ON THE AGING OF Fe-Ni-C AND Fe-Mn-C MARTENSITES

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ON THE AGING OF Fe-Ni-C AND Fe-Mn-C MARTENSITES

M. Kajatkari, K. Ullakko, J. Pietikäinen

To cite this version:

M. Kajatkari, K. Ullakko, J. Pietikäinen. ON THE AGING OF Fe-Ni-C AND Fe-Mn-C MARTEN- SITES. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-461-C4-466. �10.1051/jphyscol:1982471�.

�jpa-00222190�

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JOURNAL DE PHYSIQUE

Colloque C4, supple'ment au n o 12, Tome 43, de'cembre 2982 page C4-461

ON THE AGING OF Fe-Ni-C AND Fe-Mn-C MARTENSITES

M. K a j a t k a r i , K . U l l a k k o and J. p i e t i k z i n e n

HeZsinki University of Technology, Laboratory of Engineering Materials, Espoo, Finland

(Accepted 9 August 1982) A b s t r a c t .

V i r g i 1 and aged m a r t e n s i t e s were s t u d i e d by d i l a t o m e t r y and ~ e ~ ~ MGssbauer s p e c t r o s c o p y . D i l a t o m e t r i c measurements were made between -80°C and +10OoC w i t h one h i g h c a r b o n manganese s t e e l and t h r e e n i c k e l s t e e l s w i t h s u b z e r o M - t e m p e r a t u r e s . F o r n i c k e l s t e e l s i t was f o u n d t h a t t h e a g i n g c u r g e s , t h e c6ange o f l e n g t h vs. l o g ( t ) , w e r e l i n e a r a t t e m p e r a t u r e s below +40 C,but a t h i g h e r a g i n g t e m p e r a t u r e s t h e c u r v e s were found t o c o n s i s t o f two l i n e a r p a r t s . The c h a n g i n g o f t h e s l o p e s o c c u r r e d a t s h o r t e r t i m e s t h e h i g h e r was t h e a g i n g t e m p e r a t u r e . I n case o f t h e manganese s t e e l t h e a g i n g c u r v e s were n o t l i n e a r a t any t e m p e r a t u r e . Some p r o p o s a l s c o n c e r n i n g c l u s t e r i n g and p r e c i p i t a t i o n processes a r e made. The MSssbauer s p e c t r a were measured a t l i q u i d n i t r o g e n t e m p e r a t u r e by t r a n s m i s s i o n method. A new f i t t i n g model was developed based on t h e u t i l i z a t i o n o f Fe-Ni r e f e r e n c e s p e c t r a . The f i t t i n g f u n c t i o n c o n s i s t s o f a g r e a t number o f t h e s e p a t t e r n s c o r r e s p o n d i n g t o e v e r y p o s s i b l e n e i g h b o u r c o n f i g u r a t i o n o f t h e i r o n atom up t o t h e t h i r d n e i g h b o u r s h e l l . Each p a t t e r n i s m o d i f i e d by t h e changes i n h y p e r f i n e i n t e r a c t i o n s due t o t h e c a r b o n atoms. I n v i r g i n m a r t e n s i t e t h e c a r b o n atoms c o u l d be e x p l a i n e d t o be d i s t r i b u t e d a p a r t f r o m each o t h e r r a t h e r t h a n randomly and t o c l u s t e r d u r i n g t h e a g i n g . The p r e s e n t f i t t i n g model o f MGssbauer s p e c t r a produces e x p l i c i t v a l u e s f o r c a r b o n c o n t e n t s and t h e f r a c t i o n s o f t h e c l u s t e r phases. Carbon c o n t e n t s w e l l c o r r e s p o n d t o t h e s t r u c t u r e s , w h i c h a r e expected t o be found a c c o r d i n g t o t h e a g i n g tempera- t u r e . From t h e f i t t i n g r e s u l t s t h e volume changes o f t h e m a r t e n s i t e d u r i n g t h e a g i n g were c a l c u l a t e d . The volume changes were compared w i t h t h e v a l u e s measured by d i l a t o m e t r y . Agreement was p r o m i s i n g .

I. I n t r o d u c t i o n

The r e d i s t r i b u t i o n o f c a r b o n atoms due t o a g i n g i n i r o n - b a s e m a r t e n s i t e s has been t h e s u b j e c t o f many r e p o r t s i n r e c e n t y e a r s . Carbon atom c l u s t e r i n g and e a r l y s t a g e s o f t e m p e r i n g have been s t u d i e d by d i f f e r e n t t e c h n i q u e s e . g . hardness and r e s i s t i v i t y measurements / I / , e l e c t r o n and atom p r o b e i o n f i e l d m i c r o s c o p y / 2 - 3 / , d i l a t o m e t r y / 4 / , i n t e r n a l f r i c t i o n / 5 / and a c o u s t i c e m i s s i o n / 6 / . X-ray d i f f r a c t i o n

i s w i d e l y used and i n t e r e s t i n g r e s u l t s have r e p o r t e d l a t e l y by W i n c h e l l e t a1./7-8/.

I n case o f MFssbauer s p e c t r o s c o p y t h e r e e x i s t d i v e r g e n t v i e w s when i n t e r p r e t i n g t h e a g i n g d a t a . One c o n f l i c t i n g p o i n t i s whether c a r b o n atoms occupy o n l y o c t a h e d r a l s i t e s ,or a r e t e d r a h e d r a l s u b l a t t i c e s a l s o i n v o l v e d /9-12/.

The a i m o f t h e p r e s e n t work was t o s t u d y c a r b o n atom c l u s t e r i n g and e a r l y s t a g e s o f t e m p e r i n g by u s i n g d i l a t o m e t r y and MGssbauer spectoscopy and t o d e v e l o p e a new p r o c e d u r e t o a n a l y z e t h e MGssbauer s p e c t r a o f v i r g i n and aged m a r t e n s i t e s .

I I . E x p e r i m e n t a l p r o c e d u r e s

The s t e e l s i n v e s t i g a t e d were m e l t e d and c h i l l c a s t under a r g o n atmosphere. Then t h e y were h o t f o r g e d i n t o p l a t e s 12 mm t h i c k o r r o d s 0 15 mm. The a u s t e n i t i z i n g t e m p e r a t u r e s and c o m p o s i t i o n s o f s t e e l s ( w t % ) used i n d i l a t o m e t r y a r e expressed i n T p b l e 1 . A u s t e n i t i z i n g t e m p e r a t u r e s were chosen so t h a t t h e l o w e s t M - t e m p e r a t u r e c o u l d be reached f o r each s t e e l . These were d e t e r m i n e d by p r e l i m i n a h t e s t s . The

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982471

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JOURNAL DE PHYSIQUE

a u s t e n i t i z i n g t i m e was 30 m i n . The d i l a t o m e t r y samples were r o d s , 65 mm l o n g and

0

1,s mm, and t h e y were h e a t - t r e a t e d i n evacuated q u a r ~ c a p s u l e s . The samples were quenched i n d i l a t o m e t e r t o l i q u i d n i t r o g e n t e m p e r a t u r e and h e l d t h e r e f o r t e n m i n u t e s . Then t h e samples were immersed i n t o we1 l s t i r r e d methanol o r w a t e r b a t h s and t h e measurements were made. Time was s e t t o z e r o when t h e sample stopped expanding. The t i m e t h e a u s t e n i t i c samples s t a y e d a t room t e m p e r a t u r e b e f o r e quenching was n o t t a k e n i n t o a c c o u n t .

T a b l e 1 : The c o m p o s i t i o n and a u s t e n i t i z i n g t e m p e r a t u r e s o f s t e e l s w h i c h were used

--

i n d i l a t o m e t r y .

S t e e l C N i Mn ,T, (OC)

I n MSssbauer s t u d i e s o n l y s t e e l I l l was used h e r e . The developed f i t t i n g p r o c e d u r e o f t h e M6ssbauer s p e c t r a i s v a l i d f o r a l l o f t h e p r e v i o u s m a t e r i a l s . The measured s p e c t r a f o r t h e o t h e r s t e e l s w i l l be r e p o r t e d l a t e r . The samples were m e c h a n i c a l l y t h i n n e d . The 25 pm t h i c k a u s t e n i t e f o i l s were quenched t o l i q u i d n i t r o g e n t e m p e r a t u r e j u s t b e f o r e measurements. Carbonless r e f e r e n c e samples were p r e p a r e d b y d e c a r b u r i z i n g t h e m a r t e n s i t e f o i l s i n hydrogen atmosphere f o r s e v e r a l h o u r s a t 760'~. The Mbssbsyer measurements were made i n t r a n s m i s s i o n geometry. The r a d i o a c t i v e s o u r c e was Co :Pd. I n a l l measurements t h e a b s o r b e r was m a i n t a i n e d a t l i q u i d n i t r o g e n t e m p e r a t u r e . The MFssbauer s p e c t r a were measured f r o m v i b g i n and t h r e e d i f f e r e n t l y aged m a r t e n s i t e s . The a g i n g t i m e s were one hour a t 35 C, f u r t h e r 30 h o u r s a t 38OC and s t i l l 2.5 h o u r s a t 1 0 0 ~ ~ . The Fe-9Ni r e f e r e n c e spectrum was measured under t h e same c i r c u m s t a n c e s as t h e m a r t e n s i t e s n e c t r a .

I l l . E x p e r i m e n t a l r e s u l t s and d i s c u s s i o n

D i l a t o m e t r y . The a g i n g b e h a v i o u r o f n i c k e l s t e e l s i s p r e s e n t e d i n f i g u r e 1 . R e s u l t s a r e f o r 100% a. I t can be seen t h a t o n l y t h e s t e e l w i t h t h e h i g h e s t carbon c o n t e n t d i d n o t s h o r t e n a t t h e l o w e s t t e s t t e m p e r a t u r e , T = - 8 0 ' ~ . I t was a l s o found t h a t a t subambient t e m p e r a t u r e s s t e e l s I and I I s h o r t e n e d more t h a n s t e e l I l l b u t t h e o r d e r was changed a t h i g h e r t e s t t e m p e r a t u r e s . Theochange o f l e n g t h vs. l o g ( t ) was found t o be l i n e a r up t o a g i n g t e m p e r a t u r e +45 C , e x c l u d i n g t h e t i m e b e f o r e w h i c h t h e samples were found n o t t o s h o r t e n , w i t h i n t h e e x p e r i m e n t a l a c c u r a c y , a t t h e two l o w e s t t e m p e r a t u r e s . A t t e m p e r a t u r e s T = + ~ ~ O C and h i g h e r , a d e v i a t i o n f r o m l i n e a r b e h a v i o u r was found. The d e v i a t i o n i s n o t an a b r u p t one b u t i t i s s t r e s s e d by two s t r a i g h t l i n e s . I t c o u l d be seen b e s t w i t h s t e e l I l l . The h i g h c a r b o n manganese s t e e i

behaved i n q u i t e a d i f f e r e n t way ( F i g . 2 ) . A d i l a t i o n was f o u n d a t e v e r y t e s t t e m p e r a t u r e . Moreover t h e r e e x i s t s no l i n e a r i t y i n t h e a g i n g p l o t s . A cause f o r t h i s can be t h e c o m p e t i t i o n o f s e v e r a l d i f f e r e n t a g i n g p r o c e s s e s . When t h i s s t e e l was s t u d i e d by 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 e (TEM) some d i f f r a c t i o n p a t t e r n s due t o e p s i l o n phase were found. T h i s c o n f l i c t s t o e a r l i e r TEM o b s e r v a t i o n s f o r manganese s t e e l s /13/. A more d e t a i l e d s t u d y o f phases e x i s t i n g i n t h i s s t e e l w i l l be p u b l i s h e d e l s e w h e r e . I n l a c k o f e x a c t knowledge o f t h e m i c r o s t r u c t u r e o f t h i s s t e e l t h e changes o f l e n g t h a r e n o t c o n v e r t e d i n t o 100% m a r t e n s i t e .

I t has been s t a t e d t h a t b e f o r e a g i n g r e c o v e r y t a k e s p l a c e a t t e m p e r a t u r e s below - 5 0 ' ~ /14/. Causes f o r t h e o b s e r v e d d i l a t a t i o n s a t l o w t e m p e r a t u r e s a r e u n c l e a r . Whether t h e s e a r e due t o d i s l o c a t i o n p r o c e s s e s a n d / o r c a r b o n s e g r e g a t i o n t o d i s l o c a t i o n s must be s t u d i e d by t e c h n i q u e s o t h e r t h a n d i l a t o m e t r y o r

MGssbauer s p e c t r o s c o p y . A p o s s i b l e e x p l a n a t i o n f o r t h e a g i n g b g h a v i o u r o f n i c k e l s t e e l s i s t h a t c a r b o n c l u s t e r s a t t e m p e r a t u r e s up t o +40

-

+50 C and a t

h i g h e r t e m p e r a t u r e s c l u s t e r s s t a r t t o grow i n t o p r e c i p i t a t e s . T h i s m i g h t be t h e reason f o r t h e o b s e r v e d d e v i a t i o n s f r o m t h e l i n e a r i t y i n t h e a g i n g c u r v e s .

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MBssbauer s p e c t r o s c o p y . A p r o c e d u r e f o r f i t t i n g t h e MSssbauer s p e c t r a o f Fe-9Ni-1.4C m a r t e n s i t e was developed. The p r o c e d u r e was based on t h e u t i l i z a t i o n o f t h e Fe-9Ni r e f e r e n c e spectrum. The Fe-9Ni r e f e r e n c e s p e c t r u m was f i t t e d by s i x independent L o r e n t z i a n l i n e s w i t h f r e e parametegg. I t i s assumed t h a t t h e e f f e c t o f n i c k e l atoms on t h e h y p e r f i n e f i e l d s o f Fe i s t h e same i n t h e Fe-Ni system and i n t h e c o r r e s p o n d i n g m a r t e n s i t e . Thus i t would be unnecessary t o use c o m p l i c a t e d p h y s i c a l f i t t i n g prodedures h e r e / I S / . The f i t t i n g f u n c t i o n o f t h e m a r t e n s i t e MBssbauer s p e c t r a c o n s i s t s o f 135 s i x l i n e Fe-9Ni p a t t e r n s . Each p a t t e r n c o r r e s p o n d s t o a c e r t a i n c o n f i g u r a t i o n o f c a r b o n atoms s u r r o u n d i n g t h e p r o b e ~ e 5 7 atom and i t i s m o d i f i e d by t h e changes i n h y p e r f i n e i n t e r a c t i o n s caused by c a r b o n atoms. I t i s supposed t h a t c a r b o n i s randomly d i s t r i b u t e d i n c - a x i s o c t a h e d r a l s i t e s . Then e v e r y p a t t e r n i s combined t o t h e whole f i t t i n g f u n c t i o n o f m a r t e n s i t e spectrum by b i n o m i a l l y d i s t r i b u t e d w e i g h t s . I n t h i s p r o c e d u r e t h r e e c o o r d i n a t i o n s h e l l s a r e t a k e n i n t o a c c o u n t and so t h e w e i g h t s a r e

where n,m,l e x p r e s s t h e number o f c a r b o n atoms on t h e f i r s t , second and t h i r d s h e l l s , and C i s t h e o c c u p a t i o n p r o p a b i l i t y . A good a p p r o x i m a t i o n f o r t h e s h i f t s i n t h e e f f e c t i v e h y p e r f i n e f i e l d s due t o s o l u t e n e i g h b o u r s can be w r i t t e n as /16/

where m i s t h e number o f s o l u t e atoms i n t h e n t h c o o r d i n a t i o n s h e l l , and M i s t h e number

gf

n e i g h b o u r she1 1 s t a k e n i n t o a c c o u n t . l somer s h i f t s a r e assumed t o obey t h e same k i n d o f r u l e s . To e v a l u a t e t h e e f f e c t o f e l e c t r i c q u a d r u p o l e i n t e r a c t i o n t h e e f f e c t i v e m a g n e t i c f i e l d i s assumed t o be o r i e n t e d a l o n g t h e c - a x i s . I t i s s u f f i c i e n t t o t a k e i n t o a c c o u n t o n l y t h e f i r s t c o o r d i n a t i o n s h e l l . I f b o t h s i t e s a r e o c c u p i e d , t h e q u a d r u p o l e s p l i t t i n g i s t w i c e t h e amount produced by o n l y one c a r b o n atom. The a b s o r p t i o n f u n c t i o n used f o r f i t t i n g t h e Massbauer s p e c t r a o f t h e v i r g i n m a r t e n s i t e i s /17/

6 2 4 8

f ( v ) = C C C C I i 'nm~

v - v . , ( 3 )

i = l n=O m=O 1=0 ~ n m l ) 2

1 + 4 ( - i

where v . = v.+na.+mB.+ly.+nw+nd +m6 + I 6

~ n m l I I I I - 1 2 3 '

The a b s o r p t i o n I . , t h e l i n e w i d h t

T.

and t h e peak p o s i t i o n vi o f t h e i t h l i n e a r e o b t a i n e d f r o m th; Fe-9Ni f i t s . ~ h r & e s e t s o f p a r a m e t e r s a i , Bi, y . a r e t h e m a g n e t i c h y p e r f i n e s h i f t s o f t h e l i n e i and t h e y a r e r e l a t e d t o AHi as a . = k i q A ~ ] , Bi=S.qAH2, y.=S.vAH3, where Si i s a r e a l number d e p e n d i n g l i n e a r l y on v e l o L i t y . For t h e

1

^{i n e}

i L l

f t i s 1 and f o r t h e l i n e i = 6 i t i s - 1 . The symbol q i s a c o n v e r s i o n c o e f f i c i e n t and i t s v a l u e i s n=-0.01652 (mm/s)/kOe. The e l e c t r i c q u a d r u p o l e s p l i t t i n g i s denoted by 2 w. Isomer s h i f t s f o r t h e t h r e e s h e l l s a r e 61.6z,63. TO e v a l u a t e t h e e l e c t r i c q u a d r u p d l e s p l i t t i n g and t h e isomer s h i f t R . Kap ow s r e s u l t s

/ l o /

were u t i l i z e d a f t e r s c a l i n g t o l a r g e r e f f e c t i v e f i e l d s due t o n i c k e l atoms. The s c a l e d p a r a m e t e r s

i n mm/s a r e 6 -0.046, 6 =0.149, 6 =0.032 and 2w=0.34. Only p a r a m e t e r s AH1, AH2 and AH were d e t e r m i n e d

gy

m i n i m i z ? n g t h e mean square d e v i a t i o n between t h e computed and e x p e r i m e n t a l p o i n t s . The f i t t e d Miissbauer s p e c t r u m o f t h e v i r g i n Fe-9Ni -1.4C 3 m a r t e n s i t e i s shown i n f i g u r e 3a. The b e s t f i t was a c h i e v e d w i t h t h e parameter v a l u e s AH1=-63 kOe, AH2=-40 kOe and AH = I 5 kOe. The a c c u r a c y a c h i e v e d even w i t h o u t f i t t i n g t h e i n t e n s i t i e s o f t h e v i r g i n J a r t e n s i t e s p e c t r u m shows t h a t t h e o r i e n t a t i o n o f t h e m a g n e t i c f i e l d s a r e t h e same i n t h e b i n a r y Fe-9Ni and i n t h e v i r g i n m a r t e n s -

i t e systems. The o r i e n t a t i o n i s f a i r l y random, as e x p e c t e d . The p o s i t i o n s o f t h e

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C4-464 JOURNAL DE PHYSIQUE

l i n e s were n o t q u i t e c o r r e c t . The spectrum o u g h t t o be s l i g h t y b r o a d e r . There would be a t l e a s t two ways t o g e t a b e t t e r r e s u l t : one i s t o t a k e f u r t h e r c o o r d i - n a t i o n she1 I s i n t o a c c o u n t and t h e o t h e r i s t o assume t h a t t h e d i s t r i b u t i o n o f carbon atoms i s n o t q u i t e random. The l a t t e r assumption i s s u p p o r t e d by t h e f a c t t h a t t h e c a r b o n d i s t r i b u t i o n i n v i r g i n m a r t e n s i t e i s i n h e r i t e d f r o m a u s t e n i t e , where i t was f o u n d t o be n e a r l y o r d e r e d /17/.

When a g i n g m a r t e n s i t e c a r b o n atoms a r e assumed t o f o r m c l u s t e r s and t h u s m i n i m i z i n g t h e s t r a i n e n e r g y . O b v i o u s l y t h e c a r b o n atom d i s t r i b u t i o n i s o r d e r e d

i n c l u s t e r s . I t i s seen, nowever, t h a t e x a c t knowledge o f t h e s t r u c t u r e s o f t h e c l u s t e r s i s n o t n e c e s s a r y f o r f i t t i n g t h e MGssbauer s p e c t r a o f aged m a r t e n s i t e q u i t e w e l l , because t h e f r a c t i o n o f t h e c l u s t e r phase and s o i t s c o n t r i b u t i o n t o t h e spectrum i s s m a l l . I n t h e f i t t i n g p r o c e d u r e c a r b o n d i s t r i b u t i o n i n c l u s t e r s i s supposed t o be, f o r t h e sake o f s i m p l i c i t y , t h e same as i n v i r g i n m a r t e n s i t e , a l t h o u g h t h e carbon c o n t e n t i s h i g h e r . I n f i g u r e 4 t h e f u n c t i o n ( 3 ) i s s i m u l a t e d w i t h d i f f e r e n t c a r b o n c o n t e n t s . The a b s o r p t i o n f u n c t i o n f ( v ) o f t h e MGssbauer Z J e c t r a o f aged m a r t e n s i t e i s a c h i e v e d by c o m b i n i n g two terms o f t h e f o r m ( 3 ) , one

~ e l o n g i n g t o t h e m a t r i x and t h e o t h e r t o t h e c l u s t e r phase w i t h w e i g h t s PI and P2=1-PI and carbon c o n t e n t s C and C , r e s p e c t i v e l y . Carbon c o n t e n t s a r e connected t o each o t h e r by C Z = ( C - ~ I C , ) / ~ ~ - ~ l ) . f h e s p e c t r a were f i t t e d by v a r y i n g o n l y t h r e e parameters, t h e f r a c t i o n P2 and t h e c a r b o n c o n t e n t C 2 o f t h e c l u s t e r phase and an e x t r a parameter K f o r t h e ~ n t e n s i t i e s o f t h e l i n e s 2 and 5 . The f i t t e d s p e c t r a a r e expressed i n f i g u r e 3b-d. The o r i e n t a t i o n o f t h e e f f e c t i v e f i e l d o b v i o u s l y changes d u r i n g a g i n g t o f a v o u r t h e d i r e c t i o n o f t h e f o i l s u r f a c e r e m a i n i n g , however, random i n a s u f f i c i e n t a c c u r a c y . The maximum v a l u e o f K ( F i g . 3 d ) i s o n l y 1 . 2 2 ( 1 ) . I n measurements made i n e x t e r n a l m a g n e t i c f i e l d o f 3 kG o r i e n t e d a l o n g t h e f o i l s u r f a c e , K was 1.83 /17/.

I n f i g u r e 5 t h e a c c u r a c y o f f i t t i n g i s expressed as a f u n c t i o n o f c a r b o n c o n t e n t o f t h e c l u s t e r phase. A f t e r t h e f i r s t a g i n g t r e a t m e n t ( I h a t 3 5 ' ~ ) t h e carbon c o n t e n t o f t h e c l u s t e r phase seems t o be 22 a t % ( F i g . 5a) and i t s f r a c t i o n

i s P2=7.4(4)%. The o b t a i n e d c a r b o n c o n t e n t 22 a t % i s q u i t e c l o s e t o t h e s t o i c h i o - m e t r y o f Fe C, w h i c h i s shown t o be an e n e r g e t i c a l l y f a v o u r a b l e s t r u c t u r e t o be formed a t t h e p b e s e n t a g i n g t e m p e r a t u r e s by R. Kaplow e t a1./10/. 4 When t h e sample was aged a t 100 C ( F i g . 5c) t h e b e s t f i t was a c h i e v e d w i t h t h e c a r b o n c o n t e n t 32 a t % and w i t h t h e phase f r a c t i o n P = 1 1 . 5 ( 2 ) % . T h i s phase c o u l d be i d e n t i f i e d w i t h E o r some o t h e r t r a n s i t i o n c a r b i d e / ? 8 / . The c a r b o n c o n t e n t 32 a t % c o r r e s p o n d s t o t h e c o m p o s i t i o n Fe C . The v a l u e 25 a t % o b t a i n e d f r o m f i g . 5b m i g h t be e x p l a i n e d by t h e c o m b i n a t i o n 2 d f t h e p r e v i o u s phases, e . g . t h e p o r t i o n 2 . 9 ( 2 ) % Fe2.2C c a r b i d e and 6 . 7 ( 2 ) % t h e s t r u c t u r e Fe C .

4

Next i t w i l l be s p e c u l a t e d t h a t t h e c l u s t e r i n g o b t a i n e d f r o m t h e f i t t i n g r e s u l t s o f t h e MGssbauer s p e c t r a o f aged m a r t e n s i t e c o u l d cause t h e observed s h r i n k - i n g o f m a r t e n s i t e . By u s i n g t h e e m p i r i c a l r e l a t i o n s between t h e l a t t i c e parametres and c a r b o n c o n t e n t s f o r b i n a r y Fe-C systems n e g l e c t i n g t h e e f f e c t o f N i and t h e changes due t o a g i n g i n l a t t i c e p a r a m e t r e s /19/ a s i m p l e c a l c u l a t i o n shgws t h a t t h e volume change o f t h e u n i t c e l l i s -0.0217 A3 a f t e r a g i n g one hour a t 35 C/17/. Hence, t h e r e l a t i v e l e n g t h d e c r e a s e i s -2.9.10-4, w h i c h seems t o be i n accordance w i t h t h e v a l u e measured by d i l a t o m e t r y ( F i g . l c )

Acknowledgement

T h i s work was p a r t i a l l y s u p p o r t e d by The Academy o f F i n l a n d . R e f e r e n c e s

/ I / S p e i c h G . R . , T r a n s . TMS-AIME 245 (1969) 2553

/2/ Garwood R.D. and Thomas G., MC T r a n s .

j !

1973) 225

/3/ M i l l e r M.K., Beaven P.A. and S m i t h G.D.V., i b i d . 1 2 ~ 0 9 8 1 ) 1197 /4/ K e r l i n s V. and A l t s t e t t e r C., T r a n s . TMS-AIME 2 2 m 9 6 3 ) 9 4

/5/ Klems G.J., M i n e r R.E., H u l t g r e n F.A. and ~ i b a F ~ . , M e t . T r a n s . z ( l 9 7 6 ) 839 /6/ Shea M.M. and Harvey D.J., S c r i p t a M e t . s ( l 9 8 2 ) 135

/7/ Chen P.C., H a l l B . 3 . and W i n c h e l l P.G., Met.Trans. f i ( 1 9 8 0 ) 1323 /8/ Chen P.C. and W i n c h e l l P.G., i b i d . f i ( 1 9 8 0 ) 1333

(6)

/ g/ F u j i t a F.E., i b i d . G ( 1 9 7 7 ) 1927

/ l o /

Choo W.K. and Kaplow R., Acta Met. g ( 1 9 7 3 ) 725 / 1 1 / L e s o i l l e M. and G i e l e n P.M., Met.Trans. 3(1972) 2681 /12/ Genin J.M. and F l i n n P.A., Trans.

TMS-AIRE

242(1968) 1419 /13/ Tanaka Y . and S h i m i z u K., T r a n s . JIM 2 1 ( 1 9 8 r 3 4

/14/ W i n c h e l l P.G., Chen P.C. and H a l l B.O: M e t . T r a n s . g ( 1 9 8 1 ) 1547 /15/ Hesse J., M i j l l e r J.B. and Wiechmann B . , J. Phys. C o l l . C2 suppldment

au n03,(1979) 161

/16/ S t e a r n s M.B., Phys.Rev. =(I9661 1136 /17/ U l l a k k o K., Diploma work, 1982

/18/ W i l l i a m s o n P.L., Nakazawa K. and Krauss G . , Met.Trans. f i ( 1 9 7 9 ) 1351 / l g / Kurdjumov G.V., i b i d . z ( 1 9 7 6 ) 999

5 10 50 100 500

AGING TlME Imm)

1..

05 1 ^' ^' ^, ^.5 10 ^, 50 100 ^' ^L

^\

^t ^c500 ^c

¹

AGING TlME (mi")

F i g . 2 . R e l a t i v e l e n g t h changes v s . t i m e a t d i f f e r e n t a g i n g tempera- t u r e s ( T ) f o r manganese s t e e l . ( A l / l o n g t f o r 100% m a r t e n s i t e )

F i g . 1 . a - c R e l a t i v e l e n g t h changes vs. t i m e f o r 100% m a r t e n s i t e a t d i f f e r e n t a g i n g t e m p e r a t u r e s (T,) f o r n i c k e l s t e e l s .

5 10 50 100 5 m

AGING TIME (mln)

5 10 50 100 500

AGING TlME Imln)

A L

l. ^T,=-SOC

(7)

JOURNAL DE PHYSIQUE

F i g . 3 . MSssbauer s p e c t r a o f Fe-9Ni -1 .4C m a r t e n s i t e measured a t 77K. The s o u r c e i s C O ~ ~ : P ~ .

( a ) v i r g i n m a r t e n s i t e , ( b ) aged one h o u r a t 35OC, ( c ) f u r t h e r aged 30h a t 38OC and ( d ) s t i 1 1 f u r t h e r aged 2.5h a t 1 0 0 ~ ~ . The c e n t r a l l i n e s a r e due t o r e - t a i ned aus t e n i te .

-

CARBON CONTENT OF CLUSTER (at%)

V E L O C I T Y ( r n m / s )

F i g . 4 . The s i m u l a t i o n o f t h e Msssbauer s p e c t r a o f v i r g i n mar-

t e n s i t e . The c a r b o n c o n t e n t s a r e supposed t o be: (a) Oat%, ( b ) 3 a t % , ( c ) 6 a t % , ( d l l O a t % , i e ) 1 5 a t % and ( f ) 2 0 a t % . The o t h e r p a r a m e t e r v a l - ues a r e t h e same as i n t h e f i t t e d

s p e c t r u m shown i n f i g u r e 3a.

F i g . 5 . The f i t t i n g a c c u r a c y o f t h e MSssbauer s p e c t r a o f aged Fe-9Ni-

1.4C m a r t e n s i t e as a f u n c t i o n o f t h e c a r b o n c o n t e n t o f t h e c l u s t e r phase. A g i n g t i m e s : ( a ) l h a t 35OC, ( b ) 30h a t 38OC, ( c ) 2.5h a t 100°C c u m u l a t i v e l y as i n f i g u r e 3 .