Powdered material for thermal spraying

Details Powdered material for thermal spraying Powdered material for thermal spraying

1984-10-18

1986-09-02

Stallard, Wayland

Specialized metallurgical processes, compositions for use therei

Compositions

Loose particulate mixture containing metal particles

C22C 2900, C22C 3200

Patent

active

046094016

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a powdered material for thermal spraying comprising a mixture of a self fluxing matrix alloy powder and of a powder of at least one metal carbide.
It is already known to use powdered materials of the above kind to produce corrosion resistant and erosion resistant coatings on metallic substrates by thermal spraying such as a flame spraying. Usually the matrix alloy of the Ni--Cr--Fe--B--Si type and it generally contains substantially less than 17 percent by weight of Cr. Among the carbides used, it is tungsten carbide which is generally preferred.
The wear resistance of coatings produced with said known materials appears to be unsatisfactory in many cases in particular under the combined action of corrosion and erosion. This is the case for example in transport screws or decanter screws rotating with relatively high speed such as they are used in the chemical industry. The usual coatings formed by a matrix alloy and imbedded carbides are particularly subject to erosion under these conditions and their unsatisfactory behaviour is assumed to be due to the formation of M.sub.6 C-carbides in the transitional zone between the matrix alloy and the carbide particles as well as to the poor elasticity properties of the matrix alloy, ranging about E=19000-20000 kp/mm.sup.2 at a hardness value of about 60 R.sub.c.
The present invention aims at providing a powdered material of the kind mentioned at the beginning, which allows to obtain a coating having an increased resistance against erosion as well as against corrosion.
It has been found, unexpectedly, that an increase of the Cr-content of the matrix alloy allows to improve the elasticity of the same and to reach values of E=15000-16000 kp/mm.sup.2, without a reduction in the hardness of the alloy. Accordingly, the corrosion resistance of a layer formed with such a matrix alloy could be substantially increased. In addition it was found that such a matrix alloy with imbedded carbide particles had a subtantially improved resistance against erosion which effect is apparently due to the reduced tendancy to M.sub.6 C-formation in the transitional zone between the matrix alloy and the carbide particles and thus to the usual tendancy of the layer to become brittle.
The powdered material for thermal spraying according to the invention comprises a mixture of a self fluxing matrix alloy powder of the following composition, in percent by weight:


______________________________________ Cr 18.0-35.0% Fe 0.1-25.0% B 0.5-4.5% Si 0.5-5.5% C 0.01-2.0% Mo 0-15.0% Nb 0-2.0% Ni Remainder ______________________________________ mixture being comprised between 20 and 80 percent by weight of this mixture. According to a first preferred embodiment, the carbide used in tungsten monocarbide to provide further increased erosion resistance. In another embodiment the carbides used are in the form of fused tungsten carbide particles. In a further preferred embodiment the carbide particles are coated with Ni. In this case the protective layer on a metal substrate obtained by thermal spraying of the powder mixture of the invention becomes even more resistant against shocks since the carbide particles are more elastically imbedded in the layer. This also allows the use of carbide particles of greater size.


EXAMPLE

A transport screw of 300 millimeters diameter and 1500 millimeters length is coated on the wear exposed surface by flame spraying with simultaneous fusing to obtain a protective coating of one millimeter thickness. The flame spray material used is a mixture of a matrix alloy powder of a grain size corresponding to -125 .mu.m and of a powder of fused tungsten carbide having a grain size corresponding to -105 .mu.m. The weight ratio of the mixture is 50:50 in percent by weight and the composition of the matrix alloy, in percent by weight, was as follows:


______________________________________ Cr 22% Fe 4% B 3.5% Si 4.1% C 0.6% Mo 2.5% Nb 0.1% Ni Remainder ______________________________________ from t

REFERENCES:
patent: 3723165 (1973-03-01), Longo
patent: 4192672 (1980-03-01), Moskowitz et al.
patent: 4507151 (1985-03-01), Simm et al.

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