Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2001-03-02
2003-02-25
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C419S028000, C075S245000, C075S247000
Reexamination Certificate
active
06524525
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for producing a contact material made of copper and chromium in a proportion of 40 to 75 wt.-% copper and 25 to 60 wt.-% of chromium in the form of a semi-finished product, from which individual contact pieces are produced for use in vacuum switch devices. This invention also relates to contact materials in the form of semi-finished products for producing contact pieces for vacuum switch devices, as well as to contact pieces for vacuum switch devices.
BACKGROUND OF THE INVENTION
Contact pieces for use in vacuum switch devices, such as vacuum contactors, vacuum load disconnecting switches and vacuum circuit breakers for low voltage and intermediate high voltage, should be distinguished by low residual porosity, a low gas content, as well as high structural strength and high electrical conductivity. Moreover, the contact materials and contact pieces should be economical to produce.
The vacuum switch principle has gained worldwide acceptance for a considerable time in the high voltage range of voltages between 3 and 36 kV. The preponderant application relates to circuit breakers. Contactors up to a maximum of 12 kV are used to a lesser degree. The great dielectric stability of the vacuum switch and the completely emission-free switching, along with the further development of the production process and the accompanying reduction of costs make the vacuum switch principle also attractive for low-voltage devices. Contactors, circuit breakers and load disconnecting switches with nominal operation, or nominal currents and 100 to 1000 A, or 630 to 6300 A, at voltages up to 1000 (1500) V are mainly considered for this application of the vacuum switch principle.
The demands made on the above switch devices vary greatly. With vacuum circuit breakers for intermediate high voltage, the ability to switch off in case of a short circuit up to approximately 50 kA is preponderant. The number of operations at nominal current is around 30,000. But with a contactor, the number of operations of at least 500,000 are dominant. Moreover, no switch-off errors are permitted with a contactor, and it must be able to dependably switch off 10 to 12 times the nominal operating current and to switch on with separable weld spots, wherein the power of the drive mechanism is comparatively low. Although a vacuum contactor is not supposed to switch off a short circuit current, it must be capable, in particular in connection with low nominal operating currents, to conduct the current passed through the upstream connected fuse long enough until the fuse has interrupted the circuit. In this case the weld spots at the contact pieces must be separable.
But the vacuum circuit breaker for low voltage must have 2 to 3 times the capability of short circuit switch-off compared with circuit breakers for intermediate high voltage. Moreover, circuit breakers for low voltage are also used as motor switches, wherein no switch-off error is supposed to occur.
The load disconnecting switch, on the other hand, must be capable to switch on currents up to 20 times the nominal current. The weld spot on the contact pieces occurring during this must be separable by the drive mechanism.
Contact materials, or contact pieces, for vacuum switches on the basis of chromium and copper are known and are produced in various forms from meltable materials or in accordance with metal powder processes or sintering and infiltration methods.
A CuCr contact material for vacuum switches is known from German Patent Reference DE-OS 16 40 039, which consists of a sintered metal matrix of chromium, which is infiltrated with an infiltration substance made of copper. German Patent References DE 23 57 333 A1 and DE 25 21 504 A1 describe a sintered metal matrix as the contact material for vacuum switches, wherein aluminum or tin is added as embrittling aid to the main component, for example chromium, and this metal matrix is infiltrated with an infiltration substance made of copper, silver or alloys of these metals. Here, the sintering temperatures lie above 1200° C., while the melting temperature of the infiltration substance lies below the respective sintering temperature. Contact materials of the size and shape of the individual contact pieces are preferably produced with the above mentioned process.
A contact material for vacuum switches is also described in German Patent Reference DE 22 40 493 A1, which has a sintered metal matrix made of a metallic main component with a melting point above 1600° C. and a metallic side component with a melting point above the melting point of an infiltration substance, wherein the sintered metal matrix is impregnated with copper as the infiltration substance, for example.
A process for producing chromium-copper contact pieces for vacuum switches is known from U.S. Pat. No. 3,960,554, having a chromium content of 40 to 60 wt.-%, wherein in a first step the desired amount of chromium powder for the finished contact piece is mixed with a very small amount of copper powder and a compact is thus produced, which is subsequently sintered for producing a porous chromium matrix, which thereafter is infiltrated with a large amount of copper while sintering is continued until the chromium matrix is filled with copper in the desired proportion of 60 to 40 wt.-% Cu and 40 to 60 wt.-% Cr. This process initially operates with less than 10 wt.-% of copper powder for producing the sinter matrix, and the larger proportion of copper powder of at least 30 to 50 wt.-% is inserted exclusively by subsequent infiltration into the chromium sinter matrix.
The sintering processes for producing individual copper-chromium contact materials have the disadvantage that, because of the introduction of the entire amount of copper, or almost the entire amount of copper, in a liquid phase, namely by infiltration into the chromium sinter matrix, compacts of clearly increased size are created, which accordingly must be later worked by machining for obtaining the final shape of the desired contact piece.
A further method for producing individual contact pieces is a powder-metallurgical method, wherein a powdery mixture of the component with a high melting point, such as chromium, and of the component with a low melting point, such as copper, are pressed to form a blank, and the blank is subsequently sintered and the sinter body is further pressed, either cold or warm, for the purpose of compressing it, such as described, for example, in German Patent Reference DE 29 14 186 A, German Patent Reference DE 34 06 535 A1 and European Patent Reference EP 0 184 854 A2. With this method, the concentration of the components can be selected within a wide range, and the shape of the blanks almost corresponds to the final shape of the contact pieces. Extensive systems of hollow spaces, such as can occur in connection with inferior infiltration materials, do not appear, but such sinter materials have a residual porosity and a density, which usually is removed by at least 2% from the theoretical density of 100% and which has disadvantageous effects when used as a contact material for contact pieces for vacuum switch tubes. Thus, the efficiency of the pure sinter materials is limited.
To reduce the porosity of contact pieces made by powder metallurgy on the basis of copper and chromium for vacuum switch tubes, a two-stage method for compressing the powder compact is proposed by International Application PCT/DE89/00343, wherein powder compacts are sintered in a high vacuum, and wherein thereafter the sinter bodies are subjected to hot isostatic pressing at temperatures below the melting point of copper in a protective gas atmosphere at pressures between 200 and 2000 bar. With this very expensive process it is possible to still further reduce the porosity of the finished product and to achieve approximately 99% of the theoretical density.
A melting material of copper and chromium for contact pieces of vacuum switches is suggested by European Patent Reference EP 0 172 411 B1, wherein a compact is produced from a powder mixtu
Hauner Franz
Lietz Alfredo
Rolle Susett
Jenkins Daniel J.
Metalor Technologies International SA
Pauley Petersen Kinne & Erickson
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