The toughness of high hardness laminar composite steel as influenced by specimen and crack orientation
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P. T. LUM, is Materials Engineer, R. CHAIT, is Supervisory Metallurgist, and C. F. HICKEY, Jr., is Metallurgist with the Army Mechanics and Materials Research Center, Watertown, Mass. 02172. Manuscript submitted June 7, 1974. METALLURGICALTRANSACTIONSA
MATERIALS AND T E S T METHODS Materials The l a m i n a r c o m p o s i t e s t e e l has a h a r d and soft s i d e c h e m i c a l c o m p o s i t i o n shown in Table I. The m a t e r i a l is heat t r e a t e d by q u e n c h i n g f r o m about 815~ and t e m p e r i n g in the r a n g e 120 to 167~ With this heat t r e a t m e n t , the following a v e r a g e m e c h a n i c a l p r o p e r t i e s for both l o n g i t u d i n a l and t r a n s v e r s e d i r e c t i o n w e r e o b t a i n e d : gys ~- 200 ksi (1379 MPa), guts ~- 260 k s i (1793 MPa), e l o n g a t i o n -~ 1.0 pct and r e d u c t i o n of a r e a ~ 3.0 pct. D e s i r e d h a r d n e s s l e v e l s on the h a r d and soft side a r e Rc 60 and 50, r e s p e c t i v e l y . The d i f f e r e n c e in h a r d n e s s b e t w e e n the h a r d and soft side s t e m s m a i n l y f r o m the h i g h e r c a r b o n c o n t e n t of the h a r d side. It should be noted that the n o m i n a l h a r d n e s s r e a d i n g s a r e r e p r e s e n t a t i v e of a g r o u n d s u r f a c e w h e r e d e c a r b u r i z a t i o n has b e e n e l i m i n a t e d . The e x t e n t of d e c a r b u r i z a t i o n and through-the-thickness hardness distribution across the h a r d and soft side as well as through the i n t e r f a cial r e g i o n is shown in Fig. 2. S p e c i m e n s and T e s t s T e s t 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 0.220 in. (5.6 ram) thick plate in the o r i e n t a t i o n s shown in Fig. 3. Note that both the c r a c k a r r e s t e r and c r a c k d i v i d e r o r i e n t a t i o n ( d i r e c t i o n of c r a c k p r o p a g a t i o n p e r p e n d i c u l a r and p a r a l l e l to the i n t e r f a c e , r e s p e c t i v e l y ) w e r e m a c h i n e d . The s p e c i m e n i d e n t i f i c a t i o n g i v e n i n F i g . 3 follows the n o m e n c l a t u r e s u g g e s t e d by ASTM ( A m e r i c a n Society for T e s t i n g M a t e r i a l s ) , s w h r e the f i r s t l e t t e r r e f e r s to the d i r e c t i o n p e r p e n d i c u l a r to the p l a n e of f r a c t u r e and the s e c o n d l e t t e r to the d i r e c t i o n of c r a c k p r o p a g a t i o n . In o u r s p e c i f i c c a s e L was c h o s e n as the d i r e c t i o n of f i n a l r o l l i n g , m a k i n g T the long t r a n s v e r s e d i r e c t i o n . The d i m e n s i o n of both the c r a c k a r r e s t e r and c r a c k d i v i d e r s p e c i m e n s a r e shown in F i g . 4. The t h i c k n e s s of the s t a r t i n g m a t e r i a l l i m i t e d the t h i c k n e s s and depth of the c r a c k d i v i d e r and c r a c k a r r e s t e r s p e c i m e n s r e s p e c t i v e l y . The s t a n d a r d C h a r p y notch depth of 0.079 i n . m m ) w a s u s e d
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