The effects of thermal history and composition on the hot ductility of low carbon steels
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HOT
d u c t i l i t y is an i m p o r t a n t p a r a m e t e r in s t e e l technology, s i n c e it h e l p s to d e t e r m i n e the s u c c e s s o r e f f i c i e n c y of s o m e of the m o s t i m p o r t a n t s t e e l p r o c e s s i n g o p e r a t i o n s . Hot d u c t i l i t y p l a y s an i m p o r tant r o l e in c a s t i n g , in hot w o r k i n g by f o r g i n g o r r o l l ing and in f a b r i c a t i o n by w e l d i n g of the final p r o d u c t . While the r e s u l t s of the p r e s e n t s t u d y have p a r t i c u l a r a p p l i c a t i o n in continuous c a s t i n g , ~-s t h e y a l s o r e v e a l a g e n e r a l m e c h a n i s m c o n t r o l l i n g the hot d u c t i l i t y of low c a r b o n s t e e l s . The t e r m " h o t d u c t i l i t y " as u s e d h e r e i n r e f e r s to the amount of hot d e f o r m a t i o n a m a t e r i a l can a c c o m m o d a t e without f r a c t u r e o r s e v e r e c r a c k i n g . High s t r a i n r a t e hot t e n s i l e t e s t s w e r e u s e d to o b t a i n m e a s u r e m e n t s of this p a r a m e t e r at t e m p e r a t u r e s r a n g i n g f r o m 1400 to 2800~ (760 to 1540~ One m e c h a n i s m of f a i l u r e in hot d u c t i l i t y t e s t s c o r r e s p o n d s to a t r u e b r i t t l e f r a c t u r e in the p r e s e n c e of a liquid g r a i n b o u n d a r y p h a s e . This c o n d i t i o n is v a r i o u s l y r e f e r r e d to a s " h o t t e a r i n g " , " h o t c r a c k i n g " o r " h o t s h o r t n e s s " d e p e n d i n g upon the s p e c i f i c a p p l i cation. A m o r e c o m p l e x phenomenon, and one that is often m i s t a k e n for hot s h o r t n e s s , can b e s t be d e s c r i b e d a s " l o w d u c t i l i t y f r a c t u r e " . This type of f r a c t u r e is a c c o m p a n i e d by s i g n i f i c a n t l o c a l d e f o r m a tion, but the t o t a l s t r a i n to f r a c t u r e can a p p r o a c h z e r o . The v a r i a b l e s involved in hot d u c t i l i t y include t e m p e r a t u r e , s t r a i n r a t e , the t y p e of s t r e s s s y s t e m i m p o s e d and the m e t a l l u r g i c a l v a r i a b l e s of c o m p o s i t i o n and s t r u c t u r e . A k e y f a c t o r in d e t e r m i n i n g the m e t a l l u r g i c a l s t r u c t u r e and the r e s u l t a n t hot d u c t i l i t y is the t h e r m a l h i s t o r y of the m a t e r i a l . Although o t h e r v a r i a b l e s have been e x t e n s i v e l y studied, the influence of t h e r m a l h i s t o r y has r e c e i v e d r e l a t i v e l y l i t t l e a t t e n G. A. WILBER is Staff Metallurgist, Specialty Steels, Jones & Laughlin Steel Corp., Pittsburgh, Pa. R. BATRA is Metallurgist, Triangle Conduit and Cable Co., New Brunswick, N.J.W.F. SAVAGE is Professor of Metallurgical Engineering, and W. J. CHILDS is a former Professor of Metallurgical Engineering, Rensselaer Polytechnic Institute, Troy, N.Y.W.J. Childs is now President, Reel Vortex, Inc., Troy, New York. Manuscript submitted Apri
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