The respective roles of the packet size and the lath width on toughness
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guished at the h i g h e r m a g n i f i c a t i o n s a s s o c i a t e d with e l e c t r o n m i c r o s c o p y . At the next p a c k e t the group of f r a c t u r e p l a n e s has a d i f f e r e n t o r i e n t a t i o n and so high a n g l e d e v i a t i o n s a r e r e q u i r e d at p a c k e t b o u n d a r i e s . E v i d e n t l y , changing the a v e r a g e f r a c t u r e d i r e c t i o n at a p a c k e t boundary w i l l r e q u i r e i m p o r t a n t d e v i a t i o n s a c r o s s the f i r s t laths of the next p a c k e t . It is then p o s s i b l e to e n v i s a g e that a c e r t a i n n u m b e r of laths and c o n s e q u e n t l y the lath width w i l l play a p a r t on the f r a c t u r e t o u g h n e s s . In the two s t u d i e s i n d i c a t e d , 1'~ the lath width w as not s u b j e c t to any m o d i f i c a t i o n , which has led to the p r e s e n t study of the e f f e c t of the lath width on f r a c t u r e t o u g h n e s s . E x p e r i m e n t a l . 13 x 13 m m b a r s w e r e supplied with the c o m p o s i t i o n given in T a b l e I. Th e CCT d i a g r a m was d e t e r m i n e d in o r d e r to find the t i m e - t e m p e r a t u r e r a n g e f o r lath m a r t e n s i t e and bainite, s e e F i g . 1. T h r e e a u s t e n i t i z i n g t r e a t m e n t s w e r e u s e d to v a r y the p a c k e t si ze; 890, 1000 and l l 0 0 ~ e a c h f o r 1 h. U s i n g the d a t a f r o m the CCT c u r v e to avoid any l e t r i t e , two q u e n c h i n g r a t e s w e r e u s e d to v a r y the lath width. T h e s e r a t e s a r e plotted in dotted l i n e s in F i g . 1 and w e r e obtained by a w a t e r (the h i g h e s t r a t e ) and a m e t h y l c e l l u l o s e quench. T e m p e r i n g at 650~ f o r 1 h was p e r f o r m e d on one half of the b a r s u s e d f o r e a c h t r e a t m e n t , to g i v e a s i m i l a r d i s l o c a t i o n d e n s i t y and a s i m i l a r p r e c i p i t a t e d i s t r i b u t i o n in a l l the t e m p e r e d c a s e s and to v a r y o n c e again the lath width. Full size charpy-V specimens were machined from the b a r s and the t o u g h n e s s c u r v e s w e r e d e t e r m i n e d to give the t r a n s i t i o n t e m p e r a t u r e f o r 50 p ct d u c t i l e - b r i t tle fracture. T h e a v e r a g e p a c k e t s i z e s w e r e obtained f r o m
The Respective Roles of the Packet Size and the Lath Width on Toughness Table I. Composition on the Steel Used m Wt Pct
J. P. NAYLOR
AND
R. BLONDEAU
P r e v i o u s w o r k 1,2 has shown that the t o u g h n e s s of high s t r e n g t h low a ll o y s t e e l s having a lath s t r u c t u r e de p en d s on the p a c k e t s i z e , w h e r e a p a c k e t i s d ef i n ed as a group of p a r a l l e l laths f o r m e d f r o m one {11117. Although the laths in one p a c k e t a r e p a r a l l e l , up to s i x lath o r i e n t a t i o n s a r e p o s s i b l e , d e f in e d by the K u r j u m o v - S a c h s 3 r e l a t i o n s h i p : a {111}7 has t h r e e
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