Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Making composite or hollow article
Reexamination Certificate
2003-02-04
2004-11-23
Jenkins, Daniel (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Making composite or hollow article
Reexamination Certificate
active
06821477
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority based on Japanese Application No. 2002-066897, filed Mar. 12, 2002.
FIELD OF THE INVENTION
The present invention relates to a method of producing a copper-base sintered bearing material and, more particularly, to a method in which sintering powder is heated by a microwave oscillating device in a sintering furnace.
BACKGROUND OF THE INVENTION
As a copper-base plain bearing material used for internal combustion engines, industrial machines, or the like, the material which is produced by scattering copper alloy powder on a steel backing metal and by sintering it has generally been used. The procedure for producing this copper-base sintered bearing material is as described below. First, copper alloy powder is scattered on a steel strip and this material is caused to pass through a sintering furnace which is heated to a high temperature by an electric resistance heater. In the sintering furnace, the copper alloy powder is heated and sintered in a reducing atmosphere. Next, the material is rolled to densify the sintered layer, and succeedingly is subjected to re-sintering by a sintering furnace and rolling. Finally, heat treatment is performed as necessary.
In the above-described producing method, the temperature of material changes as follows: Although the copper alloy powder is scattered at room temperature, subsequently in the sintering furnace, the temperature reaches a sintering temperature (about 750 to 950° C. depending on the composition) in about 10 to 30 minutes, and the material is kept at that temperature for about two minutes to be sintered. Subsequently, the material goes from the sintering furnace to a cooling zone, where it is cooled for 5 to 15 minutes, so that the temperature of material decreases to room temperature.
As described above, in the sintering furnace, the material is heated to the sintering temperature in a short time, and is kept at the sintering temperature for a fixed time to be sintered. Therefore, the sintering furnace must have a high internal volume to prevent the furnace temperature from decreasing even if the copper alloy powder and the steel strip which are charged at room temperature absorb heat.
However, if the internal volume of sintering furnace is increased, the furnace wall area increases accordingly, so that much heat dissipates to the outside. Further, the volume of refractories constituting the furnace body increases, so that the quantity of heat for heating the refractories to a high temperature also increases. In particular, in a producing system in which the operation is stopped periodically, each time the producing work is started, the whole of sintering furnace must be heated from ordinary temperature, and the start-up time is long. As described above, the conventional sintering furnace produces a high heat loss, so that it is uneconomical.
SUMMARY OF THE INVENTION
The present invention has been achieved in view of the above situation, and accordingly an object thereof is to provide a method of producing a copper-base sintered bearing material, in which heat loss can be reduced to the utmost.
According to present invention, there is provided a method of producing a copper-base sintered bearing material, in which unpressurized copper or copper alloy powder or a green compact of copper or copper alloy powder is sintered in a sintering furnace, wherein a microwave oscillating device is used as heating means provided in the sintering furnace, and the copper or copper alloy powder is heated and sintered by the microwave oscillating device.
In the present invention, the method of producing it can be configured so that copper or copper alloy powder is scattered on a steel sheet, and subsequently the copper or copper alloy powder on the steel sheet is heated and sintered by the microwave oscillating device in a sintering furnace.
Also, the sintering furnace can be provided with an electric resistance heater and/or a high frequency induction heater as heating means in addition to the microwave oscillating device so that both of the microwave oscillating device and the electric resistance heater and/or the high frequency induction heater can be used. Further, the interior of the sintering furnace can be made in a reducing atmosphere or an inert gas atmosphere.
The circumstances before the present invention have been made are as follows.
(1) Conventional Concept of Microwave Heating of Metal
Conventionally, microwave heating has widely been applied to a dielectric substance (a substance that does not pass an electric current or that is difficult to pass an electric current). However, microwave heating has not been applied to heating of a conductive material such as a metal. This is based on that the mechanism of heating of a dielectric substance by microwaves is thought that the dipole of dielectric substance is oscillated by a change in electric field (intrusion of microwaves into a substance), heat is generated by the friction, and the dielectric substance is heated uniformly from the interior thereof, but in a conductive material such as a metal, the microwaves penetrate only a very thin surface portion, so that the dielectric substance has been thought to be unable to be heated.
(2) Microwave Heating of Bronze Powder Filled Into Alumina Boat
Although it had been thought that microwave heating of metal was unable to be performed as described above, the inventor tried sintering bronze powder (Cu—Sn alloy) by means of microwave heating to solve the above-described conventional problem of poor thermal efficiency of heating furnace. Microwave heating was performed to heat bronze powder, which was a specimen, filled into an alumina boat in an unpressurized state by using microwave heating batch furnace (output: 8 KW) having a general microwave oscillating device with a frequency of 2.45 GHz, which had been used for sintering of ceramics etc. As a result, although the temperatures of the boat and bronze powder were room temperature at first, the temperatures increased rapidly along with the start of microwave heating and reached to 800° C. within about 10 minutes, by which bronze powder could be sintered. When heating by microwaves was further continued from this temperature, the temperature of specimen further increased, and shortly the specimen melted.
The internal volume of the batch furnace used for this test was about 1 m3. For the ordinary furnace that uses an electric resistance heater as a heat source, since heat is taken away by the atmosphere and furnace wall at the early stage of heating start, rapid heating such that temperature is increased to 800° C. within about 10 minutes cannot be performed by any possibility.
(3) Microwave Heating of Alumina Boat Only
The result of the above-described test means that bronze powder can be heated by microwaves. However, the alumina boat used as a holding vessel for bronze powder is formed of a dielectric substance, that is, a substance that absorbs microwaves and generates heat. Therefore, it was thought that the rapid temperature rise of bronze powder is possibly caused by heat conduction from the alumina boat.
To solve this problem, the alumina boat was, singly heated (with no bronze powder being filled) by microwaves, and resultantly it was found that the temperature of alumina boat did not increase rapidly unlike the bronze powder. From this fact, it was found that rapid heating of bronze powder was not caused by heat conduction from the alumina boat.
From the above result, it was found that the temperature of bronze powder, which is a metal that has conventionally been thought to be unable to be heated by microwaves, reaches to the sintering temperature in a short time by means of microwaves, and hence a sintered material can be obtained.
(4) Microwave Heating in a State Wherein Bronze Powder is Scattered on Steel Sheet
To check whether the application to the production of a copper-base sintered bearing material in which copper alloy is sintered on a steel backing metal, which has been used widely as a bearing for in
Saitoh Yasushi
Shibayama Takayuki
Daido Metal Co. Ltd.
Jenkins Daniel
Rader & Fishman & Grauer, PLLC
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