Patent GB-809133-A (GB)
Title: Improvements in and relating to the production of hollow thermal elements
IPC: B22F5/10
Filed: 1956-04-18
Abstract: Hollow bodies are made by surrounding a core of a high melting material with a coherent, hollow porous body of refractory material, infiltrating the porous walls of said hollow body with infiltrant metal of melting point below that of the material of the body, cooling the infiltrated body, and then separating it from said core material.  The core may be machined graphite and coated with a layer of inert refractory such as thoria, beryllia &c.  The surface is then provided with a layer of e.g. titanium carbide powder mixed with up to 25 per cent of a binder metal powder, such as nickel, cobalt, iron.  The layer may be applied by spraying, to produce porosities ranging from 30-60 per cent by volume, or deposited as a liquid slurry or paint, or slip cast about the core in a confining mould to produce porosities of about 40-70 per cent by volume.  The material is then sintered by heating in an inert atmosphere, e.g. of CO or H or in a vacuum, a slow heating cycle being employed to first evaporate or decompose the vehicle in the refractory powder if this has been deposited by slip casting or painting.  The infiltrant is then applied as by spraying or wrapping a strip around the porous body and the assembly heated to e.g. about 250 DEG C. above the melting point of the infiltrant metal, and generally in the range 1200-1700 DEG C.  The assembly is preferably supported by an inert refractory.  After infiltration the core is removed, e.g. if of graphite by heating in an atmosphere oxidizing to graphite, if of stabilized lime by exposure to moist air of hot water, if of beryllium, alumina, or zirconium by sand or grit blasting.  The core may be coated with a thin layer of metal, e.g. nickel, cobalt, iron, or alloy thereof, which during sintering of the skeleton is absorbed thereinto to leave an expansion space between the core and the body. Alternatively the coating may be of the infiltrant metal and in sufficient quantity to complete the infiltration of the skeleton. The hollow porous body may be made by extrusion, pressing or injection moulding followed by curing of the refractory bonded with 5-30 per cent of a plastic.  The body is then disposed within a confining bed of a refractory oxide powder such as zirconium, which is vibrated on a jolting table to settle the powder within and around the hollow body.  The plastic is then removed by heating, and the porous body then heated at sintering temperature under inert conditions.  The skeleton is then removed, cleaned by grit blasting, again supported by refractory oxide and infiltrated.  The plastic binder may comprise polystyrene, polymethacrylates, phenolformaldehyde, cellulose acetate, cellulose acetate-butyrate, ethyl cellulose, vinylidene chloride, vinyl acetate, polyvinyl alcohols, cellulose nitrate &c. In a modification the core is of infiltrant metal and hollow and coated with a plastic containing refractory mixture and the assembly disposed in a confining bed of refractory oxide powder densified against the inside and outside walls of the body.  In a further modification the porous body is made by depositing successive layers of a liquid slurry of a suspension of the refractory on the inner wall of a porous plaster mould, the liquid of the slurry being removed through the porous wall of the mould.  The mould is then fired in an inert atmosphere or vacuum to sinter the refractory particles, the skeleton body removed from the mould and embedded in an inert refractory.  The infiltrant metals include at least one of Ni, Co and Fe or heat resistant alloys based thereon. Examples of nickel-base alloys include by percentages 80Ni 20Cr; 80Ni, 14Cr, 6Fe; 15Cr, 7Fe, 1Cb, 2.5Ti, 0.7Al, balance Ni; 58Ni, 15Cr, 17Mo, 5W, 5Fe; 95Ni, 4.5Al, .5Mn. Cobalt-base alloys include, 69Co, 25Cr, 6Mo, 65Co, 25Cr, 6W, 2Ni, 1Fe; 56Co, 10Ni, 26Cr, 7.5W; 51.5Co, 10Ni, 20Cr, 15W, 2Fe, 1.5Mn.  Iron-base aloys include 53Fe, 25Ni, 16Cr, 6Mo; 74Fe, 18Cr, 8Ni; 86Fe, 14Cr; 82Fe, 18Cr; 75Fe, 27Cr.  The infiltrant may contain up to 30 per cent by weight of at least one of Cr, Mo, and W, the sum of the metals of said group preferably not exceeding 40 per cent, substantially the balance being at least one of Fe, Co and Ni, the sum of the metals of the group being preferably at least about 40 per cent by weight of the infiltrant, which may also contain up to 8 per cent total of at least one of Cb, Ta, and V.  Strengthening elements such as Zr, Ti and Al may also be added.  The refractory powder material employed for producing the hollow elements may comprise Cr, Mo, W, V, Ta, Cb, Ti, Zr, mixtures of at least two of these metals, their refractory compounds such as carbides, silicides, nitrides, borides, and mixtures thereof.