• Aucun résultat trouvé

ATHERMAL, STRESS AND STRAIN INDUCED TRANSFORMATIONS IN MULTIPHASE STAINLESS STEELS

N/A
N/A
Protected

Academic year: 2021

Partager "ATHERMAL, STRESS AND STRAIN INDUCED TRANSFORMATIONS IN MULTIPHASE STAINLESS STEELS"

Copied!
7
0
0

Texte intégral

(1)

HAL Id: jpa-00222209

https://hal.archives-ouvertes.fr/jpa-00222209

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

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�

(2)

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

(3)

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

B

s 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

AFT

increases 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 D

where: 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 F

i s the f r e e energy change associated with the bulk material, largely determined by eflectronic configurations;

A F

i 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

i

ncl 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 F

i 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

bFy

and

A F D

may 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 .

(4)

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.

(5)

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

(6)

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

(7)

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%.

REFERENCES

1. MARQUES, F.D.S. "Morphology/Substructure Relationships i n Z i r c o n i um Based Mar- t e n s i t e s

,

Proc. ICOMAT-82.

2. MARQUES, F.D.S. Z e i t s c h r i f t

fur

MetallKunde 69 (1978) H.3, 167.

3. MARQUES, F.D.S., Proc. 4 t h I n t . Conf. S t r e n g t h o f Metals & A l l o y s , 2 (1976) 736.

4. LECROISEY, F., PINEAU, A., Met. Trans., 3 (1972) 387.

5. VENABLES, T.A., Phi 1. Mag. 7 (1962) 35.

6. LANEBORG, R., Acta Met., 12 (1964) 823.

7. MANGANON, D.C., THOMS, G., Met. Trans., 1 (1970) 1577.

8. OLSON, G.B., COHEN, M.T., Less Common Metals, 28 (1972) 107.

9 . OTTE, H.N., Acta Met., 5 (1957) 614.

10. KELLY, P.M., NUTTING, T., J. I r o n Steel I n s t . 197 (1961) 199.

Références

Documents relatifs

To test whether the vesicular pool of Atat1 promotes the acetyl- ation of -tubulin in MTs, we isolated subcellular fractions from newborn mouse cortices and then assessed

Néanmoins, la dualité des acides (Lewis et Bronsted) est un système dispendieux, dont le recyclage est une opération complexe et par conséquent difficilement applicable à

Cette mutation familiale du gène MME est une substitution d’une base guanine par une base adenine sur le chromosome 3q25.2, ce qui induit un remplacement d’un acide aminé cystéine

En ouvrant cette page avec Netscape composer, vous verrez que le cadre prévu pour accueillir le panoramique a une taille déterminée, choisie par les concepteurs des hyperpaysages

Chaque séance durera deux heures, mais dans la seconde, seule la première heure sera consacrée à l'expérimentation décrite ici ; durant la seconde, les élèves travailleront sur

A time-varying respiratory elastance model is developed with a negative elastic component (E demand ), to describe the driving pressure generated during a patient initiated

The aim of this study was to assess, in three experimental fields representative of the various topoclimatological zones of Luxembourg, the impact of timing of fungicide

Attention to a relation ontology [...] refocuses security discourses to better reflect and appreciate three forms of interconnection that are not sufficiently attended to