I/EC
Equipment
& Design Newly techniques
developed
offer solutions
isostatic
pressing
to many problems
recent industrial metallurgy
demands
equipment
and
resulting
from
for large
ingots and fabricated
powder shapes
Compacting Refractory M e t a l Powders U,
"NTIL RECENTLY, industrial use of refractory metals—i.e., tungsten, molybdenum, tantalum—has been confined to applications where the size of parts was generally small. Inception of the nuclear and space era, however, has created an urgent need for these high melting metals and their alloys in massive form for structural applications. This new demand for high strength materials capable of long service life at elevated temperatures presents a series of formidable obstacles to manufacturers of refractory metals. Not the least of these problems concerns the consolidation of metal powders
into large shapes as the initial step in fabricating useful configurations. Formerly, when the requirements were restricted to small parts, powder compaction could readily be accomplished in metal dies with mechanical presses. This method accommodates the mass production of finished shapes within close dimensional tolerances and diminutive, rough billets for subsequent reduction into forms such as wire. However, mechanical pressing has severe limitations when the size requirements become even moderately large. In this respect the significant disadvantages of mechanical pressing are :
ISOSTATIC COMPRESSION SYSTEM
56 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
• Low and nonuniform compact density • Extremely large and expensive machinery for even reasonably high pressures • High cost of suitable metallic dies • Narrow limitations of length to diameter ratios In view of these factors, two alternate compressing methods are currently being considered for possible use in obtaining uniform massive powder consolidation. One of these—explosive forming—has been successful in the formation of massive sheet metal structures. However, it is a relatively new concept which still requires extensive development work for application to powder compaction. The second possibility is isostatic compaction, a process that has been known for many years. This method, also called hydrostatic pressing, involves the application of pressure through a liquid medium on powders contained in a nonporous membrane. Although isostatic pressing is not a new idea, it was not thoroughly exploited until this recent emphasis on massive refractory metal compacts. The principal advantage of isostatic compaction stems from the fact that the pressure is applied equally from all sides. The feature of three dimensional pressure results in high and uniform density without the very high loads required with mechanically or hydraulically operated presses. For example, molybdenum and tungsten ingots approaching 12 inches in diameter and several feet
by J. C. Lachman, Hoskins Manufacturing
Co.
long have been produced isostatically. This example highlights an important application of isostatic pressing other than for the classical powder metallurgy process—i.e., compaction, sintering, and working. Ingots of the size described are produced as electrodes for subsequent melting and casting by the consumable arc and electron beam processes. Although these latter practices are rapidly replacing powder metallurgy in refractory metal production, the starting raw materials are available only as powders that must be formed initially into a massive shape. Thus, regardless of whether manufacture is by pyro or powder metallurgy methods, isostatic compaction will continue to play an increasingly important role in the refractory metals industry. The value of isostatic pressing is not confined to the production of massive shapes, however. Research programs aimed at improved flexible molds have resulted in successful production of small, intricate shapes, including hollow cylinders, nozzles, and balance weights (2). Even in cases where dimensional tolerances are very rigid, the isostatic press can be used to advantage. This is accomplished by isostatically pressing mechanically formed powder compacts to improve their density or by a special metal die arrangement that can be subjected to hydrostatic pres-
This isostatic compression reactor has a 5 0 , 0 0 0 p.s.i. air pumping system
Consumable titanium electrode was isostatically compacted
"Sicon do es not discolor under intense heat..." Left, Model ESO-1 Majestic Incinerator with smart finish of Sicon BlueGray. Below, Model C finished in attractive Sicon Aluminum
Sicon finish is applied by spray on both models. Surface phosphatized by hand wiping.
—says Bob Cox, Sales Promotion Manager of the Majestic Company, Huntington, Indiana. "Our laboratory testing has shown us," states Mr. Cox, "that Sicon fills our requirements for excellent color retention under high temperatures."
The burning of certain materials can generate heat far in excess of the normal room temperature operation of Majestic Incinerators. That's why Sicon is used—easily applied by spray —because it retains its color up to 1000°F. in aluminum or in the 500°F. range in colors. Write for Sicon Literature—today! Dept. L-4.
Sicon
®
Silicone Hi-Heat
by
Finish
MIDLAND Industrial Finishes Co. Waukegan, Illinois
Circle No. 60 on Readers' Service Card
58 A
EQUIPMENT A N D DESIGN sure while sealed in a thin plastic container. Isostatic Pressing Systems
Isostatic pressing systems of various designs having large work load capacities and high pressure capabilities are now commercially available. One such system which features simplicity of design and operational procedure is illustrated by the flow diagram. The two principal components shown are the pressure vessel and pumping system. In operation the pressure vessel reactor chamber is filled with a liquid medium such as oil or water containing a rust inhibitor. After sealing and venting, pressure is accumulated by forcing additional fluid into the chamber from the liquid reservoir. This is accomplished by a hydraulic-type, air operated pump designed to operate through the air supply normally available in most industrial plants—i.e., 90 to 120 p.s.i. Intake air is passed through an oil filter to protect the pump and valve seals while a special high pressure check valve prevents reversal of the fluid flow. Automatic pressure control is provided by the indicating contact gage that is set for the desired pressure. When this level is attained, the solenoid valve is actuated, momentarily releasing a small amount of fluid into the liquid reservoir. The pressure can be held indefinitely at any level within the capacity of the system and pressure release can be controlled at any rate. An isostatic pressing system of the type described is being used at Hoskins Manufacturing Co. for compaction of refractory metal powders. The pressure vessel, 7 feet long and 18 inches in diameter, has a 6-inch diameter by 60-inch long work chamber, and is designed for 50,000-p.s.i. working pressure at room temperature. The compactness and simplicity of the 50,000-p.s.i. capacity pumping system and controls, shown mounted in a single panel, should be given special attention. This complete isostatic pressing facility requires only 20 square feet of floor space. The efficient and economical performance of this equipment is emphasized by the fact that maintenance was not required during a three month
INDUSTRIAL AND ENGINEERING CHEMISTRY
operational period involving at least 100 pressurizations. Good safety practice dictates a barricade around isostatic pressing facilities and many designs are available in the literature (5). In the Hoskins installation, the concrete pit-type barricade was selected for ease of loading and floor space economy. Isostatic pressure vessels with work chambers as large as 16 inches in diameter and 20 feet long capable of room temperature working pressure of approximately 50,000 p.s.i. are currently available. Vessels with proportionately smaller work chambers can likewise be designed for handling pressures up to 150,000 p.s.i. Isostatic compaction facilities can also be readily adapted to mass production-type operations when quantity output requirements justify larger initial expenditures. These systems are much more elaborate than the one described above and involve the use of electric motor driven pumps in combination with hydraulic boosters and air operated intensifiers for multistage pressurization. With these arrangements maximum pressures for even the largest vessels can be obtained in a very few minutes. Quantity production installations also require quick fill and drain provisions, rapid opening and sealing enclosures, and automatic controls.
Refractory M e t a l Compaction
With the ready availability of efficient isostatic systems, production of sound and dimensionally accurate compacts depends upon development of mold filling and pressing techniques. The approach to this problem will depend to a large extent on the particular materials involved, the desired finished shape, and the required dimensional accuracy. Although some of the information presented may apply to other materials, this report deals primarily with pressing of refractory metal powders into compacts for wire and strip fabrication or consumable electrodes for subsequent arc melting. Isostatic compaction of any material into accurately dimensioned intricate shapes is a relatively unexplored field, but at least one excellent reference exists (2).
EQUIPMENT AND DESIGN Selection of a suitable nonporous, flexible m o l d is the first concern in developing hydrostatic pressing tech niques. I n t h e past, r u b b e r — i n the form of tubes, balloons, bags, or specially shaped molds—has b e e n used. T h e chief disadvantage of r u b b e r containers is t h a t the desired sizes, shapes, a n d wall thicknesses are not readily available. L a r g e producers of d i p p e d or e x t r u d e d r u b b e r shapes are r e l u c t a n t to scale their facilities for short r u n p r o d u c t i o n of special items. A possible solution to this p r o b l e m rests with small j o b shops which are willing to devote special a t t e n t i o n to requests for r u b b e r shapes of n o n s t a n d a r d dimensions. A l t h o u g h this a r r a n g e m e n t t e n d s to m a k e t h e r u b b e r con tainers relatively expensive, the extra cost c a n usually be justified by the i m p o r t a n c e a n d value of the end p r o d u c t . O t h e r disadvantages of r u b b e r containers are nonuniformity of wall thickness, r a p i d r e t u r n a b l e stretch, a n d the t e n d e n c y to fold or crease d u r i n g compaction. These shortcomings are considered second a r y in t h a t they c a n be circum vented by holding r u b b e r m o l d p r o ducers to rigid specifications a n d by special mold filling practices. Thin plastic materials have emerged recently as a substitute for r u b b e r in hydrostatic c o m p a c t i o n molds. A d v a n t a g e s for plastic con tainers are cited as uniform thick ness, low cost, availability in a wide r a n g e of sizes a n d thicknesses, slow r e t u r n a b l e stretch, high stretch-tobreak factor, a n d the thermoplastic feature t h a t permits sealing to de sired shapes (
