Coating material for the formation of abrasion-resistant and imp

Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles

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75252, B22F 102

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active

045071519

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BRIEF SUMMARY
The invention relates to a coating material in powder form for the thermal coating of workpieces, said material containing a mechanical mixture of metal powder and a powder of hard material. It is known to provide machine parts subject to heavy wear, by a thermal coating method with a coating in which hard materials are incorporated in order to increase the resistance to abrasion and impact. By way of example, cobalt-bonded tungsten carbide WC or W.sub.2 C is deposited by spraying in powder form together with a metal alloy, and the coating is simultaneously or subsequently fused. However, the carbides thereby have a tendency to oxidise and to form intermetallic phases of the form M.sub.6 C in the transition zone between the carbide particles and the matrix alloy in which they are incorporated. These intermetallic phases are very brittle and, under shock or impact loading, lead to the breaking off of the carbide particles. Furthermore, it is found that, with the application of such coatings, independently of the specific gravity of the tungsten carbides and the grain size distribution, there is a strong tendency for the carbide particles to sink or settle, so that in relatively thick layers with a thickness of for example 1.0 mm and upwards, the carbide particles are increased in quantity in the bonding zone between the basic material of the workpiece and the desposited coating. As a consequence, the coating or layer is given irregular physical properties and, in particular, presents a surface with a lower carbide content, which is not sufficiently resistant to impact and abrasion.
The invention has for its object to provide a material with which it is possible to produce coatings or layers having very high resistance to abrasion and impact and more particularly layers which, even in relatively great thickness, present constant properties throughout their entire thickness.
This is achieved with a coating material, of the type as initially referred to, by the metal powder consisting of a self-fluxing alloy based on Ni, Fe or Co, the hard material consisting of a fused tungsten carbide alloy with, in percent by weight, 3 to 7% C, 0 to 3% Fe and a maximum of 2% of other alloying elements, the remainder being W, and the particles of hard material comprising a coating of a metal with a higher melting point than that of the said self-fluxing alloy, the grain size of the coated hard material granules being smaller than 75 .mu.m and the proportion of powder of hard material in the mixture with the metal powder being between 10 and 95% by weight.
It has surprisingly been found that, by using fused tungsten carbide alloy powder coated, more particularly, with Ni, Fe or Co, in the proportions as indicated and in the selected grain size, a sinking of the hard material particles during the deposition is avoided and the formation of M.sub.6 C compounds is practically completely prevented. A possibly present acicular structure of the fused tungsten carbides further leads to an increase in the toughness of the coating or layer and hence to a further improvement in the resistance to impact and shock loads.
The coating of the hard material particles is advantageously carried out by one of the chemical, electrochemical, CVD, PVD or agglomeration processes which are known per se or an agglomeration process with subsequent sintering.
The grain size of the coated particles of hard maeterial is advantageously smaller than 62 .mu.m and the proportion of the hard powder material in the mixture with the metal powder is advantageously between 40 and 80%.
The following examples indicate various forms of application and use of the invention, which may, however, in each given case, be modified in many ways, according to the particular requirements of the respective strain or load.


EXAMPLE 1

A fused tungsten carbide alloy having the composition of 4.0% C, 0.3% Fe, the remainder W, was obtained by fusion in an induction furnace, thereafter crushed in a hammer mill and screened to a grain size smaller than 75 .mu.m. After the screening,

REFERENCES:
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patent: 3305326 (1967-02-01), Longo
patent: 3372066 (1968-03-01), Quaas
patent: 3455019 (1969-07-01), Quaas
patent: 4025334 (1977-05-01), Cheney et al.
patent: 4075371 (1978-02-01), Patel
Schwarzkopf, P., et al.; Refractory Hard Metals Borides, Carbides Nitrides and Silicides, Macmillan Co., p. 138, (1953).
Kirk-Othmer Encyclopedia of Chemical Technology, 2nd edition, vol. 4, John Wiley & Sons, p. 80, (1963).

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