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growth k i n e t i c s . N e v e r t h e l e s s , t h e i r data c l e a r l y d e m o n s t r a t e d that g r a i n r e f i n e m e n t led to higher t e n s i l e s t r e n g t h s and longer f a i l u r e t i m e s d u r i n g s t r e s s c o r r o s i o n c r a c k i n g . However, C a r t e r ' s work 19 i n d i c a t e d that Si additions to type 4340 s t e e l led to h i g h e r s t r e n g t h s and longer f a i l u r e t i m e s ( d e c r e a s e d da/dt) even though the g r a i n s i z e b e c a m e c o a r s e n e d . T h e s e o b s e r v e d opposite b e h a v i o r s in modified 4340 s t e e l was a d m i t t e d l y on two d i f f e r e n t types of m a t e r i a l , but n e v e r t h e l e s s r a i s e s the f u n d a m e n t a l q u e s t i o n of what the g r a i n size effect r e a l l y i s . F u r t h e r m o r e , r e c e n t d e r i v a t i o n s on g r a i n s i z e effects on c r a c k i n g t h r e s h o l d s and c r a c k growth k i n e t i c s 1 i n d i c a t e d KTHOtd 1/2 and (da/dt)ad -3n for e l a s t i c a l l y p r e d o m i n a n t b e h a v i o r . It was of s o m e p r a c t i c a l and t h e o r e t i c a l i n t e r e s t , then, to v e r i f y the g r a i n s i z e effect. In an effort to i m p r o v e upon p r e v i o u s i n v e s t i g a t i o n s , the e l e c t r o c h e m i c a l effects w e r e m i n i m i z e d by going to a study of s p e c i m e n s that had b e e n cathodit a l l y charged with hydrogen. A single t e m p e r of 4340 s t e e l (573 K), which gave a yield s t r e n g t h n e a r 1380 M P a (200,000 psi) and well defined Stage I c r a c k growth k i n e t i c s was chosen as the b a s e line m a t e r i a l . The r a n g e of g r a i n s i z e s studied was expanded to app r o x i m a t e l y five t i m e s that of P r o c t o r and P a x t o n . 2~ Thus, the object of the study was to e v a l u a t e t h r e s hold s t r e s s i n t e n s i t y , yield s t r e n g t h , c r a c k velocity, and mode of f r a c t u r e for a 4340 s t e e l v a r y i n g in g r a i n s i z e f r o m 20 # m to in e x c e s s of 140 # m .

EXPERIMENTAL

PROCEDURES

Double cantilever beam (DCB) specimens, as depicted in Fig. I, were used for this study. I%21,22 This specimen design was selected for its simplicity and the information that it yields. Because of its unique design, the DCB specimen has a useful stress intensity r a n g e f r o m 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 , KIC, to a p p r o x i m a t e l y 0.1 KIC. As an added benefit, u s u a l l y two or m o r e t e s t s can be obtained f r o m one s p e c i m e n . The double c a n t i l e v e r b e a m (DCB) s p e c i m e n s w e r e m a c h i n e d f r o m a c o m m e r c i a l b i l l e t of 4340 type s t e e l (Table I). Light m i c r o s c o p y e x a m i n a t i o n of m a t e r i a l VOLUME 7A, JULY 1976-953

f r o m t h i s b i l l e t showed the m a t e r i a l to contain an e s t i m a t e d 0.45 vol pct of l a r g e , p a r a l l e l , s t r i n g e r t y p e i n c l u s i