Observations of martensite formation and fracture in TRIP steels
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pct C r , 8.0 pct Ni, 4.0 p c t Mo, 1.0 pct Mn, pct Si, 0.26 pct C, Bal F e pct C r , 8.0 pct Ni, 2.0 pct Mn, 0.6 pct C, Fe
A f t e r m e l t i n g the s t e e l s w e r e f o r g e d and a u s t e n i t i z e d at 1200~ and f i n a l l y r e d u c e d v a r i o u s a m o u n t s by p r e s s ing at 460~ T h i s p r o c e d u r e is s u b s t a n t i a l l y the s a m e as r e p o r t e d e l s e w h e r e . ~ F r a c t u r e t o u g h n e s s s p e c i m e n s ~-in. thick and of a design s u g g e s t e d by W e s s e l n w e r e u s e d in this i n v e s t i g a t i o n . Fig. 1 is an i l l u s t r a t i o n of the s a m p l e on which the p r e s s i n g d i r e c t i o n and the o r i e n t a t i o n s of the v i e w s of F i g s . 2, 3, 4, and 5 a r e indicated. T o i n v e s t i g a t e the " p l a s t i c z o n e " s h a p e , s a m p l e s of 1 9 s t e e l I, r e d u c e d 45 pct at 460~ w e r e u s e d . A ~-m. fatigue c r a c k was placed in them in a c c o r d a n c e with the ASTM r e c o m m e n d e d p r a c t i c e . ~ T h e y w e r e loaded to two d i f f e r e n t l e v e l s without c o m p l e t e l y f r a c t u r i n g : 1) until a d i s t i n c t d e v i a t i o n f r o m l i n e a r i t y in the loadd i s p l a c e m e n t r e c o r d was o b s e r v e d , and 2) to one half of the l e v e l at which the d e v i a t i o n f r o m l i n e a r i t y oc-
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Observations of Martensite Formation and Fracture in TRIP Steels S T E P H E N D. ANTOLOVICH AND BTRINDAR SINGH
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s t e e l s an applied s t r e s s p r o m o t e s m a r t e n s i t e f o r m a t i o n in an a u s t e n i t i c s t e e l and t h e r e b y g i v e s r i s e to both high s t r e n g t h and good d u c t i l i t y . ~ S u g g e s t i o n s have been m a d e that under a p p r o p r i a t e c o n d i t i o n s the f o r m a t i o n of m a r t e n s i t e during s t r a i n i n g should l e a d to i m p r o v e d f r a c t u r e toughness since the t r a n s f o r m a t i o n to m a r t e n s i t e r e d u c e s the s t r a i n e n e r g y that would o t h e r w i s e be a v a i l a b l e for c r a c k e x t e n s i o n . 2'~ T h e o r e t i c a l c a l c u l a t i o n s have been made to d e t e r m i n e the e f f i c i e n c y of the t r a n s f o r m a t i o n in d i s s i p a t i n g e n e r g y with the r e s u l t that the t r a n s f o r m a t i o n is a p p a r e n t l y m o r e e f f e c t i v e than any other p r o c e s s ~ ( i . e . , 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 and m a r t e n s i t e ) . In t h e s e c a l c u l a t i o n s , the p l a s t i c zone ( a s s u m e d to be i d e n t i f i a b l e with the m a r t e n s i t i c a r e a ahead of the c r a c k tip) ,'k~h a s s u m e d to have the shape of an e l l i p s e with the s e m i m a j o r a x i s twice the length of the s e m i m i n o r a x i s . F u r t h e r m o r e , it has b e e n s t a t e d that the e x i s t e n c e of t r i a x i a l i
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