E. S. H 0 D G E
C.
B. B 0 Y E R
F. D. 0 R C U T T
GAS PRESSURE BONDING Metal parts for a variety o f industrial applications can be fabricated by this new technique.
High pressure helium at
high temperatures is used in specially modzjed autoclaves
Gas-pressure bonding developed by Battelle utilizes gas pressure at elevated temperatures for the direct denstjication, cladding, and joining of components. Components to be bonded are fabricated to final size, cleaned, and then assembled i n a disposable metallic container. T h e thin-walled container is evacuated, sealed, and placed inside a high pressure-gas autoclave and subjected to high external gas pressure at an elevated temperature. Gas pressure at the elevated temperature is transmitted through the container with suficient force to deform the components plastically and bring them into intimate contact f o r solid-state bonding, or to deform powder-metallurgy products suficiently to cause denszjication. Feasibility of thzs process has been demonstrated in preparing control, fuel, and moderator elements and assemblies for nuclear applications.
ildvantages of Gas-Pressure-Bonding -Complex
+
shapes with a minimum of development
-Lower
fabrication costs
-Close
dimensional control
-Joinzng
of dissimilar mateiials
-Simultaneous denstjication and cladding of sucli materials as uranium dioxide -Adafitability to many different types of cladding systems
T h e process has been scaled-up for jroduction of large-scale components. Although this process was initially employed to produce nuclear components, it has now been demonstrated that fhe versatility of this process will receive wide application for producing many dzferent structural members for industry.
general discussion of the pressure-bonding equip-
A ment is necessary to understand several of the unique
featcres that have been incorporated in the use of conventional high pressure autoclaves for this application. During the development of this bonding process, there has been a continuous developrAentof the types of autoclaves and operations procedures employed. Initial pressure-bonding study was coiiducted in hotwall autoclaves consisting of an externally heated Type 304 stainless steel high pressure tube. Gas pressure was supplied from helium cylinders at approximately 2000 p.s.i., and these tubes were utilized at temperatures up to 842" C. For larger specimens, autoclaves of Type 410 stainless steel, 1 inch in inside diameter, were obtained for bonding of zirconium-base systems at 842' C. and 2000 p.s.i. Specimens were held at temperature and pressure for 24 to 36 hours to achieve complete bonding. It was obvious that high pressure autoclaves which could be operated at this temperature would be required to reduce the time required for bonding materials with better high-strength properties. A coldwall, high pressure autoclave was modified to allow use of higher pressures and temperatures as well as to accommodate larger specimens. The cold-wall autoclave is generally a larger, more complex vessel, with its own internal heater, but a hotwall autoclave is simpler to design, and is smaller in size. Although reflecting a greater initial investment, the cold-wall vessel provides the basic tool for high-temperature bonding studies at high pressures with an indefinite life. AUTHORS E. s. Hodge, Assistant Chief, C.B. Boyer, Project Leader, and F. D . Orcutt, Senior Laboratory Technician are members of the fuel-element development division of Battelle Memorial Institute. T h e authors have written numerous articles and reports on various application of the gas-pressurebonding process. VOL.
54
NO. 1
JANUARY
1962
31
All nutorlaues are owtically centered in-iadiuidual cylindrical pits, feet in dinmekr and 72 to 74f e d deep. Imtallatiorvpermit m i m u r
safety with ease of operation Vessel Material
Inconel 410 stainless 4340 steel 4340 steel
Liner Material
316 stainless 410 stainless
O.D., In.
6 14 14 26
I.D., In.
Inside Length, In.
Working Pressure, In.
2 9 9 14
21 48 48 72
15,000 10,000 10,000
Cooling
The body of cold-wall vessels may be cooled by a variety of methods. Helical fluid coolant channels embodied in an inner cooling liner or external cooling jackets or coils have been successfully employed for similar applications. A coolant other than water is required to inhibit corrosion of vessels fabricated from Type 4340 steels. A single solution of 10% watersoluble oil has been recirculated through two such vessels, using a small pumping system, and a heat exchanger to the fluid from:60°,to.30" C. for approximately 350 ding cycles over a 2-year period without noticeable g; o r , deposits at the filter point. At present, ulating pressures for the fluid are 200 to 400 p.s.i. All autoclaves are vertically centered in individual cylindrical pits, 8 feet in diameter and 12 to 14 feet deep. Installation permits maximum safety with ease of operation. . .
10,000
Specimen, In.
2 2 3 6
by 10 by8 by15 by40
Max. Temp., 0
c.
815 1570 1200 1150
Heating Element
Molybdenum Kanthal Hoskins 835
Bridgman seal fabricated is used. A full Bridgman closure is used for the large closures. The former requires only a few minutes of effort while the latter may require 2 to 4 hours. Both of the rapid-closure techniques have proved satisfactory through hundreds of cycles to 10,000 p.s.i. The O-ring closure has a lower temperature limitation; consequently, the autoclaves are not operated with head temperatures in excess of 80" C. The stainless steel sealing rings employed in the modified Bridgman closures will tend to warp if temperature is permitted to rise above 205' C. while subjected to repeated pressurizing cycles. Normal operating temperatures in the metal seal-ring area range from 38" to 150" C., using the same seal ring for approximately 250 cycles to 10,000 p.s.i. To control head ' , temperatures as described and achieve temperatures and ; ' pressures in the vessels as well as holding at temperature , and pressure for times of up to 8 hours, it is necessary that the internal heater be carefully designed. 1
,
Closures
Three types of closures have been utilized for these high pressure autoclaves. For those made on a daily basis, either a metal-to-metal with O-ring or a modified
Internal Heaters for High Gar-Pressure Autoclaves
The high-pressure helium or other inert gas acts as an excellent heat transfer medium and excessive heating of
Time. hr.
Represtntotioe )resm'ng and heating cycle for a bonding run i n one of the nutoclams. ", re-bonding cyclu are performed by bringing pressure a e up simultoncourly to operational 32
INDUSTRIAL A N D ENGINEERING CHEMISTRY
mnditiom. I t ispossibb, howavn, to apply fullpressure before bringing the specimen to temperature, or to hiat the spccirm to tempeiature before applyingprcssure. Present equipmmt iscapable of either operational cycle
c
? '
.he wall or autoclave heads c a m occur unless proper design precautions are observed. Resistance heating with Chrome1 A, Kanthal, Hoskins 835 and 875 alloys, and molybdenum-wound heating elements has been used for various temperature ranges. Original efforts to design a heater which would afford a maximum hot zone included trials with simple helical elements, wound on grooved cylindrical refractory tubes. This type of heater, when operated at gas pressures of 10,000 p.s.i., produced an unstable hot zone of a very short length in the upper portion of the heater due to gas convection. To reduce this effect, heaters are now loaded into closely fitting metallic jackets closed on the bottom. Due to the increased density of the highly compressed gas, one may think of the heater as being immersed rather than placed within the convective medium. Jacketing of the heater serves as a baffle and interrupts the logical “flue” or “chimney” effect associated with a long vertically operated heater. Liners fabricated from stainless steel, Inconel, and molybdenum and fitting the inner diameter of the heater are used to further minimize convection-current patterns. Hevi-Duty Electric Co.’s semicylindrical high-purity refractory shells have been wound with helically coiled Hoskins Type 835 resistance wire for use to 1200’ C. All molybdenum heaters are externally wound on grooved refractory tubes. Mortar is not used in the hot-zone area of the heaters. This was greatly minimized furnace losses due to the differences in thermal expansion between the alumina refractory tube and the molybdenhm wire. Generally, heating rates to temperature have been made nearly equivalent for all types of heater windings. Both single-zone and three-zone furnace variations have been used. After the specimen is positioned within a heater, thermocouples are in place, and the mass has been built up, silica sand is placed in the remaining void space. The silica reduces free gas flow and minimizes the volume of gas required to pressurize the system. It is easily removed with a..cOm is reusable to temperatu can also be used to temperatures of 1430’ C. With thm heatem and loading techniques, a typical 48-inch long heater of three-zone design has been operated at 815” C. and 10,000 p.s.i. with a 41-inch hot zone of -12,,Zo C. The same heater has been operated at 1150” C . with a 24-inch hot zone. I t is essFtial.to-prevent excessive heat transfer from. the internal he is exercised in the n of an insulating .material which would block exion currents in the high pressure gas. Low pprous materials, brick, have proved ineffective. Johns Manville pun silica batting (Microquartz) was determined as a ,sacisfactory insulating material. To provide a good t h w a l barrier to 1400’ C., Microquartz is finely shredded, then tightly packed between heater and vessel wall to a density equivalent to three or four times that of the as-received ~
VOI. 5 4
NO. 1
J A N U A R Y lp6;?.
Full-leqth section of n typical threezom wiring diagram with the separate zones of Hoskinr hdicnlly coiled wire
''fcotlurd'' fogethn to efimimtnnte hot and cold spot,. Illwtrotad is a one-fourfh diamcin srclion. Zoncs; T = top; M = middlt; B = botlom
bulk material. Repeated removal of the insulation is accomplished with a n industrial vacuum sweeper which breaks down the fibrous structure even further into an easily handled and re-usable texture. Fiberfax is a less expensive glass-fiber material and can be used in place of the Microquartz as a satisfactory thermal barrier to temperatures up to 1093" C.
along the specimen; therefore, it is necessary to know the exact temperature of the specimen along its entire length. Chromel-Alurnel thermocouples have been used to 1290' C. However, three or four cycles of 4 hours each appear to be the maximum expected life. An increase 'in life by a factor of 10 can be expected for : couples operated below 1260' C. Wasaremenc c+temperaturcsaf -t3900 TO ISIOYT..' is accomplished with physically shielded platinum' platinum-lO'% rhodium couples. The wire is threaded through high-purity alumina ceramic and the resultant couple is shielded with molybdenum tubing which ' prevents reaction of the couple with insulating and filler material. During normal operation, a series of thermocouples is attached along the specimen to be bonded for tempera+ture measurement and control. Based on detailed measurements made on specimens, furnace windings, and on insulation surrounding the furnace, it is not possible to determine a precise relationship betwern the beater-element temperature and the assembly, particularly when the mass of the load is varied from cycle to cycle. Use of the high pressure helium provides a path of high thermal conductivity between heater and specimens; consequently, the furnace windings . normally do not operate at temperatures excessively higher than these of the specimens.
i
Applications and Products Fabricated Pressurization of Autoclaves
High gas pressures required by the gas pressure bonding process are achieved at Battelle with two multistage piston-type compressors. The largest compressor presently in use is a five-stage Andres Hofer 15,000-p.s.i. unit that operates with an inlet pressure of approximately 1 p.s.i.g. Available compressors have been operated in parallel and individually to increase volumetric output of gas. Helium gas was used for nearly all development studies conducted. Argon bas also been used. Helium is reclaimed after completion of each run through a step-down regulator into a series of cylinders. T o maintain a contamination-free atmosphere, a flow of argon is piped into the autoclaves while open. Upon closing, the autoclaves q e evacuated for a short period and purged with helium to a pressure of 300 p.s.i. before pressurizing. Gas lost in this purging operation, in addition to that lost during the reclaiming cycle, is less than 15% per complete cycle. Once the system is pressurized, the compressors are turned o f f . There are no high pressure leaks in the system, consequently, no drop in pressure is observed during holding at temperature and pressure for 4- to 6-hour periods.
Assemblies can be prepared by thr one-step gas pressurebonding technique at a much lower cost than { by conventional fabrication operations. The bonds , possess excellent mechanical and physical properties and since the components are fabricated to final size and inspected before bonding, excellent dimensional control is achieved. Zircaloy-clad flat-plate uranium dioxide fuel elements containing compartmentalized uraniiun dioxidk fuel i have been prepared by the process. Uranium dioxide i: contained in compartments formed by Zircaloy webs bonded to Zircaloy cladding. Piece components are . used to form the webs and edges of the compartmented : picture frame. These components can be either contained or self edgewelded. During the pressure-bonding process, at which the conditions of 843O C., and ' 10,000 p.s.i. for 4 hours appeared most satisfactory, . solid-state bonds were formed. The application of the technique for fabrication and use of pieced components can significantly lower fabrication costs. Self-bonding of niobium bas been achieved with pa- . rameters of 1150' to 1260' C. at 10,000 p s i . for 3hours . with surfices which had been prepared by etching in a nitric-hydrofluoric acid solution prior to bonding. Niobium-clad flat-plate and rod-type fuel elements and flat-plate assemblies were produced by this process. Niobium tubing was also fabricated by this technique. r Uranium dioxide clad with Type 304 stainless steel bas been fabricated into rod, tubular, and flat-plate shapes by the process. Modifications of these basic designs ~
I
~
Thermocouple Instrumentation
Accurate measurement of temperature inside high pressure autoclaves during bonding is extremely important. Metal systems being bonded may be sensitive to loss in properties as a result of temperature gradients 34
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Zirconium-clod zirconium-uranium @at assemblies have bcen successfully fabricated in a om-rtep operation. The 169 comfments to be bonded (left) nrz fobricatcd tofrnol size and assembled in tha rontoiner,
have included compartmented rods and compartmented plates. Self-Bonding Studies
~
' 1'
I
Some other material systems in which solid-state bonding has been achieved by the gas pressure-bonding technique include self-bonding of molybdenum, zirconium, aluminum, and beryllium; cladding of uranium and uranium alloys with zirconium and Zircaloy; cladding of delta-phase zirconium hydride with Types 304 and 347 stainless steel; and cladding of a dispersion of 40 volume per cent boron in titanium with titanium.
: Compaction by Gas. Pressure Bonding It has been possible to achieve densification of uranium dioxide while cladding it with Type 304 stainless steel in a single bonding operation. With this approach densification of the UOZis achieved at temperatures consider~', ably lower than that ordinarily required for sinterhg, primarily because of the high gas pressure acting in the . system. Depending on the characteristics of the initial '. oxide, final densities ranging up to 99.5% of theoretical can be realized under conditions of 3 hours at 1149' .. to 1204" C. and 10,000 p.s.i. Cermets containing 80 volume per cent UOZdispersed - in either chromium, molybdenum, niobium, or stainless steel and exhibiting densities in excess of 90% of theo' retical were successfully prepared by gas pressure-bonding .' the mixed oxide and metal powders. The technique specimens with uniform structure and was .., produced capable of simultaneously densifying and cladding - green-pressed cermet cores. ~' Similar compaction results have also been obtained during the pressure bonding of cold-pressed and tamppacked powders of zirconium hydride, titanium-boron dispersions, magnesium oxide, beryllium, beryllium oxide, Type 304-L stainless steel and alumina.
I
I!:
Db' '
~
i 7. ,
euncuated, and realcd (center). During pressur8 bonding, plastic flow brings thc components into intimate contact and bonding and cladding occur simultamously toform an integral assembly (right)
Safely Precautions Practice
All cold-wall autoclaves purchased by Battelle and designed for 10,000-p.s.i. working pressure were hydrostatically pressure-tested at 18,750 p.s.i. and room temperature. Stainless steel safety disks with a '/*inch diameter cross section and a 15,500-psi. bursting pressure rating at 22.2' C. are used. Depth micrometers are used to determine disk growth. After initial installation and after the disk seats itself undw pressure during the first pressurizing cycle, the readings are recorded initially every five or six cycles and later every 20 cycles. Special check readings are taken just before, and immediately after, any unusual higher tempera'ure runs are made. Provisions have been made to measure the vessel body diameters and record their growth. Measurements are taken to supply information on diameter change as a function of ambient temperature, and internal pressure plus vessel wall temperature to 150' C. Immediate personnel safety is primarily accomplished by isolation. Rather than using surface barricades, each autoclave was positioned below ground level in an 8-foot diameter pit of suitable depth. Free expansion of gases due to a major pressure failure is afforded in al! but one direction. All procedures required to complete pressurizing and heating cycles are performed in the control rooms. All pressure-controlling operations are manual. All tubing, valves, fittings, and accessories used are of Autoclave Engineers' 30,000-p.s.i. working-pressure series. Cross section of the majority of all high-pressure piping is a/s inch in outside diameter by '/% inch in inside diameter. I t is not unusual to move 35 to 40 feet of helium gas per minute for periods exceeding 1 hour through the diameter tubing throughout the entire pressure range without experiencing any serious temperature rise. The entire high pressure sc designed to require more than one valve t before a major pressure operation could be started. VOL 5 4
NO. 1
JANUARY 1962
35
