C o l l o q u e C 7 , s u p p l k m e n t au n o 11, Tome 47, N o v e m b r e 1986
MICROSTRUCTURAL CHARACTERIZATION OF PRIMARY COOLANT PIPE STEEL
M.K. MILLER and J . BENTLEY
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
A b s t r a c t
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Atom probe f i e l d - i o n microscopy, a n a l y t i c a l e l e c t r o n microscopy, and o p t i c a l microscopy have been used t o i n v e s t i g a t e t h e changes t h a t o c c u r i n t h e m i c r o s t r u c t u r e o f c a s t CF 8 primary c o o l a n t p i p e s t a i n l e s s s t e e l a f t e r l o n g term t h e r m a l a g i n g . The c a s t duplex m i c r o s t r u c t u r e c o n s i s t e d o f a u s t e n i t e w i t h 15%& - f e r r i t e . I n v e s t i g a t i o n o f t h e aged mater i a l r e v e a l e d t h a t t h e f e r r i t e s p i n o d a l l y decomposed i n t o a f i n e s c a l e d network o f a and a'. A f i n e G-phase p r e c i p i t a t e was a l s o observed i n t h e f e r r i t e . The observed d e g r a d a t i o n i n mechanical p r o p e r t i e s is probably a consequence o f t h e s p i n o d a l decomposition i n t h e f e r r i t e .
INTRODUCTION
The mechanical p r o p e r t i e s o f t h e c a s t s t a i n l e s s s t e e l p i p e s t h a t a r e used t o c a r r y t h e primary c o o l a n t water i n p r e s s u r i z e d w a t e r n u c l e a r r e a c t o r s a r e known t o be degraded by exposure t o e l e v a t e d t e m p e r a t u r e s i n t h e r a n g e 300 t o 400°C. The c a s t s t a i n l e s s s t e e l used f o r t h e s e p i p e s forms a duplex m i c r o s t r u c t u r e o f a u s t e n i t e and f e r r i t e . The f e r r i t e i n c r e a s e s t h e y i e l d s t r e n g t h o f t h e c a s t m a t e r i a l and a l s o r e d u c e s t h e s u s c e p t i b i l i t y t o h o t c r a c k i n g d u r i n g s o l i d i f i c a t i o n . However, l o n g t e r m t h e r m a l a g i n g produces a n i n c r e a s e i n , h a r d n e s s and t e n s i l e p r o p e r t i e s , t o g e t h e r w i t h a d e c r e a s e i n t h e impact p r o p e r t i e s , d u c t i l i t y , and toughness.
A p r e v i o u s combined atom probe f i e l d - i o n microscopy (APFIM) and a n a l y t i c a l e l e c t r o n microscopy (AEM) i n v e s t i g a t i o n o f a d u p l e x CF 8M a l l o y i n d i c a t e d t h a t two phase t r a n s f o r m a t i o n s had occured d u r i n g a g i n g which could c o n t r i b u t e t o t h e changes i n mechanical p r o p e r t i e s El]. The f e r r i t e , a f t e r a g i n g f o r 7500 h a t 400°C, had s p i n o d a l l y decomposed t o form a n i n t e r c o n n e c t e d network o f i r o n - r i c h a phase and chromium-enriched a' phase. The p r e c i p i t a t i o n o f a Glphase n i c k e l s i l i c i d e was a l s o observed i n t h e f e r r i t e . I t was s u g g e s t e d t h a t s m a l l changes i n t h e composition o f t h e a l l o y could a f f e c t t h e q u a n t i t i e s o f t h e phases p r e s e n t by a l t e r i n g t h e p o s i t i o n o f t h e m i s c i b i l t y gap o r by s u p p r e s s i n g t h e f o r m a t i o n o f G-phase [ I ] .
I n t h i s p a p e r , t h e m i c r o s t r u c t u r e p r e s e n t a f t e r extended t h e r m a l a g i n g o f a c a s t CF 8 s t a i n l e s s s t e e l , a v a r i a n t o f t h e p r e v i o u s s t e e l w i t h a lower molybdenum c o n t e n t , w i l l be p r e s e n t e d and compared t o t h e p r e v i o u s l y r e p o r t e d r e s u l t s . The t e c h n i q u e s of atom probe f i e l d - i o n microscopy, a n a l y t i c a l e l e c t r o n microscopy, and o p t i c a l microscopy were used t o c h a r a c t e r i z e t h e m i c r o s t r u c t u r e .
Chopra and Chung showed t h a t a CF 8 s t a i n l e s s steel, ( h e a t 278), s u f f e r e d a d r a m a t i c l o s s i n impact p r o p e r t i e s , d r o p p i n g t o a l m o s t 15% o f t h e i n i t i a l v a l u e a f t e r prolonged a g i n g [2]. Some m i c r o s t r u c t u r a l c h a r a c t e r i z a t i o n o f s e v e r a l h e a t s o f CF 8 and o t h e r s i m i l a r s t a i n l e s s s t e e l s was a l s o p r e s e n t e d by t h e s e a u t h o r s [2,31.
EXPERIMENTAL
The s t a i n l e s s s t e e l used i n t h i s i n v e s t i g a t i o n was a c a s t CF 8 a l l o y ( h e a t 278) from
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986741
C7-240 JOURNAL DE PHYSIQUE
Georg F i s c h e r Co. of S w i t z e r l a n d . The nominal compositions of t h i s a l l o y [2] and t h e p r e v i o u s l y examined CF 8M a l l o y [ l ] a r e given i n Table 1. The CF 8 s t e e l was examined a f t e r l a b o r a t o r y a g i n g f o r 70,000 h a t 300, 350 o r 400°C. The APFIM a n a l y s e s were conducted on t h e ORNL atom probe [4]. The AEM a n a l y s e s were performed on P h i l i p s EMQOOT/FEG and EM430T a n a l y t i c a l e l e c t r o n microscopes both equipped with EDAX 9100/70 energy d i s p e r s i v e X-ray spectrometer (EDS) and Gatan 607 e l e c t r o n energy l o s s s p e c t r o m e t e r (EELS) systems.
Table 1. Nominal composition of t h e CF 8 and CF 8M s t e e l s ( w t % ) .
RESULTS Alloy CF 8 CF 8M
O p t i c a l microscopy i n d i c a t e d t h a t t h e c a s t and aged m a t e r i a l s c o n s i s t e d of a duplex m i c r o s t r u c t u r e of a u s t e n i t e w i t h approximately 15% f e r r i t e , i n agreement w i t h t h e c a l c u l a t e d value of 19% 121. ', comparison of t h e duplex m i c r o s t r u c t u r e of t h e m a t e r i a l s aged a t 300 and 400°C is shown i n t h e t h e e l e c t r o n m i c r o g r a p h s i n f i g u r e 1. I n t h e m a t e r i a l aged a t 400°C t h e f e r r i t e had undergone a s m a l l amount o f r e v e r s i o n t o a u s t e n i t e a t t h e f e r r i t e - a u s t e n i t e i n t e r f a c e , t o a d e p t h of approximately 4 urn, w i t h t h e p r e c i p i t a t i o n of some c a r b i d e s , f i g u r e l b . Most of t h e s e p r e c i p i t a t e s were p r e f e r e n t i a l l y l o c a t e d a t t h e o r i g i n a l f e r r i t e - a u s t e n i t e i n t e r f a c e . Analysis of t h e s e p r e c i p i t a t e s by AEM and APFIM r e v e a l e d t h a t they were chromium-rich M23C6 c a r b i d e s . No c a r b i d e s o r r e v e r s i o n o f t h e f e r r i t e were observed i n t h e m a t e r i a l s aged a t 300nC, f i g u r e 1 a . The f e r r i t e had s l i g h t l y r e v e r t e d i n t h e m a t e r i a l aged a t 350°C, but no c a r b i d e s were observed.
The microhardness of t h e f e r r i t e was found t o i n c r e a s e on a g i n g , w h e r e a s t h e m i c r o h a r d n e s s o f t h e a u s t e n i t e d i d not change s i g n i f i c a n t l y . Analyses of t h e composit i o n s of t h e f e r r i t e and a u s t e n i t e by energy d i s p e r s i v e X-ray spectroscopy r e v e a l e d t h a t t h e f e r r i t e was e n r i c h e d i n chromium, s i l i c o n , and molybdenum and d e p l e t e d i n n i c k e l and manganese a s shown i n Table 2.
C r 20.2 20.8
rlg.1. A u s t e n i t e ( Y )
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f e r r i t e (6) i n t e r f a c e s i n c a s t CF 8 - s t a i n l e s s s t e e l aged a t ( a ) 300° and ( b ) 400aC. The f e r r i t e r e v e r t s t o a u s t e n i t e and M23Cg a t 400°C but n o t a t 300°C. I n (b) t h e o r i g i n a l and f i n a l f e r r i t e - a u s t e n i t e i n t e r f a c e s a r e noted a t p o s i t i o n s 1 and 2 , r e s p e c t i v e l y .N i 8.3 10.6
S i 1.0 0.81
Mn 0.28 0.79
Mo 0.13 2.5
C 0.038 0.04
N 0.027 0.042
S 0.019 0.021
P 0.008 0.016
Fe balance balance
f o r 70000 h a t 400°C (wt%).
1
A u s t e n i t e l ~ e r r i t eChromium I 20.2 1 28.6
8.4
1 ::i6
Nickel
Manganese 0.22 0.08
Molybdenum 0.26
S i l i c o n 0.99 1.42
I r o n Balance Balance
More d e t a i l e d a n a l y s e s of t h e f e r r i t e by AEM and APFIM r e v e a l e d t h a t i t had decomposed during aging. E l e c t r o n micrographs of t h e s t r u c t u r e i n m a t e r i a l aged a t 300 and 400°C a r e shown i n f i g u r e 2. The s c a l e of t h i s two p h a s e m o d u l a t e d m i c r o s t r u c t u r e was measured from t h e s e e l e c t r o n micrographs a s 4 and 9 nm i n t h e 300 and 400°C aged m a t e r i a l s , r e s p e c t i v e l y . A f i e l d - i o n micrograph of t h e same two phase m i c r o s t r u c t u r e i n t h e 400°C aged m a t e r i a l is shown i n f i g u r e 3, where t h e d a r k l y imaging a' and t h e b r i g h t l y imaging a phases a r e e v i d e n t . The p e r i o d i c i t y of t h e modulations o f t h e two phases was measured from FIM micrographs t o be 7 nm.
F i e l d e v a p o r a t i o n sequences a l s o r e v e a l e d t h a t t h e modulated m i c r o s t r u c t u r e was i n t e r c o n n e c t e d i n t h r e e dimensions, i n d i c a t i v e of phase s e p a r a t i o n by i s o t r o p i c s p i n o d a l decomposition. An atom probe composition p r o f i l e through t h e f e r r i t e , f i g u r e 4 , a l s o i n d i c a t e s t h a t t h e f e r r i t e had decomposed i n t o a chromium-enriched a' phase and an i r o n - r i c h a phase.
AEM a l s o i n d i c a t e d t h e presence of some very f i n e p r e c i p i t a t e s i n t h e f e r r i t e a s shown i n f i g u r e 5. These p r e c i p i t a t e s were i d e n t i f i e d a s G-phase from t h e i r cube- on-cube o r i e n t a t i o n r e l a t i o n s h i p with t h e f e r r i t e , t h e f a c e c e n t e r e d cubic s t r u c t u r e
(Fm3m space g r o u p ) , t h e l a t t i c e parameter of 1 . l l nm, and t h e absence of t h e 400 r e f l e c t i o n [ l ,5].
I n both 300 and 400°C aged m a t e r i a l s , d i s t i n c t bimodal s i z e d i s t r i b u t i o n s of t h e G- p h a s e p r e c i p i t a t e s were o b s e r v e d . The s m a l l e r p r e c i p i t a t e s were randomly d i s t r i b u t e d i n t h e f e r r i t e m a t r i x , whereas t h e l a r g e r p r e c i p i t a t e s were a s s o c i a t e d with d i s l o c a t i o n s . T h i s is i l l u s t r a t e d i n f i g u r e 6 which shows C-phase p r e c i p i t a t e s imaged i n dark f i e l d with a p r e c i p i t a t e r e f l e c t i o n , d i s l o c a t i o n s imaged under weak beam dark f i e l d c o n d i t i o n s , and a s u p e r p o s i t i o n of t h e s e images.
Fig. 2. Phase c o n t r a s t e l e c t r o n micrographs o f t h e f e r r i t e i n CF 8 s t a i n l e s s s t e e l showing i s o t r o p i c s p i n o d a l decomposition i n t h e m a t e r i a l s aged f o r 70000 h a t ( a ) 300 and ( b ) 400°C.
C7-242 JOURNAL DE PHYSIQUE
Distance
Fig. 3. Field-ion micrograph o f t h e Fig. 4. Atom probe composition p r o f i l e f e r r i t e i n t h e 400°C aged through t h e f e r r i t e i n t h e 400°C a l l o y showing d a r k l y imaging aged s t e e l i n d i c a t e s chromium- a' and b r i g h t l y imaging a. e n r i c h e d a' and i r o n - r i c h a phases.
Fig. 5. Dark f i + l d e l e c t r o n micrographs o f t h e CF 8 steel showing G-phase p r e c i p i t a t e s , ( a ) i n m a t e r i a l aged a t 300°C, (b) on d i s l o c a t i o n s i n m a t e r i a l aged a t 400°C, and ( c ) i n t h e m a t r i x i n m a t e r i a l aged a t 400°C.
I n s e t i n ( b ) shows f i n e f r i n g e s p r e s e n t on t h e p e r c i p i t a t e s and t h e i n s e t i n ( c ) shows a 110 o r i e n t e d d i f f r a c t i o n p a t t e r n from t h e s e p r e c i p i t a t e s . The l a r g e r p r e c i p i t a t e s were a s s o c i a t e d w i t h d i s l o c a t i o n s i n m a t e r i a l s aged a t both 300 and 400°C.
f i e l d . ( b ) weak beam d a r k f i e l d , and ( c ) s u p e r p o s i t i o n o f ( a ) and ( b ) .
The f i n e r p r e c i p i t a t e s were a p p r o x i m a t e l y 1.5 and 2 nm i n d i a m e t e r i n t h e m a t e r i a l s aged a t 300 and 400°C, r e s p e c t i v e l y . I n both aged m a t e r i a l s , t h e p a r t i c l e s were 4 t o 5 t i m e s l a r g e r on t h e d i s l o c a t i o n s t h a n i n t h e m a t r i x , presumably due t o enhanced n u c l e a t i o n a n d growth because o f t h e a s s i s t a n c e from p i p e d i f f u s i o n a 1 0 3 t h e d i s l o c a t i o n c o r e e number d e n s i t y o f t h e s e G-phase p r e c i p i t a t e s was >10 and a p p r o x i m a t e l y
lo2'
m-' i n t h e 300 and 400°C aged m a t e r i a l s , r e s p e c t i v e l y . It should b e emphasized t h a t t h e C-phase would have been overlooked due t o its s m a l l s i z e and l i m i t e d c o n t r i b u t i o n t o t h e e l e c t r o n d i f f r a c t i o n p a t t e r n s w i t h o u t t h e p r e v i o u s c h a r a c t e r i z a t i o n o f t h e CF 8M s t e e l .DISCUSSION
The r e s u l t s o f t h i s i n v e s t i g a t i o n o f t h e CF 8 s t e e l were s i m i l a r t o t h o s e r e p o r t e d p r e v i o u s l y f o r t h e CF 8M a l l o y . I n b o t h t y p e s o f s t e e l t h e f e r r i t e s p i n o d a l l y decomposed i n t o a n i s o t r o p i c network o f ci and ci' p h a s e s and G-phase p r e c i p i t a t e d . The major d i f f e r e n c e between t h e two t y p e s o f s t e e l s was t h e s i z e and number d e n s i t y o f t h e G-phase p r e c i p i t a t e s . I n t h e CF 8M m a t e r i a l aged 7500 h a t 400°C t h e G-phase p r e c i p i t a t e s were m 3 - l f r g e r . -10 nm i n d i a m e t e r , and were p r e s e n t a t a much l a r g e r number d e n s i t y , -10 m
,
compared t o t h e CF 8 t h a t was a l s o aged a t 400°C b u t f o r a l m o s t 10 times l o n g e r . T h i s l a r g e r volume of G-phase is r e l a t e d t o t h e d i f f e r e n c e s i n i n i t i a l composition between t h e two a l l o y s . The G-phase s i l i c i d e is r i c h i n n i c k e l and molybdenum [I ,5,6] which a r e p r e s e n t a t h i g h e r l e v e l s i n t h e CF 8M s t e e l t h a n i n t h e CF 8 s t e e l , Table 1.W h i l e a s m a l l f r a c t i o n o f t h e G-phase p r e c i p i t a t e s were observed p i n n i n g t h e d i s l o c a t i o n s , i t s h o u l d b e n o t e d t h a t t h e s e r e s i d u a l d i s l o c a t i o n s w i l l n o t n e c e s s a r i l y be o f t h e same t y p e , o r behave i n t h e same manner, a s t h o s e g e n e r a t e d d u r i n g shock l o a d i n g . G-Phase p r e c i p i t a t e s o n d i s l o c a t i o n s i n f e r r i t e have been p r e v i o u s l y observed by V i t e k i n s i m i l a r s t e e l s C73.
The f i n e s c a l e s p i n o d a l decomposi t i o n and t h e G-phase p r e c i p i t a t i o n i n t h e f e r r i t e b o t h c o n t r i b u t e t o t h e changes i n mechanical p r o p e r t i e s t h a t o c c u r d u r i n g aging.
However, s i n c e t h e volume o f t h e G-phase was lower t h a n t h a t observed i n t h e CF 8M
C7-244 J O U R N A L DE PHYSIQUE
s t e e l t h e degradation i n mechanical p r o p e r t i e s is p r i m a r i l y due t o t h e spinodal decomposition t h a t occurs i n t h e f e r r i t e during aging. I n a d d i t ion, t h e observation t h a t t h e i n c r e a s e i n hardness i n t h e CF 8 and CF 8M s t e e l s is s i m i l a r t o t h a t previously observed i n a s p i n o d a l l y decomposed i r o n
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30% chromium a l l o y which d i d n o t c o n t a i n any G-phase C81, a l s o suggests t h a t spinodal decomposition is t h e primary f a c t o r i n f l u e n c i n g mechanical p r o p e r t i e s .F i n a l l y , i t s h o u l d a l s o be n o t e d t h a t conclusions drawn from t h e r e s u l t s of a c c e l e r a t e d t e s t s t h a t a r e u s u a l l y performed a t 400°C C91 s h o u l d be c a r e f u l l y examined s i n c e t h e m i c r o s t r u c t u r e s t h a t develop a r e not i d e n t i c a l t o those a t 300°C.
This d i f f e r e n c e i n t h e m i c r o s t r u c t u r e was i n d i c a t e d by t h e r e v e r s i o n of t h e f e r r i t e i n t o a u s t e n i t e and t h e p r e c i p i t a t i o n of M23C6 t h a t occurred a t 400°C but not a t t h e lower temperatures o f 350 o r 300°C. The presence of these c a r b i d e s could then be a f a c t o r i n t h e f r a c t u r e process and thereby a l t e r t h e mechanical p r o p e r t i e s .
SUMMARY
Both APFIM and AEM r e s u l t s i n d i c a t e t h a t t h e chromium-enr iched f e r r i t e had decomposed i n t o a very f i n e network of chromium-enriched a ' and i r o n - r i c h a phases a s a r e s u l t of i s o t r o p i c s p i n o d a l decomposition. A c o a r s e M23C6 p r e c i p i t a t e was observed a t the f e r r i t e - a u s t e n i t e i n t e r f a c e i n t h e m a t e r i a l t h a t was aged a t 400°C.
Very f i n e G-phase s i l i c i d e p r e c i p i t a t e s were observed i n t h e f e r r i t e . A comparison between t h e r e s u l t s from t h i s CF 8 a l l o y and t h e previously reported r e s u l t s of a CF 8M a l l o y i n d i c a t e s t h a t r e l a t i v e l y small d i f f e r e n c e s i n t h e a l l o y compositions s i g n i f i c a n t l y a l t e r t h e q u a n t i t y of phases p r e s e n t i n t h e m i c r o s t r u c t u r e , The d e g r a d a t i o n i n mechanical p r o p e r t i e s is probably a consequence o f t h e spinodal decomposition of t h e f e r r i t e t h a t occurs during aging.
Acknowledgment
This r e s e a r c h was sponsored by t h e Division of M a t e r i a l s Sciences. U.S. Department of Energy, under c o n t r a c t DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. The a u t h o r s would l i k e t o thank D r . H.M. Chung o f Argonne National Laboratory f o r supplying the laboratory-aged CF 8 a l l o y and K.F. Russell f o r her a s s i s t a n c e i n specimen preparation.
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