Introduction to the Properties of Refractories and Refractory materials
Introduction to the Properties of Refractories and Refractory materials
The properties of the refractories and the refractory materials are important and fundamental to the development, improvement, quality control, and selection of refractory linings for the high temperature processes. While the properties of the refractories and the refractory materials alone cannot be used to predict the lining performance, comparisons among these properties are frequently being used as screening tools by refractory engineers, process equipment designers, and the users of refractories and the refractory materials for the selection and the upgradation of the refractory linings.
Refractories and the refractory materials need to have properties so as to contain the corrosive process environment due to the liquid metals and slags as well as provide thermal protection and mechanical stability to safeguard the furnace. The properties of the refractory and the refractory materials are to match the demands of the service for which they are put to use and hence these properties help in the selection of appropriate refractories and the refractory materials for a particular use. The properties of the refractory and the refractory materials are also important for the quality control of the refractories during their production as well as for their handling, transport, and storage. Several national and international standards exist which define various properties of the refractories and the testing methods of the properties.
A suitable selection of the refractory furnace lining can only be made with an accurate knowledge of the properties of the refractory materials and of the stresses of the materials during the service. As it is impossible to achieve the ideal values for every property, the refractory materials are to be selected to meet the most important service requirements.
Important properties of refractories which can be determined most readily are chemical composition, bulk density, apparent porosity, apparent specific gravity, and strength at ambient temperatures. These properties are frequently among those which are used as controls in the manufacturing and quality control process. The chemical composition serves as a basis for classification of refractories. The density, porosity, and strengths of the fired refractories are influenced by several factors. Among these are type and quality of raw materials, the size and ‘fit’ of the particles, moisture content at the time of pressing, pressure of pressing, temperature and duration of firing, kiln atmosphere, and the rate of cooling.
The mechanical strength of refractories is governed by the ground mass material which is between the larger grains. At the ambient temperatures, the compressive strength is typically much higher than the tensile strength. As an example, a fireclay or 50 % alumina refractory can contain glassy material in the binding phase. The glassy material is difficult to compress but can be broken quite readily in tension. The room temperature strength of a refractory is an important indicator of its ability to withstand abrasion and impact in low temperature applications and to withstand handling and transport. Other important properties determined at the ambient temperatures are porosity, permeability, and pore size distribution.
The strength of refractories at the ambient temperature can provide little or no indication of strength at the furnace operating temperatures. For example, a fireclay or 50 % alumina refractories which have good strength at ambient temperatures typically show considerably reduced strength at high temperatures where glassy face can become quite hot or even fluid.
Chemical composition serves as a basis for classification of refractories and as a guide to their chemical properties. Another important application of chemical analysis as related to the refractories is control of the quality of both the raw materials and finished products. Minor components once thought to be unimportant, are now recognized as the controlling factors in the performance of several refractories.
Among the applications of chemical analysis to control the quantity are the alumina and the alkali content of some of the clays, the alumina content of bauxites, the alumina and alkali contents of silica refractories, chromic oxide contents of the chrome ores, the carbon content of graphite, and the boron oxide level in the magnesite.
The chemical composition of a refractory material may not be the most important selection criterion, as bricks of almost identical composition can differ widely in their behaviour under the same furnace conditions. Chemical composition of a refractory alone does not permit evaluation of such properties as volume stability at high temperatures or ability to withstand stresses, slagging or spalling.
When no single kind of refractory material is adequate to meet the various conditions prevailing in different parts of the furnace, two or more kind of refractories can be used in the construction. However refractories of dissimilar chemical compositions can react chemically with each other if in contact with high temperatures. Furnace fumes, dust, and slags can accelerate these reactions.
In the high temperature portions of a furnace, it is good practice to separate reactive refractories by an intermediate course of refractory brick which does not react with either at temperature of operation, or by a joint of non-reactive bonding mortar.
Refractories are required to face different types of stresses during the period of their service. Knowledge of refractory properties helps in the appropriate selection of those refractories which can match the stresses to be faced by the refractories during the service. The most significant properties of refractories are those which enable them to withstand the stresses to which they are exposed in service at high temperatures. This also forms the basis for the classification of the properties of the refractories and the test methods of refractory materials. The four types of stresses which refractories face during their period of service are (i) chemical stresses, (ii) mechanical stresses, (iii) thermal stresses, and (iv) thermo-technical stresses.
Chemical stresses – The properties related to chemical stresses are important for having resistance against chemical attack by slags, liquid materials, gases, and flue dust which the refractories come in contact during service. The important refractory properties for meeting the chemical stresses are chemical composition, mineralogical composition and crystal formation, pore size distribution and types of pores, gas permeability and resistance to slag, glass melts, gases and vapours.
Mechanical stresses – Properties related to mechanical stresses are having importance since they determine the strength of the refractories under different service conditions. The important refractory properties for the mechanical stresses are cold modulus of rupture and deformation modulus, crushing strength, abrasion resistance, porosity, and density.
Thermal stresses – Refractories are required to protect the furnace shell from the high temperatures existing in the furnace for meeting the requirements of the technological process being carried in the furnace. Thermal stresses develop because of the two different temperatures existing on the hot and the cold side of the refractories. The important properties of the refractories for meeting the thermal stresses which the refractories are to face during service are pyrometric cone equivalent, refractoriness under load, thermal expansion under load (creep), hot modulus of rupture, thermal expansion, reheat change (after-shrinkage and after-expansion), and thermal shock resistance.
Thermo-technical stresses – Properties related to thermo-technical stresses are important for the heat flux calculations as well as the calculations for the stored heat. The important refractory properties for the thermo-technical stresses are thermal conductivity, specific heat, bulk density, melting point, and thermal capacity, and temperature conductivity.
The relationship between service stresses and the important properties of the refractory bricks during the service is given in Fig 1.
Fig 1 Relationship between stress and refractory properties
One common condition which is applicable to all furnace operation is the high temperature. A closely related combination of other thermal effects results from high temperature and the rate of the temperature change. Heat energy flows into the refractory structure when the furnace is heated and a temperature differential develops between the inside and outside surfaces. Part of heat energy is stored in the refractory structure and part flows through the walls and other parts of the furnace structure, and is lost to the outside atmosphere through radiation and convention.
Refractories are to be sufficiently refractory to which they are to withstand the maximum temperatures to which they are exposed in service. Frequently, the refractories are to be resistant to the stress effects of rapid temperature change within critical temperature intervals. The refractories can be required to withstand varying loads, abrasive action, and penetration and corrosion by solids, liquids and gases.
High temperature properties of refractories which are relevant to service conditions include refractoriness, melting behaviour, mechanical strength and load bearing capacity, changes in dimension when heated, resistance to abrasion, mechanical erosion and impact of solids, and resistance to corrosion and erosion at high temperatures by solids, liquids, fumes, and gases. Important properties which are determined at high temperatures are hot modulus of rupture, hot crushing strength, refractoriness under load, creep behaviour, spalling resistance, dimensional changes, and thermal conductivity.
When refractories are hot their linear dimensions and volumes are higher than when they are cold due to the ‘reversible thermal expansion’. With some refractories, additional volume change, normally not reversible results from mineral inversions and transformations and sometimes from other causes.
With a single brick in a furnace wall or arch, there is normally a temperature drop of many degrees between the hot face and the cold end. As a result, difference in the amount of thermal expansion in various sections of the brick causes internal stresses to develop within the brick. In a refractory structure, pressure inherent in the furnace construction, such as arch structure, is normally increased by the thermal expansion of the brickwork. Unequal volume changes resulting from rapid fluctuations in temperature can cause the development of higher stresses within the refractories than they are capable of capable of withstanding.
In common with essentially all other structural materials, refractories expand when heated and contract when cooled. Knowledge of thermal expansion behaviour of refractory materials is frequently crucial in designing of the furnace lining. If the lining is constrained by the furnace structure, destructive forces can result as the lining expands during furnace heat-up. In such case, the lining is to be designed to allow some free expansion. However, the amount of expansion allowance is not to be so much that it causes lining instability. An understanding of thermal expansion characteristics is also important in assessing the thermal shock resistance of a material. Thermal shock occurs when severe temperature gradients in a material cause internal stresses due to different thermal expansion.
If no changes of a permanent nature occur during heating, fired refractories return to their original dimensions. This characteristic is known as ‘reversible thermal expansion’. Typically, refractories which are heated to temperatures below their firing temperature show this behaviour. Magnesite and forsterite brick have relatively high rates of thermal expansion while fireclay, silicon carbide, zircon brick, and majority of insulating firebricks have relatively low rates. The thermal expansions of other refractories have intermediate values. For majority of fired refractories, thermal expansion increases at a fairly uniform rate over the working range temperature. Notable exceptions are silica bricks which undergo the greater part of their expansion below 425 deg C.
In general, the thermal expansion behaviours of unfired refractories are more complex than those of their fired counter-parts. During initial heating, drastic expansion or contractions can occur in an unfired material as a result of changes in the bonding structure, changes in mineralogy, and sintering effects. Fig 2 shows thermal expansion of different refractories.
Fig 2 Thermal expansion of different refractories
In service, refractory materials are to support a load which, at its minimum, is equal to the lining above the reference point. The pressure which is exerted depends on the height of the lining and density of the material. Hence, for applications in which the entire lining component is at high temperatures, it is important to understand the load bearing capabilities of the candidate materials. Examples of such an application are blast furnace stoves and carbon bake furnaces in which the refractories are heated relatively uniformly to high temperatures. Fig 3 shows ‘refractoriness under load’ curves of different refractory bricks.
Fig 3 Refractoriness under load curves of refractory bricks
Like majority of structural materials, refractories show creep behaviour when exposed to high temperature. Majority of refractories show two characteristic stages of creep. In the first stage which is called primary creep, the rate of subsidence declines gradually with time. In the second stage, the rate of subsidence is constant. At very high temperatures, steady state creep is sometimes followed by a tertiary creep region where the rate of subsidence accelerates and leads to catastrophic failure or creep rupture. Primary creep is normally short in duration. Hence, secondary creep normally provides a more meaningful comparison of refractories. Creep behaviours of refractory bricks cannot be predicted based only on chemistry. Important variables which affect creep behaviour are chemistry of the bonding phase and firing temperature. The result of the formation of low viscosity glassy phase is poor creep behaviour. Aggregate sizing and porosity also affect the creep behaviour with larger aggregate and lower porosity giving better creep resistance. Creep curves of refractories are at Fig 4.
Fig 4 Creep curves of refractories
When a furnace is heated, thermal energy flows, into the refractory structure, causing a temperature difference to develop between inside and outside surface of walls and roofs. Part of this thermal energy is stored in the refractory structure and its foundation, and part flows through the walls, roofs, and hearths, and is lost to the outside atmosphere by radiation and convention. The amount of heat which escapes in this manner is frequently of considerable importance in the economy of the process. In estimating the quantities of eat flowing therough the parts of the furnace, use is made of a co-efficient known as thermal conductivity or K-value of each material involved in the construction. The thermal conductivity value differs not only for different materials, but also normally for the same material at different temperatures. The thermal conductivity value is reported at the mean temperature of the test brick.
Major factors which affects the thermal conductivity of a refractory material are mineral composition, the amount of amorphous material (glass or liquid) which it contains, its porosity, and its temperature. For materials which have similar mineralogical compositions, the properties of pore space is questionably the most important factor affecting the amount of heat which flows through it at a given temperature. Within the temperature range seen in the majority of the applications, thermal conductivity decreases with incresing porosity.
At the ambient temperatures, the thermal conductivity of glass is considerably lower than the crystalline material of the same composition. With rising temperatures, the conductivity of glasses tends to increase while that of the crystalline materials tends to decrease. However, in refractory bodies consisting of crystal aggregates with limited amount of glass, the effect of the temperature is difficult to predict. The conductivity of refratory in service at high temperature is frequently changed somewhat, either by an increase in the amount of glass or liquid it contains, or by diversification of any glass which it can contain.
High conductivity is desirable for refractories used in construction needing efficient transfer of heat through brickwork, as in retorts, muffles, by-product coke oven walls, and recuperators. However, in most types of furnaces or vessels, low thermal conductivity is desirable for heat conservation, but is normally less important than other properties of the refractories. The rate of heat flow in refractory constructions can be calculated only approximately for several reasons. Other important factors which affect the quantity of heat flow through refractory lining include the emissivity of the refractory or metal shell, the kind of gases within the furnace atmosphere, and the external convection currents. Fig 5 shows thermal conductivity of different types of refractory bricks.
Fig 5 Thermal conductivity of refractory bricks
In refractory structures built of insulating firebricks or very other porous refractories, the type of furnace atmosphere can have a very appreciable effect on heat loss through the refractory walls. This is especially true for atmospheres with a high content of hydrogen. Most protective atmospheres including dissociated ammonia, exothermic gases, contain appreciable percentage of hydrogen. The conductivity of hydrogen is to be taken into consideration in calculating the heat losses through the refractory walls of controlled atmosphere, heat treating furnaces.
The thermal conductivity of hydrogen is around 7 times that of air. As a result, the presence of hydrogen gas in the pores of a porous refractory product increases the rate of heat flow through the refractory. With an atmosphere of 100 % hydrogen, the heat flow through an insulating firebrick or other refractory with similar porosity is two and a half times as high as in the atmosphere of air. At lower concentrations of hydrogen, the increase is less and the relationship is almost directly proportional to the percentage of hydrogen present. The thermal conductivities of nitrogen, carbon di-oxide, and carbon mono-oxide are around 100 %, 59 % and 97 % respectively that of air.
Spalling is the loss of fragments or spall from the face of a refractory brick or structure through cracking and rupture. In various types of furnace operation, service conditions necessarily expose the brickwork to spalling influences. The resistance of refractory brick to spalling is enhanced by the addition of spall inhibitor, proper design of the brick shapes, optimum sizing of grains, proper choice of grains, and close manufacturing control. For a given refractory, spalling can also be minimized by proper design construction, and operation of the furnace. Spalling of refractories can be of three types namely (i) thermal, (ii) mechanical, and (iii) structural.
Thermal spalling is caused by the stresses resulting from unequal rates of expansion or contraction between different parts of the brick and is normally associated with the rapid changes in temperature. Refractories bricks with the highest resistance to thermal spalling are those having the lowest average rate of thermal expansion, high tensile strength, and a texture conducive of flexibility and relief of stress. These compositions also do not show a high rate of thermal expansion through narrow temperature ranges. Other factors being equal, temperature gradients and stresses which cause spalling are least destructive in brick having the highest thermal conductivities. A refractory of a given mineral composition normally has maximum resistance to spalling when the ratio of strength to modulus of elasticity is a maximum.
The spalling characteristics of fireclay bricks are normally dependent upon the amount of free silica present in the clays, the composition of the glassy bond, and the size and ‘fit’ of the particles. The amount and the character of the glass are fixed by the quantity and kind of accessory minerals in the clays and by the time and temperature of firing. Within the temperature range at which the glass is rigid, high glass content is conducive to spalling. However, at temperatures high enough to cause the glassy bond to become somewhat viscous, fireclay brick can be highly resistant to spalling.
Relatively porous, light duty fireclay bricks are normally more resistant to spalling than denser, hard-burned bricks. Super-duty bricks of the spall resistant variety, while dense and hard burned, have high resistance to spalling and retain this resistance upon exposure to high temperatures.
Some high alumina bricks also have high resistance to spalling. In several cases, bricks are designed to generate a system of stress relieving micro-cracks to improve their spalling resistance. With other conditions being equal, higher density, lower porosity compositions tend to have poorer spall resistance than average density and average porosity compositions.
Majority of silica bricks are sensitive to rapid temperature changes at low temperatures because of the abrupt volume changes associated with the crystalline inversion of the mineral cristobalite. However, silica brick can be heated or cooled quite rapidly as long as they remain above the inversion temperature of around 650 deg C.
Burned basic bricks, in general, do not have a high resistance to thermal spalling as fireclay and high-alumina bricks. However, with proper choice of raw materials, basic bricks with improved thermal shock resistance can be produced.
Mechanical spalling of refractory brick is caused by the stresses resulting from impact or pressure. Shattering or spalling of brickwork can result from such influences as the rapid drying of wet brickwork, inadequate provision for thermal expansion, or pinching of the hot ends of brick, especially in the furnace arches. Bricks which are strongest and toughest at the operating temperatures have the highest resistance to mechanical spalling.
‘Pinch spalling’ is frequently observed in sprung brick arches since the hot end of the brick expand more than the cold end. This conduction is aggravated by rapid heating since the furnace structure cannot rapidly adjust to the expansion forces. The differential expansion can cause a concentration of arch stresses upon relatively small bearing surfaces at the inner ends of the bricks. Insulation decreases the temperature gradient through a roof arch and tends to somewhat reduce pinch spalling.
Structural spalling of a refractory unit is caused by stresses resulting from differential changes in the structure of the unit. The word ‘structural’ as used in this context does not refer to the furnace lining assembly but to the texture or structure of the lining.
Contributing to the structural spalling are changes which occur in service to the texture and composition of the hotter portion of the brick through the action of heat, the absorption of slags or fluxes and their reactions with the refractory. These alterations can result in zones in the brick which differ in the mineral content and chemical and physical properties from the unaltered portion of the brick. Following the development of a zoned structure, hot ends of a brick can spall or fall off for different reasons including differences in thermal expansion or contraction between adjacent zones, increased sensitivity to rapid changes, or the presence of shrinkage or expansion cracks.
Monolithic refractories , not having being fired prior to installation, are more resistant to thermal spalling than are their fired brick counterparts, at least until they have become vitrified or slagged in service. However, being lower in strength than fired bricks, monolithic refractories can be more subject to mechanical spalling.
Several tests are normally used to determine the relative resistance to spalling. The most common tests are (i) panel spalling test, (ii) prism spalling test, and (iii) the loss of strength test.
In several applications, the refractory is in contact with a slag or metal during service. A chemical reaction frequently results between the refractory and slag or metal. Some of the reaction products can be extremely detrimental to brick service while other reaction products result in little or no change in the service life of the refractory.
Slag attack refers to chemical reactions and solutions which corrode the surface of the refractory lining in service and reactions which can take place between the liquid slag, refractory, and the fluxing agents which have been absorbed. Erosion of the refractories frequently follows corrosion. An example is the washing away of refractory grains after the bond between grains has been dissolved by fluxing agents in the slag.
The composition of industrial slags of the refractories which they contact in service and the reaction products derived from them are exceedingly complex. The most advantageous selection of refractories frequently is governed by the chemical nature of fluxing agents present in the furnace. Alkaline materials are highly basic and the majority of metallurgical slags and metallic oxides are basic and fluxes and slags high in silica are acidic.
To determine whether a particular slag is acidic or basic, the lime to silica ratio is normally examined. As a normal rule, if the CaO / SiO2 ratio (or CaO + MgO / SiO2 + Al2O3 ratio) is higher than 1, the slag is normally considered to be basic. If the ratio is less than 1, the slag is considered as acidic. Fireclay, high alumina, or silica bricks are normally used where the corrosive slag is acidic. For highly basic slags, magnesia, chrome, or a blend of these two materials typically gives good service. Exceptions to these general principles are based on operating and reaction temperatures, reaction rates, and formation of protective glazes and coatings on the refractory surfaces.
In order to determine the relative resistance of a refractory to the slag or metal, several slag tests have been developed. Among them, the most common tests are cup slag test, drip slag test, gradient slag test, rotary slag test, dip and pin tests, and aluminum boat test.
Some of the important physical and chemical properties are described below.
Melting point – Melting point (melting temperatures) specify the ability of materials to withstand high temperatures without chemical change and physical destruction. The melting points of major elements which constitute refractory composition in pure state vary from 1,700 deg C to 3,480 deg C. The melting point serves as a sufficient basis for considering the thermal stability of refractory mixtures and is an important characteristic indicating the maximum temperature of use.
Size and dimensional stability – The size and shape of the refractories is an important feature in design since it affects the stability of any structure. Dimensional accuracy and size is extremely important to enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.
Porosity – Porosity is a measure of the effective open pore space in the refractory and is expressed as the average percentage of open pore space in the overall refractory volume. The mechanical strength of a refractory material is largely determined by the true porosity which is composed of closed pores and open pores, the latter being either permeable or impermeable. For higher mechanical strength, low porocity of the refractory bricks is aimed. The important properties with respect to porosity is its behaviour during chemical attack by liquid metal, slag, fluxes and vapour which can penetrate and thereby contribute to degradation of the refractory structure. High porosity materials tend to be highly insulating as a result of high volume of air they trap. The content of open pores of a brick is calculated from the water absorption. By using the water air displacement method, the open pores are classified either as permeable or effective or as impermeable pores.
Bulk density – The bulk density is normally considered in conjunction with apparent porosity. It is a measure of the weight of a given volume including the pore space of the refractory. It is one of the important characteristic and provides a general indication of the product quality. An increase in bulk density increases the volume stability, the heat capacity, the resistance to abrasion, and slag penetration. The bulk density is determined by means of a hydrostatic scale, according to the mercury displacement method or by measurement.
Cold crushing strength – It is a measure of the mechanical strength of the refractory brick. In furnaces, cold crushing strength is of importance, because of bricks with high crushing strength is more resistant to impact from rods or during removal of slag than a brick with a low cold crushing strength. It is a useful indicator to the adequacy of firing and abrasion resistance in consonance with other properties such as bulk density and porosity.
Pyrometric cone equivalent – It is the measurement of the refractoriness. Pyrometric cone equivalent (PCE) is the ability to withstand exposure to elevated temperature without undergoing appreciable deformation. Refractories due to their chemical complexity melt progressively over a range of temperature. This softening behaviour of the refractories is determined by PCE which consists of comparing ceramic specimen of known softening behaviour (seger or orton cones) with the cone of the refractory. Pyrometric cones are small triangular ceramic prisms which when set at a slight angle bend over in an arc so that the tip reaches the level of the base at a particular temperature if heated at a particular rate. The bending of the cones is caused by the formation of a viscous liquid within the cone body, so that the cone bends as a result of viscous flow. PCE is measured by making a cone of the refractory and firing it until it bends and comparing it with standard cone. PCE is useful for the quality control purpose to detect variations in batch chemistry changes or errors in the raw material formulation. Refractoriness points to the resistance of the refractory to the extreme conditions of heat (higher than 1,000 deg C) and corrosion when hot and liquid materials are contined while being transported and / or processed. PCE cones before and after firing are shown at Fig 6.
Fig 6 Pyrometric cones before and after firing
Refractoriness under load – Refractoriness under load (RUL) evaluates the softening behaviour of fired refractory bricks at rising temperature and at constant load conditions. RUL gives an indication of the temperature at which the brick collapses in service condition with similar load. However, under actual service conditions the bricks are heated only on one face and most of the load is carried by the relatively cooler rigid portion of the refractory bricks. Hence, the RUL test gives only an index of refractory quality, rather than a figure which can be used in a refractory design. Under service conditions, where the refractory used is heating from all sides such as checkers, partition walls etc. the RUL test data is quite significant. For RUL, samples in cylindrical shape of 50 mm height and 50 mm diameter are heated at a constant rate under a load of 0.2 N/sq mm and the change in height includes the thermal expansion and also the expansion of test equipment. The test results are taken from the recording. The initial temperature is taken at 0.6 % compression while the final temperature is taken at 20 % compression or when the specimen has collapsed.
Thermal expansion under load (creep) – Thermal expansion under load (creep) is a time dependent property which determines the deformation in a given time and at a given temperature by a refractory under stress. Refractory material is required to maintain dimensional stability under extreme temperatures (including repeated thermal cycling) and constant corrosion from hot liquid and gases. In the creep test, specimen of 50 mm diameter and 50 mm height with an internal bore for the measuring rod is heated at constant rate and under a given load (normally at 0.2 N/sq mm). After the required temperature is reached, the samples is held for 10 hours to 50 hours. The compression of the specimen, after maximum expansion has been attained, is given in relation to the test time as a measure of creep at a specified test temperature.
Volume stability, expansion, and shrinkage at high temperature – Permanent change in the dimension of a refractory due to contraction and expansion during service can take place due to (i) the changes in the allotropic forms which cause a change in specific gravity (ii) a chemical reaction which produces a new material of altered specific gravity (iii) the formation of a liquid phase, (iv) sintering reactions, and (v) due to fluxing by dust and slag or by the action of alkalis in case of fireclay refractories.
After heating to high temperatures and subsequent cooling, a permanent change in dimensions frequently occurs. This can cause either loosening of the bricks during service or the destruction of brickwork due to the pressure. Permanent linear change (PLC) on heating and cooling of the refractory bricks give an indication of volume stability of the brick as well as the adequacy of the processing parameters during manufacture. It is particularly significant as a measure of conversion achieved in the manufacure of silica refractories.
Reversible thermal expansion – Refractories like any other materials expand when heated and contract when cooled. The reversible thermal expansion is a reflection on the phase transformation which occurs during heating and cooling. The PLC and the reversible thermal expansion are followed in the design of refractory lining for provision of expansion joints. As a rule, those with a lower thermal expansion co-efficient are less susceptible to spalling.
Thermal conductivity – Thermal conductivity is defined as the quantity of heat which flows through a unit area in direction normal to the surface area in a defined time with a known temperature gradient under steady state conditions. It indicates general heat flow characteristics of the refractory and depends upon the chemical and mineralogical composition as well as the application temperature. High thermal conductivity refractories are needed for some applications such as coke ovens, regenerators etc. On the other hand refractories with lower thermal conductivity are preferred in most application since they help in conserving heat energy. Porosity is an important factor in heat flow through refractories. The thermal conductivity of a refractory decreases on increasing its porosity. Although it is one of the least important properties as far as service performance is concerned, it evidently determines the thickness of the brickwork.
Thermal shock resistance – It characterizes the behaviour of refractories to sudden temperature shocks. Temperature fluctuations can reduce the strength of the brick structure to a high degree and can lead to disintegrtaion or spalling in layers. There are several methods of determining the thermal shock resistance each having its own advantages and disadvantages.
Specific heat – The specific heat is a material and temperature related energy factor and is determined with the help of calorimeters. The factor indicates the amount of energy (calories) needed to raise the temperature of one gram of material by 1 deg C. Compared to water, the specific heats of refractory materials are very low. These values are less than one fourth of value of specific heat of water.
Abrasion resistance – The mechanical stress of refractory bricks is not caused by pressure alone, but also by the abrasive attack of the solid raw materials as they slowly pass over the brickwork and by the impingement of the fast moving gases with fine dust particles. Hence the cold crushing strength is not alone sufficient to characterize the wear of the refractories. There is no approved method for testing abrasion resistance but there are some methods available to give reference values such as Bohme grinding machine method and sand blast method etc.
Modulus of Rupture or Modulus of deformation – During thermal stress, normally combined with altered physical-chemical conditions because of infiltration, strain conditions occur in refractory brickwork which can lead to brick rupture or crack formation. In order to determine the magnitude of rupture stress, the resistance to deformation under bending stress (rupture strength) is measured. Determination of the modulus of deformation in the cold state is carried out, together with modulus of rupture, on a test bar resting on two bearing edges. In general, a high ductility is looked for in refractory bricks,i.e. a large deformation region without rupture, which means a high value of the ratio of modulus of rupture to modulus of deformation.
The modulus of rupture is defined as the maximum stress of a rectangular test piece of specific dimensions which can withstand maximum load until it breaks, expressed in N/sq mm. For hot modulus of rupture (HMOR) load is applied at a high temperature. The international standard test method is described in ISO 5013 with test piece dimensions of 150 mm x 25 mm x 25 mm.
Mineralogical composition and crystal formation – The behaviour of refractories of identical composition also depends on the type of raw materials used and on the reactions achieved during firing of the bricks. A glassy phase is more susceptible to attack by slag than a tightly interlocked crystal lattice structure. Two methods are used to identify mineralization composition. In the first method polarizing microscope or scanning electron microscope (SEM) is used. In the second method X-ray diffraction analysis is done.