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...

In the Soviet Union, graphite-containing refractory crucibles and muffles required for melting and distributing nonferrous metals and alloys are being produced using hydrostatic compaction of the bodies incorporating an argillaceous binder at a pressure of 15 N/mm 2. The service life of these products does not meet the current specifications.Based on the service conditions, the products must possess high density, strength, thermal conductivity, and thermal shock resistance and low porosity and oxidability. The forming (shaping) pressure is one of the most important factors determining the variation of these parameters.This paper deals with a study of the effectiveness of increasing the forming pressure during the production of the graphite-containing products and the technological conditions and parameters required for obtaining the products.In order to obtain refractory products by semidry compaction, the moisture content of the graphite-containing mixtures prepared using an argillaceous binder is maintained at 12%. Figure 1 shows the dependence of the strength of specimens obtained at a pressure of 15 and 60 N/mm 2 on the moisture content of the mixtures. A change in the moisture content of the mixture has virtually no effect on the ultimate bend strength Obn d of the raw specimens (green compacts) and the fired specimens and on the ultimate compressive strength Ocm of the fired specimens compacted at a pressure of 15 N/mm 2. In the case a raw specimens, Ocm was found to increase by 1.8 times with decreasing moisture content. At a compaction pressure of 60 N/mm 2, decreasing the moisture content up to 8% improves Ocm of the raw specimens by almost 3 times and Obn d by 1.5 times and, in both modes of testing, the strength of the fired specimens increases by approximately 1.3 times. In this case, the apparent density of the specimens was found to increase from 1.80-1.85 up to 1.9 g/cm a and their open porosity was found to decrease from 26-28 up to 23-25%.Thus, a significant effect due to an increased compaction pressure is observed only when the moisture content of the mixture is decreased simultaneously. This owes to the high content (38%) of the fairly plastic refractory clay in the mixture.It is not advisable to prepare a mixture having a moisture content of less than 8% since the plasticity of the argillaceous binder is not used adequately for ensuring homogeneity of the mixture and uniform coating of the binder on the particles of graphite and silicon carbide. The mixtures having a moisture content of 14% and more do not pass through the sieve easily, agglutinate, and are separated into layers after removing the compaction load.The moisture content of the mixtures can be decreased by holding (ageing) them before shaping. Figure 2 shows the dependence of the ultimate compressive strength of the as-formed specimens on the compaction pressure at different durations of holding the mixtures r prior to compaction (shaping). On increasing 9 up to 24-48 h, Ocm of the specimens increases by almost 3 times and a furt...

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