According to the degree of refractoriness, it can be divided into ordinary refractory products (1580-1770 ° C), high-grade refractory products (1770-2000 ° C) and special refractory products (above 2000 ° C).
The refractory material is an inorganic non-metallic material having a refractoriness of not less than 1,580 °C. Although the definitions set by countries are different, for example, the international standards officially published by the International Organization for Standardization (ISO) stipulate that “refractory materials with a non-metallic material or product with a refractoriness of at least 1500 ° C (but not excluding those containing a certain proportion of metals) However, refractory materials are used as structural materials for thermal equipment such as high-temperature kiln and furnace, as well as materials for industrial high-temperature containers and components, and can withstand corresponding physical and chemical changes and mechanical effects.
Most refractory materials are made from natural ores (such as refractory clay, silica, magnesite, dolomite, etc.). Nowadays, certain industrial raw materials and synthetic raw materials (such as industrial alumina, silicon carbide, synthetic mullite, synthetic spinel, etc.) are also increasing.
According to the degree of refractoriness, it can be divided into ordinary refractory products (1580-1770 ° C), high-grade refractory products (1770-2000 ° C) and special refractory products (above 2000 ° C).
According to the shape and size, it can be divided into standard bricks, shaped bricks, special bricks, large shaped bricks, as well as special products such as crucibles, dishes and tubes for laboratory and industrial use.
According to the manufacturing process, it can be divided into a mud casting product, a plastic molding product, a semi-dry pressing product, a stencil molding product from a powdery non-plastic mud material, a product cast from a molten material, and a product sawn from a rock.
Table 1 Classification of chemical mineral composition of refractory materials
Classification | Category | Main chemical composition | Main mineral component |
Siliceous products | Silica brick | SiO2 | Phosphorus quartz, cristobalite |
Quartz glass | SiO2 | Quartz glass | |
Aluminum silicate products | Semi-silicon brick | SiO2, Al2O3 | Mullite, cristobalite |
Clay brick | SiO2, Al2O3 | Mullite, cristobalite | |
High alumina brick | SiO2, Al2O3 | Mullite, corundum | |
Magnesium products | Magnesia brick (permanite brick) | MgO | Periclase, |
Magnesia-aluminum brick | MgO, Al2O3 | Periclite, magnesium aluminum spinel | |
Magnesia chrome brick | MgO, Cr2O3 | Periclase, chrome spinel | |
Forsterite brick | MgO, SiO2 | Forsterite, periclase | |
Magnesia silica brick | MgO, SiO2 | Periclase, forsterite | |
Magnesia-calcium brick | MgO, CaO | Periclase, dicalcium silicate | |
Magnesia dolomite brick | MgO, CaO | Periclase, calcium oxide | |
Magnesia carbon brick | MgO, C | Periclase, amorphous carbon (or graphite) | |
Dolomite products | Dolomite brick | CaO, MgO | Calcium oxide, periclase |
Chrome products | Chrome brick | Cr2O3, FeO | chromite |
Chrome magnesium brick | Cr2O3, MgO | Chrome spinel, periclase | |
Carbonaceous products | Charcoal brick | C | Amorphous carbon (graphite) |
Graphite products | C | graphite | |
Silicon carbide products | Si C | Silicon carbide | |
Zirconium products | Zircon brick | ZrO2, SiO2 | Zircon |
Special product | Pure oxide products | Al2O3, ZrO2 | Corundum, high temperature type ZrO2 |
CaO, MgO | Calcium oxide, periclase | ||
Other: Carbide | |||
nitride | |||
Silicide | |||
Boride | |||
Cermet, etc. |
Table 2 Classification of refractory appearance
Classification | Species |
Refractory brick (with certain shape) | Burnt brick, non-burned brick, fused brick (fused cast brick), refractory brick |
Unshaped refractory material (referred to as bulk material, no shape, construction materials according to the required shape) | Castable, ramming, projection, spray, plastic, refractory mud |
There are many methods for classifying refractory materials. There are classification methods based on the chemical mineral composition of refractories. It can characterize the basic composition and characteristics of various refractories and has practical significance in production, use and scientific research. Table 1).
In addition, refractory materials are classified according to the following indicators (see Table 2).
In the future, China's refractory industry should change from quantity to variety quality, based on China's resource conditions and use needs, research and development of high-quality high-efficiency high-aluminum and alkaline products, development of high-quality unshaped refractory materials and thermal insulation materials.
1. Composition and properties of refractory materials
The general properties of refractory materials include chemical mineral composition, microstructure, mechanical properties, thermal properties, and high temperature use properties. Some of them are properties measured at room temperature, such as porosity, bulk density, true density, and compressive strength. According to these properties, the use of refractory materials at high temperatures can be predicted; others are properties measured at high temperatures, such as refractoriness, load softening point, thermal shock stability, slag resistance, high temperature volume stability, etc., these properties Reflects the state in which the refractory is at a certain temperature, or the relationship between it and the outside world at that temperature.
1.1. Chemical mineral composition of refractory materials
Several properties of the refractory material depend on the phase composition, distribution and characteristics of the phases, ie, on the chemical mineral composition of the article. For a given raw material, that is, when the chemical mineral composition is constant, an appropriate process can be used to obtain a phase composition having certain characteristics (such as crystal form, grain size, distribution, and formation of solid solution and glass equivalent) within a certain limit. Improve the working nature of the product.
1.1.1 Chemical composition
The chemical composition is a basic property of refractory products. The chemical composition of the refractory material is generally divided into two parts according to the content of each component and its action, that is, an absolute amount of the basic component - the main component and a small amount of the subordinate component. The secondary component is the inclusion component accompanying the raw material and the additive component (addition) which is specifically added during the process.
1.1.1.1, principal component
It is a component of the refractory product that constitutes the refractory matrix and is the basis of the refractory properties. Its nature and quantity directly determine the nature of the product. The main component may be an oxide or an elemental or non-oxide compound. Refractory materials can be further divided into three categories according to their chemical properties: acid refractories, neutral refractories and basic refractories.
Acidic refractories contain significant amounts of free silica (SiO2). The most acidic refractory material is a siliceous refractory material consisting almost 94-97% of free silicon oxide (SiO2). Clay-based refractories have a lower amount of free silicon oxide (SiO2) than silicetes and are weakly acidic. A semi-silicon refractory material is in the process.
Neutral refractory materials are carbon refractories according to their strict meaning. High-aluminum refractories (Al2O345% or more) are acidic and tend to be neutral refractories. Chromium refractories are alkaline and tend to be neutral and refractory. material.
Alkaline refractories contain significant amounts of MgO and CaO, magnesia and dolomite refractories are strongly basic, chrome-magnesium and forsterite refractories and spinel refractories are weakly basic refractories.
1.1.1.2, impurity components
Most of the raw materials for refractory materials are natural minerals, which contain a certain amount of impurities in the refractory material (or raw material). These impurities are oxides or compounds which react with the refractory matrix to reduce their fire resistance, i.e., impurities commonly referred to as fluxes. For example, the main component in the chemical composition of the magnesia refractory material is MgO, and other oxide components are all impurity components. The eutectic liquid phase formation temperature of the system is lower due to the flux of the impurity component. The more the amount of liquid phase generated by the unit flux, and the faster the liquid phase increases with the increase of temperature, the smaller the viscosity, the better the wettability, the stronger the effect of the impurity flux. As can be seen from the data in Table 3, the flux strength of these oxides to SiO2 was enhanced in the following order.
Table 3 Flux effect of certain oxides on SiO2
oxygen Chemical Object | Eutectic | SiO2 content in liquid phase, % | ||||||
Balanced phase | Temperature (°C) | Liquid phase amount per % of impurities in the system (%) | Oxide content (%) | Co-melting point (°C) | 1400 ° C | 1600 ° C | 1650 ° C | |
K2O | Quartz (SiO2)-K2O.4SiO2 | 769 | 3.6 | 27.5 | 72.5 | 87.0 | 96.2 | 98.0 |
Na2O | Quartz (SiO2)-Na2O.2SiO2 | 782 | 3.9 | 25.4 | 74.6 | 86.0 | 95.8 | 97.8 |
Li2O | Phosphorus quartz (SiO2)-Li2O.2SiO2 | 1028 | 5.6 | 17.8 | 88.2 | 88.8 | 96.5 | 98.5 |
Al2O3 | Cristobalite (SiO2)-3Al2O3 2SiO2 | 1545 | 18.2 | 5.5 | 94.5 | - | 96.9 | 98.1 |
TiO2 | Cristobalite (SiO2)-TiO2 | 1550 | 9.5 | 10.5 | 89.2 | - | 92.0 | 95.4 |
CaO | Phosphorus quartz (SiO2)-CaO SiO2 | 1436 | 2.7 | 37.0 | 63.0 | - | 67.8 | 69.5 |
MgO | Cristobalite (SiO2)-MgO SiO2 | 1543 | 2.9 | 35.0 | 65.0 | - | 65.5 | 67.8 |
BaO | Phosphorus quartz (SiO2)-BaO SiO2 | 1374 | 2.1 | 47.0 | 53.0 | 53.5 | 61.2 | 67.0 |
ZnO | Phosphorus Quartz (SiO2)-2 ZnO-S | 1432 | 2.1 | 48.0 | 52.0 | - | 60.0 | 64.0 |
MnO | Phosphorus quartz (SiO2)-MnO SiO2 | 1291 | 1.8 | 55.8 | 44.2 | 45.0 | 50.4 | 52.5 |
FeO | Phosphorus quartz (SiO2)-2 FeO SiO2 | 1178 | 1.6 | 62.0 | 38.0 | 41.2 | 47.5 | 51.7 |
Cu2O | Phosphorus quartz (SiO2)-Cu2O | 1060 | 1.1 | 92.0 | 8.0 | 19.2 | 29.6 | 32.7 |
1.1.1.3, adding ingredients
In the production of refractory products, in order to promote the change in high temperature and lower the sintering temperature, a small amount of added components are sometimes added. According to its purpose and role, it is divided into mineralizers, stabilizers and sintering agents. The ignition loss, various oxide contents and other major component contents of refractory products and raw materials are usually analyzed. The percentage reduction in mass when the dried material is heated under specified temperature conditions is referred to as burning reduction.
1.1.2, mineral composition
The refractory product is a mineral composition. The nature of the product is a comprehensive reflection of its constituent minerals and microstructure. The mineral composition of a refractory product depends on its chemical composition and process conditions. For products with the same chemical composition, the performance may vary greatly depending on the type, quantity, grain size and bonding of the mineral phases formed. For example, siliceous products with the same SiO2 content may have different properties of the products due to the formation of two types of minerals, phosphorus quartz and cristobalite, which have different structures and properties under different process conditions. Even if the mineral composition of the product is certain, it will have a significant effect on the properties of the product (such as molten products) depending on the grain size, shape and distribution of the mineral phase.
The refractory material is generally a plurality of constituent bodies, and the mineral phases thereof can be classified into two types, a crystalline phase and a glass phase.
The main crystalline phase refers to a crystalline phase which constitutes the bulk of the structure of the article and has a relatively high melting point. The nature, quantity and bonding state of the main crystalline phase directly determine the nature of the article.
Matrix refers to the presence of large crystals or aggregates in the refractory material. The matrix plays a decisive role in the properties of the article, such as high temperature properties and resistance to ablation. In use, the article tends to be damaged first from the matrix portion, and the use of adjusting and modifying the matrix component of the article is an effective process for improving the properties of the article.
Most refractory products (except a few extra-high refractory products) can be divided into two categories according to their main crystalline phase and matrix composition: one is a multi-component refractory product containing a crystalline phase and a glass phase, such as clay bricks, silicon. Bricks, etc.; the other is a multi-component product containing only a crystalline phase, the matrix is ​​mostly fine crystals, such as alkaline bricks such as magnesia bricks and chrome-magnesia bricks. When these products are fired at a high temperature, a certain amount of liquid phase is generated, but the liquid phase does not form glass upon cooling, but forms a crystalline matrix, and the main crystal phases are cemented together, and the composition of the matrix crystal is different from that of the main crystalline phase. .
There are two types of microstructures of refractory products. One is a structural type of silicate (silicate crystal mineral or vitreous) conjugate crystallographic particles, and the other is a crystal lattice directly interlaced into a crystalline network, such as a high-purity magnesia brick, which is directly bonded to the structure type. The high temperature performance (high temperature mechanical strength, slag resistance or thermal shock stability) of the product is much superior to the former one; therefore, it has broad development prospects.
1.2, the structure of refractory materials
The refractory material is a heterogeneous body composed of a solid phase (including a crystalline phase and a glass phase) and pores, and a macroscopic structure between the pores and the solid phase of various shapes and sizes.
1.2.1 Porosity, bulk density, true density
Porosity, bulk density, true density, etc. are important indicators for evaluating the quality of refractories. There are ten definitions of GB/T2997: bulk density (ratio of the mass of dry material with pores to its total volume, expressed in g/cm3 or kg/m3), total volume (solid matter in materials with pores, open pores) And the sum of the volume of the closed pores), the true density (the ratio of the mass of the dry material with pores to its true volume, expressed in g/cm3 or kg/m3), the true volume (the volume of solid matter in the material with pores) , open pores (pores that can be filled with liquid during immersion), closed pores (holes that cannot be filled with liquid during immersion), apparent porosity (the ratio of the volume of all open pores to the total volume of the material with pores, in %) Representation), closed porosity (the ratio of the volume of all closed pores in the material with pores to the total volume, expressed in %), true porosity (expressed porosity and closed porosity, expressed in %), densely shaped fire resistant Product (shaped refractory products with a true porosity of less than 45%).
GB/T2997 has the principle of measurement: the mass of the sample is weighed, and then the volume is determined by hydrostatic weighing method, the apparent porosity and bulk density are calculated, or the true porosity is calculated according to the true density of the sample.
1.2.1.1 porosity
The pores in the refractory material are composed of pores in the raw material and pores between the formed particles. It can be roughly divided into three categories: 1) closed pores, which are closed in the product and not connected to the outside; 2) open pores, one section closed, the other section is connected to the outside, can be filled with fluid; 3) through the pores, through the two sides of the product It can pass through the fluid; for the sake of simplicity, the above three types of pores are usually combined into two types, namely, open pores (including through pores) and closed pores. Generally, the open pore volume accounts for an absolute majority of the total pore volume, and the closed pore volume is small. The closed pore volume is difficult to directly measure. Therefore, the porosity index of the product is usually expressed by the open porosity (also called the apparent porosity).
True porosity (total porosity) A = (V1 + V2) Χ 100% / V0, open porosity (potential porosity) B = V1 Χ 100% / V0 where: V0, V1, V2 represent total pore volume, open pores Volume and closed pore volume (CM3).
1.2.1.2 Water absorption rate
It is the ratio of the mass of all open pores in the product to the dry mass, expressed as a percentage. It is essentially a technical indicator reflecting the amount of open pores in the product. Because of its simple determination, it is directly used for identification in production. Raw material calcination quality. For well-sintered raw materials, the water absorption rate should be lower.
1.2.1.3 Bulk density
It represents the ratio of the mass of the dried product to its total volume, that is, the mass per unit volume (apparent volume) of the product, expressed in g/cm3.
Bulk density is also the main indicator to characterize the compactness of the product. When the density is high, the total area of ​​the external intrusion medium (liquid phase or gas phase) acting on the refractory material can be reduced, thereby increasing the service life, so densification is to improve the quality of the refractory material. An important route, usually in production, should control the bulk density of the raw material after calcination, the bulk density of the brick and the degree of sintering of the product.
1.2.1.3 True density
The GB/T5071 standard has two definitions: true density (the ratio of the mass of the dry material with pores to its true volume, expressed in g/cm3 or kg/m3), true volume (the volume of solid matter in the material with pores) ).
GB/T5071 standard measurement principle: the sample is crushed, ground, so that there is no closed pores as much as possible, and the dry mass and true volume are measured to measure the true density. The volume of the fines is determined using a pycnometer and a liquid of known density, and the temperature of the liquid used must be controlled or carefully measured.
True density refers to the mass per unit volume of refractory material excluding pores and can be expressed by the following formula.
d true=G/[ V0- (V1+V2)], the mass of G-dry sample in the formula, g; V0, V1, V2 - the total volume of the sample, the open pore volume, the closed pore volume, cm3 .
2. Thermal properties and electrical conductivity of refractory materials
2.1, thermal expansion
The GB/T7320 standard has two definitions: the linear expansion ratio (relative rate of change of the sample length between room temperature and test temperature, expressed in %), and the average linear expansion ratio (the temperature of the sample is increased by 1 ° C between room temperature and test temperature). The relative rate of change, in units of 10-6/°C), is shown in Table 4 for the average coefficient of thermal expansion of common refractory products.
Table 4 Average thermal expansion rate of refractory products (20-2000 ° C)
name | Clay brick | Mullite brick | Mullite corundum brick | Corundum brick | Semi-silicon brick | Silica brick | Magnesia brick |
Average coefficient of thermal expansion (10-6/°C) | 4.5-6.0 | 5.5-5.8 | 7.0-7.5 | 8.0-8.5 | 7.0-9.0 | 11.5-13.0 | 14.0-15.0 |
GB/T7320 standard measurement principle: the sample is heated to the specified test temperature at the specified heating rate, the change of the length of the sample increases with the temperature, and the linear expansion rate and the specified temperature of the sample as the temperature increases are calculated. The average linear expansion coefficient of the range and plot the expansion curve.
Thermal expansion of a refractory material refers to a physical property whose volume or length increases as the temperature increases.
2.2, thermal conductivity
YB/T4130 defines the thermal conductivity as: the amount of heat transferred per unit area of ​​the material along the direction of heat flow per unit temperature gradient per unit time. As shown in formula (1):
λ=q/(dT/dx)
Where: λ - thermal conductivity, in watts per meter Kelvin (W / (mK);
Q——heat flux per unit time, in watts per square meter (W/m);
dT/dx - temperature gradient in Kelvin per meter (K/m).
The principle of thermal conductivity measurement by YB/T4130 is: According to the basic principle of the stable heat conduction process of Fourier one-dimensional plate, the heat flow in the one-dimensional temperature field per unit time in steady state is measured by the longitudinal flow of the sample through the hot surface of the sample to the cold surface. The heat absorbed by the water flow. The heat is proportional to the thermal conductivity of the sample, the temperature difference between the hot and cold surfaces, and the area of ​​the heat absorption surface of the central calorimeter is inversely proportional to the thickness of the sample.
λ=Q.δ/(A.ΔT)
Where: λ - thermal conductivity, in watts per meter Kelvin (W / (mK);
Q——heat absorbed by water flow per unit time, in watts (W);
Δ——the thickness of the sample in meters (m);
A——the area of ​​the sample, the unit is square meters (m2);
ΔT——The difference between cold and hot surface temperature, the unit is Kelvin (K).
The heat absorbed by the water flow is proportional to the specific heat of the water, the quality of the water, and the temperature of the water:
Q=C.ω. Δt
Where: Q——heat absorbed by water flow per unit time, in watts (W);
C——specific heat of water, the unit is coke per gram Kelvin (J/(gK);
Ω—water flow in grams per second (g/s);
Δt - the water temperature rises, the unit is Kelvin (K).
Its physical meaning refers to the heat passing through the unit vertical area per unit time under a unit temperature gradient. Thermal conductivity is a physical property that characterizes the thermal conductivity of a refractory material and is equal to the heat flux density divided by the negative temperature gradient.
2.3, heat capacity
Any substance heats up when heated, but the heat required to heat up 1 °C for different substances of the same quality is different. It is usually expressed by the heat (kJ) required to heat 1 kg of material at normal pressure to raise the temperature by 1 ° C, and is called heat capacity (also called specific heat capacity).
2.4, temperature conductivity
Temperature conductivity is the temperature transfer rate when the object is heated. It determines the internal temperature gradient of the refractory material when it is quenched and hot. Temperature conductivity is expressed by the temperature coefficient (α):
α=λ/cÏ
In the formula:
Λ—the thermal conductivity of the refractory material, w/mk;
C——the isostatic heat capacity of the refractory material, kJ/kg.°C;
Ρ——volume density of refractory material, kg/m3.
Generally, the heat capacity of refractories is not much different, and their temperature conductivity mainly depends on the thermal conductivity and bulk density of the articles.
2.5, conductivity
Refractory materials (other than carbonaceous and graphite products) are poor conductors of electricity at room temperature. As the temperature increases, the electrical resistance decreases and the electrical conductivity increases. The increase is particularly remarkable at temperatures above 1000 ° C, such as heating to a molten state, which exhibits a large electrical conductivity.
3. Mechanical properties of refractory materials
The mechanical properties of refractory materials refer to the strength, elasticity and plastic properties of materials at different temperatures. The mechanical properties of the refractory are usually judged by indicators such as pressure resistance, flexural resistance, wear resistance and high temperature soft creep.
3.1, mechanical properties at room temperature
3.1.1, normal temperature compressive strength
It refers to the maximum pressure that a refractory material is subjected to per unit area at normal temperature. If it exceeds this value, the material is destroyed. If A is used to indicate the total area under pressure of the sample, and P is the ultimate pressure required to crush the sample, then:
Normal temperature compressive strength = P/A Pa
Generally, refractory materials rarely cause damage due to static load at normal temperature during use. However, the normal temperature compressive strength mainly indicates the sintering condition of the product, and the properties related to its structure, and the measuring method is simple, so it is a common inspection item for judging the quality of the product.
3.1.2, tensile, flexural and torsional strength
When used, in addition to compressive stress, refractory materials are also affected by tensile stress, bending stress and shear stress. The main factors affecting the tensile and flexural strength of refractory products are their microstructure. Fine particle structure is beneficial to these indicators. Improvement.
3.1.3, wear resistance
The wear resistance of a refractory material depends not only on the density and strength of the article, but also on the mineral composition of the article, the firmness of the structure and the combination of material particles. The normal temperature resistance is high, the porosity is low, the structure is dense and uniform, and the sintered product always has good wear resistance.
3.2, high temperature mechanical properties
3.2.1, high temperature compressive strength
High temperature compressive strength is the ultimate pressure that a material can withstand at a high temperature. As the temperature increases, the strength of most refractory products increases, with clay articles and high aluminum articles being particularly pronounced, reaching a maximum at 1000-1200 °C. This is because the viscosity of the melt formed at a high temperature is higher than the viscosity of the fragile glass phase at a low temperature. However, the bond between the particles is stronger. As the temperature continues to rise, the intensity drops dramatically. The high temperature compressive strength index of the refractory material can reflect the change of the bonding state of the product at high temperature.
3.2.2, high temperature flexural strength
High temperature flexural strength refers to the ultimate bending stress that a material can withstand at a high temperature. It characterizes the ability of a material to resist bending moments at elevated temperatures.
The high temperature bending strength is also called high temperature bending strength or high temperature breaking modulus. The maximum load that a rectangular parallelepiped specimen of a certain size at a high temperature can withstand when bent on a three-point bending device can be measured, and the bending strength can be calculated as follows:
R=3.Wl/2.b.d2
Where R is the flexural strength, Pa;
W——the maximum load applied when breaking, N;
l - the distance between two points, cm;
B——the width of the sample, cm;
d - the thickness of the sample, cm.
The high temperature strength of refractories is closely related to their actual use. Especially for evaluating the quality of alkaline direct bonding bricks, high temperature flexural strength is an important performance. If the high-temperature bending strength of the alkaline direct bonding brick is large, the shear stress generated by the temperature gradient is strong, so that the product is not easily peeled off during use. Products with high high-temperature flexural strength will also improve the impact and wear of the materials and enhance the slag resistance. Therefore, the high-temperature flexural strength is used as an indicator to characterize the strength of the product.
The high temperature flexural strength index of refractory materials mainly depends on the chemical mineral composition, structure and production process of the products.
3.2.3, high temperature creep
When the material is subjected to a constant load at a high temperature that is less than its limit, plastic deformation occurs, and the amount of deformation gradually increases with time, and even the material is destroyed. This phenomenon is called creep. Therefore, for materials at high temperatures, the strength cannot be considered in isolation, and both temperature and time factors and strength should be considered. For example, the damage of the hot blast stove bricks working at high temperatures for a long time is due to the softening of the brick body to produce plastic deformation, and the strength is significantly reduced or even destroyed. This creep phenomenon of the lattice bricks is the main cause of furnace damage.
The factors that affect high temperature creep are generally considered to be: 1) conditions of use such as temperature and load, time, atmosphere properties, etc.; 2) materials such as chemical composition and minerals; 3) microstructure. The high temperature creep curve of the material is divided into three stages, the first stage creep becomes deceleration creep (time is short); the second stage is uniform creep (minimum creep rate); the third stage is accelerated creep (creep rate) Rapid increase).
4. High-temperature use properties of refractory materials
4.1, refractoriness
The property that the refractoriness resists high temperature without being loaded and does not melt is called refractoriness. For refractory materials, the degree of refractoriness is different from the melting point. The melting point is the temperature at which the crystalline phase of the pure substance is in equilibrium with its liquid phase. However, the general refractory material is a heterogeneous solid mixture composed of various minerals. It is not a single-phase pure substance, so there is no melting point. The melting is carried out within a certain temperature range, that is, only a fixed initial melting temperature and A fixed melting end temperature. The liquid phase and the solid are present in this temperature range.
The refractoriness is a technical index. The measuring method is a truncated triangular cone made of test materials. The upper side is 2 mm long on each side, and the lower bottom is 8 mm long and 30 mm high. (The angle between the side and the vertical direction is 80) Equilateral triangle. When heated at a certain heating rate, it gradually deforms and bends due to its own weight. When it is bent until the apex is in contact with the chassis, it is the refractoriness of the sample.
The GB/T7322 standard has three definitions: refractoriness (high temperature resistance of refractory materials), standard temperature measuring cone (a truncated triangular pyramid with a certain shape and size, when it is installed and heated according to specified conditions) , can be bent in a known manner at a specified temperature called a standard temperature measuring cone), reference temperature (when the standard temperature measuring cone is placed on the frustum, when heated under the specified conditions at a specified heating rate, the cone The tip of the tip is bent to the temperature of the frustum and the principle of the refractoriness measurement. Reference temperature (bending temperature)
The GB/T7322 standard has a principle: the test cone of the refractory raw material or product is placed on the frustum together with the standard temperature measuring cone of known refractoriness, and the bending of the test cone and the standard temperature measuring cone is compared under the specified conditions. The down condition indicates the refractoriness of the test cone.
4.2, high temperature load deformation temperature
The YB/T370 standard has four definitions: load softening temperature (temperature at which a refractory product undergoes a constant pressure load under specified temperature rise conditions), maximum expansion temperature T0 (temperature at which the sample expands to a maximum value), x% Deformation temperature Tx (the temperature at which the sample compresses a certain percentage (x) of the original height from the expansion maximum), the fracture or rupture temperature Tb (the temperature at which the sample suddenly collapses or ruptures after T0); A principle (at a constant load and heating rate, the cylindrical sample is deformed by the combined action of the load and the high temperature, and the corresponding temperature of the specified degree of deformation is measured).
The load deformation index of the refractory material at high temperature indicates its resistance to the simultaneous action of high temperature and load, and also indicates that the refractory material exhibits a softening range of significant plastic deformation. The high-temperature load deformation temperature of the refractory material is determined by the pressure applied to the fixed sample, and the temperature is continuously increased. The temperature at which the sample is deformed and collapsed is called the high-temperature load deformation temperature.
The reason why the refractory load deformation curve is different depends mainly on the chemical mineral composition in the product, which depends on:
(1) The existence of crystal phase, crystal structure and traits, that is, whether the crystal forms a network skeleton or is dispersed in the liquid phase in an island shape, the former has a high deformation temperature, and the deformation temperature of the latter is mainly determined by the content and viscosity of the liquid phase. It can be seen that the microstructure has a significant influence on the load deformation temperature of the product.
(2) The number of crystal phases and liquid phases and the viscosity of the liquid phase at a certain temperature.
(3) The interaction between the crystal phase and the liquid phase, the interaction between the two changes the amount and nature of the liquid phase. In addition, the degree of compactness of the product also has a certain influence on the high temperature load deformation temperature.
4.3, high temperature volume stability
When the refractory material is used for a long period of time at a high temperature, the performance of the outer shape of the refractory material which remains stable without undergoing a change (shrinkage or expansion) is called high temperature volume temperature. It is an important indicator for assessing the quality of a product.
During the firing process, the physicochemical changes in the refractory material generally do not reach the equilibrium state at the firing temperature. When the product is subjected to high temperature for a long time, some physical and chemical changes will continue. On the other hand, in the actual firing process, for various reasons, there may be insufficiently baked products. When such products are used in a kiln and then subjected to high temperature, some firing changes continue, resulting in products. The volume changes - contraction or expansion, this irreversible volume change is called residual shrinkage or expansion, also known as reburning shrinkage or expansion. The magnitude of the change in reburning volume indicates the high temperature volume stability of the article.
The volume change during reburning can be expressed as a percentage of volume or as a percentage change in line:
LC=(L1-L0)Χ100/L0
Vc=(V1-V0)Χ100/V0
Where LC - sample reburning line change rate, %;
Vc——the rate of change in sample reburning volume, %;
L0, L1 - sequentially indicating the length of the sample before and after reburning, mm;
V 0, V 1 - sequentially represents the volume of the sample before and after reburning, cm3;
The results calculated by the above two formulas are positive values ​​indicating expansion, and negative values ​​indicating contraction. When the reburning volume changes very small, it can be considered that Vc = 3LC.
4.4, thermal shock stability
The property of a refractory material against a sharp change in temperature without damage is called thermal shock stability. It is well known that materials rise or contract with temperature rise and fall, and if the expansion or contraction is constrained and cannot develop freely, stress is generated inside the material. Such internal stress caused by thermal expansion or contraction of the material is called thermal stress. Thermal stress is generated not only under mechanical constraints, but also in temperature gradients in the homogeneous material, the difference in thermal expansion coefficients between the phases in the heterogeneous solid, and even the anisotropy of the thermal expansion coefficient in the single-phase polycrystal. Both are the source of thermal stress.
Thermal shock damage of refractory materials can be divided into two categories: one is instantaneous fracture, called thermal shock fracture; the other is cracking, spalling, then fragmentation and deterioration under the action of thermal shock cycle, and finally The overall damage is called thermal shock damage.
Conclusion: With the advancement of science and technology and the development of high-temperature industry, people's understanding of the mechanical properties of refractory materials will become deeper and deeper, and the requirements will be higher and higher. There are new mechanical properties to express the quality and durability of refractory materials. Sex. Therefore, there are more and more items that require the determination of the mechanical properties of refractories. For example, refractory fiber materials will require fiber strength determination and test methods; in the face of increasingly harsh conditions of use, the scouring resistance and wear resistance of refractories will become important properties of certain refractories, and thus it will be required to gradually establish science. Expression methods and test criteria.
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