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300M s t e e l was s u b j e c t e d to a wide r a n g e of quenched and t e m p e r e d h e a t t r e a t m e n t s . The p l a n e - s t r a i n f r a c t u r e t o u g h n e s s and the t e n s i l e u l t i m a t e and y i e l d s t r e n g t h s w e r e e v a l u a t e d . R e s u l t s i n d i c a t e that s u b s t a n t i a l i m p r o v e m e n t in t o u g h n e s s with no l o s s in s t r e n g t h can be a c c o m p l i s h e d in quenched and t e m p e r e d s t e e l by a u s t e n i t i z i n g at 1255 K (1800~ o r h i g h e r . Low f r a c t u r e t o u g h n e s s in c o n v e n t i o n a l l y a u s t e n i t i z e d 300M s t e e l (1144 K (1600~ a p p e a r s to be c a u s e d by u n d i s s o l v e d p r e c i p i t a t e s s e e n both in the s u b m i c r o s t r u c t u r e and on the f r a c t u r e s u r f a c e which p r o m o t e f a i l u r e by q u a s i - c l e a v a g e . T h e s e p r e c i p i t a t e s a p p e a r e d to d i s s o l v e in the r a n g e 1200 to 1255 K (1700 to 1800~

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c o r r e l a t i o n of m i c r o s t r u c t u r e with m e c h a n i c a l p r o p e r t i e s of s t r u c t u r a l m a t e r i a l s h a s b e e n a topic of i n t e r e s t f o r m a n y y e a r s a t t r a c t i n g the a t t e n t i o n of n u m e r o u s w o r k e r s , x-~l M o s t of t h e s e s t u d i e s have b e e n p r i m a r i l y f o c u s e d e i t h e r on m e c h a n i c a l p r o p e r t i e s o r on m i c r o s t r u c t u r a l d e t a i l s , and a s a r e s u l t , many a m b i g u i t i e s and u n r e s o l v e d q u e s t i o n s r e m a i n . It i s b e l i e v e d to be of u t m o s t i m p o r t a n c e that equal e m p h a s i s be p l a c e d on both m i c r o s t r u c t u r e and m e c h a n i c a l b e h a v i o r , and the p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n with this d u a l e m p h a s i s a s i t s c o r n e r stone. 300M s t e e l i s a w i d e l y u s e d u l t r a h i g h s t r e n g t h s t e e l , and c o n s i d e r a b l e r e s e a r c h h a s been conducted into i t s m e c h a n i c a l p r o p e r t i e s . I~ The p r e s e n t r e s e a r c h a t t e m p t s to add to t h e s e e a r l i e r s t u d i e s by e x p l o r i n g in depth the influence of c o n v e n t i o n a l quenching and t e m p e r i n g t r e a t m e n t s o v e r a s wide a temperature range as is practical for steels. The t e n s i l e s t r e n g t h and f r a c t u r e t o u g h n e s s a s w e l l a s the m i c r o s t r u c t u r e and f r a c t u r e s u r f a c e m o r p h o l o g y were evaluated for each heat treatment. E x p l a n a t i o n s a r e o f f e r e d a s to the m e c h a n i s m s r e s p o n s i b l e f o r the o b s e r v e d m e c h a n i c a l b e h a v i o r . Heat t r e a t m e n t s a r e a l s o r e c o m m e n d e d which should give an o p t i m u m c o m b i n a t i o n of s t r e n g t h and toughn e s s in p r a c t i c a l a p p l i c a t i o n s . E X P E R I M E N T A L PROCEDURE M a t e r i a l s and H