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Refractories, Structure and Properties of
Refractories, Structure and Properties of The definition of ‘‘refractory’’ employed as an adjective is variously applied; to people, it means obstinate or unmanageable, and to things such as ores, it means hard to reduce or to fuse. This latter usage is correct for refractory ceramic materials, which possess the key characteristic of refractoriness, i.e., they are difficult to fuse. Refractory ceramics are inorganic chemical substances, single or polyphase in nature, which are processed at high temperature and\or are intended for high-temperature applications. They are employed wherever a process involves treatment or exposure at elevated temperatures, e.g., the smelting of metals, the sintering of ceramics, the melting of glass, the processing of hydrocarbons and other chemicals, etc. The selection of refractories for widely varied applications depends almost exclusively on the process temperatures and how they are programmed as well as the chemistry of the materials being processed. This includes the chemistry of the atmospheres that develop during the processing. Direct contact with corrosive liquids often requires the refractory to have the highest density attainable and compositions of the respective phases that are chemically compatible with the processed material. There are new systems appearing, however, in which the refractory ceramic is designed to react with the corrosive material so as to form a resistant phase at the interface, or, in situ, in the interior of the refractory-lined confinement system: furnace, kiln, conduit, combustion chamber, etc. The properties and behaviors of refractories in service must remain at or above specified threshold levels in order for the refractories to endure for a costeffective time period. In severe exposures the refractories may only last for days or weeks, e.g., in steel ladle use or in the melting of very corrosive glasses. However, some refractories serve for a decade or more, e.g., silica brick crowns of conventional glass melters and silica bricks used in the construction of coke ovens. This article focuses on the properties and behaviors which are most important for the selection of refractories for the most demanding applications. It will also treat the difficulties encountered in the property measurements and in their use.
or plastic molds. After drying and curing the parts may be prefired or they may be installed directly into the lining of a furnace. The second category, unshaped refractories, also termed monolithics, includes the precast shapes described above as they begin as loose powders and grains mixed with binder materials. The grain\powder mixtures are combined with a liquid, usually water, and are cast, puddled, trowled, gunited, shotcreted, or hand placed into forms or onto vertical and horizontal surfaces to form monolithic masses of refractory. Such monolithics have been used for many decades but the development of high-purity calcium aluminate cements and of stable phosphate binder systems in the middle of the twentieth century resulted in a phenomenal growth in their use. Their usage is beginning to accelerate once more and monolithic refractories comprise the majority of refractory linings worldwide. In many specific applications they have completely replaced shapes, in particular, those that are pressed or extruded and then fired to develop final properties. In order to achieve certain properties and behaviors it is necessary to process refractories so that they possess a ‘‘correct’’ distribution of phases including porosity. The chemistry of the phases depends on the starting materials and the thermal processing or the exposure in situ. Because refractories as a rule are quite heterogeneous, key properties such as strength, density, elasticity, etc., depend on the interaction of the phases making up the whole texture of the product at hand. The same is true of the various behaviors of refractories, i.e., resistance to corrosion, thermal shock, abrasion, erosion, mechanical impact, etc. These various behaviors are often characterized by performance indices, e.g., the thermal shock resistance is expressed in terms of properties known to affect the resistance to sudden temperature change, thermal expansion, modulus of elasticity, tensile strength, and parameters to measure rates of heating\cooling and size and shape. Each application of a refractory places a premium on one or more of the properties and behaviors preferably determined or measured under highly simulative conditions. For example, refractories used in the door of a furnace must withstand rapid heating and cooling; refractories to contain molten glass must resist penetration, dissolution, or alteration by the glass.
1. Classification of Refractories by Processing Refractories are categorized by geometry as shaped and unshaped. The former may be shaped in the cold or warm state by pressing, stamping, extrusion, slip casting, etc., or they may be produced by foundry methods, i.e., by smelting constituent powders to a complete liquid and then casting into molds. Manufacturers are now using precast shapes, formed as a rule by using vibration to uniformly fill plaster, metal,
2. Constitution of Refractory Ceramics The principal refractory oxides behave more or less as acid, basic, or neutral chemical reactants when heated to reaction threshold temperatures. MgO and CaO are bases; SiO and ZrO are acids; and Al O and Cr O # that are # amphoteric in #their $ reaction # $ are materials behavior. Until the middle of the twentieth century virtually 1
Refractories, Structure and Properties of by the Steetley Company at Hartlepool, UK, is shown in Fig. 1 (Chesters 1973). The product is marketed in various grades based mostly on levels of impurities. The size of single crystals after sintering is a key characteristic of sintered and also fused MgO, termed periclase. Products made from these synthetics, produced by sintering or fusion continue to expand in number. Products of many of the most refractory oxides are prepared by fusion: MgO, SiO , Al O , # #and $ Cr O , ZrO , and binary oxides such as mullite # $ # spinel are available. Efforts are under way to produce calcium hexaluminate, CaO : 6Al O . These more $ expensive materials are justified for# incorporation as aggregates and powders into various products employed in steelmaking and glass melting, in particular, as they are relatively chemically inert. These high-performance starting materials have made possible the development of shaped and unshaped refractories with greatly extended wear capabilities. Longer campaigns justify a disproportionately higher investment in refractories as proven by the result that the costs of refractories per ton of iron and steel produced have been steadily declining since the early 1980s.
3. Properties and Behavior of Refractories
Figure 1 Flowchart for the seawater magnesia process at Hartlepool, UK (Chesters 1973).
all starting materials for refractory products were rocks or beneficiated rocks, e.g., clays that had been concentrated by air or water elutriation. Clay minerals from the kaolinite group especially kaolinite itself, Al O : SiO : 2H O, are widely used as placticizers # surface-active $ # # agents in many forming and inand stallation procedures. Other commonly used rocks include chromite, a complex mixture of spinel minerals, quartzite, a massive form of SiO , dolomite, (Ca,Mg)CO , magnesite, MgCO , and # bauxite, a mixture of $clay and aluminum $hydrates plus impurities. Extensive beneficiation of a rock or sediment source can yield nearly pure minerals, e.g., silica, SiO , and zircon sands, ZrSiO , which are single crystals. # From the middle of the%twentieth century, refractory manufacturers, working with chemical technologists, began to produce some synthetic versions of naturally occurring refractory minerals. Their availability made it possible to produce refractories with greatly enhanced property levels. Aluminum oxide became available in sintered (tabular) forms and fused types (white fused and brown fused). MgO was produced from seawater in the 1950s in the UK, Japan, and the USA. A flowchart of the elaborate process developed 2
The properties of refractories as inorganic solid materials designed to resist various challenges at elevated temperature are categorized much the way they are for structural ceramics, i.e., physical properties, mechanical properties, chemical properties, and thermal properties. The behavior of refractories in the face of various challenges is often represented by indices of behavior that contain selected properties in the respective indices. The challenges include: mechanical loading, temperature, temperature change, abrasion, erosion, mechanical impact, and attack by gases, liquids, and dusts.
3.1 Thermal and Mechanical Properties The mechanical properties are discussed first as the chronological first step in the design of a refractory for a high-performance lining is the determination of stresses owing to self-loading and thermal expansion. In other words, there must be sufficient strength at temperature to resist loadings that could generate stresses sufficient to cause the refractory to fail in a brittle or a ductile mode. Refractories heated to a sufficiently high temperature undergo a brittle to ductile (plastic) transition. A properly designed lining provides a thermal gradient through the lining that ensures that the load is supported elastically or by a volume segment of the refractory whose creep resistance is relatively very high. The design must
Refractories, Structure and Properties of anticipate that a significant depth of the lining may subsequently be dissolved, erode away, or be so altered that it cannot support a load. The loads and stresses that derive from heating are of two types, transient and steady state in nature. When a lining, confined or not by a rigid shell, is heated rapidly, so-called thermal stresses, compressive in nature in the hot face, can cause failure in the form of mass loss termed spelling, a slabbing off of mass after crack development parallel to the heated surface. If the shell does not confine the lining at equilibrium, the hot face will be in compression and the cold face will be in tension. If the lining is monolithic in nature, cracks will often form on the cool side, as refractories are quite weak in tension owing to their coarse texture and often high porosity. If joints between bricks or blocks are present, the tensile stresses may be largely relieved. A common practice is to confine the shell in a carefully designed manner so as to have the bulk of the lining mass in a state of compression. The dilation of the lining at the end of the heating interval is given by L∆α∆T, where L would be the circumference of a cylindrical lining, α is the thermal expansion coefficient, and ∆T is the total temperature change. The lining must withstand this induced load that is compressive and a maximum at the hot face. For an arc segment of a lining heated over ∆T, the stress is given approximately by σc l
E(TkT )α ! 1kν
(1)
where E is the modulus of elasticity and ν is Poisson’s ratio. The 1kν term accounts for the biaxial character of the stress state. Three of the key design properties are, therefore, the modulus of elasticity (MOE), Poisson’s ratio, and the thermal expansion coefficient. These properties must be known as a function of temperature for a selected refractory. They may change with time owing to sintering, crack healing, pore fraction reduction, and also because of chemical alteration. In order to predict stresses in a lining held at a single temperature, more or less, the changes in these properties must be measured under simulated conditions. In many instances the temperature cycles widely and wildly and the changes in the properties due to cycling should be determined for a realistic design calculation. The property as a function of time and temperature is termed the material function for the property and is used in finite element analysis estimation of the stresses. The compressive failure stress is measured isothermally on either full-size brick specimens, 9 ini 4" ini2" in (230 mmi114 mmi64 mm), or on 2 in # mm)# cubes or right circular cylinders cut from (50 larger shapes or unshaped refractories. The MOE is measured either statically in compression or sonically in various modes as a function of temperature in order
to obtain design data. Poisson’s ratio is usually measured using sonic techniques, by the resonant frequency or the pulse decay method. Both methods are difficult to implement at temperatures above 1000 mC. Static MOE measurements in compression or bending often show nonlinear behavior, e.g., strain softening occurs. If the load exceeds the elastic limit of the material, yielding will occur in refractories by viscous flow or by diffusional processes, usually the former. In these cases measurements must be made of the creep behavior. Data are taken, as for metals, looking for linear creep behavior so that the steadystate creep rate as a function of load (stress) and temperature can be assessed. The steady-state creep rate is given by E
Ec l Aσn exp
∆Hc RT
G
k F
(2) H
where A l a texture constant, σ l stress in compression, n l stress exponent (unity for viscous flow), and ∆H l enthalpy of the creep process. The steady-state creep rates and the values of ∆H are recorded for many pure oxides. It is not uncommon that a refractory fails to display a steady-state creep interval. The entire curve must then be used for characterization. Resistance to crack extension owing to static or dynamic loading and, especially, to thermal shock, is an important concern for refractory lining designers and process engineers. Thermal shock damage usually occurs on rapid cooling, so-called down shock. Thermal cycling that is characteristic of melting and thermal treatment processes also leads to alternating stress levels that induce crack growth. Toughness measured by KIC provides data on the resistance to crack initiation; the values usually following closely on variations in the flexural strength with temperature: KIC l σYc"/# where σ l maximum tensile stress, Y l a dimensional constant, and c l depth of a sawn notch. Figure 2 demonstrates the arrangement for three-point loading usually employed for refractories for testing at ambient and elevated temperatures. In this loading σl
3Pl 2bd #
(3)
where P l load, l l length, b l breadth, and d l depth. This is the expression for the outer fiber stress on a beam surface, the beam loaded at the center, which is the most commonly used method for testing the mechanical strength of refractories. The same loading arrangement is employed in the more relevant toughness measurement, the work of fracture (WOF). This measurement is widely 3
Refractories, Structure and Properties of standard technique for many years; however, this method imposes a very steep thermal gradient on the specimen. Other methods in use include the hot-wire technique, laser and other flash methods, and comparison methods. There is no method that is straightforward in its application to shapes, fiber forms, and loose powders. All of these can undergo thermally induced changes and thus yield constant values only over a restricted range of temperature. Thermal conductivity values determine the thermal gradient through single or composite refractory walls. Thermal loss control is crucial in the management of highly corrosive agents where a gradient is essential to suppress penetration and alteration by chemical reaction. When the temperature is changing owing to process design, loss of power, or process interruption or error, the temperature of the refractory responds depending on its thermal diffusivity, a thermal behavior defined by Dl
Figure 2 Specimen geometries for fracture toughness measurement.
performed on refractories as the results are incorporated into expressions for resistance to thermal shock. Some designers also incorporate WOF results into failure criteria for crack extension under static or cyclic loads. It has been shown that almost all refractories including dense carbon materials are not linear according to linear elastic fracture mechanics theory that requires that a specimen in which a crack has extended should experience no residual strain on unloading (Nakayama et al. 1981). Consequently, measurements of WOF can give only relative values and must be conducted so as to minimize the effects of nonlinear behavior. Most of the published data on the WOF of refractories has been taken without regard to the problems with nonlinear behavior. The thermal expansion coefficient has already been mentioned as a key property in the previous discussion on design. It is usually measured using a dilatometer following standardized procedures. It is essential to measure dilation and contraction as many refractories are nonlinear and display hysteresis effects owing to thermally induced changes in the specimens. Thermal expansion data are used in design, certification, and in research as a probe for effects caused by changes in constitution, processing, and exposure in the field. Thermal conductivity is also considered to be a key design property of refractories as design often includes heat balance estimations. It is very difficult to measure accurately. Calorimeter methods have been used as a 4
κ ρCp
where D l diffusivity, k l thermal conductivity (W m−" K−"), ρ l density (g cm−$), and Cp l specific heat. Diffusivity has units of m# s−" and is usually measured by transient methods for k, e.g., hot-wire and flash methods. Diffusivity can be considered a design parameter as it plays a major role in thermal shock damage estimation. The specific heat is normally calculated rather than measured; the density measurement is normally obtained through an Archimedes principle-based procedure. A thorough treatment of Cp measurement and calculation is available in Carniglia and Barna (1992). The measurement of k and D when using flash methods is made difficult by the coarse texture of many refractories. Data for thermal conductivity are used extensively in heat loss estimations. Many engineers have reported that cold face temperatures estimated from k data available for materials used in single or composite walls do not match values from direct measurement. This common experience is interpreted to mean that the k values are inaccurate. The thermal shock damage of refractories is a dominant wear mechanism. Measurements of specific thermal and mechanical properties make it possible to calculate parameters for the resistance to thermal shock. Norton (1949) laid the groundwork for an understanding of the issues involved in thermal shock exposure of refractories. Figure 3 illustrates thermal shock damage suffered by spheres and parallelepipeds subjected to up shock (heating) and down shock (cooling). Early standard tests of refractories focused on mass loss after alternating down shock and up shock, but many refractories suffer extensive cracking without mass loss so
Refractories, Structure and Properties of under isothermal conditions. The various TSR parameters are predictive in some cases and not in others. In general, the best measure of resistance to thermal shock of refractories is the ratio of the WOF to KIC, i.e., resistance of crack propagation compared to the resistance to crack initiation. However, shape plays a major role in the distribution of the transient stresses and the calculated parameters often do not predict relative performance in the field, as test specimens are normally simple cylinders or orthogonal cross-section rods. Another serious problem in using property test data to calculate TSR parameters is that the strength values that appear in them are often room temperature values. It is not always apparent what temperature should be used in strength testing for use in the TSR expressions. 3.2 Chemical Properties and Behaior Figure 3 Thermal shock damage to spheres and bricks.
that these tests are no longer used. Strength after down shock is a widely used measurement. Data from Ainsworth and Moore (1969) shown in Fig. 4 for dense alumina are representative of the behavior of many refractories and structural ceramics. Hasselman (1969) has elaborated a theory to explain such behavior; however, it assumes the simultaneous extension of N preexisting cracks, which is not observed experimentally. With successive thermal shocks some cracks deepen while others do not; subsequently, both branching and extension occur in a complex progression of the total damage. The major factors that are involved in the resistance of a refractory to thermal shock are included in the following expression: σhmax l
Eα(T kT ) ! " l F( β) 1kν
(4)
where σhmax l maximum biaxial surface stress for a given heat transfer, h, condition, F( β) l a function of the Biot modulus, and β l rh\k, where r l radial dimension, h l heat transfer coefficient, and k l thermal conductivity. For an infinite heat transfer rate, a parameter R is defined by R l (T kT )max l ! "
Eα(T kT ) ! " 1kν
(5)
and referred to as a thermal shock resistance (TSR) parameter. There are additional TSR parameters that reflect shape and size, heating rate, and toughnesscontrolled failure mode. They are all calculated from property data taken on simple specimen geometries
(a) Refractoriness. Some refractories are relatively quite pure, 99.5% or higher; accordingly, data on the melting, decomposition, or sublimation points of the pure compounds can be used as a guide for use. However, the large majority of refractories contain substantial levels of impurities and\or they contain glassy phases after processing. Some of them are thermally processed below their eventual use temperatures. These considerations mean that the melting points are usually not useful even as a first approximation for service temperature limits of a product. Moreover, many pure refractory oxides undergo creep at temperatures well below their melting points. Refractories for load-bearing designs are measured for refractoriness by use of creep tests; the so-called hot load test and a thermal expansion under load test (‘Annual Book of Standards’ 1998a, 1998b). Other inhouse tests are frequently employed by producers, e.g., rods are supported at the ends and then heated in steps to various test temperatures. The temperature at which the rod sags to a critical degree is deemed the threshold temperature for a nonload-bearing application. The pyrometric cone equivalent test (‘Annual Book of Standards’ 1998c) is used for silica-containing raw materials, e.g., clays and aluminosilicate fired products. A small inclined test cone is heated at a standard rate until it softens, bends, and touches the horizontal plane. The test is used to certify raw materials and products and for firing control. (b) Resistance to chemical attack. This behavior is exceedingly difficult to characterize for refractories as their texture and mineralogy has so much variation and there are so many scenarios of exposure. The principal high-temperature refractory corrodants are glass melts, slags, powders and dust, and gases or gas mixtures. The best approach to a corrosion 5
Refractories, Structure and Properties of
Figure 5 Dynamic corrosion test for refractories.
Figure 4 Strength after thermal shock data for aluminum oxide.
investigation is to characterize fully the corrodants in terms of their chemistry, temperature variation in the furnace, dynamics of corrodant movement for design of simulated test protocols, and to perform a careful characterization of the refractory. This latter should include the chemistry, mineralogy, and texture of the aggregate and the matrix or grain boundary phases. The porosity and permeability are very important, as is information on the size distribution of pores. Wettability and surface or interfacial tension data are not usually available for complex polyphase products such as commercial refractories. The temperature gradient in the refractory lining is very important. The presence of joints, mortar chemistry, and texture, and whether the system is more or less gas tight is also important. For example, the sealing of a glass tank furnace crown so as to prevent infiltration of NaOH vapor along the joints is a most effective countermeasure to sodium hydroxide attack of the refractory. Whenever possible corrosion tests are designed to simulate closely conditions at the refractory–corrodant interface. A problem is that the kinetics of corrosion in some furnaces is so slow that accelerated tests need to be employed. The dynamic finger test (Cooper et al. 1964) as illustrated in Fig. 5 is extensively used in the testing of glass or slag contact refractories; however, it relies on isothermal exposure rather than in thermal gradient exposure that is the norm in furnace walls and roofs. In general, refractories designed to challenge the most aggressive melts must be totally devoid of 6
porosity and microcracks. Glass is melted in fused cast blocks of alumina–zirconia–silica (AZS), which are less than 1% porous although porosity, grain size, and phase distribution vary widely within a block resulting from gravity effects and growth processes during solidification. Basic refractories, MgO, MgO : CaO, and spinel, are used to contain basic melts and slags, while acid refractories, S O , ZrO , SiO , 2Al O : SiO , mullite, # # # $ # etc., are used" to# contain acidic melts. Neutral refractories such as Al O and Cr O are used for certain # $ # $ glass compositions; Cr O is extremely resistant to $ fiberglass melts. attack by borosilicates, #e.g., Process changes have a great impact on refractory corrosion. The use of CaF , fluorspar, in basic oxygen # steels sharply increases furnace (BOF) melting of corrosion rates. Changing of combustion systems, fuel\air to fuel\oxygen, has made silica crowns obsolete for the containment of the combustion gases in many glass tanks. Fused refractories, especially M types, mixtures of α and β alumina, and AZS are highly resistant to the increased concentration of NaOH promoted by the substitution of O for air. # howRebonded and fused spinels are also resistant; ever, all the good performers are up to 10 times more expensive than the conventional silica refractories. The availability of new starting materials often leads to the development of more corrosion-resistant refractories. In the 1950s ‘‘high’’-purity calcium aluminate cements were developed to replace high iron-containing calcium aluminate hydraulic binders that were limited to use temperatures of about 1200 mC. The new cements, mixtures of CaO, Al O , and CaO : 2Al O plus highly reactive added# $Al O , $ # $ enabled the# production of monolithic refractories with higher service temperatures and greater corrosion resistance. The presence of SiO , often incorporated as silica fume to promote the# desired rheological
Refractories, Structure and Properties of behavior leads to formation of calcium alumina silicate phases that limit the refractoriness and corrosion resistance of the products. Availability of hydratable alumina hydraulic binders and colloidal silica binders that respectively combine with reactive counterparts to form mullite in situ has led to monolithics with unparalleled corrosion resistance. Developments in binders, matrix powders, aggregates, and additives have made it possible to obtain refractories in monolithic form that have thermal corrosion resistance levels exceeding those of fired shapes. 3.3 Physical Properties Density is a most important underlying property of refractories because it can be correlated to most of the key properties and behaviors. Porosity measurements correlate very well to mechanical, thermal, and chemical properties and to various behaviors, such that for a given product density is the generic index of quality, e.g., the quality of MgO bricks is reflected by their density values that serve as a basis for classification. Refractories almost invariably contain some porosity, up to 30% by volume in fired shapes and in monolithics. Higher porosities are induced through use of porous components or fugative phases in order to adjust the insulating properties. The size distribution and connectivity of the pores influence the permeability of a refractory, which in turn determines the rate of influx of corrodants and efflux of vapors generated during heat application, e.g., steam during the dry out of monolithics. Water is incorporated to hydrate the binders and to adjust the consistency for placement. Both forms of water are vaporized during heating to about 600 mC when the most stable to dehydration phases release chemical water. Permeability to water vapor movement to the heated face of monolithics is a key characteristic that controls excessive buildup of internal pressure that results in cracking and explosions. Permeability measurement is not straightforward for monolithics. Innocentini et al. (1999) have pointed to the inability of Darcy’s law to incorporate the total resistance to gas flow in many permeable media. They emphasize that a single value for permeability cannot account for contributions to resistance to flow measured by a pressure drop. Darcy’s law applies for fluids flowing at low velocities: P#kP# µνs " !l K 2PL "
(6)
where P and P are pressures at inlet and outlet faces, " ! which fluid velocity and viscosity P l pressure for values are obtained, K is the Darcian permeability, µ " ν is the volumetric flow rate. is the fluid viscosity, and s However, when the velocity increases as occurs in very low permeability monolithics, kinetic effects originat-
ing from turbulence and fluid inertia contribute additional resistances to flow. Two constants are therefore required and the expression for permeability becomes P#kP# µνs ρν# " !l j K K 2PL # "
(7)
where ρ l fluid density and K l new constant to # characterize high-velocity resistances. The second equation is called Forcheimer’s equation. This improvement in understanding of permeability will allow studies to determine constitutional and textural influences on the two permeability constants. This information is crucial for the correct modeling of the dewatering of monolithics. Moore et al. (1997) have reviewed the properties and factors that are essential for modeling of the critical process of drying out monolithic linings stressing that permeability data are the most critical. Data are needed for K and K from ambient to 600 mC for the various types" of low-# permeability monolithics used to line vessels to contain process liquids, e.g., ladles for holding molten steel. There are five distinct uses for the data discussed in this section on properties and behaviors: (i) certification and specification, (ii) quality control, (iii) design and modeling, (iv) development of all indices of performance, and (v) research and development. Test development activity has slowed during the period of refractory company downsizing, yet the demands on refractory products are more severe than ever. 4. Property Measurement Advances Needed in the Future The specific properties and behaviors of primary concern to the refractories technologist of the future will not change but the methods of determination will receive increasing attention due mostly to the need for the most relevant data for applications. The coarsegrained nature of most refractories requires specimen sizes large enough to overcome statistical variation in the constitution of individual test specimens. In the mechanical strength testing of monolithics, beams 2 ini2 ini8 in (50 mmi50 mmi200 mm) are employed for the measurement of the MOE, modulus of rupture (flexural strength), KIC toughness, and WOF. This practice should be followed for strength and toughness testing of all coarse-grained (top grain size of 4–6 mm or greater) refractories. For creep measurements, the specimens should be 50 mm cubes or 50 mmi50 mm cylinders or larger. Standardization of these larger specimens will yield more accurate and precise mechanical property data. Thermal conductivity is a property that is critical for design and modeling efforts and needs a major collaborative effort by international standards groups to support the development of new and improved 7
Refractories, Structure and Properties of characterization protocols are needed to produce the knowledge necessary for the design and optimal use of these refractory products demanded by users throughout the world. Researchers have developed a novel rheometer specifically adapted to the measurement of refractory monolithics (Pileggi et al. 2000). Results suggest that the instrument is capable of accurate and precise rheological characterization of the large family of refractory castables. Developments like those cited above require high levels of support for extended time periods. In particular, attention is needed to improve measurement capabilities of the following properties and behaviors: thermal shock resistance, nonlinear fracture mechanical behavior, high-temperature permeability, rheological behavior, abrasion resistance, thermal conductivity, and corrosion resistance. Figure 6 Extensometer for accurate measurement of strain in creep and MOE procedures (Moore et al. 1997).
Bibliography
measurements above 1200 mC. Flash techniques, modified to account for the coarse grain size of many refractories, are being promoted. Creep is the major failure mechanism to avoid in all high-temperature structural under load applications of refractories. A new technique that assures only the specimen dilation is measured has been perfected by Wereszczak et al. and applied to crown refractories for glass tanks. This method employing a scissors extensometer (see Fig. 6) has the capability of producing measurements with a resolution of 1 µm. The scissorstype extensometer technique for strain measurement eliminates problems with the axial extensometers that record dilations in the loading system, push rods, bearings plates, etc. The same scissors-type extensometer method is suited for measurements of the MOE in compression, another property important in design and modeling activities. A need to focus on the properties that lead to wider usage of monolithic refractories is obvious but there are major obstacles posed by the complexity of the compositions and their resulting microstructures. A contemporary high-performance castable may contain six to eight powder and aggregate particle components. The chemical additive package for consistency adjustment may contain three to five electrolytes. The rheological behavior, activated by the water addition and mode of mixing, needs to be characterized by methods that are economical and can be standardized. From the onset of drying, through the complete dewatering and heating stages, hydraulically bonded monolithics are ‘‘living’’ entities, i.e., their microstructures are continuously changing in contrast to a prefired shape or brick. New standard testing and
Ainsworth J H, Moore R E 1969 Fracture behavior of thermally shocked aluminum oxide. J. Am. Ceram. Soc. 52 (11), 628–9 Annual Book of Standards 1998a Load testing refractory brick at high temperatures; procedure C16-81. American Society of Testing and Materials, Vol. 15.01, pp. 1–4 Annual Book of Standards 1998b Standard method of measuring the thermal expansion and creep of refractories under load; procedure C832-89. American Society of Testing and Materials, Vol. 15.01, pp. 222–7 Annual Book of Standards 1998c Standard test method for pyrometric cone equivalent (PCE) for fireclay and high alumina refractory materials; procedure C24-89. American Society of Testing and Materials, Vol. 15.01, pp. 8–11 Carniglia S C, Barna G L 1992 The insulating refractory line. In: Handbook of Industrial Refractories, Principles, Types, Properties and Applications. Noyes Publications, Chap. IX, pp. 307–28 Chesters J H 1973 Refractories, Production and Properties. Iron and Steel Institute, London, p. 134 Cooper Jr. A R, Sawaddar B N, Oishi Y, Kingery W D 1964 Dissolution in ceramic systems. J. Am. Ceram. Soc. 47, 37 Hasselman D P H 1969 United theory of thermal shock fracture initiation and crack propagation in brittle ceramics. J. Am. Ceram. Soc. 52, 600–4 Innocentini M D M, Pardo A R F, Sadumi V R, Pandolfelli V C 1999 How accurate is Darcy’s law for refractories? Am. Ceram. Soc. Bull. 78 (11), 64–8 Moore R E, Smith J D, Sander T P, Severin N 1997 Dewatering monolithic refractory castables: experimental and practical experience. In: Proc. UNITECR ’97, United Int. Tech. Conf. on Refractories. American Ceramic Society, Vol. II, pp. 573–82 Morrell R 1985 Thermomechanics of properties. In: Handbook of Properties of Technical & Engineering Ceramics: Part 1. An Introduction for the Engineer and Designer. HMSO, London, Chap. 26, pp.141–58 Nakayama J, Abe H, Bradt R C 1981 Crack stability in the work of fracture tests: refractory applications. J. Am. Ceram. Soc. 64 (11), 671–5 Norton F H 1949 Spalling. In: Refractories. McGraw-Hill, New York, 3rd edn., Chap. XV, pp. 420–52 Pileggi R G, Pandolfelli U C, Paiva A E, Gallo J 2000 Novel
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Refractories, Structure and Properties of rheometer for refractory castables. Am. Ceram. Soc. Bull. 79 (1), 54–8 Wereszczak A A, Karakus M, Liu K C, Pint B A, Moore R E, Kirkland T P 1999 Compressive creep performance and high temperature dimensional stability of conventional silica re-
fractories, technical report ORNL\TM-13757. Oak Ridge National Laboratories, US Department of Energy, Sect. 3.2, pp. 5–16
R. E. Moore
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 8079–8088 9
or plastic molds. After drying and curing the parts may be prefired or they may be installed directly into the lining of a furnace. The second category, unshaped refractories, also termed monolithics, includes the precast shapes described above as they begin as loose powders and grains mixed with binder materials. The grain\powder mixtures are combined with a liquid, usually water, and are cast, puddled, trowled, gunited, shotcreted, or hand placed into forms or onto vertical and horizontal surfaces to form monolithic masses of refractory. Such monolithics have been used for many decades but the development of high-purity calcium aluminate cements and of stable phosphate binder systems in the middle of the twentieth century resulted in a phenomenal growth in their use. Their usage is beginning to accelerate once more and monolithic refractories comprise the majority of refractory linings worldwide. In many specific applications they have completely replaced shapes, in particular, those that are pressed or extruded and then fired to develop final properties. In order to achieve certain properties and behaviors it is necessary to process refractories so that they possess a ‘‘correct’’ distribution of phases including porosity. The chemistry of the phases depends on the starting materials and the thermal processing or the exposure in situ. Because refractories as a rule are quite heterogeneous, key properties such as strength, density, elasticity, etc., depend on the interaction of the phases making up the whole texture of the product at hand. The same is true of the various behaviors of refractories, i.e., resistance to corrosion, thermal shock, abrasion, erosion, mechanical impact, etc. These various behaviors are often characterized by performance indices, e.g., the thermal shock resistance is expressed in terms of properties known to affect the resistance to sudden temperature change, thermal expansion, modulus of elasticity, tensile strength, and parameters to measure rates of heating\cooling and size and shape. Each application of a refractory places a premium on one or more of the properties and behaviors preferably determined or measured under highly simulative conditions. For example, refractories used in the door of a furnace must withstand rapid heating and cooling; refractories to contain molten glass must resist penetration, dissolution, or alteration by the glass.
1. Classification of Refractories by Processing Refractories are categorized by geometry as shaped and unshaped. The former may be shaped in the cold or warm state by pressing, stamping, extrusion, slip casting, etc., or they may be produced by foundry methods, i.e., by smelting constituent powders to a complete liquid and then casting into molds. Manufacturers are now using precast shapes, formed as a rule by using vibration to uniformly fill plaster, metal,
2. Constitution of Refractory Ceramics The principal refractory oxides behave more or less as acid, basic, or neutral chemical reactants when heated to reaction threshold temperatures. MgO and CaO are bases; SiO and ZrO are acids; and Al O and Cr O # that are # amphoteric in #their $ reaction # $ are materials behavior. Until the middle of the twentieth century virtually 1
Refractories, Structure and Properties of by the Steetley Company at Hartlepool, UK, is shown in Fig. 1 (Chesters 1973). The product is marketed in various grades based mostly on levels of impurities. The size of single crystals after sintering is a key characteristic of sintered and also fused MgO, termed periclase. Products made from these synthetics, produced by sintering or fusion continue to expand in number. Products of many of the most refractory oxides are prepared by fusion: MgO, SiO , Al O , # #and $ Cr O , ZrO , and binary oxides such as mullite # $ # spinel are available. Efforts are under way to produce calcium hexaluminate, CaO : 6Al O . These more $ expensive materials are justified for# incorporation as aggregates and powders into various products employed in steelmaking and glass melting, in particular, as they are relatively chemically inert. These high-performance starting materials have made possible the development of shaped and unshaped refractories with greatly extended wear capabilities. Longer campaigns justify a disproportionately higher investment in refractories as proven by the result that the costs of refractories per ton of iron and steel produced have been steadily declining since the early 1980s.
3. Properties and Behavior of Refractories
Figure 1 Flowchart for the seawater magnesia process at Hartlepool, UK (Chesters 1973).
all starting materials for refractory products were rocks or beneficiated rocks, e.g., clays that had been concentrated by air or water elutriation. Clay minerals from the kaolinite group especially kaolinite itself, Al O : SiO : 2H O, are widely used as placticizers # surface-active $ # # agents in many forming and inand stallation procedures. Other commonly used rocks include chromite, a complex mixture of spinel minerals, quartzite, a massive form of SiO , dolomite, (Ca,Mg)CO , magnesite, MgCO , and # bauxite, a mixture of $clay and aluminum $hydrates plus impurities. Extensive beneficiation of a rock or sediment source can yield nearly pure minerals, e.g., silica, SiO , and zircon sands, ZrSiO , which are single crystals. # From the middle of the%twentieth century, refractory manufacturers, working with chemical technologists, began to produce some synthetic versions of naturally occurring refractory minerals. Their availability made it possible to produce refractories with greatly enhanced property levels. Aluminum oxide became available in sintered (tabular) forms and fused types (white fused and brown fused). MgO was produced from seawater in the 1950s in the UK, Japan, and the USA. A flowchart of the elaborate process developed 2
The properties of refractories as inorganic solid materials designed to resist various challenges at elevated temperature are categorized much the way they are for structural ceramics, i.e., physical properties, mechanical properties, chemical properties, and thermal properties. The behavior of refractories in the face of various challenges is often represented by indices of behavior that contain selected properties in the respective indices. The challenges include: mechanical loading, temperature, temperature change, abrasion, erosion, mechanical impact, and attack by gases, liquids, and dusts.
3.1 Thermal and Mechanical Properties The mechanical properties are discussed first as the chronological first step in the design of a refractory for a high-performance lining is the determination of stresses owing to self-loading and thermal expansion. In other words, there must be sufficient strength at temperature to resist loadings that could generate stresses sufficient to cause the refractory to fail in a brittle or a ductile mode. Refractories heated to a sufficiently high temperature undergo a brittle to ductile (plastic) transition. A properly designed lining provides a thermal gradient through the lining that ensures that the load is supported elastically or by a volume segment of the refractory whose creep resistance is relatively very high. The design must
Refractories, Structure and Properties of anticipate that a significant depth of the lining may subsequently be dissolved, erode away, or be so altered that it cannot support a load. The loads and stresses that derive from heating are of two types, transient and steady state in nature. When a lining, confined or not by a rigid shell, is heated rapidly, so-called thermal stresses, compressive in nature in the hot face, can cause failure in the form of mass loss termed spelling, a slabbing off of mass after crack development parallel to the heated surface. If the shell does not confine the lining at equilibrium, the hot face will be in compression and the cold face will be in tension. If the lining is monolithic in nature, cracks will often form on the cool side, as refractories are quite weak in tension owing to their coarse texture and often high porosity. If joints between bricks or blocks are present, the tensile stresses may be largely relieved. A common practice is to confine the shell in a carefully designed manner so as to have the bulk of the lining mass in a state of compression. The dilation of the lining at the end of the heating interval is given by L∆α∆T, where L would be the circumference of a cylindrical lining, α is the thermal expansion coefficient, and ∆T is the total temperature change. The lining must withstand this induced load that is compressive and a maximum at the hot face. For an arc segment of a lining heated over ∆T, the stress is given approximately by σc l
E(TkT )α ! 1kν
(1)
where E is the modulus of elasticity and ν is Poisson’s ratio. The 1kν term accounts for the biaxial character of the stress state. Three of the key design properties are, therefore, the modulus of elasticity (MOE), Poisson’s ratio, and the thermal expansion coefficient. These properties must be known as a function of temperature for a selected refractory. They may change with time owing to sintering, crack healing, pore fraction reduction, and also because of chemical alteration. In order to predict stresses in a lining held at a single temperature, more or less, the changes in these properties must be measured under simulated conditions. In many instances the temperature cycles widely and wildly and the changes in the properties due to cycling should be determined for a realistic design calculation. The property as a function of time and temperature is termed the material function for the property and is used in finite element analysis estimation of the stresses. The compressive failure stress is measured isothermally on either full-size brick specimens, 9 ini 4" ini2" in (230 mmi114 mmi64 mm), or on 2 in # mm)# cubes or right circular cylinders cut from (50 larger shapes or unshaped refractories. The MOE is measured either statically in compression or sonically in various modes as a function of temperature in order
to obtain design data. Poisson’s ratio is usually measured using sonic techniques, by the resonant frequency or the pulse decay method. Both methods are difficult to implement at temperatures above 1000 mC. Static MOE measurements in compression or bending often show nonlinear behavior, e.g., strain softening occurs. If the load exceeds the elastic limit of the material, yielding will occur in refractories by viscous flow or by diffusional processes, usually the former. In these cases measurements must be made of the creep behavior. Data are taken, as for metals, looking for linear creep behavior so that the steadystate creep rate as a function of load (stress) and temperature can be assessed. The steady-state creep rate is given by E
Ec l Aσn exp
∆Hc RT
G
k F
(2) H
where A l a texture constant, σ l stress in compression, n l stress exponent (unity for viscous flow), and ∆H l enthalpy of the creep process. The steady-state creep rates and the values of ∆H are recorded for many pure oxides. It is not uncommon that a refractory fails to display a steady-state creep interval. The entire curve must then be used for characterization. Resistance to crack extension owing to static or dynamic loading and, especially, to thermal shock, is an important concern for refractory lining designers and process engineers. Thermal shock damage usually occurs on rapid cooling, so-called down shock. Thermal cycling that is characteristic of melting and thermal treatment processes also leads to alternating stress levels that induce crack growth. Toughness measured by KIC provides data on the resistance to crack initiation; the values usually following closely on variations in the flexural strength with temperature: KIC l σYc"/# where σ l maximum tensile stress, Y l a dimensional constant, and c l depth of a sawn notch. Figure 2 demonstrates the arrangement for three-point loading usually employed for refractories for testing at ambient and elevated temperatures. In this loading σl
3Pl 2bd #
(3)
where P l load, l l length, b l breadth, and d l depth. This is the expression for the outer fiber stress on a beam surface, the beam loaded at the center, which is the most commonly used method for testing the mechanical strength of refractories. The same loading arrangement is employed in the more relevant toughness measurement, the work of fracture (WOF). This measurement is widely 3
Refractories, Structure and Properties of standard technique for many years; however, this method imposes a very steep thermal gradient on the specimen. Other methods in use include the hot-wire technique, laser and other flash methods, and comparison methods. There is no method that is straightforward in its application to shapes, fiber forms, and loose powders. All of these can undergo thermally induced changes and thus yield constant values only over a restricted range of temperature. Thermal conductivity values determine the thermal gradient through single or composite refractory walls. Thermal loss control is crucial in the management of highly corrosive agents where a gradient is essential to suppress penetration and alteration by chemical reaction. When the temperature is changing owing to process design, loss of power, or process interruption or error, the temperature of the refractory responds depending on its thermal diffusivity, a thermal behavior defined by Dl
Figure 2 Specimen geometries for fracture toughness measurement.
performed on refractories as the results are incorporated into expressions for resistance to thermal shock. Some designers also incorporate WOF results into failure criteria for crack extension under static or cyclic loads. It has been shown that almost all refractories including dense carbon materials are not linear according to linear elastic fracture mechanics theory that requires that a specimen in which a crack has extended should experience no residual strain on unloading (Nakayama et al. 1981). Consequently, measurements of WOF can give only relative values and must be conducted so as to minimize the effects of nonlinear behavior. Most of the published data on the WOF of refractories has been taken without regard to the problems with nonlinear behavior. The thermal expansion coefficient has already been mentioned as a key property in the previous discussion on design. It is usually measured using a dilatometer following standardized procedures. It is essential to measure dilation and contraction as many refractories are nonlinear and display hysteresis effects owing to thermally induced changes in the specimens. Thermal expansion data are used in design, certification, and in research as a probe for effects caused by changes in constitution, processing, and exposure in the field. Thermal conductivity is also considered to be a key design property of refractories as design often includes heat balance estimations. It is very difficult to measure accurately. Calorimeter methods have been used as a 4
κ ρCp
where D l diffusivity, k l thermal conductivity (W m−" K−"), ρ l density (g cm−$), and Cp l specific heat. Diffusivity has units of m# s−" and is usually measured by transient methods for k, e.g., hot-wire and flash methods. Diffusivity can be considered a design parameter as it plays a major role in thermal shock damage estimation. The specific heat is normally calculated rather than measured; the density measurement is normally obtained through an Archimedes principle-based procedure. A thorough treatment of Cp measurement and calculation is available in Carniglia and Barna (1992). The measurement of k and D when using flash methods is made difficult by the coarse texture of many refractories. Data for thermal conductivity are used extensively in heat loss estimations. Many engineers have reported that cold face temperatures estimated from k data available for materials used in single or composite walls do not match values from direct measurement. This common experience is interpreted to mean that the k values are inaccurate. The thermal shock damage of refractories is a dominant wear mechanism. Measurements of specific thermal and mechanical properties make it possible to calculate parameters for the resistance to thermal shock. Norton (1949) laid the groundwork for an understanding of the issues involved in thermal shock exposure of refractories. Figure 3 illustrates thermal shock damage suffered by spheres and parallelepipeds subjected to up shock (heating) and down shock (cooling). Early standard tests of refractories focused on mass loss after alternating down shock and up shock, but many refractories suffer extensive cracking without mass loss so
Refractories, Structure and Properties of under isothermal conditions. The various TSR parameters are predictive in some cases and not in others. In general, the best measure of resistance to thermal shock of refractories is the ratio of the WOF to KIC, i.e., resistance of crack propagation compared to the resistance to crack initiation. However, shape plays a major role in the distribution of the transient stresses and the calculated parameters often do not predict relative performance in the field, as test specimens are normally simple cylinders or orthogonal cross-section rods. Another serious problem in using property test data to calculate TSR parameters is that the strength values that appear in them are often room temperature values. It is not always apparent what temperature should be used in strength testing for use in the TSR expressions. 3.2 Chemical Properties and Behaior Figure 3 Thermal shock damage to spheres and bricks.
that these tests are no longer used. Strength after down shock is a widely used measurement. Data from Ainsworth and Moore (1969) shown in Fig. 4 for dense alumina are representative of the behavior of many refractories and structural ceramics. Hasselman (1969) has elaborated a theory to explain such behavior; however, it assumes the simultaneous extension of N preexisting cracks, which is not observed experimentally. With successive thermal shocks some cracks deepen while others do not; subsequently, both branching and extension occur in a complex progression of the total damage. The major factors that are involved in the resistance of a refractory to thermal shock are included in the following expression: σhmax l
Eα(T kT ) ! " l F( β) 1kν
(4)
where σhmax l maximum biaxial surface stress for a given heat transfer, h, condition, F( β) l a function of the Biot modulus, and β l rh\k, where r l radial dimension, h l heat transfer coefficient, and k l thermal conductivity. For an infinite heat transfer rate, a parameter R is defined by R l (T kT )max l ! "
Eα(T kT ) ! " 1kν
(5)
and referred to as a thermal shock resistance (TSR) parameter. There are additional TSR parameters that reflect shape and size, heating rate, and toughnesscontrolled failure mode. They are all calculated from property data taken on simple specimen geometries
(a) Refractoriness. Some refractories are relatively quite pure, 99.5% or higher; accordingly, data on the melting, decomposition, or sublimation points of the pure compounds can be used as a guide for use. However, the large majority of refractories contain substantial levels of impurities and\or they contain glassy phases after processing. Some of them are thermally processed below their eventual use temperatures. These considerations mean that the melting points are usually not useful even as a first approximation for service temperature limits of a product. Moreover, many pure refractory oxides undergo creep at temperatures well below their melting points. Refractories for load-bearing designs are measured for refractoriness by use of creep tests; the so-called hot load test and a thermal expansion under load test (‘Annual Book of Standards’ 1998a, 1998b). Other inhouse tests are frequently employed by producers, e.g., rods are supported at the ends and then heated in steps to various test temperatures. The temperature at which the rod sags to a critical degree is deemed the threshold temperature for a nonload-bearing application. The pyrometric cone equivalent test (‘Annual Book of Standards’ 1998c) is used for silica-containing raw materials, e.g., clays and aluminosilicate fired products. A small inclined test cone is heated at a standard rate until it softens, bends, and touches the horizontal plane. The test is used to certify raw materials and products and for firing control. (b) Resistance to chemical attack. This behavior is exceedingly difficult to characterize for refractories as their texture and mineralogy has so much variation and there are so many scenarios of exposure. The principal high-temperature refractory corrodants are glass melts, slags, powders and dust, and gases or gas mixtures. The best approach to a corrosion 5
Refractories, Structure and Properties of
Figure 5 Dynamic corrosion test for refractories.
Figure 4 Strength after thermal shock data for aluminum oxide.
investigation is to characterize fully the corrodants in terms of their chemistry, temperature variation in the furnace, dynamics of corrodant movement for design of simulated test protocols, and to perform a careful characterization of the refractory. This latter should include the chemistry, mineralogy, and texture of the aggregate and the matrix or grain boundary phases. The porosity and permeability are very important, as is information on the size distribution of pores. Wettability and surface or interfacial tension data are not usually available for complex polyphase products such as commercial refractories. The temperature gradient in the refractory lining is very important. The presence of joints, mortar chemistry, and texture, and whether the system is more or less gas tight is also important. For example, the sealing of a glass tank furnace crown so as to prevent infiltration of NaOH vapor along the joints is a most effective countermeasure to sodium hydroxide attack of the refractory. Whenever possible corrosion tests are designed to simulate closely conditions at the refractory–corrodant interface. A problem is that the kinetics of corrosion in some furnaces is so slow that accelerated tests need to be employed. The dynamic finger test (Cooper et al. 1964) as illustrated in Fig. 5 is extensively used in the testing of glass or slag contact refractories; however, it relies on isothermal exposure rather than in thermal gradient exposure that is the norm in furnace walls and roofs. In general, refractories designed to challenge the most aggressive melts must be totally devoid of 6
porosity and microcracks. Glass is melted in fused cast blocks of alumina–zirconia–silica (AZS), which are less than 1% porous although porosity, grain size, and phase distribution vary widely within a block resulting from gravity effects and growth processes during solidification. Basic refractories, MgO, MgO : CaO, and spinel, are used to contain basic melts and slags, while acid refractories, S O , ZrO , SiO , 2Al O : SiO , mullite, # # # $ # etc., are used" to# contain acidic melts. Neutral refractories such as Al O and Cr O are used for certain # $ # $ glass compositions; Cr O is extremely resistant to $ fiberglass melts. attack by borosilicates, #e.g., Process changes have a great impact on refractory corrosion. The use of CaF , fluorspar, in basic oxygen # steels sharply increases furnace (BOF) melting of corrosion rates. Changing of combustion systems, fuel\air to fuel\oxygen, has made silica crowns obsolete for the containment of the combustion gases in many glass tanks. Fused refractories, especially M types, mixtures of α and β alumina, and AZS are highly resistant to the increased concentration of NaOH promoted by the substitution of O for air. # howRebonded and fused spinels are also resistant; ever, all the good performers are up to 10 times more expensive than the conventional silica refractories. The availability of new starting materials often leads to the development of more corrosion-resistant refractories. In the 1950s ‘‘high’’-purity calcium aluminate cements were developed to replace high iron-containing calcium aluminate hydraulic binders that were limited to use temperatures of about 1200 mC. The new cements, mixtures of CaO, Al O , and CaO : 2Al O plus highly reactive added# $Al O , $ # $ enabled the# production of monolithic refractories with higher service temperatures and greater corrosion resistance. The presence of SiO , often incorporated as silica fume to promote the# desired rheological
Refractories, Structure and Properties of behavior leads to formation of calcium alumina silicate phases that limit the refractoriness and corrosion resistance of the products. Availability of hydratable alumina hydraulic binders and colloidal silica binders that respectively combine with reactive counterparts to form mullite in situ has led to monolithics with unparalleled corrosion resistance. Developments in binders, matrix powders, aggregates, and additives have made it possible to obtain refractories in monolithic form that have thermal corrosion resistance levels exceeding those of fired shapes. 3.3 Physical Properties Density is a most important underlying property of refractories because it can be correlated to most of the key properties and behaviors. Porosity measurements correlate very well to mechanical, thermal, and chemical properties and to various behaviors, such that for a given product density is the generic index of quality, e.g., the quality of MgO bricks is reflected by their density values that serve as a basis for classification. Refractories almost invariably contain some porosity, up to 30% by volume in fired shapes and in monolithics. Higher porosities are induced through use of porous components or fugative phases in order to adjust the insulating properties. The size distribution and connectivity of the pores influence the permeability of a refractory, which in turn determines the rate of influx of corrodants and efflux of vapors generated during heat application, e.g., steam during the dry out of monolithics. Water is incorporated to hydrate the binders and to adjust the consistency for placement. Both forms of water are vaporized during heating to about 600 mC when the most stable to dehydration phases release chemical water. Permeability to water vapor movement to the heated face of monolithics is a key characteristic that controls excessive buildup of internal pressure that results in cracking and explosions. Permeability measurement is not straightforward for monolithics. Innocentini et al. (1999) have pointed to the inability of Darcy’s law to incorporate the total resistance to gas flow in many permeable media. They emphasize that a single value for permeability cannot account for contributions to resistance to flow measured by a pressure drop. Darcy’s law applies for fluids flowing at low velocities: P#kP# µνs " !l K 2PL "
(6)
where P and P are pressures at inlet and outlet faces, " ! which fluid velocity and viscosity P l pressure for values are obtained, K is the Darcian permeability, µ " ν is the volumetric flow rate. is the fluid viscosity, and s However, when the velocity increases as occurs in very low permeability monolithics, kinetic effects originat-
ing from turbulence and fluid inertia contribute additional resistances to flow. Two constants are therefore required and the expression for permeability becomes P#kP# µνs ρν# " !l j K K 2PL # "
(7)
where ρ l fluid density and K l new constant to # characterize high-velocity resistances. The second equation is called Forcheimer’s equation. This improvement in understanding of permeability will allow studies to determine constitutional and textural influences on the two permeability constants. This information is crucial for the correct modeling of the dewatering of monolithics. Moore et al. (1997) have reviewed the properties and factors that are essential for modeling of the critical process of drying out monolithic linings stressing that permeability data are the most critical. Data are needed for K and K from ambient to 600 mC for the various types" of low-# permeability monolithics used to line vessels to contain process liquids, e.g., ladles for holding molten steel. There are five distinct uses for the data discussed in this section on properties and behaviors: (i) certification and specification, (ii) quality control, (iii) design and modeling, (iv) development of all indices of performance, and (v) research and development. Test development activity has slowed during the period of refractory company downsizing, yet the demands on refractory products are more severe than ever. 4. Property Measurement Advances Needed in the Future The specific properties and behaviors of primary concern to the refractories technologist of the future will not change but the methods of determination will receive increasing attention due mostly to the need for the most relevant data for applications. The coarsegrained nature of most refractories requires specimen sizes large enough to overcome statistical variation in the constitution of individual test specimens. In the mechanical strength testing of monolithics, beams 2 ini2 ini8 in (50 mmi50 mmi200 mm) are employed for the measurement of the MOE, modulus of rupture (flexural strength), KIC toughness, and WOF. This practice should be followed for strength and toughness testing of all coarse-grained (top grain size of 4–6 mm or greater) refractories. For creep measurements, the specimens should be 50 mm cubes or 50 mmi50 mm cylinders or larger. Standardization of these larger specimens will yield more accurate and precise mechanical property data. Thermal conductivity is a property that is critical for design and modeling efforts and needs a major collaborative effort by international standards groups to support the development of new and improved 7
Refractories, Structure and Properties of characterization protocols are needed to produce the knowledge necessary for the design and optimal use of these refractory products demanded by users throughout the world. Researchers have developed a novel rheometer specifically adapted to the measurement of refractory monolithics (Pileggi et al. 2000). Results suggest that the instrument is capable of accurate and precise rheological characterization of the large family of refractory castables. Developments like those cited above require high levels of support for extended time periods. In particular, attention is needed to improve measurement capabilities of the following properties and behaviors: thermal shock resistance, nonlinear fracture mechanical behavior, high-temperature permeability, rheological behavior, abrasion resistance, thermal conductivity, and corrosion resistance. Figure 6 Extensometer for accurate measurement of strain in creep and MOE procedures (Moore et al. 1997).
Bibliography
measurements above 1200 mC. Flash techniques, modified to account for the coarse grain size of many refractories, are being promoted. Creep is the major failure mechanism to avoid in all high-temperature structural under load applications of refractories. A new technique that assures only the specimen dilation is measured has been perfected by Wereszczak et al. and applied to crown refractories for glass tanks. This method employing a scissors extensometer (see Fig. 6) has the capability of producing measurements with a resolution of 1 µm. The scissorstype extensometer technique for strain measurement eliminates problems with the axial extensometers that record dilations in the loading system, push rods, bearings plates, etc. The same scissors-type extensometer method is suited for measurements of the MOE in compression, another property important in design and modeling activities. A need to focus on the properties that lead to wider usage of monolithic refractories is obvious but there are major obstacles posed by the complexity of the compositions and their resulting microstructures. A contemporary high-performance castable may contain six to eight powder and aggregate particle components. The chemical additive package for consistency adjustment may contain three to five electrolytes. The rheological behavior, activated by the water addition and mode of mixing, needs to be characterized by methods that are economical and can be standardized. From the onset of drying, through the complete dewatering and heating stages, hydraulically bonded monolithics are ‘‘living’’ entities, i.e., their microstructures are continuously changing in contrast to a prefired shape or brick. New standard testing and
Ainsworth J H, Moore R E 1969 Fracture behavior of thermally shocked aluminum oxide. J. Am. Ceram. Soc. 52 (11), 628–9 Annual Book of Standards 1998a Load testing refractory brick at high temperatures; procedure C16-81. American Society of Testing and Materials, Vol. 15.01, pp. 1–4 Annual Book of Standards 1998b Standard method of measuring the thermal expansion and creep of refractories under load; procedure C832-89. American Society of Testing and Materials, Vol. 15.01, pp. 222–7 Annual Book of Standards 1998c Standard test method for pyrometric cone equivalent (PCE) for fireclay and high alumina refractory materials; procedure C24-89. American Society of Testing and Materials, Vol. 15.01, pp. 8–11 Carniglia S C, Barna G L 1992 The insulating refractory line. In: Handbook of Industrial Refractories, Principles, Types, Properties and Applications. Noyes Publications, Chap. IX, pp. 307–28 Chesters J H 1973 Refractories, Production and Properties. Iron and Steel Institute, London, p. 134 Cooper Jr. A R, Sawaddar B N, Oishi Y, Kingery W D 1964 Dissolution in ceramic systems. J. Am. Ceram. Soc. 47, 37 Hasselman D P H 1969 United theory of thermal shock fracture initiation and crack propagation in brittle ceramics. J. Am. Ceram. Soc. 52, 600–4 Innocentini M D M, Pardo A R F, Sadumi V R, Pandolfelli V C 1999 How accurate is Darcy’s law for refractories? Am. Ceram. Soc. Bull. 78 (11), 64–8 Moore R E, Smith J D, Sander T P, Severin N 1997 Dewatering monolithic refractory castables: experimental and practical experience. In: Proc. UNITECR ’97, United Int. Tech. Conf. on Refractories. American Ceramic Society, Vol. II, pp. 573–82 Morrell R 1985 Thermomechanics of properties. In: Handbook of Properties of Technical & Engineering Ceramics: Part 1. An Introduction for the Engineer and Designer. HMSO, London, Chap. 26, pp.141–58 Nakayama J, Abe H, Bradt R C 1981 Crack stability in the work of fracture tests: refractory applications. J. Am. Ceram. Soc. 64 (11), 671–5 Norton F H 1949 Spalling. In: Refractories. McGraw-Hill, New York, 3rd edn., Chap. XV, pp. 420–52 Pileggi R G, Pandolfelli U C, Paiva A E, Gallo J 2000 Novel
8
Refractories, Structure and Properties of rheometer for refractory castables. Am. Ceram. Soc. Bull. 79 (1), 54–8 Wereszczak A A, Karakus M, Liu K C, Pint B A, Moore R E, Kirkland T P 1999 Compressive creep performance and high temperature dimensional stability of conventional silica re-
fractories, technical report ORNL\TM-13757. Oak Ridge National Laboratories, US Department of Energy, Sect. 3.2, pp. 5–16
R. E. Moore
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 8079–8088 9