Isothermal transformations in iron-chromium-carbon alloys
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INa p r e v i o u s
paper 1 the f e r r i t e m o r p h o l o g i e s produced in the i s o t h e r m a l y ~ a t r a n s f o r m a t i o n of an F e - C r b i n a r y alloy were r e l a t e d to the r e a c t i o n k i n e tics as a function of t r a n s f o r m a t i o n t e m p e r a t u r e . In this paper, the i s o t h e r m a l d e c o m p o s i t i o n of a u s t e n i t e in two r e l a t e d c h r o m i u m s t e e l s is i n v e s t i g a t e d in t e r m s of both the r e s u l t a n t f e r r i t e morphology and the a s s o c i a t e d c a r b i d e d i s p e r s i o n s . It will be shown that the p r e c i p i t a t e s which develop in a s s o c i a t i o n with the growing f e r r i t e do not r a d i c a l l y a l t e r the o b s e r v e d f e r r i t e morphology, (i.e. a c o m p a r i s o n with the b i n a r y F e - C r alloy shows little m a c r o s c o p i c d i f f e r e n c e in b e h a v i o r ) . S i m i l a r l y , it will be shown that for f e r r i t e c o n t a i n i n g i n t e r p h a s e p r e c i p i t a t e * the k i n e t i c data is *Interphase precipitation occurs in "sheets" parallel to the transformation front. For fuller details see Ref. 2.
c o n s i s t e n t with a n i n t e r f a c i a l l y c o n t r o l l e d f e r r i t e growth r a t e and o b s e r v a t i o n s s u p p o r t the postulated " l e d g e - t y p e " m e c h a n i s m of t r a n s f o r m a t i o n . 3-5 T h e s e r e s u l t s a r e shown to be c o n s i s t e n t with s i m i l a r s t u d i e s in the F e - C r alloy t r a n s f o r m e d at e q u i v a l e n t t e m p e r a tures. The two a l l o y s chosen for i n v e s t i g a t i o n had the nominal compositions :
In the 5Cr alloy, the b a i n i t e s t a r t (B s) and m a r t e n site s t a r t (M s ) t e m p e r a t u r e s were d e t e r m i n e d by high speed d i l a t o m e t r y , and the p r o g r e s s of the b a i n i t e r e a c t i o n u s i n g a s i m p l e i s o t h e r m a l d i l a t o m e t e r (see Ref. 7). D i s c s for 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 y were thinned i n a s o l u t i o n c o n t a i n i n g 5 pct p e r c h l o r i c acid in 2 - b u t o x y - e t h a n o l with 80 V applied voltage a t - 10~ u s i n g a t w i n - j e t p o l i s h e r . The foils w e r e e x a m i n e d in P h i l i p s EM 300 and EM 301 m i c r o s c o p e s . 3. RESULTS: THE TRANSFORMATION STRUCTURES 3.1. F e r r i t e Morphologies In both a l l o y s , the t r a n s f o r m a t i o n was i n i t i a t e d at the a u s t e n i t e g r a i n b o u n d a r i e s over the whole t e m p e r a t u r e r a n g e 800 to 400~ At lower t e m p e r a t u r e s , b o u n d a r y n u c l e a t i o n continued u n t i l site s a t u r a t i o n oc800
I % S a 9 %
~"
6
o . 70Q u4
carbide
austenite
n-
~60@
OC I,LI O. :E
F e - 1 0 p c t C r - 0 . 2 pct C F e - 5 pct C r - 0 . 2 pct C*
50G
F-
Ss
*All percentages are given as weight pet,
bainite
T h e s e c h r o m i u m levels were s e l e c t e d in o r d e r to
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