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Submitted on 1 Jan 1982
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ATHERMAL, STRESS AND STRAIN INDUCED TRANSFORMATIONS IN MULTIPHASE STAINLESS
STEELS
F. Marques
To cite this version:
F. Marques. ATHERMAL, STRESS AND STRAIN INDUCED TRANSFORMATIONS IN MUL- TIPHASE STAINLESS STEELS. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-569-C4-574.
�10.1051/jphyscol:1982489�. �jpa-00222209�
ATHERMAL,STRESS AND STRAIN INDUCED TRANSFORMATIONS I N MULTIPHASE STAINLESS STEELS
F.D.S. Marques
Department of MetaZZurgieaZ Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, U. S. A.
(Revised text accepted 4 October 1982)
ABSTRACT.- An i n v e s t i g a t i o n has been made o f athermal, s t r e s s and s t r a i n i n - duced transformations i n mu1 ti phase (duplex, t r i p 1 ex and quadruplex) s t a i n l e s s s t e e l s w i t h a chemical composition o f t h e type 301, 302 and 304. Special t h e r - momechanical treatments c o n s i s t i n g i n various sequences o f athermal transform- ations, deformation induced transformations, deformation, tempering and r e - c r y s t a l 1 iz a t i o n were developed. This r e s u l t e d i n multiphase m i c r o s t r u c t u r e s c o n s i s t i n g o f composite networks o f austeni te, f e r r i t e , athermal martensi t e and s t r e s s / s t r a i n induced m a r t e n s i t e (MFAFM) w i t h h i g h s t r e n g t h and h i g h duc- ti li t y . The e f f e c t o f dispersed p a r t i c l e s o f f e r r i t e , athermal martensi t e , d u interfaces, y t w i n and g r a i n boundaries, amount o f deformation and deforma- t i o n temperature on t h e n u c l e a t i o n , growth, morphology, substructure, s t r e n g t h and d u c t i l i t y o f t h e t r a n s f o r m a t i o n products a r e discussed.
DESIGN OF MULTIPHASE STAINLESS STEELS/EXPERIMEI\ITAL PIETHODS
During t h e p r e s e n t i n v e s t i g a t i o n f o u r types o f m i c r o s t r u c t u r e s were designed and studied: ( I ) duplex: a u s t e n i t e ( y ) and deformation induced m a r t e n s i t e (MD); (11) t r i p l e x : austenite, athermal martensite (MA) and deformation induced martensite;
(111) t r i p l e x : a u s t e n i t e , f e r r i t e ( a o r 6 ) and deformation induced martensite and ( I V ) quaduplex: a u s t e n i t e , f e r r i t e , athermal m a r t e n s i t e and deformation induced mar- t e n s i t e . The f i r s t and second type o f m i c r o s t r u c t u r e s were developed through the a p p l i c a t i o n o f type I o f t h e thermomechanical treatments and t h e t h i r d and f o u r t h type of m i c r o s t r u c t u r e s were developed through t h e a p p l i c a t i o n o f thermomechanical treatments type I 1 and type 111. Thermomechanical treatments type I were designed t o o b t a i n a d i s p e r s i o n o f deformation ( s t r e s s o r s t r a i n ) induced m a r t e n s i t e and/or ather- ma1 martensi t e w i t h i n an austeni t i c ( c o n t a i n i n g between 6 and 12 w t % n i c k e l ) , s t a i n - l e s s ( c o n t a i n i n g between 16 and 20% chromium) m a t r i x . This m i c r o s t r u c t u r e was then submitted t o thermomechani c a l treatments type I I : deformation a t d i f f e r e n t tempera- t u r e s t o 50% s t r a i n and then tempering p r i o r t o r e c r y s t a l l i z a t i o n w i t h i n the a + Y o r 6 + y phases f i e l d ; f o l l o w e d by water quenching, l i q u i d n i t r o g e n requenching and de- formation t o a s t r a i n s u f f i c i e n t t o o b t a i n a considerable volume f r a c t i o n (20.3) o f deformation induced martensite. I n order t o achieve f u r t h e r refinement these micro- s t r u c t u r e s were submitted t o thermomechanical treatments type 111, which c o n s i s t e d i n the r e a p p l i c a t i o n o f one o r various c y c l e s o f therrnomechanical treatments type 11.
This r e s u l t e d i n a v e r y f i n e micromultiphase s t r u c t u r e , w i t h a t y p i c a l average g r a i n s i z e between 1 and 5 um.
RESULTS AND DISCUSSION
K e r o s t r u c t u r e s Type I and II/ThermorneckanicaZ Treatments Type I
Abundant formation o f deformation induced ( s t r e s s and/or s t r a i n ) m a r t e n s i t e was observed i n a l l s t e e l s a t a1 1 temperatures w i t h i n t h e range 77K t o 273K. This r e s u l t - ed i n a considerable increase i n t h e diamond pyramid hardness (DPH), y i e l d s t r e n g t h
(YS) and u l t i m a t e t e n s i l e s t r e n g t h (UTS), b u t was accompanied by a s i g n i f i c a n t decrease i n the e l o n g a t i o n (Figs. 1 and 2). For t h e same amount o f deformation, the amount o f deformation induced m a r t e n s i t e and t h e increase i n DPH, YS and UTS were observed t o increase w i t h decreasing deformation temperatures f o r a l l s t e e l s i n v e s t i g a t e d . As shown i n f i g u r e s 1 and 2 a much h i g h e r volume f r a c t i o n o f m a r t e n s i t e and a much high- e r s t r e n g t h were obtained f o r a given amount o f deformation a t 77K than a t 298K.
This i s expected since the temperature range 77-298K f a l l s w i t h i n the temperature
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982489
C4-570 JOURNAL DE PHYSIQUE
Fig. 1 Effect of t h e s t r a i n and temperature on martensite formation and mechanical properties of: ( a )
& (6)302, (c)
&(d) 304 and ( e l
&( f ) 304
Bs t a i n l e s s s t e e l s . range MS-Md f o r the 302 and 304 s t e e l s and within the l a t t e r the t o t a l driving force f o r the formation of martensite
AFTincreases with decreasing deformation tempera- tures. This results in a higher amount of martensite being formed f o r the same amount of s t r a i n a t lower temperatures and a lower s t r e s s and/or s t r a i n being needed to form the same amount of martensite a t lower temperatures.
Assuming t h a t the nucleation of martensite i s heterogeneous and without speci- fying any shape o r s i z e f o r the embryos, the thermodynamic driving force f o r t h e for- mation (nucleation and growth to the measured completion) of martensite can be ex- pressed by the following equation:
AFT' - A F V
+
AF S + A F E+
A F T-
A F Dwhere: AF i s the t o t a l free energy change measured per unit volume due t o the forma- tion of martensite (athermal, s t r e s s a s s i s t e d and/or s t r a i n induced);
A Fi s the f r e e energy change associated with the bulk material, largely determined by eflectronic configurations;
A Fi s the surface energy associated with the atomic m i s f i t between the parent (austenSte) and product (martensite) phase;
AF,i s the s t r e s s which has t o be overcome t o shear the austeni t e i n t o martensi t e , which
incl udes the free energy change associated with the growth of one martensite crystal (block, l a t h o r p l a t e ) in the s t r a i n f i e l d created by previously formed martensite c r y s t a l s and
A Fi s t h e f r e e energy of the microstructural defect which i s destroyed due t o the formaeion of mar- t e n s i t e . The terms
A F,
AF,and AF, are always positive and therefore oppose the transformation a t a11 $emperatures. The terms
bFyand
A F Dmay become negative and have t o overcome the terms aFS +
AF,+
AF,in order t h a t the martensite c r y s t a l s can complete t h e i r transformation.
For each two deformation temperatures the experimentally measured differences both i n the volume fractions of martensite and in the correspondent strength ( D P H , YS, UTS) were observed to be a function of the amount of deformation, a maximum being obtained i n the deformation range of 20% to
40%s t r a i n . I t was observed t h a t in the 304B s t e e l the amount of martensite formed by deformation i n the temperature range 77-298K increased continuously and very rapidly with increasing s t r a i n through the e n t i r e s t r a i n range. However in the case of the 302 and 304 s t e e l s the amount of mar-
t e n s i t e formed exhibited plateaux between 15% and 35% s t r a i n , as shown in figure l .
zag d i s l o c a t e d and T : 304B, ~ = 2 % , 77K; (c) M : zig-zag d i s l o c a t e d : 304B, ~ = 8 %, 77K;
(d) M : v e r y f i n e 1 e R t i c u l a r nucleates a t MA: i i s l o c a t e d : 3048, E-161, 77K; (e) MD n u c l e i t e s a t T : 304, €=lo%, 298K and (f) M : l e n t i c u l a r twinned and f a u l t e d : 304,
€=15%, 298K; iRterna1 t w i n s i n p l a t e pl coneinue i n t h e a u s t e n i t e as p l a t e s P2 6 P3.
F i g . 3 The r o l e o f MA and MD on tempering and on t h e n u c l e a t i o n o f a / 6 and y d u r i n g r e c r y s t a l l i z s t i o n and o f a / 6 and y on t h e formation o f M and MD upon TMT Type 11:
(a) 304B, 673K; (6) 304, 773K and (c) 301, 1073K f o l l o w e i by 10% s t r a i n a t 298K.
C4-572 JOURNAL DE PHYSIQUE
T h i s i s a t t r i b u t e d t o a t r a n s i t i o n f r o m s t r e s s a s s i s t e d t o s t r a i n i n d u c e d m a r t e n s i t e i n t h e 302 and 304 s t e e l s between 15% and 35% s t r a i n . However i n t h e 304B s t e e l mar- t e n s i t e i s c o n s i d e r e d t o be s t r a i n i n d u c e d t h r o u g h t h e e n t i r e s t r a i n range. Those p l a t e a u x c o i n c i d e w i t h t h e l a s t s t a g e o f f o r m a t i o n o f t h e s t r e s s a s s i s t e d m a r t e n s i t e and can be e x p l a i n e d b y t h e p a r t i a l s t a b i l i z a t i o n o f t h e a u s t e n i t e r e l a t i v e l y t o t h e f o r m a t i o n of s t r e s s a s s i s t e d m a r t e n s i t e , due t o p r e v i o u s t r a n s f o r m a t i o n . T h i s i s due t o t h e t r a n s f o r m a t i o n i t s e l f and t h e f a c t t h a t t h e s p e c i f i c volume o f m a r t e n s i t e i s 4.3% l a r g e r than t h a t o f t h e p a r e n t a u s t e n i t e , which cause a d i s t o r t i o n t h a t must be accomodated by t h e g e n e r a t i o n o f d i s l o c a t i o n s , t w i n s o r s t a c k i n g f a u l t s w i t h i n t h e a u s t e n i t e . The r e v e r s e o f t h e a u s t e n i t e s t a b i l i z a t i o n ( i .e. i n s t a b i li z a t i o n r e l a t i
-
v e l y t o t h e f o r m a t i o n o f s t r a i n - i n d u c e d m a r t e n s i t e ) w o u l d be e x p e c t e d i f t h e marten- s i t e was s t r a i n - i n d u c e d , due t o t h e a u t o c a t a l i t i c e f f e c t a s s o c i a t e d w i t h t h e forma- t i o n o f n u c l e a t i o n s i t e s by p l a s t i c d e f o r m a t i o n which w o u l d s h a r p l y i n c r e a s e t h e s l o p e o f t h e curve, as observed i n t h e 3048 s t e e l . I t i s a l s o i m p o r t a n t t o e x p l a i n why t h i s s t a b i l i z a t i o n o f t h e a u s t e n i t e , observed o n l y i n t h e l a s t s t a g e s o f forma-
t i o n of s t r e s s a s s i s t e d n a r t e n s i t e ( i n t h e 302 and 304 s t e e l s ) does n o t o c c u r f o r s t r a i n l e v e l s below 15%. T h i s i s a t t r i b u t e d t o t h e f a c t t h a t t h e mechanical work (8%
t o 15%) produced d u r i n g d e f o r m a t i o n i s n o t b e i n g s t o r e d i n t h e a u s t e n i t e t h r o u g h t h e formation of d i s l o c a t i o n s , t w i n s o r s t a c k i n g f a u l t s , b u t i s i n s t e a d used as t h e s t r a i n energy need t o p r o p a g a t e t h e m a r t e n s i t e i n t e r f a c e . T h i s i s i n agreement and e x p l a i n s why no s i g n i f i c a n t number o f d i s l o c a t i o n s , t w i n s o r s t a c k i n g f a u l t s were observed i n
t h e a u s t e n i t e , even a d j a c e n t t o t h e m a r t e n s i t e p l a t e s ( 2 ) .
The n i c k e l c o n t e n t and t h e n i c k e l e q u i v a l e n t o f t h e 302 and 304 t y p e s t e e l s i s a p p r o x i m a t e l y t h e same and h i g h e r t h a n those o f t h e 304B s t e e l . T h i s r e s u l t s i n a l o w e r MS t e m p e r a t u r e f o r t h e 302 and 304 s t e e l s . T h e r e f o r e i t i s a l s o e x p e c t e d t h a t Mg ( t h e temperature below which " y i e l d i n g " o f t h e a u s t e n i t e i s i n i t i a t e d by marten- s i t e f o r m a t i o n , and above which y i e l d i n g o f t h e a u s t e n i t e i s i n i t i a t e d by p e r f e c t o r d i s s o c i a t e d s l i p , i .e. M ~ < @ < M ~ ) i n t h e 302 and 304 s t e e l s w i l l be l o w e r than t h a t o f t h e 304B s t e e l . I t i s t h e r e f o r e p o s s i b l e f o r t h e f o r m a t i o n o f m a r t e n s i t e i n t h e 302 and 304 s t e e l s t o be s t r e s s a s s i s t e d f o r s t r a i n l e v e l s below 15% and s t r a i n i n d u c e d f o r s t r a i n l e v e l s above 35%, where s i g n i f i c a n t number o f n u c l e a t i o n s i t e s have been formed by p l a s t i c d e f o r m a t i o n .
I n t h e 302 and 304 s t e e l s t h e morphology o f t h e d e f o r m a t i o n i n d u c e d m a r t e n s i t e was observed t o be c o n s i d e r a b l y i n f l u e n c e d b y t h e d e f o r m a t i o n temperature. The num- b e r o f v a r i a n t s o f d e f o r m a t i o n i n d u c e d m a r t e n s i t e was observed t o i n c r e a s e w i t h de- c r e a s i n g d e f o r m a t i o n temperatures. A t l o w e r temperatures a g r e a t e r number o f s h o r t e r and t h i n n e r p l a t e s and p l a t e v a r i a n t s were observed ( F i g . 2). Upon d e f o r m a t i o n a t 298K o n l y one o r two v a r i a n t s o f m a r t e n s i t e were observed w i t h i n each a u s t e n i t e g r a i n . However upon d e f o r m a t i o n a t 77K an average f o u r v a r i a n t s o f d e f o r m a t i o n i n - duced m a r t e n s i t e were observed. These o b s e r v a t i o n s a r e i n agreement w i t h p r e v i o u s r e s u l t s o b t a i n e d by Marques f o r z i r c o n i u m and t i t a n i u m based d e f o r m a t i o n i n d u c e d mar- t e n s i t e s (2,3) and a r e c o n s i s t e n t w i t h a s t r e s s a s s i s t e d heterogeneous m a r t e n s i t e f o r m a t i o n . The s t r e s s a s s i s t e d modes wi 11 be t h o s e whose t r a n s f o r m a t i o n s h e a r d i s t o r - t i o n i s compensated ( t h e sum AFS + AF, * + AF, i s m i n i m i zed) b y t h e a p p l i e d s t r e s s . Thus t h e a p p l i e d s t r e s s w i l l o n l y a s s l s t t h e f o r m a t i o n o f v e r y few p l a t e v a r i a n t s , t h e o t h e r b e i n g opposed. I t i s expected t h a t t h e f o r m a t i o n o f these p l a t e s w i l l p r o - duce s t r a i n s p a r a l l e l and normal t o t h e h a b i t p l a n e . T h i s i s i n agreement and ex- p l a i n s t h e morphology o f these m a r t e n s i t e s and i n p a r t i c u l a r t h e 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 s u c c e s s i v e v a r i a n t s o f m a r t e n s i t e ( F i g s . 2 and 3). The l a r g e r den- s i t y o f p l a t e s and p l a t e v a r i a n t s observed a t l o w e r temperatures can be e x p l a i n e d s i n c e a t l o w e r temperatures AFV becomes l a r g e r , AFT becomes s m a l l e r , t h e c r i t i c a l r a - d i u s decreases and t h e number o f c r i t i c a l embryos i n c r e a s e s . The s m a l l e r s i z e o f t h e p l a t e s observed a f t e r growth i s completed, a t l o w e r temperatures, can be e x p l a i n e d b y t h e h i g h e r s t r e n g t h o f t h e a u s t e n i t e and t h e r e f o r e a l a r g e r AFT.
A l t h o u g h t h e e x a c t n a t u r e o f t h e m a r t e n s i t e embryos i s n o t y e t f u l l y e s t a b l i s h - ed i t i s e x p e c t e d ( 2 ) t h a t t h e s t r e s s a s s i s t e d n u c l e a t i o n o f m a r t e n s i t e depends on t h e same n u c l e a t i o n s i t e s o r embryos which a r e r e s p o n s i b l e f o r t h e u s u a l athermal t r a n s f o r m a t i o n s . T h i s w o u l d e x p l a i n t h e e f f e c t o f t h e d e f o r m a t i o n temperature on t h e morphology o f t h e s t r e s s as;isted m a r t e n s i t e s observed i n t h e 302 and 304 s t a i n l e s s s t e e l s . However, t h e s t r a i n i n d u c e d n u c l e a t i o n o f m a r t e n s i t e depends on t h e c r e a t i o n o f new s i t e s and embryos b y p l a s t i c d e f o r m a t i o n , w i t h an expected a u t o c a t a l y t i c nu- c l e a t i o n e f f e c t . I t i s g e n e r a l l y b e l i e v e d t h a t l o c a l s t r e s s c o n c e n t r a t i o n s a c t as
tem w i t h E m a r t e n s i t e p l a t e s ( 6 ) ; i n t e r s e c t i o n o f an E p l a t e w i t h an a u s t e n i t e t w i n and/or g r a i n boundary (7,8) and s t r a i n induced s t a c k i n g f a u l t s (5,9,10) have been suggested as p o s s i b l e embryos. However i n each o f t h e above cases the r e g i o n o f i n - t e r s e c t i o n i s d e f i n e d as a l a t h o r r o d shaped volume which c o n t r o l s the morphology o f the s t r a i n induced martensi te, b u t which i s n o t s i g n i f i c a n t l y i n f l u e n c e d by t h e de- formation temperature. T h i s f u r t h e r supports t h e view t h a t t h e present deformation
in
duced martensites, observed i n t h e 302 and 304 s t e e l s are s t r e s s assisted. During t h i s i n v e s t i g a t i o n i t was observed t h a t the deformation induced m a r t e n s i t e nucleated heterogeneously a t the f o l l o w i n g favorable s i t e s : (a) annealing t w i n s w i t h i n t h e aus- t e n i t e (Fig. 2); ( b ) athermal martensite p l a t e boundaries ( F i g . 2 and 3); ( c ) austen- i t e q r a i n boundaries ( F i q . 2 ) ; and ( d ) i n t e r f a c e s between the a u s t e n i t e and f e r r i t e
(Fig. 3).
U ~ o n auenchina/reauenchins t o 77K a s i s n i f i c a n t amount o f athermal m a r t e n s i t e was observed ' i n t h e y 0 4 ~ ' s t e e l , 6 u t n o t i n the 302 o r 304 s t e e l s . This i s e x p l a i n e d by t h e lower n i c k e l content o f t h e 304B s t e e l and consequently h i g h e r Ms temperature.
This athermal martensi t e had a l e n t i c u l a r zig-zag morphology (1) w i t h a l a r g e number (>6) o f p l a t e v a r i a n t s which made i t e a s i l y d i s t i n g u i s h e d from the deformation i n - duced martensi t e (Figs. 2 and 3 ) .
The s u b s t r u c t u r e o f the deformation induced martensites was observed t o be h i g h l y f a u l t e d and mainly i n t e r n a l l y twinned (Fig. 2). This heavy i n t e r n a l f a u l t i n g i s probably associated w i t h stresses due t o the complex i n t e r a c t i o n s between t h e l a t t i c e i n v a r i a n t deformation o c c u r r i n g d u r i n g t h e martensite formation and t h e a p p l i e d s t r e s s / s t r a i n . However the s u b s t r u c t u r e o f t h e l e n t i c u l a r zig-zag athermal martensite was observed t o be d i s l o c a t e d . The f a c t o r s t h a t c o n t r o l the morphology, s u b s t r u c t u r e and the morphological and s u b s t r u c t u r a l t r a n s i t i o n s are discussed e l s e - where ( 1 ) . The athermal m a r t e n s i t e i n t h e 3048 s t e e l was observed t o p r o v i d e a con- s i d e r a b l e volume f r a c t i o n o f n u c l e a t i o n s i t e s f o r t h e formation o f deformation i n - duced martensite and t o considerably increase t h e volume f r a c t i o n o f MD upon subse- quent deformation a t a l l deformation temperatures ( F i g s . 2 and 3 ) . The r o l e of t h e athermal m a r t e n s i t e on subsequent n u c l e a t i o n o f t h e deformation induced m a r t e n s i t e i s shown i n f i g u r e 1: f o r a 304B s t e e l 100% o f MD was obtained w i t h 20% s t r a i n a t 77K and 45% s t r a i n a t 298K. However i n t h e 302 s t e e l 100% MD was obtained w i t h 40%
s t r a i n a t 77K and 50% s t r a i n a t 298K o n l y produced 65% MD. I n t h e 304 s t e e l 50%
s t r a i n o n l y produced 85% MD a t 77K and 55% MD a t 298K. Upon thermomechanical t r e a t - ments type I and f o r a given p a i r : amount o f deformation-deformation temperature, the amount o f martensite induced by deformation was observed t o be c o n t r o l l e d mainly by t h e n i c k e l content o f the austeni t e phase o f t h e p a r t i c u l a r s t e e l and t h e amount and type o f athermal m a r t e n s i t e p r e v i o u s l y formed d u r i n g quenching/requenching t o 77K.
MuZtiphase Microstructures Type 111 and IV/ThemnomechanicaZ Treatments Type I I I n o r d e r t o increase t h e s t r e n g t h (DPH, YS, UTS) o f these s t a i n l e s s s t e e l s w i t h a small decrease i n t h e i r f o r m a b i l i t y and f r a c t u r e toughness a second type of
therrnomechani c a l treatments (Table 1) was a p p l i e d t o t h e m i c r o s t r u c t u r e s obtained a t the end o f TMT type I . During the deformation o f TMT type I 1 the dispersed c r y s t a l s o f athermal m a r t e n s i t e and deformation induced m a r t e n s i t e were observed t o p l a y f o u r i m p o r t a n t r o l e s : ( a ) MA provided considerable volume f r a c t i o n o f s i t e s f o r t h e nucle- a t i o n o f MD, I n a d d i t i o n the dispersed c r y s t a l s o f b o t h MA and MD were observed to:
( b ) p r o v i d e s i t e s f o r t h e n u c l e a t i o n o f a l a r g e number o f d i s l o c a t i o n s ; ( c ) p r o v i d e s i t e s f o r t h e p r e c i p i t a t i o n o f carbide p a r t i c l e s d u r i n g tempering; ( d ) p r o v i d e s i t e s f o r t h e n u c l e a t i o n o f r e c r y s t a l l i z e d g r a i n s o f f e r r i t e and austeni te; and ( e ) impede the motion o f r e a c t i o n f r o n t s , which considerably l i m i t e d the g r a i n growth d u r i n g re- c r y s t a l l i z a t i o n . As shown i n Table 1 and f i g u r e 3 f o r a 3021304 s t e e l annealing a t 674K r e s u l t e d i n a multiphase s t r u c t u r e c o n t a i n i n g f e r r i t e , a u s t e n i t e and 80% r e t a i n - ed and tempered martensite. Annealing a t 773K r e s u l t e d i n a s i m i l a r m i c r o s t r u c t u r e b u t w i t h o n l y 20% r e t a i n e d martensite. Annealing a t h i g h e r temperatures (973K and 1073K) r e s u l t e d i n very f i n e (4-5 m: 973K and 7-10 m: 1073K) f u l l y r e c r y s t a l l i z e d austeni t e - f e r r i t e m i c r o s t r u c t u r e s . S i m i l a r m i c r o s t r u c t u r e s were obtained i n t h e 304B s t e e l , however both the r e c r y s t a l l i z a t i o n temperatures and t h e g r a i n s i t e s achieved were s h i f t e d t o l o w e r values, which i s explained by t h e r o l e played by t h e athermal martensi t e . These r e c r y s t a l 1 i zed m i c r o s t r u c t u r e s were water quenched and requenched
C4-574 JOURNAL DE PHYSIQUE
i n t o l i q u i d n i t r o g e n and f u r t h e r deformed t o various s t r a i n l e v e l s by r o l l i n g i n the temperature range 77K t o 298K. This again r e s u l t e d i n considerable amounts of t r a n s - formation o f the a u s t e n i t e i n t o m a r t e n s i t e a t a l l temperatures. The achievement o f such f i n e g r a i n s i z e w i t h o n l y one c y c l e o f TMT i s a t t r i b u t e d and explained by t h e r o l e played by the disposed c r y s t a l s o f MA and MD.
TABLE 1 Thermo-Mechanical Treatments Type I 1 (1 Cycle: 1-6) and Type I 1 1 ( 2 Cycles: 1-8). Results f o r a 302 s t e e l . Stages :
1. Heat Treatment a t 1473K f o r 1 hour.
2. Water Quenching/Liquid Nitrogen Requenching.
3. Deformation by r o l l i n g t o 50% reduction.
4. Ageing f o r 6 hours a t d i f f e r e n t temperatures.
5. Soaking a t 77K/l h o f r e c r y s t a l l i zed microstructures.
6. F u r t h e r deformation by r o l l i n g : 1 c y c l e - 1 0 % ~ / 2 c y c l e s - 2 5 % ~ . 7. Ageing f o r 6 hours a t d i f f e r e n t temperatures.
8. F u r t h e r deformation by r o l l i n g t o 10% reduction.
Phase Nature A f t e r
Temperature Phase Nature A f t e r WQ 10% Deformation
673 a + + M ~ ( 8 0 % )
77 3 a + + M ~ ( 3 0 % )
973 a + + M ~ ( 2 0 % )
1073 Recryst. Grains (4-5/2-311m) Recryst. Grains
a + y + T a a + y + T a + M g
1173 Recryst. Grains (7-10/5-6vm) Recryst. Grains
a + y + T a + y + T a + M g
MicromZ tiphase Structures Type 111 and IV/!%ermomechcmica 2 Trea tmerlts Type I I I I n o r d e r t o r e f i n e the m i c r o s t r u c t u r e s even f u r t h e r two and t h r e e cycles of TMT were c a r r i e d o u t (Table 1 , Fig. 3). T h i s r e s u l t e d i n r e f i n e d r e c r y s t a l l i z e d micromultiphase s t r u c t u r e s c o n t a i n i n g f e r r i t e and a u s t e n i t e w i t h a very h i g h d e n s i t y o f annealing twins (T,). Typical g r a i n s i z e s f o r 302 and 304 s t e e l s a f t e r two cy- cles o f TMT and r e c r y s t a l l i z a t i o n a t 1073K were between 2 and 3 pm. A f t e r t h r e e cycles o f TMT the 304B s t e e l e x h i b i t e d g r a i n s i z e s o f approximately 1.5 pm. These micromul ti phase s t r u c t u r e s were water quenched and requenched t o 1 iq u i d n i trogen and
f u r t h e r deformed t o various s t r a i n l e v e l s w i t h i n the temperature range 77K t o 298K.
This r e s u l t e d i n t h e formation o f very f i n e c r y s t a l s o f MD i n the 302 and 304 s t e e l s and i n a m i x t u r e o f very f i n e MA and MD c r y s t a l s i n t h e 304B s t e e l . These micro- s t r u c t u r e s have UTS values above 200 Kpsi and t e n s i l e elongations above 80%.
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