Clay-graphite refractory crucibles containing variable amounts of silicon carbide

Abstract

Graphite-bearing refractory crucibles with clay bonding agents are used in melting and distributing nonferrous metals; they are made here with the use of black silicon carbide. Crucibles intended for operation up to 1300~ contain 3% silicon carbide, or 60% for use up to 1500~In the first, one finds low thermal conductivity and density, with fairly high oxidation rates, which lead to loss of thermal conductivity and reduction in the working resistance, while in the second, the low proportion of binding agent and the low pressing pressures result in low strength and heat resistance, with premature failure on account of cracking.There are technological problems in making the crucibles. A large amount of refractory clay (up to 40%) is required in the shaping mixtures with 3% silicon carbide, and the operations with that clay result in considerable dust. With 60% carbide, the small amount of binding clay (only 10%) results in low strength after molding, with the surface inadequately wetted by the glaze. The glazing coating is poorly retained, which leads to a high rejection rate as well as loss of resistance during use.The presence of a certain amount of silicon carbide in the crucibles is important to the performance; it has high thermal conductivity (36 W/m.K at 875~ but part of it is oxidized during firing and use at high temperatures, which protects the main graphite component from oxidation. An optimal proportion of the carbide also gives the crucible elevated density and strength from its reinforcing effect.We examined the properties of crucibles containing 3-60% silicon carbide. The specimens were prepared from molding mixtures having various ratios between graphite, clay, and silicon carbide. Table I gives the mixture compositions.For mixture No. i, we used black silicon carbide grade 54S, grain sizes 630 (50%) and 250 Dm (50%), while for the others we used grain sizes 800 (30%) and 120~m(70%). The specimens were shaped in the usual way in routine production of crucibles at 15 N/mm 2 and were fired in an electric furnace at the final temperature of 1280~To protect them from oxidation durng firing, they were placed in a muffle composed of silicon carbide plates filled with graphite powder.We examined the major physicomechanical parameters and the oxidation. The oxidation coefficient was determined as the relative differences in mass for the fired specimens of equal dimensions after 1.5-2 h at 800~ expressed as percentages of the initial mass. We also evaluated the relative oxidation in relation to the oxidation coefficient and graphite content. Figure 1 shows the parameters as functions of silicon carbide content for mixtures 1-6. As the carbide content increases from 3 to 60%, the graphite and clay contents decrease, while the density of the material increases by about 15%, the porosity is reduced by 10-12% up to a silicon carbide content of 33-40%, but then begins to rise, while the strength has a maximum at 33-50% carbide. The oxidation coefficient mainly falls, par%icularly in that carbide content range. On...

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“…Clay bonded Si-C was selected for experimentation preferentially to Al2O3 which suffers from poor heat transfer properties as well as a lack of inertness to iron oxide at reaction temperatures due to the formation of iron aluminates 162 . Si-C was also preferable to clay-graphite due to the much-improved oxidation properties of Si-C at the extreme reaction temperatures used 163,164 .…”