Phase transformation of stainless steel during fatigue
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of the 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 a r e m e t a s t a b l e at r o o m t e m p e r a t u r e and u n d e r g o 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 s as a f o r m of p l a s t i c d e f o r m a t i o n . The f o r m a t i o n of m a r t e n s i t e s i n c r e a s e s the work h a r d e n ing r a t e , r e s u l t i n g in high u l t i m a t e s t r e s s l e v e l s with good u n i f o r m elongation. 1 F o r monotonic t e n s i l e t e s t s on s t a i n l e s s s t e e l s the s t r e s s - i n d u c e d 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 s have b e e n d e s c r i b e d on m a c r o s c o p i c (Refs. 1 t h r u 5) and m i c r o s c o p i c (Refs. 6 thru 9) l e v e l s . Only r e c e n t l y have i n v e s t i g a t o r s begin to e x p l o r e the fatigue of u n s t a b l e a u s t e n i t e , both in r e g a r d to c r a c k p r o p a g a t i o n r a t e s ~o- ~z and fatigue life. 13,14 Though t h e r e is s o m e c o n t r o v e r s y o v e r the s e q u e n c e of f o r m a t i o n , it is well e s t a b l i s h e d that an hcp (~) and a bee (or') m a r t e n s i t e m a y f o r m e i t h e r on cooling o r upon s t r e s s i n g . I I n g e n e r a l , a s the s o l u t e content o r the t e m p e r a t u r e i n c r e a s e s the a u s t e n i t e b e c o m e s m o r e s t a b l e . B r e e d i s and K a u d m a n Is have c a l c u l a t e d r e l a t i v e f r e e e n e r g i e s of the p h a s e s in the F e - C r - N i s y s t e m , and o t h e r s have d e t e r m i n e d e m p i r i c a l r e l a t i o n s for the M~6 and M~ t e m p e r a t u r e s . Not only the i n i t i a t i o n of the t r a n s f o r m a t i o n upon cooling o r u n d e r s t r e s s is i m p o r t a n t , but also the c o u r s e of t r a n s f o r m a t i o n d u r i n g s t r a i n i n g will affect the m e c h a n i c a l b e havior.l~ F u r t h e r m o r e , the l o c a l i z e d t r a n s f o r m a t i o n is i m p o r t a n t in d e t e r m i n i n g m e c h a n i c a l b e h a v i o r 13'18 and not m e r e l y the a v e r a g e a m o u n t at a given s t r e s s o r s t r a i n . N e v e r t h e l e s s , for p r a c t i c a l p u r p o s e s it is helpful to quantify r e l a t i o n s h i p s b e t w e e n m a c r o s c o p i c m e a s u r e m e n t s and fatigue b e h a v i o r . In low cycle fatigue it is f r e q u e n t l y o b s e r v e d that when the total s t r a i n a m p l i t u d e p e r cycle, AE, is held
DANIEL HENNESSY, formerly a Research Assistant, is now with Essex International, Ft. Wayne, IN. GARY STECKEL is presently a Research Assistant at the University of Illinois. CARL ALTSTETTER is a member of the MetallurgyDepartment and the Materials Research Laboratory of the University of Illinois at Urbana-Champaign,IL 61801. Manuscript submitted June 29, 1975, METALLURGICALTRANSACTIONSA
c o n s t a n t , the fatigue life can be e m p i r i c a l l y r e l a t e d , and e x p r e s s e d s i m p l y , in t e r m s of e l a s t i c s t r
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