Variables in the Load Test for Fire–Clay Refractories

The so–called load test, as applied to refractory fire–clay brick, was studied from two points of view. The present A.S.T.M. standard test procedure is found to give results, on specimens smaller than the standard size, that depend on the cross section and length of the specimen. A more informative and more reliable test, constituting the second part of the study, consists in recording the curve of deformation against time at a constant temperature. The deformation of fire–clay brick at 2100° to 2550°F. is an exponential function of temperature, and the flow up to 24 hours fits the equation for flow of glass. At constant temperature, between 2100° and 2550°F., the permanent deformation after cooling is proportional to the load for the range 15 to 40 lb. per sq. in.     

Bonding mechanism and performance of rectorite/ball clay bonded unfired high alumina bricks

In this study, the bonding mechanism and normal/high temperature performance of rectorite clay or ball clay bonded unfired high alumina bricks were investigated by using different techniques (XRD, TG-DSC, SEM, particle size distribution and rheology). The results showed that clay particles are separated into layer structural units in water due to the hydration swelling and electrostatic repulsive force, and rectorite layer structural units have larger aspect ratio than ball clay. Rectorite layer structural units form band-type structure with “face to face” after drying results in better bonding performance than ball clay (card-house structure with “edge to face”). The cold crushing strength of 9% rectorite/ball clay bonded unfired high alumina bricks after firing at the dehydroxylation temperature for 3 h reach 71 MPa and 50 MPa, respectively, and which can satisfy the strength requirement for the transportation and use of most high alumina bricks. The secondary mullitization and lower liquid phase content of ball clay bonded unfired high alumina brick under high temperature cause it has higher refractoriness under load and lower linear shrinkage than rectorite clay bonded brick. The T_0.6 refractoriness under load of 9% rectorite/ball clay bonded unfired high alumina brick are 1262.6 °C and 1580.3 °C, respectively. Thus, the 9% ball clay bonded unfired high alumina bricks have wider service temperature range than 9% rectorite bonded bricks.     

METHOD FOR DETERMINING TENSILE PROPERTIES OF REFRACTORY MATERIALS AT ELEVATED TEMPERATURES*

Equipment for testing ceramic materials to temperatures of 2000°F. was developed, and a method was devised for evaluating the bending stresses introduced by the test equipment. With this equipment, the tensile strength, stress-to-rupture characteristics, and modulus of elasticity of a sillimanite refractory were investigated at the Cleveland Laboratory of the National Advisory Committee for Aeronautics. The tensile strength varied from a minimum of 8000 lb. per sq. in. at 500°F. to a maximum of 19,000 lb. per sq. in. at 1800°F. Heat-treating the tensile specimens for one half hour at 1800°F. increased the tensile strength 35% at room temperature and 70% at 500°F. No increase in strength was noted at or above 1400°F. The stress-to-rupture in 1000 hours at 1600°F. was 8500 lb. per sq. in. The modulus of elasticity at room temperature was 20.3 × 10⁶.     

RELATION OF CRUSHING STRENGTH OF SILICA BRICK AT VARIOUS TEMPERATURES TO OTHER PHYSICAL PROPERTIES*

The failure at elevated temperatures under constant load for silica brick is reported using the Iupuy load test apparatus. The crushing strength at 1500°F, 1800°F. 2100°F, and 2400°F is recorded, as well as the crushing strength at room temperature. The size of test piece utilized normally was 1 by 1 by 2′/2 inches. A definite relationship is shown to exist between the strength at room temperature and that at elevated temperatures. The effect of variation in lime content, bats content, and fluxes is also reported. Data were obtained on brick made from three different quartzites. Additional physical data are reported to give information concerning the properties of the brick tested.     

HIGH TEMPERATURE LOAD AND FUSION TESTS OF FIRE BRICK FROM THE PACIFIC NORTHWEST IN COMPARISON WITH OTHER WELL-KNOWN FIRE BRICK1

Seventeen samples of fire-clay brick from the Pacific Northwest have been tested with twenty-seven other commercial brands of fire clay, silica, magnesia, chromite, zirconia, diaspore, silicon carbide and crystalline alumina, as well as china clay and crystalline sillimanite products made at the University of Washington. The tests show that the fire-clay brick of the Pacific Northwest vary considerably in quality. According to the high temperature load test, the majority of the local brick are among the upper grades, some are to be classed with the best fire-clay brick and one equal to the best diaspore brick. The brick tested is not the best which can be made from Pacific Northwest materials, for the kaolins in eastern Washington and northwestern Idaho give opportunity for the production of an all-kaolin fire brick. A method is suggested for testing super-refractory materials under load at high temperatures similar to the standard load test for fire clay and silica brick except that the temperatures are measured by cones, and are raised until 10% linear deformation of the brick is obtained. The rate of heating and soaking varies with the brick under test, and the principles learned from the cone fusion test are used in the application of heat. A numerical value, expressing the area under the cone-shrinkage curve, affords an easy method for comparing the high temperature load resistance of various refractories. The brick which are best able to resist deformation at high temperatures are composed of crystalline materials which have developed a recrystallized bond of the same composition. These are crystalline silica, silicon carbide, corundum and sillimanite, and they resist deformation at temperatures close to their melting points. Amorphous materials like fire clay, diaspore, bauxite or even the very refractory crystalline materials lie chromite and periclase, which depend on amorphous silicates for a bond or are contaminated with silicate impurities. will fail with the softening of the bond of the amorphous impurities. The cone fusion of the brick as a whole can not be depended upon to indicate the resistance to load at high temperatures.     

A STUDY OF THE FACTORS INVOLVED IN THE SPALLING OF FIRE CLAY REFRACTORIES WITH SOME NOTES ON THE LOAD AND REHEATING TESTS AND THE EFFECT OF GRIND ON SHRINKAGE1

Of the three factors, elasticity, coefficient of expansion and rate of temperature change, which affect spalling, the former is by far the most important. Only small differences are found between fire clay mixtures of widely varying structure and composition in the rate at which they change in temperature under like conditions of heating. The coefficient of expansion varies directly with the silica content and differences in this respect of large order were found. However, the spalling on the particular mixtures tested varied almost inversely as the coefficient of expansion. This apparent discrepancy is explained on the basis of greater elastic properties of the brick which had high expansions. The elasticity may be varied between wide limits and is sufficiently important as to overbalance the effect of greater expansion. This property is accordingly the one upon which efforts directed toward the development of non-spalling brick should be centered. It was discovered that a plastic deformation could be obtained at as low a temperature as 635°C. This gives the effect of elasticity and undoubtedly has considerable influence on spalling at the higher temperature ranges. Results are given for a number of load tests which show clearly the importance of hard firing. The secondary expansion of brick made from Pennsylvania flint clay is shown to be influenced by the temperature of reheating, as well as its rate. Detailed results showing the effect of grind and firing on the finished size of the brick included in the investigation are also given.     

CORRECTION OF AN EXTREME CASE OF CRACKING IN THE DRYING OF BRICK1

I. In the manufacture of stiff-mud brick from a highly colloidal clay of low permeability to water, drying breakage was extremely high even though the drying was conducted at a very slow rate under high humidity conditions. Laboratory investigation yielded two methods of correcting the faults: (1) Preheating the clay for half an hour at a temperature between 400 °C and 500 °C increased the permeability to such an extent that brick made from the preheated clay could be dried rapidly without cracking. (2) Coagulating chemicals, such as aluminium chloride, ferric chloride, sodium chloride, and hydrochloric acid, in conjunction with moderate additions of grog, increased the permeability and thus improved the drying properties of the clay. II. Plant scale tests using ferric chloride, sodium chloride, and grog resulted in the production of brick which could be dried safely in a reasonable time. The fired brick were improved in quality as to strength and color. III. The chemical treatment of the clay using 1% ferric chloride and 0.5% sodium chloride with 10 to 15% grog was adopted for plant operation and resulted in increased production, lower cost of manufacture, and improved quality of product.     

THE REVERSIBLE THERMAL EXPANSION OF REFRACTORY MATERIALS

The reversible thermal expansion from 15–1000°C was measured for kaolin, siliceous and aluminous fire clays, quartzite, alumina, magnesia, and carborundum, after preliminary burnings at cones 06, 9, 14 and 20, and as well as for English commercial silica bricks before and after use in a coke oven and the roof of a steel furnace. Kaolin and bauxitic fire clay after calcination have a regular reversible thermal expansion which does not vary much with the temperature of calcination. Siliceous fire clays, after calcination at cone 06 (980°C) or cone 9 (1280°C) display irregularities (departures from uniformity) in their expansion. Between 500° and 600°C they show a large expansion due to contained quartz and on cooling the contraction in that region is larger than the corresponding expansion. Moreover, the expansion between 100° and 250°C after being fired to cone 9 (1280°C) exceeds the average. After calcination at higher temperatures, cone 14 (1410°C) or cone 20 (1530°C). these materials gradually lose these peculiarities until on incipient vitrification a linear expansion similar to that of kaolin is attained. This change is due to the destruction of quartz by its interaction with the clay material and fluxes; it takes place most easily in a fine-grained, rather friable clay such as ball clay. The previous thermal treatment necessary for a particular clay in order to obtain regular expansion in use can only be determined by trial. It can be stated with confidence that in such a piece of apparatus as a glass pot or crucible, a distinct gain will result from maintenance at a high temperature for some time before use, but that the red heat of an ordinary pot arch is useless for the purpose. An increase in the porosity of a fire clay was accompanied by a corresponding decrease in expansion between 15° and 1000°C until a porosity of 50% was attained. Further increase in porosity produced very little change in the expansion. No irregularities in expansion were shown by magnesia brick, carborundum, or alumina bonded with 10% of ball clay. Welsh quartzite with lime bond, either unfired or after burning at cone 06, had a large expansion to 550 °C and a much larger expansion from 550–600 °C due to the inversion of α to β quartz while from 600–1000°C a slight contraction took place. Firing to cone 9 converted part of the quartz into cristobalite, thus increasing the expansion from 200–250°C. This conversion was considerably increased on burning for two hours at cone 14, which greatly reduced the expansion from 550–600°C with a corresponding increase of that from 200–250°C. The conversion of the quartz into cristobalite was completed by a further heating for two hours at cone 20. Determinations of refractive indices and specific gravities confirmed these results. Flint inverted to cristobalite with greater ease than quartz. Commercial silica brick consisted chiefly of cristobalite and unconverted quartz and showed a large expansion up to 300°C, followed by a considerably smaller but regular expansion to 550°C. From 550° to 600°C the rate of expansion was considerably increased, but above 600°C the change in dimensions was small. The innermost exposed layer of a silica brick after use in a coke oven was an impure glass with a steady expansion, but only half as large as that of the layers of brick behind, which was made for shelling away. A silica brick after use in a steel furnace was divided into four layers. The layer exposed to the furnace heat was practically all cristobalite and silicates, the next layer the same, the third layer showed some α to β quartz expansion as well as the α to β cristobalite expansion, while the fourth (outermost) layer exposed to air was similar to the brick before use. In these bricks exposure to high temperature had evidently completed the change from quartz to cristobalite which had been largely effected in the kiln during manufacture. Little or no tridymite had formed. The reversible thermal expansion from 15–1000°C of the commercial silica brick examined was 1.1 to 1.3%, about double that of fire clay brick.     

EFFECTS OF VARIOUS CLAYS USED IN MILL ADDITIONS ON PROPERTIES OF A TITANIA-OPACIFIED ENAMEL*

In the course of investigations made in connection with the development of titania-opacified enamels, it was observed that color, reflectance, and acid resistance were influenced by the clay used in the mill additions. In a subsequent search for a clay which would produce the best over-all enamel properties, a study of the effects of twenty clays of different types was made under standardized conditions of milling, spraying, and firing. Coatings sprayed at 30 gm. per sq. ft. were examined for bisque strength, brushing behavior, and tearing. After firing for 3 minutes at 1540°F., and at 1640°F., the reflectance, color, gloss, and acid resistance of the enamels were observed. It has been concluded that the success or failure of an enamel is determined to an important degree by the clay used in the mill addition.     

PURE BERYLLIUM OXIDE AS A REFRACTORY*

The development of a pure BeO refractory is described, using pure Be(OH)₂ as the starting material. The hydroxide is calcined at 1200° (low) to 1800°C. (high), depending on the crystal size desired. The calcine is ball milled wet in a steel mill and cast at pH 4.5 to 5.0, or the powder may be dried and mixed with 14% Carbowax 4000 added as a water solution for dry pressing at 20,000 to 30,000 lb. per sq. in. The pressed product is vitreous when fired at 1800°C. but is also volatile at that temperature.     

THE EFFECT OF CALCINED CYANITE IN PORCELAIN BODIES1

A study was made of the physical properties of porcelain bodies containing varying amounts of calcined cyanite (mullite), feldspar, and flint with a constant content of 50% clay. All bodies were made up under uniform conditions and fired to their proper maturing temperatures which varied from cones 8 to 20. Moduli of rupture for correctly fired bars varied from 5100 to 12,100 Ibs. per square inch, those bodies high in mullite being the strongest. The coefficients of linear expansion between 30 and 844° C varied from 4.3 × 10⁻⁶ to 6.6 × 10 ×⁻⁸ 6, those bodies high in mullite having the lower expansion. Thirty per cent or more calcined cyanite (50% or more total mullite as figured from chemical composition) must be present to obtain a marked effect on either of these properties. Triaxial diagrams show the position of various typical porcelain bodies and how they might be affected by additions of this material.     

USE OF ALUMINUM METAL IN THE CERAMIC INDUSTRY I,PROPERTIES OF REFRACTORIES PRODUCED WITH MIXTURES OF ALUMINUM,FIRE CLAY,AND GROG*

The addition of aluminum metal powder to fire-clay-grog mixtures greatly increased the strength of the fired brick as a result of an aluminothermic reaction between the metal and the silica in the clay and grog. Because the reaction takes place at 930°C. and causes the temperature to rise rapidly, it is necessary to heat these refractories only to 930°C. to produce hard, well-fired brick. Such products have a high load-carrying capacity at furnace temperatures and also a fair spalling resistance.     

ELASTIC BEHAVIOR AND CREEP OF REFRACTORY BRICK UNDER TENSILE AND COMPRESSIVE LOADS*

Specimens cut from 9-in, brick of nine brands of firebrick, including two high-alumina, four fire-clay, two siliceous fire-clay, and one silica, were subjected to tensile and compressive creep tests at eleven temperatures from 25° to 950°C., inclusive. The duration of each test was approximately 240 days. Small length changes, independent of stress direction (that is, compressive or tensile), occurred at the lower temperatures. The lowest temperatures at which creep was significant were (a) high-alumina brick, 700° to 850°C.; (b) fire-clay brick, 600° to 700°C.; and (c) siliceous and silica brick, 950°C. Creep results under compressive stress could not be correlated with results under tensile stress. Specimens of different brands, at 950° C. showed greatly different capacities to carry load. Repeated heatings caused growth of silica brick of approximately 0.27%. Moduli of elasticity at room temperature were determined before and after the various heat-treatments and resultant changes were recorded. The changes in moduli were 15% or greater for silica and siliceous brick and 4% or less for the fire-clay brick. The moduli of elasticity at room temperature were approximately 2.7–4.3 × 10⁶ for high-alumina brick, 0.6–1.9 × 10⁶ for fire-clay brick, 0.3–1.7 × 10⁶ for siliceous fire-clay brick, and 0.4 × 10⁶ for silica brick.     

INFLUENCE OF WATER VAPOR ON SILICA BRICK AT HIGH TEMPERATURES*

Some tests are described which show that water vapor does not cause softening of silica brick at a temperature of 2900°F in a sixteen-hour laboratory test     

INVESTIGATION OF THE EFFECT OF LOW-FIRED GROGS ON PROPERTIES OF SOFT-MUD BRICK*

Soft-mud brick were made from various mixtures of Hudson River clay and grog prepared by calcining the clay at 500°, 700°, and 900°C., and rate of drying tests were carried out. The brick were fired to cones 012, 08, and 04, and a special fast fire to cone 08 was also run. The various properties were compared with those of regular mix brick and all-clay brick. Brick made of a mixture of clay with 35% of 900°C. grog had particularly satisfactory properties such as to afford definite advantages as regards faster drying and accelerated firing.     

THE SOLUBILITY OF QUARTZ AND CLAY IN FELDSPAR1

The literature on the subject is completely reviewed. The method used involved the addition of increasing amounts of kaolin to fixed amounts of feldspar and the addition of increasing amounts of quartz to fixed amounts of feldspar. All compositions were examined microscopically and the presence of undissolved kaolin was shown by the appearance of mullite needles, while the presence of undissolved quartz was shown by the presence of cristobalite. Soda feldspar is a better solvent for both quartz and clay than potash feldspar. The solution of quartz in soda feldspar starts at about 1350°C and increases with the temperature until at 1425°C. 32 parts of quartz per 100 of feldspar are dissolved. The solution of quartz in high potash feldspar does not start until 1400°C and only 4 parts are soluble at 1425°C to every 100 parts of feldspar. The solution of clay in soda feldspar starts at 1225°C and increases with the temperature until at 1425°C thirty-six parts are soluble. The solution of clay in high potash feldspar starts at 1250°C and at 1425°C the solubility is 20.5 parts per 100 of feldspar. The solution of quartz in porcelain bodies starts at the same temperature as it does in the pure feldspar which the body contains. The amount of solution per unit of feldspar is much greater. The general trend of one of the boundary lines in the ternary system K₂O-A1₂O₃-SiO₂ has been found.     

THE MODULUS OF ELASTICITY AS AN INDICATION OF THE UNIFORMITY OF ELECTRICAL PORCELAIN1

An electrical porcelain body of the composition 30% H and G A1 china clay, 20% Dorset English ball clay, 20% Ottawa flint, and 30% Buckingham feldspar, fired to cone 11. showed a modulus of elasticity of 9.54 ± 0.02 × 10⁶ 1b. per sq. in. (mean of 164 measurements). A similar body with Kentucky special ball clay substituted for Dorset hall clay gave a value of 10.01 ± 0.02 × 10⁶ 1b. per sq. in. (mean of 69 measurements) The accuracy of the measurements was ± 1.5%. The use of the modulus as a measure on. the uniformity of electrical porcelain gave excellent results The variations were expressed in terms of statistical constants such ac: the standard deviation and the coefficient of variability.     

ZIRCONIA CEMENTS1

The effect of calcined zirconia on zirconia cement. Seven refractory cements were made having compositions of zirconia 90 per cent and plastic clay 10 per cent. (1) The shrinkage was found to be excessive in a cement containing raw zirconia and clay. The addition of 50 per cent or more of calcined zirconia practically eliminated this shrinkage and the cracking which accompanies it.(2)Strength Draw trials showed that the cement became strong at 1200°C and that it was very strong when burned at 1700°C. (3)Load tests on piers at 1500°C showed that joints of these cements did not fail in any manner at this temperature. (4) An industrial test of zirconia cement used as a wash for bungs in a malleable iron furnace showed that the life of a bung was increased about 25 per cent by the use of a zirconia wash.     

TESTS OF FIRE-BRICK MADE FROM GANISTER,FLINT CLAY,AND PLASTIC CLAY MIXTURES,WITH SPECIAL REFERENCE TO SPALLING1

Influence of the alumina-silica ratio on properties of fire-brick.—Five experimental batches of fire-brick were made by mixing various proportions of ganister, flint clay and plastic clay in such a way as to vary the silica content from 53 to 77 per cent and the alumina content from 43 to 20 per cent. (1) The fusion points were found only slightly lower than those of corresponding pure silica-alumina mixtures. (2) Load tests at high temperature showed that the behavior under compression does not depend on chemical composition so much as on other factors such as the temperature of burning. (3) The resistance to spalling , as tested by alternate heating and dipping in cold water, was found to decrease as the temperature of burning was increased from 1300° to 1400 °C. The higher silica bricks were relatively more resistant at the lower temperature but not so at 1400 °C. Therefore the substitution of ganister for flint clay increases the resistance to spalling at moderate operating temperatures but is of no advantage at 1400° or above.     

SOME INVESTIGATIONS OF A NEBRASKA CLAY1

A brief review of the literature on the fluxing action of different metallic oxides on clays is given. A series of mixtures of what seemed to be the “dirtiest” clay of four obtained, with varying amounts of pure calcium carbonate which passed a 150-mesh screen was made up into briquets and physical properties were studied on the unfired and fired specimens. The firing was done at four temperatures, 1800°F, 1900°F, 1950°F, and 2000°F. Properties observed were fusion point, drying and firing shrinkage, crushing strength, and porosities of the fired specimens. The principle of the Armstrong volumeter used for porosity determinations is explained. The results obtained indicate possibilities of materially improving qualities of brick now being produced from so-called ordinary clays but only after a very considerable amount of further work is done.