Furnace atmosphere effects on casting of eutectic superalloys
- PDF / 1,233,791 Bytes
- 4 Pages / 594 x 774 pts Page_size
- 42 Downloads / 216 Views
N I C K E L base s u p e r a l l o y e u t e c t i c s r e i n f o r c e d with t a n t a l u m m o n o c a r b i d e w h i s k e r s (NiTaC alloys) a r e being c o n s i d e r e d as p o t e n t i a l gas t u r b i n e a i r f o i l m a t e r i a l s . 1 P r e c i s i o n c a s t i n g of NiTaC a l l o y s for such shapes a s a i r c r a f t t u r b i n e b l a d e s puts s e v e r e d e m a n d s on the mold used, due both to the c h e m i s t r y of the a l loys and the p r o c e s s i n g c o n d i t i o n s . The c o m p o s i t i o n s of some t y p i c a l NiTaC alloys a r e listed in T a b l e I. The c a r b o n l e v e l s of the N i T a C alloys a r e much higher than those in t y p i c a l s u p e r a l l o y s , and s e v e r e m e t a l - m o l d r e a c t i o n s can occur v i a r e d u c t i o n of the mold by the c a r b o n in the a l l o y with an a c c o m p a n y i n g c a r b o n boil in the m e t a l . A d d i t i o n a l s e v e r e conditions a r e i m posed on the mold by the p r o c e s s r e q u i r e m e n t s for N i T a C a l l o y s - t he c r i t i c a l t h e r m a l g r a d i e n t , growth r a t e quotient (G/R) for aligned growth is about 100~ h c m -2. Consequently, c a s t i n g r a t e s a r e low (0.6 to 2 c m / h ) , and m e t a l t e m p e r a t u r e s a r e high (1700~C) in o r d e r to achieve a high t h e r m a l g r a d i e n t . The c o m b i n e d effects of p r o c e s s and c h e m i s t r y a r e thus to c a u s e long t i m e , elevated t e m p e r a t u r e e x p o s u r e for both d e f o r m a t i o n of the mold and m e t a l - m o l d r e a c tion. An a l u m i n a b a s e d mold s y s t e m was c h o s e n for e v a l u a t i o n in c a s t i n g t r i a l s . The mold s y s t e m c o n s i s t e d of a l u m i n a p a r t i c l e s bonded by s i l i c a . Mold c o m p o s i t i o n s of A1203 plus 6 to 12 wt pct SiO2 were i n v e s t i g a t e d and on a p p r o p r i a t e f i r i n g yielded a l u m i n a bonded by m u l lite. Mold p e r f o r m a n c e d u r i n g c a s t i n g was found to be independent of the mold c o m p o s i t i o n s t r i e d . T h e s e r e s u l t i n g m o l d s were v e r y sag r e s i s t a n t and m i n i m i z e d c o n c e r n s with m e c h a n i c a l d e f o r m a t i o n of the mold d u r ing s o l i d i f i c a t i o n of N i T a C c a s t i n g s . D e t a i l s of the mold c o m p o s i t i o n , c o n s t r u c t i o n , phase r e l a t i o n s h i p s , and firing procedure are reported elsewhere. 4 I n i t i a l c o n t r o l l e d c a s t i n g t r i a l s in an a r g o n a t m o s p h e r e of N i T a C - 1 3 with this mold s y s t e m yielded poor r e s u l t s with r e s p e c t to m e t a l - m o l d r e a c t i o n . Most c a s t i n g s were s e v e r e l y d e c a r b u r i z e d , and m e t a l had p e n e t r a t e d the c e r a m i c mold. A study was thus begun, the s u b j e c t of this p a p e r , to e v a l u a t e the role of f u r n a c e a t m o s p h e r e on the quality of N i T a C alloys c
Data Loading...
