Electron microprobe study of the carbon content of ferrite and bainite in Fe-C-Mo and Fe-C-Cr alloys
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Electron Microprobe Study of the Carbon Content of Ferrite and Bainite in Fe-C-Mo and Fe-C-Cr Alloys K. R. KINSMAN, E. EICHEN, AND H. I. AARONSON
S I N C E the c a r b o n c o n t e n t of the f e r r i t i c c o m p o n e n t of b a i n i t e i s of p r i m a r y i n t e r e s t i n t h i s s t u d y , it i s i m p e r a t i v e t o r e c o g n i z e a t t h e o u t s e t that t h r e e d i f f e r e n t d e f i n i t i o n s o f b a i n i t e a r e c u r r e n t l y i n u s e . 1 O n the m i c r o s t r u c t u r a l d e f i n i t i o n , b a i n i t e i s a d i s p e r s i o n of p r i n c i pally n o n l a m e l l a r c a r b i d e s f o r m e d in a s s o c i a t i o n w i t h p r o e u t e c t o i d f e r r i t e , z's T h e f e r r i t e n e e d not a p p e a r i n a Widmanstatten morphology.~ The s u r f a c e r e l i e f d e f i n i t i o n a c c e p t s a s b a i n i t e any s l o w g r o w i n g p r e c i p i t a t e p l a t e which y i e l d s a martensite-like r e l i e f effect at a f r e e s u r f a c e .4'5 T h e k i n e t i c d e f i n i t i o n i s b a s e d u p o n o v e r a l l r e a c t i o n k i n e t i c s : k i n e t i c a l l y d e f i n e d b a i n i t e is t a k e n to h a v e i t s own C - c u r v e a n d to t r a n s f o r m a p r o g r e s s i v e l y s m a l l e r p r o p o r t i o n of the a u s t e n i t e m a t r i x a s the h i g h e s t t e m p e r a t u r e of t h i s c u r v e , the k i n e t i c - B s temperature, is approached. 6 Unfortunately t h e s e d e f i nitions encompass three different p h a s e transformation p h e n o m e n a : the m i c r o s t r u c t u r a l d e f i n i t i o n r e p r e s e n t s a n o n - l a m e l l a r e u t e c t o i d r e a c t i o n , ~ surface r e l i e f is a m a n i f e s t a t i o n of the g r o w t h m e c h a n i s m of a d i s l o c a t i o n i n t e r p h a s e b o u n d a r y , ~ a n d the k i n e t i c e f f e c t s h a v e b e e n s h o w n t o be a s p e c i a l e f f e c t of c e r t a i n a l l o y i n g e l e m e n t s upon the n u c l e a t i o n , a n d e s p e c i a l l y upon t h e g r o w t h k i n e t i c s of p r o e u t e c t o i d f e r r i t e . V ' 8 T h u s a t r a n s f o r m a t i o n product which qualifies as b a i n i t e o n o n e definition o f t e n d o e s n o t o n e i t h e r o r b o t h of the o t h e r t w o . x T h i s c o m munication d e a l s with kinetic and s u r f a c e relief bainite. A t l e a s t a s o b s e r v e d by o p t i c a l m i c r o s c o p y , f e w i f a n y of the m i c r o s t r u c t u r e s o n w h i c h m e a s u r e m e n t s w e r e made qualify as microstructural bainite. P r e v i o u s l y r e p o r t e d m e a s u r e m e n t s of the c a r b o n c o n t e n t of b a i n i t e f a l l into t w o c a t e g o r i e s . P h y s i c a l p r o p e r t y a n d chemical m e a s u r e m e n t s (e.g., d i l a t o m e t r y , 9'~ c a l o r i m e t r y , ~ c h e m i c a l e x t r a c t i o n , x2 r e s i s t o m e t r y ~2,~3) often indicate significant supersaturation, parti
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