Environmental Effect on Creep Fatigue Life of Type 316FR Stainless Steel in Liquid Sodium at Elevated Temperature

Environmental Effect on Creep Fatigue Life of Type 316FR Stainless Steel in

Liquid Sodium at Elevated Temperature

Hiroshi Ishikawa 1), Hitoshi Kaguchi 2), Tomomi Otani 2), Shingo Date 1), Yukio Takahashi 3), Takanori Nakazawa 4)

1) Material and strength section, Mitsubishi Heavy Industries, Ltd., Takasago R&D Center Japan

2) A d v a n c e d N u c l e a r P l a n t E n g i n e e r i n g Section, M i t s u b i s h i H e a v y I n d u s t r i e s , Ltd. K o b e J a p a n 3) Material Science Department, Central Research Institute of Electric Power Industry Japan

4) Mechanical Engineering Department, Gunma University Japan

ABSTRACT

Advanced type stainless steel Type 316FR will be used for the next generation fast breeder reactors in Japan. This steel contains less carbon and more nitrogen than conventional stainless steels such as type 304 and 316. The material strength standard for the 316FR has already been established for design. However, the standard is based on the results of material tests in air, while actual structural material may be used in sodium environment in FBR plants. Therefore, the effect of sodium environlnent should be clarified to confmn the reliability of the design standard especially for fatigue and creep damage. Cyclically bending tests were carried out with and without hold time in sodium as well as in air. The tested materials are 316FR and conventional 304 and 316 stainless steels. Weld metal of 316FR was also tested. The sodium tests were carried out in closed sodium loop, which can control oxygen level to simulate actual plant condition.

INTRODUCTION

In Japan, various efforts are under way to develop fast breeder reactor plants which are reliable and economically acceptable. Although other options are being considered, use of liquid sodimn as a coolant is still a primary option. One of the important advances of the future FBR from the prototype reactor, Monju, is the change of main structural materials. Low-carbon type 316 stainless steel with controlled nitrogen called 316FR (Fast Reactor) is planned to be used in the future plants, while type 304 stainless steel is used in Monju, because of better creep and creep-fatigue strength of the former.

Tests for obtaining fundamental material strength data necessary for high-temperature structural design are being conducted by several organizations in Japan. Among them, the authors have conducted long-term creep- fatigue tests and developed an accurate creep-fatigue life assessment method based on ductility exhaustion concept [ 1,2]. However, all of these tests have been conducted in the air. Therefore, tests in sodium or inert gas environment are required to assess the possible environmental effects and to confirm the applicability of the life prediction method and the material strength data.

Based on this consideration, the authors conducted fatigue and creep-fatigue tests for 316FR in liquid sodium and in the air.

TEST MATERIALS

In this study 316FR was used as test material. Chemical compositions of this material are shown in Table 1, in compared with those of 316FR weld metal, type 304 and type 316 stainless steel. Details of fundamental mechanical properties of these materials were given in the literature [ 1,3]. As mentioned above, creep and creep-fatigue strength of 316FR are better than those of conventional 304 and 316 stainless steels, because lower carbon prevent precipitation of carbide along grain boundaries.

The materials used in the tests was produced by hot-rolling process and its thickness was 50 mm. Solution heat treatment was given at 1050°C for 30 min followed by water quenching. Additional heat treatment (1250°C,16hr) was applied to it prior to rolling process for homogenize chromium distribution for 316FR.

SMiRT 16, Washington DC, August 2001 Paper # 1694

TEST M E T H O D S

Material test facility for fatigue and creep-fatigue testing in liquid sodium was newly constructed for this study. The facility consists of fatigue test machine and a sodium loop. The schematic diagram of the sodium loop is shown in Fig. 1. The sodium loop consists of a main loop and a subloop. Main loop controls the sodium temperature at the test section, while the subloop controls and measures the oxygen content by a cold trap system and a plugging meter. The density of oxygen was kept below 1 ppm by keeping the temperature of the cold trap at 120°C. Test temperature was maintained at 550°C__+2°C in the test vessel. The fatigue test machine is shown in Fig.2. It is a plate bending fatigue test machine of cantilever beam type and gave cyclic displacement to test pieces. Cyclic bending displacement was given to test pieces by crank arms which were connected to rotating shaft. Rotation was transferred from the driving motor through an eccentric disc and gears. The specimen geometry is shown in Fig.3. The test machine can hold 10 specimens at the same time. Each specimen was loaded with individual strain range which was adjusted by space between the crank arms. Strain rate was adjusted at approximately 0. l%/sec. Based on the FEM analysis results, the relationship between the displacement and the strain of test piece has been calibrated. It was also confnaned that the strain prediction was accurate compared with strain gauge output in a test specimen in room temperature test. Test condition in fatigue test is shown in Table2-1. The test was conducted in liquid sodium and in air by the same type of test machines. The test condition in creep-fatigue test is shown Table2-2. In creep- fatigue tests, the displacement was held at its maximum for 1 hr. Strain range was changed in the range from 0.4% to 1.0%.

TEST RESULTS Fatigue test

The results of fatigue tests in liquid sodium and in air are shown in Fig.4 with an average fatigue curve constructed by using the results of conventional fatigue tests in air using round bar specimens of type 304, 316 and 321 stainless steel [4]. Test failure lives in air agree well with the average curve at high strain ranges but longer at small strain ranges. This is the same trend as obtained by conventional fatigue tests for 316FR and this indicates failure lives obtained by the present tests are comparable with those by the conventional tests.

It can be also seen from Fig.4 that fatigue lives obtained by in liquid sodiuna tests were longer than those obtained in the air. The fatigue strength of 316FR was similar to that of 304 or 316 stainless steel in liquid sodium, as well as in air. The weld metal specimens showed somewhat shorter lives than the base metal.

Creep-Fatigue test

The results of creep-fatigue test in liquid sodium and in air are shown in Fig.5. Although it cannot be discussed in detail because some tests are still under operation, Fig.5 indicates the tendency that creep-fatigue lives in liquid sodium were similar or longer than those obtained in the air for each material.

Summary of environmental effect in sodium

For comparison of fatigue and creep-fatigue lives of 316FR in sodium with those in the air, Fig.6 shows ratio of fatigue and creep-fatigue life in sodium to those in air. The fatigue lives in the sodium were always longer then those in the air, with a factor between 2 to 11. The fact that fatigue life in liquid sodium is longer than that in air is similar to the data of type 304 and type 316 [5,6,7,8]. In the previous study [5,6,7,8], the environment effect was reported to be smaller as the strain range decreases. However the results of the present study exhibited opposite trend and does not support this. Fatigue lives in sodium are similar to those obtained in the tests conducted in vacuum[8]. Therefore it is considered that the oxidation plays an important role for the difference of fatigue life in liquid sodium and in air.

Fracture surface & M i c r o s t r u c m r e

Typical fracture surface and microstructure of the fatigue specimen tested in liquid sodium are shown in Fig.7. They show transgranular crack propagation, similar to those tested in air. And striations were observed in crack propagation area. The number of the sub-crack on specimen tested in liquid sodium is fewer than those tested in air. Typical fracture surface and microstructure of the creep-fatigue specimen tested in liquid sodium are shown in Fig.8. They show the intergranular crack propagation at tension hold side of the specimen, similar to those tested in air.

CONCLUSION

From the test results, the following conclusions were obtained,

(1) Fatigue life of 316FR in sodium environment is three to seven times longer than that in air environment at elevated temperature. The fracture surface of fatigue tests in sodium shows transgranular crack propagation in both cases.

(2) Creep fatigue life of 316FR gains smaller effect of enviromnent than fatigue life. The creep fatigue life in sodium is equal to or slightly longer than that in air. The fracture surface shows intergranular crack propagation. (3) These results show that the design evaluation for fatigue and creep fatigue using material strength data in air will

be conservative for 316FR stainless steel. Furthermore, fatigue curve could be rationalized for the components used in sodium environment.

A C K N O W L E D G E M E N T

This work is sponsored by the Ministry of Economy, Trade and Industry in Japan. The authors would like to express their sincere gratitude for the members of the steering committee of this program (chairman: Professor G.Yagawa ,University of Tokyo)

R E F E R E N C E S

[ 1] Kurome, K., Date, S., Sukekawa, M., Takakura, K., Kawasaki, N., Tanaka, Y., "Material Strength Standard of 316FR Stainless Steel and Modified 9Cr-lMo Steel", ASME-PVP, Vol.391, Advances in Life Prediction Methodology, 1999, pp.47-54.

[2] Takahashi, Y., "Further Evaluation of Creep-Fatigue Life Prediction Methods for Low-Carbon Nitrogen-Added 316 Stainless Steel", Trans. of the ASME Journal of Pressure Vessel Technology, Vol. 121, 1999, pp.142-148. [3] Takahashi, Y., "Evaluation of Creep-Fatigue Life Prediction Methods for Low-Carbon Nitrogen-Added 316

Stainless Steel", ASME Journal of Engineering Materials and Technology, Vol. 120, 1998, pp.119-125.

[4] Wada, Y., Kawakami, Y. and Aoto, K.,"A Statistical Approach to Fatigue life Prediction for SUS304, 316, 321 Austenitic Stainless Steels", ASME PVP-Vol. 123, 1987, pp.37-42.

[5] Takahashi, Y., Toya, Y., Abe, H., "Environmental Effects on Fatigue, Creep and Creep-Fatigue Behavior of 316FR Steel", SMiRT-15, Vol.X, 1999, pp.141-148

[6] Zeman, G.J. and Smith, D.L., "Low Cycle Fatigue Behavior of Type 304 and 316 Stainless Steel Tested in Sodium at 550°C '', Nuclear Technology, Vol.42, 1979, pp.82-89

[7] Huthmann, H., "Creep-Fatigue Behavior of Type 304 and 316L(N) in Flowing Sodium", KFK-4935, 1991, pp.163-187

Table 1 Chemical composition (wt %)

Mn P S Ni

0.85 0.026 0.004 11.16

1.30 0.025 <0.005 11.40

0.81 0.023 0.003 8.86

0.95 0.019 - 12.10

Table2-1 Test condition (fatigue test) Environment

Cr Mo N

16.88 2.11 0.0754

18.60 2.04 0.0800

18.35 - -

16.70 2.19 0.0400

In liquid sodium Temperature(C)

Strain range (%)

0.7 0.5

+ +

+ +

+

550

In liquid sodium

In air

1.0 0.4 1.0 0.4

316FR + + + +

316FR + + +

weld metal

304 S.S. + +

316 S.S. + +

Table2-2 Test condition (creep-fatigue test)

Environment I In air

Strain range (%)

0.7 0.5

+ +

+ +

+ +

+

Temperature(C) Hold time (hr)

550

Strain range (%) Strain range (%)

1.0 0.7 1.0 0.7

316FR + + + +

316FR

weld metal + + + +

304 S.S. + +

316 S.S.

C Si

316FR 0.008 0.53

316FR

0.008 0.47

weld metal

304 S.S. 0.050 0.56

316 S.S. 0.045 0.54

fatigu:e

test m a c h i n e W , , ~ To Ar gas controller

I

N

vapor

pod for level meter " ~ - - - l - - ~ t e s t vessel

Na ator

economizer

I-

_{.owme l II SU pumplOOp}

'tmete r

flowmeter

plugging meter

N a sampler

N a cooler m a i n loop

p u m p

I

[ ..~___4-~._~To Ar ~ a s "~ va"p'or " controner

1, trap

storage tank

cold trap 1 cold trap2

Ix,,,,

,"

I "'--'--~

L

"

... _ _~e~.~ ,-[

~_.30 ]---~

i l v, Test piece// V,

\ i t

4

A-A view

Fig.2 Fatigue test machine

[ Na pod

~A

rank

Test piece v.~.__._..~

12.5

, I

30 54

67

, I ,

| |

l- i i

~_. 26.22

129

),

3~

101 I

l

-T.~ i I

_F]-.

'_i

-i

N-~ E t" '"

~~"'/-~

/~~~r

~ ~ . . .

Torsion

/

. I J

to

<l

¢) E

],,.

E (0 O9

10

0.1

... ~ ! ! ~ !

i:ii~:i!!!!i!:!ii:!!!~i~"~'"'

...

... in l, qu,~'~o'd,um',na,;~!!i~!ii

...

1 ~ o v ~, . . . . I i ... i ... 3 1 6 F R - W M • O --i-i i-

... i..i ... j ... J ... i .... i ... J ... i304 * O i..i...i

,

ii

x ~ ... ]i ... ii! ... .... .... i ... ! i ! ... ! ',! ...

i

... !~ ... i ... i ... :A v e r a g e c u r v e f o r 316FR, 304 and 316 ... i ... i ... ii

~ / f r o m p u s h - p u l l t e s t !

... ] ... ~ ~ : , .... [ ... o ... ~ , i ...

i

... i ... ii ... - i ... r~ .... i t... s , . . .

i

: ~i ... i ... ~ ~ i ... i

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

N u m b e r of c y c l e s t o failure Nf

F i g . 4 F a t i g u e t e s t r e s u l t

4 J

to E E

. m L

~0

10

0.1 ~

1 .E+02

... C r e e p - F a t i g u e T e s t R e s u l t ~ In liquid sodium In air )i

... 5 5 O O C ... 3 1 6 F R 1 !-I

... t H = l hr ... 3 1 6 F R - W M Q O

... - - - . . . [ 3 0 4 • ~ ...

... 316 A A ...

l _ _ _ _ _ _ _ J ~

:::::::::::::::::::::::::::::: ~ _{... :::::::::::::::::::::::::::::::::::::::: ;;;}2;.}

. . . E . . . . . . ~ - . . . . , ... , ... , ...

i ... i ...

: , , ,,

1 .E+03 1 .E+04 1 .E+05

N u m b e r of c y c l e s to failure Nf

14

~ 12

c-

-~ 10

e - -

8

N

.=.=.=_.

B++H +I

LCF • Fatigue T e s t FOl • C r e e p - F a t i g u e T e s t

- LOF A ~ t=l.0%

W L C F A ~ t = 0 . 7 %

N LCF A ~ t=0.5% El LCF A ~ t=0.4% I~ FCI A ~ t=1.0%

F C I A ~ t = 0 . 7 %

+

/!iiiiii+!!!i!l

.l-I t~

o

o+6

il

4

°

I I

I i

2

!!i!!ii!+iii

316FR 316 FR-WM 304 316

Fig.6 Comparison of fatigue and creep-fatigue life between in air and in liquid Sodium

... :.: ..~,:.,~.," :-- ... "./:r-" !-,."--.~...::: ;,. ,, . _+:.X:- ."\ 5~.\, - .~:.,

../ • ~ ,. .... ,- ,.. ,, ~,,..f ',,.,.:; "<,::~... ~..,-.'::.... . ..:~ . . . ,.

~ . . .¢. ~....,.~ .+.~. .. . . ~ + . . . . ..

10. l ~ .,~: ~5!(.~ :~:, ~ , ' .., ~ ,.~::.,.-: !:.stl~m: ~t03~,.~

(a) Fracture surface (b) Microstructure

Fig.7 Fracture surface and microstructure observation (Fatigue test in liquid sodium) (Material:316FR, Strain range:0.49%)

j . ,

. ' ~ . P - ~ ;~4.#4d1#.

(a) Fracture surface (Tension hold side) (b) Microstructure (Tension hold side) Fig.8 Fracture surface and microstructure observation (Creep-fatigue test in liquid sodium)