Process for producing a copper-chromium contact element for medium-voltage switchgear assemblies, and contact element itself

Abstract

 The invention relates to a process for producing a contact element for medium-voltage switchgear assemblies, and also to the contact element itself, in accordance with the preamble of claims 1 and 6. In order, in this case, to considerably lessen the abrasive effect which occurs during operation when the contact element is finished, and therefore to make the contact element more durable, the invention proposes that the chromium particles in the powder mixture are surface-rounded in advance by ball milling and/or by coating the particles with copper.

Description

[0001] The invention relates to a process for producing a contact element for medium-voltage switchgear assemblies, and also to the contact element itself, in accordance with the preamble of claims 1 and 6.

[0002] Contact elements for medium-voltage switchgear assemblies are often produced by means of sintering processes. In this case, not only copper but also an alloy of copper and chromium is used for increasing the resistance to erosion. In the case of the sintering processes, it is known to press what are known as green compacts with near final dimensions in advance, and these green compacts are then annealed.

[0003] Very large pressing forces are used during the pressing of the green compacts. In this case, the powder consisting of copper and chromium particles is placed in a mold and the pressing tool then compresses said particles with a high pressing force. It has been observed in this process that the extremely hard particles of the chromium powder have a considerable abrasive effect on the surface of the pressing tool. Even after a few pressing operations, this damage to the pressing tool leads to intolerable deviations in the final dimensions of the green compact and in the surface quality thereof.

[0004] The invention is therefore based on the object of improving a process for producing a contact element for medium-voltage switchgear assemblies, and also a contact element itself, in such a way as to considerably lessen the abrasive effect which occurs during operation when the contact element is finished, and therefore to make the contact element more durable during operation.

[0005] In the case of a process in accordance with the preamble of claim 1, the set object is achieved according to the invention by means of the characterizing features of claim 1.

[0006] Further advantageous refinements are specified in dependent claims 2 to 5.

[0007] In terms of a contact element, the set object is achieved according to the invention by means of the characterizing features of claim 6.

[0008] The essence of the invention in this case is that the chromium particles in the powder mixture are surface-rounded in advance. This considerably reduces the mechanically abrasive effect on the expensive pressing tool. In this case, however, the outer surface of the hard chromium particles can be rounded in different ways.

[0009] A first option is for the chromium particles to be rounded mechanically in a mill, in particular in a ball mill. The chromium powder rounded in this way is then used, for its part, for the copper/chrominum powder mixture required for production.

[0010] In a further refinement, it is provided that the chromium particles or the already mechanically rounded chromium particles are provided with a copper layer. In detail, this means the following: either this further copper coating is a further measure for the chromium particles which have already been mechanically rounded in advance, or else chromium particles which have not been mechanically rounded in advance, i.e. those which still have sharp edges, are rounded by the application of the copper layer. Both of these variants work.

[0011] In a further advantageous refinement, it is provided that the chromium particles are alternately coated with copper particles in an inert crucible or a vacuum container and are then heat-treated such that the chromium particles are covered with a copper layer, and the powder which has been only loosely sintered is then ground again, and only then pressed and sintered with near final dimensions in the form of a green compact.

[0012] This also applies to the two variants

1. mechanical rounding,

2. rounding by application of a copper layer.

[0013] During the heat treatment mentioned, heating is of course carried out up to that temperature at which the copper covers the chromium particles.

[0014] In a further advantageous refinement, it is provided that the process is carried out under vacuum, under a hydrogen atmosphere or under inert gas. Further oxidation processes are avoided in the process.

[0015] In terms of a contact element itself, the essence of the invention is that the chromium particles in the powder mixture have a surface-rounded structure before the pressing operation. This allows a low-abrasion, direct pressing operation with near final dimensions to be carried out using the pressing tool.

[0016] In a further advantageous refinement, it is stated that the structure is provided mechanically by breaking the edges in a mill, and/or the chromium powder or the chromium powder particles mechanically rounded in this way is/are rounded by vaporizing with copper. This results in a homogeneous, high-quality green compact, in the case of which the pressing tool can operate with considerably less abrasion.

[0017] Since the process for pressing green compacts is also largely known for contact elements, no further explanation is required here.

[0018] The figure therefore merely shows an example of the microstructure of chromium powder particles as are used for this process.

[0019] A first illustration shows a chromium particle which has sharp edges per se. In this case, this particle is provided with a copper layer in the manner described, and the surface of the resulting copper-coated chromium particle is therefore rounded in this way.

[0020] A further alternative is to mechanically round the chromium particles in, for example, a ball mill. In this case, the chromium particles can also additionally be coated with copper. This then no longer only has the purpose of rounding, but also producing a copper/chromium alloy which is homogeneous as a result.

[0021] The coating operation can take place by means of vaporizing, or by layering copper and chromium powder without preceding mechanical pressure, and thermal tempering to a temperature at which copper powder melts or vaporizes and therefore coats the chromium powder.

[0022] It is easily possible to subsequently mill the mixture as it is not pressed. The powder obtained is then the starting material for filling a pressing mold and for the production of a green compact.

[0023] The figure therefore firstly shows the edged chromium particle 1. In this case, two alternative procedures can be used.

1. The original shape of the chromium particle 1 has sharp edges and is therefore firstly rounded in a mill. It is then coated with copper, as described above.
or

2. The edged chromium particle 1 is rounded by a correspondingly thick copper layer 2 merely by means of the coating.

[0024] Optionally, however, the milling operation in the mill may also be used in this case.

Claims

1. A process for producing a contact element for a medium-voltage switchgear assembly, in which a green compact is produced by pressing from a powder consisting of a copper/chromium mixture and is then sintered,
wherein
the chromium particles in the powder mixture are surface-rounded in advance.

2. The process as claimed in claim 1,
wherein
the chromium particles are rounded mechanically in a mill, in particular in a ball mill.

3. The process as claimed in claim 1 or 2,
wherein
the chromium particles or the already mechanically rounded chromium particles are provided with a copper layer.

4. The process as claimed in claim 3,
wherein
the chromium particles are alternately coated with copper particles in an inert crucible or a vacuum container and are then heat-treated such that the chromium particles are covered with a copper layer, and the powder which has been only loosely sintered is then ground again, and only then pressed and sintered with near final dimensions in the form of a green compact.

5. The process as claimed in claim 4,
wherein
the process is carried out under vacuum, under a hydrogen atmosphere or under inert gas.

6. A contact element for a medium-voltage switchgear assembly, in which said contact element is produced by pressing from a green compact consisting of copper/chromium powder and is then sintered,
wherein
the chromium particles in the powder mixture have a surface-rounded structure before the pressing operation.

7. The contact element as claimed in claim 6,
wherein
the structure is provided mechanically by breaking the edges in a mill, and/or the chromium powder or the chromium powder particles mechanically rounded in this way is/are rounded by vaporizing with copper.

Drawing