*"* *

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Commission of the European Communities

COST

physical sciences Stress corrosion cracking and corrosion fatigue of steam-turbine rotor and blade materials

Report EUR 13186 EN

Commission of the European Communities

COST

Stress corrosion cracking and corrosion fatigue of steam-turbine rotor and blade materials M. O. Speidel,1 J. Denk,2 B. Scarlin2 1

1nstitute of Metallurgy CH-Eth, Zürich 2

Asea Brown Boveri CH-Baden

Edited by

J. B. Marriott Commission of the European Communities

Contract No: ECI COST-0010-CH (CH)

PARL EURO?. BibÜA

Directorate-General Science, Research and Developrrjetø c
(0

200

a) NaOH, 1 0 0 ° C , tensile b) NaOH, 160°C, tensile c) demineralized, aerated H 2 0 8 0 - 1 0 0 ° C , tensile d) condensing pure steam, 9 5 ° C , bent beam, microcracks

!

I

10'

10 J

10«

time to failure, [h]

Fig. 4

Effect of stress and environment on time to stress corrosion crack initiation in typical steam turbine rotor steels, /Ref. 12, 24,26,31/. Note that in water and condensing steam cracks are observed after several thousand hours if the specimens are stressed near the yield strength and if the environment permits pitting corrosion to develop around MnS-inclusions. After 12000 hours, microcracks are observed at very low stress levels in pure condensing steam. Lines a and b refer to Figures 5 and 6.

-28-

10=

1000 800 CO Q.

600

V>

CO CO

400 SCC, steam turbine rotor otor steels

^w

•""*"

200 - • 35%Na0H, 160°C ■ 30%NaOH, 200°C 10z

10

10 3

time to failure, [h]

Fig. 5

Stress corrosion cracking of steam turbine rotor steel s in concentrated aqueous NaOH sol utions. Effect of appl ied stress on the time to failure of originally smooth thin specimens. Different investigations under sl ightl y different conditions yiel d similar results / 2 4 , 3 1 / .

-29-

10"

1000

800 (O Q.

600 30-35NaOH, 1 6 0 - 2 0 0 ° C

(A

tø

400

V)

200

-

35%NaOH, 1 0 0 ° C 30%NaOH, 1 2 0 ° C _L 10z

10

10 3

time to failure, [h]

Fig. 6

Lower tem peratures result in longer tim es to failure by stress corrosion cracking compared to the data in Fig.' 5,/24,31/. Note in both, Figures 5 and 6 that the tim e to failure increases with lower applied stress.

■30-

10"

Stress

Corrosion

35% NaOH + 3.5% NaCI tensile

specimen

O ground

100

o- 80 DC Q.

6^

#

shot

k. +-»

w

boiling

peened

r\

'N V

40 /

J

S1 20 35% NaOt 160"C

of

123°C

surface, D3

X / o N

60

temperature

A V

\j

C

w o

at

Tests

( 5.5 mm)

v \

Cracking

'

n

u

^^

«

-m

no failij r e 10'

10'

10°

10* time to failure h

Fig. 7

Stress corrosion specimens from hvo different COST programs show similar times to failure when tested in hot concentrated NaOH solutions, Ref. /12,24/. Shot peening considerably lengthens the time to failure, Ref. / 1 2 / .

-31

10a

i

3 ID O (D

1.0

­

0.8

­

i

­

_

0.6 0.4

:

CERT

O

c o o 3 ■o a> \ e 0 s

õc3

a a> (0

o c

—

0.2 i

0 1i

1.0

8

0.6

O 3 ■o 0

0.4

¡M

io" 5

I

­

v

0.8

p

io­* strain rate, é [1/s]

V/

­

i

io' 7

CERT

0.2

­

n u IO"

8

i

i

107

106

10S

strain rate, é [1/s]

Fig. 8

Schematic representation of two possible effects of strain rate on ductility in CERT ­ SCC tests.

• 32­

1.0 0.8

o

0.6

(Q

«

w (0 i ^

0.4

O

c o +* o3

■o

a>

0.2 J

0 10

*m

N.

IO"7

2Cr1Ni, 2Cr2Ni steam turbine steels­ CERT 1 n NaHCC­3+1 n Na 2 C0 3 ­675mV (SCE), 7 0 ° C I 10*

10 a

strain rate, ë [l/s]

C

O 3

O

»

(0 0> k (D O C

o o3

TJ 0) k

1.0 0.8 0.6 3.5NiCrMoV steam turbine steels. CERT

0.4

1 n NaHC0 3 +1 n Na 2 C0 3 _|

0.2 0 10"°

J 10"'

­ 675 mV (SCE), 70°C I 10"°

10*

strain rate, é [1/s]

Fig. 9

Both effects shown in Fig. 8 are observed in reality, Ref. / 2 7 / , but the meaning and the use of such data is not clear, other than that they demonstrate qualitatively a "low susceptiblity to stress corrosion cracking".

■33­

1

1

1 1 1 1 111

1 i i 11 u p

1

­

i

I I I

• D issolved Oj of outlet water

o ­

30 % ICSCC S TCSCC + 70 % ductile

T

F ­1U0

°"° •° 1 t

_i

1



< \7

­ÜUÜ

m O ­300 0.

i

i

i i 11 i i

50 % TCSCC + 50 % ductile »­«»O (high conductivity) Î M}

O D issolved Oj of inlet water

_

niri

-

I

•

¿-¿6

i

30 % TCSCC + 70 % ductile

0

-

1 1

1­

-

­100 -­

•

.

•

§> °

: • o

°

o

°

Ol

i

-

ì­o

U0 % ICSCC E TCSCC + 60 % ductile

-

1 % ICSCC + 99 % ductile

­500 ­

Î6

T 4 i 10

Fig. 10

'

1 1 1 1 1 ll 1 100 DISSOLVED 0 2 (ppbl '

i i I ll 1 1—L­ 1000

I

1—l I I I iJ 10000

Effect of dissolved oxygen content on potential and CERT stress corrosion cracking of steam turbine rotor steel (3.5NiCrHoV) in high­purity water at 160°C, Ref./25/.

­34­

LIJ

X >

I 99% (tactile

+ 100 0

40% IGSCC

50% TGSCC

-

E

-100

v/y/y/r'^

2

-200

Y////

^»

c



"i_ CO

3/2

(D

240

k. +j

]

u

Effect of stress intensity on the growth rate of stress corrosion' cracks in various steam turbine rotor steels, exposed to water at 160°C. c K .

■36-

—i—i—i—i—i—i—i—i—r SCC in hot water, 1 0 0 ­ 2 8 8 ° C threshold stress intensity, K |SCC ,austenitic steels v iscc • K.­.­.ferritic low alloy steels O Kv. ™ , f e r r i t i c 12% Cr steel iscc

CM co

'E z o o CO

20

* ~

5* +■>

"5> C d)

Ï

15

0)

(fl CD t­

CO

o .n co

5

­

CD

( V = UK20, 95°C, steam) I

0

I

I

400

I

800

I

I

1200

L

1600

2000

yield strength, R p0 2 , [MPa]

Fig. 13

Stress corrosion threshold stress intensities, of various steels in water, Ref. /IO, 17, and 27/. Note that Ki s c c i s between 20 and 10 MN.m­3/2 fe MPaVm).

­37­

stress intensity, K t 50

10

E 10

100

[ksiViñj 200

150

—i—i—h—i—i *­!—i—r—i—r —I— steam turbine rotor steels, SCC, in H 2 0 • 3%Ni steel - ■ 2Cr­1Ni steel ▲ "clean steels",specially alloyed steels ­

,_L-

ra IU

10

u c COI«'

10

5 o

10

288°C,0 2 "374ppm,C0 2 ­4.5ppm

10'

ra

r*

10

h.

o>

¿£ U (0

10

«

10'

o u

10

u ra o "55 o o u (0 (/> a>

V) 0) 10 100

stress intensity, K j

Fig. 14

S o O)

160 o C,0 2 ­77ppm,C0 2 ­ 1.4ppm ^ T ^ * ■ A , ­ ­

to (0

O)

200

[MN-m

240 3/2

(0

J

Effect of temperature and stress intensity on the growth rates of stress corrosion cracks in steam turbine rotor steels, Ref. /IO, 17/.

■ 38­

temperature, [ °C ] 300

I

200

r­V \ . ■\\

I

10

10 ral­



1

io 10

c O

w io o

­ii

V'­Í'A

—

^i* g * á

1_

o Ü If)

­12

10

(A =D , 100°C) (V =UK20,95°C)

(A d)

_i

>_ 0)

100

10-13

i

i

i

l

i

200

i

240

stress intensity, Kz, [MNm" 3/2 ]

Fig. 18

Effect of stress intensity, oxygen, and carbon dioxide on stress corrosion crack growth rates in steam turbine rotor steels in water near 100°C. Main data base from Ref. /IO/ and / 1 7 / , but data from Ref. / 2 3 / , / 2 7 / , / 3 1 / agree very well with it.

­42­

10"

(Ol**



SCC. steam turbine rotor steels (Oli

1 0 * r­

• ■

H20,160°C H20,100°C

various oxygen contents

!" S

O O) .2 Ü (0

o 'iõ

s 1-

o o

(A= D , 80 "C) (V= UK20, 95 °C) i ' I I

CO CO

0)

10 0.00

0.20

a40

0.60

0.80

1.00

1.20

manganese in the bulk alloy,[weight percent]

Fig. 20

The manganese content of the steels has no influence on their stress corrosion crack growth rate. Steels according to Table I. The applied stress intensities range between 40 and 80 MN­m'3/ 2 . Note that data from Ref. / 3 1 / (D) and Ref. / 2 7 / (UK 20) agree with the main data base from Ref./10/,/17/.

­44­

co

10 » cg

—I 1 1 1 1 1 1 1 1 1— SCC steam turbine rotor steels

H,0, 160 °C) 2

.

.

„ various oxygen contents H 2 0, 100 °C )

$

O O)

o CO C

o '35 o w w O O

(fl CO

£

0)

(A=D,100°C) (V= UK 20, 95 °C) L _J I I I 1 L J 10 0.00 0.04 0.08 0.12 0.16

0.20

0.24

silicon in the bulk alloy, [weight percent]

Fig. 21

The silicon content of the steels has no influence on their stress corrosion crack growth rate. Steels according to Table I. The applied stress intensities range between 40 and 80 MN-m'3/2, Note that data from Ref. / 3 1 / (D) and Ref. /27/ (UK20) agree with the main data base from Ref./10/,/17/.

-45-

(O

I

IO"

-\—i—i—i—i—i—i—i—r steam turbine rotor steels

(0

io"8 h

SCC in 160°C water, various oxygen contents

S o O)

o

ni

c o » o o u

(A=D, 150°C) J

V) ID

0>

0.00

0.20

0.40

I

0.60

I

l_

0.80

1.00

molybdenum in the bulk alloy, [weight-%]

Fig. 22

The molybdenum content of the steel has no influence on their stress corrosion crack growth rate. Steels according to Table I. The applied stress intensities range between 40 and 80 MN-m" 3 / 2 . Data from Ref./IO,17/. Note the somewhat lower data from Ref./31/ (D), due partly to lower temperature.

-46-

­7

UTJ al­

10

i

1

1

1

1

r~

steam turbine rotor steels (0

S o o

SCC in 160°C water,

10

various oxygen contents

10

o (0

I o o ca co a>

W­h (A= D .iso'te)

A J 2.0

10 0.0

1.0

nickel in the bulk alloy

Fig. 23

I

L 3.0

ÏÏ 4.0

[weight­%]

The nickel content of the steels has no influence on their stress corrosion crack growth rate. Steels according to Table I. The applied stress intensities range between 40 and 80 MN­m"3/2. Note the somewhat lower data from Ref. / 3 1 / (D), due partly to lower temperature.

■47­

(O

"~I .­8

oll

10

1 1 1 1 1 1 1 1 1 1 SCC steam turbine rotor steels

A H O

¿ (O

2

1 6 0 °C )

' {various oxygen contents ■ H 2 0, 100 °C I

5 o O)

o

co o

o o

Ï

(A = D, 100 °C) (V=UK20,95°C) í­12 I I I I I L J L 10 0.000 0.002 0.004 0.006 0.008 0.010

0.012

sulfur in the bulk alloy, [weight percent]

Fig. 24

The sulfur content of the steels has no influence on their stress corrosion crack growth rate. Steels according to Table I. The applied stress intensities range between 40 and 80 MN­m'3/2. Note that data from Ref. / 3 1 / (D) and Ref. / 2 7 / (UK 20) agree with the main data base from Ref./10/,/17/.

­48­

'co'

1

col**



(0

lñ13

i

0

20

i

i

40

i

i

60

80

100

120

stress intensity, K_, [MNm"3/2]

Fig. 26

The stress corrosion crack growth rates in the plateau range appear to be independent of the degree of temper embrittlement. Steel number 9 in Table I. The fracture toughness values are taken at room temperature. Pef. / 1 0 , 1 7 / .

■50­

10 /

■co

stress corrosion cracking 3.5NiCrMoV rotor steel H 2 0, Jp0°C, aerated

ik

£ io' (Ol*­

(O

corrosion fatigue with mean stress

Fig. 34

Schematic representation of the effect of mean stress and environments on the fatigue strength of steam turbine rotor and blade steels. Note that pitting corrosion should be avoided if acceptable fatigue strength is desired.

­58­

600

400

o. S

200 -

-200

-400 mean stress

Fig.35

mean stress

(MPa)

(MPa)

Comparison of the corrosion fatigue behaviour of a conventional 12% Cr-steel and the duplex steel developed in COST 505.

-59-

cyclic stress intensity range, A K [MN-m"3/2]

Fig. 36

Schematic representation of the effect of the cyclic stress intensity range on the growth rates of fatigue cracks in metallic material s exposed to inert environments, / 3 5 / , / 3 6 / .

■ 60-

0)

"õ >> ü

\

J=

z

true corrosion fatigue

< (D

5 o

environment

O)

o

(0

o

d) 3 O)

c

o

'55 o h.

o o

AKf

cyclic stress intensity range, A K [wiN-nf 3 7 2 ]

Fig. '37

In "true" corrosion fatigue there is a dear but limited acceleration of the growth rate of fatigue cracks compared to inert environments, /36/.

-61

O

>•

\O

stress corrosion under cyclic loads

r

decreasing frequency

i /

/

o