On the toughness increment associated with the austenite to martensite phase transformation in TRIP steels
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s t e e l s a r e m e t a s t a b l e a u s t e n i t i c m a t e r i a l s that t r a n s f o r m to m a r t e n s i t e on s t r a i n i n g . ~ It has b e e n pointed out that the phase t r a n s f o r m a t i o n i s e q u i v a l e n t to a mode of p l a s t i c d e f o r m a t i o n and a s such i s capable of a b s o r b i n g s t r a i n e n e r g y d u r i n g a f r a c t u r e t e s t that might o t h e r w i s e have gone to c r a c k e x t e n s i o n , 2-4 t h e r e b y r e s u l t i n g in p o t e n t i a l t o u g h n e s s i n c r e a s e s . P r e l i m i n a r y i n d i c a t i o n s show that TRIP s t e e l s do have a high t o u g h n e s s 3-s but a s yet no e x p e r i m e n t a l work h a s been c a r r i e d out to u n a m b i g u o u s l y a s s e s the cont r i b u t i o n that the t r a n s f o r m a t i o n m a k e s to the o v e r a l l t o u g h n e s s . It has b e e n noted z that the net t o u g h n e s s c o n t r i b u t i o n f r o m the t r a n s f o r m a t i o n can be e i t h e r p o s i t i v e o r n e g a t i v e . T h i s c a n be u n d e r s t o o d i n a g e n e r a l s e n s e by noting that m a r t e n s i t e is g e n e r a l l y m o r e b r i t t l e than a u s t e n i t e so that the e n e r g y a b s o r b e d when m a r t e n s i t e f o r m s at the c r a c k tip m u s t m o r e than offset the i n t r i n s i c b r i t t l e n e s s of the m a r t e n s i t e . The p u r p o s e of t h i s i n v e s t i g a t i o n i s to d e t e r m i n e both e x p e r i m e n t a l l y and t h e o r e t i c a l l y the cont r i b u t i o n that the a u s t e n i t e to m a r t e n s i t e phase t r a n s f o r m a t i o n m a k e s to the t o u g h n e s s of TRIP s t e e l s . Such a d e t e r m i n a t i o n i s n e c e s s a r y in e v a l u a t i n g the u t i l i t y of the T R I P p h e n o m e n o n a s a toughening mechanism.
STEPHEN D. ANTOLOVICHis Associate Professor, Department of Materials Science and MetallurgicalEngineering, University of Cincinnati, Cincinnati, Ohio. BIRINDAR SINGH, formerly Graduate Research Assistant, Department of Materials Science, University of Cincinnati, is now with the Hilton Davis Company, Cincinnati, Ohio. Manuscript submitted November 23, 1970. METALLURGICALTRANSACTIONS
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A steel whose nominal composition was 9 pct Cr, 8 pat Ni, 2 pct Mn, 0.6 pct C, and 80.4 pct F e w a s i n duction melted under an inert helium atmosphere. A f t e r m e l t i n g the ingots w e r e forged to b i l l e t s whose c r o s s s e c t i o n s w e r e 1 ~ by 1 ~ in. The b i l l e t s w e r e t h e n a u s t e n i t i z e d at 1200~ and w a t e r quenched. They w e r e then cut into s m a l l b l o c k s heated to 460~ in an a r g o n a t m o s p h e r e and r e d u c e d a p p r o x i m a t e l y 75 pct in height by p r e s s i n g in i n c r e m e n t s of 20 to 40 m i l s with r e h e a t i n g b e t w e e n each p r e s s i n g . All s a m p l e s w e r e p r o c e s s e d t o g e t h e r a n d c o n s e q u e n t l
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