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Effect of cold work on the surface segregation of phosphorus in AISI 321 austenitic stainless steel
Sc~'ipta
METAI, LURGICA
Vol.
19, pp. 1199-1202, 1985 Printed in the U.S.A.
Persimmon P ~ e s s l~td. All ri!~,ht~ reserved
E F F E C T OF C O L D W O R K ON T H E S U R F A C E S E G R E G A T I O N OF P H O S P H O R U S IN A I S I 321 A U S T E N I T I C S T A I N L E S S S T E E L M.qXvrd~ + , R . S e i d l ++ , L . H y s p e c k a + , K . M a z a n e o +++ +
++ +++
V~TKOVICE Engineering and Metallurgical Research Institute, 0strava P h y s i c a l I n s t i t u t e , C z e c h o s l o v a k A c a d e m y of S c i e n c e s , Prague Technical University, 0strava, Czechoslovakia (Re¢-eived June 11, 1983) (Revised July 15, 1985) Introduct
ion
Austenitic stainless steels are known to be susceptible to stress corros i o n c r a c k i n g , c o r r o s i o n - i n d u c e d f a t i g u e , a n d h y d r o g e n embrittlement, w h i c h c a u s e p r e d o m i n a n t l y i n t e r o r y s t a l l i n e f a i l u r e . S e v e r a l p a p e r s (I-4) h a v e a s c r i b ed this t e n d e n c y to the s e g r e g a t i o n of i m p u r i t i e s , p a r t i c u l a r l y of p h o s p h o r u s , at the g r a i n b o u n d a r i e s . It has a l s o b e e n p o i n t e d out that f a c t o r s s u c h as r a d i a t i o n d a m a g e shift the d o m a i n in w h i c h this s e g r e g a t i o n o c c u r s t o w a r d s l o w e r t e m p e r a t u r e s , thus c r e a t i n g a r i s k of i n t e r c r y s t a l l i n e o m b r i t t l e m e n t during l o n g - t e r m e x p o s u r e to the s e r v i c e t e m p e r a t u r e s c o m m o n in n u c l e a r r e a c t o r s of the p r e s s u r i z e d - w a t e r or b o i l i n g w a t e r t y p e s (3). A s i m i l a r e f f e c t m a y be e x p e c t e d a f t e r c o l d w o r k i n g , w h e n the i n c r e a s e d d e n s i t y of m i c r o s t r u c t u r a l def e c t s is l i k e l y to m o d i f y the s e g r e g a t i o n p r o c e s s e s , p r o m o t i n g the d e v e l o p m e n t not o n l y of e q u i l i b r i u m , but a l s o of n o n e q u i l i b r i u m s e g r e g a t i o n p r o c e s s e s (5). A n e a r l i e r r e p o r t (6) has d e s c r i b e d h o w i n v e s t i g a t i o n s of s e g r e g a t i o n p r o u e s s e s at f r e e s u r f a c e s t b y m e a n s of A u g e r e l e c t r o n s p e c t r o s c o p y , c a n be e x p l o i t e d f o r a s s e s s i n g the s e g r e g a t i o n a c t i v i t i e s of i m p u r i t i e s , e s p e c i a l l y of p h o s p h o r u s a n d s u l p h u r , in v a r i o u s types of steel. T h e w o r k r e p o r t e d in the p r e s e n t p a p e r a p p l i e d the same e x p e r i m e n t a l t e c h n i q u e to a s c e r t a i n h o w v a r i o u s a m o u n t s of c o l d d e f o r m a t i o n a f f e c t the s e g r e g a t i o n of p h o s p h o r u s in a t i t a n i u m s t a b i l i z e d a u s t e n i t i c s t a i n l e s s s t e e l of the A I S I 321 type. Material and Experimental
Procedure
T h e A I S I 321 m a t e r i a l was t a k e n f r o m the c e n t r e of the c r o s s s e c t i o n of a h e a v y p l a t e r o l l e d f r o m a r o u t i n e c o m m e r c i a l ingot w i t h a h e a t a n a l y s i s of 0 ° 0 7 % C, 1 o 4 2 % Mn, 0 o 5 1 % Si, 0 ° 0 2 3 % P, 0 ° 0 1 0 % S, 1 8 ° 0 5 % Cr, I 0 ° 5 5 % Ni a n d 0 ° 4 5 % Ti° T h i s m a t e r i a l was s o l u t i o n a n n e a l e d at I020vC a n d t h e n w a t e r q u e n c h e d . S p e c i m e n s m e a s u r i n g 2 x 10 x 40 m m w e r e c o l d w o r k e d b y r o l l i n g on a t w o - h i g h l a b o r a t o r y m i l l to p r o d u c e o v e r a l l d e f o r m a t i o n s of 5 , 1 0 , 2 0 , 4 0 or 90 p e r cent. S u r f a c e s e g r e g a t i o n was e x a m i n e d , b y the AES m e t h o d , on s p e c i m e n s w i t h a n a r e a of 10 x 10 nun a n d a t h i c k n e s s d e t e r m i n e d by the r e s p e o t i v e d e g r e e of d e f o r mation. T h e s e s p e c i m e n s w e r e f i r s t i o n e t c h e d w i t h x e n o n ions, w i t h i n the AES a p p a r a t u s , to r e m o v e the s u r f a c e l a y e r a b o u t one ~un thick. T h e t e m p e r a t u r e d e p e n d e n c e of s u r f a c e s e g r e g a t i o n w a s e s t a b l i s h e d b y a m e t h o d d e s c r i b e d prev i o u s l y (6): the s p e c i m e n s w e r e a n n e a l e d , f o r f i v e m i n u t e s at a time, at t e m p e r a t u r e s i n c r e m e n t e d in steps of 50°C e a c h f r o m 400 to I000oc. E v e r y a n n e a l was f o l l o w e d b y c o o l i n g to less t h a n 3 0 0 o c a n d A u g e r e l e c t r o n s p e c t r o s c o p y ° Next, the s u r f a c e l a y e r f o r m e d by the s e g r e g a t i o n p r o c e s s e s w a s r e m o v e d b y ion e t c h i n g b e f o r e the s p e c i m e n was r e h e a t e d to the n e x t h i g h e r t e m p e r a t u r e ° The s p e c t r a thus o b t a i n e d w e r e e v a l u a t e d w i t h the a i d of d a t a p u b l i s h e d b y P a l m b e r g et al. (7). 1199 0036-9748/85 $3.00 + .00 Copyright (c) ]985 Per!lamon Press
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R e s u l t s and D i s c u s s i o n The t e m p e r a t u r e d e p e n d e n c e of the surface c o n c e n t r a t i o n of phosphorus is summarized in Figs. |a to If, w h i c h a p p l y to m a t e r i a l s s u b j e c t e d to various amounts of p r i o r c o l d w o r k i ~ . Fig. |a indicates that in the n o n - d e f o r m e d specimens the sur£aoe s e g r e g a t i o n of p h o s p h o r u s attains a p r o n o u n c e d p e a k at about 750°C, w h i c h c o r r o b o r a t e s e a r l i e r findings on the surface s e g r e g a t i o n of this element in types 304 a n d 321 a u s t e n i t i o stainless steels (6). W h i l e this p e a k p r o d u c e d b y a n n e a l i n g at 750°C a n d m a r k e d "A" in the diagram, was most prominent in the n o n - w o r k e d material, it also a p p e a r e d in less distinct forms in specimens that h a d u n d e r g o n e cold working. Fig. |b shows that a p r i o r deformation of 5~ shifted the m a x i m u m of phosphorus s e g r e g a t i o n to the d o m a i n m a r k e d "B", w h i c h covers the range b e t w e e n 500 a n d 650°C. B o t h this m a x i m u m and the one m a r k e d "C" in Fig° Ic, w h i c h arises a f t e r a p r i o r d e f o r m a t i o n of I0~, were d i s p l a c e d towards lower temperature intervals as the amount of d e f o r m a t i o n increased° Concurrently, more intense d e f o r m a t i o n tended to d i m i n i s h p e a k "B" but to a c c e n t u a t e p e a k "C". B o t h these phenomena, the f o r m a t i o n and gradual decline of peaks as w e l l as their shift towards lower temperatures, are a t t r i b u t a b l e to the modes in w h i c h phosphorus atoms are t r a n s p o r t e d to the free surface in the structure of c o l d - w o r k e d m e t a s t a b l e austenite. This structure is not only m a r k e d by a n i n c r e a s e d defect d e n s i t y but also contains some products of the m a r t e n s i t i c transformation. E v e n u n d e r these circumstances, p h o s p h o r u s s e g r e g a t i o n retains its c h a r a c t e r o£ a p u r e l y surface process, and the s e g r e g a t i o n layer is q u i c k l y r e m o v e d b y ion etching. This type of p h o s p h o r u s s e g r e g a t i o n dif£ers f r o m the surface s e g r e g a t i o n of p h o s p h o r u s in the ferritio structure of the F e - 3 ~ Si system, where c o l d w o r k i n g m e r e l y shi£ts the s e g r e g a t i o n maxima towards lower t e m p e r a t u r e s (8). Fig. If shows that the greatest a p p l i e d d e f o r m a t i o n o£ 90~ largely supp r e s s e d the surface s e g r e g a t i o n of phosphorus in the 650 to 750°C interval. As a first approximation, we may infer that the d e £ o r m e d austenitic structure is capable of r e t a r d i n g the p h o s p h o r u s s e ~ T e ~ a t i o n k i n e t i c s w i t h i n this temperature range° However, we must also take into account the p o s s i b i l i t y that this s t a b i l i z e d A I S I 321 steel may well d i s p l a y a p h o s p h o r u s s c a v e n g i n g effect similar to that o b s e r v e d in a F e - 3 . 5 ~ N i - 1 . 7 ~ C r system doped w i t h 600 p.p.m, of p h o s p h o r u s and 100 p.p.mo of titanium (9). Plastic d e f o r m a t i o n of this steel c o u l d c o n c e i v a b l y m o d i f y this s c a v e n g i n g effect a n d thereby contribute to the p a t t e r n of our e x p e r i m e n t a l findings° S u r f a c e s e g r e g a t i o n studies c a n also offer clues to the p r o c e s s e s by w h i c h the m a t r i x recovers a f t e r cold working. By w a y of an example, Fig. 2 presents the t e m p e r a t u r e d e p e n d e n c e s of the surface s e g r e g a t i o n o£ p h o s p h o r u s in the n o n - d e f o r m e d material~ in m a t e r i a l s u b j e c t e d to a 90~ de£ormation; and after the first and second repetition of the entire e x p e r i m e n t a l cycle c o m p r i s i n ~ successive reheating, to temperatures i n c r e a s i n g in steps of 50oc e a c h from 400 to 900 or I000°C° The d i a g T a m suggests that the r e p e a t e d r e h e a t i n ~ to a final 900 or I000oc induces a p a r t i a l r e c o v e r y of the d e f o r m e d austenitic structure, w h i c h thero£ore ~ d ~ a l l y reverts to the s e g r e g a t i o n p a t t e r n detected in the n o n - d e £ o r m e d material. If we compare these findings w i t h those o£ e a r l i e r r e s e a r c h into the surface s e g r e g a t i o n of phosphorus in v a r i o u s types of steel, we m u s t c o n c l u d e that the c h a r a c t e r of the temperature d e p e n d e n c e s of the surface c o n c e n t r a t i o n of p h o s p h o r u s is m o r e s t r o n g l y a f f e c t e d by cold w o r k i n g than b y chan~es in the c h e m i s t r y or a l l o y i n ~ of the steel (6). A p a r t i c u l a r l y important o b s e r v a t i o n is the shift in the s e ~ T e ~ a t i o n a c t i v i t y o£ p h o s p h o r u s towards lower temperatures. T h i s shift casts doubts on the s t a b i l i t y o£ the properties of austenitic stainless steels a £ t e r l o n e - t e r m exposure to temperatures w h i c h are generally b e l i e v e d to involve next to no r i s k o£ p h o s p h o r u s s e g T e ~ a t i o n in a non-def o r m e d matrix° Furthermore, it c a n not be r u l e d out that e v e n the rough m a c h i n e d surfaces of austenitic steel products, w h i c h are d e £ o r m e d to r e l a t i v e l y substantial depths, may a £ t e r p r o t r a c t e d exposure to c o m m o n service temperatures be
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enriched w i t h segregates likely to alter the kinetics of corrosion and contribute to the nucleation of surface cracks (3).
]?,Ol
processes
What has so far been discovered about the short-term kinetics of phosphorus segregation to the free surfaces of cold-worked AISI 321 steel confirms that this segregation has to be taken seriously, not only w i t h regard to surface corrosion prooesses~ but, in view of the qualitative or semi-q~antitative relationship between segregation at the free surfaces and at high-angle grain boundaries (I0,11), also as a factor liable to accelerate interorystalline crack p r o p a g a t i o n b y such mechanisms as interorystalline stress oorTosion cracking, corrosion-induced fatigue, or hydrogen embrittlement (3,4,12). Cone lus ion The study reported in this paper examined h o w 5 to 90% of prior cold deformation affected the surface segregation activity of phosphorus in AISI 321 steel after annealing at successively higher temperatures in the 400 to |O00°C range, as detected by Auger electron spectroscopy. Increasing amounts of deformation were found to cause a pronounced surface segregation of phosphorua in the 400 to 600°C interval, a phenomen not encountered in the non-deformed material. Repeated reheating to successively higher temperatures up to the solution annealing temperature induces a gradual reversion to the segregation pattern observed in the non-deformed material. This type of phosphorus segregation may hold serious implications for the gradual deterioration of the properties of austenitio stainless steel components durin~ their long-term exposure to temperatures in the range in which these processes occur. References I. A.Joshi, E.F.Stein: Corrosion, 28, 321, (1972). 2. R.L.Cowan, G.M.Gordon: Interg~raJanuLar stress corrosion crackir~E and hydrogen embrittlement of Fe-Ni-Cr alloys; Stress corrosion cracking and hydrogen embrittlement of iron base aLLoys, ed. R.W.Staehle, NACE, Houston, (19Z7).
3. H.Hanninen: Int.MetaLs Rev., 24, 85, (1979). 4. M.Habashi, J.Galland: M~m. Sci.Rev.Mgtallurg., 79, 311, (1982). 5. K.T.Aust, R.E.Hannea~ann, P.Niessen, J.H.Westbrook: Acta Metall.,
16, 291,
(1968). 6.
M.Tvrd~, R.Seidl, L.Hyspeek~, K.Mazanee: Scripta Metall., 19, 51, (1985). 7. P.W.Palmberg, E.G.Riach, R.E.Weber, N.C.MoDonald: Handbook of Auger electron spectroscopy, Phys. Electronics Industries, Ed~r~, MA, (1972). 8. R.Seidl: U n p u b L i s h e d Findings of the Physical Institute of the Czechoslovak Academy of Sciences in Prague, (1984). 9. H.Ohtani, H.C.Feng, C.J.McMahon,Jr.: MetaLl. Trans., 7a, 1123, (1976). 10. E.D.Hondros, M.P.Seah: Int.Metals Rev., 22, 262, (1977). 11. P.Dumoulin, M.Guttmann: Mat.Sol.Eng., 42, 249, (1980). 12. C.L.Briant: Corrosion, 36, 497, (1980).
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T e m p e r a t u r e dependences of the surface s e g r e g a t i o n of phosphorus after 9 0 % of c o l d d e f o r m a t i o n ( 0 ) and after the first ( A ) and second ( D ) r e p e t i t i o n of the successive h e a t i n g cycle. The b r o k e n line represents the as received condition.
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Effects of 5 to 90% cold d e f o r m a t i o n on the temperature dependences of the surface s e g r e g a t i o n of phosphorus.
I
1000
METAI, LURGICA
Vol.
19, pp. 1199-1202, 1985 Printed in the U.S.A.
Persimmon P ~ e s s l~td. All ri!~,ht~ reserved
E F F E C T OF C O L D W O R K ON T H E S U R F A C E S E G R E G A T I O N OF P H O S P H O R U S IN A I S I 321 A U S T E N I T I C S T A I N L E S S S T E E L M.qXvrd~ + , R . S e i d l ++ , L . H y s p e c k a + , K . M a z a n e o +++ +
++ +++
V~TKOVICE Engineering and Metallurgical Research Institute, 0strava P h y s i c a l I n s t i t u t e , C z e c h o s l o v a k A c a d e m y of S c i e n c e s , Prague Technical University, 0strava, Czechoslovakia (Re¢-eived June 11, 1983) (Revised July 15, 1985) Introduct
ion
Austenitic stainless steels are known to be susceptible to stress corros i o n c r a c k i n g , c o r r o s i o n - i n d u c e d f a t i g u e , a n d h y d r o g e n embrittlement, w h i c h c a u s e p r e d o m i n a n t l y i n t e r o r y s t a l l i n e f a i l u r e . S e v e r a l p a p e r s (I-4) h a v e a s c r i b ed this t e n d e n c y to the s e g r e g a t i o n of i m p u r i t i e s , p a r t i c u l a r l y of p h o s p h o r u s , at the g r a i n b o u n d a r i e s . It has a l s o b e e n p o i n t e d out that f a c t o r s s u c h as r a d i a t i o n d a m a g e shift the d o m a i n in w h i c h this s e g r e g a t i o n o c c u r s t o w a r d s l o w e r t e m p e r a t u r e s , thus c r e a t i n g a r i s k of i n t e r c r y s t a l l i n e o m b r i t t l e m e n t during l o n g - t e r m e x p o s u r e to the s e r v i c e t e m p e r a t u r e s c o m m o n in n u c l e a r r e a c t o r s of the p r e s s u r i z e d - w a t e r or b o i l i n g w a t e r t y p e s (3). A s i m i l a r e f f e c t m a y be e x p e c t e d a f t e r c o l d w o r k i n g , w h e n the i n c r e a s e d d e n s i t y of m i c r o s t r u c t u r a l def e c t s is l i k e l y to m o d i f y the s e g r e g a t i o n p r o c e s s e s , p r o m o t i n g the d e v e l o p m e n t not o n l y of e q u i l i b r i u m , but a l s o of n o n e q u i l i b r i u m s e g r e g a t i o n p r o c e s s e s (5). A n e a r l i e r r e p o r t (6) has d e s c r i b e d h o w i n v e s t i g a t i o n s of s e g r e g a t i o n p r o u e s s e s at f r e e s u r f a c e s t b y m e a n s of A u g e r e l e c t r o n s p e c t r o s c o p y , c a n be e x p l o i t e d f o r a s s e s s i n g the s e g r e g a t i o n a c t i v i t i e s of i m p u r i t i e s , e s p e c i a l l y of p h o s p h o r u s a n d s u l p h u r , in v a r i o u s types of steel. T h e w o r k r e p o r t e d in the p r e s e n t p a p e r a p p l i e d the same e x p e r i m e n t a l t e c h n i q u e to a s c e r t a i n h o w v a r i o u s a m o u n t s of c o l d d e f o r m a t i o n a f f e c t the s e g r e g a t i o n of p h o s p h o r u s in a t i t a n i u m s t a b i l i z e d a u s t e n i t i c s t a i n l e s s s t e e l of the A I S I 321 type. Material and Experimental
Procedure
T h e A I S I 321 m a t e r i a l was t a k e n f r o m the c e n t r e of the c r o s s s e c t i o n of a h e a v y p l a t e r o l l e d f r o m a r o u t i n e c o m m e r c i a l ingot w i t h a h e a t a n a l y s i s of 0 ° 0 7 % C, 1 o 4 2 % Mn, 0 o 5 1 % Si, 0 ° 0 2 3 % P, 0 ° 0 1 0 % S, 1 8 ° 0 5 % Cr, I 0 ° 5 5 % Ni a n d 0 ° 4 5 % Ti° T h i s m a t e r i a l was s o l u t i o n a n n e a l e d at I020vC a n d t h e n w a t e r q u e n c h e d . S p e c i m e n s m e a s u r i n g 2 x 10 x 40 m m w e r e c o l d w o r k e d b y r o l l i n g on a t w o - h i g h l a b o r a t o r y m i l l to p r o d u c e o v e r a l l d e f o r m a t i o n s of 5 , 1 0 , 2 0 , 4 0 or 90 p e r cent. S u r f a c e s e g r e g a t i o n was e x a m i n e d , b y the AES m e t h o d , on s p e c i m e n s w i t h a n a r e a of 10 x 10 nun a n d a t h i c k n e s s d e t e r m i n e d by the r e s p e o t i v e d e g r e e of d e f o r mation. T h e s e s p e c i m e n s w e r e f i r s t i o n e t c h e d w i t h x e n o n ions, w i t h i n the AES a p p a r a t u s , to r e m o v e the s u r f a c e l a y e r a b o u t one ~un thick. T h e t e m p e r a t u r e d e p e n d e n c e of s u r f a c e s e g r e g a t i o n w a s e s t a b l i s h e d b y a m e t h o d d e s c r i b e d prev i o u s l y (6): the s p e c i m e n s w e r e a n n e a l e d , f o r f i v e m i n u t e s at a time, at t e m p e r a t u r e s i n c r e m e n t e d in steps of 50°C e a c h f r o m 400 to I000oc. E v e r y a n n e a l was f o l l o w e d b y c o o l i n g to less t h a n 3 0 0 o c a n d A u g e r e l e c t r o n s p e c t r o s c o p y ° Next, the s u r f a c e l a y e r f o r m e d by the s e g r e g a t i o n p r o c e s s e s w a s r e m o v e d b y ion e t c h i n g b e f o r e the s p e c i m e n was r e h e a t e d to the n e x t h i g h e r t e m p e r a t u r e ° The s p e c t r a thus o b t a i n e d w e r e e v a l u a t e d w i t h the a i d of d a t a p u b l i s h e d b y P a l m b e r g et al. (7). 1199 0036-9748/85 $3.00 + .00 Copyright (c) ]985 Per!lamon Press
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R e s u l t s and D i s c u s s i o n The t e m p e r a t u r e d e p e n d e n c e of the surface c o n c e n t r a t i o n of phosphorus is summarized in Figs. |a to If, w h i c h a p p l y to m a t e r i a l s s u b j e c t e d to various amounts of p r i o r c o l d w o r k i ~ . Fig. |a indicates that in the n o n - d e f o r m e d specimens the sur£aoe s e g r e g a t i o n of p h o s p h o r u s attains a p r o n o u n c e d p e a k at about 750°C, w h i c h c o r r o b o r a t e s e a r l i e r findings on the surface s e g r e g a t i o n of this element in types 304 a n d 321 a u s t e n i t i o stainless steels (6). W h i l e this p e a k p r o d u c e d b y a n n e a l i n g at 750°C a n d m a r k e d "A" in the diagram, was most prominent in the n o n - w o r k e d material, it also a p p e a r e d in less distinct forms in specimens that h a d u n d e r g o n e cold working. Fig. |b shows that a p r i o r deformation of 5~ shifted the m a x i m u m of phosphorus s e g r e g a t i o n to the d o m a i n m a r k e d "B", w h i c h covers the range b e t w e e n 500 a n d 650°C. B o t h this m a x i m u m and the one m a r k e d "C" in Fig° Ic, w h i c h arises a f t e r a p r i o r d e f o r m a t i o n of I0~, were d i s p l a c e d towards lower temperature intervals as the amount of d e f o r m a t i o n increased° Concurrently, more intense d e f o r m a t i o n tended to d i m i n i s h p e a k "B" but to a c c e n t u a t e p e a k "C". B o t h these phenomena, the f o r m a t i o n and gradual decline of peaks as w e l l as their shift towards lower temperatures, are a t t r i b u t a b l e to the modes in w h i c h phosphorus atoms are t r a n s p o r t e d to the free surface in the structure of c o l d - w o r k e d m e t a s t a b l e austenite. This structure is not only m a r k e d by a n i n c r e a s e d defect d e n s i t y but also contains some products of the m a r t e n s i t i c transformation. E v e n u n d e r these circumstances, p h o s p h o r u s s e g r e g a t i o n retains its c h a r a c t e r o£ a p u r e l y surface process, and the s e g r e g a t i o n layer is q u i c k l y r e m o v e d b y ion etching. This type of p h o s p h o r u s s e g r e g a t i o n dif£ers f r o m the surface s e g r e g a t i o n of p h o s p h o r u s in the ferritio structure of the F e - 3 ~ Si system, where c o l d w o r k i n g m e r e l y shi£ts the s e g r e g a t i o n maxima towards lower t e m p e r a t u r e s (8). Fig. If shows that the greatest a p p l i e d d e f o r m a t i o n o£ 90~ largely supp r e s s e d the surface s e g r e g a t i o n of phosphorus in the 650 to 750°C interval. As a first approximation, we may infer that the d e £ o r m e d austenitic structure is capable of r e t a r d i n g the p h o s p h o r u s s e ~ T e ~ a t i o n k i n e t i c s w i t h i n this temperature range° However, we must also take into account the p o s s i b i l i t y that this s t a b i l i z e d A I S I 321 steel may well d i s p l a y a p h o s p h o r u s s c a v e n g i n g effect similar to that o b s e r v e d in a F e - 3 . 5 ~ N i - 1 . 7 ~ C r system doped w i t h 600 p.p.m, of p h o s p h o r u s and 100 p.p.mo of titanium (9). Plastic d e f o r m a t i o n of this steel c o u l d c o n c e i v a b l y m o d i f y this s c a v e n g i n g effect a n d thereby contribute to the p a t t e r n of our e x p e r i m e n t a l findings° S u r f a c e s e g r e g a t i o n studies c a n also offer clues to the p r o c e s s e s by w h i c h the m a t r i x recovers a f t e r cold working. By w a y of an example, Fig. 2 presents the t e m p e r a t u r e d e p e n d e n c e s of the surface s e g r e g a t i o n o£ p h o s p h o r u s in the n o n - d e f o r m e d material~ in m a t e r i a l s u b j e c t e d to a 90~ de£ormation; and after the first and second repetition of the entire e x p e r i m e n t a l cycle c o m p r i s i n ~ successive reheating, to temperatures i n c r e a s i n g in steps of 50oc e a c h from 400 to 900 or I000°C° The d i a g T a m suggests that the r e p e a t e d r e h e a t i n ~ to a final 900 or I000oc induces a p a r t i a l r e c o v e r y of the d e f o r m e d austenitic structure, w h i c h thero£ore ~ d ~ a l l y reverts to the s e g r e g a t i o n p a t t e r n detected in the n o n - d e £ o r m e d material. If we compare these findings w i t h those o£ e a r l i e r r e s e a r c h into the surface s e g r e g a t i o n of phosphorus in v a r i o u s types of steel, we m u s t c o n c l u d e that the c h a r a c t e r of the temperature d e p e n d e n c e s of the surface c o n c e n t r a t i o n of p h o s p h o r u s is m o r e s t r o n g l y a f f e c t e d by cold w o r k i n g than b y chan~es in the c h e m i s t r y or a l l o y i n ~ of the steel (6). A p a r t i c u l a r l y important o b s e r v a t i o n is the shift in the s e ~ T e ~ a t i o n a c t i v i t y o£ p h o s p h o r u s towards lower temperatures. T h i s shift casts doubts on the s t a b i l i t y o£ the properties of austenitic stainless steels a £ t e r l o n e - t e r m exposure to temperatures w h i c h are generally b e l i e v e d to involve next to no r i s k o£ p h o s p h o r u s s e g T e ~ a t i o n in a non-def o r m e d matrix° Furthermore, it c a n not be r u l e d out that e v e n the rough m a c h i n e d surfaces of austenitic steel products, w h i c h are d e £ o r m e d to r e l a t i v e l y substantial depths, may a £ t e r p r o t r a c t e d exposure to c o m m o n service temperatures be
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enriched w i t h segregates likely to alter the kinetics of corrosion and contribute to the nucleation of surface cracks (3).
]?,Ol
processes
What has so far been discovered about the short-term kinetics of phosphorus segregation to the free surfaces of cold-worked AISI 321 steel confirms that this segregation has to be taken seriously, not only w i t h regard to surface corrosion prooesses~ but, in view of the qualitative or semi-q~antitative relationship between segregation at the free surfaces and at high-angle grain boundaries (I0,11), also as a factor liable to accelerate interorystalline crack p r o p a g a t i o n b y such mechanisms as interorystalline stress oorTosion cracking, corrosion-induced fatigue, or hydrogen embrittlement (3,4,12). Cone lus ion The study reported in this paper examined h o w 5 to 90% of prior cold deformation affected the surface segregation activity of phosphorus in AISI 321 steel after annealing at successively higher temperatures in the 400 to |O00°C range, as detected by Auger electron spectroscopy. Increasing amounts of deformation were found to cause a pronounced surface segregation of phosphorua in the 400 to 600°C interval, a phenomen not encountered in the non-deformed material. Repeated reheating to successively higher temperatures up to the solution annealing temperature induces a gradual reversion to the segregation pattern observed in the non-deformed material. This type of phosphorus segregation may hold serious implications for the gradual deterioration of the properties of austenitio stainless steel components durin~ their long-term exposure to temperatures in the range in which these processes occur. References I. A.Joshi, E.F.Stein: Corrosion, 28, 321, (1972). 2. R.L.Cowan, G.M.Gordon: Interg~raJanuLar stress corrosion crackir~E and hydrogen embrittlement of Fe-Ni-Cr alloys; Stress corrosion cracking and hydrogen embrittlement of iron base aLLoys, ed. R.W.Staehle, NACE, Houston, (19Z7).
3. H.Hanninen: Int.MetaLs Rev., 24, 85, (1979). 4. M.Habashi, J.Galland: M~m. Sci.Rev.Mgtallurg., 79, 311, (1982). 5. K.T.Aust, R.E.Hannea~ann, P.Niessen, J.H.Westbrook: Acta Metall.,
16, 291,
(1968). 6.
M.Tvrd~, R.Seidl, L.Hyspeek~, K.Mazanee: Scripta Metall., 19, 51, (1985). 7. P.W.Palmberg, E.G.Riach, R.E.Weber, N.C.MoDonald: Handbook of Auger electron spectroscopy, Phys. Electronics Industries, Ed~r~, MA, (1972). 8. R.Seidl: U n p u b L i s h e d Findings of the Physical Institute of the Czechoslovak Academy of Sciences in Prague, (1984). 9. H.Ohtani, H.C.Feng, C.J.McMahon,Jr.: MetaLl. Trans., 7a, 1123, (1976). 10. E.D.Hondros, M.P.Seah: Int.Metals Rev., 22, 262, (1977). 11. P.Dumoulin, M.Guttmann: Mat.Sol.Eng., 42, 249, (1980). 12. C.L.Briant: Corrosion, 36, 497, (1980).
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T e m p e r a t u r e dependences of the surface s e g r e g a t i o n of phosphorus after 9 0 % of c o l d d e f o r m a t i o n ( 0 ) and after the first ( A ) and second ( D ) r e p e t i t i o n of the successive h e a t i n g cycle. The b r o k e n line represents the as received condition.
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Effects of 5 to 90% cold d e f o r m a t i o n on the temperature dependences of the surface s e g r e g a t i o n of phosphorus.
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