US 3695597 A
In manufacturing sintered bodies, a compacted charge of powder forming a billet is enclosed in a sealed casing of soft metal such as steel from which the air has been evacuated. The casing and powder are subjected to pressure at high temperature in a furnace chamber to sinter the powder. Before its introduction into the chamber, the charge is heated outside the furnace to treating temperature. The charge is inserted from below into the furnace, after which the bottom opening of the furnace is closed and pressure medium is introduced. The furnace has an insulating sheath within the furnace chamber depending from the top of the chamber and terminating near its bottom. The material of the casing is so chosen that there will be substantially no interchange of material especially of carbon, with the material of the compacted billet.
Description (OCR text may contain errors)
Umted States Patent [151 3,6 Lundstrom [4 Oct. 3, 1972  FURNACE FOR HEAT TREATING  References Cited RESSURE OBJECTS UNDER P UNITED STATES PATENTS [721 Inventor: 3,628,779 12/1971 Lundstrom ..266/5 E we en Primary ExaminerGerald A. Dost  Assignee: Allmanna Svenska Elektrlska Ak- A" Jennings Bailey, Jr.
tiebolaget, Vasteras, Sweden [22 Filed: Sept. 1, 1970 ABSTRACT In manufacturing sintered bodies, a compacted charge  Appl' of powder forming a billet is enclosed in a sealed cas-  Foreign n m priority Data ing of soft metal such as steel from which the air has been evacuated. The casing and powder are subjected Oct. 24. I968 Sweden l436 /63 to pressure at high temperature in a furnace chamber to sinter the powder. Before its introduction into the chamber, the charge is heated outside the furnace to treating temperature. The charge is inserted from Related Apphcauon Data below into the furnace, after which the bottom open-  Continuation-impart o Set. Oct. ing of the furnace is closed and pressure medium is in- 16, 1969, abandoned. troduced. The furnace has an insulating sheath within the furnace chamber depending from the top of the chamber and terminating near its bottom. The material of the casing is so chosen that there will be substan  US. Cl. ..266/2 any no interchange of material especially of carbon gf 6/2 R I R with the material of the compacted billet.
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saw 3 OF 4 INVENTOR. HANS LUNDSTR'OM FURNACE FOR HEAT TREATING OBJECTS UNDER PRESSURE RELATED APPLICATIONS Thisapplication is a continuation-inpart of application Ser. No. 866,885, filed Oct. 16, 1969 and now abandoned.
BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to a method, when pressure-sintering powder bodies using a vertical furnace provided with pressure chamber, for simultaneously treating a material at high temperatures, up to l,500 C., and high pressure, preferably 500 bar and above.
2. The Prior Art A furnace for treating objects under high pressure is earlier disclosed in the US. Pat. application, Ser. No. 855,911 filed on Sept. 8, 1969 and entitled Furnace for Heat-Treating Objects Under High Pressure, assigned to the assignee of the present application, now US. Pat. No. 3,598,378, issued Aug. 10, 1971.
If metal powder is enclosed in a sheath, the sheath evacuated, sealed and, together with its contents, subjected to high pressure and high temperature simultaneously, the metal powder sinters to a solid body at a lower temperature than at atmospheric pressure.
By sintering under high pressure it is possible to achieve greater density than with sintering at atmospheric pressure and thus better properties of the finished product in many respects. The method seems to have great advantages in the production of alloys which, with conventional casting of molten metal into billets, easily become coarse-grained and have a tendency to segregate upon solidifying. By manufacturing these alloys of fine-grained powder and then sintering bodies formed from the powder with hot isostatic compression, a homogenous and very finegrained structure is obtained.
SUMMARY OF THE INVENTION Hitherto the method has only been suitable for laboratory use or for production on a very limited scale. However, the method according to the invention is suitable for industrial production so that pressuresintering can be a realistic alternative in many fields to other methods of manufacture. The method is essentially characterized in that the evacuated casing filled with powder is heated at atmospheric pressure outside the furnace provided with a pressure chamber, inserted from below in the furnace chamber which is heated to sintering temperature and located inside the pressure chamber, through a charging opening, and the pressure chamber closed, a pressure medium such as helium or argon is pumped into the pressure chamber, pressure and temperature are set at suitable values for sintering for a predetermined length of time, the pressure is reduced, the pressure chamber opened and the charge taken out through the charging opening in the bottom of the furnace. For certain purposes the pressure medium may be nitrogen or hydrogen. The powder in the evacuated sheath may be isostatically compacted before heating at atmospheric pressure before insertion into the pressure sintering furnace.
The invention also relates to a furnace for carrying out the method.
Furnace for pressure sintering normally comprise a cylindrical pressure chamber suspended in a stand and consisting of a high pressure cylinder with end closures projecting into it, a press stand which surrounds the high pressure chamber during the compressing operation and takes up the axial forces which a pressure medium in the pressure chamber, usually an inert gas, exercises on the end closures, insulation arranged in the pressure chamber between a furnace chamber and the walls of the pressure chamber, the insulation consisting of a cylindrical sheath with insulating lid and bottom, and transport members to move the press stand between a charging position and a compressing position. Furnaces having high pressure cylinders with threads at their ends and threaded end closures which take up the axial forces are practical at reasonable pressures, but are very risky at high pressures. Argon is usually used as pressure medium. At high pressure argon has very high density but at the same time low viscosity, only 4-5 times that of air at atmospheric pressure, and is thus very mobile. Since, with respect to the density, it also has great heat capacity, it is important that the insulation surrounding the furnace chamber itself is designed in such a way that the least possible convection is obtained between the furnace chamber and the inner walls of the pressure chamber so that the heat losses are not unreasonably great. Conventional furnaces have apermanent lower end closure provided with insulation, as the bottom. This supports an insulating cylindrical sheath and heating elements. The furnaces are charged from the top. It is therefore necessary to have a detachable lid for the sheath, which involves considerable sealing problems. These are superable if the furnace can be sealed and opened cold. If the seal leaks gaps may arise through which hot gas may seep from the furnace chamber. The lid seal cannot be cooled directly and it is hardly possible to obtain a reliable seal if the lid is placed on a hot furnace. So that the internal parts of the furnace shall not be subjected to temperature shocks when the lid is opened and the hot gases arise and are replaced by cold air, the furnace must cool before it is opened. Rapid cooling of the inner metal of the sheath may also damage the insulating layer inside the metal so that it cracks and the insulating capacity decreases. Furthermore, it is not advisable for the hot sheath or heating elements to be subjected to oxidizing atmosphere. If there is molybdenum in the sheath of heating elements they are destroyed in a very short time. If air penetrates into the furnace chamber it must be evacuated and rinsed to remove air particles, and this takes time. All this contributes to a lengthy cycle time which, in view of the high cost of the furnace, is a great disadvantage.
The furnace according to the invention is designed so that it can be kept continuously filled with inert gas and thus also continuously heated to operating temperature since the insulating sheath and heating elements are not subjected to rapid alterations in temperature or contact with oxygen in the air. It can thus be emptied without previous cooling and charged with heated new powder bodies, especially billets of high-speed steel for rolling mills. It takes an extremely long time to heat billets to an even temperature all the way through, especially large billets. With the process suggested the billets can be heated to sintering temperature at least substantially at atmospheric pressure in furnaces of conventional type.
The time for an operating cycle can thus be kept low and the costs will be considerably lower than with conventional furnaces. Pressure-sintering powder will thus become a method of manufacturing which can be generally used, for example for manufacturing ingots from powder. The method is particularly advantageous for such alloys where fine-grained structure, uniform alloying or bubble-free goods cannot be achieved by conventional casting methods. The furnace is characterized substantially in that the upper end closure of the pressure chamber is permanently arranged in the high pressure cylinder, that the insulating sheath and the insulating lid are suspended in the upper end closure and that the lower closure supports the insulating bottom and the charge and can be raised and lowered by means of a lifting mechanism while the furnace is being charged from below. This design prevents hot furnace gas flowing to the gap' between the sheath and the walls of the pressure chamber if there are leaks between the insulating sheath and the insulating lid. The seal between the insulating lid and the sheath therefore is not of the same vital importance as in earlier construction. It is, furthermore, permanent and is not opened when the furnace is charged. It is also cooled effectively, which considerably reduces the likelihood of leakage. The insulating lid is designed or inserted in the sheath so that a closed space is formed between the lid and the end closure, this space communicating with the furnace chamber only through a pressure-equalizing opening. This space can be used for connecting heating elements to lead-in cables and facilitates assembly and dismantling of the furnace.
In one embodiment of heat body a tube surrounding the furnace chamber is suspended in the insulating lid. The heating elements of the furnace are suspended on the outside of the tube and can be held in position with the help of vertical bars having slots through which the elements pass. The tube may even be provided with internal, vertical protective bars. If the tube finishes a little way below the insulating lid and if it is suspended in this by means of said bars, openings are obtained through which the elements can be drawn to the bushings in the lid. Electric current and pressure medium are of course supplied through the stationary upper end closure. It is convenient to make the lower end closure in two parts with an annular part permanently suspended in the high pressure cylinder and a lid arranged inside this cylinder, which can be raised and lowered and which supports the insulating bottom and the charge. An insulating ring is attached to the annular part and projects up into the insulating sheath. When heating elements are suspended on a tube, the ring is pressed up into a gap between the sheath and the tube so that a labyrinth seal is obtained in the lower part of the furnace between the gap between the high pressure cylinder and the sheath and the furnace chamber itself. In this embodiment it is possible to effectively protect the insulating sheath from contact with the air when the lid of the end closure has been removed. By continuously supplying gas to the gap between the high pressure cylinder and the insulating sheath, a downwardly flowing gas stream is obtained which in the lower part is deflected and forced into the gap between the insulating sheath and the tube carrying the heating elements. The continuous supply of gas, for example argon, to this gap ensures that the insulating sheath and the heating elements are always situated in a controlled inert atmosphere which, even at high temperatures, does not damage the material. Any air can be limited to the lower part of the furnace chamber. If the tube surrounding the furnace chamber is of a material, for example inconel, which is resistant to oxidization even at high temperatures, this small quantity of air need not cause any material damage. The invention thus permits great freedom of choice as to the material for most of the vital components of the furnace. Radiation protection means may also be built into the annular part of the end closure and are automatically lowered when the bottom is lowered so that the sealing surface between the bottom andthe annular part is protected during charging against heat radiation from the charge. The annular part may also form a protective or guiding ring which prevents the charge from being inserted in such a way that the inner parts of the furnace come into contact with the charge and are damaged.
A further matter of importance when selecting the material for the casing is to prevent the migration or interchange of material between the casing and the enclosed billet of a powder mixture to be sintered. The migration of carbon must be particularly observed.
BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described with reference to the accompanying drawing:
FIG. 1 shows a side view of a furnace, partly in section,
FIGS. 2 and 3 sections of a part of the lower and upper parts of the high pressure chamber, respectively, and
FIG. 4 a horizontal section through the high pressure chamber.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings, 1 designates a press stand which is movable between the position shown in the drawing and a position where it surrounds the high pressure chamber 2. The press stand is of the type consisting of yokes 3 and 4, spacers 5 and a strip mantle 6. The stand is supported by wheels 7 running on rails 8. The high pressure chamber 2 is supported by a pillar 9. It contains a high pressure cylinder consisting of an inner tube 10 and a surrounding strip mantle 11 with end rings 12 holding the strip mantle together axially and forming grips for brackets by which the high pressure chamber is attached to the pillar 9. The chamber 2 has an upper end closure 13 which projects into the tube 10 of the high pressure chamber. Between the tube and the end closure is a sealing ring 14. The furnace chamber 15, in which the charge 16 is inserted, is surrounded by an insulating sheath 17 consisting of three concentric metal tubes 18, 19 and 20, layers 21 and 22 of insulating material and two end rings 23 and 24. The sheath 17 is suspended by means of the ring 23 in the upper end closure 13 and connected to this lid closure in a gas-tight manner. In the upper part of the sheath 17 is an insulating lid 25 consisting of a metal casing 26 filled with insulating material 27 and provided with bushings 28 for electric conductors to heating elements 47. The casing 26 is attached to an attachment ring 29 clamped between the ring 24 and the end closure 13. In the ring 29 is a groove for the sealing rings 30 and 31. Between the lid 25 and the upper end closure 13 a closed space 32 is formed which communicates with the furnace chamber only through a pressureequalizing opening 33. In the upper end closure is a lead-in channel 34 with a sealing member 36 for a feeding conductor 35. In the closure is also a channel 37 for the supply of pressure medium and a groove for the sealing ring 14. The channel 37 opens into the gap 39 between the inner wall of the tube 10 and the outer tube 18 in the sheath 17. The heating elements 47 are supported by a tube 40 which surrounds the furnace chamber 15. The elements are attached to the outer side of the tube 40 by means of rails 41 having slots through which the heating elements can pass. The tube 40 is suspended in the lid 25 by means of the protective rails 43 on the inside of the tube. Between the lid 25 and the tube 40 is a gap 44, so that openings are formed through which the connecting part of the heating elements can pass from the gap 46 between the tube of the sheath l7 and the tube 40. Above the upper end closure 13 is a pressure plate 45 with slots for the electric conductor 35.
The pressure chamber has a lower end closure consisting of an outer annular part 50 permanently attached to. the high pressure cylinder and a lid 51 projecting into this part. In both parts of the end closure are slots for sealing rings 52 and 53. A metal ring 54 is attached to the lid 51. In this is a thick layer of insulating material 55 and a plate 65 on which the charge is placed. This plate is preferably made of a material having good heat conductivity and may be provided with radial channels 66 and 67 between the periphery and a central space 68. If the plate 65 has good heat conductivity the central parts of the billet 16 can be supplied with heat through the plate. When the gas in the space is cooled it circulates from the gap 69 in through the channel 66 and out through the channel 67, which contributes to more rapid heating of the central parts of the billet. On the ring 50 is an insulating ring 56 which projects into the gap 46 so that between the lower part of the furnace chamber 15 and the gap 39 a labyrinth seal is formed. In the ring-shaped part 50 metal rings 57 and 58 are arranged which fall a little way away from each other during charging when the lid 51 is lowered so that the sealing surface in the ring 50 against which the seal 53 abuts is protected from heat radiation from the charge 16 during charging. The lid 51 is attached by brackets 59 to a casing 60 which runs along a guide 61 attached to the pillar 9. The casing can be lowered by an operating cylinder 62, the operating rod 63 of which is attached at bracket 64 on the casing 60.
The casing enclosing the powder is subjected to high temperature during sintering and a material interchange may take place between the powder and the material of the casing. For alloying substances having high diffusion ability such as, for example, carbon, a considerably conversion may take place. It is therefore important that the material chosen for the casing should have approximately the same carbon activity at the sintering temperature as the material in the powder enclosed in the casing. It has been found that the carbon activity for a casing material of steel plate having 0.10% C, 0.20% Si and 0.35% Mn and powder material containing 0.85% C, 4.0% Si, 6% W, 5% Mo, 2% Va and the remainder Fe is approximately the same. When the sintering takes place at l,l50 C. at a pressure of l kbar, the alterations in the boundary layer for the powder material mentioned are negligible.
In lieu of soft low-carbon steel, other materials such as soft stainless steel and other metals may be used, which are sufficiently deformable to permit the exertion of most of the pressure to the enclosed powder billet and which do not interchange elements, especially carbon, to a substantial degree with the material of the billet.
The device is utilized in the following manner: A charge in the form of a cylindrical billet 16, consisting of a powder enclosed in a casing from which air has been evacuated, is heated to about the sintering tem perature in a preheating furnace, not shown. This heated billet is placed upon the metal plate 65, carried by the lower lid 51 in lowered position. The lid 51 is then lifted by the cylinder 62 to the position shown in the drawings. The press stand 1 :is then moved to the right from the position of figure l to a position where it surrounds the pressure chamber 2. Pressure medium from a pressure medium source (not shown) preferably heated, is then supplied to the pressure chamber through the channel 37 in the upper end-closure 13. The upwardly and downwardly directed forces upon the upper end and lower-end closures, produced by the pressure medium, are taken up by the press stand 1. During the pressure sintering of the billet 16, the temperature is adjusted and kept at a level suitable for the treatment by the heating element 47, connected to a current source by a feeding conductor 35. After the treatment, and after lowering the pressure in the pressure chamber, the press-stand l is moved back to the position shown in FIG. 1, whereafter the lid 51 with billet 16 is lowered and the treated billet replaced by another preheated billet to be sintered.
When the furnace is loaded from below, a bubble of inert gas will remain inside the pressure chamber and prevent air from ruining hot furnace parts, such as the insulating sheath or electric heating element if the furnace is opened when still hot. It will therefore be unnecessary to cool the furnace between chargings. The powder body, for instance, in the form of a billet, can be heated to the treating temperature outside the high pressure furnace. The heating time for a large diameter billet is 5-10 hours and the necessary treating time in the high pressure furnace perhaps about 10 percent of this time. The cost of a preheating furnace furthermore is only a fraction of the cost of the high pressure furnace.
The invention is of course not limited to the embodiment shown, Many variations are feasible within the scope of the following claims.
l. Furnace for use in a method of manufacturing sintered bodies which comprises enclosing a charge of powder in a casing, evacuating air from the casing, sealing the casing, inserting the casing and its contents into a furnace having a furnace chamber which is enclosed in a pressure chamber, subjecting the sealed casing to high pressure at high temperature so that the powder sinters, the evacuated casing being heated outside the furnace, inserted from below into the furnace chamber which is heated to sintering temperature and located inside the pressure chamber through a charging opening, the pressure chamber being closed and a pressure medium being pumped into the pressure chamber while setting the pressure and temperature for sintering, for a predetermined length of time, which furnace comprises a cylindrical pressure chamber including a high pressure cylinder with a furnace chamber therein, upper and lower end closures projecting into said cylinder, insulation in the pressure chamber between said furnace chamber and the walls of the pressure chamber, which insulation comprises an insulating sheath with insulating lid and bottom, and in which the upper end closure of the pressure chamber is permanently secured in the high pressure cylinder, the insulating sheath and the insulating lid are suspended in the upper end closure and the lower end closure supports the insulating bottom and the charge and can be raised and lowered while the furnace is being charged from below.
2. Furnace according to claim 1, in which resistance elements to heat the furnace are suspended in the upper end closure.
3. Furnace according to claim 1 in which the insulating sheath is connected in a gas-tight manner to the upper end closure.
4. Furnace according to claim 3, in which there is a space between the insulating lid and the upper end closure and the furnace chamber through a pressureequalizing opening connecting said space to the furnace chamber.
5. Furnace according to claim 1, comprising a tube suspended in the insulating lid and surrounding the furpace and resistance elements to heat the furnace supported by said tube.
6. Furnace according to claim 5, in which the resistance elements are arranged on the outside of the supporting tube.
7. Furnace according to claim 6, comprising vertical rails with horizontal slots facing the tube, the resistance elements being supported in said rails and passing through said slots.
8. Furnace according to claim 7, in which the vertical rails are carried by the tube carrying the resistance elements.
9. Furnace according to claim 6, in which the tube carrying the resistance elements is suspended in the insulating lid by means of said rails and at a distance from the lid so that openings are formed between lid and tube between the rails.
10. Furnace according to claim 5, in which there are gas-tight bushings in the insulating lid for the resistance elements.
11. Furnace according to claim 1, including supply conduits for said resistance elements passing through the upper end closure.
12. Furnace according to claim 1, in which the lower end closure comprises a ring-shaped part permanently suspended in the high pressure cylinder and a lid inside this ring-shaped part which can be raised and lowered and which supports the insulating bottom and the char e.
13 Furnace according to claim 12, in which the ringshaped part of the end closure supports an insulating ring which projects into the insulating sheath so that between this and the ring a narrow gap is formed.
14. Furnace according to claim 12, in which in the ring-shaped part of the end closure is arranged a ringshaped axially lowerable radiation protection which, when the lid is lowered, is lowered and protects the sealing surface in the ring-shaped part from radiation from the charge.