Metallographic Investigation on the Cladding Failure (2)

8/16/2019 Metallographic Investigation on the Cladding Failure (2)

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N U CLEA R EN G I N EERI N G A N D D ESI G N 16 ( 1971) 20 5- 2 22 . N O RTH - H O LLA N D PU BLI SH IN G CO MPA N Y

M E T A L L O G R A P H I C I N V E S T I G A T I O N O N T H E C L A D D I N G F A I L U R E

I N T H E P R E S S U R E V E S S E L O F A B W R

T a t su o K O N D O , H a j im e N A K A J I M A a n d R y u k i c h i N A G A S A K I

Reactor Materials Laboratory Japan Atomic Energy Research Institute Tokai-Mura lbaraki-Ken Japan

Received 2 February 1971

Samples were remo ved for m etal lograph ic analyses f rom one o f the c racked po r t ions of the m ult i- layer s tainless-

s teel weld over lay in the JPDR pressure vessel. Metal iographic s tructure and microch emical analysis were consis ten t with

each othe r in suggesting that th e fai lure was caused by the s ignif icant dep let ion in the chrom ium c onte nt in the weld

metal . The lack in th e co mp osit iona l balance was foun d to have or iginated at th e overlaying of th e f ir s t layer on the dis-

s imilar base metal . Morphological features o f the cracks indica ted the o perat io n o f s tress cor rosion cracking dur ing the

pr ocess of c r ack p r opaga t ion . The chem ica l comp os i t ion of the w e ld meta l , the r mal h i s tor y , t ens i le st res s and envi r on-

ment w er e judged to have f avour ed the occur r ence of c racking. Some of th e c r acks w er e f ound to have r eached the

base metal across the weld bou ndar ies and fur ther pen etrated into the vessel wall in the form of local ized cor rosion .

The a t tac k w as of p i t t ing- type cor r osion and i t w as cons ide r ed to have been acce le r a ted by the concen t r a t ion- ce l l

ac t ion due to the f or m at ion of a concent r a t ion gr ad ien t o f d i sso lved oxygen in w a te r w i th in the n a r r ow c r acks . I n

consid erat ion of the fu ture se cur i ty of the vessel , containing th e de fects in th e cor rosion-res is tant l iner , some possible

proble ms and fur ther necessary s tudies were discussed. The proba ble inf luence of the high - temperatu re aqueo us envi-

ronm ent o n the fatigue cha racter of the reacto r vessel was po inted ou t .

1 I n t r o d u c t i o n

C r a c k i n g f a i l u r e i n s t a i n l e s s - s te e l c l a d d i n g o f s t e e l

v e s se l s h a s b e e n r e c o g n i z e d t o b e o n e o f t h e t e c t /n i c a l

p r o b l e m s i n t h e d e v e l o p m e n t a n d m a i n t a i n a n c e o f l i gh t -

w a t e r c o o l e d r e a c t o r s [ 1 ] . J P D R , t h e J a p a n e s e e x p e r i-

m e n t a l b o i l i n g - w a t e r t y p e r e a c t o r , h a d e x p e r i e n c e d e x -

t e n s iv e c r a c k i n g f a i l u re i n i t s w e l d - o v e r l a y c l a d d i n g a t

t h e i n n e r w a l l o f t h e t o p h e a d . T h i s f a i lu r e w a s f o u n d

a f t e r 1 3 0 0 0 n e t h o u r s o f o p e r a t i o n . T h r o u g h v i su a l

o b s e r v a t i o n t h e c r a c k s a p p e a r e d t o b e l o c a t e d m a i n l y

a t t h e a r e a s w h e r e a l l t h r e e l a y e r s w e r e m a n u a l l y w e l d e d ,

t h e s o u n d a r e a s h a v in g b e e n m a d e b y t h e a u t o m a t i c

m a c h i n e w e l d i n g a f t e r m a n u a l l y w e l d i n g t h e i n i t i a l s in g l e

l a y e r . A s f a r a s t h e t o p - h e a d w a s c o n c e r n e d a l l t h e

v i s ib l e d e f e c t s h a d b e e n r e p a i r e d b y g r i n d i n g o f f t h e

d e p o s i t a n d r e w e l d i n g , e x c e p t t h a t a f e w t y p i c a l c r a c k s

w e r e l e ft u n r e p a i r e d f o r l a te r m o n i t o r i n g p u r p o s e s .

S i n ce t h e s a m e f a b r i c a t io n p r o c e d u r e w a s e m p l o y e d

f o r th e b o t t o m h e a d , w h i c h i s b e l o w t h e r e a c t o r c o r e

a n d d i f f i c u l t t o r e p a i r , t h e d e t e r m i n a t i o n o f t h e c a u s e

a n d m e c h a n i s m o f t h e c r a ck i n g h as b e e n c o n s i d e r e d

t o b e o f p r a c t i c a l i m p o r t a n c e c o n c e r n i n g t h e f u t u r e

m a i n t a i n a n c e a n d t h e s c h e d u l e d r e v i s io n fo r t h e p o w e r -

u p p r o g r a m * . D e p e n d i n g o n w h e t h e r t h e c r a c k i n g

r e s u l te d f r o m s o m e m e t a l l u r g i c a l c a u s e t h r o u g h a p u r e l y

m e c h a n i c a l p r o c e s s o r t h r o u g h i n t e r a c t i o n o f m a t e r i a l

w i t h t h e e n v i r o n m e n t s u c h a s st r e s s -c o r r o s i o n c r a c k i n g ,

t h e p o s s i b i l i t y o f e x i s t e n c e a n d / o r e x t e n t o f t h e c r a c k s

w o u l d b e d i f f e r e n t . A m o n g t h e s e v er a l i n f l u e n c i n g

f a c t o r s t h e s u s c e p t i b i l i t y o f a n a u s t e n i t i c s t a in l e s s

s t ee l t o t h e s t r e s s- c o r r o s io n c r a c k i n g i s p a r t i c u l a r l y

i n f lu e n c e d b y t h e o x y g e n c o n t e n t o f w a t e r a t e l e v a te d

t e m p e r a t u r e s [ 2 , 3 ] .

I n t h e J P D R u n d e r t h e s t e a d y - s t a t e o p e r a t i o n , t h e

o x y g e n c o n t e n t s a re a b o u t 0 . 3 a n d 3 5 p p m i n t h e

l i q u i d p h a s e n e a r t h e b o t t o m - h e a d s u r fa c e a n d i n

t h e w a t e r - s t e a m t w o - p h a s e f l o w a t t h e t o p - h e a d a r e a

r e s p e c t i v e l y .

T h e p r e s e n t r e p o r t i n c l u d e s t h e r e s u l t s o f a m e t a l -

l o g r a p h i c s t u d y o n c r a c k - s a m p l e s r e m o v e d f r o m t h e

t o p h e a d . D i r e c t e x a m i n a t i o n o f t h e m e t a l l o g r a p h ic

f e a t u r e o f t h e c r a c k s a n d s t r u c t u r e a s w e l l as t h e m i c r o -

c h e m i c a l a n a l y s i s g a ve s u b s t a n t i a l i n f o r m a t i o n a b o u t

* JPDR-II Power-Doubling Project.


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206

T. Ko ndo et aL Cladding failure in pressure vessel

Main steam

p p _ e j ~

w e t c a r b o r u n d u m p a p e r s u p t o 8 0 0 m e s h , a n d a c e t y l -

c e l l u l o s e r e p l i c a s w e r e t a k e n . A n y s u r f a c e e t c h i n g w a s

a p p l ie d o n l y t o t h e s a m p l e s r e m o v e d f r o m t h e v e s se l,

s o t h a t n o c h e m i c a l c o n t a m i n a t i o n r e m a i n e d w i t h i n t h e

c r a c k s t h a t w e r e t o b e l e f t in t h e r e a c t o r s y s t e m . T h e

r e p li c as w e r e e x a m i n e d t h r o u g h a n o p t i c a l m i c r o s c o p e .

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Fig. 1. Location o f the sam ple extraction.

t h e c a u s e a n d p r o c e s s o f th e f a i l u r e , a n d p r e p a r e d a

b a s i s f o r d i s c u s s i n g s o m e p o s s i b l e e f f e c t s o f t h e f a i l u r e

o n t h e s a f e t y o f t h e p r e s s u r e v e ss e l i n f u t u r e o p e r a t i o n s .

T h e a n a l y s e s w e r e d i r e c t e d t o t h e f o l l o w i n g p h a se s :

1 . F e a t u r e s o f t h e m e t a l lu r g i c a l s t r u c t u r e a n d c h e m -

i ca l c o m p o s i t i o n o f t h e w e l d m e t a l a t t h e f a il e d

a n d u n f a i l e d p o r t i o n s .

2 . T h e m o r p h o l o g y o f t h e c r a c k s a n d i t s r e l a ti o n

t o t h e t y p e o r n a t u r e o f f r a c t u r e .

3 . E f f e c t o f t h e c r a c k i n g o n t h e b a s e m e t a l.

2 E x p e r i m e n t a l

2 . 1 . S u r f a c e o b s e r v a t i o n

T h e s u r f a c e o f t h e f a i l e d c l ad d i n g w a l l a b r a d e d w i t h

2 .2 . S a m p l e e x t r a c t i o n

T h e l o c a t i o n o f t h e c r a c k s t o b e e x t r a c t e d w a s

r e la t iv e l y r e m o t e f r o m t h e n o z z l e p a r t a s s h o w n i n

f i g . 1 . A t b o t h s i d e s a l o n g a c r a c k d e e p g r o o v i n g s

w e r e c u t b y m e a n s o f a w a t e r - c o o l e d g r in d i n g b l ad e

t o le a v e a p i e c e o f a b o u t 8 m m w i d e a n d 2 4 m m l o n g .

U s i n g c u t t i n g s o f a b o u t 1 2 m m d e e p , t h is p o r t i o n

w a s c h il le d w i t h l i q u id n i t r o g e n a n d t h e n r e m o v e d

b y k n o c k i n g o f f w i t h a h a m m e r t o s e p a r a te i t i n b r i t-

t le f a s h i o n . T h e s a m p l e s s o r e m o v e d c o n t a i n e d 6 t o

8 m m t h i c k s t a i n l e s s - s t e e l o v e r l a y c l a d d i n g a n d 4 t o 6

m m t h i c k b a s e - m e t a l p o r t i o n s . A s l i g h t l y s m a l l e r

s a m p l e w a s a ls o r e m o v e d i n t h e s a m e w a y . A f t e r t h e

r e m o v a l o f t h e s e s a m p l e s t h e v e s s e l w a l l w a s g r o u n d

s m o o t h s o t h a t n o a p p r e c i a b l e s t r e s s c o n c e n t r a t i o n

r e s ul ts , b u t n o a d d i t i o n a l p r o t e c t i o n s u c h a s cl a d-

d i n g w a s a p p l i e d .

2 . 3 . M e t a l l o g r a p h i c o b s e r v a t i o n

T h e s a m p l e s w e r e s l i c e d w i t h a m e t a l l o g r a p h i c

c u t t i n g b l a d e i n c o o l i n g w a t e r i n t h e m a n n e r a s i l -

Reactor Vessel

Third Layer al l Surf ace

{ 3 0 8 L ]

S e c o n d L a y e r ~ ' ~ /~ • ~ I

[ 3 08 L } ~ - ~ l I

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Fig. 2. Sketch of the samp le removed from the inner wall of

the vessel . Each plane developed by the sectioning is marked

for identification.


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T. Kon do e t al . Cladding fai lure in pressure vessel 207

lus t ra ted in f ig . 2 . The p lanes developed by the s l i c ing

w er e mar k ed f o r i d en t i f i c a t i o n p u r p o s es an d t h e n eces -

s a r y m e t a l lo g r a p h i c t r e a t m e n t s w e r e a p p l ie d . T h e o t h e r

an a l y s e s , i n c lu d i n g mi c r o - ch em i ca l an a l y si s , h a r d n es s

t e s t in g an d r ep l ica - an d s can n i n g e l ec t r o n - m i c r o s co p i c

o b s e r v a t i o n s w e r e mad e o n t h e s ame p l an es s o t h a t a l l

t h e r e s u l t s co u l d b e c o n s i s t en t l y i n t e r p r e t ed .

S i n ce a w i d e r an g e o f e r ack w i d t h s w as ex p ec t ed ,

p a r t i cu l a r c a r e w as ex e r c i s ed o n t h e m e t a l l o g r ap h i c

p o li sh i ng f o r t h e p u r p o s e o f e x a m i n i n g t h e c r a c k m o r -

p h o l o g y . In f ac t t h e r e w e r e f i n e c r ack s w i t h t h e d e t ec t -

e d w i d th s b e l o w 1 0 0 0 A . T h e p r o c e d u r e e m p l o y e d

was as fo l lows :

S t e p 1 . F r o m 3 2 0 u p t o 1 2 0 0 m e s h w e t a br a s io n w i t h

c a r b o r u n d u m p a p e r .

S t ep 2 . F r o m 1 5 u p t o 0 . 3 / am w e t ab r a s i o n w i t h d i a -

m o n d p as t e an d n ap l e ss c l o t h o n a d i s c- p o l is h e r .

S t ep 3 . L i g h t ch emi ca l e t ch i n g o f t h e b a s e me t a l w i t h

d i l u t e N i ta l s o l u t i o n an d t h e s u b s eq u en t e l ec t r o -

ly t i c e t ch ing o f the s ta in less -s tee l po r t ion in 10%

ch r o mi c ac i d s o l u t i o n .

T h e s a m p l e s p r e p a re d w e r e e x a m i n e d t h r o u g h a n o p -

t i c a l mi c r o s c o p e , a scan n i n g e l ec t r o n mi c r o s co p e ( J O E L ,

E S M - 2 0 0 ) a n d a 1 0 0 k V e l e c t r o n m i c r o s c o p e .

2 .4 .

M i c r o - c h e m i c a l a n a l y s is

T h e d i s tr i b u ti o n o f m a j o r c o n s t i tu t i o n a l e l e m e n t s

i n t h e c l ad d i n g l ay e r w as meas u r ed b y t h e l i n e- s can-

n i ng t e c h n i q u e i n a n e l e c t r o n - p r o b e X - r a y m i c r o

an a l y s e r ( XMA h e r e a f t e r ) . T h e r eco r d ed s i gn a ls w e r e

ca l i b r a t ed w i t h t h o s e f r o m s t an d a r d a l l o y sp ec i men s t o

co n v e r t th e s e i n t o t h e ab s o l u t e co n t e n t s i n w e i g h t p e r

c e n t . Q u a n t i ta t iv e d e t e r m i n a t i o n o f t h e c a r b o n c o n -

t e n t b y X M A i s n o t a c c u r a t e e n o u g h , t h e r e f o r e t h e

s t a n d a r d c h e m i c a l d e t e r m i n a t i o n w a s e m p l o y e d , i n

w h i ch each s li ce o f t h e s p ec i men a s s h o w n i n f i g . 2

w a s c o m p l e t e l y o x i d iz e d i n t h e o x y g e n s t r e a m , a n d

t h e g en e r a t ed ca r b o n d i o x i d e w as ab s o r b ed i n e l ec t r o -

l y t e t o d e t e r m i n e t h e c o n c e n t r a t i o n o f c a r b o n a t e

i o n s b y e l ec t r o l y t i c t i tr a t i o n .

3 R e s u l t s

f i n i sh ed b y o r d i n a r y p a p e r ab r a s i o n . A t y p i ca l ap -

p ea r an ce o f t h e c r a ck ed s u r f ace a t t h e v e s se l in -

ner w al l i s as shown in f ig . 3 . which w as rep l i c a ted

a f t e r l ig h t s u r f ace ab r a s io n w i t h 6 0 0 me s h w e t c a r -

b o r u n d u m p ap e r . A simi la r o b s e r v a t i o n w as ma d e

o n t h e e x t r a c t e d s a m p l e s w h i c h w e r e f u r h t e r e x -

am i n ed b y g iv in g me t a l l o g r ap h i c e t ch i n g . W h en

v i ew ed a t t h e ac t u a l s u r f ace o f t h e c l ad d i n g lay e r ,

t h e c r ack i n g w as j u d g ed t o b e m o s t l y o f i n t e r g r an u l a r

n a t u r e . T h e g r a i n b o u n d a r i e s s eem ed t o h av e s epa -

r a t ed b o t h i n t h e n o r m a l an d s h ea r in g s tr e s s d i rec -

t i o n s . O b s e r v a t i o n t h r o u g h t h e s can n i n g e l ec t r o n

m i c r o s c o p e s ho w s t h e s m o o t h a n d s h a rp f r a c t u r e

s u r f ace s w i t h i n t h e c r ack s , in d i ca t i n g t h a t t h e f r ac -

t u r e o c c u r r e d a t t h e b o u n d a r y w i t h o u t m u c h r es is -

t a n c e f r o m p l as ti c d e f o r m a t i o n .

Th e sec t ion al p lanes para l l e l to the vesse l wal l

w e r e m ac r o - e t ch ed a s s h o w n i n fi g. 4 . T h e p h o t o -

g r ap h s n u m b e r e d a s 1 , 2 a n d 3 c o r r e s p o n d t o t h o se

o f p l an es 1 , 2 an d 3 i n fi g. 2 , w h i ch h ad b een l o ca t ed

a p p r o x i m a t e l y 2 , 4 a n d 6 m m f r o m t h e s u r f ac e o f

t h e c l ad d i n g r e s p ec t i v e l y b e f o r e s li ci ng . E x a mi n a t i o n

o f t h e s e s ec t io n s an d i n t e r c o n n ec t i o n o f t h e se l e ad

t o a t h r ee d i m en s i o n a l imag e o f t h e c r ack d i s t r i b u t i o n .

I t is co n c l u d ed , t h en , t h a t t h e c r ack s a r e f o u n d o n l y

i n s o me p a r t i cu l a r b ead s , an d t h e r e a r e ce r t a i n a r ea s

t o w h i ch c r ack s n ev e r d i d ex t en d . I n a d d i t i o n t o t h e

ab o v e ch a r ac t e r it i s a l so n o t ed t h a t t h e e t ch i n g

ch a r ac t e r i s c l ea r l y d i f f e r en t b e t w een t h e s e c r ack ed

an d u n c r ack ed a r ea s . T h e p l an e n o . 3 co n t a i n s c r ack s

co n s i d e r ab l y w i d en ed b y l o ca l i z ed co r r o s i o n . T h i s

f ea t u r e i s a ls o ap p a r en t w h en v i ew ed a t t h e v e r t i c a l

s ec t i o n s . T h e c r ack s f o r m ed i n t h e c l ad d i n g l ay e r

w o r k a s n a r r o w w a t e r ch an n e l s . T h e s o ca l led c r ev ice

c o r r o s io n w o u l d h a v e b e e n i n o p e r a t i o n . T h e d a r k

a r ea ap p ea r e d i n t h e p l an e co r r e s p o n d s t o t h e b a s e

me t a l , an d ev i d en t l y t h e c r ack s h ad r each ed t h e c l ad -

d in g -b ase m e t a l b o u n d a r y .

F ig . 5 s h o w s s o m e d e t a il ed f ea t u r e s o f t h e c r ack ed

a r ea w h i c h h a d b e e n lo c a t e d b y a b o u t 2 m m b e l o w t h e

c l ad d i n g s u r f ace . Cr ack s a r e co n c en t r a t ed a t t h e a r ea s

w h e r e c o a r se a u s t e n i te d e n d r i te s a r e f o r m e d . T h e m o d e

o f f r ac t u r e i s p r ed o m i n an t l y i n t e r g r an u l a r i n t h i s p l an e .

3 . 1 . M a c r o s c o p i c o b s e r v a t i o n s

By ca r e f u l me t a l l o g r ap h i c s u r f ace p r ep a r a t i o n i t w as

f o u n d t h a t t h e r e w e r e a c tu a l ly m a n y m o r e c r a c ks in t h e

d en s i t y an d l en g t h t h an r ec o g n i zed o n t h e s u r f ace s

3 .2 . M e t a l l o g r a p h i c o b s e r v a t i o n s

T h e n o . 1 p l an e co n t a i n s t w o t y p i ca l l y d i f f e r en t

s t r u c t u r e s . T h e s o u n d an d d e f ec t ed p o r t i o n s a r e co n -

t r a s t ed i n f i g. 6 ( a ) an d ( b ) r e s p ec t i v e l y . T h e s o u n d p o r -


4/18

208 T . K o n d o e t a l . C l a d d i n g f a i l u r e i n t h e p r e s s u r e v e s s e l

I

i

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l m m

. i i

i I

0 1 m m

F i g . 3 . R e p l i c a p h o t o g r a p h s o f t h e c r a ck s l e f t u n r e p a ir e d f o r f u t u r e m o n i t o r i n g p u r p o s e .

t i o n i s a t y p i c a l w e l d s t r u c t u r e f o r a n a u s t e n i t i c s t a i n le s s

s t e e l c o n t a i n i n g s e v e r a l p e r c e n t s o f f er r i ti c p h a s e w h i l e

t h e fa i l e d p o r t i o n c o n s i s t s o n l y o f a u s t e n i t e p h a s e .

A c a r e f u l p o l i s h i n g o f t h e p la n e n o . 3 w h i c h i s t h e

c l o s e s t t o t h e b a s e m e t a l i n t h e t h r e e p l a n e s o b -

s e rv e d w i t h 0 . 3 / a m d i a m o n d p a s t e r e v e a le d s o m e

f a i n t t r a c e s o f m i c r o f i s s u r e s . I n o b s e r v i n g t h e s e m o r e

v i v i d l y t h e s u r f a c e w a s e l e c t r o p o l i s h e d i n t h e p e r c h l o r i c


5/18

T. K on do e t al. Cladding failure in pressure vessel 2 0 9

F i g 4

S e m i - m a c r o s c o p i c v i e w o f t h e s e c t i o n a l p l an e s c u t p a r a ll e l t o t h e v e ss e l w a l l ( E l e c t r o - e t c h e d i n c h r o m i c - a c i d s o l u t io n s ) .

D i s t a n c e f r o m c l a d d i n g s u r f a c e ; 2 ra m - p l a n e 1 , 4 m m - p l a n e 2 a n d 6 m m - p l a n e 3 .


6/18

210 T. Ko nd o et al . Cladding failure in pressure vessel

0 5 m m

Fig. 5. Optical micrograph of the sectional plane 1. (Electro-etched).

a c id - e tha no l m ix tu r e a t r e l a t i ve ly h igh c u r r e n t de ns i ty ,

so t ha t no a dd i t i ona l s t r uc tu r a l e t c h ing oc c u r s , a nd

obse r v a t ion on ly o f t he se f i s su r es i s m a de a va i la b l e . T he

r e su l ts ob t a ine d a r e show n in f ig . 7 ( a ) a nd ( b ) , t he

op t i c a l a nd r e p l i c a - e l e c t r on m ic r og r a phs r e spe c t ive ly .

T he p a t t e r n s r e ve a l e d a re i de n t i f i e d f r o m ~ he se f e a tu r e s

t o b e o f t h e m a r t e n s i ti c p h a s e t h a t a p p e a r s t y p i ca l i n

a s ta in l e ss - s te e l w e ld m e ta l de po s i t e d o n a l ow - a l loy

s te e l. T h e s h a r p li n e s a n d t h e a c c o m p a n y i n g l o n g

s h a d o w s i n t h e m i c r o g r a p h ( b ) i n d i c a te t h e p r e s e n c e

o f m ic r o f i s su r e s . T he e l e c t r on - sc a nn ing m ic r og r a phs

o f va r ious m a gn i f i c a t i ons i n f i g. 8 f o r t he s a m e p l a ne

c lea r ly revea l such f i ssures . Bas ica l ly the mar tens i te

f o r m a t i o n w o u l d m a k e t h e s t r u c t u r e s e n s it iv e t o

c racking , but in th is case the presence of such f i s -

s u r es in t h e o b s e r v e d d e n s i ty m a y n o t p r o v i d e t h e

c ond i t i on f o r t he c r a c k ing be c a use o f pos s ib l e d i f -

f i c u l ty i n s t r es s c onc e n t r a t i on . I n f a c t N or r i s e t a l .

[ 4 ] ha s r e po r t e d , i n t he m e ta U ogr a ph ic obse r va t ion o f

the s im i l a r c l add ing f a i l u r e i n t he E lk R ive r R e a c to r ,

t h a t t h e c r a ck s e x t e n d e d i n t o t h e l a y e r w i t h m a r t e n -

s i te ha d be e n de f l e c t e d t o t he l a t e r a l d i r e c ti ons ,

w i t h o u t r e a ch i n g t h e c l a d d in g - b as e m e t a l b o u n d a r y .

I n t he p r e se n t l im i t e d c a se no pa r t i c u l a r r e l a t i on

w a s f o u n d b e t w e e n t h e c r a ck s t h a t r e a c h e d t h e

b o u n d a r y a n d t h e m a r t e n s i t i c p h a s e, s in c e t h e f o r m a -

t i on o f t he l a t t e r w a s no t e x t e ns ive .

3 .3 . C r a c k m o r p h o l o g y

I n o r de r t o e s t im a te t he s e que nc e o f t he c r a c k ing

som e ve r t i c a l s e c t i ons w e r e e xa m ine d . T he s e c t ions

a l so r e ve a l e d t he c r a c k t i p s w he r e t he ba se m e ta l w a s


7/18

T. Kon do e t aL Cladding failure in pressure vessel

211

a ) b )

|

lO 1um

Fig. 6. Optical micrographs of sound (photo a) and cracked (photo b) areas of cladding. The micrographs were taken from plane 1.

a ) , , b ) ,

1 ~um 1 ~m

Fig. 7. Optical (photo a) and replica electron (photo b) micrographs of a grain boundary region in plane 2 (Electropolished in

ethanol-perchloric acid mixture) .


8/18

2 1 2 T . K o n d o e t a l . C l a d d i n g f a i l u r e i n p r e s s u r e v e s s e l

J

lO tum

| . . . . . ~ J

1 t u r n

•

1,urn

F ig . 8 . E le c t ro n - s c a n n in g m ic ro g ra p h s o f p a r t ly -m a r t e n s i t i c a u s t e n i t e s t ru c tu re in p l a n e 3 .


9/18

T. Kon do e t aL Cladding failure in pressure vessel 213

CLADDING

B S E

METAL

Imm

Fig. 9. Optical micrograph of the failed portion in sectional plane A (Electro-etched).


10/18

214 T. Ko ndo et al. Cladding failure in pressure vessel

a f f e c t e d by the a r r iva l o f t he c r a c ks a nd the r e su l t a n t

d i r ec t c o n t a c t w i t h h o t w a t e r. T h e m o d e s o f e x t e n s io n

o f t he c r a c ks a r e t yp i c a l l y show n in f i g . 9 . F r o m the se

m o r p h o l o g i c a l f e a t u re s s h o w n , i t m a y b e n a t u r a l t o

suppose t ha t t he c r a c ks w e r e f o r m e d f i r s t a t t he c l a d -

d ing su r f a c e , o r a t l e a s t in t he v i c in i t y o f i t , a nd the n

p r opa ga t e d t ow a r d the ba se m e ta l . I n t h i s p r oc e s s t he

c r a c ks a re c o n s i d e r ed t o h a v e b r a n c h e d . A n o t h e r f a c t

t o be no t e d i s t ha t t he c r a c ks be c om e pa r t i a l l y t r a ns -

g r a nu la r a s t he y e x t e nd in to t he i nne r pa r t s o f t he

c l a dd ing . M or e t r a nsg r a nu la r c r a c ks w e r e f oun d in t he

ve r t i c a l c r o s s s e c t i ons t ha n in e i t he r t he o u t e r su r f a c e

o r t he ho r i z on ta l s e c t i on , t he p l a ne no . 1 . T he th r e e

l a ye r s o f the o ve r l a y , w h ic h shou ld be d i s c r e t e ly

v i s ib l e in ve r t i c a l s e c t i ons o f o r d ina r y m u l t i - l a ye r

ove r l a y t ha t a r e f a b r i c a t e d i n t he s a m e w a y a s the

p r e se n t s a m p le , a r e no t c l e a r e ve n by de e p e t c h ing in

the se pa r t i c u l a r s a m p le s . I t i s t hus sugge s t e d t ha t e x -

c e s sive sha f f l i n g be tw e e n the l a ye r s m igh t ha ve

c a use d the e x t e ns ive d i l u t i on o f t he c l a dd ing m e ta l

w i th t he ba se m e ta l du r ing the r e pe a t e d pa s se s o f t he

w e ld ing ope r a t i on . T he t i p s o f t he b r a nc he d c r a c ks e x -

t e n d a l m o s t v e rt i c al l y t o w a r d t h e w e l d b o u n d a r y , a n d

e s se n t i a l ly no in t e r f e r ing l a ye r s s e e m to ha ve e x i s t e d

a ga in s t t he p r opa g a t ing c r a c ks .

3 . 4 . M i c r o c h e m i c a l d e t e r m i n a t i o n s o f m a j o r e l e m e n t s

i n t h e w e l d m e t a l

A th in su r f a c e l a ye r o f the f a i l e d pa r t o f t he c l a dd ing

w a s m e c h a n i c a l l y r e m o v e d a n d c h e m i c a l a n a ly s i s w a s

m a d e o n t h e m a j o r c o n s t i t u t io n a l e l e m e n t s . T h e r es u l ts

a re tabula ted in tab le 1 .

r a t i o f o r sound type 308L m e ta l a r e 19 a nd 2 . 1

r e spe c t ive ly . F u r th e r de t a i ls o f t he e l e m e n t d i s t ri -

bu t ion c a n be de p ic t e d by the qua n t i t a t i ve l i ne s c a n -

n ing o f t he ho r i z on ta l a nd ve r t i c a l c ro s s s e c t ions

us ing the X M A . I t is aga in c on f i r m e d tha t t he de -

p l e t i on i n c h r om ium ha s a d i r e c t r e l a t i on t o t he oc -

cur rence of the c racking . The resul ts shown in f ig . 10

w e r e ob t a ine d f r om the ho r i z on ta l p l a ne no . 1 , t he

p l a ne show n in f i g. 4 . T he r e f o r e , i t h a s be e n de m on-

s t r a t e d t ha t t he c h r om ium de p le t i on i s t he o r ig in o f

the obse r ve d d i f f e r e nc e i n t he m e ta l l u rg i c a l s t r uc tu r e

o f t he f a il e d po r t i o n f r o m tha t o f t he sound a r e a ,

a nd he nc e su c h a l a c k in the c om pos i t i on a l ba l a nc e

i s r e spons ib l e t o t he f a i l u r e . T he de p le t i on o f c h r o -

m i u m i s m o r e p r o n o u n c e d i n t h e l a y er s c l os e t o t h e

base m eta l a s seen in f ig . 11 which i s the l ine - scan-

n ings f r om the ve r t i c al p l a ne B , t he p l a ne sho w n in

f ig . 9 . T he de p le t i on goe s dow n to a lm o s t 13% a t t he

loc a t ion c o r r e spond ing to t he f i r s t l a ye r o f ove r l a y

w e ld ing . T h i s l a ye r c o r r e sponds t o t he l a ye r m a de

o f 3 0 9 M o e l e c t r o d e s , a n d n o r m a l l y a l ar g e c o n t e n t

o f c h r o m i u m a n d n i c k e l s h o u ld b e f o u n d .

D i s t r i b u t io n o f m o l y b d e n u m a n d c a r b o n i s e x p e c t -

e d t o g ive m or e subs t a n t i a l i n f o r m a t ion on the s e -

q u e n c e o f t h e a c t u a l w e l d i ng o p e r a t i o n , s in c e b o t h

e l e m e n t s c a n be c ons ide r e d to w or k a s t r a c e r s i n t he

t y p e 3 0 8 L w e l d d e p o s i t . In p a r t i c u la r t h e c a r b o n

c on te n t i n t he c l a dd ing l a ye r ough t t o i nd i c a t e t he

e x t e n t o f d i l u t io n b y t h e b a s e m e t a l. F i r s t ly m u c h

d i f fu s e d d i s t r i b u t io n o f m o l y b d e n u m a s s ee n i n fi g .

11 m a y be t he r e su l t o f e x t e ns ive m ix ing be tw e e n

the l a ye r s . T he r e su l t o f c a r bon de t e r m ina t ion ha s

Table 1

Chem ical composition of the surface layer of the cracked area.

Sample no. C Si Mn Ni Cr Cr/Ni

1 0.05 0.29 1.27 10 .26 17 .82 1.74

2 0.06 0.25 1.26 9.80 16 .8 2 1.72

Unit: w eight per cent

R e f e r r in g t o t h e s t a n d a r d c o m p o s i t i o n f o r t h e w e l d

d e p o s i t o f t y p e 3 0 8 L s t ee l , a p p r e c i a b le d e p l e t i o n i n

c h r o m i u m i s n o t e d , w h i l e o t h e r c o n s t i t u e n t s f a l li n g

w i t h i n n e a r l y n o r m a l r a ng e s . I n c o n s e q u e n c e t h e s o

c a l l ed n i c ke l - t o - c h r o m iu m r a t i o i s a bo u t 1 .7 . T he typ i c a l

v a lu e s o f c h r o m i u m c o n t e n t a n d n i c k e l - t o - c h r o m i u m

subs t a n t i a t e d t h i s , a s c a n be s e e n in f ig . 12 . A no the r

p r o b l e m i n c o n n e c t i o n w i t h t h e e x c e ss i ve i n c o r p o r a t -

i ng o f t he b a se m e ta l i n to t he c l a dd ing i s t he m e ta l -

l u r g i c a l e f f e c t o f c a r bon . C a r bon c on ta ine d in a n

a us t e n i t i c s t a in le s s s t e e l t e nds t o p r e c ip i t a t e p r e -

f e r e n t i a l l y a t g r a in bounda r i e s a s c h r om ium c a r b ide .


11/18

T. Ko nd o e t al . Cladding failure in pressure vessel

215

2 0

-

15

t 0

0 C

Q)

Horizontal Secion

#-

c r

.. .. .. .. .. .. . i _~ i . . . . ,, - I ,,4_.i ............................................ ~ I

F a i le d , , S o u n d

I i i

, , I

0 2 4 6 8 t 0

D I S T A N C E , m m

F ig . 1 0 . D i s t r ib u t io n o f c h ro m iu m a n d n ic k e l i n s e c t io n a l p l a n e 1 (XM A l in e - sc a n n in g ).

, -- 2 0

o_

t - -

Z

u . J

( . 3

Z

0

( . 3

I : ~ i N ~ 2

~ M o o

_ _ _ L i ~ _ _ ~

0 1 ~ - - - - - - - 1 - - , . .. i

0 2 4

C la d din g A - 3 0 2 B

Base meta l

V e r t i c a l S e c t i on

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

6 8 t 0

D IS T A N C E F R O M C L A D D I N G S U R F A C E , m m

F ig . 1 1 . D i s t r ib u t io n o f c h ro m iu m , n i c k e l a n d m o ly b d e n u m in th e v e r t i c a l s e c t io n , p l a n e B (XM A l in e- s c a n nin g ) .


12/18

2 1 6 T . K o n d o e t a l . C l a d d i n g f a i l u r e i n p r e s s u r e v e s se l

O. 13

O.12

O.10

0 . 0 8

0 .06

0 . 0 4

0 .02

O.OO

p i i 1 1 T i I 4

s I

Sample Thickness

Ov e r l a y Ba s e M e ta l

BOundary

I I I 1 I I l I

0 1 2 3 4 5 6 7 8

D I S T A N C E F R O M C L A D D I N G

S U R F A C E , m m

Fig. 12.

D i s t r i b u t io n o f c a r b o n i n t h e c l a d d i n g

as a

f u n c t io n o f

di s tance f rom the surface .

_ C L A D D I N G

/

/

3 0 0 2 0 0 I 0 0

B SE MET L

L ~ .o_.o-----o~

o -~ ' 1

1 0 0 2 0 0

3

D I S T A N C E F R O M W E L D B O U N D A R Y , p m

Fig. 13.

M i c r o - k n o o p h a r d n e s s a t t h e c l a d d in g - b a s e m e t a l

b o u n d a r y .

z.O0

Table 2

Range of C Cr

d is ta nce f r o m A v e ra g e D i l u t io n A v erag e

cladding su rfa ce content ( ) content

(m m) ( ) ( )

N i

Dilution Average Dilution

(%) content (%)

(%)

0.2 - 1.3 0.079 47.6 17.0

1.9 - 3.0 0.095 59.8 16.0

40.0 9.5 40.0

60.0 9.0 60.0

T h e a s s o c i ated ch ro m i u m d ep l e t i o n i n th e v ic i n i ty o f

such gra in boundaries i s a wel l known cause o f the

sensitiz ation t o intergranular corr osion and stress-cor-

ros ion fa ilures. The high carbon con tent o f the present

samples are ref lected in the h igh hardness o f the c ladding

meta l . The m icro-k noo p hardness data shown in f ig . 13

indicate m uc h elevated hardne ss particularly in the vi-

c in i ty o f the weld boundary . The decrease in the hard-

ness in the adjacent part in the base meta l m ay b e resul t

o f d i f fus iona l carb on transfer up on he at ing during the

welding operat ion and the subsequent s tress removal

annealing at 650°C. The carbon transfer from a ferritic

to an austenitic-al loy steel is natural s ince the activity

of carbon i s much di f ferent between these two materia l s .

T h e h a rd n es s o b s erv ed w a s n o t l e s s th a n K H N 3 5 0 i n

the fa i led areas even a t the p lace a few m m apart from

the fus ion l ine .

20

A A is o o t e l

_

. . . . . or.o

X

Q

N M + F

J

F ( F e r r i t e )

1 '0 12 1L. 1 '6 1 '8 2'0 2'2 2/ . 26 28

% O r + % M o + 1 . 5 ~ % S i

F i g . 1 4 . R e s u l ts o f c h e m i c a l a n d X M A a n a l y s e s p l o t t e d o n t h e

Schaef f ers s d iagram. The di lut ion direct ions for overlaying on

A 3 0 2 B a r e i n d i c a t e d w i t h b l o c k l i n e s . T h e n u m b e r s a t t h e

p l o t i n d i c a t e t h e d i s t a n c e i n m m f r o m t h e c l a d d i n g s u rf a c e .


13/18

T. Ko nd o et al . Cladding failure in pressure vessel 217

l l

~

J

C 2

J

0 . 1 m m

Fig. 15. Corrosion of th e base metal at the ends o f the cracks in plane B. The mlcrograph (2) was taken after the surface shown

i n the micrographs (1) w as abraded off by 0 .2 m m o f th ickness.

F o r c o n v e n i e n c e t h e w e l d i n g p r o c e s s c a n b e d e-

s c r ib e d i n t e r m s o f t h e d i l u t i o n b y t h e b a s e m e t a l a s fo l -

l o w s . T h e f i r st l a y e r w a s w e l d e d w i t h

3°9Mo

e l ec -

t r o d e s t o f o r m a la y e r o f a v er a g e c o m p o s i t i o n , 1 4 %

c h r o m i u m , 8 % n ic k e l a n d 0 . 1 2 2 % c a r b o n .

T h e b a s e m e t a l , A 3 0 2 B , c o n t a i n e d 0 . 6 5 % n i ck e l a n d

0 , 1 9 % c a r b o n a n d t h e d e p o s i t o u g h t t o h a v e s o l i d i f i e d

w i t h 2 2 % c h r o m i u m a n d 1 2 % ni c k el i f n o d i l u t io n

b y t h e b a s e m e t a l w e r e i n v o lv e d .

F r o m t h e a b o v e d i s c r e p a n c ie s t h e c a l c u l a t e d d i lu -

t i o n c o e f f i c i e n t s ar e 3 6 . 4 a n d 3 5 . 3 % o n t h e b a s e s

o f c h r o m i u m a n d n i c k e l c o n t e n t r e s pe c ti v el y . T h e re -


14/18

218 T. Ko nd o et al. Cladding failure in pressure vessel

. . . . °

: I

Cladding

ase Me tal

Fig. 16. Corrosion of the base metal at the ends of the cracks in plane A, which is perpendicular to plane B.

f o r e a p p r o x i m a t e l y 3 6 % d i l u ti o n b y t h e b a s e m e t a l

w o u l d h a v e p r o v i d e d t h e f i r s t c a u s e o f t h e c o m p o s i .

t i ona l unba l a nc e i n t he w e ld de pos i t .

T h e s u b s e q u e n t s t e p s o f w e l d in g w i t h t y p e 3 0 8 L

e l e c t ro d e s o n t h e 3 0 9 M o w e l d d e p o s i t c a n b e s u b ,

j e c te d t o t h e s i m il a r j u d g e m e n t b y a s s u m in g t h a t

t h e w e l d m e t a l o f t y p e 3 0 8 L w i t h 0 . 0 4 % c a r b o n ,

1 9% c h r o m i u m a n d 1 1% n i ck e l w a s d i lu t e d b y t h e

f a u l t y w e l d d e p o s i t o f t h e t y p e 3 0 9 M o w i t h 0 . 1 2 %

c a r bo n , 14% c h r om ium a nd 8% n ic ke l . T he r e su l t s

o f c a l c u l a t i on a r e show n in t a b l e 2 .

T he a bove r e su l t s i nd i c a t e t he oc c u r r e nc e o f e x -

t e ns ive m ix ing be tw e e n the o ve r l a ye d l a ye rs .

A c c o r d ing to W yl ie [ 5 ] ge ne r a l al l ow a b le d i lu -

t i o n o f t y p e 3 0 9 M o w i t h A 3 0 2 B i s n o t m o r e t h a n

a r ound 16% , w h i l e t he e x t e ns ive d i l u t i on by 35%

a s in t he p r e se n t c a se m a y c o r r e spon d to t he c ond i -

t i o n o f f o r m i n g w e l d m e t a l o f e i t h e r f u l l y a u s t e n it i c

o r pa r t l y m a r t e ns i t i c a us t e n i t e s t r uc tu r e . I f t he w e ld -

ing o f t he f i r s t l a ye r w i th 309M o w e r e no r m a l , t he

s u b s e q u e n t o v e r l a y in g w i t h 3 0 8 L w o u l d n o t h a v e

d e v i a t e d t o o m u c h f r o m t h e s t a n d a r d c o m p o s i t i o n

e ve n i f t he e x t e ns ive m ix ing a s f ou nd in t he se l a ye r s

w a s t he c a se . T he r e su l t s o f t he a b ove de sc r ibe d

c h e m i c a l a na l y s es c a n b e s u m m a r i z e d o n a S c h a e f f e r ' s

d i a g r a m f o r s t a in le s s - st e e l w e ld de po s i t o f d i s s im i l ar

w e lds a s show n in f i g . 14. T he f i gu r e s i n t he d i a g r a m

ind ic a t e t he l oc a t ion o f s a m p l ing in t e r m s o f d i s t a nc e

f r om the c l a dd ing su r f a c e . T he b loc k l i ne s in t he

d i a g r a m ind i c a t e t he d i l u t i o n d i r e c t i on l i ne s f o r

t y p e s 3 0 9 M o a n d 3 0 8 L w e l d d e p o s it s o n a n A 3 0 2 B

s te e l ba se . A ny c om pos i t i ona l va r i a t i on due t o m ix ing

is to fa l l on these l ines . The resul ts depic t tha t the

d i lu t i on i n t he g ive n m u l t i l a ye r w e ld ing w a s o f c on -

s ide r a b l e e x t e n t , a nd the r e su l t a n t s t r uc tu r a l f e a tu r e s

a g r ee w e ll w i th t he m e ta l l og r a ph ic s t r uc tu r e s obse r ve d .

3.5. T h e e f f e c t o f t h e c l a d d i n g f a i lu r e t o t h e b a s e m e t a l

A s a l r e a dy no te d in f i gs . 4 a nd 9 , som e o f t he c r a c ks

ha d r e a c he d the ba se m e ta l . S inc e t he ba se m e ta l i s m uc h

le s s c o r r o s ion r e s i s t a n t c om pa r e d to t he w e ld ove r l a y ,

d i r ec t c o n t a c t w i t h t h e h o t w a t e r t h r o u g h t h e c r a ck s

m a y r e su l t i n r a p id c o r r o s ion a t t a c k .

I n t he ve r t i c a l s e c t i on sho w n in f i g . 15 c r a c ks a r e

se e n to ha ve e x t e n de d in to t he ba se m e ta l a c r o s s the

f us ion l ine i n t he f o r m o f l oc a li z e d c o r r o s ion p e ne t r a -

t i ons . C ons ide r ing a th r e e - d im e ns ion a l ge o m e t r y , t he a t -

t a c k e d p o r t i o n s a r e t h o u g h t t o h a v e e x t e n d e d a s

d i t c h e s l y ing a long the t i p s o f the c r a c ks . T he c om -

pa r i son o f t he t op a n d bo t to m p i c tu r e s in f ig . 15 i n -

d i c a t e t h i s. I t ough t t o b e no t e d t ha t t he r e i s a s li gh t

t e n d e n c y o f a c o r r el a t i o n b e t w e e n t h e f e a t u re o f t h e

c o r r o s io n p e n e t r a t i o n a n d t h e d i r e c t i o n o f t h e m a j o r

pe r m a ne n t s t r a in l e f t a s t he op e n ing o f t he c r a c ks . T he

m ic r og r a ph in f ig . 15 w a s t a ke n f r om p l a ne B , w h ic h

l ie s pa ra l l el t o t he d i r e c t i on o f l a rge pe r m a ne n t s t r a in .


15/18

T. Ko nd o et al . Cladding failure in pressure vessel 219

Cladding

aseMetal

Fig. 17. Corrosion penetration at the cladding-base metal boundary in plane B.

W hi le i n p l a ne A , w h ic h i s pe r pe n d ic u l a r t o p l a ne

B , t he a t t a c k i s s e e n to ha ve e x t e nd e d r a the r l a t e r a l -

l y a long the f u s ion l i ne a s show n in f ig . 16 . I t i s no t

c l e a r , how e ve r , f r om the se l im i t e d m a te r i a l s w he the r

the c o r r o s ion pe ne t r a t i on a c tua l ly ha s a c e r t a in r e l a -

t i on w i th t he s t r e s s a ct i ng on the c r a c k t i p s o r no t .

I t i s , t he r e f o r e , sugge s t e d t ha t a ny f e a s ib i l i t y o f a

m e c ha noc he m ic a l e f f e c t , suc h a s s t r e s s c o r r o s ion o r

c o r r o s ion f a t i gue unde r t he g ive n pa r t i c u l a r e l e c t r o -

c h e m i c a l c o n d i t io n , m u s t b e j u s t i fi e d b y f u r t h e r e x -

pe r im e n ta l s t ud i e s .

T h e v e r y r a n d o m a n d c o m p l i c a t e d f e a tu r e o f t h e

c o r r o s i o n . r ea c t i o n i n t e r fa c e a t t h e a t t a c k e d p o r t i o n

c a n be s e e n in f ig . 17 , t he m a gn i f i e d ve r s ion o f f i g ,

15 . T h i s i nd i c a t e s t he oc c u r r e n c e o f p i t t i ng - typ e c o r -

r o s ion . T he t r e nd o f p i t t i ng i n a na r r ow c r e v i c e , suc h :

a s i n t he c r a c k obse r ve d , ha s be e n p r e d i c t e d , s i nc e

t h e e l e c t ro c h e m i c a l c o n d i t i o n i n c l u d in g t h e f o r m a t i o n

o f a n o x y g e n - c o n c e n t r a t i o n c e ll in s u c h p o r t i o n m a y

le a d to p i t t i ng a t t a c k .

4 Discuss ion

4 . I . F o r m a t i o n a n d g r o w t h o f c r ac k s i n t h e s t a in l es s -

s t e e l c l a d d i n g

I t ha s be e n show n th a t t he r e i s a c ons i s t e n t a g r e e -

m e n t a m o n g t h e m e t a l lu r g i c al s tr u c t u r e , c h e m i c a l

c o m p o s i t i o n a n d t h e o c c u r r e n c e o f t h e c r ac k s . T h e y

c a n b e c o r r e l a te d t o e a c h o t h e r i n t e r m s o f t h e e x -

c e ss ive d i l u t i on du r ing the w e ld ing o f d i s s im i l a r

m a t e r ia l s . T h e r e s u l t an t p r o n o u n c e d d e p l e t i o n i n


16/18

220 T . K o n d o e t a l. C l a d d i n g s f a i l u r e i n p r e s s u r e v e s s el

c h r o m i u m h a d t h e l ar g es t i n f l u en c e o n t h e f o r m a t i o n

o f t he a us t e n i t e s ing le - pha se s t r uc tu r e , be ing s e ns i ti ve

to c r a c k ing unde r t he g ive n se r v ic e c ond i t i ons . T he

c r a c ks w e r e no t obse r ve d a t t he t im e o f i n s t a l l a t i on

o f t he ve s se l a s f a r a s t he dye pe n e t r a n t t e s t a f t e r t he

s t re s s r e m ov a l he a t ing h a d ind i c a t e d . T h us t he f a i lu r e

i s t hough t t o ha ve oc c u r r e d du r ing the r e a c to r ope r a -

t i o n p e r i o d , a lt h o u g h e x a c t e s t i m a t i o n o f t h e t i m e o f

oc c u r r e nc e i s d i f f ic u l t .

F r o m t h e o b s e r v e d m o r p h o l o g i c a l f e a t u r e o f

c r a c k ing , t he c r a c ks a r e t houg h t t o ha ve in i t i a t e d a t

t h e c l a d d in g s u r f ac e a n d t h e n p r o p a g a t e d t o w a r d t h e

ba se m e ta l a c r o s s t he tw o w e ld bounda r i e s . T he e x -

i s t e nc e o f t r a nsg r a nu la r c r a c ks i s e v ide nc e t ha t t he

c r a c k ing i s due t o o the r t ha n o r d ina r y i n t e r g r a nu la r

f i s su r ing o f a f u l l y a us t e n i t i c w e ld m e ta l w h ic h , ge -

ne r a l l y , is o f i n t e r g r a nu la r na tu r e [ 5 ] . T he c r a c ks a r e

c onc e n t r a t e d i n t he t op he a d a r e a w he r e t he e nv i r on -

m e n ta l c ond i t i on i s c ha r a c t e r i s t i c bo th o f inc r e a se d

o x y g e n c o n c e n t r a t i o n a n d c o n t i n u o u s c o n d e n s a t i o n

o f s t e a m a t t he w a l l su r fa c e . T he l a t t e r c ond i t i on c a n

c a use a c c um ula t ion o f im p ur i t i e s in w a te r c a r r i e d

ove r by s t e a m . A l thou gh the e xa c t l e ve l o f e f f e c t i ve

i m p u r i t y a t t h e c l a d d in g s u r f ac e c a n b y n o m e a n s b e

e s t i m a t e d , t h e c o n t r i b u t i o n o f s u c h a f a c t o r c a n n o t

be i gno r e d in c ons ide r ing the c a use a nd p r oc e s s o f

c r a c k ing . I n t h e l a s t de c a de , s t r e s s - c o r ro s ion c r a c k ing

i n a h i g h - t e m p e r a t u r e d e i o n i z e d w a t e r e n v i r o n m e n t

h a s c o m e u n d e r a t t e n t i o n a n d m o r e t h a n s e v er a l

c a ses ha ve be e n r e p o r t e d t o ha ve oc c u r r e d i n t he

nuc l e a r - r e a c to r w a te r e nv i r o nm e n t s [1 ] .

Austeni te s ingle phase i s more sens i t ive to the s t ress-

c o r r o s ion c r a c k ing tha n a us t e n i t e p lu s f e r r i t e s t r uc tu r e

[ 6 ] . T he c ond i t i ons t o be s a t i s f i e d f o r t he ope r a t i on

o f s t r e s s - c o r ro s ion c r a c k ing o f a n a us t e n i t e s t a in l e ss

s t e e l i n h igh - t e m pe r a tu r e w a te r m a y be : 1 . T e ns i l e

s t r es s a bove a c e r t a in c r i t i c a l l eve l ; 2 . O x yge n c on -

t e n t i n t he e nv i r onm e n t ; 3 . C h lo r ide o r o the r i ons i n -

c lud ing hyd r ox ide [ 2 , 3 , 7 ] . I n pa r t i c u l a r t he d i s so lve d

o x y g e n p l a y s a p a rt i c u la r r o l e in p r o m o t i n g t h e p h e n o m -

e n o n [ 8 ] .

T h e w a t e r q u a l i t y o f t h e J P D R t h a t h a s b e e n m a i n -

t a lne d i s t yp i c a l l y de sc r ibe d by the f o l l ow ing f i gu r e s ;

spe c i f i c e l e c t ri c a l c ondu c t iv i t y c h r o l ide 6 ppb a nd

o x y g e n 0 . 3 ( li q u i d p h a s e a t th e b o t t o m h e a d ) t o 3 5

p p m . ( s t e a m - w a t e r t w o p h a s e f l o w re g i o n a t t h e t o p

he a d ) . T he t e ns i le s t re s s e s t im a te d to ha ve a c t e d on

t h e s u r fa c e o f th e c l a d d i n g w a s a ro u n d 2 2 k g / m m 2 .

F o r j u s t i f y ing the f e a s ib i l i t y o f t he oc c u r r e nc e o f

s t r e s s -c o r r o s ion c r a c k ing in t he g ive n r e a c to r w a te r

c ond i t i on , ve r y l im i t e d i n f o r m a t ion i s a va i la b l e a t

t he p r e se n t da t e . S t a e h l e [ 2 ] ha s t e s t e d t ype 304

s t e e l i n a c i rc u l a t i ng a u toc l a ve sy s t e m unde r t he c on -

d i t i ons c ove r ing those a t a t yp i c a l B W R . I n a t t e m p t ing

a n a p p r o x i m a t e j u d g e m e n t o n t h e p r e s e n t s u b j ec t a

r ough e x t r a po la t i on i s m a d e a s show n in f i g. 18 . T he

p lo t i nd i c a t e s t ha t t he r e i s a pos s ib i l i t y o f c ra c k ing

i n t h e J P D R a q u e o u s e n v i r o n m e n t w i t h in 1 n e t y e a r

o f d u r a t i o n w h e n f u l ly a u s t e n i te w e l d m e t a l i s k e p t

unde r t he t e n s il e s t r e ss no t l es s t ha n 15 kg /m m 2 .

B o th t he s t r e ss re m o va l he a t ing a t 650°C a nd the

f ina l su r f a c e g r ind ing [ 9 ] w ou ld ha ve g ive n the m e ta l

som e a d d i t i ona l su sc e p t ib i l i t y t o s t r e s s - c o r r o s ion c ra c k -

ing.

T h e r e i s a n o t h e r s u p p o r t i n g p i e c e o f e v i d en c e t h a t

t he s t r es s c o r r o s ion - c r a c k ing i s r e spons ib le f o r t he

f a i lu r e . R e c e n t r e su l t s o f t he r e a c to r su r ve i ll a nc e t e s t s

f o r c ra c k d e t e c t i o n a t t h e b o t t o m h e a d h a v e i n d i c a te d

tha t t he c r a c ks f o r m e d in the b o t to m - he a d a nd she ll -w a ll

a r e a s w e r e m uc h l e s s i n num be r a nd de p th t ha n in t he

top - he a d . T he l i qu id pha se , w he r e t he bo t tom - he a d

and she l l -wa l l a reas were e xp ose d , i s mu ch less aggres-

s ive in s t r ess-cor ro s ion t r en d s ince i t s ox yg en leve l i s

a b o u t t w o o r d e r s o f m a g n i t u d e l e s s t h a n t h e s t e a m -

w a te r pha se . C onse que n t ly , t he o r ig in a nd the s e que nc e

o f t he f a i l u r e ca n be c on c lude d to be , r e spe c t ive ly , t he

1

c

E

, _ g l

z

( j 1 0

o

i f -

- 1 ~ .

l ~ m O ~15Kg[mr13 ~0eC

~ l O O d o p I ~

I

-lda7

~ ~ ~ . . ~ - ~ 1 5 8p p m 1 1 2 ,1 5 K g /I T m 2 2 6 0 C

6 p p b : .1 -

%

1 0 2 1 0 1 1 1 0 1 1 0 0 3 1 0 z

N a C I p p m

Fig. 18. Extrapolation o f literature data [2] for an equivalent

material in testing the feasibility of stress-corrosion cracking in

JPDR pressure ve s s e l


17/18

T. Ko nd o et al . Claddings failure in pressure vessel

221

i m p r o p e r w e l d i n g t o f o r m a c r a c k s e n s it iv e m a t e r i al

a n d t h e s t r e s s -c o r ro s i o n a c t io n o f t h e e n v i r o n m e n t

to ass is t the c racking process .

4.2. C o r r o s io n o f t h e b a s e m e t a l a n d i t s r e l a ti o n t o

t h e f u t u r e i n t e g r i t y o f t h e v e s s e l

T he ve r y i rr e gu la r na tu r e o f t he c o r r o s io n in t e r -

f a c e obse r ve d a t t he l oc a l i z e d a t t a c k o f t he ba se m e ta l

i n fi g . 17 , is t he r e su l t o f r a p id p i t t i ng r e a c t ion . R e c e n t

r e su l ts c onc e r n in g the p i t t i ng c o r r o s ion in t he c r e v i ce s

o f A 3 02B s t e e l w i th s t a in l e s s -s t e el ove r l a y ha ve ind i c a t e d

a b o u t 2 0 t i m e s h ig h e r p e n e t r a t i o n r a t e a s c o m p a r e d

t o t h a t o f f r e e l y - e x p o s e d s u rf a c es o f A 3 0 2 B s t e el i n

w a te r a t 270°C [ 10 ] . I n t he na r r ow c r e v i c es l i ke those

f o r m e d b y t h e c r a c k s ar e a p t t o c o n t a i n o x y g e n - c o n -

c e n t r a t i o n g r a d i e n t b e t w e e n t h e t i p s o f t h e c r ac k s a n d

the su r f a c e a r e a s i n a que ous e nv i r onm e n t . I n t he c o r -

r o s ion o f s t e el i n h igh - t e m p e r a tu r e w a te r d i s so lve d

o x y g e n p l a y s a n i m p o r t a n t r o l e , a n d t h e c o n c e n t r a t i o n

g r a d i e n t p r ov ide s a c e rt a in e l e c t r om o t ive f o r c e r e su l t -

i ng i n a c onc e n t r a t i on c e ll f o r m a t ion in t he c r e v i c e

ope n ing . S uc h a s i t ua t i on , pe r ha ps i nc lud ing the c on -

t r i bu t ion o f e l e c t r oc he m ic a l d i s s im i l a r it y o f t he s t a in -

less- s tee l c ladding and the base meta l , i s be l ieved to

f a v o r a p a r ti c u l a r a n o d i c c o n d i t i o n t o p r o m o t e p i t -

t i ng c o r r o s ion , w he r e t he l ow - a l loy s t e e l f a il s t o m a in -

ta in s tab le pass iv i ty .

I n p r inc ip l e t he ba se m e ta l e xpose d to w a te r a t t he

b o t t o m o f t h e c r e v ic e s f o r m e d b y t h e c r a c k s in t h e

c l a dd i n g m a y b e p o l a r i z e d t o w a r d a n o d i c d i r e c t io n

b y e i t h e r o f t h e a b o v e t w o e l e c t ro c h e m i c a l f a c t o r s.

I t m u s t b e n o t e d t h a t t h e r e c a n b e c e r t a in e f f e c t

o f s u c h c h e m i c a l e n v i r o n m e n t o n t h e l o w c y c le f a t ig u e

o f t he p r e s su r e ve s se l due t o t he s t r e s s cyc l ing a c c om -

p a n y i n g t h e r e a c t o r - o p e r a t i o n c y c le s .

T he low c yc l e f a t i gue o f s t e e l ves se ls ha ve be e n a

po te n t i a l sub j e c t o f r e a c to r - s t r uc tu r a l s a f e ty p r ob -

l e m s , a nd s tud i e s ha ve be e n c onc e n t r a t e d i n t he f a t i gue

a na ly s is in a tm o sphe r i c c ond i t i ons . T he r e f o r e , e s se n ti a l -

l y n o i n f o r m a t i o n h a s b e e n a v a i la b le o n t h e e f f e c t o f

t h e h i g h - t e m p e r a t u r e w a t e r e n v i r o n m e n t o n t h e f a t ig u e -

c r a c k p r o p a g a t i o n .

A l though the c r a c ks obse r ve d in t he p r e se n t

s tudy w e r e no t t ho se c r e a t e d a t h igh - s t r e ss r e g ions

l ike - noz z le c o r ne r s , t he r e w e r e d i f f e r e n t f e a tu r e s

i n t h e f o r m o f c o r ro s i o n a t t a c k a t t h e t i p s o f t h e

c r a c ks de pe nd ing on the d i r e c t i on o f s t re s se s a c t i ng

on the c r a c ks . T he ve s se l ha d e xpe r i e nc e d a bou t

50 c yc l e s o f t e n s i l e s tr e s s w a ve s due t o t he op e r a -

t i on sh i f t up t o t he da t e o f t he s a m p l ing . I t is no t

c l e a r w h e the r t he c ha r a c t e r i s t i c sha pe o f t he c o r -

r ode d po r t i o n s e e n in fi g . 15 a nd 17 is the r e su l t o f

c o r r o s ion f a t i gue o r no t . H ow e ve r , i t shou ld be

na tura l to suspec t the e f fec t of the aggress ive -

c o r r o s io n e n v i r o n m e n t o n t h e p r o p a g a t i o n o f f a t ig u e

c r a c ks a s a f u tu r e p r ob le m . C onc e r n ing the e nv i r on -

m e n ta l e f f e c t on t he f a t i gue o f p r e s su re - ve s se l s t e e ls ,

C r o o k e r a n d L a n g e [ 11 ] h a v e re p o r t e d t h a t A 3 0 2 B

s te e l i n s im u la t e d bo i l e r w a te r i s c r a c ke d a f e w t im e s

f a s t e r t ha n in a i r unde r r e p e a t e d b e nd ing . W e l l i nge r

[ 12 ] ha s de m ons t r a t e d t he e f f e c t o f s t r e s s c yc l ing

r a t e t o be m uc h in f lue nc ing f o r a s t e e l i n t he num be r

o f c yc l e s t o f a i lu r e i n 60 ° C w a te r . T he in f lue nc e

o f s e ve r a l f a c to r s on f a t i gue o f s t e e ls a r e be ing c r i t -

i c al l y t es t e d i n h i g h - t e m p e r a t u r e w a t e r s y s t e m b y

the a u tho r s [ 13 ] , a n d som e p r e l im ina r y r e su l ts

ha ve ind i c a t e d a subs t a n t i a l e f f e c t o f e nv i r onm e n t

shou ld be t a ke n in to a c c oun t i n c ons ide r ing the l ow

cyc le fa t igue behaviour of s tee l pressure vesse ls pa r t icu-

l a r ly w he n th e i r c o r r o s ion - r e si s t i ng l i ne a r c on ta in de f e c t s .

~ Fhe m a jo r c on t r o l l i ng f a c to r s a r e t e m p e r a tu r e , s t r e s s

o r s t r a in a m p l i t u de a nd c yc l ing r a t e , s t r e s s -w a ve pa t -

t e r n , w a te r qua l i t y a nd so f o r th . A m or e c r i t i c a l su r ve y

on the de f e c t - c on ta in in g ve s sel s m a y be m a de a va i l-

a b l e a f t e r t he se s tud i e s ha ve be e n m a de .

5 . C onc lu s ions

1 . T he f a i lu r e o f t he w e ld - ove r l a y c l a dd ing w a s c a use d

b y t h e m i s c o m p o s i t i o n o f t h e w e l d m e t a l d u e t O i m -

p r o p e r w e l d in g o p e r a t i o n . T h e f a il u r e c o r r e s p o n d s

d i r e c tl y t h e d e p l e t i o n i n c h r o m i u m a n d t h e a s s o c ia t e d

m e ta l l u r g i c a l - s t r uc tu r e .

2 . T he de p le t i on i n c h1 ' om ium i s t he r e su l t o f e xc e s s ive

d i l u ti o n o f t h e f i rs t l a y e r o f t y p e 3 0 9 M o w i t h A 3 0 2 B

b a s e m e t a l , an d t h e s u b s e q u e n t o v e r la y i n g w i t h t y p e

3 0 8 L m a t e r i a l w a s n o t s u c c e ss f u ll y m a d e . T h e m i -

n i m u m c h r o m i u m c o n t e n t o b s e r v e d w a s a b o u t 1 3 % ,

a nd the s t r uc tu r e o f t he de pos i t e m e ta l w a s e i t he r

s ingle aus teni te or pa r t ia l mar tens i t ic aus teni te in

the fa i led regions .

3 . O n the ba s i s o f t he c r a c k m or pho logy , t he m e ta l -

l u r g i c a l s t r uc tu r e a nd the e nv i r onm e n ta l c ond i t i ons ,

s t r e s s - co r r o s ion c r a c k ing i s t hou gh t t o ha ve be e n in

o p e r a t i o n .


18/18

222

4. The cracks reached the base metal across the fusion

lines have extended int o the base metal in the form

of an aggregate of c orrosi on pits. The pitt ing corro-

sion was thought to be the result of the crevice ef-

fect within the cracks.

5. A probabili ty and the necessity of studies concernin g

the corrosion fatigue were poin ted out.

cknowledgements

Thanks are due Dr. R.D. Wylie of Sowthwest Re-

search Institu te, Texas, USA and Prof. Y. And o of

University of Tok yo for discussions and the encoura-

gement of this work. The sup port by the JPDR Pres-

sure Vessel Safety Evaluation Committ ee is appreciated.

Assistance by Messers T. Kikuyam a and Y. Katan o

was very helpful in preparing electron micrographs.

References

[ 1 ] E.A. Wimunc, How Serious Are Vessel Cladding Failures?

Power Reactor Technology 9 (1966) 101-109.

[2] R.W. Staehle, Circulating Autoclave System for Stress

Corrosion Cracking Studies, Stress Corrosion Testing,

ASTM-STP-425 (1967) 248-273.

[3] W.L. Williams,Chloride and Caustic Stress Corrosion of

Austenitic Stainless Steel in Hot Water and Steam, Cor-

rosion 13 (1957) 539-545.

[4] E.B. Norris et al., The Second Inspection of the Elk

River Reactor Pressure Vessel after Operation, AEC

Report, (1967) SwRI-1228-P.

[5] R.D. Wylie, et al., Weld Deposited Cladding of Pressure

Vessels, Brit. Weld. J., August (1965) 378-393.

[6] H.H. Uhlig and R.A. White, Some Meailurgical Fac-

tors Affecting Stress Corrosion Cracking of Austenitic

Stainless Steels, Trans. A.S.M., 52 (1960) 830-847.

17] W.E. Berry, Some Facts About Stess Corrosion Cracking

of Austenitic Stainless Steels in Reactor Systems., Re-

actor Materials 7 (1964) 1-13.

[8] V.V. Gerasimov et al., Corrosion Cracking of 1Kh

18N9T Steel;Corrosion of Reactor Materials: A col-

lection of articles, AEC Report AEC-tr-5219 (1960)

146-151.

[9] M.G. Fontana et al., Metal Progress 86 (1961) 99.

[10] T. Kondo et al., A Study on the Pitting Corrosion in

High Temperature Water of a Low Alloy Steel within

the Simulated Cracks, Japan Atomic Energy Res. Inst.

Report (1970) JAERI-memo-4019.

[ 11 ] T.W. Crooker and E.A. Lange, Low Cycle Fatigue

Crack Propagation in A201B, A302B and A517F

Pressure Vessel Steels., Welding J. Res. Suppl., July

(1967) 322S-328S.

[12] K. Wellinger, op cit. and D. Uebing, Nuclear Eng.

Design 3 (1967) 421.

[13] T. Kondo et al., The Effect of Frequency of Loading

on the Fatigue Crack Propagation of A low Alloy Steel

in High Temperature, Deionized Water, Japan Atomic

Energ. Res. Inst. Report, (1970) JAERl-memo-4022.

Documents

Metallographic Investigation on the Cladding Failure (2)