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S c r i p t a M E T A L L U R G I C A V o l. 2 3, p p. 9 9 5 - 1 0 0 0 , 1 9 8 9 P e r g a m o n P r e s s p l c

P r i n t e d i n t h e U . S . A . A l l r i g h t s r e s e r v e d

F R A C T U R E M E C H A N I S M I N C O A R S E G R A IN E D H AZ

O F H S L A S T E E L W E L D S

S . L e e , B . C . K i m a n d D . Y . L e e

D e p a r t m e n t o f M a t e r i a l s S c ie n c e a n d E n g i n e e r i n g

P o h a n g I n s t i t u t e o f S c i e n c e a n d T e c h n o l o g y

P o h a n g , K y u n g b u k , 7 90 K o r e a

( R e c e i v e d J a n u a r y i i, 1 9 8 9 )

( R e v i s e d A p r i l 6 , 1 9 8 9)

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

R e c e n t l y , v e r y l ow t o u g h n e s s v a l u e s o f h e a t - a f f e c t e d z o n e s ( HA Z) h a v e b e e n n o t i c e d i nh i g h s t r e n g t h l ow a l l o y (H SL A) s t e e l s w e l d e d w i t h m u l t i - p a s s s u b m e r g e d a r c w e l d i n g

p r o c e d u r e s [ 1 , 2] . M e t a l l o g r a p h i c a n a l y s e s o f H SL A s t e e l w e l d s r e v e a l s i g n i f i c a n t l y d i f f e r e n t

r e g i o n s i n HA Z m i c r o s t r u c t u r e s . F o r e x a m p l e , i n s i n g l e p a s s w e l d s t h e r e a r e f o u r

c h a r a c t e r i s t i c r e g i o n s i n t h e H AZ d e p e n d i n g o n t h e p e a k t e m p e r a t u r e t h a t t h e r e g i o n w a s

e x p o s e d t o d u r i n g t h e w e l d t h e r m a l c y c le ; a c o a r s e g r a i n e d r e g i o n , a f i n e g r a i n e d r e g i o n , a n

i n t e r c r i t i c a l r e g i o n , a n d a s u b c r i t i c a l r e g i o n . I n m u l t i - p a s s w e l d s th e s e r e g i o n s u n d e r g o

m u l t i p l e t h e r m a l c y c l e s , r e s u l t i n g i n a n i n h o m o g e n e o u s a n d c o m p l i c a t e d d i s t r i b u t i o n . I t i s

n o w g e n e r a l l y a c c e p t e d t h a t t h e c o a r s e g r a i n e d r e g i o n h a s t h e lo w e s t t o u g h n e s s b e c a u s e i t

c o n s i s t s o f t h e f o l lo w i n g u n f a v o r a b l e m i c r o s t r u c t u r a l f e a t u r e s , v ~z ., l a r g e p r i o r a u s t e n i t e g r a i n

s i z e , u p p e r b a i n i t e , m a r t e n s i t e - a u s t e n i t e ( M -A ) c o n s t i t u e n t s , a n d m i c r o a l l o y p r e c i p i t a t e s [ 3 -7 ] .

T h e M -A c o n s t i t u e n t , w h i c h i s a l s o c a l l e d a h i g h - c a r b o n m a t e n s i t e i s l a n d , i s a d u a l p h a s e

m i c r o s t r u c t u r e f o r m e d i n t h e c o a r s e g r a i n e d H AZ w h e n t h e t h e r m a l c y c l e o f a s u b s e q u e n t w e l d

p a s s h a s a p e a k t e m p e r a t u r e i n t h e i n t e r c r i t i c a i t w o p h a s e ( O + ~ ) r e g i o n [ 3 - 5 ] . I n t h e

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

g r a i n b o u n d a r i e s a n d b a i n i t i c l a t h b o u n d a r i e s , a n d t h e a u s t e n i t e i s s t a b i l i z e d to r o o mt e m p e r a t u r e o r t r a n s f o r m s t o m a r t e n s i t e u p o n c o o li n g f r o m t h e i n t e r c r i t i c a l t e m p e r a t u r e .

A m o n g t h e a b o v e m i c r o s t r u c t u r a l f e a t u r e s i n t h e c o a r s e g r a i n e d H AZ, t h e M -A

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

a n d c r a c k s u s c e p t i b i l i t y [ 6 ,7 ] , a l t h o u g h t h e r e l a t i v e r a n k i n g o f t h e m i c r o s t r u c t u r e s d e p e n d s

o n t h e c h e m i c a l c o m p o s i t i o n s a n d t h e l o ca l t h e r m a l c y c l e s i n c l u d i n g p o s t w e l d h e a t t r e a t m e n t s .

I n t h i s p a p e r , t h e s p e c i f i c r o l e s o f t h e M - A c o n s t i t u e n t i n t h e p r o c e s s o f v o i d a n d m i c r o c r a c k

i n i t i a t i o n w e r e i n v e s t i g a t e d i n a 5 0 0 M P a c l a s s H S LA s t e e l . S p e c i f i c a l l y , b y e x a m i n i n g t h e

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

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

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

T h e e x p e r i m e n t w a s c a r r i e d o u t o n a 4 4 m m t h i c k p l a t e o f a n o r m a l i z e d B S 4 3 6 0 G r .5 0 D

s t e e l p r o v i d e d b y P o h a n g S t e e l C o. T h i s i s a h i g h - s t r e n g t h g r a d e s t e e l w i t h a t e n s i l e

s t r e n g t h o f a b o u t 5 00 M P a , w h o s e c h e m i c a l c o m p o s i t i o n i s 0 . 1 3 C - 0 . 4 S i - l . 4 3 M n - 0 . 0 1 9 P - 0 .0 0 1 S -

0 . 0 6 5 A I - 0 .3 C u - 0 . 2 2 N i - 0 .0 1 7 T i - 0 . 0 2 1 N b . T h e s p e c i m e n s w e r e s u b j e c t e d t o a w e l d s i m u l a t i o n ,l e a d i n g to c o a r s e g r a i n e d H AZ. T h e h e a t i n g r a t e w a s 10 ° C / s. A f t e r r e a c h i n g t h e p e a k

t e m p e r a t u r e o f 1 35 0 0C a n d a r e s t i n g t im e o f a b o u t 5 - 1 0 s e c , t h e s p e c i m e n s w e r e c o o l e d d o w n

w i t h a c o o l i n g t im e f r o m 8 0 0 ° C t o 5 00 °C ( d ts / s) o f 4 2 s . T h i s c o r r e s p o n d s t o a s u b m e r g e da r c w e l d i n g o f a 44 m m t h i c k p l a t e w i t h a h e a t i n p u t o f 5 0 k J / c m . T h e p e a k t e m p e r a t u r e o ft h e s e c o n d t h e r m a l c y c l e (T p 2) w a s v a r i e d b e t w e e n 6 0 0 0C a n d 1 2 0 0 0C . T h e C h a r p y V - n o t c h

s p e c i m e n s w i t h T - L o r i e n t a t i o n w e r e m a c h i n e d f r o m t h e o v e r s i z e d C h a r p y b l a n k s a f t e r t h e r m a l

s i m u l a t i o n t r e a t m e n t s a n d w e r e t h e n t e s t e d a t - 1 0 °C a n d - 8 0 °C .

I n o r d e r t o i n v e s t i g a t e t h e m i c r o m e c h a n i s m o f f r a ct u r e a n d d e t e r m i n e t h e s t ra i n

r e q u i r e d f o r v o i d i n it i at i on u n d e r h i g h t r ia x ia l s t r e s s f i e ld s , t e n si l e t e s t s a n d s u b s e q u e n t

9 9 50 0 3 6 - 9 7 4 8 / 8 9 $ 3 . 0 0 + . 00

C o p y r i g h t ( c) 1 9 8 9 P e r g a m o n P r e s s p l c



9 9 6 F R AC T U RE I N W EL DS V o l . 2 3 , N o . 6

m e t a i l o g r a p h i c a n a l y s e s w e r e c o n d u c t e d u s i n g c i r c u m f e r e n t i a l ly - n o t c h e d s p e c i m e n s . T h e

m a c r o s c o p i c s t r a i n a t a d i s t a n c e f ro m t h e p o i n t o f m i n im u m r a d i u s ( ~z ) c a n b e o b t a i n e d f r o mt h e s t r a i n d i s t r i b u t i o n p r o f i l e o f t h e t e n s i l e s p e c i m e n u s i n g t h e e v a p o r a t e d c a r b o n g r i d

m e t h o d [ 8 , 9 ] . T h e t e n s i l e s p e c i m e n s p u l l e d a n d u n l o a d e d p r i o r t o f r a c t u r e w e r e s e c t i o n e dp a r a l l e l t o t h e t e n s i l e a x i s , p o l i s h e d , e t c h e d a n d e x a m i n e d i n a S E M t o i d e n t i f y t h e l o c a t i o n

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

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

R e s u l t s a n d D i s c u s s i o n

T h e c o a r s e g r a i n e d H AZ o f t h e H S LA s t e e l w e l d s c a n b e r o u g h l y c a t e g o r i z e d i n t o f o u rr e g i o n s a c c o r d i n g t o t h e r e h e a t i n g t e m p e r a t u r e a s fo l lo w s : ( a) s u b c r i t i c a l l y r e h e a t e d z o n e

( SR C G H A Z), t h e z o n e r e h e a t e d b e l o w A c l , ( b ) i n t e r c r i t i c a l l y r e h e a t e d z o n e ( IC R C G H A Z), t h ez o n e r e h e a t e d b e t w e e n A c l a n d A ce , ( c) s u p e r c r i t i c a l l y r e h e a t e d z o n e ( SC R C G H A Z), t h e z o n e

r e h e a t e d a b o v e A c~ a n d b e l o w a b o u t 1 2 0 0 0C , a n d ( d ) u n a l t e r e d c o a r s e g r a i n e d z o n e ( U A CG

H A Z) , t h e z o n e t h a t i s n o t r e h e a t e d o r r e h e a t e d a b o v e 1 2 00 ° C.

T h e r e s u l t s o f C h a r p y t e s t s i n th e s i m u l a t e d c o a r s e g r a i n e d H AZ a r e s h o w n in F i g u r e

1 . I t c a n b e s e e n f r om F i g u r e 1 t h a t t h e c o a r s e g r a i n e d HA Z e x h i b i t s l o w C h a r p y i m p a c t

e n e r g y i n t w o c a s e s . T h e f i r s t i s th e o n e s i m u l a t e d b y a s i n g l e t h e r m a l c y c le w i t h a p e a k

t e m p e r a t u r e o f 1 3 50 0C ( UA C G H AZ) a n d t h e s e c o n d i s t h e o n e s i m u l a t e d b y d o u b l e t h e r m a l

c y c l e s w i t h Tp i n t h e r a n g e o f 7 5 0 oC t o 8 0 0 0C ( IC R C G H A Z). M i c r o a t r u c t u r e s o f t h e s e t w o

z o n e s a r e m a i n l y c o m p o s e d o f u p p e r b a i n i t e . W h e n t h e s p e c i m e n s w e r e s p e c i a l l y e t c h e d b y

u s i n g a 2 - s t a g e e l e c t r o l y t i c e t c h i n g t e c h n i q u e [ 7 ] , M - A c o n s t i t u e n t s w e r e r e v e a l e d i n t h e

c o a r s e g r a i n e d b a i n i t i c m i c r o s t r u c t u r e a . F i g u r e 2 i s a t y p i c a l S EM m i c r o g r a p h o f M - A

c o n s t i t u e n t s i n t h e s im u l a t e d I C RC G HA Z. B e c a u s e t h e s e M - A c o n s t i t u e n t s a r e k n o w n t o b e

f o r m e d a l o n g t h e b a i n i t e l a t h e [ 3 ] , t h e i r m a j o r c o n f i g u r a t i o n i s s t r i n g e r t y p e . A r e l a t i v e l y

s m a ll a m o u n t o f b l o c k y t y p e M - A c o n s t i t u e n t s o c c u r s a s w e ll . T h e v o l u m e f r a c t i o n o f M - A

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

F i g u r e 1 . B y c o m p a r i n g t h e a m o u n t o f M - A c o n s t i t u e n t s a n d C h a r p y e n e r g y , o n e f i n d s t h a t

t h e t o u g h n e s s v a l u e s o f t h e c o a r s e g r a i n e d H AZ a r e s t r o n g l y a f f e c t e d b y t h e a m o u n t o f M - A

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

t h e c o a r s e g r a i n e d H AZ.

T h e f r a c t u r e s u r f a c e s o f C h a r p y i m p a c t s p e c i m e n s a n d c i r c u m f e r e n t l a l l y n o t c h e d t e n s i l es p e c i m e n s w e r e e x a m i n e d t o d e f i n e t h e n a t u r e o f t h e f r a c t u r e s u r f a c e s . A ll t h e f r a c t u r e

s u r f a c e s o f b o t h s p e c i m e n s , p r e s e n t e d i n d e t a i l e l s e w h e r e [ 2 ] , w e r e r o u g h a n d f i b r o u s i n

a p p e a r a n c e . Q u a n a t i t a t i v e m e a s u r e m e n t s o f t h e d i m p l e s i z e s o n m a n y f r a c t u r e s u r f a c e s

r e v e a l e d t h a t t h e f e a t u r e s o n t h e f r a c tu r e s u r f a c e s w e r e t h e s a m e fo r b o t h s p e c i m e n

g e o m e t r i e s . T h u s , i t i s s u g g e s t e d t h a t th e m i c r o s t r u c t u r a l p r o c e s s e s w h i c h l e a d t o f r a c t u r e

a r e t h e s a m e f o r b o t h t e s t s i n t h i s s i m u l a t e d s t e e l HA Z e v e n t h o u g h t h e l e v e l o f t h e s t r a i n

r a t e i s q u i t e d i f f e r e n t .

T o i d e n t i f y a n d q u a n t i f y t h e m i c r o m e c h a n i c a i f r a c t u r e p r o c e s s e s i n v o l v e d i n v o i d a n dc l e a v a g e m i c r o c r a c k i n i t i a t i o n , t h e r e s u l t s o f t e n s i l e t e s t a a n d s u b s e q u e n t m e t a l l o g r a p h i c

s t u d i e s o f t h e c i r c u m f e r e n t i a l l y - n o t c h e d s p e c i m e n s s u b j e c t e d t o t h e d o u b l e c y c l e s w i t h Tp z o f

8 0 0 oC w h e r e t h e IC R C G H AZ w a s o b t a i n e d w e r e a n a l y z e d . F i g u r e 3 ( a ) s h o w s t h e

m i c r o s t r u c t u r e b e n e a t h t h e f r a c t u r e s u r f a c e s o f t h e t e n s i l e s p e c i m e n s f r a c t u r e d a t - 1 0 °C .

V o i ds a r e o b s e r v e d t o i n i t i a t e b y t h e c r a c k i n g a n d d e c o h e s i o n o f t h e M -A c o n s t i t u e n t s f r o m

t h e m a t r i x . O n c e n u c l e a t e d , v o i d s g r o w m a i n l y in t h e d i r e c t i o n o f t e n s i l e a x is ; t r a n s v e r s eg r o w t h o c c u r s o n l y i n t h e l a t e s t a g e s o f n e c k i n g . W h e n t h e t e s t t e m p e r a t u r e w a s l o w e r e d t o- 8 0 oC, a n u m b e r o f m i c r o c r a c k s a l o n g a z i g - z a g p a t h w e r e o b s e r v e d i n s t e a d o f c o n n e c t e dv o i d s ( se e F i g u r e 3 ( b ) ) . A s t h e M -A c o n s t i t u e n t i s c o n s i d e r e d t o b e b r i t t l e p h a s e , t h e M -A

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

a r e a p p l i e d . I t i s i n t e r e s t i n g t o n o t e t h a t t h e s e m i c r o c r a c k s w e r e e a s i l y f o r m e d f r o m t h es t r i n g e r - t y p e M - A c o n s t i t u e n t s a t t h e o r i e n t a t i o n o f 4 0 - 5 0 d e g r e e t o t h e t e n s i l e a x i s w h e n t h e

M - A c o n s t i t u e n t s t r i n g e r s w e r e a p p r o x i m a t e l y p a r a l l e l t o t h e t e n s i l e a x i s a s s h o w n in F i g u r e3 ( c) . C o n v e r s e l y , o n l y f e w m i c r o c r a c k s w e r e o b s e r v e d w h e n t h e s t r i n g e r - t y p e M - A

c o n s t i t u e n t s w e r e a l i g n e d n o r m a l to t h e t e n s i l e ax i s. T h i s t y p e o f i n i t i a l c r a c k i n g o f M - A

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

b y M i l l e r a n d S m i t h [1 0 ]. F i g u r e 4 s h o w s a i d e a l i z ed s k e t c h o f s h e a r c r a c k i n g m e c h a n i s m i n

M - A c o n s t i t u e n t s . W h e n t h e s p e c i m e n i s u n d e r t e n s i l e s t r e s s e s , s m a l l v o i d s w h i c h a r e

n u c l e a t e d i n i t i a l l y a t M -A c o n s t i t u e n t s p r o m o t e s h e a r i n g o f t h e b a i n i t i c m a t r i x o n t h e s l ip



V o l . 2 3 , N o . 6 F R A C T U R E I N W E L D S 9 9 7

plane. Th ese initial voids then link up to form a micro crack if the shear bec ome s large

e n o u g h . I n t h e c a s e w h e r e c r a c k g r o w t h o c c u r s b y c le av ag e, t h e f i rs t m i c r o c r a c k f o r m e d i s

capable of prop agat ing catastropically as a cleavage crack. Thus, the primar y rol e of M -A

constituents wh os e size is considera bly larger than that of cementite lamsllae in pearlite

colonies is to initiate the microc racks by the the sam e crack ing m ec ha ni sm as the cementite

lamaliae.

Figur e 5 sh ow s the relationship bet wee n the p erce ntag e of M -A constituent associated

with voids (%VM) an d the macrosc opic strain (¢z) in a not ched tensile spec imen subjecte d to

t h e I C R C G H A Z s i m u l a t i o n . E l o n g a t e d M n S i nc lu si on p a rt ic le s g e n e ra l l y d e c o h e r e d e v e n a t

mac ros cop ic strains well bel ow the strains at wh ic h voids initiated at M - A constituents.

Examination of the fracture surfaces in the S E M using E DS indicated that less than 5 pct of

the voids contri buting to fracture originated at Mn S particles, sh ow in g that vo id initiation at

M- A constituent is the domi nant initiation mecha nism. As s ho wn in Figur e 5, % V M increases

slowly at first, approxima tely linearly with strain, an d then increases mo re sharpl y with

i nc rea s i ng s t ra i n . The smoot h i nc rea se of %VM appea r s t o be t he re su l t o f r e l a t i ve l yex t ens i ve and un i form d i s t r i bu t i on of vo i ds ev i dence d by t he homogeneous de forma t ion al ong

t he cen t e r l ine of f rac t u red t ens i l e spec i mens . The min imum s t ra i n s a t whi ch vo i ds appea r(E,~) ar e ver y low, i .e . , in the ra ng e 0.13 ~ 0.15 as ind icat ed by arro ws in Fig ur e 5, andnea r l y i ndep endent o f t he t e s t i ng t empera t u re s i n con t ra s t t o t he t rue f rac t ur e s t ra i ns whi ch

were found t o depend pr edomi nan t l y on t he t e s t t empe ra t ure s . Consequent l y , t he l a ck ofto ug hn es s in the simula ted ICRCG HAZ is mainly at tr ib ut ed to low val ues of Cv~ re su lt i ng fromprema t u re vo i d nuc l ea t i on a t M-A cons t i t uen t o r b r i t t l e f r ac t ure o f M-A cons t i t u en t .

Based on t he above cons i d e ra t i on s , i t appea r s t ha t l ow Cha rpy en e rgy i n loca l b r i t t l ezone re su l t s f rom t he forma t i on of M-A cons t i t uen t s , whi ch a re ea s i ly c rac ked i n t he ca se of

appl y i ng ex t e rna l s t r e s se s . I t c an be t hus conc l uded t ha t t he amount o f M-A cons t i t uen t sshould be lowered to improve the toughness of the HAZ.

S u m m ~ r 7

1. The e f fec t s o f M -A cons t i t uen t on t he mi c romechani sm of f rac t ure p roce sse s i n t he co a rse

gra ine d HAZ of HSLA stee l welds were inv est iga t ed by examining the ini t i a t ion of voids andmi c roc racks i n sec t i oned t ens i l e spec i mens .

2. The coa rs e gr a i ned HAZ i nc l udes t wo l oca ll y b r i t t l e mi c r os t ru c t ure s , whose t oughn ess va l uesa re s t rong l y a f fec t ed by t he amount o f M-A cons t i t uen t s .

3. Voids and mi c roc rac ks a re obse rv ed t o i n it i a t e a t t he M -A cons t i t uen t s by t he s hea rc r a c k i n g p r o c e s s , i . e . , c rack i n g or decohes i on of t he M-A cons t i t uen t s a t t he or i en t a t i on of 40-50 deg ree to the tens i le axis when the M-A con st i t uen ts a r e appro ximate ly para l le l to the

tensi le axis .

4. The void ini t i a t ion s t r a in in the coars e gr a ine d HAZ are v ery low because of prem atu re void

nuc l ea t i on or b r i t t l e f rac t ure a t M-A cons t i t uen t s , conf i rmi ng t ha t t he M -A cons t i t uen t i s t hemai n met a l l u rg i cal f ac t or whi ch gove rn s t he t ou ghness o f t he coa rse gr a i ned HAZ.

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

Thi s work was suppor t ed by t he Resea rch Ins t i t u t e o f Indu s t r i a l Sc i ence and

Techno logy (RIST) und er con t ra c t numbe r 8-118-A. Use of the RIST Technica l Serv ice CenterFac i l i t y i s a l so gra t e fu l l y acknowl edged .

R e f e r e n c e s

1. J. G. Youn and H. J. Kim: P r o c . o f t h e 2 n d C o n f . o n M e c h a n i c a l B e h a v i o r s o f M a t e r i a ls ,

Seoul, Korea, pp. 35-43, (1988) (Korean).2. B. C. Kim, J. H. Eom, C. S. Lee, S. Lee and D. Y. Lee: "Local Brittle Zone of Offshore

S t ruc t u ra l S t ee l We lds" , submi t t ed t o J . o f t h e Korean W e l d i n g S o c i e t y , (1989).3. J. Y. Koo and A. Ozekcin: " W e l d i n g M e t a l l u r g y o f S t r u c t u r a l S t e e l s " , Ed. J. Y. Koo, Pub.,

A/ME, pp. 119-135, (1987).4. D. P. Fairchild: i b i d , pp. 303-318.5. R. Denys and H. I. McHsnry: Proc. of t he 7 t h I n t . Conf. on O f f s h o r e M e c h a n i c s a n d A r c t i c

E n g i n e e r i n g , Houston, Texas, pp. 379-385, (1988).



9 9 8 F R A C T U R E I N W E L D S V o l . 2 3, N o . 6

6 . T . H a z e a n d S . A i h a r a : ibid, p p . 5 1 5 - 5 2 3 .

7 . K . U c h i n o a n d Y . O h n o : ibid, p p . 1 5 9 - 1 6 5 .

8 . S . L e e : S c r . Mete11., 2 2 , p p . 5 9 - 6 4 , ( 1 9 8 8 ) .9 . I t. C o u q u e , R . J . A s a r o t J . D u f f y , a n d S . L e e : Met.all. Trans. A, 1 9 A , p p . 2 1 7 9 - 2 2 0 6 , ( 1 9 8 8 ) .1 0 . L . E . M i l l e r a n d G . C. S m i t h : J. Iron S L e e l Inst., 2 0 8 , p p . 9 9 8 - 1 0 0 5 , ( 1 9 7 0 ) .

15 0

0

T - - , 3 5 o . c Id t 4 / a - - 4 2 $ e c

( ) : V O L . F R A C T IO N O F M - A

( o . ~ o r I r a ) L .

t ~ c - " ~ ' s R c GC R e '--" -" --~ ' ~ C ' U " ~ ~

( c )

F IG . 1 . C h a r p y i m p a c t e n e r g y v s . p e a k t e m p e r a t u r e o f t h e

s e c o n d t h e r m a l c y c l e ( Tp =), s h o w i n g t w o l o c a l b r i t t l e z o n e s ,

U A CG H AZ a n d I CR C G H AZ . P a r e n t h e s e s i n d i c a t e t h e v o l u m e

f r a c t i o n o f M -A c o n s t i t u e n t s .

F IG . 2 . S c a n n i n g e l e c t r o n m i c r o g r a p h o f t h e s i m u l a t e d I C RC G

H AZ p r e p a r e d b y th e 2 - s t a g e e l e c t r o l y t i c e t c h i n g t e c h n i q u et o o b s e r v e M - A c o n s t i t u e n t s .



V o l . 2 3 , N o . 6 F R A C T U R E I N W E L D S 9 9 9

FIG. 3. Scanni n g e l ec t ron mi c rograp hs of no t ched roundtensi le spec imens sec t ioned para l le l to the tens i le axiss h o w i n g t h e m i c r o s t r u c t u r e b e n e a t h t h e f r a c t u r e s u r f a c e s o fthe s imulate d ICRCG HAZ fr act ur ed at (a) -10 °C and (b)- (c)-80 °C. The tens i le axis i s ver t ic a l for the micro grap hs .



1000 FRACTURE IN WELDS V ol . 23 , No. 6

0 1 i I b

c d

FZG. 4. Schematic des cripti on of shear crac king proc ess ofM-A cons ti tu ents , afte r Miller and Smith [10].

0 3 2 O

" r

I - - 1 5 ,

L ~

00U3

U3

., ~ 5 '

I

I i t

0

SIM. ICRCG HAZd ~ / 5 = 4 2 sac

0 TESTED AT -1 0° C• TESTED AT -6 0 "0 #

!I

/ /• t

i ~ , ! s I i

0 . 4 0 . 6

£ z

0 . 8 ' 1 . 0

FIG. 5. The peroentage of M-A consti tuents associated withvoid s vs. the macroscopic stra in (~,) in the notched tensilespe cime n su bj ec te d to ICRCG HAZ simulation. Arrow showsvoid ini t ia tion st rain (E~ ) at which voids were f i rs tde t ec t ed .

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