Elevated-temperature strengthening of iron alloys by titanium
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s t u d i e s have b e e n made in this l a b o r a t o r y of the p r o p e r t i e s of s u b s t i t u t i o n a l solid s o l u t i o n s of i r o n . In m a n y c a s e s , 0.15 wt pct Ti* was added to the alloy, *Compositionsare expressedin wt pct throughoutthis paper. and the r e s u l t i n g s p e c i m e n s w e r e s p e c i a l l y h e a t - t r e a t e d to c o m b i n e all the i n t e r s t i t i a l i m p u r i t i e s (C, N, O) with the TI. 1 In this way, the influence of the m a j o r a l l o y i n g e l e m e n t on the l o w - t e m p e r a t u r e p r o p e r t i e s of i r o n could be d e t e r m i n e d without the c o m p l i c a t i o n s caused b y the p r e s e n c e of i n t e r s t i t i a l s o l u t e s . F a i l u r e to o b s e r v e s t r a i n aging, Snoek peaks, or r e s i s t i v i t y changes b e t w e e n 23 and 200~ indicated that the i n t e r s t i t i a l i m p u r i t i e s had indeed b e e n r e m o v e d f r o m solution. F u r t h e r m o r e , the p r o p e r t i e s of the a l loys at low t e m p e r a t u r e a p p e a r e d to r e f l e c t the p r e s ence of the s u b s t i t u t i o n a l solute alone. However, the T i - b e a r i n g a l l o y s exhibited a n o m a l o u s b e h a v i o r at e l e vated t e m p e r a t u r e s . F o r e x a m p l e , in the r a n g e 300 to 600~ T i - b e a r i n g a l l o y s of Fe with Ni, Mn, Si, or Pt exhibited a plateau in the f l o w - s t r e s s - t e m p e r a t u r e c u r v e , a peak in hot h a r d n e s s , and s e r r a t e d yielding. *-4 N e i t h e r d i s p e r s i o n h a r d e n i n g b y Ti(C, N) p a r t i c l e s n o r s o l i d - s o l u t i o n h a r d e n i n g b y Ti would produce h a r d ening peaks at e l e v a t e d t e m p e r a t u r e s . Such peaks r e s u l t f r o m diffusional p r o c e s s e s which b e c o m e o p e r a tive only at e l e v a t e d t e m p e r a t u r e s where t h e r e is s u f f i c i e n t a t o m i c mobility. P r e c i p i t a t i o n of t i t a n i u m c a r b i d e s or n l t r i d e s on d i s l o c a t i o n s is one such p r o c e s s which could lead to h i g h - t e m p e r a t u r e s e c o n d a r y h a r d e n i n g peaks. Another is the r e o r i e n t a t i o n of solute c l u s t e r s in the s t r e s s field of a mobile d i s l o c a t i o n . P r e c i p i t a t i o n effects would lead to t r a n s i e n t h a r d e n i n g which would d i s a p p e a r a f t e r long t i m e s b e c a u s e of the e x h a u s t i o n of solute. C l u s t e r r e o r i e n t a t i o n m a y be a s t a b l e s t r e n g t h e n i n g m e c h a n i s m which could be of use in developing c r e e p - r e s i s t i n g s t e e l s . T h e r e f o r e , this i n v e s t i g a t i o n was u n d e r t a k e n to e x a m i n e the c h a r a c t e r of this h i g h - t e m p e r a t u r e s t r e n g t h e n i n g , in p a r t i c u l a r its s t a b i l i t y . L. J. CUDDYand H. E, KNECHTELare Senior Scientist and Scientist, respectively, U. S. Steel Corp. Researc
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