AMES IIABORATORY RESEARCH AND DEVELOPMENT REPORT

U. S. A. E. 0.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

Metals, Ceramics and Materials (UC-25) TID 4500, December 15, 1960 .,

UNITED STATES ATOMIC ENERGY COMMISSION . . I _

. I , . . . . . Reeearch and Development Report

ARC MELTING IN THE TUNGSTEN ..

ELECTRODE FURNACE.

D. E. Williams and H: L. Levingston

November, 196 1

Ames Laboratory at

Iowa State University of Sc ience and Technology F . H. Spedding, Director Contract W-7405 eng-82

This repor t is distributed according to category Metals, Ceramics and Materials (UC-25) a s l isted in TID-4500, Decemher 15, 1960.

Legal Notice

This repor t was prepared a s an account of Government sponsored work. Neither the United States , nor the Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty of representation, express o r implied, with respect to the accuracy, completeness, o r usefulnesa of the information contained iii this report, or, that the uoe of any in.iormation, apparatus, method, o r process disclosed in this report may not infringe privately owned rights; o r

B. Assumes any liabilities with respect to the use of,, o r for damages resulting f rom the use of any information, apparatus, method, o r process disclosed in this report.

As used in the above, "person acting on behalf of the Commissiontt includes any employee o r contractor of the Commission, o r employee of such contractor , to the extent that such employee o r contractor of the Commission, o r employee of such contractor p repares , disseminates, o r provides access to, any information pursuant to his employment o r contract with the Commission, o r his employment with such contractor.

Pr in ted in USA. Pr ice $ . 50 . Available from the

Office of Technical Services U. S. Department of Commerce Washington 25, D. C.

IS-262

CONTENTS

Page

ABSTRACT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .'. . . . . . . 5

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . '. . . . . . . . . . . . 5

HISTORICAL REVIEW.. . . . ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 . .

OPERATION O F THE ~ R C FURNACE . :. . . . . . . . . . . . . . . . . . . . . 15 . "

GENERAL COMMENTS . . . . . . . . . . . . .' . . . . . . . . . . . . . . . . . . . . . . . 16

REFERENCES ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

ARC MEETING IN THE TUNGSTEN ELECTRODE FURNACE

D. E. Will iams and H. L. Levingston

ABSTRACT

An a r c furnace is descr ibed which employs a non-

consumable tungsten e lectrode and a water-cooled

copper hear th . It has been used success.ful1y for mel t ing

r e f r ac to ry meta le and al loys . The furnace is equipped

with a vacuum sys t em, a n i ne r t ga s supply, and a n 800-

a m p e r e direct-current power supply.

INTRODUCTION

The development of nuclear r e a c t o r s , m i s s i l e s and high speed

a i r c r a f t hasprompted a n i n t e r e s t i n the high-melting me ta l s . Un-

fortunately, these m e t a l s p r e sen t difficulties i n mel t ing and fabr i -

cation. Because of the i r affinity for oxygen, they m u s t be me l t ed

in a vacuum o r under a n i ne r t a tmosphere . Most o.f the ord inary

r e f r ac to ry c ruc ib les a r e a t tacked by these m e t a l s in the molten

s ta te , and some a r e mechanically weak a t the t empera tu re s in-

volved. Many of these me ta l s and . their a l loys can be p r e p a r e d by

6 1

the methods of powder metal lurgy, but the products made by this

p r o c e s s do not fi l l a l l needs. As a consequence, r e sea rch

meta l lurg is t s have made much use in recent yea' is o f t h e kind o f ..

. furnace descr ibed in this repor t . . . .

HISTORICAL REVIEW

The idea of an a r c furnace i s not new. ]In 1905, von Bolton (1)

designed and built an a rc- furnace with a water-cooled hearth for

melting tantalum. Little m o r e attention was given to the possibility.

of using this type of furnace until 1939 when Kroll designed equip-

ment for melting titanium. Kroll modified von, Bolton's design by

substituting thoriated tungsten for the tantalum electrode tip. During,

and especially a f te r World War 11, an increasing number of investi-

ga to r s found use .for such .furnaces until they have become relatively

common laboratory equipment. They have been used .for melting

titanium (2 , 3, 4, 5, 6 ) , zirconium (7), molybdenum (5), chromium (8),

tungsten (9) and qther meta ls . . . . .

The .furnace descr ibed in this report is shown in Fig. 1.

I t s essent ia l p a r t s a r e the melting chamber, the movable electrode,

the vacuum pumping sys tem with i t s accessory gauges and valves, . .

and the power unit. A line .from a tank of helium enters the pumping

sys tem near the chamber. Water supply l ines run to the individually

jacketed top, bottom and s ides of the melting chamber and to the

electrode shaft.

Fig. 1 - A Tungsten Arc Furnace for Laboratory Melting.

The melting chamber is shown in Figs. 2, 3, 4. The apparatus

depicted in these figures differs from the equipment shown in the photo-

graph in Fig. 1 by having a ball and sliding shaft arrangement instead of

a metal bellows at the entrance of the movable electrode into the chamber.

Either arrangement gives satisfactory results. The bellows i s a simple

device but occasionally develops leaks where it i s soldered to other parts

and may crack a t a fold. The ball and sliding shaft a re more complicated

to construct but require less maintenance and a re easier to manipulate

during melting.

Figure 2 shows the cylinder, o r barrel of the melting chamber, and i ts

attachments. The cylinder is made of copper tubing about 8 in. inside

diameter and has flanges for the attachment of the top and bottom plates

ehown in Figs. 3 and 4, respectively. The pumping system may be isolated

from the melting chamber by the vacuum valve when helium ie to be admitted.

The hy,draulic jack is used to hold the lower plate (Fig. 4) against the lower

f lang~. of the barrel.

The top plate is shown in Fig. 3. It is attached to the barrel by 6 bolts

through holes near the rim. A rubber gasket makes the joint vacuum tight,

An important component of the top section i s the movable electrode that

passes through the center of theplate. By means of the sliding shaft and

ball combinatian, the operator can move the tungsten tip of the electrode

to any desired point in the melting chamber. Much of the heat that enters

the electrode tip ie removed through the water-cooled shaft.

Figure 4 shows the bottom plate which carries the hearth or crucible

and is also the negative electrode. ' It is insulated from the barrel by a

OBSERVAT PORT

Fig. 2 - The Barre l of the Melting Chamber.

10

ELECTRODE COOLANT. INLET

OF ELECTRODE

STAINLESS STEEI ... SHEATH AROUND MAlN TUBE OF ELECTRODE

KNURLED NUT

MAlN TUBE OF ELECTRODE

- SPLIT COLLET

TUNGSTEN ELECTRODE TIP

Fig. 3 . - The Top of the Melting Chamber.

I-, CAVITY

Fig. 4 - The Bottom of the ~ e l t i n g Chamber.

12

1/2-in. - thick bakelite r ing which occupies a position between the lower

flange of the b a r r e l (F ig . 2) and the bear ing sur face of the bottom plate.

The bottom plate and the r ing assembly a r e held f i rmly against the flange

on the b a r r e l by the hydraulic jack shown in Fig. 2.

The crucible cons is t s of a 1/2-in. -thick plate of copper in which th ree

cavi t ies , each about 1/4-in. deep by 1 3/4-in. in d iameter , 'have been

formed. A space between the lower sur face of the hear th plate and the

ma in plate s e r v e s a s a water jacket t o cool the hear th plate. Water l ines

a r e a t tached by Snaptite connectors. E l ec t r i c power is connected to the

plate by a plug and jack a r rangement not shown.

An al ternat ive c ruc ib le assembly was constructed in which the hear th

plate can be easi ly removed f rom i t s water jacket. The plate r e s t s on top

of the jacket which i s r a i s e d and lowered by a pneumatic piston instead of

the hydraul ic ' jack. A water- t ight s ea l i s made by an O-r ing between the

hear th plate and the top flange of the water jacket. This a r r angemen t

p e r m i t s the rapid exchange of hear th plates of different cavity design and

.facilitates cleaning and maintenance.

Additional detai ls of the construction o.f the sl iding shaft and ball

mechanism a r e shown in Fig. 5. The polished s ta in less s t ee l s leeve

surrounding the e lectrode s l ides through the polished s ta in less steel.

bal l and i s guided by the th ree O-r ings . These O-r ings a r e held in '

posit ion by the b r a s s cyl indr ical s p a c e r s that , in combination with the

knur led nut, s e r v e to c o m p r e s s the r ings slightly and force them

to b e a r .firmly ,on both the shaft and the in te r ior of the ball s o that

the combination i s vacuum tight. Rings and shaft a r e lubr icated

with si l icone vacuum g r e a s e . The ball r e s t s in a contoured cavity

Fig. 5 - Details of the Bal l and Movable Shaft Mechanism.

in the boss and is held firmly against the O-ring b$ a bakelite

'pressure ring. The fop plate is, cut back a s shown so that the

shaft can be turned to the side a s far a s desired.

For evacuation of the melting chamber prior to the introduction

of helium o r argon, both mechanical and diffusion pumps a r e used.

The pressure in the evacuated chamber is measured by a National

Research Corporation ionization gauge.

Power i s supplied from a Westinghouse 800 ampere (maximum)

DC welder. The positive terminal of the welder i s connected to

t'he movable electrode. A Westinghbuse high frequency stabilizer

.is used to initiate the a rc . With the stabilizer turned on, a spark

I ' discharge passes continuously between the electrode and the hearth.

Power from the welder is switched off and on by a foot switch and

regulated by a remote control unit operated from a switch on the

panel.

OPERATIONOFTHEARC FURNACE

The furnace is constructed,so that several charges can be l

melted without opening the chamber. The f irst of these to be

melted i s a getter charge of zirconium o r other suitable material

for the absorption of, contaminants introduced with the inert gas o r

relene.ed .from the.charnber walls by the heat of the a rc . In

ordinary operati& the getter charge provides sufficient puri-

fication of the inert gas.

Introduction of the inert gas usually is performed in'the

followin'g manner,

After cha rges have been placed on the hear th pla te , the furnace

- 5 is c losed and evacuated to a p r e s s u r e of 10 m m Hg o r l e s s . The

valve to the vacuum line i s then c losed and helium admit ted t o the

des i red p r e s s u r e , In some ins tances , th is p r e s s u r e i s re la t ively

l.ow but usually mel t ing i s done a t helium p r e s s u r e s of 1 to 1. 3 a tm.

Although the a r c produces considerable mixing of the molten

meta l , the bottom one- th i rd to one- half of the charge usually

remains solid because of the cooling action of the water-cooled

hear th plate; accordingly, completely homogeneous alloys a r e

difficult to p r epa re . The hornogeniety i s improved by remel t ing

and inverting the alloy s eve ra l t imes . The inverting is done by

manipulating the sa.mples with the e lectrode t ip while the power

is turned off but usually with the s tab i l i ze r s t i l l operating. The

spa rk discharge furn i shes sufficient light fo r t he ,ope ra to r to s e e

the in te r io r of the furnace.

The crucible i s cleaned a f te r melt ing each s e t o.f ingots with

a 1: 1 solution o.f n i t r i c acid and water . The ac id solution i s

removed with water a n d the crucible r i n sed with acetone.

A

GENERAL COMMENTS

contaminat ion o.f the ingot f rom the copper hear th o r the

tungsten tip o rd inar i ly i s negligible. U ~ l e s s the ingot actually

s t i cks to the hearth , no copper i s likely to be introduced. Con-

tamination with tungsten may occur i.f the e lectrode t ip has ac tua l

contact with the molten alloy.

9

16

The mechanical pump should be operated continuously to

prevent moisture from collecting on the interior of the .melting

chamber. If the apparatus has been allowed to remain open for

more than a short time, several getter mel ts should be ma.de in ,,

succession.

REFERENCES

P. Werne r von Bolton: "Das Tantal , se ine Dars te l lung und seine

Eigenschaf ten." Z. E lek t rochem. (1905) - 11, p. 45-51.

2. W. Krol l : "The P rod~ ic t i on o.f Ductile Titanium. I ' T r a n s .

E lec t rochem. Sac. (1940) - 78, p. 35-47.

3. Otto Winkler: "Schm'elzen von Metal len ohne Tiegelreakt ion. "

Z. Metallkunde (1953) 44, p. 333-341. - 4. R. S. Dean, J. R. Long, F. S. War tman and E . T. Hayes:

"Ductile Titanium - I t s Fab.rication and Phys ica l P r o p e r t i e s . '' T r a n s . Am,. Inst . Mining, Met. E n g r s . (1946) - 166, p . 382-398.

5. Rober t M. P a r k e and John L. Ham: "The Melting of Molybdenum

in the Vacuum Arc . " T r a n s . Am. Inst . Mining, Met. Eng r s .

(1947) - 171, p. 416-427. ,

6. P. J. Maddex and L. W. Eastwood: "Continuous Method of

Producing Ductile Titanium. I ' J. Metals (1950) - 2, p. 634-640.

7. W. J. Kroll; A. W. Schlechten and L. A. Yerkes : "Ductile

Zirconium f r o m Zircon Sand. " T r a n s . E lec t rochem. Soc. (1946)

89, p. 263-274. - 8. H. L. Gi lber t , H. A. Johansen and R. G. Nelson: "Forging of

Arc-Mel ted Chromium. " T r a n s . Am. Inst . Mining, Met. E n g r s .

(1953) 197, p. 63-65. - 9. H. B. Goodwin and C. T. Greenidge: "Forgeable Arc-Mel ted

Tungsten. " Metal P r o g r e s s (1951) - 59, p. 812-814.