The effect of homogenization treatment and hot isostatic pressing on microporosity in cast steel
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I C R O P O R O S I T Y is known to be an i m p o r t a n t m i c r o s t r u c t u r a l c h a r a c t e r i s t i c affecting m e c h a n i c a l p r o p e r t i e s of c a s t i n g s and of wrought m a t e r i a l p r o d u c e d f r o m cast ingots. 1-4 M i c r o p o r e s may be g e n e r a t e d e i t h e r by s h r i n k a g e o c c u r r i n g d u r i n g the t r a n s i t i o n f r o m liquid to solid or by e x s o l u t i o n of d i s s o l v e d g a s e s , or m o r e f r e q u e n t l y by a c o m b i n a t i o n of both. s'6 The a m o u n t of m i c r o p o r o s i t y can, t h e r e f o r e , be r e d u c e d by d e c r e a s i n g the gas content in the m e l t and by i n c r e a s i n g feeding through d i r e c t i o n a l s o l i d i f i c a t i o n . Very little i n f o r m a t i o n has b e e n p u b l i s h e d on the effect of high t e m p e r a t u r e h o m o g e n i z a t i o n on m i c r o p o r o s i t y in cast m e t a l s and in p a r t i c u l a r on the p o s s i b i l i t y of its e l i m i n a t i o n through heat t r e a t m e n t . However, the e l i m i n a t i o n of m i c r o p o r o s i t y d u r i n g s i n t e r i n g of m e t a l powder c o m p a c t s has b e e n the s u b j e c t of m a n y p u b l i c a tions. D u r i n g the i n i t i a l stage of s i n t e r i n g it is g e n e r ally accepted that neck growth o c c u r s through a bulk diffusion m e c h a n i s m 7'8 and that v a c a n c i e s diffuse f r o m the s u r f a c e of the p o r e s to g r a i n b o u n d a r i e s which act as v a c a n c y s i n k s . 9'1~ The effect of a t m o s p h e r e on the s i n t e r i n g b e h a v i o r of a l u m i n a powder c o m p a c t s was i n v e s t i g a t e d by Coble 11 who concluded that c o m p l e t e e l i m ination of p o r o s i t y was p o s s i b l e only if d i s c o n t i n u o u s g r a i n growth was c o n t r o l l e d and if the a m b i e n t a t m o s p h e r e was hydrogen, oxygen, or v a c u u m . No e l i m i n a tion of p o r o s i t y was o b s e r v e d when the a m b i e n t a t m o s p h e r e was h e l i u m , argon, n i t r o g e n , o r a i r . Pogrebnoi12'13 studied the evolution and e l i m i n a t i o n of p o r o s i t y d u r i n g h o m o g e n i z a t i o n of h y p e r e u t e c t i c steel, by w a t e r - q u e n c h i n g the s t e e l f r o m l l 0 0 ~ and holding the s p e c i m e n s i s o t h e r m a l l y at v a r i o u s t e m p e r a t u r e s above Acm. He c o n f i r m e d that e l i m i n a t i o n of M. BASARAN, R. MEHRABIAN, and M. C. FLEMINGS are Graduate Student, Assistant Professor, and ABEX Professor, respectively, Department of Metallurgyand Materials Science, M.I.T., Cambridge, Mass. 02139. T. Z. KATTAMISis Associate Professor, Department of Metallurgy, University of Connecticut, Storrs, Conn. 06268. Manuscript submitted December 1, 1972. METALLURGICALTRANSACTIONS
m i c r o p o r e s by v a c a n c y d i s c h a r g e at g r a i n b o u n d a r i e s and d i s l o c a t i o n s is quite s l u g g i s h at low t e
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