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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

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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

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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.

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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

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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.

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