US3427011A

US3427011A - High pressure furnace

Info

Publication number: US3427011A
Application number: US681857A
Authority: US
Inventors: Charles B Boyer; Franklin D Orcutt
Original assignee: Battelle Development Corp
Current assignee: ASEA PRESSURE SYSTEMS Inc; Autoclave Engineers Inc; Huntington National Bank; Mitsubishi International Corp
Priority date: 1967-11-09
Filing date: 1967-11-09
Publication date: 1969-02-11
Anticipated expiration: 1986-02-11

Description

Feb. l1', 1969 C. B. Boven ET AL 3,427,011

HIGH PRESSURE FURNACE File'd Nov. 9, 1967 Sheet of '2 Tini :o7 2' I NVEN TORS CHARLES B. BOYER & FRANKLIN D. ORCUTT FI G l GRAY, MAsE, a. DuNsoN ATTORNEYS Feb. l1, 1969 y c. B. BOYER ETAL 3,427,011

HIGH PRESSURE FURNACE Filed Nov. 9,V 1967 y Sheet 2 of 2 CHARLES B. BOYER & FRANKLIN D. ORCUTT F I 2 BY GRAY MAsE aDUNsoN ATTORNEYS United States Patent O 3,427,011 HIGH PRESSURE FURNACE Charles B. Boyer and Franklin D. Orcutt, Columbus,

Ohio, assignors to The Battelle Development Corporation, Columbus, Ohio, a `corporation of Delaware Filed Nov. 9, 1967, Ser. No. 681,857 U.S. Cl. 263--40 14 Claims Int. Cl. F27d 7l/00; F27b 1/22; H05b 1/00 ABSTRACT F THE DISCLOSURE A high-pressure furnace comprising a pressure vessel having at least two coaxial cylindrical shells positioned therein, the shells being combined with various sealing means to provide 1) a closed cylindrical inner work chamber surrounded by a closed annular chamber having a height substantially greater than its width and (2) a closed cylindrical insulating chamber having a diameter substantially greater than its height at one end of the inner chamber. Heating elements are arranged adjacent the innermost shell to provide at least two independent heating zones within the inner chamber and means are provided for restricting the flow of pressure medium between the various chambers while equalizing the pressure therein.

Background of the invention This invention relates generally to high-pressure furnaces and more particularly to furnaces capable of maintaining within the work receiving chamber pressures suiiicient to effect solid-state bonding or compaction of materials.

The furnace of this invention is particularly adapted for use in hot-gas-pressure bonding and hot-isostatic cornpaction of materials. When used for these purposes the furnaces are commonly termed autoclaves Briefiy, the gas-pressure bonding process is a process which utilizes gas pressure at elevated temperatures for joining metallic or ceramic components. The parts to be joined are shaped to final size, cleaned, and assembled into a flexible, pressure-tight metallic container. The component is then placed in a high-pressure furnace such as the furnace of the present invention and subjected to a high external inert gas pressure at a prescribed elevated temperature. The isostatic gas pressure is uniformly transmitted through the flexible container and forces all of the mating surfaces of the component into intimate contact. The mating surfaces are held under pressure at an elevated temperature for a suicient time to cause solid-state bonding. The only deformation which occurs during the joining process is the amount required to bring the parts into intimate contact. Therefore, the dimensional tolerances achieved in the component are essentially comparable to those of the individual parts. The joints achieved by this process have the same physical, mechanical, and chemical properties as the parent materials and in most cases the original joint interface cannot be detected by microscopic examination or non-destructive techniques.

The hot-isostatic compaction process is used to cornpact metallic, cermet, or ceramic powders or preforms into complex, fully dense structures. Loose powders, vibratory compacted powders, explosively impacted powders, or cold isostatic pressed preforms are enclosed in a flexible, metallic, thin-walled envelope, which is evacuated and sealed prior to being placed in a high-pressure autoclave where it is subjected to an isostatic gas pressure at elevated temperatures.

The feasibility of bonding and densifying various materials with the one-step hot-isostatic bonding and compaction processes has been successfully demonstrated. How- ICC ever, the limitations of available high-pressure furnaces have precluded general acceptance of these processes as production processes. Therefore, concurrent with the development of the gas-pressure bonding and compaction processes, there has been continuous development of the furnaces, or autoclaves, employed. Initial developments of the bonding process were conducted in hot-wall autoclaves in which the heat was supplied from an external source to the specimens through the pressure vessel wall. Limitations of size and temperature-pressure capabilities of the hot-wall unit led to the installation of a resistance heating element within the pressure vessel. These internally heated furnaces could be cooled by inner cooling liners or external cooling jackets to allow extended cycle times and were generally referred to as cold-wall autoclaves. Improvements in the preferred cold-wall type autoclaves are discussed in the following articles: E. S. Hodge, C. B. Boyer, and F. D. Orcutt, Gas Pressure Bonding, Industrial and Engineering Chemistry, vol. 54, January 1962, pp. 30-35; and C. B. Boyer, F. D. Orcutt, and R. M. Conaway, High Temperature High Pressure Auto` claves, Chemical Engineering Progress, May 1966, pp. 99-106.

For the lower temperature ranges, up to 2900 F., and internal work chamber diameters up to 9 inches, conventional autoclaves will perform satisfactorily at pressures up to about 10,000 p.s.i. Demands for higher temperatures, higher pressures, greater work chamber capacities and greater versatility have led to numerous studies to better the understanding of conditions which exist within high pressure autoclaves. It was recognized from these studies that problems of size and those resulting from higher pressures could be minimized through the use of additional heating zones and various baffles to restrict convective flow of the pressure medium. Despite these findings, the largest autoclave that it has heretofore been possible to construct was limited to an inner Work chamber 131/2 inches in diameter and 60 inches in length. This autoclave utilized a pressure vessel having an internal cylindrical cavity 27 inches in diameter and 108 inches in length. Two concentric cylindrical shells were positioned coaxially within the pressure vessel cavity with the innermost shell defining the cylindrical inner work chamber. Resistance heating elements were helically wound on the exterior surface of the innermost shell to provide five independent, longitudinally adjacent hea-ting zones within the work chamber. When this autoclave was operated at 10,000 p.s.i. and l700 F., the effective capacity of the work chamber was reduced rby one-half as the total usable hot zone measured only 13% inches in diameter by 30 inches in length. A further increase in pressure to 15,000 p.s.i. caused the total usable hot zone to decrease to 24 inches in length.

The fluctuations in size and stability of hot zones as pressures are increased and the limited capacity of present funnaces have been the two most significant factors limiting the acceptance by industry of the high-pressure isostatic bonding and compaction processes as production processes. Present furnaces have not been versatile enough in terms of range of product sizes and materials that they are capable of processing to justify the extremely high initial investment. The long processing times required for isostatic bonding or compaction in many instances would make it economical to use such processes only for large parts or structures, many of which are larger than present furnace capacities. Therefore, before the high-pressure bonding and compaction processes could become commercially useful, it was necessary to find a Way to increase the capacity and stability of the furnaces employed.

The present invention makes it possible to provide highpressure furnaces with significantly increased capacities and which can be operated in a variety of different pressure and temperature ranges without affecting the size or stability of the internal hot zone. It has been found that surrounding the cylindrical inner work chamber with a closed annular chamber having a height substantially greater than its width, providing a closed cylindrical insulating chamber having a diameter substantially greater than its height at one end of the inner Work chamber, and substantially sealing both ends of the work chamber, will significantly improve the performance of a high-pressure furnace. When the chamber assembly is combined with a multiple zone internal heater and the free flow of pressure medium between the various chambers is impeded, the capacity of the furnace can be increased significantly. The present invention allows the work chamber capacity t be doubled while using the same external pressure vessel (internal cavity 27 inches by 108 inches) which with previous furnace designs could provide a maximum work chamber capacity of only 131/2 inches in diameter and 60 inches in length.

A furnace according to the present invention has been constructed with an internal work chamber 18 inches in diameter and 70 inches in length. This furnace has been operated at 10,000 p.s.i. and 1700 F. with no reduction in the effective capacity of the Work chamber. The total usable hot zone measured 18 inches in diameter by 70 inches in length or 100 percent of available capacity as compared with a maximum of 50 percent achieved in the best prior design. This furnace has also been tested at 15,000 p.s.i. and 1400a F. with no reduction in effective capacity. The results of tests have also shown that power requirements are appreciably lower and that it is not necessary in many instances to fill the volume around the work piece with a material capable of restricting the free flow of pressure medium within the inner work chamber. Thus, the present invention provides a high-pressure furnace with significantly increased capacity, stability and efficiency. Present indications are that even larger furnaces can be constructed according to this invention in which hot zones approximating 100 percent of available capacity will be present over a wide range of operating temperatures and pressures. This advancement in furnace design will for the first time provide furnaces of sufficient capacity and versatility to make it economically feasible for industry to accept high-pressure bonding and compacting processes as production processes.

Summary of the invention A typical high-pressure furnace according to this invention comprises a gas-tight cylindrical pressure vessel, means for maintaining superatmospheric pressures within the pressure vessel, a first cylindrical shell positioned within the pressure vessel and defining an inner chamber, at least one additional shell coaxially surrounding the first shell `with the additional shell nearest the first shell being spaced therefrom to form an annular chamber therebetween having a height substantially greater than its width, a plurality of heating elements adjacent the first shell arranged to provide at least two independent heating zones within the inner chamber, closure means sealing at least the first shell and the additional shell nearest thereto at one end the first shell, means sealing the end of the first shell opposite the closure means, cover means sealing at least the additional shell nearest the first shell and positioned to provide a cylindrical insulating chamber having a diameter substantially greater than its height, the insulating chamber being coaxial with the inner chamber and adjacent the means sealing the end of the first shell opposite the closure means, circumferential sealing means between the first shell and the additional shell nearest thereto adjacent the means for sealing the first shell, and means disposed to impede the flow of pressure medium between the pressure vessel and the inner chamber, the annular chamber, and the insulating ychamber for equalizing the pressure therein.

The heating elements may be wound in a serpentine or substantially sinusoidal pattern about support members projecting from the first shell. Typically the support members are positioned in circumferential rows about the exterior of the first shell and the heating elements are wound circumferentially about the first shell to provide substantially cylindrical, longitudinally adjacent heating zones. Additional independently-controlled heating elements may be provided adjacent the closure means or the means for sealing the first shell. The closure means Imay also be provided with a substantially closed cylindrical insulating chamber having a diameter substantially greater than its height.

Brief description 0f the drawings FIG. l is an isometric view in partial vertical section of the upper portion of a high-pressure furnace embodying this invention.

FIG. 2 is a sectional view taken vertically through the center of the removable shell assembly as removed from the pressure vessel assembly of FIG. 1.

FIG. 3 is lan elevational view of the support plate as removed from the shell assembly showing the base heating element.

Description! of the preferred embodiment Referring to FIG. l, high-pressure furnace 11 comprises generally a gas-tight pressure vessel assembly 13 having a removable, hollow cylindrical shell assembly 15 (FIG. 2) positioned coaxially therein. The pressure vessel assembly 13 includes an elongated hollow cylindrical body 17 which is open at both ends. The body 17 is typically constructed of high strength steels or steel alloys such as A.I.S.I. Type 410 Stainless Steel or S.A.E. Type 4340 Modified Steel, the selection of body material depending primarily on the desired pressure capabilities of the furnace. Closure of the body 17 at the upper end is accomplished with a pressure-tight sealing head 19 which comprises a -main head 21 secured by a main nut 23, a service head 25 secured coaxially within the main head 21 by a service nut 27, and double O-

ring seals

29 and 29 internally sealing the main head and service head respectively. The service head 25 and service nut 27 are incorporated in the main head 21 to allow for simple and quick loading and maintenance of the furnace without disturbing the electrical and pressure connections. The service head 25 is provided with eye bolts 31 to facilitate removal of either the service head alone or the entire sealing head as a unit.

Electrical leads 33 for the internal heating elements are passed through appropriate openings in the main head 21 and are electrically insulated from the head. The leads 33 pass through pressure-tight fittings 35 (only one shown) in the external surface of the main head 21 and connect with an external power source (not shown).

As a safety precaution a rupture disc assembly 37 is installed in the main head 21. The disc assembly 37 may be of the type commercially available from Autoclave Engineers, Inc., Erie, Pa. lt is constructed so as to burst when the pressure within the pressure vessel exceeds a predetermined level.

Also extending through pressure-tight fittings in the main head 21 are the high-pressure line 39 for ingress of the pressure medium and the vacuum line 41 for an initial rough purging of the furnace atmosphere. The high-pressure line 39 and the vacuum line 41 connect respectively with an external compressor and vacuum pump (not shown). Thermocouple lead wires 43 also extend through an opening in the head 21 and exit through a pressuretight fitting 45. The thermocouple wires 43 are connected to a plug 46 which connects to external instrumentation (not shown) for monitoring temperatures within the furnace.

The lower end of the body 17 is sealed with a pressure-tight sealing head (not shown) similar to the upper sealing head 19 although a separate service head and nut are not used. The separate service head and nut may also be eliminated from the upper sealing head 19, especially in small diameter furnaces. Other forms of sealing heads may be used to seal the body 17 without departing from the scope of the invention. Typical closure heads which have been employed include full Bridgman closures, modified Bridgman closures, and metal-to-metal with an O-ring type closures. Various closure heads that may be employed are described in Comings, E. W., High-Pressure Technology, McGraw-Hill, New York, N.Y. (195-6).

Still referring to FIG. l, the pressure vessel assembly 13 also includes a hollow cylindrical inner liner 47 having a spiral-grooved outer surface which is contiguous with the inner wall of the body 17 such as to form a spiral channel between the inner liner 47 and the body 17. The pressure vessel assembly 13 is cooled using a coolant liquid which is circulated by means of a pumping system (not shown) through coolant line 49, the spiral-grooved channel of the liner 47, and out an exit line (not shown) to an external heat exchanger (not shown). Other forms of cooling liners may be employed or the pressure vessel assembly may be cooled with an external cooling jacket. In certain low temperature operations the cooling liner may be omitted. Coolant liners are generally used to permit extended operation without overheating the vessel walls.

FIG. 2 illustrates the preferred embodiment of the cylindrical shell assembly as removed from the pressure vessel assembly 13. rIhe shell assembly 15 includes four coaxially disposed, hollow cylindrical shells the outermost of which is a loading shell 51. Positioned coaxially within the loading shell 51 and spaced therefrom is a support shell 53 which coaxially surrounds and is spaced outwardly from a heat-shielding shell 55. The innermost shell is the heater shell 57 which is spaced from the heat-shielding shell 55 to form an annular chamber 59 therebetween having a height substantially greater than its width. The heater shell 57 provides an elongated, cylindrical inner chamber 61 which is disposed to receive a workpiece. The heater shell 57, the heat-shielding shell 55, the support shell 53, and the loading shell 51 are generally constructed of high strength materials. Stainless steel may be employed where operating temperatures do not exceed l800 F. Refractory materials may be used when extremely high temperatures are required. The particular material employed and the wall thickness of these shells will depend, of course, on the pressure and temperature requirements of the furnace.

The loading shell `51 is closed on the bottom and has opposing perforated brackets 62 (only one shown) at its upper end to facilitate loading and unloading of the shell assembly 15 from the pressure vessel assembly 1-3. The bottom of the loading shell 51 is tightly packed with a thick layer of a thermally insulating material 63 which is capable of blocking convection currents in high pressure medium. Such a material is Johns-Manville Corporations Microquartz (spun silica batting). The support shell 53, which is also closed on the bottom, is suspended within the loading shell on the layer of insulating material 63. The annular space between the loading shell 51 and the support shell 53 is also tightly packed with the insulating material 63 to restrict the llow of pressure medium between shells. In the preferred embodiment Microquartz is finely shedded and then tightly packed bet-ween the shells to a density equivalent to three or four times that of the as-received bulk material. This provides a good thermal barrier to 2500 F. The annular chamber 59 is ordinarily not packed with an insulating material, although higher temperature requirements may make the utilization of an insulating material within that chamber desirable.

Still referring to FIG. 2, a disc-shaped base plate 65 is positioned inside an-d at the bottom of the support shell 53. The base plate `65 is constructed of an electrically insulating material such as the asbestos-Portland cement composition marketed by Johns-Manville Sales, Inc., under the trade name Transitef A stepped cylinder having two annular steps of decreasing diameter `forms a support plate 67 which is positioned coaxially within the support shell 53 with the end of the plate 67 having the largest ydiameter being contiguous with the upper surface of base plate 65. The support plate 67 is also constructed of an electrically insulating material such as Transitef The annular upper surface of that step of the support plate 67 which has the largest diameter (the lower step) supports the heat-shielding shell 55 coaxially within the support shell `53. The annular upper surface of the second step supports the heater shell 57 coaxially within the heat-shielding shell 55. The upper end of the support plate y67 extends into the heater shell 57 for a short distance. The support plate 67 substantially seals both the annular chamber 59 and the inner chamber 61 at the lower end. A thermally insulating 4material 68 such as alumina is used to ll the annular space between the support shell 53 and the heat-shiel-ding shell 55.

A base heating element 69 is recessed in a serpentine groove in the upper surface of the support plate 67 (see FIG. 3). The heating element 69 is generally a resistance wire of any suitable resistance heating material (Chromel A, Kanthal, Hoskins 835 and 87S alloys, molybdenum, etc.) depending, of course, on temperature requirements. A graphite or tungsten-rhenium element may be used when extremely high temperatures are required. The shape and position of the heating element 69 are not critical to the invention as the element Imay =be shaped in any convenient configuration and rmay be positioned above, within, or below the support plate 67 according to individual preferences and requirements.

Referring again to FIG. 2, the base heating element electrical leads 71 (only one shown) extend downward through openings in the support plate 67 and exit through grooves in the bottom surface of the support plate to the annular space between the support shell -53A and the heat-shielding shell 55 where they connect with two of the lead rods 73. There are a total of twelve lead rods 73 (only six shown) annularly spaced from each other and extending longitudinally through the annular space between the support shell 53 and the heat-shielding shell 55. Two of the lead rods are for the circuit of the base heating element `69 and the remaining ten lead rods are for the respective circuits of the five independently controlled heating zones to be described later. A support bracket 75 of an electrically insulating material such as Transite secures the lead rods 73 to the upper end of the support shell 53. The lead rods 73 are preferably small diameter, metallic rods with tubular ceramic insulators 77 positioned at intervals along their length to prevent electrical contact with the adjacent shells. Flexible leads 79 are used to connect the lead rods 73 with the electrical leads 33 in the main head 21 (FIG. l).

As illustrated in FIGS. 1 and 2, a plurality of studs 81 project `from the exterior surface of the heater shell 57 in circumferential rows. The studs -81 may be welded, bolted or otherwise joined to the heater shell 57. Spoolshaped members 83 of an electrically insulating, temperature-resistant material such as lava are positioned coaxially on the studs 81 and secured by a washer 85 and a locking pin 87. Acceptable lava spools are commercially available from 4Lava Corporation of America, Chattanooga, Tenn. The heater shell 57 in the particular embodiment shown supports twenty circumferential rows of the spools 83 which may be `divided into ten longitudinally adjacent pairs of two adjacent rows each. Resistance heating elements 89 which are generally wires or straps of Chromel A, Kanthal, Hoskins 835 or 875 alloys, molybdenum, or other suitable resistance heating material (depending on temperature requirements) are wound alternately above and below the spools 83 of each pair of circumferential rows in a serpentine or substantially sinusoidal pattern depending on the location and spacing of the spools. As shown in FIG. 2, the heating elements 89 of the bottom two pair of circumferential rows are connected in series to a single, independent source of power to provide a substantially cylindrical heating zone shown as Zone at the bottom of the chamber 61. The power leads 91 (only one shown) for the Zone 5 heating elements extend downward through the annular chamber 59 and the support plate 67 and exit through grooves in the bottom of the support plate to the annular space between the support shell 53 and the heat-shielding shell 55 where they connect with two of the lead rods 73- to complete the circuit. The heating elements 89' of the third and fourth, fth and sixth, seventh and eighth, and ninth and tenth pairs of circumferential rows, respectively, are connected in the same manner as those of the Iiirst and second pairs of rows to complete their individual circuits and provide a total of tive substantially cylindrical, longitudinally adjacent heating zones within the inner chamber 61, each zone having an independently controlled power source.

For greater temperatures, pressures, and/or capacities, additional heating elements may be required. For example, an independent heating element similar to the base heating element 69 may be installed adjacent to the cover .'109 :at the end of the heater shell 57 opposite the support plate 67. (Coiver -9 lwill be described later.) For larger diameter shells, each substantially cylindrical heating zone may be divi-ded into sectors with lthe heating elements of each sector being independently controiled. Other methods and/ or patterns of mounting the heating elements 89 such las helical winding may be used as long `as the lrequisite independent heating zones are present. The hea-ting elements may be arranged -to provide heating zones shaped other than cylindrical if the heating of the zones can be controlled .to :provide uniform heating throughout the entire inner chamber 61. The heating elements 89 Imay be mounted on the inner surface of the heater shell 57 or they may be supported by other means such as a lattice-type frame positioned adjacent to the heater shell 57 on either side of it.

Five sheathed thermoco'uples 93 (one for each heating zone) are positioned longitudinally within the inner chamber l61 adjacent the inner surface of the heater shell 57. Clamp 95 secures the thermocouples 93 to the heater shell 57 in parallel and annularly spaced relationship (not apparent in drawings). The junction of each of the five thermocoiuples is positioned so as to measure the temperature within one of the live heating zones. A sixth thermocouple 97 is positioned in the support plate 67 near its center 4and .measures the temperature at the bottom of the chamber 61. The thermocouple leads 99, which are generally sheathed in a thermally insulating material '(not shown) such as high-purity alumina or thoria, ex- -tend downward through openings in the support plate 67 Iand exit through groves in the bottom ofthe support plate to the annular space between the support shell 53 and the heat-shielding shell 55. T he thermocouple leads 99 then pass upwardly between the support shell and heat-shieldin g shell in protective tubes 101 of .a material such as stainless steel to the top of the support shell 53. Bracket 10-3, also constructed of an insulating material such as Transite, secures the protective tubes 101 to the support shell 53 in a parallel and annularly spaced relationship. The leads `99 extend a short distance from the upper ends of the tubes 101 and are connected to a quick-disconnect plug I105. The plug 105 engages with a jack 107 (FIG. 1) which is connected to thermocouple lead Wires 43 in main head 21 to complete the circuit. More than six thermocou-ples may be employed for monitoring hot zone temperatures where desirable.

Referring again to FIG. 2, the heater shell 57 is sealed at the upper end by a disc-shaped cover member 109. The heat-shielding shell 55 extends longitudinally beyond the upper end of the heater shell 57 and is sealed at that end by a second disc-shaped cover member 11'1. The cover member 1111 is spaced outwardly from the cover member 109 to provide la substantial-ly cylindrical insulating chamber 113, having a diameter substantially lgreater than its height, therebetween. -The insulating chamber 113 is coaxial -with the inner chamber 61.

Annular

lipped rings

115 and 117 are welded or otherwise joined to the heater shell 57 and the heat-shielding shell 55, respectively, near their upper ends. The ring 11115 interlocks with the ring 1117 to substantially seal the upper end of the chamber 59. The interlocking rings also complete enclosure of the insulating chamber 113.

IT he cover members 109 and lll-1 have central threaded holes 119 -to receive eye bolts l(not shown) used to facilitate handling. The opposing studs 121 are fastened to the heat-shielding shell at the upper end to iacilita-te removal of that shell.

11n an alternate embodiment the insulating chamber 113 is formed by constructing the cover members, the logi-tudina-lly extending portion ofthe heat-shielding shell, yand the interlocking rings as a single cover mem-ber which is positioned at the upper end of the heater and heat-shielding shells. The particular configuration is not critical so long as a substantially closed cylindrical chamber having a diameter substantially greater than its height is provided adjacent to and coaxial with the inner Work chamber at its upper end and that the upper end of the annular chamber 59 is substantially sealed.

It has been found that surrounding the inner work chamber 61 with the closed narrow annular chamber 59 and the closed thin cylindrical chamber 113 substantially reduces `or eliminates the convective ilow of pressure medium `ar-ound the work chamber. Lt is believed that confvective flow of the pressure medium .around the work chamber is the primary cause of instability in the hot zone, especially when such ilow is in the form of convective loops. It is believed that convective loops are not able to form a readily in the closed narrow chambers which surround the work chamber of the present invention. In order to achieve the optimum results it is necessary that the annular chamber 59 be substantially sealed yat both the top and bottom and that the lthin cylindrical chamber l113 be a separate and dist-inet chamber.

For greater temperatures, pressures and/or capacities, an -additional insulating chamber similar to the insulating chamber 1f13 may be provided in the support plate 67. Suc-h additional chamber should be Ia substantially closed cylindrical chamber, coaxial with the inner chamber, tand having a diameter substantially :greater than its height. It may also be desirable to provide additional closed annular chambers outwardly of the chamber 59 or to divide the chamber 59 into several longitudinallyr adjacent chambers.

It is necessary to provide means for equalizing the pressures within the various chambers of the furnace to prevent the chamber walls from collapsing under pressure. In operation, the temperature and pressure are generally increased simultaneously to predetermined levels. It has been fotlnd to be advantageous to retard or impede the free flow of pressure medium between chambers during the pressurization cycle so that convective ilow of pressure medium Within or between chambers is substantially reduced. There are several ways in which the free ow of pressure medium can be impeded. In the preferred embodiment, the members which define the various chambers fit together loosely to allow pressure medium to leak slowly through the joints. For example, the lower edge of the heat-shielding shell 55 does not form a complete annular seal where it contacts the support plate 67, but is rather irregularly finished or formed to allow slow leakage of pressure medium into the annular chamber 59. Similar irregular, loose-tting joints between the cover members 1091and 111 and their respective shells, between the heater shell 57 and the support plate 67, and between the annular

lipped rings

115 and 117 also restrict the free ilow of the pressure medium into and between chambers. The insulating material 68 between the support shell 53 and the heatshielding shell 55 also serves to impede the free flow of pressure medium into the various chambers.

Other means may be employed in addition to or in place of those already mentioned for retarding or impeding the free flow of pressure medium. These include small holes or vents, tightly packed fibrous filters, and various valve mechanisms. They may be used alone or in combination with other means so long as they achieve the desired result of controlling the free flow of pressure medium between the various chambers such that the pressure within the chambers is eventually equalized without damage to the furnace components and, at the same time, sulbstantially reducing or eliminating convective fiow of pressure medium within or between chambers.

An inert gas such as helium or argon gas is preferred for use as the pressurizing medium although other gases or even liquids may be acceptable for particular operations. Due to the cost of helium and argon gas and to convenience of operation, the gas is recycled in storage tanks after the bonding or compacting operations. At very high temperatures and pressures helium gas has been found preferable for control of the hot zone and of the convective circulation of the dense gas. Multistage compressors are used to pressurize the gas. These compressors have oil separators which remove virtually all lufbricating oil from the gas. Since it is important that no oil or moisture be carried into the autoclaves when isostatic bonding or compacting is being done, additional filters (not shown) are contained in the pressurizing line 49.

In a typical gas-pressure bonding operation, the workpiece is loaded into the inner chamber 61 of the shell assemlbly where it is generally supported on a layer of a material such as tabular alumina. For certain applications the volume around the workpiece may be filled with bubble alumina, silica, copper shot, or similar materials to prevent excessive convective currents Within the inner chamber `61. Previous designs almost always require filling the volume around the workpiece to Iprevent formation of convective currents. It has unexpectedly been found that with the present design the filler material is rarely necessary.

The shell assembly 15 with the workpiece positioned therein is loaded into the pressure vessel assembly 13 with the

cover members

109 and 111 removed. The sealing head 19 is instated and the entire assembly is subjected to at least one bake-out cycle, generally at a pressure of about 300 p.s.i. and temperatures from 900 to l100 F. In each bake-out cycle, upon reaching temperature the power is turned off and the pressurizing medium exhausted from the vessel. The vessel is then placed under vacuum for a predetermined period of time. The

cover members

109 and 111 are then instated and an insulating material such as Microquartz is packed between the cover member 111 and the sealing head 19. The pressure vessel assembly 13 is sealed and temperature and pressure are increased to predetermined levels and held for a predetermined period of time. At completion of the bonding operation, the power is shut off and the temperature is allowed to decrease. The pressure decreases only slightly during cooling of the hot zone and is usually not removed until a predetermined temperature has *been reached. However, certain applications often require special heating and cooling conditions, such as removal of the pressure at maximum temperature or heating to temperature prior to pressurization, and the invention per-mits such versatility.

The furnaces according to this invention are normally operated in the vertical position with each furnace being located in an individual cylindrical pit. This type of installation permits maximum safety with ease of operation. The removable shell assembly 15 of the present invention allows the workpiece to be preheated externally of the pressure vessel assembly 13 and transported to the pressure vessel assembly just prior to treatment. The electrical and thermocouple leads may be quickly connected after the shell assembly is lowered into the pressure vessel assembly. After treatment the shell assembly may be removed from the pressure vessel assembly for cooling and removal of the workpiece. While the first shell assembly is cooling, a second preheated shell assembly can be cycled in the pressure vessel assembly. Utilizing preheated removable shell assemblies the down-time of the pressure vessel assembly can Ibe minimized and turn-around time for an autoclave can -be reduced to under 12 hours. These time savings also contribute to commercial acceptance of highpressure technology.

It should be understood that construction of a highpressure furnace according to this invention is not limited to the materials specifically mentioned herein, but that the materials employed will depend on the temperature and pressure requirements of each particular furnace. The use of furnaces according to this invention is not limited to gas-pressure bonding or compacting processes, but may be extended to any high-pressure, high-temperature operation, whether it requires the maximum operational capabilities of the furnace or not.

It is to be understood that the foregoing detailed description is given merely by way of illustration and that many variations and modifications may -be made therefrom without departing from the true spirit and scope of the invention.

What is claimed is:

1. A high-pressure furnace comprising:

(a) a gas-tight cylindrical pressure vessel dening an outer chamber;

(b) means for maintaining superatomspheric pressures within `said outer chamber;

(c) a first cylindrical shell defining an inner chamber positioned coaxially within sai-d pressure vessel and spaced therefrom;

(d) at least one additional cylindrical shell positioned within said pressure vessel coaxially surrounding said first shell and spaced therefrom, the additional shell nearest said first shell being spaced therefrom to form an annular chamber therebetween having a height substantially greater than its width, said additional shell nearest said first shell also being spaced from said pressure vessel;

(e) a plurality of heating elements adjacent said first shell and arranged to provide at least two independent heating zones within said inner chamber;

(f) closure means sealing at least said first shell and the additional shell nearest thereto at one end of said first shell;

(g) means sealing the end of said first shell opposite said closure means;

(h) cover `means sealing at least the additional shell nearest said first shell and positioned to provide a substantially closed cylindrical insulating chamber having a diameter substantially greater than its height, said insulating chamber being coaxial with said inner chamber and adjacent said means for sealing the end of said first shell opposite said closure means;

(i) circumferential sealing means between said first shell and the additional shell nearest thereto adjacent said means sealing the end of said first shell opposite said closure means; and

(j) means disposed to impede the free ow of pressure medium between said outer chamber and said inner chamber, said annular chamber, and said insulating chamber for equalizing the pressure therein.

2. A furnace according to claim 1 wherein said heating elements are wound in a serpentine pattern about support members projecting from sai-d first shell.

3. A furnace according to claim 2 wherein said heating elements are wound in a substantially sinusoidal pattern about said support members.

4. A furnace according to claim 3 wherein said support members are :positioned in circumferential rows about said first shell and said heating elements are wound circumferentially about said iirst shell to provide substantially cylindrical, longitudinally adjacent heating zones.

5. A furnace according to claim 2 wherein said support members project from the exterior surface of said first shell.

6. A furnace according to claim 5 wherein said first shell is metallic and said support members are insulated therefrom.

7. A Vfurnace according to :claim 1 wherein at least one additional heating element is positioned adjacent said closure means, said additional heating element being independently controlled.

8. A furnace according to claim 7 wherein said additional heating element is recessed in the surface of said closure means within said inner chamber.

9. A furnace according to claim 7 wherein at least one additional heating element is positioned adjacent said means sealing the end of said first shell opposite said closure means, said additional heating element being independently controlled.

10. A furnace according to claim 1 wherein said cover means, said circumferential sealing means, and said means for sealing said rst shell are a single member.

11. A furnace according fto claim 1 wherein said olosure means is provided with a substantially closed cylindrical insulating chamber, coaxial with said inner chamber, lhaving a diameter substantially I'greater than its height.

1.2. A furnace according to claim 1 wherein the additional shell nearest said rst shell is coaxially surrounded by two additional shells, the outermost of which is a loading shell for removing the shells contained therein from said pressure vessel and the next innermost of which is a heat shielding shell.

13. A furnace according to claim 1 wherein at least a portion of said inner chamber is filled with a material capable of restricting the free flow -of pressure medium within said inner chamber.

14. A furnace according to claim 1 wherein said annular chamber is divided into -at least two longitudinally adjacent annular chambers.

References Cited UNITED STATES PATENTS 1,880,806 10/1932 Ciof 13-31 2,987,788 y6/1961 Lyman 13-31 X 3,240,479 3/1966 Shea et al. 263-41 JOHN I. CAMBY, Primary Examiner.

U.S. Cl. XR. 13-31