Materials in the AI203--ZrSiO, system, as research has shown [1][2][3], are of great interest in view of their high thermal-shock resistance and strength, including performance at elevated temperatures [4].Two methods are known for obtaining suspensions of mixed composition in the AI203--ZrSi04 system: staged suspension and mixing of suspensions. Using both methods, the starting materials consist of powders, dry milled in metal mills, followed by washing to remove tramp iron in hydrochloric acid [5]. In obtaining slips by the first method the ZrSiO~ is added to water with a regulated pH (2.0), and after 5-min mixing the alumina is added, followed by combined mixing for 30 min, and casting.Using the second method individual suspensions are first obtained with the optimum parameters; then they are mixed in the stated ratios [6]. The porosity of casts from the first method came within the range 45-48%, and for the second 35-40%.In this article we examine the results of deyeloping a method for obtaining mixed suspensions in the system AI20s (80%)--ZrSi04 (20%) with increased concentrations, their bonding properties, and the density of the cast articles.Taking previous work [7,8] into account, suspensions were obtained by direct combined wet milling in a ballmill using corundum linings and grinding bodies.The original materials were powdered electrocorundum (about 99% AI=Oa) as specified by OST 2MT71-5-78 and zircon (65.5% Zr02, 32.7% SiO=) as stated by OST 48-82-81 with a specific surface measured with the PSKh-8 apparatus equal to 4000 and 4500 cm2/g. To obtain a high concentration wet milling was done with portion charging of the material [7,8]; and in this case the additions of HCI were used to keep the pH of the medium close to the optimum for deflocculation (pH = 3.5-4.0). The first stages of grinding were done on the electrocorundum (as being the more difficult to grind).Before adding the 20% ZrSi04 the suspension had a density of 2.38 g/cm 3 (C V = 0.46), a viscosity of 1.5 ~ and a predominant concentration of particles measuring not more than i0 ~m. After the zircon had been added combined milling was done in 20 h.The density of the slip thus obtained was 2.60 g/cm 3 (C V = 0.54), water content 17%, and pH = 4.0, and was characterized by a relatively polydispersed grain size composition (Fig. i): the content of particles of diameter up to 5 Dm was 60%.The peculiarities of the resulting slips were the aggregative instability during storage (aging), and the marked coagulation during mechanical mixing.This was due to the fact that the pH of the suspension on standing increased into the nonoptimum region, which was accompanied by a rise in the rheological factors, and as a result in the porosity of the casting (Fig. 2).From an analogy with data in [9] the increase in the pH of the resulting mixed suspension can be explained by the contribution of A1203.It is known that an increase in the pH value of aluminous suspensions in the acid region according to [9] can be explained by the reaction NaA102 + 4HCI = NaCI § A...
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.
As is well known [1][2][3][4], the isostatic method of pressing is one of the prospective methods in the production of refractories. The authors*of the present article developed a mold for making crucibles 150 mm high and 220 mm in diameter by the isostatic method using a "free matrix" scheme. The mold is shown in Fig. 1. It consists of a rigid matrix-ring 1, fitted on a thread in a flange 2 placed on the columns 3, and spring loaded on the axle of the press mold with springs 4. The external rigid-plunger 5 is connected with the upper plate 6. The internal elastically deformable plunger (press buffer) 7 made of polyurethane is fitted to the rigid cylindrical table-pusher 8 connected by means of a shaft 9 and an ejector 10 with the press ejector 11. The annular ejector 12 is installed concentrically on the table-pusher 8 on a cross piece 13, connected with a locking device 14. Two electromechanical vibrators 15 of the type PV-66 are fitted to the flange 2, and the intermediate plate 16 --to the bottom plate 17. The regulating supports 18 are placed on the upper plate 6.
It is known [i, 2] that the strength of the mullite-bearing materials increases at 1000-II00~ owing to stress relaxation resulting from softening of the glassy phase. This property can be realized to certain extent in the structural components like backing tiles (lining) and die inserts working at a temperature of 900-I000~ under a stress (specific load) of 100-200 MPa with a long safe life. For this purpose, a material having a granular structure is particularly used. It is obtained by the cold casting method and is based on a system consisting of mullitic chamotte (binder) -a mixture of chamotte (0.4-l-mm fraction) and the No. i0 -No. 50 electrocorundum (filler) [3].However, mullite-bearing granular materials cannot be used for higher service temperatures (1000-1100~and stresses up to 200-300 MPa with simultaneous reduction in the cost of production of the components (for example, engraving on the previously fired preforms without using a diamond tool). At II00~ their ultimate compressive strength is not high (200 MPa) and during the turning operation, the coarse grains show pitting and the pores are opened up.In view of this, we carried out additional investigations for obtaining a mullite-corundum material with a dense structure and improved thermomechanical characteristics. The production technology of this material is also based on the principle of casting in gypsum molds using highly concentrated suspensions with a filler [4].In order to prepare the suspension, we used a mullitic chamotte made from the scrap of the MLS-62 products containing 63%* AI20 s (GOST 24704-81). The chamotte was subjected to wet milling using corundum balls in the mills lined with quartz according to the previously described technology [3, 4]. The suspension had a density of 2.30-2.40 g/cm 3 when the volumetric concentration of the solid phase amounted to 0.65-0.70; the discharge (flow) time recorded in the Engler viscosimeter was 1-1.5 min at pH = 9-10; and the dispersed phase had the following granular composition: the O.O01-O.Ol-mm fraction 30-35% and the 0.01-0.05 mm fraction 65-70%.During the grinding process 3-5% amorphous silica was ground from the quartz lining. During firing, amorphous silica aids the sintering process owing to the formation of a glass phase and facilitates the process of formation of secondary mullite by interacting with the electrocorundum filler.As a filler we used electrocorundum (OST 2 MT-71-5-78, grades 23A and 24A) in the form of fine micropowder M5 or ground powder No. 10-No. 50. The filler was subjected to dry milling using corundum balls in ball and vibrational mills having corundum lining. All the obtained powders of electrocorundum had a particle size less than 0.05 mm.*Here and elsewhere, weight contents are given.
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