A study of strengthening mechanisms in tempered martensite from a medium carbon steel
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P r e v i o u s s t u d i e s of s t r e n g t h e n i n g m e c h a n i s m s in quenched and t e m p e r e d s t e e l s h a v e b e e n b a s e d a l m o s t e n t i r e l y on m i c r o s c o p y . The d e n s i t y and d i s t r i b u t i o n of r e s i d u a l d i s l o c a t i o n s in t h e s e m a t e r i a l s h a v e g e n e r a l l y been p o o r l y c h a r a c t e r i z e d and t h e r e i s doubt a s to t h e c o n t r i b u t i o n of s u b s t r u c t u r e to s t r e n g t h . In the p r e s e n t w o r k , a 0.42 p c t C s t e e l h a s been h a r d e n e d and t e m p e r e d f o r 1 h r at s e v e n t e m p e r a t u r e s in the r a n g e 250 ~ to 550~ The s t r u c t u r e s have b e e n s t u d i e d b y X - r a y d i f f r a c t i o n line p r o f i l e a n a l y s i s a s w e l l a s by e l e c t r o n m i c r o s c o p y . The m e a s u r e d 0.2 p c t offset y i e l d s t r e n g t h , o v e r t h e whole t e m p e r i n g t e m p e r a t u r e r a n g e , could b e r e p r e s e n t e d by kl
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w h e r e D i s the m e a n p l a n a r s p a c i n g of c a r b i d e p a r t i c l e s , p i s the d e n s i t y of r e s i d u a l d i s l o c a t i o n s in the a s - t e m p e r e d s t r u c t u r e , and b i s t h e B u r g e r ' s v e c t o r of a d i s l o c a t i o n in i r o n . The s t r e s s ao w a s found to b e about 35 k g m m m -2 f o r the s t e e l u s e d and p r o b a b l y i n c l u d e s an a p p r e c i a b l e c o n t r i b u t i o n f r o m i n c r e m e n t a l i n t e r s t i t i a l s o l i d solution s t r e n g t h e n ing. T h e c o n s t a n t s kl and k2 h a v e v a l u e s which a r e c o n s i s t e n t with both t h e o r y and p r e v i o u s e x p e r i m e n t a l w o r k with the O r o w a n m e c h a n i s m and s t r a i n - h a r d e n i n g , r e s p e c t i v e l y . The d i s l o c a t i o n s u b s t r u c t u r e c o n t r i b u t i o n a c c o u n t e d for m o r e than 35 p c t of the y i e l d s t r e n g t h of the s t e e l t e m p e r e d at t e m p e r a t u r e s up to 400~
S E V E R A L p r e v i o u s i n v e s t i g a t o r s h a v e s t u d i e d the s t r u c t u r e s of t e m p e r e d m e d i u m to high c a r b o n m a r tensite, using X-ray diffraction, 1 transmission elect r o n m i c r o s c o p y , 2 and e l e c t r i c a l r e s i s t i v i t y 3 m e t h o d s . N e a r l y a l l of the c a r b o n is b e l i e v e d to b e out of s o l u tion and to b e p r e s e n t a s a c a r b i d e d i s p e r s i o n a f t e r t e m p e r i n g at about 250~ f o r 1 h r . The lath s t r u c t u r e of the m a r t e n s i t e a p p e a r s to b e r e t a i n e d on t e m p e r i n g up to ~450~ b e y o n d which it i s p r o g r e s s i v e l y r e p l a c e d b y a n e t w o r k d i s l o c a t i o n s u b s t r u c t u r e and by s u b g r a i n s . T h e d e n s i t y of r e s i d u a l d i s l o c a t i o n s within the m a r t e n s i t e l a t h s a p p e a r s to d e c r e a s e with i n c r e a s i n g t e m p e r
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