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.
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