US3679807A

US3679807A - Die-furnace, especially for the fabrication of sintered products

Info

Publication number: US3679807A
Application number: US141800A
Authority: US
Inventors: Jacques Carcey
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1970-05-12
Filing date: 1971-05-10
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: CH532440A; GB1313462A; BE766576A; DE2123105A1; NL7106443A; FR2087235A5

Abstract

The die-furnace comprises a cylindrical die of refractory material, a banding assembly which surrounds the die externally and transmits a radial compressive force to the die by means of packing material, an induction heating device located within the packing material and constituted by a cooled primary coil connected to an external high-frequency power supply and by a coaxial secondary circuit which surrounds the die and in which a current is induced by the primary coil.

Description

[ 1 July 25, 1972 United States Patent Carcey 77 man 33 11. 0 1 a/ 9 1 "1 2,908,739 10/1959 Rummel 3,311,695 3/1967 Kasper 3,303,258 2/1967 Junkev...........................

FOREIGN PATENTS OR APPLICATIONS [54] DIE-FURNACE, ESPECIALLY FOR THE FABRICATION OF SINTERED PRODUCTS [72] Inventor:

France Jacques Carcey, Rives, France [73] Assignee: Commissariat A LEmergie Atomique,

Paris, France May 10, 1971 eynolds norney-Cameron, Kerkam & Sutton ABSTRACT Primary Examiner-J. V. Truhe Assistant Examiner-B. A. R

[22] Filed:

[21] Appl. No.:

ounds the die and in which a current isi ry coil.

nduced by the prima- References Cited 5 Claims, 2 Drawing Figures UNITED STATES PATENTS 2,437,127 3/1948 Richardson............................219/149 DIE-FURNACE, ESPECIALLY FOR THE FABRICATION OF SINTERED PRODUCTS This invention relates to an apparatus comprising a pressure die which is primarily intended to permit the compression of a powder to be sintered and a furnace which is combined with the die and serves to heat said powder to a high temperature, all the conditions which are necessary for the sintering process being consequently brought together within a Single apparatus.

The chief aim of the invention is to increase the performances of conventional dies, to permit at the same time the possibility of a larger geometry for the parts employed and, while improving the uniformity of temperatures through the sintered product, to limit the thermal gradients between the heating source and the product through the die itself, thereby reducing the differences in thermal expansion during operation as a result of temperature variations. A further object of the invention is to provide a simple and relatively inexpensive assembly comprising a die of refractory material which is capable of being employed a large number of times without any danger of seizure, in particular at the time of displacement of plungers which carry out within the die the compression of the product to be sintered. Finally, a further object of the invention is to permit immediate application of the die-furnace under consideration to all the usual sintering processes and especially to those in which the compression of the product to be sintered is an isostatic compression obtained by means of a fusible metal or pressure-transmitting agent as well as to the processes in which said isostatic compression is accompanied by mechanical creep or plastic flow as a result of deformation and reduction in volume of an intermediate casing.

To this end, the die-furnace under consideration essentially comprises a die of a type known per se and constituted by a cylindrical element of refractory material, a banding assembly which surrounds the die externally, packing material placed between the die and the banding assembly for transmitting to the die the radial force which is produced by said assembly and an induction heating device located within the packing material and constituted by a primary winding having circular turns connected externally of the die-furnace to a highfrequency electric power supply and a completely closed secondary circuit in which a current is induced by the primary winding and which surrounds the die while being co-axial with said primary winding.

Apart from this main characteristic feature, a die-furnace as constructed in accordance with the invention also has a number of other related features which are preferably to be considered in combination but could also be considered separately and are primarily concerned with the following aspects:

the banding assembly is constituted by a series of superposed open loops insulated from each other and associated with mechanical means for positional locking of each loop with respect to the next in order to form a rigid non-deformable block the primary winding of the heating device is constituted by a tubular inductor which is cooled in particular by water, the primary circuit being passed through the banding assembly by means of a coaxial passage, the external portion of said passage being connected to ground in order to prevent induction phenomena within the banding assembly the secondary circuit comprises at least two concentric loops in closed relation the metallic sleeve of the secondary circuit is provided with cut-out portions in order that the compression forces applied by the banding assembly should be transmitted to the die by means of the packing material the packing material is constituted by a refractory substance and especially by concrete made from an insulating cement mixed with grains of high-density alumina. Advantageously, the concrete which is employed has a variable particle-size distribution within its mass between the banding assembly and the die the die-furnace is rigidly fixed to the cooled internal wall of a leak-tight outer enclosure for creating a vacuum, said enclosure being provided with an opening for establishing a communication with a pump set and dynamic seals providing a leak-tight passage through the enclosure wall for plungers which serve to compress the product to be sintered and penetrate axially into the die the product to be sintered can be surrounded if necessary by a deformable casing which is placed directly within the die.

Further properties of a die-furnace in accordance with the invention will be brought out by the following description of one exemplified embodiment and of a number of modes of application, reference being made to the accompanying drawings in which FIG. 1 is a view in perspective and in partial cross-section showing the die-furnace under consideration FIG. 2 is an axial half-sectional view of said die-furnace.

In FIG. 1, the reference numeral 1 designates the apparatus or die-furnace as a whole comprising a die 2 of a type which is known per se and designed in the form of a heat-resisting circular cylinder which is open at both ends and formed of sintered alumina or any other material which is capable of carrying out this function. A product 3 to be sintered at a suitable temperature and pressure is placed within said cylinder and is either a ceramic or metallic refractory powder which has been pre-densified in the cold state in the most suitable manner or a powder of the same type which is placed within a sealed staticvacuum enclosure. By reason of the high axial pressures to which the product to be sintered is subjected within the die 2 and which give rise in particular to deformation of the product so that an appreciable radial force is applied by said product against the internal surface of the die, said die must be maintained externally in order that it may be permitted to withstand the stresses developed therein.

To this end and in accordance with the invention, the diefumace under consideration is provided at its periphery with a banding assembly 4 which will be described in greater constructional detail with reference to FIG. 2, said assembly being so arranged that a system of forces directed towards the axis of the die 2 is continuously applied to this latter. The region which is located between the die 2 and the banding assembly 4 is filled with packing material 5 preferably consisting of refractory concrete which, in the exemplified embodiment herein described, is formed of insulating cement and grains of high-density alumina.

Moreover, in accordance with another essential arrangement of the invention, the die-furnace l is associated with an induction heating device constituted by a primary winding 6 and a completely closed secondary circuit which is formed in the embodiment illustrated in FIG. 1 by means of two

coaxial sleeves

7 and 8 which surround the die 2 and are joined together by means of radial portions 9 and 10. The primary winding 6 is preferably of the cooled inductor type consisting of a tubular conductor 12, a cooling fluid 11 which is preferably water being circulated within said conductor. The primary winding 6 is passed through the banding assembly 4 by means of a coaxial device 14 which is also cooled the highfrequency current is admitted at the center the return to the exterior which is insulated from the admission by means of a suitable insulator 13 is preferably the high-frequency ground. This penetration device prevents any stray induction within the banding assembly 4.

Reference will now be made to FIG. 2 which shows more precisely a few constructional details of the die-furnace under consideration. in particular, it is apparent from this figure that the external banding assembly 4 is constituted by means of a series of open and superposed loops 15 which are metallic, for example, and insulated from each other, said loops being disposed in an irregular manner and associated with interlocking means constituted by the packing material and/or parts of high-density alumina which serve to secure the abovementioned loops with respect to each other in order to constitute a rigid non-deformable block at the periphery of the die-furnace. In order that the banding forces arising from the above-mentioned assembly 4 should be transmitted through the packing material 5 to the die 2, the

sleeves

7 and 8 which constitute the secondary circuit of the heating device are provided with a series of openings such as 17 and 18 which are suitably distributed across the surface of said sleeves.

The die-furnace can also be arranged so that the external surface of the banding assembly 4 should be coupled to the wall 19 of a sealed enclosure 20 which completely surrounds said assembly and is connected by means of a duct 21 to a pump set (not illustrated) which serves to create a vacuum within the enclosure and especially around the product 3 to be sintered which is placed within the die 2. In order to carry out the compression of the product 3, the enclosure 20 is associated with two plungers which are designated respectively by the

reference numerals

22 and 23 and pass through the end-

walls

24 and 25 of said enclosure with interposition of

dynamic seals

26 and 27, said

plungers

22 and 23 being adapted to engage axially within the die 2. Finally, temperature-measuring instruments and especially thermocouples 28 are placed at different points of the die-furnace l and especially within blind-end bores 29 which are formed within the die 2 itself.

In the example described in the foregoing in which the diefurnace l is placed within the interior of a sealed enclosure 20 for compressing the product 3 in a dynamic vacuum by virtue of the

seals

26 and 27 the practical construction of the equipment makes it necessary to ensure that the winding 6 should pass through the enclosure wall by means of a vacuum-tight coaxial passage 30. By way of alternative and in an essentially equivalent manner insofar as the basic design of the die-furnace is concerned, the sealed enclosure 20 could be dispensed with and the product 3 to be sintered could be simply placed within a closed vacuum-type casing 31 which maintains a static vacuum within the product, in which case the die-furnace is placed in the surrounding atmosphere. In the alternative embodiment just mentioned, the compression of the product 3 is naturally carried out in a static vacuum.

The die-furnace as hereinabove described makes it possible to carry out with suitable values of pressure and temperature any known sintering technique and in particular the techniques which are contemplated in French Pat. No. 1 548 603 as filed in the name of Commissariat a lEnergie Atomique.

In the first case, the axial thrust of the plungers which penetrate into the die is converted by means of a metal which is capable of melting at the operating temperature into an isostatic pressure on the product which is accordingly subjected to simple compression, thereby resulting in rearrangement of the product by plastic flow. In the second case, the axial thrust is converted by means of a pressure-transmitting agent which can be either a fusible metal or a refractory powder at the operating temperature into isostatic compression on the product together with a plastic flow action produced by means of a deformation and reduction in volume of a deformable casing which surrounds the product to be sintered. As has already been indicated, the above mentioned operations can be carried out according to requirements either in a dynamic vacuum or in a static vacuum the last-mentioned solution has the advantage of simplification of the apparatus as a whole and especially of removing the problems involved in degassing of the packing material of the die-furnace itself. In the case of the method which results simultaneously in isostatic compression and in plastic flow, the product to be sintered under pressure is advantageously pre-densified in the cold state and in a vacuum, then placed within a compression casing which is deformable at operating temperatures, said casing being surrounded by suitable envelopes or shells which contain a pressure-transmitting agent and prevent any diffusion of this latter into the product itself this second method can be carried out either in a static or dynamic vacuum.

Whatever method may be chosen, the action of the

plungers

22 and 23 on the product 3 within the die 2 produces high radial thrust forces on the internal surface of said die. Owing to the nature of the refractory material which constitutes the die, this latter usually has low tensile strength and consequently high mechanical fragility. This disadvantage is completely removed by virtue of the external banding assembly 4 and by reason of the fact that this latter produces action on the die 2 through the intermediary of the packing material 5 while applying radial forces which are readily transmitted through the

openings

17 and 18 formed in the

sleeves

7 and 8 of the secondary heating circuit.

The die-furnace under consideration has another significant advantage which arises from the particular design concept of the heating device itself and makes it possible especially to limit the value of the thermal gradient within the die 2 proper. In point of fact, the heat build-up in the charging loop of the secondary circuit as a result of a resistive effect and the simultaneous heat build-up in the assembly consisting of product and casing under the induction heating effect makes it possible to maintain the die in a uniform temperature zone. Furthermore, the product itself may be heated by induction by means of the secondary circuit on condition that said product has the property of conductivity at the operating frequency, in which case the heat buildup takes place within the product itself.

The die-furnace under consideration makes it possible under the conditions described in the foregoing to contemplate either continuous or non-continuous sintering of any ceramic, refractory or metallic material at high temperatures which can attain 1500" C or more under pressures ranging from a few Kg/cm" to 2 T/cm or more. Moreover, the design concept of the apparatus as a whole is such that hot delivery of the sintered product from the mold can readily be carried out while retaining said product in a zone which is not subjected to mechanical stresses during cooling. The furnace makes it possible to form on the product a uniform-temperature zone which may be localized within the product itself if necessary. The structural design of the primary winding and of its grounded coaxial passage within the banding assembly prevents stray-induction effects, in particular within the loops of the banding assembly. The packing material can be chosen so as to have a different particle-size distribution according to the zones which are occupied and to constitute in particular an insulating material which plays a contributory role in improving the interlocking of the loops of said assembly.

A number of other advantages also arise from the foregoing description of the die-furnace and the most significant of these are as follows v The design concept of the heating device permits efficient banding by virtue of its transformer and cut-out portions while at the same time carrying out either direct sintering of metallic materials under pressure or alternatively sintering of refractory or ceramic substances if use is made of a sealed vacuum enclosure or an unsealed enclosure of metallic type. The die-furnace also has greater potentialities on the one hand as a result of banding by consolidation through the heating device and on the other hand by virtue of the uniformity of temperatures in the die and in the product as achieved by simultaneous heat build-up in the charging loop of the secondary circuit and in the assembly consisting of product and casing. Hot take-out, continuous pressing, incorporation with a pressing and evacuating system or incorporation with a standardized modular assembly for hot-pressing can be contemplated.

It will clearly be understood that the invention is not limited in any sense to the exemplified embodiment which has been more especially described with reference to the accompanying drawings but extends on the contrary to all alternative forms.

What we claim is: 1

1. A die-furnace especially for the fabrication of sintered products, comprising a die of a type known per se of a cylindrical element of refractory material, a banding assembly surrounding said die externally, packing material between said die and said banding assembly transmitting to said die the radial force produced by said assembly, an induction heating device within said packing material a primary winding for said device having circular turns connected externally of the die-furnace to a high-frequency electric power supply and a completely closed secondary circuit for said device in which a current is induced by said primary winding and surrounding and heating the external surface of said die and coaxial with said primary winding.

2. A die-furnace according to claim 1, wherein said banding assembly comprises a plurality of superposed open loops insulated from each other and interlocked by said packing material to form a rigid non-deformable block.

3. A die-furnace according to claim 1, wherein said secondary circuit includes at least two concentric loops in closed relation.

4. A die-furnace according to claim 3, wherein said loops are a metallic sleeve having cut-out portions whereby the compressive forces produced by said banding assembly are transmitted to said die by said packing material.

5. A die-furnace according to claim 1, said die-furnace being rigidly fixed to the cooled internal wall of a leak-tight outer enclosure supporting a vacuum, an opening in said enclosure communicating with a pump set and a leak-tight passage through said enclosure for plungers compressing the product to be sintered and axially entering said die.

Claims

1. A die-furnace especially for the fabrication of sintered products, comprising a die of a type known per se of a cylindrical element of refractory material, a banding assembly surrounding said die externally, packing material between said die and said banding assembly transmitting to said die the radial force produced by said assembly, an induction heating device within said packing material , a primary winding for said device having circular turns connected externally of the die-furnace to a high-frequency electric power supply and a completely closed secondary circuit for said device in which a current is induced by said primary winding and surrounding and heating the external surface of said die and coaxial with said primary winding.