ] Tensdc propemes of the alloys before and after =rradlatmn.

M Suzuki et al

/ Small specimen

test techmques

1025

Table 2 Elastic-plastic fracture toughness (JIc) of the alloys before and after irradiation determined using 0.4DCT (disc-shaped compact tension) specimens

JIc [kJ/m2]

i:.!i0 oo o~

F-82

F-82H

HT-9

initial lrrad

initial irrad,

initial ffrad.

387

371

214

465

500

i

F-82

i

|

|

F-S2H n

D

s.

\

E

I

|

|

AR I r r a d o •

Elastic

i/| ],.~ ~WL

•

/

HT-9 Q . AR:A ..... ivea Irrad 600K Ix1024n/m 2

400

I.

402

dllA~

~ ~ d ~ ~ /

/ /~

(E>IMeV}

300

v

>

0

2OO

100

0

1.0

2.0

Fig 2. Relationship between elast,c-plastlc fracture toughness (Jlc) determined by DCT test and equivalent fracture strain (gqf) determined by SP test for F-82, F-82H and HT-9 before and after Irradiation at 600K to 1 2× 1024 n / m 2. (The number shows that ~'qr is calculated from eqs (!) or (2))

p~ I ~ cq

"O u~ m

F-82

O4

600K. I 2x1024nlm2[E>IMeV

c e-

~

Lr~

v

SP-DBTT(Irrad ) J I 3 K SP-DBTT(Unsrrad ) 94K

0.3

~PTDBTT Shsft

~ i~,' ~

~[

.e I,.~ ,~ ~

[gK

P ==o2

"O

== 11.

0 O

__1 100

I

I 200

I

I 300

Temverature

I

I, 400

I

I

(K)

Fig 3 SP energy as a function of test temperature for F-82 after irradiation The middle lines show the estimated En curve using umrradlated Welbull parameter (m) The upper and lower lines show the upper (95% failure probability) and lower (5% failure probability) bounds of SP energies

1026

M Suzuki et al / Small specimen test techmques

06

F-82H

~100

600K I 2 x 1 0 2 4 u / m 2 ( E > l M e V )

,..,O5

SP D B T T ( I r r a d ) SP D B T T ( L ~ l r r a d

"04

'

I

'

103K ) 107K

SP DBTT S h i f t

4K

so

~03

,-82 .

TIAOo: " = 5"C/10MPa"

t--

n

02

/

01

0 [

100

I

I

[

[

I

I

t

2OO 3OO 40O Temperature ( K )

50O

0

Fig 4 SP energy as a function of test temperature for F-82H after ~rradiatlon. The middle hnes show the esUmated E o curve using umrradmted Wetbull parameter (m) The upper and lower hnes show the upper (95% fadure probability) and lower (5% fadure probability) bounds of SP energies where a is the correlation coefficient ranging from ~ 0.35 to ~ 0.45 [8-11] A certain number of test runs are required for an accurate determination of D B T r . T o minimize the test runs, M~sawa et ai. proposed a test procedure to determine the D B T T with the aid of statistical analysis [12]. They adopted the We,bull distribution to express the probabihty distribution of SP energy at a certain test temperature. The following cumulative distributmn function was used to determine the distribution parameters, F(X)

(4)

= l - exp[ - ( x/E,1)"],

where m (shape parameter) and En (scale parameter) are distribution parameters. The detailed procedure is described m the literature [12]. In the present experiment, only 5 to 6 specimens were testcd at tempera-

HT -___S9 500K, I 2 x 1 0 2 4 n l m 2 [ E > I M e V )

SP-DBTT(Irrad )

~03

SP-DBT'r(UnIr-ad

v

SP-DBTT S h i f t

)

[54K 154K OK

~ 02 q) ¢ffl O.t

I

IO0

200

I

I

300

t

I

400

I

o

I

F-82H

( O )

-

, I , 50 100 150 200 I_rcreasa__= in 0.2~ off set stress ( M P a )

Fig 6. Charoy DBTT shifts estimated from SP test results are

plotted as a function of the change in 0.2% off set stress for F-82, F-82H and HT-9

tures from 113 to 300 K for each material. Test results are shown in fig. 3 to 5 The D B T T was defined as the temperature corresponding to the middle energy between that at the cross-point of two SP energy curves and at 50 K. S P D B ' I T s were determined using the same distribution parameters ( m ) obtained for the unirradlated specimens. F r o m the results on unirradiated specimens [12], D B T T s of F-82 and F-82H could be evaluated with six specimens with reasonable accuracy (_+ 5 K) On the o t h e r hand, the DB'VI" of HT-9 was hard to determine with six specimens, because large scatter was observed, possibly due to its heterogeneous dual-phase structure. The C V N - D B T r shift predicted by SP tests were plotted aga,nst the increase in 0 2% off set stress m fig 6. C V N - D B T r shift was estimated by assuming a = 0.4 ,la eq (3). The line which represents AT/oe02 ~ 5 K / 1 0 MPa is shown for comparison. (This is often observed for pressure vessel steels [13].) Although pronounced irradtation-mduced hardening ,s observed m HT-9, large inherent scatter peculiar to this steel also seemed to make the estimation of DBT'I" difficult after trradtatmn. The results on F-82 and F-82H are very reasonable. While the radiatmn-mduced change of D B ' [ ' r can possibly be measured by the SP test method, the number of specimens cannot be reduced for an accurate determination That Is, judging from the study by Misawa et al. [12] on umrradiated material, as many as 20 specimens would be desirable for a reliable determination of DBTT.

[

500

Tem0erature ( K ) Fig 5. SP energy as a function of test temperature for HT-9 after irradiation The m,ddle hnes show the estimated E o curve using umrradtated Welbull parameter (m) The upper and lower hnes show the upper (95% fadure probability) and lower (5% failure probab,hty) hounds of SP energies.

4. Conclusions

It has been confirmed that the SP test is an effecUve method to acqmre fracture related properties (DBTT, J i c ) on ,rradmted specimens using 3 m m d,ameter T E M disks. J , c versus gqf had the same relatranship after irradmtmn Though only five to six spec,-

M. Suzukt et al. / Small specrmen test techmques

mens could be tested for D B T r tests in the present experiment, the results indicate that the determination of DBT]" should be possible with a sufficient number of specimens

Acknowledgements The authors would hke to thank Messers A. Umino, T. Koya and T. Tobita for their help m the experimental work. Professor H. Takahashl of Tohoku University and Dr. S. Jitsukawa of J A E R I are also acknowledged for their valuable advice.

References [1] J M Balk, J. Kamcda and O Buck, Scr, pta Metall 19 (1983) 1443.

1027

[2] J.M Baik, J. Kameda and O. Buck, ASTM-STP 888 (1986) 92 [3] H. Takahashl, M A Khan and M. Suzuki, J. Test and Eval 8 (1980) 63. [4] M Eto et al. to be published. [5] X. Mao, T. Shoji and H. Takahashi, J. Test. and Eval. 15 (1987) 30. [6] X. Mao and H. Takahashi, J. Nucl. Mater. 150 (1987) 42. [7] T. Mtsawa, S Nagata, N. Aokl, J. lshizaka and Y. Hamaguchi, J Nucl. Mater. 169 (1989) 225. [8] T Misawa~ T. Adachl, M. Saito and Y Hamaguchl, J. Nucl Mater. 150 (1987) 194. [9] J Kameda, Acta Metall. 344 (1986) 2391. [10] T Matsush,ta. M.L Saucedo, M. Yotsutsujl, T. Shoji and H Takahashl, Trans. Japan So¢ Mecb Engineers. (in Japanese) 55 (1989) 1619. [11] M. Suzuki, M Eto, K. Fukaya, Y. Nishtyama, T. Kodaira. T. Oku, M Adacht, A. Umino, I. Takahashl, T. Misawa and Y Hamaguchi, J Nucl. Mater. 179-181 (1991) 441. [12] T. Misawa, IC Suzuki, M. Satto and Y. Hamaguchl, J. Nucl Mater 179-181 (1991)421 [13] G R Odette. P Lombrozo and R A Wullaert, ASTMSTP 870 (1980) 840.