A note on the microstructure of reheated, quenched, and tempered low carbon-manganese-niobium steel
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b i l i t y in f e r r i t e and a fine c a r b i d e p r e c i p i t a t e f o r m e d during air cooling. The softer ferrite better resists f r a c t u r e . T h e i n c r e a s e in i m p a c t t r a n s i t i o n t e m p e r a t u r e beyond 0.60 wt p c t i s m o s t p r o b a b l y due to s u b stantial solid solution hardening effects overshadowing the c a r b i d e p r e c i p i t a t i o n e f f e c t s . F u r t h e r , h i g h e r s i l i c o n c o n t e n t s a r e r e p o r t e d to p r o m o t e c a r b i d e p r e c i p i t a t i o n n e a r and at the g r a i n b o u n d a r i e s with obvious e m b r i t t l i n g r e s u l t s . 4 A l l of the s i l i c o n i n t h e s e e x p e r i m e n t a l s t e e l s is a s s u m e d to be p r e s e n t in s o l i d solution, not in oxide p h a s e s . T h i s i s a r e a s o n a b l e a s s u m p t i o n c o n s i d e r i n g the d e o x i d a t i o n p o w e r of the a l u m i n u m p r e s e n t . 1~ Y i e l d S t r e n g t h . In an e v a l u a t i o n of t h e effect of s i l i con on l o w e r y i e l d point it i s e a s i e s t to t r e a t s e p a r a t e l y the effect on t h e g r a i n s i z e i n d e p e n d e n t t e r m go and the g r a i n s i z e d e p e n d e n t t e r m ky d -1/2 of the P e r c h e q u a tion. Since s i l i c o n i s a s u b s t i t u t i o n a l s o l u t e a t o m , it i n c r e a s e s (to. An i n c r e a s e in go of f r o m 6,000 to 14,000 p s i (41.3 to 96.5 M P a ) p e r wt p c t s i l i c o n has b e e n r e p o r t e d . 1'%%7'x~ In the p r e s e n t study go w a s found to i n c r e a s e c o n t i n u o u s l y with s i l i c o n content at a r a t e of 17,000 p s i (117.2 Mlaa) p e r wt p c t s i l i c o n . P i c k e r ing and G l a d m a n 6 r e p o r t a 10,800 p s i (74.5 MPu) inc r e m e n t in go p e r vet pct s i l i c o n , and F r a z i e r et al ~ found go to be i n c r e a s e d about 14,000 p s i (96.5 MPa) p e r wt p c t s i l i c o n . It i s o f t e n a s s u m e d that ky c a n b e t r e a t e d a s a cons t a n t . H o w e v e r , in t h i s w o r k ky w a s found to d e c r e a s e f r o m about 3500 l b / i n . 2 m m -~/z (77.8 x 103 k g - m -~n) to about 2100 l b / i n , z m m -~/2 (46.7 • 10 s k g - m -~/z) a s s i l i con i n c r e a s e d f r o m 0 to 0.71 wt p c t . The i n i t i a l e l 1 6 0 4 - V O L U M E 7A, O C T O B E R 1976
f e c t of s i l i c o n on ky i s m o s t p r o b a b l y one of s i l i c o n r e d u c i n g the g r a i n b o u n d a r y s t r e n g t h by d e p l e t i o n of carbon from solid solution as carbide prec
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