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T H E t e m p e r a t u r e for the s t a r t of the a u s t e n i t e to m a r t e n s i t e t r a n s f o r m a t i o n in s t e e l s , M s , has b e e n l i n k e d to the s h e a r s t r e n g t h of the p r i o r a u s t e n i t e phase by s e v e r a l i n v e s t i g a t o r s . I - ~ Schiell p r o p o s e d that p l a s t i c d e f o r m a t i o n of a u s t e n i t e above the M d t e m p e r a t u r e a c t s to m e c h a n i c a l l y s t a b i l i z e the a u s t e n i t e , thus lowering the M s t e m p e r a t u r e . T h i s has s u b s e q u e n t l y b e e n c o n f i r m e d by Kelly and Nutting. 2 P h i l i b e r t 3 and C r u s s a r d 4 have both s u g g e s t e d m o d e l s to account for the t r a n s f o r m a t i o n which a r e b a s e d upon the r e s i s t a n c e of the a u s t e n i t e m a t r i x to the d e f o r m a t i o n a c c o m p a n y i n g the f o r m a t i o n of m a r t e n s i t e . P h i l i b e r t d e s c r i b e d the r e s i s t a n c e to d e f o r m a t i o n in t e r m s of s e g r e g a t i o n in the a u s t e n i t e . W o e h r l e , Clough, and A n s e l l 5 i n v e s t i g a t e d this a r g u m e n t u t i l i z ing r a p i d quench r a t e s p r i o r to t r a n s f o r m a t i o n i n o r d e r to r e d u c e the extent of c a r b o n s e g r e g a t i o n in the a u s t e n i t e d u r i n g q u e n c h i n g . H e r e the M s t e m p e r a t u r e i n c r e a s e d with quench s e v e r i t y tending to c o n f i r m the p o s t u l a t e . L e s l i e and Sober 6 have r e c e n t l y shown that at i n t e r m e d i a t e c a r b o n c o n t e n t s , r e a r r a n g e m e n t of c a r b o n a t o m s d u r i n g and a f t e r q u e n c h i n g i s the m a j o r c o n t r i b u t o r to the s t r e n g t h of m a r t e n s i t e . Gaggero and Hull 7 have shown that the a u s t e n i t e - m a r t e n s i t e t r a n s f o r m a t i o n in thin foils of F e - 3 0 . 2 Ni alloys o c c u r s 100~ above the M s m e a s u r e d for bulk s p e c i m e n s . They explained this b e h a v i o r a s r e s u l t i n g f r o m the lower d e f o r m a t i o n c o n s t r a i n t in thin foils as c o m p a r e d to bulk s p e c i m e n s . S i m i l a r o b s e r v a t i o n s have b e e n made by Hull 8 and W a r l i m o n t . 9 Yeo 1~has shown that a l though s m a l l additions of f e r r i t e s t a b i l i z e r s t e n d to r a i s e the M s due to t h e i r effect on the r e l a t i v e c h e m ical free e n e r g i e s of the p h a s e s , l a r g e r additions of EDWARD M. BREINAN is Senior Research Scientist, United Aircraft Research Laboratories, East Hartford, Conn. GEORGE S. ANSELL is R. W. Hunt Professor and Chairman, Department of Matedais Engineering, RensselaerPolytechnic Institute, Troy, N. Y. Manuscript submitted May 14, 1969. METALLURGICALTRANSACTIONS

the s a m e e l e m e n t s d e p r e s s the M s. Yeo s u g g e s t e d that the d e p r e s s i o n of the M s r e s u l t e d f r o m solid solution s t r e n g t h e n i n g of the a u s t e n