Effect of repeated allotropic transformation on sintering of iron powder
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Effect of Repeated AIIotropic Transformation on Sintering of Iron Powder SHIRO KOHARA L i t t l e has been r e p o r t e d on the effect of r e p e a t e d a l l o t r o p i c t r a n s f o r m a t i o n on s i n t e r i n g of i r o n powder. Hausner 1 r e p o r t e d that cyclic s i n t e r i n g of e l e c t r o lytic iron powder through the ~ - 7 t r a n s f o r m a t i o n t e m p e r a t u r e r e s u l t e d in c o n s i d e r a b l y i n c r e a s e d d i a m e t e r shrinkage, but not in t h i c k n e s s shrinkage, so that volume shrinkage was s i m i l a r to that of i s o t h e r m a l s i n t e r i n g . Cizeron and L a c o m b e ~ studied the effect of t h e r m a l cycling a c r o s s the ~ - 7 t r a n s f o r m a t i o n t e m p e r a t u r e on s i n t e r i n g r a t e of carbonyl i r o n powder and found that r e p e a t e d t h e r m a l cycling r e s u l t e d in an a c c e l e r a t e d s i n t e r i n g r a t e . Powell 3 investigated c y c l i c s i n t e r i n g of iron, cobalt, and titanium powders and concluded that t h e r m a l cycling through the t r a n s f o r m a tion t e m p e r a t u r e brought about no i m p r o v e m e n t in p r o p e r t i e s over i s o t h e r m a l s i n t e r i n g . Thus, e x p e r i m e n t a l r e s u l t s r e p o r t e d in the l i t e r a t u r e a r e incons i s t e n t . It was noticed in the c o u r s e of the p r e s e n t work that when the e x t e r n a l load was applied d u r i n g c y c l i c s i n t e r i n g the effect of t h e r m a l cycling b e c a m e m o r e m a r k e d and r e p r o d u c i b l e ; f u r t h e r m o r e , when adequate load was applied the a c c e l e r a t i n g effect on s i n t e r i n g r a t e was r e m a r k a b l e only in cyclic s i n t e r ing, but negligible in i s o t h e r m a l s i n t e r i n g . T h e r e f o r e , e x p e r i m e n t s were c a r r i e d out to investigate the effect of r e p e a t e d a l l o t r o p i c t r a n s f o r m a t i o n on s i n t e r i n g of i r o n powder under s m a l l c o m p r e s s i v e load. The m a t e r i a l used in the p r e s e n t study was r e d u c e d iron powder with an a v e r a g e p a r t i c l e s i z e of 29 ~ m . The i m p u r i t y levels of the powder w e r e 0.005 pct C, 0.007 pct P, 0.006 pct S, and 0.06 pct Si. Specimens of 5 mm in diam and 12 mm in height w e r e compacted at a p r e s s u r e of 196 MN//m ~ and s i n t e r e d in an a t m o s p h e r e of mixed gas of hydrogen and argon (1 : 1). L i n e a r shrinkage of a s p e c i m e n during s i n t e r i n g was r e c o r d e d by means of a v e r t i c a l type d i l a t o m e t e r . T e m p e r a t u r e was changed c y c l i c a l l y from 25~ above to 25~ below the o~-7 t r a n s f o r m a t i o n t e m p e r a t u r e of iron (910~ * including an i s o t h e r m a l p e r i o d
composed of a heating and a cooling p e r i o d without the i s o t h e r m a l p e r i o d . Cyclic s i n t e r i n g s were c a r r i e d out with v a r i o u s lengths of cycle, keeping the width of t
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