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...

The primary supplier of graphite-containing refractory parts (crucibles, muffles, baths) is Luga Abrasive Plant, in which at present in production of refractories by the plastic method a significant volume is occupied by manual operations including the preparation of the prepared clay for a crucible, transportation of it, forming of the crucible, etc.In perspective these operations are subject to mechanization, which involves a change in certain criteria of quality of forming mixtures.The purpose of this work was determination of qualitative criteria for evaluation of the quality of clay-graphite mixtures with a granular filler, silicon carbide, for the production of refractory parts by manual and mechanical methods.One of the criteria for evaluation of the quality of a mixture under plant conditions is the moisture content such as 22-25% for crucibles and baths and 18-20% for muffles.The mixtures for production of crucibles on potter's wheel type machines operating by the method of rolling out with the use of a pattern are subject to organoleptic evaluation by the purchaser of the mixture for the possibility of working of it in forming.Such an analyisis requires significant experience and dors not guarantee accuracy.

A method for estimating the thermostability of refractories by the change in the frequency of natural oscillations of a specimen after a single thermal shock is presented. The method has been tested in estimating the thermostability of silicon carbide plates on silica binder produced by the Volzhskii Abrasive Plant. At the same time, a direct dependence of the apparent density on the frequency of natural oscillations of a specimen is established, the method is quicker and less laborious than the conventional one.The operating ability of refractory products is determined to a large extent by their therrnostability, because of which the determination of this characteristic in the production of refractories and in the development of new kinds of refractories is indispensable. The technique for determination of thermostability is specified in the State Standard GOST 7875 -83. It is laborious and requires a large number of tests, and the result depends on an overly large number of the factors associated with destruction of specimens. Because of this the thermostability of produced refractories is tested only irregularly, and often with violation of the rules of testing.A rapid method for determination of thermostability is proposed below. It is based on the hypothesis of "equivalent crack formation," which assumes a dependence of the mechanical properties of the material on the formation of microcracks resulting from sharp cooling of a specimen without its destruction [1].In particular, a method is known by which the thermostability and the associated rated modulus of elasticity are determined by the change in the velocity of propagation of ultrasound in a specimen. It has been established that the ultrasonic method provides an objective estimate of the destruction of the specimen and makes it possible to determine the thermostability in a single loading cycle [2]. This method, however, relies on the use of rather intricate equipment.The proposed rapid method is based on using known acoustic devices of the "Zvuk" series [3]. Noting that the velocity of propagation of ultimate oscillations is proportional to the natural oscillation frequency, the thermostability can be evaluated by the ratio of the frequency of natural oscilla-1 Volzhskii Research Institute for Abrasives and Grinding, Volzhsk, Russia. 129tions of a specimen after a thermal shock to its value before the thermal shock.In the method proposed, bar-shaped specimens of size 20 x 20 x 120 mm or 25 x 25 x 120 mm are cut from refractory products. A specimen is placed vertically in the measuring stand of a "Zvuk-107" instrument arid the frequency of forced oscillations is measured up to the onset of resonance. Four or five measurements are made with change in the fastening point of the specimen, and the arithmetic mean value is then determined. The specimens are then placed in a basket made of heat-resistant wire and subjected to a thermal shock in a furnace at a temperature of 850°C with a hold for 30 min. After that the specimens are cooled in a flow of cold w...