Fracture Mechanismincoarse Grained Haz
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Scripta METALLURGICA Vol. 23, pp. 995-1000, 1989 Pergamon Press plc Printed in the U.S.A. All rights reserved FRACTURE ME CHA NISM INCO ARSE GRAINED HAZ OF HSLASTE EL WEL DS S. Lee, B. C. Kim and D. Y. Lee Department of Materials Science and Engineering Pohang Institute of Science and Technology Pohang, Kyungbuk, 790 Korea (Received January ii, 1989) (Revised April 6, 1989) Introduction Recently, very low toughness values of heat-affected zones (HAZ) have been noticed in high strength low alloy (HSLA) steels welded with multi-pass subm erged arc welding procedures [1,2]. Metallographic analyses of HSLA steel welds reveal significantly different regions in AZ microstructures. For example, in single pass welds there are four characteristic regions in the HAZ depending on the peak temperature that the region was exposed to during the weld thermal cycle; a coarse grained region, a fine grained region, an intercritical region, and a subcritical region. In multi-pass welds these regions undergo multiple thermal cycles, resulting in an inhomogeneous and complicated distribution. It is now generally accepted that the coarse grained region has the lowest toughness because it consists of the following unfavorable microstructural features, v~z., large prior austenite grain size, upper bainite, martensite-austenite (M-A) constituents, and microalloy precipitates [3-7]. The M-A constituent, which is also called a high-carbon matensite island, is a dual phase microstructure formed in the coarse grained HAZ when the thermal cycle of a subsequent weld pass has a peak temperature in the intercriticai two phase (O + ~ ) region [3-5]. In the intercritical region, austenite nucleates and grows preferentially along the prior austenite grain boundaries and bainitic lath boundaries, and the austenite is stabilized to room temperature or transforms to martensite upon cooling from the intercritical temperature. Among the above microstructural features in the coarse grained HAZ, the M-A constituent is the main factor causing deterioration of toughness because of its high hardness and crack susceptibility [6,7], although the relative ranking of the microstructures depends on the chemical compositions and the local thermal cycles including post weld heat treatments. In this paper, the specific roles of the M-A constituent in the process of void and microcrack initiation were investigated in a 500 MPa class HSLA steel. Specifically, by examining the initiation of voids and microcracks in sectioned tensile specimens, we identified the micromechanism of fracture processes and related it to the observed Charpy impact energy. Experimental The experiment was carried out on a 44 mm thick plate of a normalized BS4360 Gr.50D steel provided by Pohang Steel Co. This is a high-strength grade steel with a tensile strength of about 500 MPa, whose chemical composition is 0.13C-0.4Si-l.43Mn-0.019P-0.001S- 0.065AI-0.3Cu-0.22Ni-0.017Ti-0.021Nb. The specimens were subjected to a weld simulation, leading to coarse grained HAZ. The heating rate was 10 °C/s. After reaching the peak temperature of 1350 0C and a resting time of about 5-10 sec, the specimens were cooled down with a cooling time from 800 °C to 500 °C (dts/s) of 42 s. This corresponds to a submerged arc welding of a 44 mm thick plate with a heat input of 50 kJ/cm. The peak temperature of the second thermal cycle (Tp 2) was varied betw een 600 0C and 1200 0C. The Charpy V-notch specimens with T-L orientation were machined from the oversized Charpy blanks after thermal simulation treatments and were then tested at -10 °C and -80 °C. In order to investigate the micromechanism of fracture and determine the strain required for void initiation under high triaxial stress fields, tensile tests and subsequent 9 9 5 0036-9748/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc
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Transcript of Fracture Mechanismincoarse Grained Haz
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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
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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
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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).
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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 .
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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 .
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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 .
