The ferrite increases the yield strength of the cast material and also reduces the
... Some microstructural characterization of several heats of CF 8 and other ...
MICROSTRUCTURAL CHARACTERIZATION OF PRIMARY COOLANT PIPE STEEL M. Miller, J. Bentley
To cite this version: M. Miller, J. Bentley. MICROSTRUCTURAL CHARACTERIZATION OF PRIMARY COOLANT PIPE STEEL. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-239-C7-244. .
HAL Id: jpa-00225935 https://hal.archives-ouvertes.fr/jpa-00225935 Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destin´ee au d´epˆot et `a la diffusion de documents scientifiques de niveau recherche, publi´es ou non, ´emanant des ´etablissements d’enseignement et de recherche fran¸cais ou ´etrangers, des laboratoires publics ou priv´es.
JOURNAL DE PHYSIQUE C o l l o q u e C 7 , s u p p l k m e n t au no 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 - 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 aging. 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 ferrite. 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 reduces the s u s c e p t i b i l i t y t o hot cracking during 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 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 mechanical p r o p e r t i e s El]. 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 The 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 . 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 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 prolonged a g i n g [2]. 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
JOURNAL DE PHYSIQUE
C7-240
Georg Fischer Co. of Switzerland. The nominal compositions of t h i s a l l o y [2] and the previously 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 aging f o r 70,000 h a t 300, 350 o r 400°C. The APFIM The AEM analyses were performed a n a l y s e s were conducted on t h e ORNL atom probe [4]. 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 spectrometer (EELS) systems. Table 1. Nominal composition of t h e CF 8 and CF 8M s t e e l s ( w t % ) . Alloy
Cr
Ni
Si
Mn
Mo
C
N
S
P
Fe
CF 8 CF 8M
20.2 20.8
8.3 10.6
1.0 0.81
0.28 0.79
0.13 2.5
0.038 0.04
0.027 0.042
0.019 0.021
0.008 0.016
balance balance
RESULTS 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 with approximately 15% f e r r i t e , i n agreement with t h e ', comparison of t h e duplex m i c r o s t r u c t u r e of t h e c a l c u l a t e d value of 19% 121. 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 small 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 Most of approximately 4 urn, with 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 . these 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 these p r e c i p i t a t e s by AEM and APFIM revealed t h a t they were chromium-rich M23C6 carbides. 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 did not change s i g n i f i c a n t l y . Analyses of t h e composit ions 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 revealed t h a t t h e f e r r i t e was enriched i n chromium, s i l i c o n , and molybdenum and depleted i n n i c k e l and manganese a s shown i n Table 2.
rlg.1.
-
Austenite ( Y ) f e r r i t e (6) ( a ) 300° and ( b ) 400aC. The but not a t 300°C. In (b) the a r e noted a t p o s i t i o n s 1 and
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 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 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 2, r e s p e c t i v e l y .
Table 2.
EDS a n a l y s i s of t h e compositions o f a u s t e n i t e and f e r r i t e a f t e r aging f o r 70000 h a t 400°C (wt%).
1 Chromium Nickel Manganese Molybdenum Silicon Iron
I
1 ::i6
Austenite l ~ e r r i t e 20.2 1 28.6 0.22 0.26 8.4 0.99 Balance
0.08
1.42 Balance
More d e t a i l e d analyses 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. Electron 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 phase modulated 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 A f i e l d - i o n micrograph of t h e same two 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 . 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 darkly imaging a' and t h e b r i g h t l y imaging a phases a r e evident. 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 evaporation sequences a l s o revealed t h a t the 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 spinodal 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 shown i n f i g u r e 5. on-cube o r i e n t a t i o n (Fm3m space group), r e f l e c t i o n [ l ,5].
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 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 cuber 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 centered cubic s t r u c t u r e 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
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 Gp 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 matrix, 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 . This 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 spinodal 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.
Fig. 5.
Field-ion micrograph o f t h e f e r r i t e i n t h e 400°C aged a l l o y showing d a r k l y imaging a' and b r i g h t l y imaging a.
Fig. 4. Atom probe composition p r o f i l e through t h e f e r r i t e i n t h e 400°C aged s t e e l i n d i c a t e s chromiume n r i c h e d a' and i r o n - r i c h a phases.
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.
Fig. 6 .
E l e c t r o n micrographs o f t h e 400°C aged CF 8 s t e e l showing t h e c o a r s e r Gphase p r e c i p i t a t e s 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 , ( a ) p r e c i p i t a t e d a r k 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 dislocation core 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 without t h e previous 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 both types of s t e e l the f e r r i t e spinodally 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 dislocations, it should be noted 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
J O U R N A L DE PHYSIQUE
C7-244
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 - 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 c o a r s e M23C6 p r e c i p i t a t e was 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. 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. REFERENCES [I]
C21 C31
C4l C53 C61 C71 C81 C91
M i l l e r , J . Bentley, S.S. Brenner and J.A. S p i t z n a g e l , J . de Physique, (1984). O.K. Chopra and H.M. Chung, P r o c . 1 3 t h Water Reactor S a f e t y Research Information Meeting, National Bureau a f Standards, Gaithersburg, (1985) H.M. Chung and O.K. Chopra, Proc. 2nd I n t . Symposium on Environmental Degradation of M a t e r i a l s i n Nuclear Power Systems Water Reactors, Monterey, e d s . J .T. Roberts, J .R. Weeks and G.J. Theus, American Nuclear Society, p.289 (1985) M.K. M i l l e r , J . d e p h y s i q u e , 493 (1986); 499 (1986) J . B e n t l e y . M.K. M i l l e r , S.S. Brenner and J . A . S p i t z n a g e l , Proc. 43rd. Electron Microscopy Society of America, ed. C.W. Bailey, San Francisco Press, San Francisco, p.328 (1985) F.X. S p i e g e l , D. Bardos, and P .A. Beck, Trans. AIME., 3 ,575 (1 963) J.M. Vitek. Met. Trans, i n p r e s s . S.S. Brenner, M.K. Miller and W.A. S o f f a , S c r i p t a Met., 16, 831 (1982) C. Slama, P. Petrequin, S.H. Masson, and T. Mager, SMET Conf. Seminar 6 , "Assuring S t r u c t u r a l I n t e g r i t y of S t e e l R e a c t o r P r e s s u r e Boundary Componentsm, Monterey, August 1983. M.K.
2, 385
-
z,
z,
