The oxidation behavior of some Ni-Cr-Al alloys at high temperatures

  • PDF / 1,813,063 Bytes
  • 9 Pages / 612 x 792 pts (letter) Page_size
  • 71 Downloads / 193 Views

DOWNLOAD

REPORT


T H E oxidation b e h a v i o r of N i - b a s e s u p e r a l l o y s is g e n e r a l l y quite c o m p l e x due to the p r e s e n c e of m a n y alloying e l e m e n t s . The v a r i o u s c o m p o n e n t s f o r m oxides of v a r y i n g s t a b i l i t y and c o m p l e x i t y . However, the r e a c t i o n s both at the s u r f a c e and in the s u b s c a l e v a r y not only with the c o m p o s i t i o n of the alloy, but also with the r e a c t i o n t i m e and t e m p e r a t u r e , oxidizing e n v i r o n m e n t , s u r f a c e condition, and p r e t r e a t m e n t . In o r d e r to get a b e t t e r u n d e r s t a n d i n g of these effects on the oxidation b e h a v i o r of s u p e r a l l o y s it is i m p o r tant to s t a r t with s i m p l i f i e d a l l o y s . In n i c k e l - b a s e s u p e r a l l o y s , c h r o m i u m and a l u m i n u m a r e two of the m o s t i m p o r t a n t alloying e l e m e n t s , s u g g e s t i n g that the study s t a r t s with these additions. The p u r p o s e of this i n v e s t i g a t i o n was to a d j u s t the c o n c e n t r a t i o n l e v e l of c h r o m i u m and a l u m i n u m in N i - C r - A 1 alloys in o r d e r to obtain o p t i m u m oxidation r e s i s t a n c e . A l u m i n u m additions to high t e m p e r a t u r e N i - C r a l loys g e n e r a l l y r e s u l t in i m p r o v e d oxidation r e s i s t a n c e through f o r m a t i o n of an a-A1203 l a y e r or a s c a l e c o n taining both a-A1203 and Cr203. R e p o r t e d s t u d i e s on the effect of a l u m i n u m additions to N i - C r in the open l i t e r a t u r e ~-~ have been l i m i t e d and m o s t l y c o n c e n t r a t e d on alloys w h e r e the c h r o m i u m c o n c e n t r a t i o n v a r i e s between 15 and 30 wt pct. The p r e v i o u s s t u d i e s have b e e n confined to a t m o s p h e r i c p r e s s u r e and m a i n l y a g r e e that an i n c r e a s e d oxidation r e s i s t a n c e is obtained when the e x t e r n a l s c a l e m a i n l y c o n s i s t s of A1203. G u l b r a n s e n and Andrew 2 r e p o r t that addition of 4 pet A1 to N i - 2 0 C r * *The indicated compositions represent weight percent.

alloy does not yield any a p p r e c i a b l e i m p r o v e m e n t in the oxidation b e h a v i o r . T h i s does not a g r e e with s u b s e q u e n t s t u d i e s . 1'3-5 W a s i e l e w s k i 3 studied c o m m e r c i a l n i c k e l - b a s e s u p e r a l l o y s c o n t a i n i n g d i f f e r e n t Cr-A1 r a t i o s and concluded that alloys c o n t a i n i n g 6 to 9 pet C r and 4 to 9 pet A1 exhibit an o p t i m u m oxidation resistance. Scott, Wood, and Hobby 4 have made a c o m p a r i s o n of the s c a l i n g b e h a v i o r of F e - C r - A 1 , N i - C r - A 1 , and INGARD A. KVERNES and PER KOFSTAD are Senior Scientist, Divisionof Metallurgy,and Head-Department of Materials Research, respectively, Central Institute for Industrial Research, Oslo, Norway. Manuscript submitted May 24, 1971. METALLURGICAL TRANSAC