A high-temperature vacuum induction furnace

VACUUM Classified Abstracts

III

O

--

V a c u u m Processing Techniques Contd. ,

--

III

Melting and Sintering of Metals In Vacuo United Kingdom. T h e a b s e n c e of air in m e t a l sintering, p u r i f y i n g , h e a t t r e a t m e n t or c a s t i n g processes r e s u l t s in a p r o d u c t w i t h lower g a s c o n t e n t , g r e a t e r a c t i v i t y a n d t o u g h n e s s , h i g h e r d e n s i t y , f r e e d o m f r o m blow holes and i m p r o v e d electric a n d t h e r m a l c o n d u c t i v i t y values. Gas c a n dissolve in a m e t a l either b y f o r m i n g a simple s o l u t i o n or b y f o r m i n g a c o m p o u n d w i t h t h e m e t a l . I n t h e first case t h e a m o u n t of d i s s o l v e d g a s is d e t e r m i n e d b y Q = Kp½eT, w h e r e K is a c o n s t a n t , T t h e t e m p e r a t u r e a n d p t h e g a s p r e s s u r e over t h e m e t a l . T y p i c a l for t h e second case is o x y g e n c o n t a m i n a t i o n g i v i n g t h e m e t a l oxide. T h e dissociation p r e s s u r e p of t h e oxides a n d t h e dissociation t e m p e r a t u r e T are related b y log p = A - B / T , w h e r e A a n d ]3 are c o n s t a n t s . F o r iron oxide, for i n s t a n c e , t h e dissociation p r e s s u r e is 1 0 ~ m m . H g a t 1,5500C. A t e m p e r a t u r e of 2,500°C is required to raise t h i s p r e s s u r e to 1 m m . as w o u l d be r e q u i r e d in a v a c u u m f u r n a c e to p r o v i d e for a n effective r e a c t i o n t o w a r d s lower o x y g e n c o n t e n t . I n s i n t e r i n g one h a s to deal w i t h t h r e e t y p e s of gases w h i c h m a y be p r e s e n t in t h e pores of a sinter : (1) gases w h i c h dissolve in t h e metal, (2) g a s e s w h i c h diffuse t h r o u g h t h e m e t a l a n d (3) gases w h i c h n e i t h e r dissolve n o r diffuse t h r o u g h t h e m e t a l . All t h r e e c o m b i n e in slowing d o w n t h e o r d i n a r y s i n t e r i n g process. B y s i n t e r i n g in vacuo t h e effects of all t h r e e t y p e s of gases c a n be r e d u c e d b y a factor of a b o u t l04. Details are given of a v a c u u m f u r n a c e w h i c h is h i g h - f r e q u e n c y h e a t e d a n d designed to h a n d l e m e l t s of 50 to 100 lbs. of a n o n - m a g n e t i c steel alloy. I t is a d v i s a b l e n o t only to m e l t t h e m a t e r i a l in v a c u u m b u t also to c a s t t h e i n g o t in v a c u u m . V a r i o u s d e s i g n s facilitating b o t t o m p o u r i n g or tilt p o u r i n g in v a c u u m are described a n d s h o w n in illustrations. Sommaire : Th6orie et p r a t i q u e de la fusion s o u s vide, la fonderie et le frittage de m d t a u x , s o n t donndes, suivis de la description ddtaill~e d ' u n four ~ vide.

A High-Temperature V a c u u m Induction Furnace United States. Details are g i v e n of a f u r n a c e w h i c h h a s been developed specially for t h e p u r p o s e of m e a s u r i n g t h e v i s c o s i t y or o t h e r p h y s i c a l p r o p e r t i e s of c e r a m i c m a t e r i a l s a t h i g h t e m p e r a t u r e . A n i m p o r t a n t r e q u i r e m e n t was t h e a t t a i n m e n t of t e m p e r a t u r e s b e y o n d 2,000°C a n d t h e m a i n t e n a n c e of t h e s a m e for a period long e n o u g h to facilitate p r o p e r r e a d i n g s a n d o b s e r v a t i o n s . T h e f u r n a c e c o n s i s t s of a water-cooled b r a s s shell 5 inches in d i a m e t e r , ~ inch t h i c k a n d 18 inches long. T h e cooling t u b e is soldered to t h e shell in t h e f o r m of a widely spaced helix. T h e t o p a n d b o t t o m p l a t e s of t h e f u r n a c e are r e m o v a b l e a n d sealed w i t h silicone r u b b e r gaskets. T h e h e a t is supplied b y a h i g h - f r e q u e n c y g e n e r a t o r w i t h a n o u t p u t of 10 k w a t frequencies f r o m 200 to 600 kc. T h e i n d u c t o r coil is led into t h e f u r n a c e t h r o u g h c o m p r e s s i o n r u b b e r fittings. T h e s u s c e p t o r t u b e - - t h e a c t u a l h e a t i n g e l e m e n t - - i s m a d e f r o m m o l y b d e n u m b u t as t h i s m a t e r i a l t e n d s to e v a p o r a t e a t v e r y h i g h t e m p e r a t u r e s t a n t a l u m is preferable. T h e s u s c e p t o r t u b e is fitted inside a fused silica cylinder w h i c h in t u r n fits closely inside t h e i n d u c t o r coil. T u n g s t e n - m o l y b d e n u m t h e r m o c o u p l e s a r e u s e d for t e m p e r a t u r e m e a s u r e m e n t s b u t h a v e n o t p r o v e d fully s a t i s f a c t o r y . T h e o p e r a t i o n a l p r e s s u r e is 1 p H g or b e t t e r a n d is p r o d u c e d b y a n oil diffusion p u m p coupled w i t h a m e c h a n i c a l fore p u m p . T h e r e q u i r e d v a c u u m c a n be o b t a i n e d w i t h i n 30 m i n u t e s . Sommaire: D~tails d ' u n m o u f f l e c o n s t r u i t s p ~ c i a l e m e n t d a n s le b u t de m e s u r e r la viscositd ou d ' a u t r e s propri6t~s p h y s i q u e s de m a t ~ r i a u x de c ~ r a m i q u e a u x t e m p e r a t u r e s ~lev~es.

0

An Improved V a c u u m Fusion Furnace United States. T h e presence of gases in c o m b i n a t i o n or m solutions, e v e n in m i n u t e q u a n t i t i e s , m a y g r e a t l y affect t h e p h y s i c a l properties of m e t a l s . F o r i n s t a n c e , a c o n s i g n m e n t of nickel to be u s e d for v a c u u m v a l v e c a t h o d e s w a s f o u n d to c o n t a i n o x y g e n of t h e order of 0.008 to 0.02%. T h i s a m o u n t c a n seriously i m p a i r electron emission. To d e t e r m i n e t h e a m o u n t a n d k i n d of gaseg p r e s e n t in a m e t a l t h e v a c u u m fusion f u r n a c e m e t h o d is u s u a l l y e m p l o y e d . A s p e c i m e n of t h e m e t a l is m e l t e d in a g r a p h i t e crucible b y m e a n s of a h i g h f r e q u e n c y i n d u c t i o n cell. Oxides are r e d u c e d b y c o m b i n i n g o x y g e n w i t h c a r b o n a n d t h e r e s u l t i n g CO t o g e t h e r w i t h o t h e r liberated g a s e s is p u m p e d off for analysis. T h e s u c c e s s of t h i s process v e r y largely d e p e n d s o n t h e design of t h e furnace. W h e n o n l y s m a l l s a m p l e s of t h e order of 1 g r a m or less are available, t h i s m e t h o d o f t e n lacks precision a n d reliability. A v a c u u m f u r n a c e is described here w h i c h gives good r e s u l t s f r o m s m a l l samples. It e s s e n t i a l l y consists of a q u a r t z tube, 2 inches in d i a m e t e r a n d 6 inches long, s u s p e n d e d from glass h o o k s b y m e a n s of p l a t i n u m wires inside a P y r e x glass e n v e l o p e of 3 i n c h e s d i a m e t e r . T h e q u a r t z t u b e c o n t a i n s a g r a p h i t e crucible r e s t i n g on g r a p h i t e p o w d e r loosely p a c k e d to r e t a i n t h e h e a t g e n e r a t e d in t h e crucible a n d also to p e r m i t t h e free flow of gases to be p u m p e d out. T h e crucible is ~ i n c h in d i a m e t e r a n d 3 i n c h e s long. I t is p r o v i d e d w i t h a g r a p h i t e f u n n e l w h i c h is split l e n g t h w i s e to a v o i d e d d y - c u r r e n t s . A n o p e n i n g a t t h e b o t t o m of t h e o u t e r glass e n v e l o p e facilitates t h e i n s e r t i o n a n d r e m o v a l of t h e q u a r t z t u b e a n d associated c o m p o n e n t s . S a m p l e s of t h e m e t a l to be s t u d i e d are a d m i t t e d to t h e f u r n a c e t h r o u g h a b r a n c h t u b e c o n n e c t e d to a g r o u p of sealed tubes, w h e r e u p to 12 s a m p l e s c a n be stored. F e r r o u s s a m p l e s a r e m o v e d f r o m t h e s t o r a g e t u b e s to t h e f u r n a c e b y m a n i p u l a t i n g a s m a l l m a g n e t o u t s i d e a n d n o n - f e r r o u s s a m p l e s are stored in little c u p s m a d e f r o m m a g n e t i c m a t e r i a l w h i c h c a n be s u i t a b l y s h i f t e d a n d u n l o a d e d in t h e s a m e way. T h i s a r r a n g e m e n t m a k e s it possible to s t u d y several s a m p l e s w i t h o u t b r e a k i n g t h e v a c u u m seal. T h e t e m p e r a t u r e is m e a s u r e d b y a n optical p y r o m e t e r . E v a c u a t i o n is carried o u t b y a 2 - s t a g e m e r c u r y diffusion p u m p coupled w i t h a m e c h a n i c a l fore p u m p . T h e f u r n a c e h a s b e e n u s e d for d e t e r m i n a t i o n s o n iron, m o l y b d e n u m , copper, g e r m a n i u m , m i c r o p h o n e c a r b o n a n d nickel, d u r i n g t h e last t w o years. S a m p l e s as s m a l l as 0.1 g r a m h a v e b e e n e m p l o y e d . Sommaire: O n ddcrit en ddtail u n n o u v e a u four de fusion sous vide, p e r m e t t a n t l'emploi de tr6s p e t i t s dchantillons, j u s q u ' h u n m i n i m u m de 0.I g r a m m e

January, 1952

vacuum Vol. I I No. 1

Abstracg No. and References

4~/III

Article by E. D. Malcolm J. Set. Instrum. Suppl. 1, 1951 63-66

46/III

Article by H. T. Smyth R. H. Meinken & L. G. Wisnyi J. Amer. Cerapniv 34, May 1951 161-163

47/III

Article by W. G. Guldner BdL Lab. Rev. 29, Jan. 1951 18-20

93