Phase transformational kinetics and hardenability of low-carbon, boron-treated steels
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I-1IGH s t r e n g t h , a c c e p t a b l e t o u g h n e s s , w e l d a b i l i t y , and f o r m a b i l i t y a t a m i n i m u m c o s t in t e r m s of a l l o y i n g and p r o c e s s i n g a r e the d e s i r e d c h a r a c t e r i s t i c s of the high s t r e n g t h s t r u c t u r a l s t e e l s . L o w - c a r b o n , l o w - a l l o y s t e e l s m a y d e v e l o p t h e i r s t r e n g t h a s the r e s u l t of a u s t e n i t i z i n g and quenching p l u s t e m p e r i n g , or s i m p l y by c o n t r o l l e d cooling a f t e r hot r o l l i n g , followed by s u i t a b l e t e m p e r i n g t r e a t m e n t . A knowledge of the e f f e c t s of a l l o y i n g e l e m e n t s on the t r a n s f o r m a t i o n s of a u s t e n i t e u n d e r c o n d i t i o n s of continuous cooling i s , t h e r e f o r e , e s s e n t i a l for the s e l e c t i o n of a l l o y c o m b i n a tions b e s t s u i t e d to a v a i l a b l e p r o c e s s i n g e q u i p m e n t . B o r o n t r e a t m e n t of the l o w - c a r b o n a l l o y s t e e l s has been found a d v a n t a g e o u s for d e l a y i n g of the f e r r i t i c p e a r l i t i c t r a n s f o r m a t i o n s and for the e x t e n s i o n of the t i m e r a n g e of the i n t e r m e d i a t e (bainitic) t r a n s f o r m a tion. B a r d g e t t and R e e v e 1 showed that l o w - c a r b o n m o l y b d e n u m - b o r o n s t e e l s m a y a c q u i r e high l e v e l s of t e n s i l e s t r e n g t h with a wide r a n g e of a i r - c o o l e d s e c tion s i z e s . I r v i n e , P i c k e r i n g , H e s e l w o o d , and A t k i n s 2-4 s t u d i e d the p h a s e t r a n s f o r m a t i o n s , m i c r o s t r u c t u r e s , and m e c h a n i c a l p r o p e r t i e s of the l o w - c a r b o n b a i n i t i c s t e e l s of the 0.5 p c t M o - B type and a l s o b o r o n - t r e a t e d s t e e l s containing Cu, W, Ni, Mn, C r , Mo, V, and T i . The c r u c i a l r o l e of m o l y b d e n u m and tungsten in d e l a y ing the f e r r i t i c - p e a r l i t i c t r a n s f o r m a t i o n s and in e x tending the t i m e r a n g e of the i n t e r m e d i a t e (bainitic) W. W. C1AS is Senior Research Metallurgist, Research Laboratory, Climax Molybdenmn Company of Michigan, Ann Arbor, Mich. 48106. Manuscript submitted April 21, 1972. METALLURGICAL TRANSACTIONS
t r a n s f o r m a t i o n was c o n f i r m e d d u r i n g t h e s e s t u d i e s . B o r o n - t r e a t e d and r e f e r e n c e l o w - c a r b o n C r - M o , C r - N i - M o , M n - C r - M o , and M n - C r - N i - M o s t e e l s w e r e s t u d i e d by C i a s , 5 and the continuous cooling t r a n s f o r m a t i o n d i a g r a m s w e r e e s t a b l i s h e d for t h e m and c o m p a r e d on the b a s i s of the r e s p e c t i v e v a l u e s of the ferrite start half-cooling time. In o r d e r to p r o v i d e c o m p r e h e n s i v e i n f o r m a t i o n on the f o r m a t i o n of s t r u c t u r e s on continuous cooling, and on the
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