Pulverulent filler and method for applying a wear-resistant...

Electric heating – Metal heating – For deposition welding

Reexamination Certificate

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C219S076100, C219S119000, C219S146230

Reexamination Certificate

active

06603090

ABSTRACT:

FIELD OF THE INVENTION
The invention related to a pulverulent filler for applying a wear-resistant layer comprising a nickel alloy matrix and intercalated (tungsten carbides and) vanadium carbides to a surface which is to be protected, using the metal spraying route, in particular arc welding or plasma welding.
BACKGROUND OF THE INVENTION
Wear-resistant layers of this nature are usually applied to the surface of tools, such as implements used in mining, in deep drilling, in the ceramics industry and the like. Wear-resistant layers are generally used to counteract the progressive loss of material on the surface of a solid body. Such loss of material, also known as wear, generally results from mechanical causes, i.e. generally from contact with and movements relative to another body. Consequently, tribological investigations have made it possible to develop various processes for applying metal coatings, including metal spray coatings. These coatings are regularly applied to particularly large workpieces or workpieces which are only to be treated in certain areas. In this case, the metal, which is in wire or powder form, is melted by a fuel gas mixture or by an arc and is thrown onto the workpiece which is to be treated in the form of fine droplets, by means of compressed air. The adhesion to the surface is purely mechanical, for which reason the latter is generally roughened to an average roughness by sand-blasting.
In practice, processes for applying wear-resistant layers of the basic structure described above which are based on oxyacetylene welding processes or electric arc welding processes are known. When using electric arc welding to apply wear-resistant layers, consumable stick electrodes or flux-cored electrodes which comprise small alloy tubes with a finely particulate filling are customarily used. A distinction is generally drawn between stick electrodes or flux-cored electrodes on the basis of DIN 8571, and in the context of the invention such electrodes are distinguished by a nickel casing and a powder filling on the inside.
In addition to the abovementioned deposition welding processes, it is generally known to apply pulverulent fillers directly onto a substrate in the course of plasma-arc powder surfacing. In this case, work is always carried out in such a way that the pulverulent filler can be fed from a storage container to the associated plasma torch via a metering device, with the aid of a carrier gas. Consequently, it is possible to do without a welding electrode.
The nickel alloy matrix essentially serves to accommodate and support intercalated (tungsten) carbide grains. These carbide grains form the actual hard-facing, which is mainly responsible for the antiwear and friction properties of the wear-resistant layer applied. According to the prior art, small nickel tubes containing an accurately specified mixture of tungsten carbides and a suitable amount of alloying elements are used as the welding electrodes.
This results, for the wear-resistant layer, in, for example, a nickel-(chromium)-boron-silicon matrix, in the form of a corresponding alloy (cf. DE 40 08 091 C2).
Moreover, in the context of arc welding for applying wear-resistant layers, it is generally known to use small tubes made from a nickel base alloy or from a cobalt base alloy (cf., for example, DE 87 16 743 U1). The wear-resistant layers with a nickel alloy matrix which are applied during electric arc welding cannot always be produced with satisfactory results. Moreover, the welding work using an electric arc produces further drawbacks which are attributable to the high thermal load imposed on the tungsten carbides, since the tungsten carbides may decompose, specifically, in the most part, into the two hard phases tungsten monocarbide WC and ditungsten carbide W
2
C. Particularly the latter, ditungsten carbide, is more brittle, owing to its lower modulus of elasticity (compared to WC) and is therefore less suitable for incorporation in a wear-resistant layer (cf. U.S. Pat. No. 2,137,471).
Also, the temperatures which prevail in the arc may lead to a high level of carburization, i.e., as it were, at the extremely high temperatures in the plasma discharge space, graphite is deposited in the alloy matrix. This is to be avoided at all costs, since the matrix alloys are to have as low a carbon content as possible, in order to counteract embrittlement.
At any rate, the prior art may lead to flaking of the hard-facing, particularly under impact loads or dynamic loads. In addition, the matrix basic structure (which is soft by comparison with the hard phases) may wear prematurely, particularly under high abrasive loads with mineral particles of <20 Tm, so that the tungsten carbide hard materials or tungsten carbide grains which are actually to be protected or supported are washed out. Consequently, damage to the wear-resistant layer results not only from the fact that the (di)tungsten carbide hard-facing which is formed is unable (no longer able) to withstand high loads, but also from the fact that the alloy matrix (which in itself is protective) is additionally abraded.
In addition, in the event of abrasive-adhesive loads, such as for example those which are found in the case of pressure worms used in the ceramics industry, the use of tungsten-carbide-reinforced protective layers is restricted, in view of the fact that in this application the hard tungsten carbide materials or grains which are found in the hard-facing lead to adhesion of the ceramic material. Consequently, enormous drive powers are required in order to ensure continuous production operation with a sufficient throughput. At any rate, the fillers, welding electrodes and production processes for wear-resistant layers based on tungsten carbides as described above are entirely unable to satisfy requirements. The invention wishes to provide a remedy to these problems .
SUMMARY OF THE INVENTION
The invention is based on the technical problem of providing a pulverulent filler for applying a wear-resistant layer comprising a nickel alloy matrix and intercalated carbides to a surface which is to be protected, using the metal spraying route, which filler produces an improved microstructure morphology of the nickel alloy matrix and optimized sliding properties for the wear-resistant layer. Moreover, it is intended to provide a suitable method for applying such wear-resistant layers.
To solve this object, the subject matter of the invention comprises pulverulent fillers. The invention furthermore relates to a method for applying a wear-resistant layer comprising a nickel alloy matrix with intercalated tungsten carbides and/or vanadium carbides to a surface which is to be protected, via the metal spraying route, in particular arc welding or plasma welding, on the basis of the pulverulent fillers according to the invention.
In the course of the invention, a controlled improvement in the microstructure morphology of the nickel alloy matrix and an optimization of the wear resistance have been achieved by the controlled use of vanadium or vanadium carbide. Specifically, the heat-related tungsten carbide decomposition described above is compensated for, because the vanadium br vanadium carbide which has dissolved in the molten pool lads to primary carbide precipitations out of the molten pool. These primary carbide precipitations mainly involve vanadium carbide VC, i.e. in particular vanadium nitride VN, for example, is not formed. The production of ditungsten carbide is also suppressed. The same applies to vanadium pentoxide V
2
O
5
.
At any rate, the high carburization of the nickel alloy matrix outlined above is successfully counteracted, by the very addition of vanadium which, in dissolved form, is converted into vanadium carbide with any carbon which is formed. These primary carbides or vanadium carbide nuclei result in a fine-grained (and desired) solidification of the remaining molten pool or of the nickel alloy matrix. The fact that carburization is, as it were, automatically avoided makes it possible to successfully prevent embrittlement of

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