Fatigue strength correction factors for carbon and low-alloy steels in oxygen-containing high-temperature water

Nuclear Engineering and Design 129 (1991) 293-306 North-Holland

293

Fatigue strength correction factors for carbon and low-alloy steels in oxygen-containing high-temperature water Makoto Higuchi

a

and Kunihiro Iida b

a Research Institute, Ishikawajima-Harima Heavy Industries Co., Ltd., Isogo-ku, Yokohama 235, Japan b Department of Mechanical Engineering Shibaura Institute of Technology, Minato-ku, Tokyo 108, Japan

Received September 1990

Strain-controlled fatigue tests were conducted on carbon and low-alloy steels in high-temperature water containing controlled amounts of oxygen, in order to determine quantitatively the reduction of fatigue strength due to factors such as the strain rate and the environmental testing conditions such as temperature and dissolved oxygen content. For each of these factors, the effect on fatigue strength was separately quantified and parametric equations were derived with reference to the basic fatigue curve obtained in air at room temperature. Using the formulas thus obtained, generalized expressions for predicting the expected fatigue life of a given material under specified combinations of strain amplitude and environmental conditions were deduced. Based on these generalized expressions, "fatigue strength correction factors for environmental effects" are proposed, which can be conveniently used in combination with the design fatigue curve given in the ASME Boiler and Pressure Vessel Code, Section III, to account for fatigue strength reduction under severe service conditions.

1. Introduction The design fatigue curves given in the A S M E Boiler and Pressure Vessel Code, Section III, are widely applied in deriving the design strength of structural components for light-water reactors. Although these curves were derived applying safety factors for fatigue strength and life, it appears that sufficient account was not taken of the effect of fatigue strength reduction in a severe service environment such as oxygen-containing hightemperature water. The first instance known to the present authors of published data concerning this effect was based on the results from bending fatigue tests performed on carbon steel at the Dresden-1 BWR power station [1]. This report was followed by others also on carbon steel based on pipe tests in which high-temperature water was circulated through a pipe subjected to cyclic loading [2], or on fatigue crack initiation tests applied to blunt-notched compact-type (CT) specimens in high-temperature water [3]. The foregoing reports, however, do not contain sufficient data from tests conducted with adequate variation of the applied cyclic stress or strain or of the environmental conditions to permit full parametric analysis. Since 1980, the present authors have been engaged in a study of the fatigue characteristics of carbon and low0029-5493/91/$03.50

alloy steels under controlled axial strain cycling in high-temperature water environment. These results have been published in reports covering the fatigue strength reduction behavior as a function of strain rate [4-6]. Subsequently, the study was extended to an examination of the dependence of this behavior on the effects of water temperature and dissolved oxygen content, as well as of strain rate. The present report contains new acquired data concerning the environment-dependent effects observed on fatigue behavior, together with an attempt at quantification and parametrization of these factors, from which new fatigue strength correction factors for environmental effect Ke,, are proposed.

2. Materials tested and testing method Two structural materials widely applied in nuclear equipment construction were chosen for testing: (a) carbon steel pipe (STS42 in JIS G 3455, equivalent to A S M E SA333 Gr.6) , and (b) forged low-alloy steel (SFVV3 in JIS G 3212-1977, equivalent to A S M E SA508 C1.3). The carbon steel sample from a pipe was of 318.5 mm nominal outside diameter and 17.4 m m wall thickness; the specimens were taken from the mid-

© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d )

294

M. HiguchL K. lida / Fatigue strength correction factors

Table 2 Principal factors governing environmental and test conditions

8 ~+ - 0.0~

Medium Temperature Pressure Electrical conductivity pH Dissolved oxygen content Water flow rate Fatigue mode Wave shape Strain rate rising phase

1

II G.L. --

106

Fig. 1. Shape and dimensions of test specimens.

wall thickness of the material as received a n d m a c h i n e d with the axis coinciding with that of the pipe. The forged low-alloy steel sample was taken from a plate of 165 m m thickness, normalized, tempered, then q u e n c h e d a n d retempered, a n d followed by annealing for stress relief; the specimens were sampled from positions corres p o n d i n g to " and ~ plate thickness a n d m a c h i n e d with axis coinciding with the principal direction of forging. N o account was taken of any possible differences in material properties that might exist between specimens sampled from either of the two positions in the plate thickness. Table 1 gives the chemical compositions and mechanical properties of the sample materials. Figure 1 shows the shape of the u n n o t c h e d cylindrical specimens. The tests were performed using a servo-hydraulic fatigue testing machine c o m b i n e d with an autoclave connected to a loop that circulated deionized water, arranged as shown schematically in fig. 2. The testing t e m p e r a t u r e was parametrically varied in the range from room temperature to 290°C, a n d the pressure was set at a level that ensured that the water remained in the liquid state at the testing temperature. The c o n t e n t of dissolved oxygen was controlled by regulating the rate of argon gas and oxygen blown into a tank provided for the purpose in the loop circuit. The water flow in the autoclave was set in virtually stagnant condition, in the

Deionized water Room temp. to 290°C 3.9-8.0 MPa < 0.2 rtS/cm 5-7 0.01-20 ppm 60 l / h Axial strain cycling Triangular/Saw tooth 0.4-0.0001%/s (variable) 0.4%/s (constant) - 1 (fully reversed)

falling phase Strain ratio

expectation that it would accelerate the effect of corrosion on fatigue strength. Fully reversed strain cycling ( R = - 1 ) was applied in triangular or saw-tooth (i.e. a slow rising p h a s e a n d a fast falling phase) waves. The strain rate was set at 0 . 4 % / s for triangular wave; for the saw-tooth wave, the rate was varied parametrically in the range 0.0001 to 0.4 % / s for the rising phase, a n d held at 0.4 % / s for the falling phase. Variation of strain rate in the failing phase was k n o w n from a previous study to exert almost no influence on the fatigue life in h i g h - t e m p e r a t u r e water environments. The principal factors that governed the e n v i r o n m e n tal and other test conditions are given in table 2.

3. T e s t

results

For STS42 c a r b o n steel, the relation between applied strain a m p l i t u d e a n d fatigue life was f o u n d to be as presented in fig. 3 for various c o m b i n a t i o n s of water

Table 1 Chemical compositions (wt%) and mechanical properties of materials Material

C

Si

Mn

P

S

Ni

Cr

Cu

Mo

V

AI

STS42 (A333Gr.6) 0.20 0.31 0.93 0.020 0.015 0.02 SFVV3 (A508C1.3) 0.20 0.29 1.45 0.003 0.003 0.68 0.12 0.03 0.55 0.01 0.023

T 0.2% ( ° C) yield strength (MPa)

Ultimate Elongation Reduction tensile (%) of area strength (%) (MPa)

RT 290 RT 290

489 451 606 575

302 218 471 419

41.0 30.0 30.4 20.8

80.0 73.0 73.5 72.6

M. Higuchi, K. lida / Fatigue strength correctionfactors

295

Monitor m

Lr-u~ ~

,e.

-~PressureRegulator

rn

El

f'

I I[ Autoclave I I1:: ~,;~-~,~

II I

I I1',, ~] ][~Specimenl

I I ~::....~I i:Ii,

or A',,I'~L.~ ] c3 I U ~ I

o

(~)DOl.o~o.~lPressurelI ~1

I ~ I

"E:~

-~J ~ I

_ I

FatigueTestMachine

C]-:ChlorJne, DO:Dissolved oxygen EC:Electrical conductivity, PII:pH

Pump

Fig. 2. High-temperature deionizcd water circulating loop.

temperature and strain rate. The solid line drawn in the diagram represents the best-fit curve obtained from tests in air at room temperature, and the dashed line is the ASME best-fit curve for carbon steel [7]; the chain line is the ASME design fatigue curve incorporating a safety factor. The solid line for the air data obtained from the present study is seen to coincide approximately with the ASME best-fit curve. The corresponding data for the SFVV3 low-alloy

l0 -1

'

'

' ='"'1

'

'

' ''"'1

"~, .

"~\

' I''"l

'

in w,t°~

"~,.

"~

,o-=

'

\.,,

T

I~ R~=-I

_2_0 __o

10-1 ~

' i'--t

-" o.4 I. o.o, I olol

I"

0.004

4)

",,X

I ~nW_"~"~

I t~

,oo ~ o , fl

.

0.0001

-~ ,oo Oo::, I~'~

~

o4

2,0

-~-

~ ~ /

"~

'

z~

RT Air

~

I''"1

*

i DO=eppm

. :~

'

steel are presented in fig. 4. Fatigue strength reduction by increasing temperature and dissolved oxygen content, and by decreasing strain rate is seen to be qualitatively similar to the case of STS42. However, compared with STS42, S F W 3 shows a smaller reduction of fatigue strength, and no data fall below the ASME design fatigue curve. The original experimental data on STS42 and SFVV3 are tabulated in the Appendix. With very few exceptions, all the fatigue life plots

15

0.4

-

0

o,0,

-

.~

10-2

,

Fit Curve

-.

RT Air

'

~. E m -

~.,:u. c .... I01

102

103

\ ~u;ve 104

I0

s

1 10 s

Experimental Fatigue Life N25 (cycles)

Fig. 3. Fatigue test results, with testing temperature and strain rate varied parametrically, for STS42 carbon steel.

0.

ASME Design Fatigue Curve

]0-3

101

102

103

Experimental Fatigue Life

-~ 104

l0 s

10e

N25 (cycles)

Fig. 4. Fatigue test results, with testing temperature and strain rate varied parametrically, for SFVV3 low-alloy steel.

M. Higuchi, K. lida / Fatigue strength correction factors

296 i

z

,

,

r

. . . .

I

r

r

'

I ' ' '

r

'

'

'

D O (ppm) Temp. ~.oZo,~ 8 (°c)

10 0

'

' ' ' ' r

,

,

i

I

, , r T

i

l l l i I

~ : ~ .

z _J

n,

o

10 -z

e~

n, i

Re=--I h

lO-~'10-4

i

,

i

i

i

i

i

iJ

i

10-3

i

i

i

i

i

i iI

i

i

i

i

i

i

10-2 Strain Rate ~T ( % / 5 )

i

~ta=O 006

i[

i

i

i

10-1

10o

Fig. 5. Fatigue life ratio vs. strain rate for STS42 carbon steel.

are found at levels below the solid line representing the basic fatigue curve from tests in air at room temperature. The extent of fatigue strength reduction below the basic fatigue curve has been accentuated with increasing water temperature, with increasing dissolved oxygen content and with decreasing strain rate. In the extreme case seen for very slow strain rates with STS42, the data have fallen even below the ASME design fatigue curve. On the other hand, at relatively small strain amplitudes,

'

<

z z

100

e~

'

'

,

. . . .

,

'

'

'

the environmental factors have had little effect on the fatigue strength with both types of steel. As regarding the strain rate, among the factors in question governing the environmental and test conditions, it has already been reported by the present authors [6] that a linear relation exists between the logarithms of the fatigue lives and strain rate in the range between 0.0001 and 0.4%/s in the case of STS42 steel subjected to strain cycling at 250°C in deionized water containing

,

. . . .

~

,

,

,

,

. . . .

i

,

,

,

SF'VV 3 (A508-3) in Water

] /

--E3~-

R,~=--I, Eta=O.O06

]

- 0 ~ . . - : ~--:-~__________~--''I~

~

-

~

~

,

. . . .

~ :

<=

._] Iz o ~"

10-I

DO !ppm)

I Temp. I

Ioo,lo=i, l, l
I"1"1- l*12g°l

...I

h 10-2 10-4

......

16'-3

......

i0'-2

.

.

.

.

.

.

.

.

.

.

.

Strain Rate ~T (°~o//$) Fig. 6. Fatigue life ratio vs. straining rate for SFW3 low-alloy steel

.

.

lOO

297

M. HiguchL K. lida / Fatigue strength correction factors

Z

/in wat,~

Z

100

_

0

z

~----._._

I]

/Triangle or Saw Tooth RIRIR"IRIRE=I ~ta=0 006 II ~ ,

l0 o

Iz o

o

[

[Wo~<

'= tO-t / '~

DO (ppm) o~ T 8 0 . 2 0.01 (.o/S) O [] ZX 0.4

s S~T S ~ A ~

ITriangle

[ or Saw Tooth I

•

[RE=

~'ta=0. 006

•

•

l i . Water

0.0[

3~6)6)

--I,

~

~

"5 10-~ --

L o

•

•

_

•

~ !5o j

........

~ 200 ]

q

..........

.....A I

J

'

'

Temperature

2~0

/

',~w-------~

I

LD~ L o *,[

0'o

"

(c) l

,

10-2

(°C)

,

,

= ....

I

'

'

= I ll,ll

i

I

i

I lll~l

10-~ 10o Dissolved Oxygen Content (ppm)

Fig. 7. Fatigue life ratio vs. testing temperature for STS42 carbon steel.

Fig. 9. Fatigu,

8 p p m oxygen, as shown in fig. 5. This relation is expressed by

performed under various conditions are shown for STS42 and SFVV3 steels respectively in figs. 5 and 6, where the ordinate represents the fatigue life ratio R LAR = N25w/N25 A, i.e. the ratio of fatigue life in high-temperature water to that in air at room temperature. This quantity has the advantage of not being sensitive to small changes in the applied strain amplitude, and will for this reason be adopted hereafter in this report for representing fatigue strength reduction in high-temperature water. The lines in figs. 5 and 6 show a tendency to

N25 ~ ( ~ T ) P

(1)

where the symbols have the meanings given in the nomenclature list (this applies to all symbols appearing hereafter). It has been confirmed for the steel SFVV3 at 150°C in water containing 1 ppm oxygen (see fig. 6). The index P - - giving the slope of the straight line representing the above relation - - has been termed the strain rate dependence factor of fatigue life. This factor appears to vary with temperature and with dissolved oxygen content, as well as with the type of steel. Examples of the assumed linear relation derived from tests

"1

ratio vs. dissolved oxygen content for STS42 carbon steel.

'

'

' ....

I

'

<

z

r

[

Air

Water (DO, ppm) 8 002 0.01

SFVV3 (A508-3) in Water & Air

~

0

A

Triangle

0.01

--

•

•

RE=-

•

or Saw ,

,

, i ,,,

l in Water ~ta# 0.006, RE=--I W a v e : Triangle or Saw Tooth

z Tooth

Eta=0.006

er

z n,

I ,,"1

z

I

~T (~/S) 0.4

'

r sFvv 3 (A508-3)

< [

1

10 ~

o

10o

.o

J J

LL i0_~

~T (%Is) 0.4 0.01 [] • zx • 0 • 0 •

LL ,

I

J

I

100 200 Temperature (°C)

r

300

Fig. 8. Fatigue life ratio vs. testing temperature for S F W 3 low-alloy steel.

10-1

,; 1 -2

L

Temp. (C) 290 200 tSO IO0

I l=,,l

10-1

J

, . i .... I

10o

i

i i L Jill

101

Dissolved Oxygen Content (ppm)

Fig. 10. Fatigue life ratio vs. dissolved oxygen content for S F W 3 low-alloy steel.

298

M. Higuchi, K. lida / Fatigue strength correction factors

steepen their slope with increasing temperature and dissolved oxygen content. In figs. 7 and 8, the fatigue life ratio is plotted against temperature. For STS42, but not for SFVV3, there appears to be a tendency for the fatigue life ratio to reduce sharply at temperatures greater than 200°C in tests at relatively high dissolved oxygen content and slow strain rate. Plotted against dissolved oxygen content, the fatigue life ratio varies as shown in figs. 9 and 10. As was noted in figs. 3 and 4, the fatigue life of STS42 is more sensitive to differences in dissolved oxygen content than is seen with SFVV3. Moreover the plots show a sharp fall around or beyond 0.2 ppm at relatively high temperatures and slow strain rates.

For the two types of steel in question, the curves thus obtained are: (for STS42)

'taA = 0"231N25 °'472 + 0.00108,

(4)

(for SFVV3)

(t~, = 0-419N2~A °'56s + 0.00140.

(5)

For the data from tests in high-temperature water, using eq. (2), eqs. (4) and (5) become: (for STS42)

(taw

= 0.231(N25p(gT)-P) -°'47z + 0.00108, (for SFVV3)

(taw

.

*

_p.

= 0.419( N2sp ( ( T )

4. Discussion 4.1. Prediction o f the ( - N water environments

curve f o r high-temperature

The foregoing tests conducted on the two types of steel in question indicated that the fatigue lives of both were significantly influenced by all the three factors studied of strain rate, temperature and dissolved oxygen. The effects exerted individually by each of these factors have been examined by separate parametric analysis of each factor, and the effects have been quantified by equations relating the magnitude of the factor to fatigue life. In figs. 5 and 6 the fatigue life ratio is plotted against strain rate. Despite appreciable scatter in the data, all the plots are seen to indicate a tendency to converge towards l % / s strain rate, i.e., the fatigue life of specimens tested under different environmental conditions tends to approach the value in air at room temperature. This observation has led to the expression Nz5w = N25A(;T) P or RLA . = ( ; T ) p.

(2)

't.A = A(N25A) B + C

where A, B and C can be determined for a given material by least-squares treatment of the plotted data.

+0.00140.

(7)

(for STS42) P = 0.l + M N

(8)

where - M=0;

D O < 0 . 1 ppm; N = 0 . 2 1"/100; T < 100°C. M = ( D O - 0.1)/0.1; 0.1 < D O < 0.2 ppm; N = 0.2;

-

100 < T < 2 0 0 ° C . - M = 1.0; D 0 > 0 . 2 200)/100; T > 200 o C.

ppm;

N = 0.2 + 0.4 ( T -

(for SFVV3) P = 0.1 + M N

a- 1.0

i

.E

I

i

J

h 0.6

w 0.4 t3 o) I~ 0.2

(9)

STS42 (A333-6) } in Water / Triangle or Saw Tooth / R ~ = - - I , Eta=0.006 j

~ o.8i

I

O [] O

DO (ppm) 8 0.2 0.1 0.01

/

I'' f " 0

(3)

-0.568

)

The index P represents the dependence of fatigue life on the strain rate is plotted in figs. 11 and 12 against test temperature, and in figs. 13 and 14 against dissolved oxygen content. With a view to conservative prediction of fatigue life, the highest values obtained from tests have been adopted for P, as indicated in figs. 11 to 14 by the dashed lines, from which empirical expressions can be derived for P as follows:

c

Consequently, the ( - N curve for tests in air at room temperature can be considered to represent the basic fatigue curve for the type of steel in question. The basic fatigue curve is expressed by the generalized c - N curve relating the strain amplitude flaA to fatigue life N25A:

(6)

t

c~

[] I

I

A I

I oo 2oo Temperature (°C)

.O

[]

~

o

///

A

I

300

Fig. 11. Strain rate dependence factor of fatigue life vs. testing temperature for STS42 carbon steel.

M. Higuchi, K. /ida 0.5

,

I

/

Fatigue strength correctionfactors .... n

i

,

SFVV 3 (A508-3) SFVV3 [__ Watel in Water l l A0 0.4 Triangle Triangh or Saw Tooth I ~ b. R~=-R ~ = - - I , ~ta=0.006 17 0.3 / I- . . . . .

~ 0.2 /

/

/

i

10° 0.2

001201

i

.--o

h

[~

o

n

Triangle

I

Ji R E = - I ,

I

'

" ' ' ....

,

.

I or

Saw

Tooth

Eta=0.006 Value

C "U C

0

a

# m

........

lio Water

Estimated

i

/

........

[s vv3
(3.

n

299

/

O.

n,' lO_X

121

121

A

~

t-

0

I

I

I

i

300

I O0 200 Temperature (=C)

2 ,HI

Fig. 12. Strain rate dependence factor of fatigue life vs. testing temperature for SFVV3 low-alloy steel.

,,

O3 ,

10-2

J i P ....

I |0

. -1

,

, , ....

I

. * .,.

100

10 l

Dissolved Oxygen C o n t e n t (ppm)

Fig. 14. Strain rate dependence factor of fatigue life vs. dissolved oxygen content for SFVV3 low-alloy steel. where - M=0; DO 0 . 2 ppm; N = 0 . 1 7 5 + 0 . 0 7 5 ( T 200)/100; T > 200°C. In the above expressions, M and N are coefficients depending respectively on the dissolved oxygen content and on the test temperature. The abrupt changes affecting the variation of P at 100 and 200 ° C and at 0.1 and 0.2 ppm cannot at present be explained from physical grounds, but this behavior has been borne out by clear experimental evidence, and this fact should substantiate the validity of eqs. (8) and (9) for all practical purposes.

....

i

L

Q"

u.

8

,

,

, , ....

J Temp.

u

('C)

[ Z3 [

290

O[~

250

.

[

,

• , ....

u

. . . . . .

STS42(A333-6)

J i n Water -]

Triangle

or Saw

Tooth

lo° i~_l___j_~o___ J m , = - , , Eta=0.00S IV l =oo I . . . . . . . . . . . . . . . . . . . .~

, []

._~ ...........

c

"~ lO-Z - <) (,1'}

',,,,I

,

,

,,,,,,1

,

, ,t,,,,I

A J ,i,

10-2

IO-i 10o 10z Dissolved Oxygen C o n t e n t (ppm)

Fig. 13. Strain rate dependence factor of fatigue life vs. dissolved oxygen content for STS42 carbon steel.

Using eqs. (6) to (9), it should be possible to make conservative predictions of the fatigue life to be expected of various types of steel under different strain rates and environmental water conditions. In what follows, the validity of such predictions is verified by comparison of the predicted curves with experimentally determined data. For the case taken as an example of environmental water containing 8 ppm oxygen, the predicted curves are shown together with the experimentally determined data in figs. 15 and 16 respectively for STS42 at 250°C and for SFVV3 at 290°C; strain rate is the variable parameter. All the predicted curves fit the experimental plots quite well, except in the range of very long fatigue lives. In figs. 17 and 18, the predicted fatigue life is plotted against experimentally determined data for all combinations of testing and environmental conditions. The plots fall fairly well along the 45 o equivalence line, although a slight tendency towards over-conservativeness can be seen for higher temperatures (250°C and above), and for water containing oxygen around 0.2 ppm. This is also the case with SFVV3 at low strain amplitudes (resulting in fatigue lives above 105 cycles). The higher than expected fatigue strength shown at higher temperatures can be attributed - - as already mentioned - - to dynamic strain aging at blue brittleness temperatures. Note that the case of 0.2 ppm oxygen, for which a similar overconservative prediction is seen, corresponds to the region in figs. 13 and 14 where the strain rate dependence factor P was observed to undergo a transition, and that the higher value of P was already adopted

M. Higuchi, K. /ida / Fatigue strength correction factors

300 10-1 ~

,,,11,,,l

, .,,I ...

I

, ,,, ....i

IL

#

L

"X.fo

"~%

/ [-

®

10 -2

, ,,i ....i

1 ,,, ....i

x,~ ,

L %, "<.'. lo~ez ""~-. I @,o~, "L'-, L-~Z

•

,,?

I Strain Rate (%/s) l j

I

I

II 0.4

0.4/ I

~

,L 0.01

0,4 I 1

• 0.004 = 0.00, • o.ooo,

0.4 I 0.4 1 ] o,4 i I

DO=8ppm

/ R6=--I

ET

I L_____"-.

, ,,i,,,~.l

(A333"6) ] [ 250C Water | -STS42

~'C / J

-0.472

".

(1)

10-312-

Predicted E - N

~_

Relation:

-.~

eta=0' 231 (Nzsp" @T'/2)-0"472+0' 00108

_~

, i iliilil

4x10-41

10 z

i i lllllll 10 2

i i ilillil 10 3

Experimental

and P r e d i c t e d

Fig. 15. Predicted fatigue curves, compared with

, , ~l~,,,i

, , ~li,H I

Fatigue

I I lllllll

106

107

L i v e s Nzs (Cycles)

4.2. Fatigue strength correction factors Ke, for environmental effect In the ASME design fatigue curve, the safety margin that is provided does not appear to account for circumstances in which there is a significant reduction of

i i rllill I

i i tllllT ~

i i il,H q

i f ,iii11

?~O~Z_~/~,

SFVV3 (A508-3) 1290°C Water

~.O~-~-j~¢

IOO=8ppm

o

Ira=-, o

~ 0.004 o,4 J

~"~%"~', \--~\ "~,,,

® "10

I I llllll[

10s

experimentally determined plots, for STS42 carbon steel, at 250 ° C.

for this oxygen content for the purpose of predicting the fatigue life. The method proposed here for predicting fatigue life can be considered to provide a generally conservative correction to account for fatigue strength reduction by strain rate and environmental variables.

10- ]

l I lllllll 104

~ ~ ~ x ~ x

10-;

Strain Rate (%/s) iT ~C I

.

0.4

0.4 I

~'ta=0.419 N25 + 0 . 0 0 , 4 0 (RT, Air)

`

c"

m 10_ 3

Predicted

E-N

Relation: • -I/3.-0.568

~'ta=0.419 (N25P" £T

4x10-4 101

i I 1Ii1111

102

Experimental

i I }liitli

)

I

+0.00140

i II11111

103 and P r e d i c t e d

l I tltili[

104 Fatigne

l0 s lives

i i ililitl

1 l

lllll

106

107

N2s (Cycles)

Fig. 16. Predicted fatigue curves, compared with experimentally determined plots, for SFVV3 low-alloy steel, 290 o C.

301

M. Higuchi, K. lido / Fatigue strength correction factors I0~ STS42 (A333-6) I in Water /Triangle or Saw Tooth /

-~10 !

the corresponding expression for tests at high temperature in water becomes

. / ' 4[>/~'

j

a.

,=.,

<~y

Z ¢~ 104

B

Temp

o

~-

0.4

290

•

,~/

"~ 103 LL

o

Z

102

Q,.

-6--

~

l

~---

, t .......

i,,,,I

io 2

',~A = (%aW -- C ) fPB + C.

, , ,i,,..l

io a

Experimental

, , ,i,,,,[

io ~

,,o o,, 0.01 I-8

Fatigue

0.2

20 t

, , ,i . . . . . . .

1o ~

(11)

Ken

l-8

E taA £ taW

( taA - - ( taW

-- 1 +

£ taW

l*

. IO ~

L i f e N25 ( c y c l e s )

(12)

The correction factor K¢n is defined as the ratio of fatigue strengths between that for room temperature in air and that for high temperature in water:

t-s

1-8

•

io;

I--8

o.2

100

IO

From eqs. (10) and (11),

O. I o.ou

2oo ~5

0.2 0.1

o,

250

; ~ m

+ c.

o o,

_1 0

)

Eta w = . 4 (

•

1o 7

= 1

.

- 1)(,.w

- c)

Ctaw

Fig. 17. Predicted vs. experimentally determined fatigue life values for STS42 carbon steel

= 1 + ( i ~ P - - 1)(1 - c-~Cw)

fatigue strength due to environmental conditions. To account for this reduction in the case of high-temperature water environments, an attempt has been made to derive a correction formula in the form of a factor Kc, which can be applied to the basic fatigue curve prescribed in the ASME code for air at room temperature. Expressing the generalized fatigue curve for tests at room temperature in air by ',aA = A(N25) s + C,

(10)

(13)

where C is the presumed fatigue limit that can be derived individually for a given material by applying eqs. (6) and (7). The product of C and the elasticity modulus of the relevant material was found to be dose to the value of design stress intensity S m ( = 206 MPa for STS42, and 277 MPa for SFVV3). Hence the correction factor is given as follows: (For STS42) Ken = 1 eta £ 0.00108 Ken

=1

+ ('T

0"472(0"1+MN, -

1)(1

0.00108)(ta

(14)

eta > 0.00108 1

0

6

~

( F o r SFVV3) Ken = 1 eta < 0.00140 K . . = 1 + (Cr °'sss(°A +Mlv) - 1)(1

0.00140 / E ta

(15)

]

(ta > 0.00140 °

I°'k

.7 I , . o 0('C) . o(ppm) o _/<~WI~

/ "~

Temp.

"

O

/

~-

v

/

/

~

u

10t 101

102

DO

i A

1-8

I~,

2oo

('c) (ppm) l e -

~

tO2

~

9~A

103 Experimental

104

o.5 ~ -

---

o.~ I~ °.L I ~ u.uu

105

I Ig

o.2

o.i

o.ol 1-s

-

o.~,

,oo i-e 0.2

106

107

F a t i g u e Life N2s ( c y c l e s )

Fig. 18. Predicted vs. experimentally determined fatigue life values for SFVV3 low-alloy steel.

where the values of M and N are derived for STS42 and S F W 3 respectively from eqs. (8) and (9). Multiplying by the value of K=, - - derived using eq. (14) or (15), whichever is applicable - - the given strain amplitudes applied in the present study result in t: .~ equivalent plots marked in the fatigue diagrams of figs. 19 and 20 respectively for STS42 and SFVV3. With few exceptions, the plots are seen to fall along the basic fatigue curve, or else deviate toward the higher strength (i.e. conservative) side of the curve. This demonstrates the validity of the correction factor Kc, proposed here for bringing the predicted fatigue life plots onto the basic fatigue curve given in the ASME code.

M. Higuchi, K. /ida / Fatigue strength correctionfactors

302 10-1

........

~

........

~ • m-

""~

•

"=•

STS42 (A333"6) in Water

[r I,r,.o,,. or I

"~ 10 -3

/ t ~=-' .

101

Med,om .oe

.

RT Air

o

150

02 001

~

100

02

.

°

]

1

C* O

~,t~ ....

l0 s

o

'-'

ASME Best ~ . m ~ l

I

.

~" 200 L~

~~ °

- "~1"~.'9. ""

- .... .-_~-I

E

~. 100

11

[] 0

f0 s

Fig. 19. Equivalent plots of fatigue test results derived applying correction factor K¢. to experimentally applied strain amplitude compared with basic fatigue curves, for STS42 carbon steel.

4.3. Mechanism of fatigue strength reduction in high-temperature wa ter enoironments The concurring factors normally considered to account for a reduction of fatigue strength under severe environmental conditions are (a) general corrosion, (b) pitting corrosion, (c) stress corrosion cracking. Of these factors, general corrosion evenly covering the whole specimen surface can be ruled out, considering the high purity of the water used in the present experiments (deionized water with very small oxygen contents), and the short duration of the test.

r-.~

~T

L

¢

t

........ [ ' Temp. DO

I ........ r Medium Line

...r

"'.~\

[ i~c, i,ppo,

/

"..

L~J

11-8 0.5

o, 0,15

290

t -

"'.~ "'..'~ ®

1~2 Io I

•" ' , ~ . CI~<~ n

' ''"'1 ....... Temp. DO

,o) (opm)

~

10-2

R~=--I I ~ '

I I

10-3i ........ I |0 ! 102

........

l 103

-)%

........

t 104

(ppm)

There remains the possibility of pitting corrosion. Figure 21 is based on data reported by Mizuno [8], indicating the region in terms of temperature and dissolved oxygen content in which pitting corrosion was observed to occur on unstressed coupons• The region marked by a dashed line, representing that in which fatigue strength reduction was observed in the present study, only overlaps that of pitting corrosion in a narrow zone around 250°C in the range of high dissolved oxygen content• However, in the case that a load causes plastic strain is slowly repeated, the region of pitting corrosion would presumably be wider than in the present instance, and in such case pitting corrosion might to some extent contribute to fatigue strength reduction. Strain-rate sensitive stress corrosion cracking in carbon and low-alloy steels in high-temperature water

0.3

SA533-B A 150~C [ SSRT Test ~ 2O0C n Pure Water 250 C © 280 c

o.ol ,-n

~ra

150

02 n',

~

,oo

,-s

o,

-"""-.m

........

10 l

100

,oo o, 0.1

~,

ASME. Best Fit Curve

10-]

, , ,hll.l

Fig. 21. Region susceptible to fatigue strength reduction compared with that susceptible to pitting corrosion, for steel in deionized water.

6(]

i

, . .i.,,.1

Dissolved Oxygen

E~10-2

i sFvv, ~,,o~,1

i . , i....i

i-s

~

I o.i ~ I o.ol ~

" ~

•

Pitting

Experimental Fatigue Life N2s (cycles)

10-11 - . . . . . . . . L•

o

"-.

l0 s

104

Pitting on SA333 - 6 O No ) Few • Many {81 (after Mizuno) Degradation in Fatigue on STS 42 [ ] No [ ] Slightly • Much

I-8

2oo r 0 2

*

%'%.*

,oothl

102

250 (I --8) 0.2 ' O.i

-~-

/

Ixt] ID • O 13 Fatigue Degrade \ O O [] [3~~11 • IB

Temp(~c) p~Om

•

°°

I

DO

I --8 O (re) L (ppm) 0.4 [] 290 0.2 ~

~

I

.~

•

-~ O ~m~@

N-2 •~ I

"

Ld

Temp!

~" ' " ' . ~

• g

~"

300

I

I 105


I

,. \

L

'

?"

......

,, 10e

"~ 2(] nt

\

o.

~k.•..o=

Experimental Fatigue Life N2s (cycles)

Fig. 20• Equivalent plots of fatigue results derived applying correction factor K¢n to experimentally applied strain amplitude compared with the basic fatigue curves, for SFVV3 lowalloy steel.

%

'

o'.4

'

o'.8

'

1.'2

Dissolved Oxygen

Fig. 22. R e d u c t i o n

'

t'.e

'

2'.o

'

(ppm)

o f area vs. dissolved oxygen c o n t e n t w i t h

testing temperature as parameter for SA533-B [12].

M. Higuchi, K. lida / Fatigue strength correction factors

303

Appendix 1 Strain controlled fatigue test results Material: STS42, Environment: Water

T

DO

( o C)

(ppm)

290

8.0

~T (%/s)

~¢ (%/s)

0.4

0.4

0.01 0.4 0.2 0.1 0.01 250

20.0 8.0

0.01 0.4 0.01 0.4 0.01 0.4 0.01 0.01 0.4

0.4

0.04 0.01

0.004 0.001

0.2

0,1 0.01 200

8.0

0.0001 0.4 0.01

0.01 0.4 0.01 0.4 0.01

1.0 0.2

0.4 0.01

0.01 0.01

0.4

Material: S F W 3 , Environment: Water Cta 0.006 0.003 0.006 0.003 0,006 0,006 0,006 0,006 0,006 0,006 0,006 0,006 0,018 0,010 0,006 0,004 0,003 0,0025 0.002 0.006 0,010 0.006 0.004 0.003 0.025 0.002 0.0019 0.010 0.006 0.006 0.003 0.0025 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.0017 0.006 0.003 0.006 0.006 0.0017 0.006 0.006 0.006

N25 (Cycles) 2065 8460 207 938 377 2530 430 3140 1768 3060 1896 544 221 523 1930 4625 14100 56400 97000 938 247 418 994 1840 3700 7700 > 42343 93 255 112 370 545 50 4220 530 452 515 2126 4520 2239 2800 73220 1060 13220 1725 4280 237000 1664 2680 1574

T

DO

~T

~C

( o C)

(ppm)

(%/s)

(%/s)

290

8.0

0.4

0.4

8.0

0.01 0.004

1.0 0.5 0.2

0.01 0.01 0.4 0,01 0,01 0,01 0,4 0,01

0.15 0.1 0.01

200

8.0

0.2

0,4 0,01 0,4 0,01 0,01 0,4 0.01 0.01 0.4 0,01 0,4 0.01 0.4 0.04 0,01 0.004 0.0004 0.4

0.1 0.01

0.01 0.01 0.4

0.01

0.01

1.0

0.4 0.01 0.4 0.01

1.0 0.3 0.2 0.1 0.01 150

100

8.0 8.0 1.0

0.2

0.4

0.4

0.4

ta

0.006 0.006 0,004 0.003 0.0025 0.0024 0.0022 0.002 0.006 0,006 0.004 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0,006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.006 0.002 0.006 0.006 0.006 0.006 0.006 0,006 0.006 0.006 0.006 0.006

N25 (Cycles) 1660 1920 5702 8080 25450 > 400000 355771 > 380000 780 355 1383 788 1190 3625 2054 1924 1990 3540 2343 3540 3460 1880 1845 1020 1100 2680 2115 2065 4160 3218 1420 720 1850 1820 930 878 542 3004 2745 117400 1043 1376 2130 2240 1556 1745 2480 1177 3400 1400

304

M. HiguchL K. lida / Fatigue strength correction factors

containing oxygen is characterized by a lowering of elongation and reduction of area in slow strain rate tests (SSRT) that vary with strain rate, temperature and dissolved oxygen content [9-12]. The data reported by Shoji [12] are reproduced in fig. 22, and show how the reduction of area is lowered with increasing content of dissolved oxygen and with increasing test temperature. The range of temperature and of dissolved oxygen content in which the reduction in ductility occurs is quite similar to that in which the fatigue strength reduction was observed in the present study. This suggests that strain-rate sensitive stress corrosion cracking was probably a strong factor in the mechanism of fatigue strength reduction in high-temperature water environment. This type of stress corrosion cracking is considered to be caused by the alternate fracturing and healing of a protective oxide film on the surface, which is accompanied by exposure of the new generated crack surface and its active dissolution. At relatively low strain amplitudes, the protective film can be considered to be maintained intact, thus preventing reduction of fatigue strength in this range of strain amplitudes. 5. Conclusion Axial strain low-cycle fatigue tests were conducted on carbon and low-alloy steels in an environment of

high-temperature deionized water containing controlled amounts of dissolved oxygen. The effect on fatigue strength and life was examined as a function of the variables of strain rate, testing temperature and dissolved oxygen content. The principal results obtained from the tests are as follows: (1) With both carbon and low-alloy steels, a significant reduction of fatigue strength was observed in the h i g h - t e m p e r a t u r e deionized o x y g e n - c o n t a i n i n g water, compared with the corresponding values from tests in air at room temperature. The reduction of fatigue strength was observed in general to be accentuated with increasing applied strain amplitude and a reduction in strain rate. The extent of fatigue strength reduction was greater in the case of the carbon steel compared with the low-alloy steel. At small strain amplitudes (below 0.2%), the fatigue strength was not reduced to any appreciable extent by differences in the conditions of strain or of environment. (2) The fatigue strength reduction observed in hightemperature oxygen-containing water is considered to be attributable to a mechanism analogous to that of stress corrosion cracking seen in slow strain rate tests. (3) For both types of steel and for various combina-

Appendix 1 (continued) Material: SFVV3, Environment: Water

Material: STS42, Environment: Water T

DO

iT

gc

( o C)

(ppm)

(%/s)

(%/s)

150

8.0

0.4

0.4

0.2

0.01 0.4

0.01 100

8.0

0.2 20

8.0

0.01 0.4 0.01 0.4

0.01 0.4 0.01 0.4 0.01

0.4

0.4

eta

N25

(Cycles) 0.006 0.004 0.002 0.0017 0.0015 0.0014 0.006 0.006 0.0017 0.006 0.006 0.006 0.006 0.006 0.0017 0.006 0.006 0.006 0.006 0.006

1840 7325 46900 54950 309800 398000 948 3070 119500 1580 3260 2298 2980 2570 64050 1065 2325 1385 2425 2214

T ( o C)

DO (ppm)

gT (%/s)

gC (%/s)

~ta

N25 (Cycles)

M. Higuchi, K. lida / Fatigue strength correctionfactors tions of strain rate and environmental conditions, an exponential relation was found to relate fatigue life with strain rate, expressed by the formula N2sw (~T) p for the range of strain rate ~T from 0.0001 to 0.4%/s. The straight lines drawn through the plots for these two values of strain rate on a log-log diagram extrapolated to 1%/s strain rate indicated a tendency for all lines to converge towards the same basic value of fatigue strength obtained from tests in air at room temperature. From this observation, the expression N25w = N25A (~T) p was derived. The index P in this expression - - termed the strain rate dependence factor of fatigue life, and which represents the slopes of the straight lines - - proved to be dependent on the testing temperature and on the dissolved oxygen content in a complex way. (4) The dependence of the index P on testing temperature and on dissolved oxygen content was determined for the two types of steel studied, and the relation was represented by parametric equations. Using this relation, it should be possible to estimate roughly the fatigue life of these steels in high-temperature water up to 290°C under various combinations of strain rate and environmental conditions. Expressions have been derived for predicting fatigue life, formulated so as to provide conservative estimates for component design. The estimated fatigue life data were compared with experimentally determined values and found to be sufficiently accurate and conservative for practical applications. (5) A fatigue strength correction factor for the environmental effect, Ken, is proposed for use in converting the values of fatigue life in air at room temperature to the values of fatigue life that would be relevant to service in oxygenated high-temperature water. The value of Ken is dependent on type of steel, rate of strain applied, environmental temperature and content of oxygen dissolved in the water.

The authors express deep appreciation of their contribution of the resulting test data, and of the kind permission given by Dr. Shohachiro Miyazono of the same Institute to publish these data.

Nomenclature A,B,C DO Ken M

N

~5

~sA

/•25P N2sw

P

R LAR

Sm Acknowledgements The main substance of the present paper was reported at the 6th International Conference on Pressure Vessel Technology, Beijing, September 11-15, 1988. Of the fatigue tests performed in the present study, those on SFVV3 steel were conducted by the TFC Subcommittee of the Nuclear Research Committee under the Japan Welding Engineering Society, and sponsored by the Japan Atomic Energy Research Institute.

305

T eta CtaA

Ct a w

ic iT

Material constants in the expression A(N25)s + C (where C the presumed fatigue limit) Dissolved oxygen content (ppm) Fatigue strength correction factors for environmental effect Coefficient - - depending on the dissolved oxygen content - - in the expression P = 0.1 + MN Coefficient - - depending on the testing temperature - - in the expression P = 0.1 + M N Experimental fatigue life in strain-controlled fatigue test (cycles); suffix 25 means the number of cycles to a 25% drop in tensile peak stress (at the maximum tensile strain in hysteresis loop) from the maximum value in the characteristic cyclic curve of tensile stress in a test Experimental fatigue life obtained in straincontrolled tests in air at room temperature (cycles) Predicted fatigue life in high-temperature water (cycles) Experimental fatigue life obtained in straincontrolled test in high-temperature water (cycles) Strain rate dependence factor of fatigue life (exponent index in the expression REAR= (~T) P) Fatigue life ratio ( = N2swfN2s A obtained from tests under equal strain amplitude) Design stress intensity value prescribed by the design code Temperature (°C) Applied strain amplitude Strain amplitude applied in tests in air at room temperature Strain amplitude applied in tests in high-temperature water Strain rate in falling phase of straining (%/s); constant in the experiment Strain rate in rising phase of straining (%/s); variable in the experiment

306

M. Higuchi, 1(- lida / Fatigue strength correction factors

References [1] D.A. Hale, S.A. Wilson, J.W. Kass and E. Kiss, Trans. ASME J. Eng. Mater. & Technol. 103 (1981) 15-25. [2] D. Weinstein, EPRI Research Project 1248-1 Final Report No. NP-2406 (1982). [3] T.A. Prater and L.F. Coffin, Trans. ASME J. Pressure Vessel Technol., 109 (1987) 124-134. [4] M. Higuchi and H. Sakamoto, Trans. Iron and Steel Inst. of Japan 24 (1984) B196. [5] M. Higuchi and H. Sakamoto, J. Iron and Steel Inst. of Japan 71 (1985) 101-107.

[6] K. lida, H. Kobayashi and M. Higuchi, IIW XIII-1164 (1985). [7] T. Mizuno, S. Pedneker, Z.S. Smialowska and D.D. MacDonald, NACE CORROSION 81 (1981) Paper No. 21. [8] H. Uemura and T. Kawamoto, Boshoku Gijutsu (Corros. Eng.) 30 (1981) 276. [9] J. Kuniya, I. Masaoka and R. Sasaki, Boshoku Gijutsu (Corros. Eng.) 32 (1983) 264. [10] J. Kuniya, M. Kanno, I. Masaoka and R. Sasaki, Boshoku Gijutsu (Corros. Eng.) 32 (1983) 649. [11] T. Shoji and J. Congleton, private communication.