GB820654A - Improvements in and relating to the coating of refractory metal compound composites - Google Patents

Abstract

In a method of producing cladded thermal elements, e.g turbine blades, by coating with a ductile heat resistant metal a composite base comprising refractory grains of the carbides, borides, nitrides or silicides of Ti, Zr, Cr, Mo, W, V, Cb, or Ta or mixtures thereof in a matrix of heat resistant binder metal, containing a low melting phase based on the interaction of the binder metal and refractory grains, the heating which effects bonding is carried out so that at least one of the materials is maintained at a solidus temperature approaching within 100 DEG C. of its lowest melting point in the presence of a liquid bonding phase, for no longer than 120 minutes, to inhibit excessive diffusion of embrittling agents into the coating. Either the low melting phase of the base may be utilized as the bonding agent, the temperature used being 100 DEG C. or more below the melting point of the coating material; or the base may be maintained at a temperature no nearer than within 100 DEG C. of the melting point of its lowest melting phase, the coating material being up to 250 DEG C. above its melting point. Again a bond promoting pre-coat alloy generally similarly in composition to the principal coating and which melts at the temperature employed, may be utilized. The thickness of any such intermediate layer should not exceed one quarter that of the principal coat. A protective atmosphere such as hydrogen, nitrogen, helium, argon; or a sub-atmospheric pressure, may be provided. The Specification describes the application of the coating metal by spraying, centrifugal casting, and the lightly pressing on of a thin sheet. In Example 1 a body of TiC grains bonded with a high temperature resistant Ni alloy after grit blasting and degreasing is pre-coated using a powder spray torch with approximately 0.002 inch of a low melting point alloy containing 3 to 5% B, up to 1% C, 1 to 3.5% Si, less than 5% total of Fe, Mn and Mg, the balance being Ni-Cr in the ratio 4 parts Ni to 1 part Cr. It is bonded by electric induction heating in a vacuum furnace at 1050 DEG -1100 DEG for fifteen minutes. After cooling and grit blasting the specimen is spray coated with about 0.01 inch of alloy comprising more than 95% Ni-Cr (4 parts Ni to 1 part Cr) and the balance Fe, Mg, C, Si, its melting point being of the order 1400 DEG C. The whole is re-heated in the induction vacuum furnace at 1100 DEG C. for 1 hour. Alternatively the preliminary heating to bond the pre-coat is omitted. In Example 3 a similar base material and a 0.01 inch sheet of a Ni-base alloy comprising 14% Cr, 6% Fe and balance Ni with a melting point of 1395 DEG C. were cleaned and degreased. The sheet was placed around the carbide body, the assembly put in a mould internally simulating a turbine nozzle vane, the halves of the mould were held together by a weight exerting about 1 lb. per sq. inch, and the whole heated in a vacuum furnace at 1290 DEG C. for 1 hour. In Example 4 a TiC skeleton infiltrated with an 80% Ni, 20% Cr alloy was coated by centrifugal casting with an alloy comprising 52% Co, 27% Cr, 12% Ni and 9% W which was melted and heated at 1400 DEG C. The infiltrated body was maintained for about half a minute between 1000 DEG to 1100 DEG C., i.e. more than 100 DEG C. below the lowest melting phase which is in the neighbourhood of 1200 DEG C. Other alloys instanced as applicable are Nickel base with (a) 13 to 15% Cr, 6 to 7% Fe; (b) 13 to 16% Cr, 15 to 19% Mo, 3.5 to 5.5% W, 4 to 7% Fe; (c) 4 to 6% Al; Cobalt base with (a) 25% Cr, 6% Mo; (b) 26% Cr, 10% Ni, 7.5% W; Iron base with (a) 16 to 20% Cr, 6 to 10% Ni; (b) 25% Ni, 16% Cr, 6% Mo. Precoat alloys may comprise an iron group metal with at least one of Mg up to 20%, B to 5%, P to 12%, Si to 4%, Mn to 2%, C to 2%, the total of these alloying ingredients not exceeding about 20% of the alloy.