Deformation of pearlite
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INTRODUCTION E V E N though the i r o n c a r b i d e (cementite) in p e a r litic c a r b o n s t e e l s occupies only o n e - t e n t h to one-fifth of t h e i r v o l u m e , the d e f o r m a t i o n and s t r e n g t h of such s t e e l s a r e profoundly affected by its p r e s e n c e . The work h a r d e n i n g of d r a w n p e a r l i t i c s t e e l w i r e s is an exponential function of t r u e s t r a i n , ~ as c o n t r a s t e d to that of i r o n w i r e , which work h a r d e n s l i n e a r l y . 2-4 At a t r u e s t r a i n * of 4, the p e a r l i t i c wire is 4 to 5 *In thispaper, true strainis definedas e = ln(Ao/A), whereA andAoare the finaland originalcrosssectionalareas. t i m e s s t r o n g e r than f e r r i t i c w i r e . C e l l u l a r c o a l e s cence, the kind of d y n a m i c r e c o v e r y which c o n t r o l s the s u b s t r u c t u r a l r e f i n e m e n t of f e r r i t e in c a r b o n f r e e i r o n d u r i n g cold working, 2'5 a p p e a r s to be a l most completely inhibited in p e a r l i t e . 1 Also, effects of the s c a l e of the s u b s t r u c t u r e on s t r e n g t h a r e c o m p l e t e l y d i f f e r e n t . The s t r e n g t h of p e a r l i t e is p r o p o r t i o n a l to the r e c i p r o c a l of the s q u a r e root of the i n t e r l a m e l l a r spacing, ~ w h e r e a s the s t r e n g t h of f e r r i t e is p r o p o r t i o n a l to the r e c i p r o c a l of the l i n e a r - i n t e r c e p t cell size,* at l e a s t for d r a w n w i r e s , z *Thisis not the wholestory,however:see Langfordand Cohen.6
DEFORMATION AND STRENGTH OF CEMENTITE In o r d e r for p e a r l i t e to work h a r d e n as it does, l a m e l l a r c e m e n t i t e m u s t be ductile,* n o n d e f o r m i n g , *GilSevillanohas recentlyreviewedand presentedevidencefor the plasticity of pearliticcementite.42 h a r d p a r t i c l e s do not b r i n g about e x p o n e n t i a l s t r a i n
h a r d e n i n g in c a r b o n s t e e l s 7 or other m a t e r i a l s . 3 In spite of this a p r i o r i evidence, I have s e a r c h e d the l i t e r a t u r e for evidence of the p l a s t i c d e f o r m a t i o n of c e m e n t i t e . Fig. 1 shows that t h e r e is an effect of t h i c k n e s s of the c e m e n t i t e p l a t e s on t h e i r d e f o r m a b i l i t y . However, the n a t u r e of this e v i d e n c e so f a r p r e c l u d e s any q u a n t i t a t i v e d e s c r i p t i o n of the m a g n i tude of the s i z e effect. Some w o r k e r s have shown obvious b r i t t l e f a i l u r e of c o a r s e c e m e n t i t e plates by o p t i c a l and e l e c t r o n m i c r o s c o p y , s-u Other w o r k e r s p r o d u c e d d i s l o c a t i o n s in c o a r s e c e m e n t i t e by p l a s t i c d e f o r m a t i o n of the s t e e l . 12'13 A t h i r d group p r e s e n t e d i n d i r e c t evidence for p l a s t i c i t y of c e m e n t i t e , such as t h i n n i n g of p l a t e s or r e d u c t i o n of i n t e r l a m e l l a r s p a c ings by
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