JPS616218A - Powder metallurgical manufacture of electric contact piece of copper chromium solid solution for vacuum enclosed switch - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The object of the invention is a powder metallurgical production method for copper-chromium solid solution electrical contact pieces for vacuum-sealed switches.

From German Patent Application No. 2 346 179, chromium powder and copper powder with a powder particle size of 10 μm to 250 μm are mixed together, a contact piece preform is made from this powder mixture by pressing, and the preform is placed in a fluid or solid phase. sintering, followed by a relative density of at least 98 inches (2
It is already known to produce contact pieces for vacuum-sealed switches by final consolidation in a press operation in order to achieve a porosity of less than %. The proportion of chromium in these contact pieces is between 40 and 65% by volume, and the corresponding proportion of copper is 60% by volume.
35 to 35 volumes.

Further, from West German Patent Application No. 291.4186,
Chromium powder with a particle size of 74 to 150 μm and copper powder with a particle size of 35 to 150 μm are mixed together, contact pieces/preforms are pressed from this powder mixture, and these preforms are sintered in a solid phase in a vacuum. However, it is also known to produce contact pieces for vacuum-sealed switches by selecting the sintering temperature and the sintering time in such a way as to reach a relative density of at least 97 cm, corresponding to a porosity of not more than 3%.

The proportion of chromium in these contact pieces is 10 to 40% by weight.
, the proportion of copper is correspondingly ω% by weight.

From West German Patent No. 2822956, powder particle size 100μ
A chromium powder with a particle size of less than 5 μm and a copper powder with a powder particle size of more than 5 μm are mixed together, and from this powder mixture a chromium-copper pellet is first made by thermal granulation, and then a contact piece/preparation is made from the pellet by a pressing operation. Making contact pieces for vacuum-sealed switches by making compacts and sintering them in a hydrogen atmosphere until they reach a relative density of at least 90% corresponding to a porosity of not more than 10% in the solid phase. is publicly known. In these contact pieces, the proportion of chromium ranges from 1% to 5%, and the proportion of copper correspondingly ranges from 5% to 5%.
It is 0% by weight.

The #1 or most publications have discussed other methods, namely sintering of copper-chromium-powder mixtures in the fluid phase, or infiltrating sintered chromium frameworks with steel melts, or chromium powders in steel melts. We manufacture contact pieces for vacuum-sealed switches from copper and chromium by stirring them into

These methods will not be considered in detail here. The production of contact pieces by sintering a copper-chromium powder mixture in the solid phase has quickly proven to be well suited.

The invention therefore only refers to a method working with sintering in the solid phase.

The present invention relates to a powder metallurgical method for producing electrical contact strips for vacuum-sealed switches by sintering a copper-chromium-powder mixture in the solid phase, which provides optimum properties with respect to breaking capacity, burnout, dielectric strength and welding resistance. The problem is to provide a guide to the contact strips provided. This object is achieved by a method with the features of patent claim 1. Advantageous embodiments of the invention are defined in the claims 2 to 11.

Chromium powder is usually made by grinding. The chromium powder produced in this way usually exhibits a particle size distribution similar to a Gaussian distribution. Some known methods of manufacturing copper-chromium contact pieces produce coarse powder particles exceeding a predetermined particle size from chromium powder derived from grinding (West German Patent No. 2822956).
, and in many cases also a particulate fraction of less than a predetermined particle size (West German Patent Application Nos. 2914186 and 234).
No. 6179) Separate by sieving. It has already been proposed to screen out which particle size categories are inside. However, in all cases the original normal distribution of the particle size of the powder can be observed again. This is because only a certain range is separated from this normal distribution.

In contrast, according to the invention, a chromium powder is used for producing the contact piece preform, which no longer exhibits the original normal distribution and is no longer recognizable. Rather, an attempt is made to achieve a somewhat homogeneous particle size distribution, which means that the chromium powder produced by grinding is divided in the usual way into fractions of various particle sizes by a sieving process, and these fractions are then separated into fractions of different particle sizes. are mixed with each other so that in the new mixture there is a more or less uniform particle size distribution, ie there is more fine and coarse particle size in the mixture than in the raw powder with a Gaussian particle size distribution. Ideally, in order to achieve a uniform particle size distribution, the original chromium powder is divided into a very large number of correspondingly narrow particle size fractions by multiple sieving processes, and then these are re-divided in specific proportions. must be mixed with each other. However, in practice it is sufficient to divide the chromium powder obtained by grinding into relatively wide particle size fractions.

According to the invention, therefore, a division into particle size fractions each having a width of .mu.m is proposed.

The relatively uniform particle size distribution, apart from the state of the art proposed by the present invention, has a positive effect on the glass on the one hand on the small welding tendency of the contact pieces and on the other hand on the breaking capacity and dielectric strength, and on the other hand has a positive effect on the somewhat uniform particle size distribution of the chromium. The particle size distribution acts so that various properties of the contact piece after long-term operation, which inevitably affect the structure or surface of the contact piece due to electric arcing, do not change in a weighted manner.

The powder particle size distribution shown in claim 2 has proven to be particularly suitable. The powder mixture from which the contact strip preform is made preferably contains 35 to 40% by weight chromium, with the balance being copper.

Particle size selection is much easier for copper powder than for chromium powder. Preferably, fine-grained copper powder with an average particle size of less than 5 μm is used.

After the copper powder and chromium powder are thoroughly mixed together in the predetermined proportions, a contact piece/preform is then formed by pressing. Appropriate cold pressing of the powder avoids oxidation of the chromium. The height of the Luss pressure is determined by the density of the preform to be achieved. The density of the preform should, on the one hand, be high enough to reach the target relative density of 93-97% in the subsequent sintering process without increasing the sintering time. On the other hand, the preform can degas during the sintering process and the copper (as long as it is oxidized)
It must also be kept low so that it can be returned.

The preform exhibits a relative density of 75 to 85 inches with respect to the highest theoretically achievable density of the copper-chromium material (the highest density is reached when there is no more pore volume in the contact piece). We should strive to do so.

Due to the subsequent sintering process, the preform should reach a relative density of 93-97%. With this formulation, the present invention departs somewhat from what has heretofore been considered necessary in the industry.

That is, it was conventionally believed that the higher the relative density of the copper-chromium contact piece, the more advantageous the various properties of the copper-chromium contact piece.

Therefore, until now we have always tried to make the density of the contact pieces as high as possible (West German Patent Application No. 291418
No. 6: at least 97%; No. 2346179: at least 98%). In addition, it is difficult to achieve a particularly high density through the sintering process alone, so conventionally the preform after sintering was usually final consolidated by hot pressing to achieve the target high density. . However, when we follow the prescription for chromium powder particle size distribution selection according to the invention and take care to ensure that the chromium particles precipitated in the copper remain smaller than 1 μm by appropriate finishing heat treatment of the contact piece, the lower It has already been confirmed that the optimum properties of the copper-chromium contact piece can be obtained also in terms of density.

The relative density of 93 to 97% to be reached according to the invention can be achieved by sintering a cold pressed preform, without the need for final consolidation by hot pressing operations. However, if the sintering conditions are chosen such that the relative density targeted for the contact piece is nevertheless still not achieved by the sintering process, the sintering process is combined with final consolidation by hot pressing. You could also do that.

Sintering occurs in the solid phase, i.e. in the component with the lower melting point (
should be carried out at a temperature below the melting point of copper). Preferably sintering should be carried out at a temperature of 1030-1070°C. The sintering time depends on the chosen sintering temperature. In other words, the higher the sintering temperature selected, the shorter the sintering time. The chromium powder particles resulting from the milling process are highly irregular and jagged in shape.

It was observed that the jaggedness of the chromium particles was gradually smoothed out by the sintering process. The inventors recommend that the parameters of the sintering process (sintering temperature and sintering time) be chosen in such a way that the chromium particles change their shape as much as possible during the sintering process.

According to our experience, this is extremely important for the glass properties of the contact piece. The preform should be reduced during the sintering process. Therefore, the sintering process should temporarily proceed in a hydrogen atmosphere.

Preferably, the preform is kept in a hydrogen atmosphere only during the heating phase, while the actual sintering process is at 1030-1070°C.
The process is carried out in a vacuum at a temperature of , thereby allowing degassing of the preform.

Chromium can only be dissolved in copper in small quantities (maximum solubility is approximately 0.8% by weight at the above sintering temperatures).

As the temperature decreases, the solubility of chromium in copper decreases. As a result, when the preform is cooled, the chromium dissolved in the copper during the previous sintering process partially precipitates out.

Here, care is taken to ensure that the chromium particles precipitated in the copper are smaller than 1 μm by appropriate finishing heat treatment. Such fine chromium deposition can be achieved by extremely rapid cooling of the contact piece. To achieve this, the sintering furnace could be flushed with cold protective gas, especially hydrogen, at the end of the scheduled sintering time. However, in view of the high heat capacity of the sintering furnace, the contact pieces are cooled gradually in the sintering furnace (as is customary in the sintering process in general) and are instead subjected to a melting heat treatment in a protective gas after removal from the sintering furnace. It is advantageous to carry out the finishing treatment in hydrogen (preferably at a temperature of 960 DEG to 1030 DEG C.) and subsequent rapid cooling. Rapid cooling can be carried out by forcing the contact piece that comes out of the continuous furnace after melting heat treatment onto a metal plate that is being cooled, and blowing a low-temperature protective gas at the same time.

Highly effective cooling of the metal plate can be carried out, for example, with liquid nitrogen.

The selected particle size fractions are mixed in the following proportions to form chromium powder.

0 to 30 μm 10% by weight 30 to 60 μm 15 〃 60 to 90 μm 15 〃 90 to 120 μm 15 〃 120 to 150/7 m 15 //150 to 180 μm 15 〃 180 to 210 μm m 15 〃 100% by weight This chromium powder mixture is an average powder Particle size is approximately 110μm
It is. 35 parts by weight of this chromium powder mixture and 65 parts by weight of fine-grained copper powder with an average particle size not exceeding 5 μm are mixed together intensively for about 1 hour and cold pressed without additives to give a relative density of about 80%. (equivalent to pore volume of 20%). The press pressure required for this is 5500 to 6
500 bar (550-650 MN/m2). The preform thus produced is transferred to a sintering furnace where it is heated in a hydrogen atmosphere to a sintering temperature of 1030-1070°C, subsequently held at this temperature for 2 hours, and then The heating is interrupted and the mixture is gradually cooled down. The sintered contact piece has a relative density of approximately 95%. After removal from the sintering furnace, finishing treatment is carried out in a continuous furnace by melting heat treatment at a temperature of 1000° C. in a hydrogen atmosphere, also for a period of 1 hour. Subsequently, the contact piece (coming out of the continuous furnace) is quenched by pressing it onto a cooled metal plate under the action of a cold hydrogen shower.

Example ■ The above method is modified to shorten the duration of the sintering process to the addition period. The sintered contact piece then has a relative density of only about 90 inches. It is therefore final consolidated by a short hot isostatic pressing operation at a temperature of 100,000 and a pressure of 200 bar (-20 MN/m2), so that the same <
A relative density of 95% is reached.

Otherwise, the method proceeds in the same manner as in the first embodiment.

The contact piece made by the partially modified method differs from the contact piece made by the method shown earlier in that the chromium particles in the contact piece still maintain a large jagged shape as a result of the shortened sintering process. There is.

Claims (11)

[Claims] (1) Powder metallurgy manufacturing method of copper-chromium solid solution electrical contact piece for vacuum-sealed switch, comprising the following steps: a) 20 to 70% by weight of copper powder and 30 to 8% by weight of chromium powder;
0% by weight; the chromium powder then has a particle size of 0 up to 210 μm, and also particle size fractions of 0 to 30 μm, 30 to 60 μm, 60 to 90 μm.
, 90 to 120 μm, 120 to 150 μm, 150
a particle size distribution exhibiting approximately equal proportions normalized to the respective fraction widths between 180 and 180 μm and between 180 and 210 μm, and an average particle size lying between 100 μm and 120 μm; b) contact piece/preform from the powder mixture; c) sintering the preform in the solid phase, i.e. until reaching a relative density of 93 to 97% of the theoretically attainable maximum density at a temperature below the melting point of the copper; d) in copper. Final heat treatment of the contact piece so that the precipitated chromium particles (diameter) remain smaller than 1 μm. (2) The method according to claim 1, wherein the chromium powder used has the following particle size distribution: 0 μm to 30 μm 10% by weight 30 μm to 60 μm 15 〃 60 μm to 90 μm 15 〃 90 μm to 120 μm 15 〃 120 μm to 150 μm 15 〃 150 μm to 180 μm 15 〃 180 μm to 210 μm 15 〃 100% by weight (3) A method according to claim 1 or 2, characterized in that the average particle size of the copper powder used is smaller than 5 μm. (4) A method according to one of claims 1 to 3, characterized in that the preform is produced by a cold pressing operation. (5) A patent characterized in that the copper-chromium-powder mixture is pressed under such pressure that the preform reaches a relative density of 75 to 85%, preferably 80% of the theoretically attainable maximum density. A method according to one of claims 1 to 4. (6) A method according to one of claims 1 to 5, characterized in that the preform is sintered at a temperature of 1030 to 1070°C. (7) The method according to any one of claims 1 to 6, characterized in that the preform is sintered in vacuum. (8) The method according to claim 7, characterized in that the preform is temporarily kept in a reducing atmosphere during the heating phase. 9. Process according to claim 1, characterized in that the contact piece is finished by solution heat treatment in a protective gas and subsequent rapid cooling. (10) Melting heat treatment in hydrogen atmosphere from 960 to 1030
10. A method according to claim 9, characterized in that it is carried out at a temperature of .degree. 11. Process according to claim 1, characterized in that the powder mixture contains 35 to 50% by weight of chromium, the balance copper.