Transformation from austenite in alloy steels
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Transformation from Austenite in Alloy Steels
R . W. K. H O N E Y C O M B E R. F. Mehl Medalist
T h i s p a p e r i s c o n c e r n e d w i t h t h e d i r e c t t r a n s f o r m a t i o n of a u s t e n i t e a t h i g h t e m p e r a t u r e s to form ferrite and alloy carbide dispersions. The ferrite/austenite interfaces vary from h i g h e n e r g y r a n d o m b o u n d a r i e s t o low e n e r g y p l a n a r b o u n d a r i e s w h i c h g r o w b y s t e p p r o pagation, while the alloy carbide morphologies include a pearlitic form, fine fibers and f i n e b a n d e d a r r a y s of p a r t i c l e s . I t i s s h o w n t h a t t h e s e m o r p h o l o g i e s a r e c l o s e l y r e l a t e d t o t h e m o d e of g r o w t h of t h e f e r r i t i c m a t r i x . T h e r o l e of v a r i o u s a l l o y i n g e l e m e n t s o n t h e c a r b i d e d i s p e r s i o n i s e x a m i n e d , a n d t h e e f f e c t s of o t h e r m e t a l l u r g i c a l v a r i a b l e s o n the banded dispersions are discussed, including factors which influence the dispersion s t a b i l i t y . T h e m e c h a n i c a l p r o p e r t i e s of d i r e c t l y t r a n s f o r m e d a l l o y s t e e l s a r e s h o w n to d e p e n d l a r g e l y o n t h e f e r r i t e g r a i n s i z e a n d t h e s t a t e of t h e c a r b i d e d i s p e r s i o n . M i c r o a l l o y e d s t e e l s s u b j e c t e d t o c o n t r o l l e d r o l l i n g p r o v i d e a n e x c e l l e n t e x a m p l e of t h e a c h i e v e m e n t of h i g h s t r e n g t h a n d t o u g h n e s s l e v e l s b y c o n t r o l of t h e s e v a r i a b l e s . T h e p a p e r f i n a l l y a t t e m p t s to s h o w h o w s u c h b e n e f i t s c a n b e a c h i e v e d i n low a n d m e d i u m a l l o y steels, and in particular where resistance to creep failure at elevated temperatures is an important property.
THE
classical heat treatment for transformable steels has involved quenching from the high temperature austenitic condition to form martensite which, b e c a u s e of i t s i n h e r e n t l y b r i t t l e n a t u r e , m u s t b e s u b sequently tempered to provide an acceptable comp r o m i s e b e t w e e n h i g h s t r e n g t h a n d t o u g h n e s s . It i s n o t t h e o b j e c t i v e of t h i s p a p e r t o e x p l a i n t h i s w e l l t r o d d e n path, but to look more closely at the direct transform a t i o n of a u s t e n i t e t o p h a s e s f o r m e d a t h i g h t e m p e r a tures, e.g. ferrite and ferrite-carbide aggregates
which in many cases provide an alternative route to acceptable mechanical properties. T h e s e e d s of t h i s a p p r o a c h t o a l l o y s t e e l s w e r e sown by Davenport and Bain in their classical work on t h e 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 w h i c h n o t only revealed new phases such as bainite, but more i m p o r t a n t l y d e m o n s t r a t e d t h e e f f e c t s of a l l o y i n g e l e m e n t s o n t h e k i n e t i c s
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