Enhancement of nitrogen solubility in ingot iron by shock loading
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PLASTICd e f o r m a t i o n
i n c r e a s e s the s o l u b i l i t y of n i t r o g e n in a f e r r i t i c s t e e l b e c a u s e the c o n c e n t r a t i o n of s i t e s f o r n i t r o g e n a t o m s in l a t t i c e d e f e c t s i s i n c r e a s e d . The m a i n difficulty in i n v e s t i g a t i n g t h i s p h e n o m e n o n h a s b e e n in obtaining a s u f f i c i e n t e n h a n c e m e n t of s o l u b i l i t y f o r p r e c i s e d e t e r m i n a t i o n and i n t e r p r e t a t i o n . The m a r t e n s i t i c t r a n s f o r m a t i o n to hcp e and the s u b s e q u e n t s p o n t a n e o u s r e v e r s i o n to b c c m a r t e n s i t e which o c c u r in i r o n d u r i n g the p a s s a g e of an i n t e n s e s h o c k wave p r o d u c e a l a r g e c o n c e n t r a t i o n of l a t t i c e d e f e c t s . The p r e s e n t e x p e r i m e n t s w e r e m a d e to d e t e r m i n e w h e t h e r e x p l o s i v e s h o c k l o a d i n g of i r o n would p r o d u c e an i n c r e a s e in n i t r o g e n s o l u b i l i t y l a r g e enough f o r c o n v e n i e n t e x a m i n a t i o n . In m o s t of the t e s t s the s p e c i m e n s w e r e n i t r o g e n i z e d in a m m o n i a + h y d r o g e n m i x t u r e s at 400~ a f t e r a s t a b i l i z i n g a n n e a l in vacuum at the s a m e t e m p e r a t u r e . C o m p l e m e n t a r y e x a m i n a t i o n s by light m i c r o s c o p y , e l e c t r o n m i c r o s c o p y , and m i c r o h a r d n e s s t e s t i n g w e r e conducted on the s a m e m a t e r i a l . M A T E R I A L S AND PROCEDURE
s u r f a c e d a m a g e , p i e c e s (3 by 0.85 by > 0.017 in) w e r e r o u g h - c u t with a w a f e r i n g m a c h i n e , f i n i s h e d by s u r f a c e g r i n d i n g to a u n i f o r m 0.017 in. t h i c k n e s s , and finally s h e a r e d in half. The s h e a r e d s t r i p s w e r e a n n e a l e d for 96 h r at 400~ in e v a c u a t e d P y r e x c a p s u l e s , p i c k l e d in dilute n i t r i c a c i d to r e m o v e s u r f a c e m a t e r i a l d i s t u r b e d by m a c h i n i n g , and f i n a l l y n i t r o g e n i z e d in flowing m i x t u r e s of a m m o n i a + h y d r o g e n at 400~ The p r o c e d u r e s u s e d Were e s s e n t i a l l y the s a m e a s t h o s e d e s c r i b e d in a p r e v i o u s p a p e r . 1 The 400~ t e m p e r a t u r e a l l o w s r e a s o n a b l y r a p i d ' n i t r o g e n a f i o n of f a i r l y t h i c k s p e c i m e n s without r e c r y s t a l l i z a t i o n but with s o m e d e c r e a s e in d e f e c t c o n c e n t r a t i o n by r e c o v e r y . The a n n e a l i n g in v a c u u m was to allow any m i c r o s t r u c t u r a l c h a n g e s at 400~ to o c c u r u n i f o r m l y t h r o u g h the c r o s s s e c t i o n of the s p e c i m e n s without the c o m p l i cation of n i t r o g e n c o n c e n t r a t i o n g r a d i e n t s . Such p r e t r e a t e d m a t e r i a l h a s a r e s i d u a l a r r a y of n i t r o g e n
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