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.

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.

No abstract

Refractory mortar is used in the preparation of block construction ceramics.The working capacity of such articles is mainly assessed by the quality of the joints which is provided by a combination of properties in the mortar that is used, and especially its cementing properties and its thermal shock resistance.One of the main factors affecting the cementing strength of the mortar after high-temperature treatment is the composition of the filler in the refractory mortar, which should be close to the chemical composition of the material being cemented.The present work was concerned with a study of a refractory mortar having a mullitecorundum-zirconia composition with a phosphate bond for cementing mullite-corundum blocks, corundum and chamotte (fireclay) bricks.The blocks were made by water casting of mullite containing suspensions with the introduction of finely milled electrocorundum.The properti~ of the blocks were described in [i].The composition of the refractory mortar contained mixed filler in the form of components present in the composition of the cemented specimens: mullite-corundum chamotte, and alumina, In order to increase the plasticity we added fireclay, and to increase the thermalshock resistance --zircon.All the components were added in the finely dispersed (minus 0.08 mm) state which contributed to the more complete reaction with phosphate binder, and moreover, enabled us to obtain thinner joints which is especially important in the creation of construction products.The investigation involved using refractory mortar containing mullite-corundum powder together with alumina (47%), zircon (20%), fireclay (4%), and aluminoborophosphate bond (ABFS) with a density of 1.56 g/cm 3 (TU 113-08-10-17-83).Before use the bond was diluted to a density of 1.40-1.45 g/cm 3, and was added in amounts of 29%, which ensures the necessary consistency.After careful mixing the solution was applied in a thin layer to the surface of two articles to be bonded.The excess mortar was pressed out during placement.