LONG TERM THERMAL AGING OF TYPE CF 8 STAINLESS STEEL

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LONG TERM THERMAL AGING OF TYPE CF 8 STAINLESS STEEL

M. Miller, J. Bentley, S. Brenner, J. Spitznagel

To cite this version:

M. Miller, J. Bentley, S. Brenner, J. Spitznagel. LONG TERM THERMAL AGING OF TYPE CF 8 STAINLESS STEEL. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-385-C9-390.

�10.1051/jphyscol:1984964�. �jpa-00224451�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°12, Tome 45, decembre 1984 page C9-385

LONG TERM THERMAL AGING OF TYPE CF 8 STAINLESS STEEL

* -+- M.K. Miller, J . Bentley, S.S. Brenner and J.A. Spltznagel Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

*University of Pittsburgh, Pittsburgh, PA 15261, U.S.A.

+

Westinghouse R&D Center, Pittsburgh, PA 15235, U.S.A.

Résume - La microscopie ionique de champ et l'analyse par sonde atomi- que et la microscopie électronique par transmission ont été utilisées pour caractériser la microstructure des tuyaux du circuit de refroisis- sement primaire de réacteurs nucléaires à eau pressurisée et déterminer les changements qui interviennent à long terme lors de leur vieillisse- ment à basse température. L'acier inoxydable de fonderie de type CF 3 forme un composé d'austenite et de ferrite. Au vieillissement, des pré- cipités de 10 nm de diamètre que l'on peut identifier comme une phase G, se forment sur la ferrite. La matrice de ferrite se décompose aussi en un fin mélange d'une phase de chrome enrichie et d'une phase de fer enrichie semblables aux produits de transformation dans les systèmes de fer-chrome-X qui se décomposent d'une manière spinodale.

A b s t r a c t - Atom P r o b e F i e l d - I o n M i c r o s c o p y and T r a n s m i s s i o n E l e c t r o n M i c r o s c o p y h a v e b e e n u s e d 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 a n d t o d e t e r m i n e t h e c h a n g e s t h a t o c c u r d u r i n g l o n g - t e r m l o w - t e m p e r a t u r e a g i n g o f p r i m a r y c o o l a n t p i p e s u s e d 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 . The c a s t CF 8 t y p e s t a i n l e s s s t e e l f o r m s a m i x t u r e of a u s t e n i t e a n d f e r r i t e . On a g i n g 10 nm d i a m e t e r p r e c i p i t a t e s , t e n t a t i v e l y i d e n t i f i e d a s G - p h a s e , formed i n t h e f e r r i t e . The f e r r i t e m a t r i x a l s o d e c o m p o s e d i n t o a f i n e s c a l e m i x t u r e o f a c h r o m i u m - e n r i c h e d p h a s e a n d a n i r o n - r i c h p h a s e s i m i l a r t o t h e t r a n s f o r m a t i o n p r o d u c t s i n i r o n - c h r o m i u m - X s y s t e m s t h a t s p i n o d a l l y d e c o m p o s e .

I - INTRODUCTION

The l o n g t e r m m e c h a n i c a l i n t e g r i t y of t h e p i p e s u s e d t o c a r r y t h e p r i m a r y c o o l i n g w a t e r i n a 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 i s of t h e u t m o s t i m p o r t a n c e f o r s a f e o p e r a t i o n . T h e p i p e s a r e d e s i g n e d f o r a s e r v i c e l i f e of 40 y e a r s . H o w e v e r , i t i s w e l l known t h a t e x p o s u r e t o t e m p e r a t u r e o v e r t h i s e x t e n d e d p e r i o d may a p p r e c i a b l y a l t e r t h e m e c h a n i c a l p r o p e r t i e s o f m a t e r i a l e v e n a t r e l a t i v e l y l o w s e r v i c e t e m p e r a t u r e s . The p i p e s a r e f a b r i c a t e d o u t o f s t a i n l e s s s t e e l t o r e d u c e c o r r o s i o n and a s s o c i a t e d p r o b l e m s a t t h e s e r v i c e t e m p e r a t u r e of 300°C. They a r e up t o 1 m i n d i a m e t e r and 10 cm t h i c k a n d , b e c a u s e of t h e i r l a r g e s i z e , a r e m a d e b y w e l d i n g c a s t s e c t i o n s . T h i s t y p e of c a s t s t a i n l e s s s t e e l f o r m s a d u p l e x 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 a p p r o x i m a t e l y 15 t o 20 % 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 of 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 . As t h e p i p e s a r e e x t e r n a l t o t h e r e a c t o r t h e y a r e n o t e x p o s e d t o a n y s i g n i f i c a n t l e v e l s of r a d i a t i o n t h a t may i n f l u e n c e t h e a g i n g b e h a v i o r .

A c o m b i n e d atom p r o b e and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y s t u d y was p e r f o r m e d 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 o f t h i s c a s t s t a i n l e s s s t e e l a n d t o d e t e r m i n e t h e c h a n g e s t h a t o c c u r d u r i n g l o n g t e r m l o w t e m p e r a t u r e t h e r m a l a g i n g .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984964

JOURNAL DE PHYSIQUE

I1 - EXPERIMENTAL

The b u l k c h e m i c a l c o m p o s i t i o n of t h e CF 8 t y p e s t a i n l e s s s t e e l u s e d i n t h i s i n v e s t i g a t i o n i s g i v e n i n T a b l e 1.

TABLE I . Bulk Chemical C o m p o s i t i o n of CF 8 S t a i n l e s s S t e e l

Element C r N i Mo Mn S i C P S N Fe

w t % 20.8 1 0 . 6 2 . 5 .79 . 8 1 - 0 4 .016 . 0 2 1 - 0 4 2 b a l a n c e a t % 2 2 . 1 9.9 1 . 4 - 7 9 1 . 6 .18 .027 .037 .165 b a l a n c e

The s t e e l was examined i n b o t h t h e a s - c a s t , unaged c o n d i t i o n a n d a l s o a f t e r t h e r m a l a g i n g f o r 7 5 0 0 h o u r s a t 6 7 3 K

4 0 0 ~

~ T h i s t e m p e r a t u r e i s 100 K h i g h e r t h a n t h e n o r m a l s e r v i c e t e m p e r a t u r e s o a s t o a c c e l e r a t e t h e m i c r o s t r u c t u r a l c h a n g e s t h a t may o c c u r d u r i n g s e r v i c e [ l ] .

F i e l d - i o n m i c r o g r a p h s w e r e r e c o r d e d u s i n g neon a s t h e i m a g i n g g a s w i t h a s p e c i m e n t e m p e r a t u r e between 78 and 90 K. Atom p r o b e m i c r o c h e m i c a l a n a l y s e s were p e r f o r m e d i n t h e a b s e n c e of i m a g i n g g a s .

111 - RESULTS

The m i c r o s t r u c t u r e of t h e a s - c a s t m a t e r i a l was found t o b e a m i x t u r e o f a u s t e n i t e w i t h 1 9 % f e r r i t e . A g i n g a t 4 0 0 ~ ~ p r o d u c e d a c h a n g e i n t h e r e l a t i v e m i c r o h a r d n e s s o f t h e s e t w o p h a s e s . T h e h a r d n e s s o f f e r r i t e s u b s t a n t i a l l y i n c r e a s e d w h i l e t h e h a r d n e s s o f t h e a u s t e n i t e r e m a i n e d e s s e n t i a 11 y c o n s t a n t .

Two m a r k e d c h a n g e s i n t h e m i c r o s t r u c t u r e a n d m i c r o c h e m i s t r y o f t h e f e r r i t e w e r e f o u n d t o a c c o m p a n y l o w t e m p e r a t u r e a g i n g . Numerous s m a l l , r o u g h l y s p h e r i c a l p r e c i p i t a t e s w e r e f o u n d d i s t r i b u t e d t h r o u g h o u t t h e f e r r i t e p h a s e a s shown i n t h e f i e l d - i o n and d a r k f i e l d t r a n s m i s s i o n 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 s 1 and 2 r e s p e c t i v e l y . The p r e c i p i t a t e s e x h i b i t e d b r i g h t l y i m a g i n g c o n t r a s t i n t h e f i e l d - i o n m i c r o g r a p h s . The p r e c i p i t a t e s were of u n i f o r m s i z e , a p p r o x i m a t e l y 1 0 i a m e t e r , and were p r e s e n t a t a number d e n s i t y o f a p p r o x i m a t e l y

;:23:-$ . T h i s t y p e o f p r e c i p i t a t e w a s n o t o b s e r v e d i n t h e a s - c a s t m a t e r i a l . T h e a u s t e n i t e p h a s e i n t h e a g e d m a t e r i a l a l s o s h o w e d n o s u c h p r e c i p i t a t e s , f i g u r e 3. The b r i g h t l y i m a g i n g s p o t s i n t h e f i e l d - i o n m i c r o g r a p h a r e p r o b a b l y d u e t o t h e p r e s e n c e o f m o l y b d e n u m a n d s i l i c o n .

Atom p r o b e s e l e c t e d a r e a m i c r o c h e m i c a l a n a l y s i s r e v e a l e d t h a t t h e p r e c i p i t a t e s i n t h e a g e d f e r r i t e w e r e a 1 l o y s i l i c i d e s . T h e a v e r a g e c o m p o s i t i o n of 5 p a r t i c l e s i s g i v e n i n T a b l e 2.

T a b l e 2 . Average C o m p o s i t i o n of P r e c i p i t a t e s

a t

S i l i c o n N i c k e l I r o n Molybdenum Chromium Carbon

27.7 % 24.0

20.6 % 1 3 . 0

12.0 % 1.0 %

k3.4 k 3 . 2 2 3 . 1 ? 2 . 5

2 . 5 2 0 . 7

S e l e c t e d a r e a 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 o f t h e [ O O l ] , [Oil] a n d

[ I l l ] z o n e s o f t h e s 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 m a t r i x a r e shown i n

f i g u r e 4. The d i f f r a c t i o n p a t t e r n s w e r e c o n s i s t e n t w i t h a b a s i c f c c

c r y s t a l s t r u c t u r e (Fm3m s p a c e g r o u p ) w i t h some a d d i t i o n a l f e a t u r e s . A

l a t t i c e p a r a m e t e r o f 1.09 nm was m e a s u r e d f o r t h e p r e c i p i t a t e . The

d i f f r a c t i o n p a t t e r n s a l s o r e v e a l e d t h a t t h e p r e c i p i t a t e s had a cube-

o n c c u b e o r i e n t a t i o n r e l a t i o n s h i p w i t h t h e f e r r i t e m a t r i x .

F i g u r e 1. F i e l d - i o n m i c r o g r a p h of C F 8 s t a i n l e s s s t e e l a g e d 7500 h a t 4 0 0 ~ ~ showing s m a l l b r i g h t l y i m a g i n g p r e c i p i t a t e s i n t h e f e r r i t e m a t r i x .

F i g u r e 2. Dark f i e l d TEM m i c r o g r a p h of t h 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 m a t r i x u s i n g a p r e c i p i t a t e r e f l e c t i o n .

C9-388 JOURNAL DE PHYSIQUE

Figure 3. FIM and TEM micrographs of the austenite showing the absence of precipitation.

Figure 4. Selected area electron diffraction patterns of the [ 0 0 1 ]

[011] and [ I l l ] zones showing

precipitate reflections in

addition t o the ferrite matrix

reflections.

Atom p r o b e a n a l y s e s were a l s o made of t h e f e r r i t e p h a s e , a v o i d i n g t h e s i l i c i d e p r e c i p i t a t e s . The c o m p o s i t i o n p r o f i l e s r e v e a l e d t h a t t h e chromium-enriched f e r r i t e m a t r i x had s e p a r a t e d i n t o a n e x t r e m e l y f i n e s c a l e d m i x t u r e of a n i r o n - r l c h p h a s e and a chromium-enriched p h a s e a s shown i n f i g u r e 5. T h e a b s o l u t e c o m p o s i t i o n s o f t h e t w o p h a s e s a r e p r o b a b l y more e x t r e m e , a s t h e p r o b e a p e r t u r e was l a r g e r t h a n t h e s i z e o f c h r o m i u m - e n r i c h e d r e g i o n s a n d t h e r e f o r e some a v e r a g i n g o f t h e c o m p o s i t i o n of t h e two p h a s e s o c c u r r e d . T h i s p h a s e s e p a r a t i o n was n o t r e s o l v e d i n t h e f i e l d - i o n m i c r o g r a p h s p a r t l y b e c a u s e of t h e e x t r e m e l y f i n e s c a l e and p a r t l y b e c a u s e o f t h e p r e s e n c e of t h e b r i g h t l y i m a g i n g s i 1 i c i d e p r e c i p i t a t e s which a 1 t e r s t h e 1 o c a l imaging c o n d i t i o n s . The e l e c t r o n m i c r o s c o p e f a i l e d t o g l v e a n y i n d i c a t i o n o f t h l s p h a s e s e p a r a t i o n .

0 1 2 3 4 5 6 7

DISTANCE (nm)

F i g u r e 5. Atom p r o b e c o m p o s i t i o n p r o f i l e i n t h e f e r r i t e p h a s e showing p h a s e s e p a r a t i o n i n t o a i r o n - r i c h and a chromium-enriched p h a s e .

I V - DISCUSSION

The s m a l l s i l i c i d e p r e c i p i t a t e s i n t h e f e r r l t e p h a s e w e r e t e n t a t i v e l y

i d e n t i f i e d a s G-phase The c o m p o s i t i o n , t h e f c c c r y s t a I s t r u c t u r e

w i t h t h e weak o r a b s e n t ( 4 0 0 ) r e f l e c t i o n i n 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 , t h e 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 w i t h t h e f e r r i t e

m a t r i x , and t h e l a t t i c e p a r a m e t e r a l l s u p p o r t t h i s i d e n t i f i c a t i o n . G-

p h a s e may b e r e g a r d e d a s t h e s i l i c i d e e q u i v a l e n t t o t h e M23C6 c a r b i d e

w i t h a c o m p l e x f c c c r y s t a l s t r u c t u r e c o n t a i n i n g 1 1 6 a t o m s p e r u n i t

c e l l . The m o d e l o f G - p h a s e i s b a s e d o n N i S i 7 T i 6 w l t h v a r i o u s

e l e m e n t s S u c h a s C r , F e , Mo, Mn, V , Nb, T a , H i t a n d Z r s u b s t i t u t i n g

f o r t h e t l t a n i u m a n d n i c k e l [ 2 , 3 ] . T h i s s u b s t i t u t i o n r e s u l t s i n a

s e r i e s o f G - p h a s e s w i t h v a r i a b l e composition and l a t t i c e p a r a m e t e r .

C9-390 JOURNAL DE PHYSIQUE

T h e c o m p o s i t i o n o f t h e f e r r i t e p h a s e i s i n t h e r a n g e w h e r e a m i s c i b i l i t y g a p p r o b a b l y e x i s t s a t low t e m p e r a t u r e s . I n d e e d t h e p h a s e s e p a r a t i o n o f t h e f e r r i t e m a t r i x i n t o a chromium-enriched and a n i r o n - r i c h p h a s e i s s i m i l a r t o t h a t p r e v i o u s l y o b s e r v e d i n iron-chromium [ 4 ] a n d i r o n - c h r o m i u m - m o l y b d e n u m s y s t e m s t h a t u n d e r g o i s o t r o p i c s p i n o d a l d e c o m p o s i t i o n w i t h i n a m i s c i b i l i t y gap.

The p r e s e n c e o f b o t h t h e s i l i c i d e p r e c i p i t a t e s a n d t h e f i n e s c a l e s p i n o d a l d e c o m p o s i t i o n o f t h e f e r r i t e m a t r i x c o u l d c o n t r i b u t e t o t h e c h a n g e s o b s e r v e d i n t h e m e c h a n i c a l p r o p e r t i e s . U n f o r t u n a t e l y t h e e f f e c t s of e a c h d e c o m p o s i t i o n i n t h i s a l l o y a r e d i f f i c u l t t o i s o l a t e b y h e a t t r e a t m e n t a s b o t h p h a s e t r a n s f o r m a t i o n s o c c u r i n t h e s a m e t e m p e r a t u r e r a n g e . H o w e v e r , t h e i n c r e a s e i n h a r d n e s s t h a t was o b s e r v e d i s s i m i l a r i n m a g n i t u d e t o t h a t p r e v i o u s l y f o u n d i n a s p i n o d a l l y decomposed i r o n - 30 % chromium a 1 l o y [ 4 ] .

R e l a t i v e l y s m a l l c h a n g e s i n t h e chromium and molybdenum l e v e l s w i l l a l t e r t h e p o s i t i o n of t h e m i s c i b i l i t y g a p and t h e volume f r a c t i o n of t h e i r o n - r i c h a n d c h r o m i u m - e n r i c h e d p h a s e s . T h i s c o u l d a f f e c t t h e morphology o f t h e t r a n s f o r m a t i o n p r o d u c t s i n t h e m i s c i b i l i t y g a p and h e n c e a l t e r t h e m e c h a n i c a l p r o p e r t i e s and a g i n g b e h a v i o r . S i m i l a r l y a r e d u c t i o n i n t h e l e v e l of s i l i c o n and p e r h a p s molybdenum i n t h e a l l o y would s u p p r e s s o r r e d u c e t h e q u a n t i t y o f G-phase t h a t i s p r e c i p i t a t e d . The r e s u l t s p r e s e n t e d h e r e a r e t h e r e f o r e v e r y s p e c i f i c t o t h e m a t e r i a l a n d h e a t t r e a t m e n t a n d may v a r y c o n s i d e r a b l y i n e a c h i n d i v i d u a l c a s t i n g d e p e n d i n g o n a l l o y c o m p o s i t i o n .

The r e s u l t s o f t h e s t u d y h a v e p o i n t e d o u t t h e c o m p l e x i t y o f t h e d e c o m p o s t i o n p r o c e s s e s t h a t o c c u r a t l o w t e m p e r a t u r e i n C F 8 t y p e s t a i n l e s s s t e e l and h a v e shown t h e n e c e s s i t y of u s i n g t h e atom p r o b e a n a l y s i s t o d e t e c t t h e e x t r e m e l y f l n e - s c a l e p h a s e s e p a r a t i o n t h a t may o c c u r i n t h e f e r r i t e . F u r t h e r s t u d i e s a r e r e q u i r e d t o e v a l u a t e t h e e f f e c t s of t h e v a r i o u s p a r a m e t e r s .

Acknowledgments.

R e s e a r c h s p o n s o r e d p a r t l y by t h e D i v i s i o n o f M a t e r i a l s S c i e n c e s , U.S.

D e p a r t m e n t o f E n e r g y , u n d e r c o n t r a c t DE-AC05-840R21400 w i t h M a r t i n M a r i e t t a Energy Systems, Inc., by t h e Department of M e t a l l u r g i c a l a n d M a t e r i a l s E n g i n e e r i n g , U n i v e r s i t y of P i t t s b u r g h and by W e s t i n g h o u s e R e s e a r c h and Development C e n t e r . We s h o u l d l i k e t o t h a n k D r . G. D. W.

S m i t h f o r u s e o f t h e O x f o r d Atom P r o b e f o r some o f t h e a n a l y s e s a n d D r . E. A. Kenik f o r h e l p f u l d i s c u s s i o n s .

R e f e r e n c e s .

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