200, 020, and 002 X-ray peaks in tempered Fe-18 Ni-C martensites
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P. G. WINCHELL
Separate 200, 020, and 002 X - r a y peaks were r e c o r d e d for 0.0, 0.4, and 0.8 wt pct c a r b o n (18 pct Ni) m a r t e n s i t e s after t e m p e r i n g b e t w e e n 25 and 500~ The c a r b o n b e a r i n g m a r t e n s i t e s studied h e r e have b e e n t e m p e r e d i n i t i a l l y enough to e l i m i n a t e the "high t e t r a g o n a l i t y " 002 peak u s u a l l y r e c o r d e d for a s - q u e n c h e d m a r t e n s i t e and the p r e s e n t r e s u l t s apply to t e m p e r e d m a r t e n s i t e only. The peak m a x i m u m is t a k e n to d e t e r m i n e the l a t t i c e p a r a m e t e r and the peak shape is r e c o r d e d . At all c a r b o n l e v e l s and a f t e r all t e m p e r i n g t r e a t m e n t s , the " c " p a r a m e t e r is l a r g e r than or equal to the " a " or " b " . The r e l a t i v e e n l a r g e m e n t is v e r y s m a l l (0.08 pct) for the lowest c a r b o n level and for any c a r b o n level after s e v e r e t e m p e r i n g (500~ for 15 min). F o r the two h i g h e r c a r b o n alloys t e m p e r e d at t e m p e r a t u r e s below 400~ (for 15 min) the " c " p a r a m e t e r is s i g n i f i c a n t l y l a r g e r than the " a " a n d " b " and for the 0.4 wt pct C alloy the " b " is s i g n i f i c a n t l y s m a l l e r than the " a " w h e r e as in the 0.8 pct C alloy the " b " is s l i g h t l y l a r g e r than the " a " . Within e x p e r i m e n t a l e r r o r the m e a n v o l u m e of the unit cell does not change d u r i n g the t e m p e r i n g studied h e r e and is n e a r l y unaffected by the i n i t i a l carbon content. This i n d i c a t e s that little (at m o s t 0.1 wt pct) carbon is dissolved in tempered martensite. In the low carbon alloy the peaks are symmetric and sharpen symmetricallyduring tempering. In the higher carbon alloys the peaks are nearly symmetric and sharp after severe tempering. After less severe tempering the 002 peak is asymmetricallybroadened toward lower 0 values (higher lattice parameters) whereas the 200 and 020 peaks are asymmetricallybroadened toward higher 0 values corresponding to lower lattice parameters. This collection of results is tentatively interpreted as being due to strains in martensite due to transformation induced substruct u r e and p r e c i p i t a t e d c a r b i d e s . A N i m p o r t a n t e x p e r i m e n t a l a s p e c t of the u n d e r s t a n d ing of t e m p e r i n g is p r o v i d e d by the X - r a y d i f f r a c t i o n s t u d i e s b y K u r d j u m o v and Lysak,1 Cohen and c o w o r k ers, 2'3 and Lysak and coworkers.4 By using a single austenite crystal transformed to a preferred orientation of martensite, these authors have used oscillation cameras to record separately the 200, 020, and 002 martensite spots. They have all observed that the 002 spot broadened and often split into two distinct spots during tempering at or slightly above room temperature. Changes in the 200 and 020 spots are less impressive and have attrac
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