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R E S I D U A L S T R E S S I N C L A D A N D U N C L A D P U L P D I G E S T E R S

A . G a r n e r , N . J . S t e a d , D .L . S i n g b e i l

P u l p a n d P a p e r R e s e a r c h I n s t i t u t e o f C a n a d a

3 8 00 W e s b r o o k M a l lV a n c o u v e r , B . C .

C a n a d a V 6 S 2 L 9

A B S T R A C T

R e s id u a l s t r e s s w a s m e a s u re d in c l a d a n d u n c l a d c a rb o n s t e e l d ig e s t e r s b e fo re a n d a f t e r s t re s s r e-

l i e f a n n e al . L a b o r a t o r y m e a s u r e m e n t s w e r e a l s o m a d e o n t e s t p l a te s . S t re s s r e l ie f a n n e a l w a s f o u n d to

b e s u c c e s s fu l f o r c a rb o n s t e el , b u t o n ly ma rg in a l ly s o fo r a u s t e n i ti c s t a in l e s s s te e l c l a d d in g .

I N T R O D U C T I O N

T h e l a rg e p r e s s u re v e s s e l s c a l l e d d ig e s t e rs a r e u s e d to c o o k w o o d c h ip s i n t h e k r a f t p ro c e s s . T h e y

can be up to 8 .2 m d ia m eter x 61 m h ig h (27'dp x 200 ' ) . Th ey can be sub jec t to s t ress cor ros io n c rack in g

i f ma d e f ro m c a rb o n s t e e l ( 1 ). T o m in im i s e th i s r i s k , t h e s e d ig e s t e r s ar e p o s t -w e ld h e a t - t r e a t e d (P W H T )

o n - s i t e a f te r e re c t i o n t o r e l i e v e r e s id u a l s t re s s . I n N o r th A m e r i c a , d ig e s t e r s c a n a l s o b e ma d e p a r t l y o r

fu l l y f ro m 3 0 4 L ro l l - c l a d c a rb o n s t e e l a n d th e s e t o o a r e P W H T . U n l ik e c a rb o n s t e e l , s t a in l e s s st e e l s

s u c h a s 3 0 4 L o r 3 0 9 L w e ld d e p o s i t a r e n o t g e n e ra l l y s u s c e p t ib l e t o s t r e s s c o r ro s io n c r a c k in g in a lk a l in e

s u lp h id e kr af t: l i q u o rs . H o w e v e r , c r a c k in g o f 3 0 4 L c l a d d in g i s k n o w n to o c c u r i n t h e v a p o u r p h a s e o f

kra f t d iges te rs (2 ,3 ) .

D i r e c t m e a s u r e m e n t o f re s i d u a l s t re s s w a s u n d e r t a k e n d u r i n g c o n s t ru c t i o n o f t w o c o n t i n u o u s d i -

g e s t e r s , o n e c l a d a n d o n e u n c l a d . S t r e s s me a s u re m e n t s w e re a l s o ma d e o n fu l l t h i c k n e s s l a b o ra to ry

w e ld m e n t s ( 4 ) . O n e o f t h e ma in o b j e c t i v e s w a s t o a s s e s s r e s id u a l s tr e s s l e v e l s a f t e r P W H T o f a c l a d v e s -

sel .

Copyright

• 200 0 by NACE In ternat iona l .Reque sts or permiss ion to pub l ish th is man uscr ip t in any form in par t or in whole mus t be in wr i t ing to NACEte r na ti ona l , C on fe r ences D iv i s i on , P .O . B ox 218340 , H ous ton , Texas 77218 - 8340 . The ma te r i a l p resen ted and t he v iew s exp r essed i n t h i s

pap er are so le ly those o f the author (s) and are not necessa r i ly endorsed b y the Ass oc ia t ion. Pr in ted in U .S.A.

00595CORROSION2000

Paper No.



E X P E R I M E N T A L T E C H N I Q U E S

Laboratory WeidmentsR o u g h b l a n k s f o r th e c i r c u l a r p a t c h t e st w e l d s ( C P T W ) s p e c i m e n s w e r e f l a m e - c u t fr o m 3 8 m m

( 1 .5 " ) th i c k p la t e s o f t h e A S T M A 5 1 6 G r a d e 7 0 s t ee l. T h e b l a n k s w e r e t h e n m a c h i n e d t o a d i a m e t e r o f

2 5 4 m m ( 1 0 " ) a n d o p p o s i n g 1 52 r n m ( 6 " ) d i a m e t e r g r o o v e s w e r e c u t i n to e a c h f a c e o f th e s p e c i m e n s .

E a c h s p e c i m e n w a s s h i e l d e d m e t a l ar c ( S M A ) w e l d e d w i t h 3 t u rn ( 0 .1 2 5 i n c h ) d i a m e t e r E 7 0 1 8 w e l d

e l ec t rodes (F i gu re s 1 ,2 ). Spec i m ens w ere p rehea t ed t o 95°C (200°F ) be fo re w e l d i ng and t he m ax i m um

w e l d i n te r p a s s te m p e r a t u r e w a s r e s tr i c te d t o 1 5 0 ° C ( 4 0 0 ° F ) . A i r - b l a s t c o o l i n g w a s e m p l o y e d t o s p e e d

c o o l i n g b e t w e e n w e l d p a s s e s a n d t o i n d u c e s o m e w h a t h i g h e r s tr e ss l e v e l s in t h e s a m p l e s . E a c h g r o o v e

w a s f i ll e d i n w i t h b e t w e e n 1 5 a n d 1 8 w e l d p a s s e s . D u r i n g w e l d i n g , t h e s p e c i m e n s w e r e r e s t r a i n e d b y

t a c k - w e l d i n g t h e m t o t h e w o r k b e n c h t o p r e v e n t d i s to r t io n ( a n d t h u s t o m i n i m i z e r e l i e f o f r e si d u a l

s tr e ss e s) . E a c h s p e c i m e n w a s f ir s t w e l d e d c o m p l e t e l y o n o n e s id e , th e n o n t h e o t h e r a n d t h e f a c e s o f t h e

s p e c i m e n s w e r e s u b s e q u e n t l y s k i m m e d o n a l a t h e t o a s m o o t h , p la n a r p r o f i le .

Figure 1: SMAW welding o f the 254 turn (10 inch) diameter circularweld patch test (CWPT) specimen.

Figure 2: Circular we ld patch test (CWPT ) specimen between weldingpasses.



T h e C P T W s p e c i m e n s w e r e P W H T s t re s s r e l ie v e d f o l l o w i n g g u i d e li n e s f or p o s t - w e l d h e a t t r e a tm e n t

g i v e n i n th e a p p l i c a bl e p o r ti o n s o f t h e A S M E B o i l e r a n d P r e s s u r e V e s s e l C o d e . S p e c i m e n s w e r e

w r a p p e d i n s t a in l e s s s te e l f o il t o p r e v e n t e x c e s s iv e o x id a t i o n , a n d h e a t e d t o 5 9 0 ° C ( 1 1 0 0 ° F ) f o r o n e a n d

a h a l f h o u r s . T h e y w e r e t h e n l e f t in t h e f iw n a c e a n d a l l o w e d to c o o l s l o w ly b a c k t o r o o m t e m p e r a tu r e .

I n c o n e l 8 2 ( A W S E R .N iCr -3 ) a n d 3 0 9 L ( A S T M A 1 6 7 ) w e ld o v e r l a y s w e r e a l s o a p p l i e d t o p r e v i -

o u s l y p r e p a r ed C P T W s p e c im e n s . A l l w e l d o v e r l a y w a s a p p l i e d b y a m e t a l in e r t g a s ( M I G ) w e l d i n g

p r o c e s s . S p e c im e n s w e r e m o u n te d h o r i z o n t a l l y o n a r o t a t i n g p e d e s t a l a n d a n a u to m a t i c w e ld in g h e a d

w a s u s e d t o a p p l y th e o v e r l ay . S p e c i m e n s w e r e p r e h e a t e d t o 9 5 ° C ( 2 0 0 °F ) a n d a m a x i m u m i n te r p a ss

w e ld t e m p e r a tu r e o f 1 5 0 ° C ( 3 0 0 ° F ) w a s m a in t a in e d a t a l l t im e s . A p p r o x im a te ly 8 p a s s e s w e r e a p p l i e d

o v e r t h e c a r b o n s t e e l w e ld o n b o th f a c e s o f e a c h s p e c im e n , s t a r ti n g o n th e o u t s id e d i a m e te r a n d w o r k in g

to w a r d s t h e c e n t r e o f t h e s p e c im e n .

D i g e s t e r T e s t s

O n - s i t e m e a s u r e m e n t s w e r e m a d e i n tw o K a m y r c o n t in u o u s d ig e s t e r s d u r in g c o n s t r u c t i o n . I n t h e

u n c l a d v e s s e l , r e s id u a l s t r e s s e s w e r e m e a s u r e d c lo s e t o tw o c i r c u m f e r e n t i a l w e ld s e a m s b e f o r e a n d a f t e rP W H T . T h e w e l d s e a m s c h o s e n w e r e t h e u p p e r tr a n s it i o n w e l d ( w e l d " E " ) a n d a w e l d i n th e c o o k i n g

z o n e ( w e ld "N " ) . C i r c u m f e r e n t i a l w e ld s a r e l a b l e d i n a lp h a b e t i c a l o r d e r fr o m th e t o p o f t h e d ig e st e r . I n

e a c h c a s e , s t r e s se s w e r e m e a s u r e d i n b a s e m e ta l c l o s e t o t h e f u s io n l in e . M a te r i a l s a n d p r o c e d u r e s p e c i -

f i c a t i o n s w e r e t y p i c a l fo r d ig e s t e r c o n s t r u c t i o n a n d c lo s e ly m a tc h e d t h e l a b o r a to r y w e ld m e n t s . T h e

P W H T w a s c a r ri e d o u t o n th e c o m p l e t e v e s s e l u s i n g g a s f i re d d i re c t h e a ti n g f o l l o w i n g s ta n d a r d A S M E

p r o c e d u r e .

T h e s e c o n d d ig e s t e r t e s t e d w a s m a d e f i' om 3 0 4 L r o l l c l a d A S T M A 5 1 6 G r a d e 7 0 c a r b o n s t ee l . T h e

c l a d la y e r w a s 3 m m ( 0 .1 2 5 i n c h ) t hi c k . C i r cu m f e r e n t ia l w e l d s w e r e S A W E 7 0 1 8 w i t h 3 0 8 L ( A S T M

A 1 67 ) G T A W c a p p i n g pa s s es . A s i m i l a r A S M E P W H T p r o c e d u r e w a s p e r f o r m e d o n t h is c o m p l e t e v es -

s e l a f t e r e r e c t i o n . Re s id u a l s t r e s s e s w e r e m e a s u r e d o n tw o w e ld s e a m s o n t h e i n s id e c l a d s u r f a c e o n3 0 8 L w e ld m e ta l b e f o r e a n d a f t e r P W H T in s im i l a r lo c a t i o n s .

R e s i d u a l S t r e ss M e a s u r e m e n t s

T h e b l i n d h o l e d r i l l i n g m e th o d w a s u s e d t o m e a s u r e r e s id u a l s u r f a c e s t r e s s e s i n t h e t e s t s p e c im e n s

( 5 ). A M e a s u r e m e n t G r o u p R S - 2 0 0 m i l l i n g g u id e a n d h ig h - s p e e d ( 4 0 0 ,0 0 0 r p m ) a i r t u r b in e w e r e e m -

p lo y e d , a lo n g w i th a M o d e l P - 3 5 0 A s t r a in i n d i c a to r a n d S B- 1 s w i t c h a n d b a l a n c e u n i t. M e a s u r e m e n t s

G r o u p E A - 0 6 - 0 6 2 R E - 1 2 0 o r T E A - 0 6 - 0 6 2 R K - 1 2 0 s t ra i n g a g e r os e t te s w e r e u s e d t h r o u g h o u t t h e w o r k

a n d S S W h i t e F G d e n t a l c a r b id e b u r r s w e r e u s e d t o d r i l l t h e h o l e s .

Be f o r e r e s id u a l s t r e s s e s w e r e m e a s u r e d , t h e s u r f a c e a t t h e d e s i r e d l o c a t i o n w a s p r e p a r e d t o a 6 0 0 g r i t

f i n i s h . T h e s m o o th e d a r e a w a s t h e n t h o r o u g h ly c l e a n e d a n d a s t r a in g a g e r o s e t t e c e m e n te d o n to t h es p e c im e n ( F ig u r e 3 ). A f t e r c o n n e c t i n g t h e r o s e t te t o t h e s t r a in i n d i c a to r t h r o u g h t h e s w i t c h a n d b r id g e

u n i t , t h e m i l l i n g g u id e w a s p o s i t i o n e d o v e r t h e r o s e t t e a n d a h o l e d r i l l e d t h r o u g h t h e c e n t r e o f t h e ro s e t t e

in 0 .0 0 5 " i n c r e m e n t s ( F ig u r e 4 ) . A f t e r e a c h i n c r e m e n t , t h e d r i l l w a s s t o p p e d a n d t h e r e l i e v e d s t r a in s

m e a s u r e d . I n m o s t c a s e s , t h e m a jo r i t y o f t h e r e s id u a l s t r e s s e s w e r e r e l i e v e d i n t h e f i r s t t h r e e o r f o u r i n -

c r e m e n t s . T h e n o m in a l h o l e d i a m e te r w a s 1 .6 m m ( 0 .0 6 2 " ) a n d t h e m a x im u m h o le d e p th d r i l le d w a s 1 .3

m m ( 0 .05 0 " ) . M e a s u r e d s t r a in v a lu e s w e r e c o n v e r t e d t o r e s id u a l s t re s s b y c o n v e n t io n a l a n a ly s e s . A l l r e-

s idua l s t ress va lues repor ted a re the to ta l s t ress re l ieve d a f te r d r i l l ing a ho le dep th o f 1 m m (0 .040") .



No te t h a t r e s id u a l s t r es s m e a s u r e m e n t s o f t h is t y p e a r e g e n e r a l l y c o n s id e r e d t o b e a c c u r a te t o w i th in

a b o u t _ +10 % . As t h e r e s id u a l t e n s i l e s t r e ss e s a p p r o a c h t h e y i e ld s t r e n g th o f t h e m a te r i a l ( >5 0 % ) , t h e a c -

c u r a c y o f t h e m e a s u r e m e n t d e c r e a s e s d u e t o p l a s t ic d e f o r m a t io n a r o u n d t h e d r i l l e d h o l e . T h e p r e c i s i o n

o f t h e r e s id u a l s tr e s s m e a s u r e m e n t t e c h n iq u e wa s d e t e r m in e d b y m e a s u r in g t h e r e s id u a l s t r es s p r e s e n t i n

a b lo c k o f A5 1 6 Gr a d e 7 0 s t e e l f r o m w h ic h r e s id u a l s t r e s s e s h a d b e e n r e m o v e d b y s e a l i n g i t i n f o il ,

h e a t i n g i t t o 7 0 0 ° C ( 1 3 0 0 ° F ) f o r 2 4 h o u r s , a n d t h e n f u r n a c e c o o l in g . Re s id u a l t e n s i l e s tr e s s e s m e a s u r e d

in t h e t e s t b lo c k we r e l e s s t h a n 6 M P a ( 0 .8 k s i ) .

Figure 3: Two strain gage rosettes mounted on a multipass 308L stainless steel

weld. Also sh own are support mou nts for the milling guide.

Figure 4: P ositioning the drill prior to blind hole drilling in-situ, w ith a high-

speed turbine drill and a millin g guide.



Re s u l t s

T h e h i g h e s t t e n s il e s tr e ss e s m e a s u r e d o n e a c h k i n d o f w e l d m e n t a n d c o n d i t i o n a r e s h o w n i n T a b l e 1 .

I t c an be seen t ha t t he PW H T w as e f f ec t i ve i n r em ov i ng r e s i dua l t en s i l e s t r e s s i n ca rbon s t ee l i n t he

l a b o r a t o r y a n d f i e ld w e l d m e n t s . S t r e ss e s c l o se t o t h e y i e l d w e r e l o w e r e d t o n e g l i g ib l e v a lu e s b y P W H T .

C l a d w e l d m e n t s i n th e l a b o r a t o r y C P T W w e r e f o u n d t o h a v e r e l a t i v e l y m o d e r a t e r e s i d u a l s tr e ss a f t e r

w e l d c l add i ng . In con t r a s t, c l ad d i ges te r s w e re foun d to hav e h i gh r e s i dua l s t r e s se s i n t he w e l d m e t a l (68

t o 8 6 k s i ) w h i c h r e m a i n e d q u i t e h i g h a f te r P W H T ( 4 2 to 5 1 k s i) .

D i s c u s s i o n

A s e x p e c t e d a n d i n - li n e w i t h o t h e r m e a s u r e m e n t s i n d ig e s te r s ( 6 -8 ) , P W H T w a s f o u n d t o l o w e r

s t re s s i n ca rbon s t ee l t o a po i n t w he re s t re s s co r ro s i on sh ou l d no t be a conce rn . T he sam e ca nno t be s a i d

fo r s t a in l e s s s t ee l c l ad ves se l s , w h e re PW H T w as foun d on l y pa r t i a l l y e f f ec t i ve i n low er i ng s t re s s i n

309L s t a i n le s s s t ee l w e l d m e t a l . T h i s f i nd i ng i s subs t an t i a l l y con f i rm e d by t heo re t i ca l and ex pe r i m e n t a l

ve r i f i ca t i ons f rom o t he r i ndus t r i e s fo r aus t en i ti c s t ee l c l add i ng on t he i n s i de o f a ve s se l o r o n a f l a t su r -

f a ce , c o n s t r a i n e d b y c a r b o n s te e l ( 9 -1 1 ) . T h e l o w e r c o e f f i c ie n t o f t h e r m a l e x p a n s i o n o f c a r b o n s t ee l

m eans t ha t t he ca rbon s t ee l she l l sh r i nks l e s s on coo l i ng a f t e r PW H T and t h i s cons t r a i n s t he s t a i n l e s s

c l a d d i n g in t e n s io n . T h e c l a d d i n g w i ll r e m a i n i n t e n s i o n a t a m b i e n t a n d o p e r a t in g ( > 1 7 0 ° C ) t e m p e r a -

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

ro l l i ng o f t he c l ad p la t e , a re r e l i eved du r i ng PW H T and so a re no l on ge r p re sen t to coun t e rac t t he coo l -i ng - re l a t ed t ens i l e s tr e s s i n t he c l ad l aye r .

S t a i n l e s s s t ee l s a r e no t gene ra l l y su scep t i b l e t o s t r e s s co r ro s i on c rack i ng i n t yp i ca l k ra f t l i quo r s .

H o w e v e r , f i e l d e x p e r i e n c e h a s s h o w n t h a t s tr e ss c o r r o s i o n d o e s o c c u r i n v a p o u r - s p a c e c l a d d in g . S i m i -

l a r l y , a c i d c l ean i ng w i t h H C I a l so ca r r i e s a r isk o f s tr e s s co r ro s i on fo r aus t en i t i c s ta i n l e s s st eel s . T h i s

p a p e r h a s s h o w n t h a t a u s te n i ti c s ta i n le s s s t ee l c l a d d i n g m u s t d e p e n d o n m a t e r i a l i m m u n i t y r a t h e r th a n

s t re s s r educ t i on to avo i d s t r e s s co r ro s i on .

I t h a s r e c e n t l y b e c o m e a c o m m o n E u r o p e a n p r a c t ic e t o m a k e d i g e s t er s f r o m s o l id d u p l e x s t a in l e ss

s te e l, t y p i c a l l y 2 2 0 5 . T o a v o i d u n w a n t e d p h a s e t r a n s f o r m a t i o n s , c o n v e n t i o n a l P W H T a r e n o t d o n e o n

2205 o r o t he r du p l ex s t a i n l e s s s tee l s. T he se s t ee ls a r e s i gn i f i can t l y m o re r e s i s t an t t o s t r e s s co r ro s i onc rack i ng t han t he au s t en i ti c s ta i n l e s s s tee l s u sed fo r c l add i ng i n m o s t N or t h A m e r i can d i ges t e r s.

Co n c l u s i o n s

• D i r e c t o n - s i te m e a s u r e m e n t s i n k r a ft p u l p d i g e st e rs h a v e s h o w n t h a t p o s t w e l d h e a t t r e a tm e n t c a n b e

succes s fu l fo r s t r e ss r e l i e f o f ca rbo n s t ee l bu t i s on l y m arg i na l l y so fo r aus t en i t ic s t a i n le s s c l add i ng ;

m o de ra t e l y h i gh s t r e s se s w e re m easu r ed o n t he i n t e r i o r s t a i n le s s s t ee l d i ges t e r su r f ace a f te r PW H T .

• C l ad su r f aces can be sub j ec t t o vapo u r phase c rack i ng i n d i ges t e r s s ince r e s i dua l s tr e s s leve l s can

r e m a i n h i g h e v e n a f t e r P W H T .

A c k n o w l e d g e m e n t s

W e w o u l d l i k e t o a c k n o w l e d g e th e h e l p o f A n d r 6 M e n a r d a n d F r e d H e r b e r t in p r e p a r in g t h e l a b o r a-

t o r y w e l d m e n t s . T h e s u p p o r t a n d e n c o u r a g e m e n t o f m e m b e r s o f t h e T A P P I D i g e s t e r C r a c k i n g R e s e a r c h

C o m m i t t e e , t h r o u g h w h i c h t h is w o r k w a s p a r t i a ll y fu n d e d , i s a c k n o w l e d g e d w i t h t h a n k s .



T a b l e 1

T e s t L o c a ti o n M a x i m u m M e a s u r e d R e s i d u a l S t re s s

A s W e l d e d

MP a ( k s i )

A f t er P W H T

MP a ( k s i )

1 . C i r c u l a r p a t c h t e s t w e l d s

Ca r b o n s t ee l we l d m e t a l

3 0 9 L we l d o v e r l a yI n c o n e l 8 2 we l d o v e r l a y

2 . D i g e s t e r s

Ca r b o n s t e e l t o p w e l d

Ca r b o n s t ee l b o t t o m w e l d

Cl a d t o p we l d

C l a d b o t to m w e l d

2 3 0 ( 3 3 )

170 (24)1 0 0 ( 1 5 )

4 4 0 ( 6 4 )

4 6 0 ( 6 6 )

590 (86)

4 7 0 ( 6 8 )

- 1 4 ( 2 )

30 (4 )

7 ( 1 )

360 (52)

3 0 0 ( 4 3 )

R e f e r e n c e s

1 . S ingbe i l , D.L . , 'Pu lp and Pape r Canada",Vol . 87 , (3 ) pT93 (1986) .

2 . Cla rke , S . J . and S ingbe i l , D.L . , "Pu lp and Paper Indus t ry Cor ros ion P rob lems" , NACE Vol . 8 , p294

(1995) .

3 . W e n s l e y , A . " P u l p a n d P a p e r I n d u s t r y Co r r o s i o n P r o b l e ms " , V o l . 9 p 2 7 ( 1 9 9 8 ) .

4 . S ingbe i l , D.L . and G arne r , A. , "M ate r ia l s Pe r fo rm ance" , V o l . 27 , p38 , (1988) .

5 . AS T M E 8 3 7 - 8 9 " S t a n d a r d T e s t Me t h o d f o r De t e r mi n i n g Re s i d u a l S t r e s s e s b y t h e Ho l e - Dr i l l i n g

S t r a in - Ga g e M e t h o d " , Am e r i c a n S o c i e t y fo r T e s t in g a n d M a t e r ia l s ( 1 9 8 9 ).

6 . M o s k a l , M . D. " P u l p a n d P a p e r In d u s t r y Co r r o s i o n P r o b l e ms " , N A CE Vo l . 5 p l , ( 1 9 8 6) .

7 . T h o m a s , P . E . a n d Wh y t e , D . M . , " Ap p i t a " Vo l . 3 7 , p 7 4 8 ( 1 9 8 4 ).

8 . M o s k a l , M . D. , " p u l p a n d P a p e r I n d u s t r y Co r r o s i o n P r o b l e ms " , NA CE , V o l . 4, p 3 2 , (1 9 8 3 ) .

9 . J anov ic , A and Luc ia , A.C . , " In t . J . P res . Ves . & P ip ing" , Vo l . 22 , p l 11 (1986) .

10 . Faure , F . , "P roc . 4 h In t. Conf . on R es idua l S t re s s" , p1188 (1994) .

11 . P luye t t e , E . and Sprane l , J .M. , "P roc . 4 h In t. C one on R es idua l S t re s s" , p860 , (1994) .