Mechanical behavior of superplastic ultrahigh carbon steels at elevated temperature
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WALSER
AND
OLEG
D. SHERBY
Ultrahigh carbon (UHC) s t e e l s have been investigated for t h e i r s t r e n g t h and ductility c h a r a c t e r i s t i c s from 600 to 850~ It has been shown that such UHC s t e e l s , in the carbon range 1.3 to 1.9 pct C, a r e s u p e r p l a s t i c when the m i c r o s t r u c t u r e consisted of fine equiaxed f e r r i t e or austenite g r a i n s (~1 ~zm) s t a b i l i z e d by fine s p h e r o i d i z e d cementite p a r t i c l e s . The flow s t r e s s - s t r a i n - r a t e r e l a t i o n s obtained at v a r i o u s t e m p e r a t u r e s can be quantitatively d e s c r i b e d by the additive contributions of grain boundary (superplastic) c r e e p and s l i p (lattice diffusion controlled) c r e e p . It is p r e d i c t e d that s u p e r p l a s t i c c h a r a c t e r i s t i c s should be o b s e r v e d at n o r m a l forming r a t e s for the UHC s t e e l s if the g r a i n size can be s t a b i l i z e d at 0.4 ~m. The UHC s t e e l s were found to be r e a d i l y r o l l e d or forged at high s t r a i n - r a t e s in the w a r m and hot range of t e m p e r a t u r e s even in the a s - c a s t , c o a r s e grained, condition.
R E C E N T L Y a c l a s s of ultrahigh carbon s t e e l s (1.3 to 1.9 pet C), combining s u p e r p l a s t i c c h a r a c t e r i s t i c s at elevated t e m p e r a t u r e with good r o o m t e m p e r a t u r e p r o p e r t i e s , was developed. T M The b a s i s of our ability to make plain carbon s t e e l s s u p e r p l a s t i c was the attainment of ultrafine s t r u c t u r e s with g r a i n s i z e s in the o r d e r of 0.5 to 5 ~m in s i z e and cementite p a r t i c l e s about 0.1 g m in s i z e . The high volume fraction of cementite p a r t i c l e s p r e s e n t (20 to 35 vol pct) maintains the fine g r a i n size at w a r m t e m p e r a t u r e . S e v e r a l t h e r m a l - m e c h a n i c a l p r o c e s s i n g methods were d e v e l oped to obtain these fine s t r u c t u r e s . l , 2 Not only should such hypereutectoid s t e e l s exhibit s u p e r p l a s t i c i t y below the A~ c r i t i c a l t e m p e r a t u r e , 723~ but they should a l s o be s u p e r p l a s t i c above the Ax t e m p e r a t u r e . This is possible because the anstenite g r a i n s obtained upon t r a n s f o r m a t i o n should also be fine g r a i n e d (since many nuclei exist f r o m the p r i o r fine g r a i n e d f e r r i t e ) . F u r t h e r m o r e , the fine austenite g r a i n s should r e m a i n fine b e c a u s e of the p r e s e n c e of the undissolved cementite p a r t i c l e s . This condition should yield a wide range of t e m p e r a t u r e where s u p e r p l a s t i c flow can be expected and F i g . 1 i l l u s t r a t e s the p r e d i c t e d influence of carbon on the expected range for s u p e r p l a s t i c flow. Thus, for a 1.9 pct C s t e e l , s u p e r p l a s t i c ity may be expected to be observed f r o m 600 to 850~ a d e s i r a b l e c h a r a c t e r i s t i c from
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