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RAO,

D. M. R. TAPLIN,

AND

P. RAMA

RAO

The influence of p o l y c r y s t a l g r a i n s i z e in the r a n g e 18/~m to 184/~m on the t e n s i l e b e h a v i o r of an a u s t e n i t i c s t a i n l e s s s t e e l c o n t a i n i n g by wt pct 21 Cr , 14 Mn, 0.68 N and 0.12 C has b e e n i n v e s t i g a t e d o v e r the t e m p e r a t u r e r a n g e 298 to 1273 K. D e c r e a s i n g g r a i n s i z e has b e e n shown to i n c r e a s e the flow s t r e s s at s m a l l s t r a i n s in a c c o r d a n c e with the H a l l - P e t c h r e l a t i o n s h i p at t e m p e r a t u r e s up to 873 K. The v a r i a t i o n of the H a l l - P e r c h c o n s t a n t s with t e m p e r a t u r e is influenced by d y n a m i c s t r a i n ageing b e t w e e n 575 and 775 K. Above 875 K, e s p e c i a l l y at low s t r a i n - r a t e s a r e v e r s a l of the H a l l - P e t c h c o r r e l a t i o n o c c u r s and the flow s t r e s s d e c r e a s e s with d e c r e a s i n g g r a i n s i z e , The r e l a t i o n s h i p b e t w e e n d u c t i l i t y and t e m p e r a t u r e is m a r k e d by a m i n i m u m d u c t i l i t y at about half the a b s o l u t e m e l t i n g t e m p e r a t u r e and t n t e r g r a n u l a r c a v i t a t i o n is o b s e r v e d . A d e c r e a s e in g r a i n s i z e g e n e r a l l y e n hanced the d u c t i l i t y in this t e m p e r a t u r e r e g i m e w h i l s t at fine g r a i n s i z e s this t r e n d was r e v e r s e d . T h e s e r e s u l t s a r e e x p l a i n e d in t e r m s of a c o m b i n a t i o n of a G r i f f i t h - O r o w a n type f r a c t u r e c r i t e r i o n and an i n t e r g r a n u l a r void s h e e t m e c h a n i s m of f r a c t u r e . A s e r i e s of n i c k e l - f r e e a u s t e n i t i c s t a i n l e s s s t e e l s has b e e n d e v e l o p e d in India to o v e r c o m e the s h o r t a g e of s t a i n l e s s s t e e l s c a u s e d by the n e a r l y to ta l a b s e n c e of indigenous n i c k e l o r e s . A c r i t i c a l a s s e s s m e n t of the c o r r o s i o n and oxidation c h a r a c t e r i s t i c s of t h e s e s t e e l s v i s - a - v i s the s t a n d a r d n i c k e l - b e a r i n g 18-8 type a u s t e n itic s t e e l s has b e e n m a d e by Nijhawan e t a l . ~ I n the p r e s e n t p a p e r the t e n s i l e b e h a v i o r of one of t h e s e s t e e l s has b e e n i n v e s t i g a t e d in the t e m p e r a t u r e r a n g e 298 to 1273 K (25 to 1000~ The p r i n c i p a l a i m was to e x a m ine the i n f l u en ce of g r a i n s i z e on the t e n s i l e flow s t r e s s , h a r d n e s s , d u c t i l i t y and f r a c t u r e c h a r a c t e r i s t i c s of the s t e e l o v e r a wide r a n g e of t e m p e r a t u r e . A q u a n t i t a t i v e a n a l y s i s of the flow s t r e n g t h data h a s b e e n m a d e using the H a l l - P e t c h r e l a t i o n s h i p . 2 At t e m p e r a t u r e s above about 0.5 T m (where T m is the a b s o l u t e m e l t i n g t e m