The role of grain size on the ductile-brittle transition of a 2.25 Pct Cr-1 Pct Mo steel
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AND
P. E. J. FLEWITT
The d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e of a 2 1/4 pct C r - 1 pct Mo s t e e l has been m e a s u r e d u s i n g 'V' notch Izod i m p a c t s p e c i m e n s for an u n e m b r i t t l e d and e m b r i t t l e d 2 1/4 pct C r - l p c t Mo s t e e l with p r i o r a u s t e n i t e g r a i n s i z e s within the r a n g e 40 to 150 pal. The m i c r o s t r u c t u r e of this s t e e l was upper b a i n i t e . The v a r i a t i o n of yield s t r e n g t h with g r a i n s i z e obeys a H a l l - P e t c h r e l a t i o n s h i p . The d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e was found to have a p r o n o u n c e d g r a i n s i z e dependence for both u n e m b r i t t l e d , 15 K m m 'z2, and e m b r i t t l e d , 19 K m m ~/2, s p e c i m e n s . The b a i n i t e colony size was found to v a r y as the p r i o r a u s t e n i t e g r a i n s i z e . F r o m the low t e m p e r a t u r e q u a s i - c l e a v a g e facet s i z e , together with m e t a l l o g r a p h i c o b s e r v a t i o n s of c r a c k path, it has been concluded that b a i n i t e colony s i z e r a t h e r than p r i o r a u s t e n i t e g r a i n size is the effective g r a i n s i z e .
1. INTRODUCTION
THE
effect of g r a i n size on the d u c t i l e - b r i t t l e t r a n s i tion t e m p e r a t u r e of i r o n and i r o n b a s e alloys has b e e n studied by s e v e r a l i n v e s t i g a t o r s . l - ~ In g e n e r a l the t r a n s i t i o n t e m p e r a t u r e , T, is found to v a r y with the g r a i n size, d, a c c o r d i n g to a r e l a t i o n s h i p of the f o r m :
T=A
lnd
[1]
where A is a constant. 4 Since low alloy s t e e l s a r e f r e q u e n t l y used with a m a r t e n s i t i c o r b a i n i t i c m i c r o s t r u c t u r e it is i m p o r t a n t to e s t a b l i s h which component of the m i c r o s t r u c t u r e is the effective value for d. Cons e q u e n t l y c o n s i d e r a t i o n has b e e n given to the c o n t r i b u tion of p r i o r a u s t e n i t e g r a i n size, m a r t e n s i t e o r b a i n i t e lath s i z e or cell size, and the s u b d i v i s i o n of the a u s t e n i t i c g r a i n s by p a r a l l e l laths of b a i n i t i c f e r r i t e or m a r t e n s l t e into packets s-~ In p a r t i c u l a r N a y l o r and K r a h e 1~ have shown that for an upper b a i n i t e d e c r e a s ing the packet or colony size of the laths r e s u l t s in an i n c r e a s e in yield s t r e n g t h and a r e l a t i v e i m p r o v e m e n t in t o u g h n e s s . Consequently they i m p l y that the lath colony size is the d i m e n s i o n s which b e c o m e s the effective g r a i n size, d in Eq. [1] of the m a t e r i a l . More r e c e n t l y they have a t t e m p t e d to r e l a t e the cleavage f r a c t u r e a s s o c i a t e d with the b r i t t l e f a i l u r e mode dir e c t l y to the morphology and c r y s t a l l o g r a p h y of a b a i n i t e colony. ~ In
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