Journal of Nuclear Materials 155-157 (1988) 953-956 North-Holland, Amsterdam

953

INFLUENCE OF NEUTRON IRRADIATION AT 430°C ON THE FATIGUE PROPERTIES OF SA 316L STEEL

W. VANDERMEULEN, W. HENDRIX, V. MASSAUT and J. Van de VELDE

SCK/CEN, Boeretang 200, 2400 Mel, Belgium

Fatigue tests have been carried out at 430 o C on hour-glass shaped specimens of the CEC reference heat of SA 316L

stainless steel. The tests were performed under constant total axial strain control with a triangular fully reversed wave shape at

frequencies of 0.5, 0.05 and 0.005 Hz. Specimens irradiated at 430 o C to doses of 9-12 dpa and helium contents of 80 to 145

appm showed a fatigue life reduction by about a factor of two, compared to unirradiated specimens. The cyclic stress is found

to be strongly increased by the irradiation. The test frequency influences the fatigue hardening slightly but it does not affect

the fatigue life.

1. Introduction 2.2. Specimen shape

Fatigue will be one of the main material problems

for the next generation of fusion reactors. As a conse- quence, an important part of the European Fusion

Technology programme is devoted to the study of the influence of helium and of neutron irradiation-induced

defects on the fatigue properties of structural materials. In the framework of this programme fatigue specimens

of the European reference material, type AISI 316L stainless steel, have been irradiated in the BR2 reactor

at 430” C up to 12 dpa and a helium content of 145 appm. In the following the results of the first post- irradiation tests, carried out at 430°C, will be pre-

sented.

2. Experimental details

2.1. Material

The material consisted of the CEC AISI 316L refer- ence heat with chemical composition as shown in table

1. This material was received as a 30 mm-thick plate. Because it was observed that the properties were not uniform throughout the thickness an attempt was made

to homogenize the material by swaging. Cylinders were machined from the plate in the rolling direction, hot swaged from 30 to 11 mm diameter and further cold swaged to rods of 8.5 mm. Finally an annealing treat-

ment of 15 min at 105O’C was applied.

The specimen used was identical to the one de- scribed by Grossbeck and Liu [l], i.e. an hour-glass

shape with 3.2 mm diameter and a radius of 12.5 mm.

After machining, the gauge section was ground and polished mechanically in the longitudinal direction.

2.3. Testing conditions and procedure

The fatigue testing conditions for irradiated and

unirradiated specimens were identical. All tests were performed in air at 430” C. The strain was measured with a diametral extensometer. An analogue strain com- puter was used to convert diametral to axial strain. A triangular wave shape with constant total axial strain-range was used. In order to check the precision of the strain measurement the Young’s modulus was de- termined on each specimen by loading it in the elastic domain. The values obtained were usually within & 15%

of these found in the literature. The strain range for each test has been corrected, taking into account the

deviation found on the Young’s modulus. Most tests were performed at a frequency of 0.5 Hz.

In order to examine the existence of a possible strain-rate

effect some tests were run at 0.05 and 0.005 Hz. The use of fixed frequencies, rather than fixed strain-rates, is justified by the fact that the different regions of a reactor first wall will deform at the same frequency and

not at the same strain-rate.

Table 1

Chemical analysis of the CEC Reference Heat of AISI 316L (content in wt%, duplicate analyses from supplier)

C Ni Cr Mn cu MO Si

0.026 12.23 17.41 1.81 0.20 2.3 0.45 0.022 12.34 17.62 1.84 0.20 2.3 0.46

co S P Ta N2 B a)

0.17 0.001 0.026 0.01 0.06 7

0.17 0.001 0.027 0.01 0.06 8

a) ppm.

0022-3115/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

954 I+’ Vandermeulen er al. / Influence of neutron irradiation on fatigue properties

Table 2 Irradiation data for BR2 irradiation of type 316L fatigue

specimens

Position Dose He-content Mean Maximum

WRT) fappm) temperature ‘) temperature

(dpa) (“C) (“Cl

Top 9.1 82 410 442

Centre 11.9 145 422 441

Bottom 9.1 105 407 433

‘) For reactor power greater than 80% of maximum.

2.4. Irradiation conditions

The irradiation was carried out in the BR2 reactor in the Fafuma I rig [2]. The rig atmosphere consisted of a He-Ne gas mixture. The temperature was measured by

means of thermocouples located inside specimens at the top, centre and bottom position of the rig. The helium

concentration was determined by helium release mea- surements. Details on dose, irradiation temperature and

helium content are given in table 2.

3. Results

3.1. Unirradiated material

Fig. 1 shows the strain-range versus the number of cycles to rupture for the unirradiated material. The tests were performed at frequencies of 0.5, 0.05 and 0.005 Hz. For strain-ranges of 0.4 to 2.0% this corresponds to

strain-rates of 0.2 to 1.0 X 10-2s-‘, 0.2-1.0 X 1O-3 SC’ and 0.2-1.0 X lop4 s-i, respectively. It can be seen that the points form a single scatterband and that from the present data no influence of test frequency on the

fatigue life can be detected. Fig. 2 shows the stress amplitude as a function of the number of cycles for some typical cases. The general behaviour of the material is to harden until rupture. Between about lo2 and some

lo3 cycles however, a slight softening may occur. This tendency to soften becomes more pronounced with in- creasing strain-range and frequency.

3.2. Irradiated material

The irradiated specimens have been tested at fre- quencies of 0.5 Hz for strain-ranges between 0.34 and 1.10% and at frequencies of 0.005 Hz for strain-ranges between 0.50 and 1.12%. No effect of the test frequency could be found within the domain examined. At strain-ranges above 0.5% dose variations from 9 to 12 dpa and helium concentration variations from 85 to 145 appm have no effect on the fatigue life. For strain-ranges below 0.5% however the higher dose, higher helium content specimens are found on the lower side of the

2.0

1.0

0.8

x 0.7

- % 0.6

E c 0.5 .$

5 0.4

0.3

0.2

t-

X 0.5 Hz _-I 0 0.05 Hz

0 0.005 Hz --

10’ [y/es to rupture

Fig. 1. Fatigue test results on unirradiated CEC ref. SA AISI

316L at 430°C showing the absence of a frequency effect on

fatigue life.

scatterband. In fig. 3 results on unirradiated and irradi- ated specimens are compared. For clarity no distinction according to frequency or dose has been made. It can be seen that the scatterband for the irradiated material lies somewhat below the band for the unirradiated speci-

mens. The mean effect is a reduction by about a factor of two on the number of cycles to rupture or a decrease of the strain-range with 0.1 S. Fig. 4 shows some typical fatigue hardening curves for the irradiated material. It can be seen that after irradiation the flow stress of the material is about twice as high as for unirradiated material. (Notice the different stress scale ranges of figs.

2 and 4.) The general trend of the curves is for the stress to rise during the first lo2 to lo3 cycles and then to decrease until rupture occurs. This trend is very weak for small strain-ranges but becomes more pronounced with increasing strain-range. Specimens having the lowest doses present a different behaviour. They show more hardening during the first lo2 to lo3 cycles as compared with the high dose specimens (curves 1, 2 and 5 in fig. 4). Any effect of frequency is small. There appears to be a tendency for the low frequency tests to show less softening in the range above lo3 cycles (Ex- trapolation of curves 1, 4 and 6 in fig. 4).

W. Vandermeulen et al. / Influence of neutron irradiation onjatigueproperties

4. Discussion

As the present paper is an interim report of continu-

ing work the discussion will be limited to the most

obvious observations.

4.1. Effect of irradiation

The most dramatic effect observed is the high cyclic stress needed to deform the irradiated specimens as

compared to the unirradiated ones. A similar flow stress increase has also been observed for tension tests after

irradiation at 475O C [3]. Extensive similar work on

stainless steels confirm that the effect is due to radiation

induced defect clusters and not to ageing. The strong

0 non-irradiated

t-

t-

t-

_L

Cycles to rupture

Fig. 3. Fatigue test results at 430°C on unirradiated and

irradiated AISI 316L, showing the small effect of irradiation

on the fatigue life. No distinction has been made according to

dose or frequency.

effect of irradiation on the cyclic stress contrasts with its small influence on the number of cycles to rupture.

This is quite surprising since crack initiation as well as crack growth can be expected to change due to irradia- tion. Crack initiation should be retarded up to total strain-ranges of 0.6% because the deformation becomes almost entirely elastic due to irradiation hardening. A slight change of the slope of the strain-range versus number of cycles to failure curve at 0.6% may be due to

this effect (fig. 3). More tests will be done to confirm this. The crack growth rate on the other hand should

increase due to the higher stress intensity factor at a given crack length unless this effect is compensated by the higher flow stress of the material. As the present tests do not allow to distinguish the crack initiation and

crack growth phases it is not possible to draw conclu- sions on the effect of irradiation on their extent. How- ever, since the total effect on the fatigue life is small and practically independent of the strain-range the most likely conclusion is that crack initiation and/or growth are only slightly affected. A pronounced irradiation effect on both stages would require the somewhat un- likely coincidence that they would be affected in almost equal but opposite ways. Very similar results on Type AISI 316 stainless steel have been reported by Gross- beck and Liu [l].

Curve

1 2 3 4 5 6 7 8 9 10

Strain range Freq 1%) I Hz I

0.20 0.5 0.30 0.5 0.39 0.5 0.50 0.5 0.62 0.5 0.60 0.05 0.65 0.005 0.98 0.5 100 0.05 117 0.005

I 1

10-f 100 10' 102 Number of cy

Fig. 2. Cyclic hardening of unirradiated SA AISI 316L at

430 o C. Notice the small effect of frequency above 10 * cycles.

956 W. Vandermeulen et al. / Influence of neutron irradiation on fatigue properties

4.2. Frequency effects

It has been found that the frequency (or strain-rate) has little effect on the number of cycles to rupture. This

is in contrast to the results of Van der Schaaf et al. [4] who found a lifetime decrease by a factor of two for a

strain-rate decrease of two orders of magnitude. No explanation has been found for this discrepancy but it

should be stressed that the effects observed are small

751

701

651

601

p 551

Q g so1

P

S 451 + In

401

351

301

1 10'

1 /’

z 3

/

Y /

5

2 7

8 -

t

--!-- r ilrve

i

1 1

Strain range 1%)

100 100 1.10 1. 12 0.58 0.63 0.58 0.49 0.34 ,

I

Freq. I Hz I

0.005 0.5 0.5 0.005 0.5 0.005 0.5 0.5 0.5 I

Dose I dpa I 9.7 9.1 11.7 11.7 9.1 11.6 11.3 10.4 11.6

-1 -’ 100 10’ ld 103 10’ 10’

Number of cycles

from a practical point of view. A small frequency effect on the cycle hardening is clearly shown in fig. 2. As this effect is much smaller than the change brought about by irradiation, it is not surprising that it is not affecting the fatigue life.

4.3. Dose effects

The few low dose specimens show higher cyclic

hardening during the first 10 cycles. It is hard to explain

this on the basis of the lower dose and helium content although it might be that the high dose specimens present a more “over-aged”-like defect structure. An alternative explanation could involve the slightly lower

irradiation temperature of these specimens which were loaded in the upper and lower regions of the rig (table

2).

5. Conclusions

Although testing of additional specimens is continu- ing, it appears that the following observations are not likely to be changed by further results.

Irradiation at 430°C to 9-12 dpa and helium con- tents of 85-145 appm has a limited effect on the

fatigue life of solution annealed AISI 316L. The cyclic hardening is considerably increased. Test frequency has no effect on the fatigue life in the range of 0.5 to 0.005 Hz, irrespective of whether the material is irradiated or not. Cyclic hardening is

slightly dependent on the test frequency. The fatigue life does not depend on dose or helium content within the range 9-12 dpa and 85-145 appm. A dose effect on cyclic hardening may exist but has

not been demonstrated unambiguously.

This work is performed under CEC contract Eura- tom-Belgian State 100-82-l FUA B.

References

Fig. 4. Cyclic hardening of irradiated AISI 316L showing the

effect of dose in the first 10 cycles and a very small frequency

effect above 10’ cycles. Tests conducted at 430 o C.

111

PI

[31

[41

M.L. Grossbeck and K.C. Liu, Nucl. Technol. 58 (1982)

538.

F. Moons et al., in: Proc. Twelfth Symposium on Fusion

Technology, Jtilich, 1982, Eds. CEC (Pergamon Press, Ox-

ford, 1982) p. 803.

W. Vandermeulen et al., in: Proc. Fourteenth Symposium

on Fusion Technology, Avignon, 1986, Eds. CEC (Per-

gamon Press, Oxford, 1986) p. 1025.

B. van der Schaaf, ibid. ref. [3], p. 993.