EP0099066B2 - Process for manufacturing a composite article from chromium and copper - Google Patents

Description

The invention relates to a method for producing a composite material from chrome and copper as a contact material for medium-voltage vacuum circuit breakers.

The composite material CrCu with about 40 to 60% Cr has already proven itself as a contact material for vacuum circuit breakers. The component Cu ensures sufficient electrical and thermal conductivity, while the framework material Cr both reduces burn-off and, with its low melting point of about 2 173 K compared to tungsten, eliminates the risk of harmful thermal electron emission. In addition, the Cr greatly reduces the tendency of the contact pieces to weld and has good gettering properties.

Due to the miscibility gap in the Cr-Cu system, only powder metallurgical processes can be considered for the production of the composite material CrCu for the desired concentration range of about 40 to 60% Cr content. The most common is the production of compacts from Cr powder or CrCu powder mixtures, the pores of which are filled with liquid Cu after sintering. Such sintering impregnation processes and also the other known powder metallurgy processes are difficult to master because of the tendency of chromium to oxidize. In particular, there is a risk of poor porosity or impregnation errors due to poor wettability of individual grain areas or the formation of a passive layer. Even if they are only in the order of 5 to 50 µm, they can impair the switching behavior. In practice, this results in a certain spread in the breaking capacity.

From DE-A-25 21 504 a method for producing an electrode for vacuum switches or vacuum spark gaps is known, in which sintered bodies made of chromium in particular are stacked separately by spacers in a vacuum furnace and a copper disc is placed on the stack. After evacuation to 2 x 10- ² Pa, the arrangement is first set to 1050 ° C, i.e. a temperature below the eutectic temperature (1075 ° C 1348 K) in the chrome-copper system, and then to 1090 ° C, i.e. a temperature above the heated eutectic temperature. This is to result in a simultaneous or immediately successive impregnation of the individual chrome disks, it being necessary to ensure that the process is carried out close to the thermodynamic equilibrium state. This process is known as so-called continuous watering.

From DE-B1-25 36 153 a method for producing multilayer contact pieces for vacuum medium-voltage circuit breakers is also known, in which a pressed or sintered body made of, in particular, chromium is impregnated in a non-porous manner under vacuum, for which purpose the pressed or sintered body in a crucible is completely impregnated from iron or stainless steel with oxygen-free copper and then the impregnating crucible is at least partially removed. If a compact is made for this, impregnation at a temperature of 1150 ° C is possible.

DE-B1-25 36 153 already mentions that, as an alternative to producing a pressed or sintered body, chrome powder could also be poured into the crucible, so that the sintering and impregnation process can take place in a single heat treatment process. The specific procedure is not described.

In other known methods such. B. porous blanks produced by pressing or pouring metal powder, which either consist of pure Cr powder or in which one or more other powder additives are mixed with the Cr powder to achieve a liquid phase during sintering. The subsequent sintering in a high vacuum or pure protective gas at temperatures from 1 573 K to 1 773 K leads to the desired formation of sinter bridges between the powder grains, so that the structural strength increases, which enables problem-free handling of the porous sintered blanks after the sintering process. In a further operation, the blanks are then placed in impregnation molds or placed on impregnation pads, receive an amount of impregnation metal, in this case copper, corresponding to the pore volume, and are again heated in a high vacuum or pure protective gas above the melting temperature of the impregnation metal. In this case, infiltration of the porous framework occurs due to capillary forces.

• Beim Umchargieren der Öfen zwischen Sinter- und Tränkprozeß kommt es bei den stark getteraktiven Cr-Gerüsten zu einer Neubelegung der Gerüstoberfläche mit dünnen Oxid- bzw. chemiesorbierten Gashäuten, die die Benetzung mit dem flüssigen Tränkmetall erschweren. Aus thermodynamischen Gründen treten diese Oxidationsprozesse bereits unterhalb von etwa 1 000 K selbst im Hochvakuum und in reinem Schutzgas auf, da sich in wirtschaftlich anwendbaren Ofen keine Sauerstoffpartialdrücke unter 10-^s Pa erzielen lassen. Als Resultat dieser Erscheinung treten Tränkfehler auf, die sich in Form von Mikrolunkern und Poren äußern.

With the impregnation processes described above for the production of the Cr-Cu composite materials, however, despite the most careful working method, completely impeccable impregnations cannot be achieved: There are essentially three reasons for this:

• When the furnaces are re-charged between the sintering and impregnation processes, the highly getter-active Cr frameworks are re-coated with thin oxide or chemically sorbed gas skins, which make wetting with the liquid impregnation metal more difficult. For thermodynamic reasons, these oxidation processes already occur below around 1,000 K even in a high vacuum and in a pure protective gas, since no oxygen partial pressures below 10 ^s Pa can be achieved in economically applicable furnaces. As a result of this phenomenon, impregnation errors occur, which are expressed in the form of microholes and pores.

Due to the sintering process and the associated formation of sintered bridges, poorly accessible pore areas are obtained which are not or only incompletely reached by liquid impregnation metal. This is also the possibility of reducing substances such. B. to bring carbon over the liquid impregnated metal phase to the framework metal, so that in these residual pore areas, which result from the formation of sintered bridges, residual oxides are present which impair the switching capacity of the material.

The stiffening effect of solid sintered bridges considerably reduces the possibility of the framework material for deformation. If the Cr framework impregnated with Cu or alloys thereof is cooled from the infiltration temperature of the liquid impregnation metal, a volume deficit arises because of the different thermal expansions between Cr and Cu, which cannot be compensated for by a common, uniform shrinkage of framework and impregnation metal. This known phenomenon can also lead to imperfections and microporosities which are invisible in the light microscope and which can deteriorate the quality of the material for high-performance switching tasks.

Attempts have been made to avoid these disturbances. So z. B. Cr powder and Cu powder are mixed, this avoids direct contact of the Cr grains as far as possible and there are no or only isolated deformation-preventing sinter bridges in the subsequent sintering process. Although this manufacturing process eliminates the steric hindrance of the Cr particles, a sufficient switching capacity cannot be achieved with such a material. The reason for this is the interaction between the Cu powder, which is usually contaminated with about 500 ppm of oxygen, and the getter-active Cr powder. Even below 1 273 K, that is to say 1 000 ° C, the oxidation-friendly Cr powder is oxidized when Cu ₂ 0 dissociation sets in. Due to the high heat of oxidation of the Cr, stable surface oxides are formed which cannot be removed by normal vacuum degassing.

The invention is therefore based on the object to develop a new method with which it is possible to produce a high-quality contact material made of chrome and copper, which meets the requirements of vacuum medium-voltage circuit breakers up to 36 kV operating voltage and breaking currents above 30 kA, and in which the aforementioned sources of error as well as the use of Cu powder with a high oxygen content are avoided.

According to the invention, the object is achieved in that a method of the type mentioned is carried out in the sequence of method steps a) to g) with the parameters for pressure, temperature and holding time specified therein. Specifically, Cr powder is poured into a degassed mold, a piece of low-oxygen copper is placed on the Cr powder, then the mold is closed with a porous lid, then the mold is degassed in a high vacuum oven at room temperature until a pressure of less than 10- ² Pa is reached, then increased the oven temperature to the highest possible temperature below the melting temperature of copper, this temperature is approximately maintained for one hour constant, which reached a constant internal furnace pressure of less than 10- ² Pa, and then, without intermediate cooling, the furnace temperature is further increases to a final value of 100 K to 200 K above the melting temperature of the copper and maintain this temperature for about 20 to 30 minutes, after which the porosity contained in the Cr powder fill is completely filled with the liquid copper.

The furnace temperature just below the melting point of copper in a technical implementation can be 1273 K +28 K, an internal furnace pressure in the range of 10 ^-3 Pa preferably being achieved.

With the teaching according to the invention, a method is shown with which the so-called "powder drinking" already mentioned in the prior art can be carried out for the first time in an economical manner. The specific choice of parameters enables a comparatively fast procedure to be carried out, a high-quality contact material for use as switch contacts in vacuum medium-voltage circuit breakers being produced.

Chromium-produced or electrolytically produced chromium can be used for the process according to the invention. In the first case, the Cr powder should have a particle size distribution of 50 μm to 200 μm, but preferably with proportions of at least 150 gm; in the second case the particle size can be below this, in the range from 25 µm.

Furthermore, it has proven to be expedient to use a working form made of graphite, because carbon is soluble in the liquid copper in a small amount and is therefore used as a reducing agent for Cr oxide impurities via a transport in the liquid phase.

It is particularly advantageous in the invention that no strength-increasing sintering process with the formation of stable sintering bridges is carried out, but that the Cr powder fill located in a mold is used directly. The pore volume of the powder filling can be completely filled with liquid copper without re-loading the furnace and additional handling of sintered blanks, so that a practically non-porous composite material results.

The invention is described in detail using the following exemplary embodiments:

also unterhalb der Schmelztemperatur von Kupfer (TSm - 1 356 K), ist die eigentliche Entgasungstemperatur erreicht, die für eine Stunde, mindestens jedoch aber bis wieder ein Ofeninnendruck weniger als 10-² Pa erreicht ist, beibehalten wird. Anschließend wird ohne Zwischenabkühlen die Temperatur weiter erhöht, bis zu einem Endwert, der 100 K bis 200 K oberhalb des Schmelzpunktes von Kupfer liegt. Die Temperatur kann z. B. 1 473 K sein, wobei bei dieser Temperatur nach etwa 30 Minuten ein praktisch vollständiges Ausfüllen der Poren in der Cr-Schüttung mit flüssigem Kupfer erreicht ist. Bei einem anderen Ausführungsbeispiel wird elektrolytisch hergestelltes Chrom verwendet, das einen maximalen Sauarstoffgehalt von ebenfalls 500 ppm hat. Das daraus erzeugte Cr-Pulver kann aber in diesem Fall eine Teilchengrößenverteilung haben, die kleiner als bei aluminothermisch hergestelltem Chrom ist, beispielsweise mit Teilchengrößen ab 25 µm. Ansonsten werden die einzelnen Verfahrensteilschritte entsprechend dem ersten Beispiel durchgeführt. Nach vollständiger Porenfüllung wird der gemäß obigen Beispielen hergestellte Rohling unter Vakuum abgekühlt. Nach dem Erkalten kann der Cr-Cu-Verbundblock in Kontaktstücke der erforderlichen Geometrie zerlegt werden. Werden metallographische Anschliffe des Werkstoffes hergestellt, so ist erkennbar, daß der mit dem erfindungsgemäßen Verfahren hergestellte Verbundwerkstoff praktisch keine festigkeitssteigernden Sinterbrücken und praktisch keine Poren aufweist. Mit dem neuen Verfahren können somit reproduzierbar auf Cr-Cu-Basis Kontaktstücke erzeugt werden, welche geeignete Eigenschaften für Mittelspannungs-Vakuum-Leistungsschalter haben.When using aluminothermally produced chromium with a maximum oxygen content of 500 ppm, the Cr powder produced therefrom is filled into a previously degassed graphite mold with a particle size of at least 150 μm. The crucible has e.g. B. a diameter of 85 mm and a length of 250 mm and is filled to a height of about 180 mm with Cr powder. Oxygen-poor copper is placed on the Cr powder as a solid piece that fills the remaining crucible contents. The crucible is closed with a porous graphite lid and initially while degassed in a high vacuum oven at room temperature until a pressure in the range of 10- ³ Pa, is thus achieved less than 10- ² Pa. Then starting the heating, which is always interrupted when the pressure rises above 10- ² Pa. At a temperature of about

ie below the melting temperature of copper (TSM - 1356 K), the actual degassing temperature is reached which is maintained for one hour, however, but to a furnace internal pressure is reached less than 10- ² Pa again at least. The temperature is then increased further without intermediate cooling until a final value which is 100 K to 200 K above the melting point of copper. The temperature can e.g. B. 1 473 K, at this temperature after about 30 minutes a practically complete filling of the pores in the Cr bed is achieved with liquid copper. In another embodiment, electrolytically produced chromium is used, which also has a maximum oxygen content of 500 ppm. In this case, however, the Cr powder produced can have a particle size distribution which is smaller than in the case of chromium produced by thermothermal means, for example with particle sizes from 25 μm. Otherwise, the individual process substeps are carried out in accordance with the first example. After complete pore filling, the blank produced according to the above examples is cooled under vacuum. After cooling, the Cr-Cu composite block can be broken down into contact pieces of the required geometry. If metallographic cuts of the material are produced, it can be seen that the composite material produced with the method according to the invention has practically no strength-increasing sintered bridges and practically no pores. With the new process, contact pieces can be reproducibly produced on Cr-Cu basis, which have suitable properties for medium-voltage vacuum circuit breakers.

In the exemplary embodiments described on the basis of Cr-Cu, further elements can be used as additives in a manner known per se: for example, titanium and zircon as alloy components for copper can be used to improve the getter properties; on the other hand, iron, cobalt or nickel can be added to the Cr powder to thereby improve the wetting properties.

The handling of the additives mentioned for Cr-Cu composites is manageable in connection with the invention and does not change anything fundamentally in the production process described.

Claims (7)

1. Process for the production of a composite material of chromium and copper as contact material for medium voltage-vacuum-power switches, comprising the following process steps:

a) Cr-powder is poured into a degassed working mould,

b) a piece of oxygen-poor copper is placed on the Cr powder,

c) the working mould is then closed with a porous cover,

d) then the working mould is degassed in a high-vacuum furnace at room temperature until a pressure of less than 10-² Pa is reached,

e) the furnace temperature is then increased to the highest possible temperature below the melting temperature of copper (T_SMCu = 1083°C = 1356 K),

f) this furnace temperature is held constant for about an hour, after which a constant internal furnace pressure of less than 10-² Pa is reached,

g) then, without intermediate cooling, the furnace temperature is again increased to a final value of 100 K to 200 K above the melting temperature of the copper and this temperature is maintained for about 20 to 30 minutes, after which the porosity comprised in the Cr-powder mass is completely filled with the liquid copper.

2. Process according to claim 1, characterised in that the furnace temperature in process step e) is

3. Process according to claim 1, characterised in that the pressure in process step d) and f) is in the region of 10-³ Pa.

4. Process according to claim 1, characterised in that when aluminothermally-produced chromium is used the Cr powder produced therefrom has a particle size distribution of between 50 µm and 20 µm.

5. Process according to claim 4, characterised in that Cr powder with a particle size with components of at least 150 µm is used.

6. Process according to claim 1, characterised in that when electrolytically produced chromium is used the Cr powder produced therefrom has a particle size distribution of between 25 µm and 200 µm.

7. Process according to one of claims 1 to 6, characterised in that a working mould made of graphite is used.