GB1586033A

GB1586033A - Apparatus for gas pressure bonding and hot isostatic pressing

Info

Publication number: GB1586033A
Application number: GB1114778A
Authority: GB
Original assignee: Autoclave Engineers Inc
Current assignee: Autoclave Engineers Inc
Priority date: 1977-03-24
Filing date: 1978-03-21
Publication date: 1981-03-11
Also published as: FR2385060B1; DE2812888A1; FR2385060A1; DE2812888C2

Description

(54) APPARATUS FOR GAS PRESSURE BONDING AND HOT ISO STATIC PRESSING (71) We, AUTOCLAVE ENGIN EERS, INC., a corporation of the Commonwealth of Pennsylvania, United States of America, having a principal place of business at 2930 West 22nd Street, Erie, Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns an apparatus for gas pressure bonding, and hot isostatic pressing or the like in which a workpiece may be treated at elevated teingeratures and pressures.

In prior art high pressure furnaces, uniform temperature distribution was sought either by circulating a pressurized atmosphere through the furnace or by carefully arranging the heating elements of the furnace and individually controlling them. The present invention is concerned with improvements in the gas circulation technique wherein no mechanical means are used to promote circulation.

Prior art methods of promoting gas circulation without mechanical apparatus such as fans are taught in United States Patents Nos. 3,419,935 to Pfeiler et al, 3,543,062 to Smith and 3,571,850 to Pohto, for exarnple.

In United States Patent No. 3,548,062 to Smith, a technique is disclosed in the specification for promoting gas circulation comprising a baffle between the workpiece and the heating elements positioned more or less uniformly along the inside wall of the baffle such that gases are drawn up through the space between the baffle and the heating elements and are permitted to flow down over the workpiece being cooled Iby transferring heat to the workpiece. As the workpiece heats up, the hot gases passing the workpiece are not as rapidly cooled and therefore the oirculation of the gases slows down.In United States Patent No. 3,419,935 to Pfeiler et al, and in 3,571,850 to Pohto, gas circulation is promoted by a liner between the workpiece and a furnace wall, and by heating gas just below the workpiece and allowing the gas to rise transferring heat to the workpiece as it flows past, and then permitting the gas to flow back down between the liner and the furnace wall. These furnaces have been limited to the use of metal resistance elements due to the configuration of the hearth and furnace bottom. This is a sulbstantial disadvantage as graphite and silicon carbide heating elements offer distinct advantages in certain environments.

There currently exist numerous uses for apparatus that treat a specimen or workpiece at high pressures and high temperatures including, for example, gas pressure bonding furnaces and hot isostatic pressing apparatus.

In these apparatus, it is typical to treat a workpiece at 1000"C and 15,000 psi although these are not the maximum temperature and pressure conditions encountered. Suitable apparatus for these applications generally comprises a furnace within a pressure vessel or autoclave. The furnace provides the heat to the workpiece and protects the vessel from excessive temperature. The vessel maintains the furnace and the workpiece at the desired pressures.

For a given pressure, the diameter of the pressure vessel determines the minimum safe thickness of the vessel wall. To avoid extremely heavy vessels, it is desirable to reduce the vessel diameter as much as possible.

Stated another way, the space between the interior of the vessel lining and the workpiece should be very small even though this is the space occupied by the furnace.

In most processes, it is essential that the temperature of the workpiece be extremely uniform. Oltherwise, problems may result from differential thermal expansion of the workpiece. Thus, the furnace portion of the high pressure-high temperature apparatus must dis tribute the heat evenly to the workpiece.

According to the present invention, there is provided an apparatus for gas pressure bonding, hot isostatic pressing or the like in which a workpiece may be treated at elevated temperatures and pressures, said apparatus comprising an elongate cylindrical pressure vessel within which there is disposed a heatinsulating hood, a hearth upon which a workpiece may rest, said hearth being enclosed within the hood and being set upon an elongate refractory pedestal, a cylindrical heating element selected from the group carbon, graphite and a silicon carbide defining an electrical resistance path, said heating element being disposed about and spaced from said pedestal below the hearth extending substantially entirely along the length of the pedestal; and cylindrical heat-reflecting means disposed about the pedestal and heating element, said hearth being a substantially solid disc-like structure having a diameter greater than the diameter of the top of the pedestal and shielding any workpiece upon the hearth from most of the direct radiation of the heating element, said pedestal, hearth, heating element and heat-reflecting means being arranged to permit convection to transfer heat from the heating element to a workpiece placed upon the hearth and to minimize transfer of heat to the workpiece by radiation.

The present invention enables the diameter of the pressure vessel to be minimized while at the same time providing for even dis tribution of heat to the workpiece in a way to obtain uniform workpiece temperature.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a sectional view through an apparatus according to one embodiment of this invention, Figure 2 is a plan view in section of the apparatus of Figure 1, Figure 3 is a sectional view of another embodiment of this invention, Figure 4 is a plan view of the apparatus of Figure 3, Figure 5 is a sectional view through still another embodiment of this invention, Figure 6 is a plan view in section of the apparatus of Figure 5, and Figure 7 is a perspective view of a pedestal and heating element of the embodiment of Figure 5.

In Figure 1 there is shown a pressure vessel 1, 2 within which they are disposed a heatinsulating hood 4, a cylindrical heat-reflecting means 9, and a heating element 8. A workpiece 7 is shown as being supported upon a hearth 6, which is set upon an elongate refractory, pedestal 5.

More specifically, referring to Figure 1 there is shown an elongate cylindrical pressure vessel or autoclave comprising a base 1 and an inverted hat-shaped shell 2. The base of the shell 2 has a flange provided with openings through which fastening means 3 enable the shell 2 to be secured to the base 1. An O-ring or gasket 21 provides a pressure tight seal between the base 1 and shell 2. The base 1 or the hood 4 is provided with openings (not shown), which are connected to means for pressurizing the interior of the vessel, for example, with an inert atmosphere. Pressures up to 15,000 psi are commonly employed The thickness of the shell 2 depends upon the pressures to be contained and the diameter of the shell. Typically the shell 2 is made from high strength steel.

The pedestal 5 is set upon the base 1 and supports the hearth 6. The hearth 6 should be strong enough to support the workpiece 7 at working temperatures. The pedestal 5 should have as low a heat capacity as possible.

In this way, more of the energy introduced into the furnace is available to heat the workpiece 7 and less is required to heat the pedestal 5. The hearth 6, which is placed on top of the pedestal 5 has a diameter greater than the diameter of the pedestal 5. This enables the base of the workpiece 7 to be greater than the top of the pedestal 5.

Preferably the pedestal 5 comprises a foot 22 supporting a furnace bottom 23 somewhat above the base 1. The foot 22 and the furnace bottom 23 may be constructed of carbon steel. Mounted upon the furnace bottom is a heat and eleotrically insulating support 24 which may be made from a refractory heat insulating or high aluminium castable material.

The support 24 has a raised inner portion of small diameter upon which is mounted a graphite pedestal extension 25. The hearth 6, which is made of graphite, is mounted on the pedestal extension. An anchor 26 fixed in the support 24 slidably engages the graphite pedestal extension 25 to ensure alignment.

Surrounding the pedestal but circum ferentially spaced from and thus not in con taot therewith is a cylindrical carbon or graphite, or SiC electrical resistance heating element 8. The heating element 8 may be made from a hollow right cylinder which is cut from alternate axial directions to provide a sinuous electrical path through the heating element (see Figures 1 and 2). On the other hand, the heating element 8 may comprise a cylindrical cage of rods 8' having caps 8'a spanning each pair, with adjacent rods forming pairs joined at the top (see Figures 3 and 4). Two conducting rings, one 8'b with external teeth and another 8'c with internal teeth are arranged around the pedestal to form bases to support the cylindrical rods 8' and to provide the pairs of rods with electrical current. This arangement is particularly good when the heating element is comprised of SiC.

Electrical connecting means 27 and 28 are provided through the base of the vessel to supply an electrical current at an appropriate voltage level to the heating element 8.

In a preferred embodiment, the furnace bottom 23 and the insulating support 24 have openings therein to permit graphite or carbon or silicon carbide rods 29 threaded to the heating element 8 to pass into the space below the furnace bottom. Here electrically conducting means 30 couple the rods 29 to -terminals 31 which are connected to an electrical conduit passing through the base 1.

The heat-reflecting means 9 is a cylindrical refractory heat-reflecting shield 9 provided about the periphery of the heating element 8. Its principal function in the embodiment shown in Figures 1 and 2 is to prevent direct radiation from the heating element 8 to the hood 4.

As shown in Figure 1, the heat-reflecting means may comprise two elements, a shorter ceramic or graphite refractory tube 35 radially outward of the heating element 8 and a refractory metal hood, for example Inconel (Registered Trade Mark) 601 enclosing the workpiece, pedestal and heating element.

As shown in Figures 1 and 2, the heatreflecting means 9 comprises a sealed hood made of a refractory metal. Circulation takes place within the hood due to the heat losses through the side and top d the hood as follows: hot gas is accelerated upwardly fromthe heating element 8 along the side of the hood. As the hot gas moves along the side of the hood (and upon reaching the top) it is cooled; it becomes heavier; and it flows back down toward the heating element 8. This circulation enhances temperature uniformity in the workpiece 7. If the hood 9 was insulated, then the hot gas would slowly rise and stagnate at the top of the hood moving back downward only very slowly. This latter situation might result in a temperature gradient along the workpiece 7.

Referring now to Figures 3 and 4, there is shown an emblodiment wherein a radiation shield 9' comprises an heat-reflecting insulating refractory, say a lightweight insulating brick or refractory castable material. In this instance, no separate ceramic heat shield (such as shield 35 of Figure 1) is necessary.

The shield 9' has the heat insulating characteristics of the insulating brick or castable material. Thus the top and bottom of the shield 9' must be vented as shown in Figure 3. Vents or holes 45 in the top of the heat insulated shield are preferably centrally spaced apart and are of such area as to permit sufficient flow to maintain temperature uniformity in the workpiece. Return vents or holes 46 at the base of the hood enable the complete circulation around the hood. Comparison of the shield embodiments of Figures 1 and 3 may be summarized as follows: if not heat insulated, the shield is not vented - if heat insulated, then it is vented.

Figures 5, 6 and 7 relate to an embodiment of this invention which has proved to be particularly satisfactory. The autoclave or pressure vessel 1, 2 and the hood 4 are substantially as shown in Figures 1 and 3. Also the foot 22, furnace bottom 23, insulating support, and the electrical connections 27, 28, 29, 30 and 31 through the base are substantially the same.

In the embodiment of Figures 5 to 7, however, the pedestal comprises an elongate hollow carbon or graphite cylinder 51 with radially extending plates 52 (see Figure 7).

The heating element comprises a cage of five hollow cylindrical carbon or graphite heating elements 53 in a circular arrangement around the pedestal having their own leads and thus being separately controllable. Cuts from alternate axial directions provide a sinuous electrical path through the heating elements.

Each cylindrical heating element 53 has a configuration similar to the sole element shown in Figures 1 and 2. It is contemplated that each element 53 will be powered up and down together. It is unlikely that they would be controlled to different power levels unless, of course, there was a failure in the leads to one element or in the element itself. Such a failure would not require a shutdown under most conditions as it would qnly affect onefifth of the power input capability.

In this embodiment, the hearth 6 is provided with a plurality of holes 61 therein to permit convection of the pressurized gases heated in the vicinity of the heating elements 53 to flow upwardly to the workspace above the hearth. The holes 61 are located and sized to minimize heat transfer to the workpiece by radiation.

The radially extending plates 52 help to transfer heat from the elements 53 to the gases by absorbing heat transferred by radiation and releasing heat transferred by conduction. The carbon, graphite and SiC elements have the advantage of being able to handle more power through their unit surfaces than refractory metal heating elements. This results in faster heating rates for the vessel. This embodiment is particularly good for high power levels and rapid heating rates.

In the embodiment of Figures 5, 6 and 7 the shield 9 of the embodiments of Figures 1 and 3 which extends the entire length of the vessel along the pedestal and workspace is replaced by a heat-reflecting graphite tube 35 radially outward of the heating elements 53 which serves to prevent direct radiation from the elements 53 to the hood 4 and to channel convection flow. Openings 46 in the base of the tube 35 permits incoming flow of convection currents. A cylindrical basket 65 rests upon the hearth 6. The basket 65 is made of refractory metal wire mesh or the like and while relatively impermeable to gas is not a good heat insulator. The basket 65 is arranged to be raised and lowered into the workspace with the workpiece resting upon the basket bottom 66 which acts as a second hearth separated from the hearth 6 be blocks 67.When the bottom 66 rests on the blocks 67, an annular space exists between the hearth 6 and basket bottom 66 enabling the convection flow upward along the side of the workpiece. The basket 65 und tube 35 together perform the shielding and channeling functions of shield 9 in Figures 1 and 3.

One particular autoclave furnace, similar to that shown in Figures 5 to 7 in all respects essential to this invention, has a workspace of about 44 inches in height and 18-1/2 inches in diameter. This particular furnace was wired with thermocouples in the work- space positioned one inch above the bottom, at the top and substantially equally spaced therebetween designated T7, T8, T9 and T10 respectively. Thermocouples (five) were placed adjacent the heating elements. Since they always registered the same temperature, they are collectively designated T1. A thermocouple was positioned just outside the openings 46 and was designated T14. Other thermocouples were placed around the furnace, but are not pertinent to this discussion.

The thermocouples enabled determination of workspace temperature uniformity on heating and at the hold temperature under various load conditions. The steady state hold conditions for a no-load test, a half load (250 lbs.) test and a full load (500 lbs.) test are set forth in the following table.

Thermocouples (degrees Centigrade) Load Condition T1 T7 T8 T9 T10 T14 Power Consumption Empty 1278 1227* 1200 1209 1210 1102 50-55 Kw Hall full 1270 1215 1205 1205 1200 1030 50-55 Kw Full 1270 1210 1205 1202 1200 1130 50-55 Kw *(Thermocouple placed along edge of bottom rather than one inch above bottom) The table establishes that for autoclave furnaces according to this invention, the work space will have a uniform temperature (within 10 to 20"cm at the steady state or hold con ditions. The furnace was in each case pressurized to 15,000 psi.

The temperature uniformity on heat-up depends upon the load. For the most difficult case, full load, the temperature spread within the workspace varied up to 1000C for heating rates of about 300 Centigrade degrees per hour. The temperature spread diminished as the hold temperature was approached. During heat up, the power input varied from 100 to 120 Kw's.

Cooling a laded furnace is a substantially different problem. Even for slow cooling rates of say, 1500 Centigrade per hour temperature differences develop (up to 300 degrees Centi grade). However, most processes and work pieces can sustain the temperature gradients in the workspace. Also, the peak gradient does not develop until the topmost portion of the workspace has cooled considerably, to say, 700"C.

The temperature 'between the thermo couples T19 and T1, set forth in the table, represents the driving force for assuring a circulation of convection currents. The gases passing the heating elements increase in tem peratures between 100 and 150 degrees. In the embodiment described, during steady state conditions, a substantial portion of the tem- perature drop occurs between the top of the tube 35 and the openings 46. In this embodi ment, heat transfer across the tube 35 is sufficiently restricted to permit the temperature gradient and asssociated convection currents to develop.The small annular space between the top of the tube 35 and the space between the hood 4, on one side, and the tube 35 and basket 65 on the other actually results in a slight temperature increase in the gas as it moves between the top of the basket and the bottom of the basket in the space between the basket and the hood. This may, in part, account for the excellent hold temperature uniformity in the workspace. While this em bodiment is most similar to that disclosed in Figure 3, in which gas flow is promoted in the space adjacent the hood, it is not essential that the space be insulated from the workspace.

The insulating hood 4 is the principal heat insulation separating the workpiece and the heating element from the pressure vessel shell.

The hood is designed to minimie heat trans fer to the shell and to have a low eat capacity.

A number of hood designs are possible. One shown in Figure 1 comprises a stainless steel inner lining 40 and a carbon steel outer lining 41 with ceramic fiber heat insulation 42 therebetween. Other hood structures might com prise no inner sheet and refractory insulating brick in place of the fibers. An additional axial heat shield may be placed at the upper end of the hood 43 for best results. It should be a refractory metal such as Inconel (Registered Trade Mark). As with the pedestal, the less heat energy is absorbed by the hood, the more is available for raising the temperature of the workpiece. Hence, the heat capacity of the hood should be minimized.

The heating element according to this invention is located completely below the workpiece and thereby does not occupy space between the workpiece 7 and the hood 4. This enables the diameter of the hood and therefore the shell to be reduced with the advantages described above. Surprisingly, it has been found that even though the heating element is completely below the workpiece, the furnace with the workpiece therein has good temperature uniformity at elevated temperatures.

This is achieved without the use of mechanical means to induce convection currents in the furnace interior. The simple structure described above, in a manner not completely understood, provides the uniform heating of the workpiece by convection and without direct radiation or conduction.

WHAT WE CLAIM IS: 1. An apparatus for gas pressure bonding, hot isostatic pressing or the like in which a workpiece may tbe treated at elevated temperatures and pressures, said apparatus com prising an elongate cylindrical pressure vessel within which there is disposed a heat-insulating hood, a hearth upon which a workpiece may rest, said hearth being enclosed within the, hood and being set upon an elongate refractory pedestal, a cylindrical heating element selected from the group carbon, graphite and a silicon carbide defining an electrical resistance path, said heating element being disposed about and spaced from said pedestal below the hearth extending substantially entirely along the length of the pedestal; and cylindrical 'heat- reflecting means disposed about the pedestal and heating element, said hearth being a sub stantially solid disc-like structure having a diameter greater than the diameter of the top of the pedestal and shielding any workpiece upon the hearth from most of the direct radiation of the heating element, said pedestal, hearth, heating element and heat-reflecting means being arranged to permit convection to transfer heat from the heating element to a workpiece placed upon the hearth and to minimize transfer of heat to the workpiece by radiation.

2. An apparatus according to claim 1 wherein the pedestal is supported above the base of the pressure vessel on a foot which provides a space 'between the pedestal and the base for electrical terminals and electrical connecting means.

3. An apparatus according to claim 2 in which carbon, graphite or silicon carbide rods

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

The table establishes that for autoclave furnaces according to this invention, the work space will have a uniform temperature (within

10 to 20"cm at the steady state or hold con ditions. The furnace was in each case pressurized to 15,000 psi.

The temperature uniformity on heat-up depends upon the load. For the most difficult case, full load, the temperature spread within the workspace varied up to 1000C for heating rates of about 300 Centigrade degrees per hour. The temperature spread diminished as the hold temperature was approached. During heat up, the power input varied from 100 to 120 Kw's.

Cooling a laded furnace is a substantially different problem. Even for slow cooling rates of say, 1500 Centigrade per hour temperature differences develop (up to 300 degrees Centi grade). However, most processes and work pieces can sustain the temperature gradients in the workspace. Also, the peak gradient does not develop until the topmost portion of the workspace has cooled considerably, to say, 700"C.

The temperature 'between the thermo couples T19 and T1, set forth in the table, represents the driving force for assuring a circulation of convection currents. The gases passing the heating elements increase in tem peratures between 100 and 150 degrees. In the embodiment described, during steady state conditions, a substantial portion of the tem- perature drop occurs between the top of the tube 35 and the openings 46. In this embodi ment, heat transfer across the tube 35 is sufficiently restricted to permit the temperature gradient and asssociated convection currents to develop.The small annular space between the top of the tube 35 and the space between the hood 4, on one side, and the tube 35 and basket 65 on the other actually results in a slight temperature increase in the gas as it moves between the top of the basket and the bottom of the basket in the space between the basket and the hood. This may, in part, account for the excellent hold temperature uniformity in the workspace. While this em bodiment is most similar to that disclosed in Figure 3, in which gas flow is promoted in the space adjacent the hood, it is not essential that the space be insulated from the workspace.

The insulating hood 4 is the principal heat insulation separating the workpiece and the heating element from the pressure vessel shell.

The hood is designed to minimie heat trans fer to the shell and to have a low eat capacity.

A number of hood designs are possible. One shown in Figure 1 comprises a stainless steel inner lining 40 and a carbon steel outer lining

41 with ceramic fiber heat insulation 42 therebetween. Other hood structures might com prise no inner sheet and refractory insulating brick in place of the fibers. An additional axial heat shield may be placed at the upper end of the hood 43 for best results. It should be a refractory metal such as Inconel (Registered Trade Mark). As with the pedestal, the less heat energy is absorbed by the hood, the more is available for raising the temperature of the workpiece. Hence, the heat capacity of the hood should be minimized.

The heating element according to this invention is located completely below the workpiece and thereby does not occupy space between the workpiece 7 and the hood 4. This enables the diameter of the hood and therefore the shell to be reduced with the advantages described above. Surprisingly, it has been found that even though the heating element is completely below the workpiece, the furnace with the workpiece therein has good temperature uniformity at elevated temperatures.

This is achieved without the use of mechanical means to induce convection currents in the furnace interior. The simple structure described above, in a manner not completely understood, provides the uniform heating of the workpiece by convection and without direct radiation or conduction.

WHAT WE CLAIM IS: 1. An apparatus for gas pressure bonding, hot isostatic pressing or the like in which a workpiece may tbe treated at elevated temperatures and pressures, said apparatus com prising an elongate cylindrical pressure vessel within which there is disposed a heat-insulating hood, a hearth upon which a workpiece may rest, said hearth being enclosed within the, hood and being set upon an elongate refractory pedestal, a cylindrical heating element selected from the group carbon, graphite and a silicon carbide defining an electrical resistance path, said heating element being disposed about and spaced from said pedestal below the hearth extending substantially entirely along the length of the pedestal; and cylindrical 'heat- reflecting means disposed about the pedestal and heating element, said hearth being a sub stantially solid disc-like structure having a diameter greater than the diameter of the top of the pedestal and shielding any workpiece upon the hearth from most of the direct radiation of the heating element, said pedestal, hearth, heating element and heat-reflecting means being arranged to permit convection to transfer heat from the heating element to a workpiece placed upon the hearth and to minimize transfer of heat to the workpiece by radiation.
2. An apparatus according to claim 1 wherein the pedestal is supported above the base of the pressure vessel on a foot which provides a space 'between the pedestal and the base for electrical terminals and electrical connecting means.
3. An apparatus according to claim 2 in which carbon, graphite or silicon carbide rods

extend through openings in the foot, one end of said rods engaging the heating element and the other end thereof being connected by electrical conducting means to said electrical terminals.
4. An apparatus according to any preceding claim wherein the heat-reflecting means comprises a ceramic refractory tube disposed radially outwardly of the heating element, and a refractory metal hood enclosing the workpiece, pedestal and heating element.
5. An apparatus according to any preceding claim wherein the heating element has axial cuts therein to define a sinuous electrical path.
6. An apparatus according to any of claims 1 to 3 wherein the heat-reflecting means comprises an heat insulating hood having vents in the top thereof and near the base thereof.
7. An apparatus according to any preceding claim wherein the heating element is a cage of cylindrical rods in a circular arrangement around the pedestal, the rods being paired and having a bridge across adjacent pairs enabling a continuous electrical path.
8. An apparatus as claimed in any preceding claim in which the heating element is circumferentially spaced from the pedestal.
9. An apparatus for gas pressure bonding, hot isostatic pressing or the like in which a workpiece may be treated at elevated temperatures and pressures, said apparatus comprising an elongate cylindrical pressure vessel within which there is disposed a heat-insulating hood; a hearth upon which a workpiece may rest, said hearth being enclosed within the hood and being set upon an elongate refractory pedestal; a plurality of hollow cylindrical carbon or graphite heating elements each defining a sinuous electrical resistance path, said heating elements being circumferentially spaced from said pedestal below the hearth and extending substantially entirely along the length of the pedestal; and cylindrical heat-reflecting means disposed about the pedestal and heating elements, said hearth being a substantially solid disc-like structure having a diameter greater than the diameter of the top of the pedestal and shielding any workpiece upon the hearth from most of the direct radiation of the heating element, said pedestal, hearth, heating elements and heat-reflecting means being arranged to permit convection to transfer heat from the heating elements to a workpiece placed upon the hearth and to minimize transfer of heat to the workpiece by radiation.
10. An apparatus according to claim 9 wherein the pedestal is supported above the base of the pressure vessel on a foot which provides a space between the pedestal and the base for electrical terminals and electrical connecting means.
11. An apparatus according to claim 10 in which carbon or graphite rods extend through openings in the foot, one end of said rods engaging the heating elements and the other end thereof being connected by electrical conducting means to said electrical terminals.
12. An apparatus according to any of claims 9 to 11 wherein the heat-reflecting means comprises a ceramic refractory tube having heat insulating properties and disposed radially outwardly of the heating elements, and a refractory metal basket mounted upon the hearth and enclosing the workpiece above the hearth.
13. An apparatus as claimed in any of daims 9 to 12 wherein the pedestal has a cylindrical shape and is provided with plates extending radially outwardly thereof.
14. An apparatus for gas pressure bonding substantially as hereinbefore described with reference to and as shown in Figures 1 and 2, or in Figures 3 and 4, or in Figures 5 to 7 of the accompanying drawings.