- What is refractory material?
- Grouping of refractory materials
- Refractory, heat-insulating materials
- Properties of refractory materials
What is refractory material?
Every material, which has the melting point over 1500 °C, is called refractory material.
Analysing from another aspect, every material, which has the service temperature over 900 °C, is called refractory material. Materials used below this temperature are called heat-resisting materials.
According to a less accurate wording we call every material refractory, which permanently resists the varied requisitions in an industry furnace on high temperature without significant change. It suggests that refractory materials have to sustain not only high temperature, but loads coming from dead-weight, abrasive and chemical effects when built in structures.
Analysing from another aspect, every material, which has the service temperature over 900 °C, is called refractory material. Materials used below this temperature are called heat-resisting materials.
According to a less accurate wording we call every material refractory, which permanently resists the varied requisitions in an industry furnace on high temperature without significant change. It suggests that refractory materials have to sustain not only high temperature, but loads coming from dead-weight, abrasive and chemical effects when built in structures.
Grouping of refractory materials
Refractory materials can be grouped according to various aspects:
- Chemical composition
- Silica products (silica bricks and mortars)
- Aluminium-silica products (fire-bricks, high alumina bricks, concretes, and mortars)
- Basic products (basic bricks and basic masses)
- Carbonic products
- Silicon-carbide (SiC) products
- Zirconia products
- Special products (oxide and non-oxide, BN3)
- Refractoriness
- Refractory (SK18-35)
- High refractoriness (SK36-42)
- Very high refractoriness (over SK42)
- Porosity
- High density (0-16%)
- Dense (16-30%)
- Light (30-75%)
- Production technology
- Shaped refractory materials (refractory bricks and forms)
- Non-shaped refractory materials (refractory concretes, refractory mortars, refractory glues, etc.)
Refractory, heat-insulating materials
Refractory, heat-insulating materials have significantly less bulk density (~0,4-1,6 g / cm3) and higher porosity (~45-80 tf%) than traditional dense refractory materials, and accordingly the coefficient of thermal conductivity is much less.
Can be grouped according to production technology:
- with blowing additive
- with foaming
- with gas generation
- with volatile
material.
These are very often combined.
These are very often combined.
According to outer form we differentiate:
- Shaped (pressed, drawn, cast, vacuum formed)
- Non-shaped
Refractory, heat-insulating materials can be:
- Ceramic fibre, rock cotton insulating materials
- Calcium silicate materials
- Micro-porous insulating materials (WDS)
- Insulating bricks
Properties of refractory materials
Refractoriness
Refractoriness of refractory material is the temperature, at which the truncate cone shaped sample made of the refractory material curves under its own weight so, that the top of cone reaches its bottom.
Refractory materials are usually made of more components having varying melting points, so there is no definite melting point. The component having the lowest melting point melts on high temperature, and when the quantity and viscosity of the melt exceeds a certain critical value, it distorts under its own weight, and loses its structural stability.
Value of refractoriness is not identical with the service temperature.
Marking: SK (Seger cone) – this is more common
or PK (pyrometric cone)
Refractory materials are usually made of more components having varying melting points, so there is no definite melting point. The component having the lowest melting point melts on high temperature, and when the quantity and viscosity of the melt exceeds a certain critical value, it distorts under its own weight, and loses its structural stability.
Value of refractoriness is not identical with the service temperature.
Marking: SK (Seger cone) – this is more common
or PK (pyrometric cone)
Chemical composition
Usually we study the Al2O3, TiO2, SiO2, Fe2O3, CaO, MgO, Na2O+K2O, Cr2O3 content of refractory materials. Depending on the type of refractory material we study other components, too; for example SiC, BaO.
Unit: %
Unit: %
Bulk density
Bulk density is the ratio of the weight of refractory material and the volume measured with pores.
Unit: g/cm3, kg/dm3, t/m3
Unit: g/cm3, kg/dm3, t/m3
Hydration
The hydrating ability of refractory materials shows that during impregnation of open pores how much water is hydrated compared to the dry mass of the sample.
Unit: %
Unit: %
Porosity
Refractory materials are usually not gas-compact, as these contain smaller, larger closed or open pores.
- Open or apparent porosity is the ratio of the volume of open pores to the total volume of the sample, including the volume of pores.
- Complete porosity is the ratio of total volume of pores to total volume of the sample, including the volume of pores.
Unit: %
- Open or apparent porosity is the ratio of the volume of open pores to the total volume of the sample, including the volume of pores.
- Complete porosity is the ratio of total volume of pores to total volume of the sample, including the volume of pores.
Unit: %
Strength
In the most cases the strength of refractory material to be applied has quite an important role, as sometimes this is the most important figure.
Usually the compression strength is studied, but to measure the bending- or tearing strength could be necessary, where it is needed.
The compression strength is the resistance of refractory material to compressive stress.
It could be cold compression strength, or hot compression strength.
Unit: kp/cm2; N/mm2; Mpa
Softening under load
Usually the compression strength is studied, but to measure the bending- or tearing strength could be necessary, where it is needed.
The compression strength is the resistance of refractory material to compressive stress.
It could be cold compression strength, or hot compression strength.
Unit: kp/cm2; N/mm2; Mpa
Softening under load
The built-in refractory materials are exposed to various pressures during working. The weight of lining gives compressive stress, but shear, and bending stress may occur, too. So, the softening under load is an important data considering the application.
Softening under load is typical of behaviour of refractory material during usage, and gives the highest temperature of usage.
Unit: °C
Softening under load is typical of behaviour of refractory material during usage, and gives the highest temperature of usage.
Unit: °C
Secondary shrinkage, growth
Secondary volume changes of partly burnt or unburnt refractory materials occur on high temperature.
Its numeric value is the percentage of change related to pre-heating, original length, and volume.
Maximal service temperature is the temperature, at which the value of linear secondary shrinkage reaches 1,5%.
Unit: %
Its numeric value is the percentage of change related to pre-heating, original length, and volume.
Maximal service temperature is the temperature, at which the value of linear secondary shrinkage reaches 1,5%.
Unit: %
Thermal expansion
During a furnace building it is important to know the thermal expansion of the refractory materials used to calculate the dilation (expansion) joints.
Usually the linear thermal expansion is determined in the percentage of original length.
Its value varies depending on the temperature, so the thermal expansion factor is referred to a certain temperature, or range of temperature.
Unit: %
Besides, there is cubical expansion as well.
Usually the linear thermal expansion is determined in the percentage of original length.
Its value varies depending on the temperature, so the thermal expansion factor is referred to a certain temperature, or range of temperature.
Unit: %
Besides, there is cubical expansion as well.
Resistance against temperature change
(thermal shock resistance, chilling, abschreck)
(thermal shock resistance, chilling, abschreck)
The effect of temperature changes is that crackles may occur on the refractory bricks. The reason for this is the significant quantity of glassy phase in the bricks, inside pressure, phase changes occurring during burning.
We express the resistance against temperature change with a heat variation (chilling) number, which shows, that how many times the studied refractory material stood the heating and cooling, until the 50% of area exposed to heat variation started cracking.
Unit: number of cycles
We express the resistance against temperature change with a heat variation (chilling) number, which shows, that how many times the studied refractory material stood the heating and cooling, until the 50% of area exposed to heat variation started cracking.
Unit: number of cycles
Thermal conductivity
Refractory materials are not insulating perfectly, and let the some heat through. We call this property thermal conductivity. It depends also on the temperature of the refractory material.
Unit: W/mK
Unit: W/mK
Relative density
Relative density is the ratio of weight and volume of powdered, dried, pore-free refractory material.
Unit: kg/dm3, g/cm3
Heat capacity
Unit: kg/dm3, g/cm3
Heat capacity
Heat capacity is the heat quantity, which increases the temperature of a unit of refractory material by 1°C. Its numeric value depends on the temperature.
Unit: kJ/kg0C
Unit: kJ/kg0C
Gas permeability
Gas permeability is the property of porous refractory material, which lets air, or gas through as an effect of pressure difference. It shows the kind of porosity of material (pore dimension, pore spreading).
The gas permeability coefficient is perm, marked: Pm
The material has 1 Pm gas permeability coefficient, if during 1 second 1 cm3, 1 poise viscosity gas flows through 1 cm2 surface of the 1 cm long test sample as an effect of 1 dm/cm2 pressure difference.
The gas permeability decreases while the temperature increases, as the viscosity of gases is increasing.
The gas permeability coefficient is perm, marked: Pm
The material has 1 Pm gas permeability coefficient, if during 1 second 1 cm3, 1 poise viscosity gas flows through 1 cm2 surface of the 1 cm long test sample as an effect of 1 dm/cm2 pressure difference.
The gas permeability decreases while the temperature increases, as the viscosity of gases is increasing.
Slagging
The slagging of refractory materials is very important especially for furnace linings contacting the melts.
The kinds of slag effect are so various, and the composition, viscosity and circulation circumstances of the slag are so different, that reproducing among laboratory conditions is not, or quite limitedly possible.
The most common test method is the so-called jar-method: we make a 50 mm hole into the test sample, we place the corrosion agent (glass, slag, etc.) in. Then we heat the test sample in a furnace on given temperature. After cooling down, we cut the test sample and we determine the extent of scouring and infiltration.
The kinds of slag effect are so various, and the composition, viscosity and circulation circumstances of the slag are so different, that reproducing among laboratory conditions is not, or quite limitedly possible.
The most common test method is the so-called jar-method: we make a 50 mm hole into the test sample, we place the corrosion agent (glass, slag, etc.) in. Then we heat the test sample in a furnace on given temperature. After cooling down, we cut the test sample and we determine the extent of scouring and infiltration.
Solubility (in acids, alkalis)
Special kind of refractory material is the acid-proof material.
Acid-proof refractory materials resist the chemical effects of acid gases, acid fluid and alkalis. The mix of concentrated vitriolic acid and concentrated nitric acid is used for testing the acid-proof materials.
The numeric value of solubility is determined by the per cent weight reduction occurring after solvent effect in proper circumstances.
Acid-proof refractory materials resist the chemical effects of acid gases, acid fluid and alkalis. The mix of concentrated vitriolic acid and concentrated nitric acid is used for testing the acid-proof materials.
The numeric value of solubility is determined by the per cent weight reduction occurring after solvent effect in proper circumstances.
Electric resistance and conductivity
Refractory materials are not conductive on normal temperature, so their resistance is quite large. However, if the temperature is increased, the resistance will decrease and some refractory materials become semiconductor over 1000 °C. Conductivity is significantly influenced by contaminations, especially the iron-contaminations are damaging.
Deformation under permanent load (creep)
Refractory materials, which get significant load during use besides the temperature, may specially deform.
The reason is that in refractory materials melt-phase is forming on 1000-1100 °C, and its viscosity and quantity, as well as the quantity, dimension, form of grains and pores determines the rheological (flowing, deformation) properties of the material.
The creep of built-in refractory materials is generated by slag- or glass melt, or flue dust. The properties of built-in materials are changing as an effect of melt or flue dust. Materials may show 10-20% deformation under quite small load compared to the original size. The creep is going quite slowly, on especially low temperature. This process doesn’t stop, and continues upon permanent load by almost constant speed.
The reason is that in refractory materials melt-phase is forming on 1000-1100 °C, and its viscosity and quantity, as well as the quantity, dimension, form of grains and pores determines the rheological (flowing, deformation) properties of the material.
The creep of built-in refractory materials is generated by slag- or glass melt, or flue dust. The properties of built-in materials are changing as an effect of melt or flue dust. Materials may show 10-20% deformation under quite small load compared to the original size. The creep is going quite slowly, on especially low temperature. This process doesn’t stop, and continues upon permanent load by almost constant speed.
Abrasion resistance
The abrasion of built-in refractory material can happen variously: dissolution in slag-, or iron melt, cracking, slacking by gas or other agents and mechanical abrasion. The latest occurs when the lining on quite low temperature (600-1200 °C) is exposed to some abrasive effect (e.g.: shaft furnace).
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