One of the trends in replacing design elements made of costly nickel alloys for volume isothermal working of heat-resistant alloys at 850-I000~ is the use of insulating backing tiles and reinforcement of the stamps with ceramic inserts.It is desirable to use muliitecontaining refractories for making these components because the presence of the glass phase increases the mechanical strength in the range 800-I000~[i]. Large mullitic articles can be obtained by the slip method [2]. The rheological and bonding properties of mullite suspensions have now been thoroughly investigated [3][4][5]. However, data on the production of ceramic material of a granular structure for operation at elevated temperatures are almost unavailable.It is known [3] that it is desirable to use mullite and electrocorundum as the filler for granular specimens.The present article* makes an assessment of the temperature relationship for the strength of the specimens based on mullite suspensions with various original concentrations of A1203 with and without filler, and shows the possibility of using such materials for design elements.The starting material for making the specimens consisted of scrapmullite refractories in fractions minus 0.05 mm, and high-alumina chamotte VGSh-85 (intermediate product) produced by the Semiluksk refractories factory.The suspensions were prepared by wet ba!imilling using quartz (bearing in mind the pickup of amorphous silica in fractions minus 0.05 mm) and corundum linings.
Magnesial refractories whose basic mineral component is periclase occupy a dominating position among the main refractories [I, 2]. The magnesial products are fired or unfired and are produced in the form of molded refractories and concretes [3][4][5]. It is well known that periclase refractories have excellent refractoriness, slag-and metal-resistance, but a low thermal-shock resistance as a result of the s%gniflcant thermal-expansion of MgO. However, under special conditions [20], it is possible to obtain adequately thermal-shock resistant, pressed periclase refractories in which cracks first appear after 9-12 heat cycles from 1300~ to running water and which will withstand more than 20 thermal shocks under plasma heating to 2100~The special features of such refractories are the significant (up to 40%) concentration of coarse (5-10 mm) fractions and a fragmentary structure formed after sintering at 1700~The grain composition of the pressed and fired refractories is close to that of the unfired periclase concrete obtained by vibro-molding Or casting [3,4]. Normally their composition includes 75% of fused periclase or cullet in fractions of 0-15 mm; periclase cement (finely milled periclase with a predominant particle size of 0.06 mm); and 7-18% of binder (normally a solution of sodium polyphosphate, density 0 = 1.35-1.45 g/cm S or water glass, p = 1.30-1.35 g/cmS). The periclase refractories are often used in furnaces for smelting high-quality heat-resistant alloys [3, p. 188] and therefore an important requirement is that they contain no low-melting oxides, for example Na20, in the added binder.In the present paper we report some results of a study of the process for obtaining vibro-cast periclase refractories based on the fine-and coarse-grained molded systems using as the binder a low-concentration (5%) HCI solution. Such materials have a high mechanical strength in the initial (unfired) state and can be considered as concretes. We also studied the process of sintering the obtained vibro-cast materials and show that their properties are comparable with those of the normal fired periclase refractories. The production of such materials was based on the tendency of the finely milled periclase in conjunction with the HCI solution to form an air-hardening bonding with a fairly high mechanical strength and limited hydration [6].As in the previous studies [6,7] we used perlclase with a higher concentration of MgO (98.02%). The studies were carried out on two grain compositions selected from the results of previous studies: a fine-grained composition (the filler was periclase with a particle diameter Df = 0.1-3 mm) and a coarse-grained (filler was periclase with Df = 0.i-i0 mm). In the first case (Fig. i, curve i), the mass fraction of the finely dispersed perlclase (cement) was 30% and in the second, 20%. This was made necessary because of the need to reach a maximum density of the mixture on molding. In the case of the fine-grained composition the disperseness of the cement was characterized by a specific surface o...
Aqueous and thermoplastic slips have been used in casting corundum components for use in the refractories and metallurgical industries. The characteristics of these components are given. This company has researched making ceramic materials and developing technologies based on them for various areas in industry, including metallurgy. Developments that have found practical use include the technology for making corundum components by casting, and those components have been successfully used. Corundum ceramics are fairly familiar and are widely used for their properties: resistance to corrosive media and viability at high temperatures (melting point of corundum 2050°C). Table 1 [1] gives the compositions and properties of electromelted refractories.A corundum-mullite material developed by this company has been used in crucibles for melting nickel alloys. The material of such crucibles should not interact with the metal and should not contaminate the alloy and should work at temperatures above 1600°C. The materials from which such crucibles are traditionally made are as follows: periclase (magnesite), magnesian spinel, corundum, mullite, and combinations of them. At many metallurgical organizations, nickel alloys are melted in crucibles made of periclase with the addition of corundum and a binding agent. Recently, more frequent use has been made of cast crucibles from corundum, corundum-mullite, corundum-spinel, and periclasespinel materials [2].The corundum-mullite material of grade OTM-959 is made by slip casting from a highly concentrated suspension (slip) with granular filler. The slip is made in ball mills by wet grinding of the initial material and with the addition of an electrolyte to accelerate the grinding. This has given mullite and corundum slips. Figure 1 shows the effects of firing temperature on the strength and shrinkage of slip castings from fused mullite and electrocorundum. The strength of the electrocorundum castings increases with firing temperature up to 1750°C, while that for castings of mullite after firing at 1580°C is reduced, so electrocorundum was chosen as the basis for high-temperature components, while mullite was added to the corundum slip along with other additives as a filler. The shrinkage of the slip castings at a firing temperature of 1750°C was up to 5%, while when a filler was added, it did not exceed 1%.Adding large mullite grains to finely divided slip corundum basis provides a microcrack structure in a material and raises considerably the thermal resistance. The basic
A corundum-mullite material has been developed for service at high temperatures under heavy-duty load-carrying conditions and in corrosive media. The material can be used to fabricate a wide range of industrial products: burner stones, stools, stands, supports, protecting sleeves, etc.OTM-959-grade corundum-mullite material for service under conditions of high temperature, mechanical loading, and corrosive medium has been developed. The material is prepared from aqueous suspensions with the addition of a granular filler at the stage of shaping. The major raw precursors are electrofused corundum and fused mullite, which are readily available and not very much expensive materials, both maintaining strength at high temperatures.Using an electrofused corundum in the presence of an electrolyte, a highly concentrated aqueous suspension (a slip) with a density of 2.7 -3.0 g/cm 3 and good binding properties was prepared by wet grinding in a ball mill.Prior to shaping, a granular binder was added to the slip, and the mixture was thoroughly homogenized to a slurry and was cast into plaster molds. The green performs were dried and then sintered at 1450 -1550°C in air. This technology showed very low (if any) sintering shrinkage, which made it possible to fabricate complex-shaped large-sized components with a mass of up to 60 kg. Depending on the component's dimensions and service conditions, the amount of the granular filler and its fractional composition might vary. To enhance thermal stability, 5% graphite could be added to the precursor mixture.Properties of the OTM-959 material are given below:
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