Method and apparatus for pressurizing hot-isostatic pressure vessels

A method and apparatus for the rapid, high-capacity pressurization of hot-isostatic pressing vessels, specifically of the type adapted for the hot-isostatic pressing of alloy shapes from powder metallurgy alloy charges. This is achieved by pressurizing with argon gas which is obtained by pumping cryogenic liquid argon at relatively low pressure to a relatively higher pressure and vaporizing said pumped liquid argon to produce said gas, which gas after use in the vessel for compacting is reclaimed and reliquefied for reuse.

It is well known to produce various alloy articles, such as high speed 
steel articles and titanium or superalloy articles, by compacting alloy 
powder charges in a hot isostatic pressing vessel, commonly termed an 
autoclave, to achieve articles with densities of substantially 100% of 
theoretical density. Since autoclaves are of expensive construction and 
operation, it is advantageous from the standpoint of providing an 
economical manufacturing practice to provide for rapid pressing cycles. 
This facilitates increased autoclave production rates, thereby lowering 
the cost per cycle of the product produced thereby. 
Most hot isostatic pressing vessels embody a piston or diaphragm gas 
compressor. Systems of this type have relatively small capacities for 
pressurization of the vessel and thus require a plurality of gas 
compressors to achieve pressure levels adequate for hot isostatic 
compacting. With vessels of relatively increasing size, coupled with the 
desire for more rapid pressurization of the vessel and shorter compacting 
cycles, the piston and diaphragm gas compressors conventionally used for 
the purpose have become increasingly more disadvantageous from the 
economic standpoint. 
It is consequently desirable to utilize a more rapid pressurization system, 
and for this purpose cryogenic liquid argon pumps discharging into a 
vaporizer to produce argon gas at the high pressures required for hot 
isostatic compacting have been considered for the purpose. These systems 
convert cryogenic liquid argon, which is at a relatively low pressure, 
into a gas at a pressure of for example 20,000 psi. This gas, which is 
discharged from a vaporizer, is introduced into the autoclave to 
pressurize the same to pressure levels suitable for hot isostatic 
compacting. Pressurizing systems of this character, however, have not been 
used in commercial hot isostatic pressing vessels, because after 
completion of a compacting cycle the argon gas is discharged from the 
vessel either to the atmosphere or to a gas-storage vessel for subsequent 
use unassociated with the autoclave. Consequently, in view of the 
relatively high expense of obtaining a continuous supply of liquid argon 
for conversion to high-pressure argon gas, pressurizing systems embodying 
cryogenic liquid argon pumps and assoicated vaporizers have not been used 
in association with commercial hot isostatic pressing vessels. 
It is accordingly the primary object of the present invention to provide a 
method and apparatus for pressurizing hot isostatic pressing vessels by 
the use of cryogenic liquid argon pumps in association with argon 
vaporizers, whereby upon completion of a compacting cycle the argon gas 
discharged from the autoclave may be cooled to a temperature sufficient to 
produce cryogenic liquid argon. This liquid argon is stored for subsequent 
introduction to a liquid argon pump used in association with an argon 
vaporizer, whereby the stored liquid argon is converted to a gas at 
pressures sufficient for hot isostatic compacting. In this manner, a 
closed system is provided, and the argon is reused during each compacting 
cycle with only minimal argon gas loss.

Broadly the invention involves a hot isostatic compacting system having a 
gas pressure vessel which is adapted for heating and pressurization to 
levels sufficient for hot isostatic compacting of for example powder 
charges of prealloyed high speed steel, titanium base alloys and 
superalloys. The vessel is pressurized to pressures on the order of for 
example 10,000 to 20,000 psi by the introduction thereto of argon gas. 
During compacting the powder charge according to conventional practice is 
at an elevated temperature. For this purpose heating may be achieved 
outside the autoclave prior to introducing the charge thereto, within the 
autoclave or a combination of both. After compacting, the argon gas is 
exhausted from the vessel. In accordance with the invention the argon gas 
so exhausted is collected and stored, preferably in a series of tanks. The 
gas from these tanks may then be introduced to means for cooling the same 
to a temperature sufficient to convert the gas to cryogenic liquid argon. 
Means for achieving liquefaction is a heat exchanger wherein the coolant 
is liquid nitrogen. The cryogenic liquid argon is stored for further use. 
When pressurization of the vessel is required for a compacting cycle, the 
stored liquid argon by means of a cryogenic liquid pump is pumped from 
storage to increase the pressure thereof and to a vaporizer which converts 
the pumped cryogenic liquid argon at the increased pressure into a gas at 
pressure sufficient for use in the vessel for hot isostatic compacting. 
Accordingly, the gas from the vaporizer is introduced to the vessel for 
hot isostatic compacting. Upon completion of compacting the argon gas is 
exhausted from the vessel and stored for recycling, which includes 
reliquefaction as described. 
Stored argon gas is reliquefied by heat exchange with a coolant at a 
maximum temperature of -303.degree. F. at 1 atm and in accordance with the 
preferred embodiment of this invention the coolant employed is liquid 
nitrogen, which is typically at a temperature of -321.degree. F. at 1 atm. 
The liquid argon is pumped by the use of a cryogenic liquid pump to 
increase the pressure thereof to a selected level. This liquid argon, at 
said increased pressure level, is introduced to a vaporizer operated at a 
temperature sufficient to heat said liquid argon to a temperature within 
the range of 72.degree. to 120.degree. F. to vaporize the same thereby 
further increasing the pressure of the argon. These vaporization 
temperatures are sufficient to achieve argon gas at a pressure within the 
range of 5000 to 30,000 psi and more typically 10,000 to 20,000 psi. 
To facilitate transfer of the stored argon gas from the vessel to the heat 
exchanger for reliquefaction, it is preferred to store the gas in a 
plurality of gas-storage tanks connected in parallel with each tank being 
at a relatively lower pressure than the preceding tank. In this manner, 
the pressure of the gas upon exhausting of the vessel may be retained 
during storage and used in transmitting the gas from storage to the heat 
exchanger for liquefaction. In this regard during transfer to the heat 
exchanger the gas would be preferably removed from the highest pressure 
tank initially and then progress sequentially to the relatively lower 
pressure tanks. During this transmittal of the gas from storage to the 
heat exchanger for liquefaction, it is preferred that the pressure of the 
gas, prior to reaching the heat exchanger, be decreased to promote 
subsequent liquefaction. Specifically the gas pressure could be decreased 
from a storage pressure of about 2000 to 2500 psi to a pressure of about 
35 psi. 
With respect to the single FIGURE of the drawing, there is shown 
schematically an embodiment of apparatus suitable for the practice of the 
present invention. The apparatus includes an autoclave or gas pressure hot 
isostatic vessel 10 having a top opening 12 therein to permit loading and 
unloading of a workpiece (not shown) typically in the form of a powder 
metallurgy charge for compacting. The opening 12 may be selectively opened 
and sealed by a closure 14 transported by an overhead carriage 16. A crane 
(not shown) may be used for loading and unloading of the workpiece. The 
vessel 10 may contain heating means (not shown) for heating the workpiece 
upon introduction to the vessel. Also associated with vessel 10 is an air 
discharge line 18 having a valve 20 and vacuum pump 22. The vessel also 
has an argon-gas discharge line 24 and associated valve 26. Likewise 
vessel 10 has line 28 and associated pressure relief valve 30. Argon line 
32 connects the vessel 10 with a series of argon gas storage tanks 34A, 
34B and 34C. Line 32 has associated filters 36 and 38, valve 40 and 
pressure transducers 42 and 44, with pressure transducer 42 being located 
downstream of valve 40 and pressure transducer 44 being located upstream 
of valve 40. Line 32 also contains a pressure transducer 46 in association 
with argon storage tanks 34 and valves 48A, 48B and 48C. A relief valve 50 
is provided at the downstream end of line 32. An argon gas line 52 
connects the argon gas storage tanks 34 with heat exchanger 54. In line 52 
between the gas storage tanks and the heat exchanger 54 is a valve 56. The 
heat exchanger 54 has a liquid nitrogen line 58 with associated valves 60 
and 61 at the entry, which if fed by a liquid nitrogen source (not shown), 
and exit ends, respectively, of the heat exchanger 54. From the argon exit 
end of the heat exchanger 54 there is a liquid argon line 62 connecting 
the heat exchanger and providing for liquid argon transfer from the heat 
exchanger to a liquid argon storage tank 64. The line 62 has a valve 66 
operated by a liquid argon level control potentiometer 68 adapted to 
control the valve in accordance with the level of the liquid argon exiting 
from the heat exchanger 54. From the exit or discharge from liquid argon 
to storage tank 64 there is a line 70 for transmittal of liquid argon from 
storage tank 64 to a cryogenic liquid argon pump 72, associated argon 
vaporizer 74 and the vessel 10. In association with liquid argon line 70 
there is a valve 76 at the discharge from the liquid storage tank 64, a 
subcooler 78, pressure transducer 80, temperature transducer 82 and valve 
84, which is adjacent the argon gas entry to the vessel 10. 
In the conventional manner a powder metal charge for compacting (not shown) 
is loaded into the vessel 10 through top opening 12 by means of overhead 
crane (not shown). Thereupon the carriage 16 moves closure 14 into sealing 
engagement with opening 12. Air is removed from the vessel via line 18 by 
opening valve 20 and operating pump 22. Valve 20 is closed upon completion 
of air removal. The powder metal charge is heated to an elevated 
temperature suitable for hot isostatic compacting. The powder metal charge 
is compacted while at said temperature by the introduction of argon gas to 
the vessel 10, which sequence is well known in the art. 
After compacting, if there is heating means within the autoclave such is 
shut off and the pressure in the autoclave begins to drop. Typically when 
the autoclave temperature reaches about 500.degree. F., the valve 40 in 
line 32 is opened to permit the argon gas to be transmitted from the 
vessel 10 to argon gas storage tanks 34. The gas is introduced to the 
tanks in sequence beginning with tank 34A and ending with tank 34C. For 
this purpose associated valve 48A would be opened. Upon the filling of 
tank 34A associated valve 48A would be closed and valve 48B would be 
opened to admit argon gas to tank 34B. Relief valve 50 is provided should 
the pressure within the line 32, such as during opening and closing of 
valves 48 of gas storage tanks, exceed a selected maximum. pressure 
transducers 42 and 44 monitor the argon gas pressure at the downstream and 
upstream sides of valve 40, respectively, and provide an indication of the 
pressure drop across the valve to permit operation of the valve in a 
manner suitable to facilitate storage in the tanks 34. If desired, the 
transducers 42 and 44 may in the well known manner automatically operate 
valve 40 in response to electrical signals compared to a set point, which 
signals are proportional to the argon gas pressure at the transducer. 
Suitable filters 36 and 38 are provided to remove any foreign material 
from the gas. Typical gas discharge and reclamation from vessel 10 will 
begin when the vessel has cooled to a temperature of about 500.degree. F. 
Likewise, typically at this time the autoclave will be at a pressure of 
about 7000 to 8000 psi and thus the gas will be transmitted from the 
autoclave to the gas storage tanks 34 which will be typically at a 
pressure of about 200 psi. By sequentially storing the argon gas, the tank 
34A, which is the first tank to be filled, will be at a typical pressure 
of 2300 psi. The second tank 34B typically will be at a pressure of about 
1000 to 1200 psi and the final tank 34C typically will be at a pressure of 
about 200 to 300 psi. When the argon gas pressure in the vessel 10 reaches 
about 200 or 300 psi, the valve 40 is closed as are the valves 48 to the 
argon gas storage tanks 34. Valve 26 in line 24 is opened to discharge the 
remainder of the gas to the atmosphere via line 24. Upon the completion of 
this operation the valve 26 is again closed. 
In accordance with the invention reliquefaction of the stored argon gas is 
achieved in the following manner. Valve 48A associated with argon gas 
storage tank 34A is opened as is valve 56 in line 52. Accordingly, argon 
gas from tank 34A is transmitted through the cryogenic heat exchanger 54 
(typically of aluminum plate fin construction) via line 52. Simultaneously 
valves 60 and 61 in liquid nitrogen line 58 are opened. Accordingly, the 
argon gas, after initial cooling resulting from pressure drop across valve 
56, passes through the heat exchanger and is cooled by heat exchanger with 
countercurrent flow of liquid nitrogen at a temperature of about 
-305.degree. F., whereupon the argon gas is liquified so that at line 62 
at the argon exit end of the heat exchanger 54 cryogenic liquid argon is 
produced. The temperature range at which argon is in the liquid state is 
relatively narrow ranging from -308.degree. F. to -302.degree. F. at 1 
atm. Hence, it is desirable to control the pressure of the liquid nitrogen 
flow through the heat exchanger 54 to prevent lowering the temperature of 
the argon during heat exchange to the freezing point typically 
-308.degree. F. at 1 atm. Valve 66 in line 62 in response to the output 
signal of the liquid level control potentiometer mounted on the side of 
the liquid control vessel 68 maintains a predetermined amount of liquid 
argon in the control vessel to insure a steady flow to the liquid storage 
tank 64. The valve 66 is operated to insure a positive pressure and flow 
from the heat exchanger to the liquid argon storage tank 64. Liquid argon 
is stored in tank 64 for subsequent used in converting the liquid argon at 
a relatively low pressure into a gas at high pressure suitable for 
effecting hot isostatic compacting in vessel 10. 
Prior to the introduction of high-pressure argon gas to the vessel 10, the 
workpiece is introduced thereto and the vessel is sealed in the manner 
earlier described. After sealing of the vessel air is removed therefrom 
via line 18 by opening valve 20 and pumping the air from the vessel to the 
atmosphere by pump 22. After removal of all air from the vessel pumping is 
discontinued and valve 20 is closed. 
To begin pressurization of vessel 10 valve 76 in line 70 is opened to 
permit liquid argon flow from storage tank 64 through subcooler 78 to the 
cryogenic liquid argon pump 72. Subcooler 78 is provided to improve the 
efficiency of pump 72. The vaporizer 74 converts the cryogenic liquid 
argon at a temperature typically at -303.degree. F. to argon gas at a 
pressure on the order of 10,000 to 20,000 psi and at a temperature within 
the range of 72.degree. to 120.degree. F. The pressure level of the gas 
discharged from the vaporizer 74 will depend upon the pressure of the 
liquid argon entering the vaporizer from the pump and the vaporizer 
operating temperature. With the valve 84 being open the argon gas from the 
vaporizer 74 enters the vessel 10 to increase the pressure therein. The 
pressure and temperature of the gas from the vaporizer is monitored by 
transducer 80 and transducer 82, respectively. Relief valve 30 in line 28 
is provided to permit pressure relief should the gas pressure within the 
vessel 10 exceed a selected maximum for safe operation. As the argon gas 
from the vaporizer 74 enters the vessel 10 the pressure therein is 
increased to a level sufficient to achieve hot isostatic compacting of the 
heated powder metallurgy charge within the vessel. Upon completion of 
achieving desired compacting pressure the valve 76 is closed to stop 
liquid argon flow from the storage tank 64 via pump 72 to the vaporizer, 
and the operation of the pump is discontinued. Valve 84 is closed and the 
pressure and temperature within the vessel 10 are permitted to remain a 
specific time to complete compacting of the powder metallurgy charge. 
After the specified time the heater (not shown) within the vessel 10 is 
turned off and the pressure and temperature decreased. The sequence with 
regard to argon gas reclamation and storage from the vessel, as earlier 
described, is then started. 
Argon gas is necessary in the present invention for purposes of 
pressurization in that its temperature in the liquid state relative to the 
liquid nitrogen is such to permit reliquefaction of argon gas by heat 
exchange with liquid nitrogen. For this purpose the temperatures are 
typically -321.degree. F. at 1 atm for liquid nitrogen and -303.degree. F. 
at 1 atm for liquid argon. In addition, liquid nitrogen is readily 
available for use as coolant in the heat exchanger because of both its 
abundance in the atmosphere and its being a by-product in the commercial 
production of both argon and oxygen gases. 
As a specific example of the practice of the invention employing apparatus 
similar to that shown in the FIGURE of the drawing typical, specific 
conditions of pressure, temperature and liquid and gas flow volumes would 
be as follows: 
Argon gas is stored in the gas storage tanks 34 at typical pressures of 
2300 psi in tank 34A, 1500 psi in tank 34B, and 300 psi in tank 34C. To 
start the reliquefaction of the argon gas stored in the gas storage tanks 
34, valve 48A is opened and argon gas at 2300 psi flows into line 52. 
Valve 56 in line 52 is then opened to allow the argon gas to flow into the 
cryogenic plate fin heat exchanger 54 at a typical pressure of 35 psi. 
Simultaneously with the opening of valve 56, valves 60 and 61 in line 58 
are opened to allow the countercurrent flow of liquid nitrogen through the 
cryogenic plate fin heat exchanger 54. Because of the narrow temperature 
range of argon liquid -308.degree. to -302.degree. F. at 1 atm, the liquid 
nitrogen -321.degree. F. at 1 atm is pressure-controlled as it enters 
through valve 60, raising its boiling point to preclude freezing of the 
argon in the heat exchanger. Nitrogen gas is vented out valve 61 after 
extracting all possible refrigeration from the nitrogen liquid and the 
cold nitrogen gas. The argon liquefied and subcooled in the cryogenic heat 
exchanger 54 passes into line 62 where it is collected in the liquid 
control vessel 68 which regulates its flow into the argon liquid storage 
tank 64. The liquid argon is stored at a typical pressure of 30 to 40 psi 
in the storage tank 64. Argon gas from tanks 34A and 34B is taken down to 
a typical pressure of 1000 psi before stopping the reliquefaction process. 
At that time, valves 48A and 48B are closed along with valve 58. 
Simultaneously, the valve 60 in the liquid nitrogen line 58 is closed. 
When the liquid nitrogen and nitrogen gas have been bled from the 
cryogenic heat exchanger 54, valve 61 in line 58 is closed. 
To perform the hot isostatic pressing operation, the following steps are 
taken. After loading the vessel 10 and sealing it with the closure 14, the 
vessel 10 is flushed with argon gas. This is accomplished by opening valve 
48C on tank 34C allowing argon gas to flow into line 32. Valve 40 is now 
opened to allow argon gas to flow into the vessel 10 for typically 2 
minutes, at which time valve 40 is closed and valve 26 in line 24 is 
opened until the vesel 10 pressure is typically 2 psi. The remaining air 
and argon gas is removed from the vessel 10 by turning on vacuum pump 22 
then opening valve 20. After pumping the vessel 10 down to typically 1000 
microns, the valve 20 is closed and the vacuum pump 22 turned off. 
The vessel 10 is then pressurized with argon gas by opening valve 40 in 
line 32 until it is equalized with tank 34C pressure typically 100 psi. At 
this time, internal heating of the vessel 10 is started in a conventional 
manner utilizing an internal furnace (not shown). Valve 48C is then closed 
and valves 48A and 48B are opened to equalize the pressure in the vessel 
10 with storage tanks 34A and 34B typically at 600 psi. At that time, 
valves 48A, 48B and 40 are closed isolating the vessel 10. 
The pressure is then increased in the liquid argon storage tank 64 to 
typically 100 psi. Valve 76 is then opened to allow liquid argon to flow 
from the liquid argon storage tank 64 into and through the subcooler 78 
into line 70. The subcooler cools the liquid argon by means of a boiling 
bath of cryogenic liquid. The boiling bath liquid can be either liquid 
nitrogen or liquid argon from the storage tank 64. If liquid nitrogen is 
used, the gas is vented to the atmosphere; if liquid argon is used the 
argon gas is returned to the storage tank 64. The boiling liquid absorbs 
its heat of vaporization from the liquid argon being subcooled, reducing 
its temperature. This cooling is desired to insure a sufficient net 
position suction head to the pump to prevent boiling and resulting in 
cavitation in the pump when the additional heat energy from the pump is 
absorbed by the process stream. 
Pump 72 and the vaporizer unit 74 are comprised of conventionally several 
direct electric heating modules typically capable of 150 kw total input. 
These modules typically consist of the liquid argon coils in a solid block 
which also contains the heater elements. The subcooled liquid argon enters 
the pump 72 at a rate of 5.4 gpm and pressure of 50 psi typically where 
its pressure is raised to typically 50 to .about.15,000 psi. The pump 72 
discharges the liquid argon into the vaporizer unit 74 where it is heated 
and vaporized. The argon gas discharged from the vaporizer 74 is typically 
from 50 to 15,000 psi and at a temperature range from 70.degree. to 
130.degree. F. with a flow rate of 600 scfm. 
As the vaporizer discharged argon gas pressure reaches that of the vessel 
10, which is typically at 600 psi, as measured by the pressure transducer 
80, valve 84 is opened. The vessel 10 is then pumped to a working pressure 
of 15,000 psi typically with the furnace hot zone being at a typical 
temperature of 2200.degree. F. The time typically required to reach 15,000 
psi in a vessel typically 5 feet ID.times. 14 feet IL containing a furnace 
hot zone at 2200.degree. F. of 4 feet ID.times. 10 feet IL would be 11/2 
to 4 hours. 
When the desired vessel 10 pressure is reached, typically 15,000 psi valve 
84 is closed and simultaneously pumping is stopped by closing valve 76, 
and shutting down the subcooler 78, pump 72 and vaporizer 74. 
After the compacting cycle time, typically in the range of 3 to 6 hours, 
the power to the furnace is turned off and the power metal charge allowed 
to cool. Typically in 4 hours in the vessel 10 described above, the load 
temperature would be 1000.degree. F. and the pressure 9000 psi. At this 
time, reclaim of the argon gas within the vessel is initiated by opening 
valves 40 and 48A. When tank 34A reaches 2300 psi, valve 48A is closed and 
valve 48B is opened until the pressure in the vessel 10 and tank 34B 
equalized at a typical pressure of 1500 psi. At this time, valve 48B is 
closed and valve 48c is opened until the pressure in the vessel 10 and 
tank 34c equalizes at a typical pressure of 300 psi. Valves 48c and 40 are 
then closed and valve 26 opened to exhaust the remaining 300 psi of argon 
gas within the vessel 10 out through line 24 to the atmosphere. When the 
vessel 10 pressure reaches atmospheric, valve 26 is closed and the closure 
14 is removed. Unloading of the vessel 10 is accomplished in a 
conventional manner. 
It is understood that the temperature limits recited herein for argon and 
nitrogen are for operation at a pressure of 1 atmosphere, and that these 
recited temperatures will change in accordance with changes in pressure.