Fatigue crack growth behavior of weld heat-affected zone of type 304 stainless steel in high temperature water

ELSEVIER

Nuclear Engineering and Design 153 (1994) 27-34

Nuclear Engineenng and Design

Fatigue crack growth behavior of weld heat-affected zone of type 304 stainless steel in high temperature water Masao Itatani "', Juichi Fukakura a, Masayuki Asano ", Masaaki Kikuchi b, Noriyuki Chujo b " th,avy Apparatus Engineering Laboratory. Toshiba Corporathm 2-4 Suehiro-cho, Tsurumi-ku. Yokohama 230, Japan t, Applied Metallurgy and Chemistry Department, Toshiba Corporation, 8 Shinsugita-cho, lsogo-ku, Yokohama 235, Japan

Abstract

The fatigue crack growth behavior of the weld heat-affected zone (HAZ) of type 304 stainless steel in high temperature water which simulates the boiling-water reactor environment was investigated to clarify the effects of welding residual stress, cyclic frequency f and thermal aging on crack growth rate. A lower crack growth rate of the HAZ than of the base metal was observed in both the high temperature water and the ambient air caused by the compressive residual stress. The crack closure point was measured in the high temperature water. The effect of the welding residual stress on the crack growth rate of the HAZ can be evaluated separately from the environmental effect through the crack closure behavior. The high temperature water increased the crack grewth rate at a cyclic frequency of 0.0167 Hz but did not affect it much at 3 and 5 Hz. The crack growth behavior of the thermally aged HAZ at 400 C for 1800h was almost the same as that of the unaged material tested at 0.0167 and 5 Hz in the high temperature water.

I. Introduction

There have been many investigations on the stress corrosion cracking (SCC) and the corrosion fatigue of structural steels such as low alloy steel, austentic stainless steel and Ni-base alloy in high temperature water, which aim to assess the structural integrity of light-water reactor (LWR) power plants. The fatigue crack growth behavior of these materials in high temperature water is becoming clear by great research efforts. However, the crack growth behavior of the welded joint is more complicated than the base metal because of the combined effects of welding residElsevier Science S.A. SSD! 0029-5493(94)00810-L

ual stress and microstructural change introduced by welding. Thus it is necessary to clarify the mechanisms of acceleration or deceleration of fatigue crack growth rate (FCGR) of the heataffected zone (HAZ) in the high temperature water in order to understand better the fatigue crack growth of the welded joint and to predict accurately the FCGR. Furthermore, material degradation which may be introduced by long-term plant operation is one of the great concerns when considering the plant life management and life extension. The str~ctt,,"al materials used for the main components of LWRs are thought to be degraded by neutron irradia-

28

.U. ltat,mi et al./.Vu,'h'ar ~a.~,hw,'ring am/Dexign 153 (1994) 27 34

tion, thermal aging, t~ltigue and other mechanisms. So it is also important to evaluate the effects of these aging mechanisms on the crack growth behavior of the structural steels in an LWR water environment. In this paper, the crack growth behavior of the HAZ of type 304 stainless steel was studied in high temperature water which simulated the boilingwater reactor (BWR) water environment in view of the crack closure behavior due to welding residual stress. The effect of thermal aging on the crack growth behavior was also discussed.

Table 2 Mechanical properties of solution-treated type 304 stainless steel Material

av (MPa)

al~ (MPa)

~:r I%)

Type 304

231

579

66

Base metal

Roll ing direction

Weld metal

2. Experimental procedure Base metal

The type 304 stainless steel plate of 40ram thickness received solution heat treatment at !!00 C for 0.8 h. The chemical compositions and mechanical properties in ambient air of the base metal alter solution heat treatment are shown in Table 1 and 2 respectively. Butt-welded joints were made by submerged arc welding with type 308L weld metal so that the welding line is parallel to the rolling direction of the plate. The welding current, voltage and speed were 450 A, 30 V and 3 2 0 m m m i n ~ respectively. Some of the joints were thermally aged at 400 C for 1800 h. Specimens used for the fatigue crack growth tests were prepared from these welded joints. Sidegrooved IT compact tension (CT) specimens of the HAZ were prepared from both the aged and the unaged joints as shown in Fig. ! so that the crack grows in the HAZ along the welding line. A specimen of the base metal was also prepared from the original plate. Fatigue crack growth tests were conducted using an electrohydraulic testing machine with an autoclave. Fig. 2 shows the testing facilities. Specimens were cyclically loaded in high temperature water at 288 C and 7.8 MPa. The dissolved oxygen and conductivity were controlled to 0.2 ppm

40ram Fig. I. Preparation of HAZ specimens.

and 0.21,tScm -~ respectively during the tests. The loading conditions are listed in Table 3. All the tests except one were conducted by the AKincreasing method. For a test on the aged HAZ at a stress ratio R of 0. ! in water, the AK-decreasing method was adopted. For the aged HAZ at R = 0 . 1 in water, the cyclic frequency was changed from 0.0167 to 5 Hz for a single specimen. For the unaged HAZ at R = 0.1 in water, one specimen was loaded at 5 Hz aud another specimen was loaded at 0.0167 and 3 Hz alternately during the test as shown in Table 3. The loading waveform was sinusoidal. Crack lengths were monitored by the unloading compliance method based on the measurement of the crack-opening displacement (COD) by a COD gauge mounted at the mouth of the specimen, The crack closure points were also measured in some specimens to evaluate the effect of the welding residual stress on the crack growth of the HAZ.

Tahle I Chemical composition of iype 304 stainless steel Element Amount (wl.%)

C {).{}6

Si 0.65

Mn 0.91

P 0.030

S 0.002

Ni 8.99

Cr 18.5()

Mo 0.13

N 0.039

Fe Balance

M. Itamni et al. / Nuch,ar Eng#u,ering attd Design 15.; (1994) 27 34

29

Bubbling Tank AutocI ave

(Ion Exchanger)

,i Test

I

ing~ Heat

Exchanger

I

[

I

Senso~ [

I[High Pressure

:: .........

::

I

Filter

I, ili::i::i::iiiilCoo, I " Coolant

Fig. 2. Fatigue-testi:ig facilities.

Table 3 Loading conditions for the fatigue crack growth tests Material

Environment

Stress ratio

Cyclic frequency {Hz)

Loading procedv,e

Base metal, unaged HAZ, unaged

0. I

HAZ, unagcd

Air (room temperature) Air (room temperature; Water (288 C)

AK increase AK incre;'.se AK inc'ease

HAZ, aged

Water (288 C)

0. I 0.5

5 5 0.1)167 ~ 3" 5 0.0167 -* 5 h 0.0167

0.1 0.1

AK decrease /t,K increase

" T w o cyclic frequencies were altecnately adopted. I, The cyclic frequency was changed from 0.0167 to 5 Hz.

3. Results and discussion

3.1. O'aek growth behavior of the heat-qffected zone in high temperature water Fig. 3 shows tile relationships between the crack growth rate da/dN and the stress intensity factor range AK at R = 0.1. This figure also shows the crack growth data obtained by Hishida et al. (1986) for the base metal of type 304 stainless steel in high temperature water under similar testing conditions. This reference material containing 0.051 wt.% C was solution annealed. The FCGR at

0.0167 Hz is markedly higher than the others in high temperature water. The FCGR at 3 Hz in water lower than those at 5 Hz in water and in ambient air. The effect of cyciic frequency on the FCGR decreases with increasing AK and the FCGR is not affected by the cyclic frequency and/or the environment at AK above 45 MPa m ~;2. The FCGR of the HAZ is lower than that of the base metal in air at 5 Hz. It is also found that the FCGR of the HAZ at 0.0167 Hz is lower than that of the base metal at 0.02 Hz in water obtained by Hishida et al. (1986). The lower FCGR of the HAZ is thought to be caused by the welding

.11. itatani et al./ ,¥u('h'ar Engin('('ring and Design 153 119941 27 34

30

J 304SS R:0 1 '

!

R01t I

Base Metal f 0 02Hz 10 3

Triangular Ilaveform H~h,da el al

10 3 x

Y..

o

I0 4

10 4 RO I

,'"

Z

,."~

10 s

i

~---~-~.....

I0 5

..... r . r ,,,A,,T

5 ',m

288 C Water ~ 0 0167Hz t l Base M e t a l

.~: [ U,,ao,'~ !2aa c ,a,e, r--3--~:::Ii A~lecl

'

1

10 6

i

f

t

I

~__

I

I

I

I

20

30

40

50

AK

f 0 02H:

!er,al ] R Sym Unaged 0 1 ','

288C Water

'l"r ~angu1,1T W,lveform H~h~da e t a ~

288 C Water r---~ g-0161 ~ 0 :S,•

: _ _

"HA~ R T ~n A,r R 0 1

Aged

01 • 05O

i

I

I

30

40

50

I -,

10 6

,

80

I

0

(MPaml '2)

20 AK

I

I

80

(MPam1'2)

Fig. 3. Effects of cy~iic frequency and thermal aging on the fatigue crack gro~vlh behavior of the ilAZ of type 304 stainless steel ( S S ) RT. roonl tctnperature.

Fig. 4. Effect of stress ratio on the fatigue crack growth hehavior of the HAZ of type 3114 stainless steel [SS): RT.

residual stress as discussed later. The lower F C G R of the H A Z at 3 Hz in water than those at 5 Hz ill water and in ambient air is contrary to our expectation when considering only the effect of cyclic fi'equency on the F C G R in the environment. This particular behavior of the H A Z is also thought to he causcd by the difference in the welding residual stresses of individual specimens. Fig. 4 shows the effect of stress ratio on the F C G R of the H A Z ill high temperature water compared with the data of Hishida et al. (1986). It is observed that the F C G R at R = 0.5 is higher than that at R = 0.1 and the effect of stress ratio on the F C G R decreases with increasing AK for both the H A Z and the base metal. The effect of stress ratio on the F C G R of the H A Z is greater than that on the base metal. The difference between the F C G R s of thc H A Z and the base metal in water is pronounced at R = 0.1, while not so marked at R =0.5. Although the crack closure point was not measured at R = 0 . 5 , the crack opening ratio U generally becomes higher with increasing stress ratio. Thus the effect of the resid-

ual stress on the crack closure at R = 0.5 may be smaller than that tit R = 0.1. Consequently the difference between the F C G R s of the H A Z and the base metal due to the residual stress must be small at such high R.

roonl

lenlperalure.

3.2. ('rack clo.vure hehul,ior o./' the heat-a.[l'ected SOll("

It is found that the F C G R of the H A Z is lower than that of the base metal in both high temperature water and ambient air. The residual stress introduced by the welding is thought to be a cause of the lower F C G R of the HAZ than that of the base metal. Fig. 5 is a schematic illustration of the residual stress ar: distributions of the butt-welded plate and the specimens taken from it (Ota, 1986), here, at: is tile residual stress perpendicular to the welding line. It is understood that the crack tip of the CT specimen is always ill the compressive residual stress field. When tile material is subjected to cyclic loading, the residual stress is thought to affect the crack closure behavior, especially at low

M. ltatani et al./ Nuch'ar Engineering and Design 153 (1994) 2 7 34 f

l

Welded plate Tensile

t i l l l / f f l I f M~ A I l r d w l i r f • Vlrlrlr I K I I I 4 r ~ / u l Jl~ • r r

/

,•TtfI

l

"I Center cracked type specimen (CCT)

Compact type specimen (CT)

Fig. 5. Residual stress distribution of the butt-welded joint and specimens prepared I'rom the joint (Ota, 1986).

stress ratios. So the crack closure point was measured by using the unloading compliance method during some fatigue crack growth tests. Fig. 6 shows the relationships between the crack-opening ratio U and AK. The crack-opening ratio is defined as U = (K,.,,x - K,,p)/(Km,,~ - K,.i.)

0 ~ ,,0

•

o~ao ..'

0 0

0.8 <306

.,'

,o

¢o

Ao

<3

Mater~al Environment f(Hz) Sym Base Unaged RT. in Air 5 • R.T. in Air 5 D Unaged 0. 0167 ~7 288"C Water HAZ 3 O 0.0167 V Aged 288°C Water 5 •

"0.4

,,--=

0.2

0

10

I

I

20

40 30 Ag (MPam1/2)

I

|

50

I

I

80

Fig. 6. Crack closure behavior of the HAZ of type 304 stainless steel (SS): RT. room temperature.

31

in which K,,p is the crack-opening stress intensity factor. It is found that the crack-opening ratio of the HAZ is lower than that of the base metal in the comparatively low AK range. The value of U of the HAZ increases with increasing AK and become unity at AK above 45 MPa m 1/'-, which point the effect of cyclic frequency on the I R disappears, as shown in Fig. 3. The crack-opening ratio of the unaged HAZ in high temperature water is lower than those of the aged HAZ in water and the unaged HAZ in ambient air. The value of the residual stress of the specimen may depend on the location of the specimen cut from the welded joint. For the unaged HAZ in high temperature water, data at both 0.0167 and 3 Hz can be recognized to be located on the same trend curve. These data were obtained from a single specimen changing the cyclic frequency alternately during the test, so that the influence of the scattering residual stress on the crack closure point is excluded. Since the clear effect of cyclic frequency on the crack closure point is not observed, the wedge effect due to the oxide is negligible in these frequency ranges. Therefore the difference between the crack closure behavior of the unaged HAZ and the aged HAZ in high temperature water and the unaged HAZ in ambient air is considered to be caused by the variation in the residual stress of the specimens. Fig. 7 shows the relationships between the crack growth rate d a / d N and effective stress intensity factor range AK~,. ( = U AK) which is considered to be the true driving force of fatigue crack growth. It is found that the FCGR of the HAZ is almost the same as that of the base metal in ambient air. The FCGR of the HAZ at 3 Hz in high temperature water is slightly higher than that in ambient air. The environmental effect on the FCGR appears clearly in Fig. 7 through the increase in the FCGR according to the decrease in the cyclic frequency. It can also be seen that the thermal aging does not affect the d a / d N vs. AKin. relationships. 3.3. Sensitization by weMing and thermal aging

On comparison of the results of fatigue crack growth tests of the aged HAZ with the unaged

32

M. Itatani et al. / ?¢ut'h'ar Enghteerhag and Design 15.t (1994) 27-34 i

~S--~

~7 V

_

,-,

.SP

/

\

/

Polarization)

0.01M-KSCN

t-

"oj

Z "O

Peak Current Densi ty la (Anodie

~

O,5M-H2SO 4 / tn + t--

10 3

10 -4

Ir la

~ '

/

~

I

/

~

Potential

V

Peak Cur rent

Density I ( Reverse

Ir

"ID

Material

10 5

EnvirOnment

Base]Unaged I

Unaged Aged

I

10

20

AKeff

Sym

-

I

I

I

30

40

50

+

Polarization Curve during EPR Test

Fig. 8. Definition of DOS value obtained from the current

288"C

HAZ

10 6

f(Hz)

RT in Air 5 • R T in A~f 5 [] 0 0167 ' ,7 Water 3 0 0.0167 • 288~C Water 5 •

density potential curve.

80

(MPam1/2)

Fig. 7. da/dN vs. AK,,r relationship for the HAZ of type 304 stainless steel (SS): RT, room temperature.

HAZ, it can be recognized that the effect of thermal aging at 400 C for 1800 h on the FCGR is not pronounced in two cyclic frequency levels. Generally speaking on HAZ of type 304 stainless steel of normal compositions is sensitized by the welding itself, and susceptible to intergranular SCC in high temperature water. In this study, electrochemical potentiokinetic reactivation (EPR) measurements were conducted before and after the thermal aging for both the base metal and the HAZ to evaluate the degree of sensitization (DOS). The DOS values are defined as the ratio It~l,, where I, is the peak current density during the potential rise and Ir is the peak current density during the potential drop as shown in Fig. 8 (Umemura, 1992). The measurements were performed at two points for both the aged and the unaged base metal and at five points for both the aged and the unaged HAZ. The measurements for the HAZ were conducted on the plane which is coincident with the crack plane in the HAZ about 2 mm from the fusion line. Table 4 shows the DOS value of each measurement. Although there is a little data scatter, the average DOS values

show the development of the sensitization in the HAZ by the welding and the thermal aging. A DOS value over 5% means that type 304 stainless steel is sensitized (Umemura, 1992); therefore the thermally aged HAZ is sensitized, but not so much as to influence the crack growth behavior. This is consistent with the fact that no marked differences are observed between the fracture appearance of the aged and the unaged HAZ, as shown in the next section. Table 4 Results of degree of sensitization measurement Material

HAZ. HAZ. HAZ. HAZ. HAZ.

DOS 1"',,,)

unaged unaged unaged unaged unaged

3.9 ") 2.9 2.3 Average, 3.0 2.5 3.2

Base metal, unaged Base metal, unaged

0.0"[Averag e " O.7 1.3 J '

HAZ. HAZ, HAZ. HAZ. HAZ,

aged aged aged aged aged

Base metal, aged

Base metal, aged

2.2 "] 14.1 17.1 Average, 10.2 8.6 9. I 0. I'~v> 5.3 J"~

e r'age,

2.7

M. hatani et al. / Nudear Enghleerhlg and Design 153 (1994) 27 -34

33

3.4. Crack path and fi'acture surface observation It is found that the thermal aging at 400 "C for 1800 h does not much affect the FCGR of the HAZ in high temperature water, but increases the DOS slightly. Then the crack paths and fracture surfaces of the specimens were observed to investigate the influences of the environment and the thermal aging on the microstructural aspect of fracture process. Figs. 9(a)-(c) show the crack paths of the unaged HAZ in ambient air, the unaged HAZ at 5 Hz in high temperature water and the aged HAZ at 0.016 Hz in high temperature water respectively; the latter shows the most accelerated crack growth rate. The crack path in ambient air is almost straight. On the contrary, the crack paths in high temperature water zigzag and are branched. In spite of the difference of the FCGR and the DOS, no distinct difference is observed between the fracture appearances of the unaged HAZ at 5 Hz and the aged HAZ at 0.0167 Hz in high temperature water.

200.~

.---,4m, t a ~ Unaoed. R.T.

m air.

f 5Hz, ~K 23MPam '

I

""'~""

•

200~

( b ) Unaged. 288 C water, f 5Hz, &K 20MPam ' ;

e

'"4 200.., ( C ) Aqcc. 288:

water, f 0. 0167Hz. :K 23MPam

Fig. 9. Crack path of the HAZ in ambient air and high temperature water, where the arrows show crack growth directions.

(a)Unaged. R T. , n a , r .

f 5Hz. IK2311Pam

b ) Unaged. 288C water, f 5Hz. AK 2B##Pam' ~

(c)kgec. 288: mater, f O. OI67Hr, "K 231JParn

"

Fig. 10. Fracture surface of HAZ formed in ambient air and high temperature water, wher the arrows show crack growth directions.

Figs. 10(a)-(c) show the fracture surfaces observed by scanning electron miscroscopy corresponding to Figs. 9(a)-(c) respectively. These figures show that the fracture surfaces in high temperature water are rougher than that in ambient air. From Figs. 9 and 10, it is clear that the microscopic crack morphologies are similar in the aged HAZ and the unaged HAZ and the crack grows in transgranular mode even in the aged HAZ specimen at the lowest cyclic frequency. Although not all the fracture surfaces are shown, there are no differences between the fracture morphologies of the unaged HAZ and the aged HAZ in high temperature water. It is found that the FCGR of HAZ accelerates with decreasing cyclic frequency with no intergranular SCC. This result of HAZ coincides with the observation for base metal reported by Hishida et al. (1986). The effect of cyclic frequency on the FCGR has been reported in the case of pure CF (Suresh, 199 ! ). Although the details of the mechanism of acceleration are not clear, some mechanisms for acceleration such as irreversibility of

34

M. Itatani et al. / Nuch,ar E:,agmeering and Design 153 (1994) 27 34

cyclic slip by oxidation or anodic dissolution in tha transgranular mode can be considered.

Appendix A: Nomenclature

da/dN 4. Conclusions Fatigue crack growth behavior of the HAZ of type 304 stainless steel in high temperature water has been investigated using CT specimens prepared from the butt-welded joint in view of the effect of the welding residual stress and the cyclic frequency. The influence of the thermal aging has also been discussed. The following conclusions were obtained. (l) The FCGR of the HAZ is lower than that of the base metal, since the crack tip of the HAZ specimen is always in the compressive residual stress field. (2) The effective stress intensity factor range AKCtr based on the crack closure measurement is a reasonable parameter for evaluating the fatigue crack growth behavior in the residual stress field. (3) The FCGR of the HAZ increases with increasing stress ratio R and is more sensitive to R than that of the base metal because of the compressive welding residual stress at the crack tip of the HAZ specimen. (4) Thermal aging at 400 ~C for 1800 h raises the sensitization of the HAZ slightly but does not affect the fatigue crack growth behavior in high temperature water. (5) Fracture surfaces are observed to examine the effect of cyclic frequency on the FCGR of the HAZ. It was found that the fatigue crack grows in a transgranular mode and branched in high temperature water. No distinct difference in the fracture appearance of the HAZ under the influence of the thermal aging and cyclic frequency was observed in spite of the FCGR or the DOS change.

f

Km,,x

Kmi,

AK AK,m R U

fatigue crack growth rate (mm cycle- ') loading cyclic frequency (Hz) peak current density during the potential rise in the EPR measurement (A mm -2) peak current density during the potential drop in the EPR measurement ( A m m -2) maximum stress intensity factor ( MPa m I/-') minimum stress intensity factor ( MPa m t/-') crack-opening stress intensity factor ( MPa m I/2) stress intensity factor range (MPa m I/2) U AK, effective stress intensity factor range, (MPa m ~/2) Kmin/Km,.,x, stress ratio crack-opening ratio

Greek letter ary

residual stress perpendicular to the welding line (MPa)

References M. Hishida, M. Saito, K. Hasegawa, K. E,omoto and Y. Matsuo, Experimental study on crack growth behavior for austenitic stainless steel in high temperature pure water, Pressure Vessel Technol. 108 (1986) 226-233. A. Ota, Information on characteristic of fatigue crack growth in welded joints for design or inspection of structures, JHPI, 24(!) (1986)40-47 (in Japanese). F. Umemura, Y. Hanai and T. Kawamoto, Development of automatic sensitization detector and its application, Ishikawajima-Harima Eng. Rev. 22(5) (1922) I-6 (in Japanese). S. Suresh, Fatigue of Materials, Cambridge University Press, 199 I, pp. 378- 380.