ε AND α' MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS

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ϵ AND α’ MARTENSITE FORMATION AND REVERSION IN AUSTENITIC STAINLESS STEELS

K. Guy, E. Butler, D. West

To cite this version:

K. Guy, E. Butler, D. West. ϵ AND α’ MARTENSITE FORMATION AND REVERSION IN

AUSTENITIC STAINLESS STEELS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-575-C4-

580. �10.1051/jphyscol:1982490�. �jpa-00222210�

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JOURNAL DE PHYSIQUE

Colloque C4, szipple'ment au n o 12, Tome 43, de'cembre 1982 page C4-575

E AND a ' MARTENSITE FORMATION AND REVERSION I N A U S T E N I T I C STAINLESS STEELS

K . Guy

,

E . P . ~ u t l e r * and D.R.F. west*

Engelhard Industries Ltd, Chessington, England

" ~ m ~ e r i a l CoZZege o f Science & TechnoZogy, London, England (Accepted 9 August 1 9 8 2 )

A b s t r a c t

-

The s t r u c t u r e s o f a ' and ^Em a r t e n s i t e i n 18/8 and 18/12 s t e e l s , observed u s i n g e l e c t r o n microscopy techniques a r e r e p o r t e d . S t r u c t u r a l changes a s s o c i a t e d w i t h t h e r e v e r s e t r a n s f o r m a t i o n s o f t h e s e phases t o a u s t e n i t e , y, on h e a t i n g , t o g e t h e r w i t h c o m p o s i t i o n a l v a r i a t i o n s a r e p r e s e n t - ed and mechanisms o f b o t h t h e d i r e c t and r e v e r s e t r a n s f o r m a t i o n s a r e d i s c u s s d .

I n t r o d u c t i o n - I n Fe-Ni-Cr a u s t e n i t i c s t a i n l e s s s t e e l s , two m a r t e n s i t i c phases f o r m a f t e r c o o l i n g below MS o r d e f o r m a t i o n below Md. Êm a r t e n s i t e has an hcp s t r u c t u r e and forms as p l a t e s , w h i l e bcc a ' m a r t e n s i t e forms as l a t h s . Mechanisms f o r t h e y ⁺Êt r a n s f o r m a t i o n have been proposed (1-4) which e i t h e r p r e d i c t t h e t r a n s f o r m a t i o n t o o c c u r by a r e g u l a r (2,3) o r a random ( 4 ) f a u l t i n g process on a l t e r n a t e { I l l } planes. The r e v e r s e t r a n s f o r m a t i o n o f Ê+ y has a l s o been r e p o r t e d ( 5 ) t o Y o c c u r by s i m i l a r d i s l o c a t i o n m o t i o n a t t h e p l a t e "ends", causing them t o s h r i n k . The a ' m a r t e n s i t e l a t h s f o r m i n bands and c o n t a i n a h e a v i l y d i s l o c a t e d s u b s t r u c t u r e . Various h a b i t planes (1,6,7) and o r i e n t a t i o n r e l a t i o n - s h i p s (7-1 1) have been r e p o r t e d . I n general t h e o r i e n t a t i o n i s g i v e n as b e i n g u n i q u e l y e i t h e r Nishiyama-Wasserman (N-W) o r Kurdjumov-Sachs (K-S) (7,8), w i t h some a u t h o r s r e p o r t i n g b o t h t o o c c u r s i m u l t a n e o u s l y (9-11). The mechanism o f t h e r e v e r s e t r a n s f o r m a t i o n o f a ' + y i s u n c l e a r . Both athermal ( 1 2 ) and d i f f u s i o n a l (12,13) processes have been r e p o r t e d , t o g e t h e r w i t h t h e t r a n s f o r m a t i o n proceeding i n d i s t i n c t stages ( 1 4 ) . Various m i c r o s t r u c t u r a l f e a t u r e s have been observed w i t h i n r e v e r s e d y, i n c l u d i n g s t a c k i n g f a u l t s , t w i n s and d i s l o c a t i o n a r r a y s (15,16).

The p r e s e n t paper r e p o r t s m i c r o s t r u c t u r a l o b s e r v a t i o n s made on an 18/8 s t e e l c o o l e d t o -196°C ( p r o d u c i n g 20%a1 and 1 0 % ~ ) and an 18/12 s t e e l r o l l e d a t -196OC t o g i v e a 20% r e d u c t i o n i n t h i c k n e s s ( p r o d u c i n g 20%a1 and 3 0 % ~ ) . Changes produced on h e a t i n g up t o 900°C a r e a l s o r e p o r t e d .

Experimental Procedure

-

The compositions (wt%) o f t h e two s t e e l s s t u d i e d a r e as f o l l o w s :

Table I Composition o f S t e e l s (wt%)

Fe C r N i C N M n S i Mo T i V Co P S

1

18/8 Bal 18.03 7.94 .039 .011 1.08 0.2

.

01 <0.1 .04 <.01

.

⁰¹ ^.006

18/12 Bal 17.97 11.9 .013 .010 1.03 0.32 .005 (0.1 .04 <.01 .009 .005

R o l l i n g t h e 18/12 s t e e l was c a r r i e d o u t b y immersing s t r i p s , s o l u t i o n t r e a t e d a t 1050°C, i n l i q u i d n i t r o g e n b e f o r e d e f o r m a t i o n . Each r o l l i n g pass was l i m i t e d t o a 5% r e d u c t i o n i n t h i c k n e s s . The a ' c o n t e n t was determined u s i n g a magnetic balance and t h e E b y X-ray d i f f r a c t i o n techniques. T h i n f o i l s were prepared f r o m 3mm diameter d i s c s . M i c r o a n a l y s i s was c a r r i e d o u t u s i n g EDX a n a l y t i c a l equipment on a JEOL 120 CX STEM. I n s i t u h e a t i n g experiments were conducted w i t h i n an AEI EM7, lMeV e l e c t r o n microscope, operated a t 500 kV. Reversion o f b u l k samples was p e r - folm?d oy s a l t b a t h h e a t i n g .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982490

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JOURNAL DE PHYSIQUE

R e s u l t s

1. Observations o f m a r t e n s i t i c s t r u c t u r e s .

-

F i g . 1 shows t h e a ' - l a t h s m a r t e n s i t e i n t h e 18/8 s t e e l , h a v i n g a 12251, h a b i t plane and l y i n g i n bands, bounded b y p l a t e s o f ^E. D i f f r a c t i o n evidence shows s t r e a k e d hcp E r e f l e c t i o n s , due t o t h e t h i n p l a t e morphology, w i t h maxima a t p o s i t i o n s c o n s i s t e n t w i t h t h e p r e v i o u s - l y observed ( 1 ) o r i e n t a t i o n r e l a t i o n s h i p o f (111$/~0_001),, [&I] / / [1~101,.

Two bcc p a t t e r n s a r e e v i d e n t , however, w i t h one, L1111 c o n s i z t e n t w i t h t h e K-S r e l a t i o n s h i p (111) / / (01 I),'

,

[701Iy// [711],', and t h e o t h e r [ I 0 0 1

i n d i c a t i n g t h e N-W r e l a l i o n s h i p ( 1 1 ) ( O l l ) a , [ I/ Dark f i e l d imaging r e v e a l e d i n d i v i d u a l l a t h o r i e n t a t i o n s as l a b e l l e x i n Fig. 1 ; a ' c o n t a i n s a h i g h d i s l o c a t i o n d e n s i t y , and i n F i g . 1 t h r e e d i s t i n c t s l i p - t r a c e s a r e e v i d e n t w i t h i n t h e K-S l a t h s , c o n s i s t e n t w i t h (Oll),~, (101),1? and (110),1 s l i p planes.

I n t h e deformed 18/12 s t e e l ( F i g . 2), a p a r t f r o m t h e h i g h e r ^Ec o n t e n t and a more h e a v i l y worked s t r u c t u r e i n much o f t h e r e t a i n e d y t h e a ' s t r u c t u r e i s v e r y s i m i l a r t o t h e c o o l e d 18/8 m a t e r i a l . The ^E i s c l e a r l y f a u l t e d and a t t h e i n t e r s e c t i o n o f many y s l i p bands i s i n an e a r l y stage o f f o r m a t i o n .

2. I n - s i t u h e a t i n g .

-

No s t r u c t u r a l changes o c c u r r e d on h e a t i n g t h e s e m a r t e n s i t i c s t r u c t u r e s up t o 250°C. Between 250-400°C, below t h e a '

.%

o f e i t h e r s t e e l , (540°C f o r t h e 18/8 and 470°C f o r t h e 18/12) s t a c k i n g f a u l t s w i t h i n t h e r e t a i n e d Y s h r i n k ( F i g . 3), as deduced f r o m changes i n t h e f r i n g e c o n t r a s t (eg F i g . 3b).

S i m i l a r e f f e c t s were found f o r b o t h t h e ^Ew i t h i n t h e r e t a i n e d a u s t e n i t e and t h a t a s s o c i a t e d w i t h a ' bands ,(Fig. 4).

3. TEM o b s e r v a t i o n s on m a t e r i a l heated i n h u l k form.

-

Samples o f b o t h s t e e l s were h e a t e o r an 30 m i n between 1 0-900°C. On h e a t i n g between t h e A and 600°C, t h e r rev:rEed t o adtwinned substruct:re. Above 600°C, a s u b q r a i n s t r u c h r e was p r o - duced. On h o l d i n g a t 600°C f o r 2 mins a twinned s t r u c t u r e formed which was r e p l a c e d b y subgrains, a f t e r 30 mins, ( F i g . 5).

Samples were a l s o h e l d f o r t i m e s o f up t o 10,000 min a t 550°C. F i g . 6 demon- s t r a t e s v a r i o u s aspects o f t h e twinned s t r u c t u r e . Most o f t h i s r e v e r t e d s t r u c t u r e has t h e same o r i e n t a t i o n as t h e r e t a i n e d y ( l a b e l l e d y,,). However, i n t h e c e n t r e o f t h e band i s r e v e r s e d a u s t e n i t e y2, r n i s o r i e n t a t e d t o t h e r e s t b y a 90" r o t a t i o n about a < l l O > y a x i s . Some a ' m a r t e n s i t e i s a l s o s t i l l p r e s e n t . Also, w i t h t h e samples h e l d a t 550°C, s t e p s (arrowed) were observed a l o n g a ' / a ' i n t e r f a c e s d u r i n g t h e e a r l y s t a t e s o f t r a n s f o r m a t i o n and a l o n g a l / r e v e r s e d y boundaries a f t e r

l o n g e r times, ( F i g . 7 ) .

4. M i c r o a n a l y s i s .

-

I n t h e 18/8 s t e e l h e l d a t 550°C ( T a b l e 11) a s i g n i f i c a n t decrease i n Ni c o n t e n t i s produced i n t h e remaining bcc s t r u c t u r e and an i n c r e a s e i n N i and decrease i n C r i n t h e twinned, r e v e r s e d y.

TABLE I 1 COMPOSITIONAL DATA OF THE 18/8 STEEL COOLED TO -196°C AND AGED AT 550°C

Time (mins) Retained y Remaining a ' Twinned y

% C r %Ni %Cr %Ni % C r %Ni

D i s c u s s i o n

1. E - m a r t e n s i t e .

-

The f r i n g e d n a t u r e o f t h e E - m a r t e n s i t e , ( F i g . 2), demonstrates t h a t t h i s i s n o t a p e r f e c t hcp s t r u c t u r e , b u t can be considered t o be h e a v i l y f a u l t e d , o r e q u a l l y t o be a network o f s t a c k i n g f a u l t s w i t h i n t h e y phase. I t has been

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proposed (4,17) t h a t c forins p r i o r t o the a ' by t h e random overlapping o f y

s t a c k i n g f a u l t s , generated e i t h e r by a decrease i n s t a c k i n g f a u l t energy on c o o l i n g o r by d i s l o c a t i o n c r o s s - s l i p mechanisms a t t h e i n t e r s e c t i o n o f y s l i p planes,

(Fig. 2). On h e a t i n g above %25O0C, t h e E -t y t r a n s f o r m a t i o n t h e n appears t o proceed i n an opposite manner. Thus, considering t h e phase t o be a "bundle" o f y s t a c k i n g f a u l t s , these f a u l t s s h r i n k (Figs. 3 & 4), decreasing i n l e n g t h and number through- o u t the ^Ephase u n t i l an f c c s t r u c t u r e remains, c o n t a i n i n g d i s l o c a t i o n s and a few

"remnant" s t a c k i n g f a u l t s ( 5 ) .

2. a'-martensite.

-

I t has been r e p o r t e d (9,10,11) t h a t the two o r i e n t a t i o n r e l a t i o n s h i p s (K-S and N-W) can e x i s t simultaneously i n c e r t a i n a l l o y systems. The d i f f r a c t i o n evidence i n Fig. 1 demonstrates t h a t t h i s i s so f o r t h e 1818 s t e e l . Both r e l a t i o n s h i p s were a l s o found i n the 18/12 s t e e l . It has been suggested (10) t h a t t h e r e l a t i o n s h i p changes d u r i n g t h e course o f the transformation, w i t h N-W predominant i n the e a r l y stages, a concept c o n s i s t e n t w i t h t i e f e a t u r g o f F i g . 1.

However, t h i s i s n o t w h o l l y t y p i c a l o f the amount o f each o r i e n t a t i o n found and l a r g e a ' l a t h s were observed i n the N-W o r i e n t a t i o n . The two r e l a t i o n s h i p s d i f f e r by a 5'16' r o t a t i o n about a <I

lo>,'

a x i s

.

E l e c t r o n d i f f r a c t i o n i s accurate t o w i t h i n ^.l.ZO(1 I ) , so i t i s , t h e r e f o r e , n o t p o s s i b l e t o d i s t i n g u i s h as t o whether more than one p r e c i s e o r i e n t a t i o n r e l a t i o n s h i p e x i s t s as proposed by Rao (11) o r whether a range o f o r i e n t a t i o n s occurs, c l u s t e r i n g c l o s e t o N-W and K-S as demonstrated by S a r i kaya ( 9 ) using m i c r o d i f f r a c t i o n techniques. The d i s l o c a t i o n d e f e c t s t r u c t u r e o f a ' may a r i s e by i n h e r i t a n c e o f f a u l t s from t h e f a u l t e d c . Con- s i d e r i n g the o r i e n t a t i o n r e l a t i o n s h i p s between t h e t h r e e phases y, ~ & a ' , s l i p on one {IIO},' plane c o u l d be i n h e r i t e d from (0001) f a u l t s . F u r t h e r s l i p on t h i s plane and the o t h e r two, could then o n l y be a r e s u l t o f t h e t r a n s f o r m a t i o n shape s t r a i n . However, w i t h o u t knowing t h e p r e c i s e s l i p systems and amount o f shear, i t i s d i f f i - c u l t t o determine t h e i r p r e c i s e r o l e .

The reverse t r a n s f o r m a t i o n a ' + y can occur i s o t h e r m a l l y and athermal l y (12- 14). Fig. 7 and Table I 1 p r o v i d e supporting evidence f o r a d i f f u s i o n a l transforma- t i o n mode o c c u r r i n g close t o t h e AS, i n v o l v i n g movement o f stepped (18, 19) a l / y i n t e r f a c e s and compositional changes. A f u r t h e r proposal (16) has been t h a t a sub- g r a i n reversed y s t r u c t u r e i s formed by the athermal t r a n s f o r m a t i o n o f neighbouring a ' l a t h s having small o r i e n t a t i o n a l v a r i a t i o n s , t h e twinned s t r u c t u r e being

produced by t h e recovery o f s t a c k i n g f a u l t s . However, i n t h i s i n v e s t i g a t i o n , stack- i n g f a u l t s a r e o n l y associated w i t h t h e ^E+ y t r a n s f o r m a t i o n and as t h e a ' + y t r a n s f o r m a t i o n proceeds a t 600°C a twinned s t r u c t u r e forms p r i o r t o subgrains. I t i s t h e r e f o r e proposed t h a t t h e twinned s t r u c t u r e i s associated w i t h the a ' -t y t r a n s f o r m a t i o n , i e a ' ⁺ twinned y + y. The subgrains a r e then formed by the recovery, a t temperatures a t and above 600°C, o f t h i s twinned substructure.

I n general the reversed ^yhas the same o r i e n t a t i o n as t h e r e t a i n e d Y, though i n areas close t o t h e c e n t r e o f the l a t h bands y o f a d i f f e r e n t o r i e n t a t i o n i s produced (Fig. 7). y o f the same o r i e n t a t i o n i s formed by t h e movement o f t h e o r i g i n a l a i l y boundaries, s h r i n k i n g t h e l a t h s . y o f a d i f f e r e n t o r i e n t a t i o n i s associated w i t h y forming w i t h i n the a ' l a t h bands by a n u c l e a t i o n and growth process. F u r t h e r observations i n v o l v i n g t h e a ' ⁺y t r a n s f o r m a t i o n have been r e p o r t e d e l sewhere by t h e present authors (20,21)

.

Conclusions

1. P l a t e s o f E-martensite a r e formed, w i t h a f a u l t e d s t r u c t u r e , i n a s s o c i a t i o n w i t h the a'-martensite, o r i e n t a t e d w i t h t h e close packed planes and d i r e c t i o n s o f both E and y para1 l e l

.

2. a ' - m a r t e n s i t e , {225} l a t h s form w i t h an i n t e r n a l d i s l o c a t i o n substructure, predominantly on t h r e e

{ T I O ~ , ~

s l i p planes. These do n o t have a s i n g l e unique o r i e n t a t i o n r e l a t i o n s h i p .

3. The reverse t r a n s f o r m a t i o n of E -+ y i s the d i r e c t opposite o f i t s formation, o c c u r r i n g by an u n f a u l t i n g mechanism.

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C4-578 JOURNAL DE PHYSIQUE

4. The t r a n s f o r m a t i o n a ' -t y proceeds b o t h a t h e r m a l l y and b y d i f f u s i o n a l processes p r o d u c i n g y o f t h e same o r i e n t a t i o n as t h e r e t a i n e d phase, h a v i n g a t w i n n e d s u b - s t r u c t u r e which subsequently r e c o v e r s t o a s u b g r a i n s u b s t r u c t u r e . A u s t e n i t e o f a d i f f e r e n t o r i e n t a t i o n , however, can a l s o be produced w i t h i n t h e a ' l a t h bands.

Acknowledgements

-

a r e made t o P r o f e s s o r D.W. Pashley FRS f o r t h e p r o v i s i o n o f r e s e a r c h f a c i l i t i e s , and t o SERC and t o AERE H a r w e l l f o r s u p p o r t .

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4. F u j i t a , H. and Veda, S. , T c t a . Met., (1972) 759.

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(1977) 435.

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7. K e l l y , P.M., Acta. Met., 1 3 7 1 9 6 5 ) 635.

8. Suzuki, T., Kojima, H., Suzuki, K., Hashimoto, T. and I c h i h a r a , M., Acta. Met., 25 (1977) 1151.

9. ~ F i k a y a , M., Proc. 39th Annual EMSA Meeting, (1981) 364.

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11. Narasimha-Rao, B.V., Met. Trans., 10 (1979) K45.

12. K e s s l e r , H. and P i t s c h , W., Acta. Kt., 15 (1967) 401.

13. Jana, S. and Wayman, C.M., Trans. AIME, z 9 (1967) 1187.

14. Coleman, T.H. and West, D.R.F., M e t a l Science,

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(1975) 342.

15. B r e e d i s , J.F., Trans. AIME,

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(1966) 218.

16. Smith, H. and West, D.R.F., J. Mat. Sci., 8 (1973) 1413.

17. Brooks, J.W., L o r e t t o , M.H. and Smallman, K.E., Acta. Met., 27 (1979) 1829.

18. Kinsman, ICR., Eichen, E. and Aaronson, H.I., Met. Trans., 6 A ( 1 9 7 5 ) 303.

19. Rigsbee, J.M. and Aaronson, H.I., Acta. Met.,

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(1979) 351, 20. Guy, K.B., Ph.D t h e s i s , U n i v e r s i t y o f London, 1981.

21. Guy, K.B., B u t l e r ,

E.P.

and West. D.R.F., M e t a l Science ( t o b e p u b l i s h e d ) .

F i g . 1

-

18/8, c o o l e d t o -196"C, showing E m a r t e n s i t e and bands o f a ' m a r t e n s i t e i n r e t a i n e d y . Both N-W and K-S r e l a t i o n s h i p s a r e represented, as deduced f r o m t h e d i f f r a c t i o n p a t t e r n a n a l y s i s .

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F i g . 2

-

18/12, r o l l e d by 20% a t -196"C, showing a ' obeying d i f f e r e n t v a r i a n t s o f b o t h N-W and K-S o r i e n t a - t i o n r e l a t i o n s h i p s .

Fig. 3

-

18/8, cooled t o -196°C. ( a ) RT s t r u c t u r e , ( b ) a f t e r h e a t i n g t o 250°C f o r 2 mins, ( c ) a f t e r h e a t i n g t o 450°C f o r 5 mins.

F i g . 4

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18/12, r o l l e d by 20% a t -196°C. ( a ) RT s t r u c t u r e , ( b ) a f t e r h e a t i n g t o 425°C f o r 7 mins, ( c ) a f t e r h e a t i n g t o 475°C f o r 2 mins.

(7)

C4-580 JOURNAL DE PHYSIQUE

Fig. 5

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18/12, r o l l e d by 20% a t -196OC then aged a t 600°C f o r 30 mins, showing y t w i n s and subgrains w i t h i n t h e r e v e r t e d regions.

Fig. 6

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18/8, cooled t o -196°C then aged a t 550°C f o r 10,000 mins, showing twinned r e v e r t e d y.

Fig. 7

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18/8, cooled t o -196°C then aged a t 550°C f o r 10,000 mins, showing steps, as arrowed on a ' / y boundaries.