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The cooling regime of the melts having the composition of mullite is important during mullite formation [I-3].This paper deals with a study of the effect of the fused materials of the mullite system having different quantities and sizes of mullite crystals on the properties of the mullite products.Fused materials of the mullite system were obtained by melting a mixture of the G-O alumina and quartz sand containing more than 99% SiO 2 in an arc furnace and a high-frequency furnace.The content of the main components (AI203 + SiO 2) in the fused materials exceeded 99%.Cooling of the melt was carried out at different rates by obtaining blocks and ingots of different volumes in molds of different sizes and, also, by blowing a stream of the melt using compressed air and by water quenching.When using an induction furnace, rapid melting of the mixture occurs in a small volume and it cools down rapidly after moving out from the melting zone. Wetried out four different methods for cooling the melts obtained in the arc furnace [i-4].In the first case, the melt was poured into cooled molds in which the material solidifies over a period of several days and crystallization of mu!!ite occurs in the blocks weighing approximately 1 ton.In the second case, the material (3-80 kg) was poured into molds in which cooling of the ingots occurs 5-50 times faster than in the large blocks.In the third case, the melt was poured into water where the material cools down and solidifies in the form of granules.In order to attain the maximum cooling rate, the melt was blown using compressed air; in this case, spheres having sizes ranging np to 3 mmwere obtained.When carrying out continuous melting in the induction furnace ensuring movement of the ingot at a rate of 0.75-1.00 mm/min, the solution solidified in the form of an ingot having a hexagonal cross section and an internal incircle diameter of 60 mm.The phase composition of the material and its physicochemical properties are given in Table i. The obtained results indicate that all the samples have a similar chemical composition although in the ingots obtained in the induction furnace and in the spheres, the content of AlzO~ and the alumina modulus are slightly less than those Observed in the other samples. Mullite crystallizes most completely during prolonged cooling of the melt in the molds.The weight content of mullite in the solidified blocks was found to be 94,84%. Acicu!ar mullite crystals having a size of approximately 10 mm are directed perpendicular to the mold walls.In the ingots, the weight content of mullite is slightly less than that obtained in the large blocks and amounts to 86.24%.The length of the mullite crystals present in the ingots amounts to 0.5-5 ram. They exhibit a less distinct ordered directionality as compared to the crystals formed in the blocks.In the granules, the weight content of mullite varies from 0 up to 91.9%1In the fine granules (up to 40 ~m), mullite is present only in the embryonic state. On the other hand, in the granules measuring i-i0 mm, it is well ...

The production of such nozzles is being introduced at the Krasnoarmeisk factory .Body compositions 4-6 and 62-a were used to make a trial batch of baddeleyite inserts which were fixed into mullite-corundum bases, impregnated with pitch, and tested in the ladles for casting steel into ingot molds, and with continuous casting.Tests confirmed that the inserts with the higher thermal-shock resistance (6 and 62a) during casting of manganese steels from 170-ton ladles at the Donets factory had lower wear. The wear of the edges due to the movement of the gate valve equals 3-5 mm (for 4 and 5 the edge wear reached 7 mm). This means we can recommend these inserts for industrial testing in steel casting.

The Ukrainian Institute of Refractories has developed and introduced a production method for high-temperature, mullite--corundum nozzles for lining induction vaccum crucible furnaces designed for melting heatresistant alloys based on nickel, whose other components are chromium, tungsten, and cobalt.The properties of the nozzles before service are: open porosity 15.3-17.9%, apparent density 3.07-3.11 g/cm 3, compressive strength 81-99 MPa, refractoriness under load of 0.2 MPa above 1750~ thermal-shock resistance (1300~ --water) more than 25 cycles, content* of A1203 90.81%, SiO 2 7.85%, HF-insoluble residue 95.92%.Industrial tests established that the life of induction furnaces lined with mullite--corundum nozzles reaches 400 heats. This is 3 times longer than the life of rammed linings made from fused magnesite. Using nozzles there is approximately a 10% reduction in scrap castings through nonmetallic inclusions.The main parameters for melting are: alIoy melting temperature 1650-1700~ melting time 20-30 rain, including melting and drainoff 15-25 rain, preparing the nozzle and off-loadingtheblanks 5-7 rain. During each cycle the nozzle is subjected to sudden heating and cooling within the range 1700-800~At least every 4 heats the walls and bottom are cleaned mechanically to remove metal encrustation and oxide films. The main cause of cessation of operations is the formation and later deveiopment of cracks in the nozzle wails. Clearly, sudden variations in temperature and mechanical action during cleaning help develop the cracks.The reduction in the nozzle's wall thickness during a campaign (400 heats) does not exceed 55 mm.The present authors studied nozzles after service in order to explain the physieochemieal processes taMng place in them.Petrographic studies showed that three zones can be seen in the nozzles: least change, transition, and working (see Table 1).The least-changed zone is similar to the refractory before service. The structure is of a compacted granular type, white in color. The zone consists mainly of grains of corundum with predominant sizes of 0.05-0.6 mm and mulIite and bonding mass consisting of translucent fine-grained substance (particles measuring 2-4#m more rarely up to 20#m. The glass phase content is moderate. * Here and subsequently indicates weight proportions. Trace *Contains also 2-3% nickel alloy and 3-5% NiO.