Introduction to refractory materials

Some more informations about refractory materials and also have some descriptions about high grade refractory materials.

1. Introduction to Refractory Materials Rahul Mitra M.Sc , 3 rd Semester, Roll No-13 Department of Materials Science

2. Contents What are refractory materials? Differences between ceramics and refractories Examples Properties 2 special tests for refractory materials Basic functions of refractories Types of refractories High grade refractories

3. What are R efractory M aterials R efractory materials are non-metals of construction, capable of withstanding high temperatures without appreciable deformation under service conditions. ASTM C71 defines refractories as "...non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above 1,000 °F (811 K; 538 °C).“ E.g.- Alumina(Al 2 O 3 ), Zirconia(ZrO 2 ) The property to withstand high temperature without appreciable deformation under various service condition is termed as “ Refractoriness ”

4. How it is different from Ceramics Ceramics can be covalent network and/or ionic bonded. But refractory materials contains only covalent bond. Ceramics materials consist of both metallic and non-metallic components . But refractories are mainly inorganic oxides.

5. Examples Alumina (Al 2 O 3 ) Silica (SiO 2 ) Zirconia (ZrO 2 ) Silicon carbide ( SiC ) Crome magnesite Magnesia ( MgO ) Fire clay refractories Carbide and nitride Hafnium carbide Tantalum hafnium carbide (have highest melting point among all, 4215 °C)

6. Properties Melting point: Possesses very high T m . The T m of refractory materials is usually > 1300°C. Size and Dimensional Stability: It refers to the resistance of a material to any volume change which may occur on prolonged exposure to temperature. Dimensional changes can be either reversible or irreversible. Reversible: Directly related to coefficient of thermal expansion. Irreversible; Due to phase transformation, resulting either in contraction or expansion. Porosity: Volume of open pores as % of total refractory volume. Low porosity = less penetration of molten material and lesser insulation property. Bulk density: Amount of refractory material within a volume (kg/m3) High bulk density = high volume stability, high heat capacity and high resistance to abrasion and slag penetration. Strength: Refractories are hard and brittle. Strengths of refractories are usually reported in terms of the three-point bend strength or the flexural strength, frequently called the modulus of rupture (MOR). They possesses high MOR. Cold crushing strength (CCS): Ability to resist failure under compressive load. Thermal Expansion: Thermal expansion is very low in refractory materials.

7. Contd. Creep at high temperature: Deformation of refractory material under stress at given time and temperature. High creep resistant. Spalling : It refers to cracking, splitting, or flaking of the material when subjected to sudden temperature change. There may have some causes like - i) D ue to uneven heating or cooling. ii) Compression due to volume expansion of a whole structure of refractories from a rise of temperature, sufficient to cause shear failures. iii) Differences in coefficient of thermal expansion between surface and body, brought about by slag penetration, gas penetration etc. Thermal shock resistance: Refractory materials are greater resistance to thermal shock. It can be specified by a thermal stress resistance factor K , calculated by- K = k σ (1- μ )/ α E. k= coefficient of thermal conductivity, σ = breaking strength, α = thermal expansion coefficient, μ = Poisson's ratio. Thermal and electrical conductivity: thermal conductivity and electrical conductivity is very low, so can be used for thermal insulation purpose. Slag and metal resistance: It is essential that the refractory materials should not react at high service temperature with the substances that will be in contact with them. It depends on the nature of the refractory material.

8. Two special tests for R efractory M aterials Pyrometric cone equivalent Temperature at which a refractory will deform under its own weight is known as its softening temperature which is indicated by PCE . These cones are pyramidal in shape and have a height of 38 mm of a triangular base and 19 mm long sides. They are allowed to heat under 10°C per min as a result of they undergo fusion at a definite temperature. This temperature at which the fusion or softening of the test cones occur, is shown by its apex touching the base. R UL evaluates the softening behavior of fired refractory bricks at rising temperature and constant load conditions. The refractoriness under load is usually tested under a load of 2 kg/cm 2 for dense refractories and 1 kg/cm 2 for porous insulating materials. The temperature at which the specimen starts to deform and eventually fails, usually due to shearing, is measured. Refractoriness under load(RUL) Other names: Seger c ones Orton cones Staffordshire Cones

9. B asic functions of Refractories Refractories perform four basic functions 1. They act as a thermal barrier between a hot medium (e.g., flue gases, liquid metal, molten slags, and molten salts) and the wall of the containing vessel. 2. They insure a strong physical protection, preventing the erosion of walls by the circulating hot medium. 3. They represent a chemical protective barrier against corrosion. 4. They act as thermal insulation, insuring heat retention.

10. Types of Refractories On the basis of chemical nature: 1)Acid refractories 2)Basic refractories 3)Neutral refractories Based on fusion temperature: Normal refractory: fusion temperature of 1580 ~ 1780 °C (e.g. Fire clay) High refractory: fusion temperature of 1780 ~ 2000 °C (e.g. Chromite) Super refractory: fusion temperature of > 2000 °C (e.g. Zirconia) High Grade Refractory Based on method of manufacture Dry press process Fused cast Hand molded Formed (normal, fired or chemically bonded) Un-formed (monolithic-plastic, ramming and gunning mass, castables, mortars, dry vibrating cements.) Un-formed dry refractories.

11. High G rade Refractories While dealing with higher temperature (>1500 °C) , it is seen that the traditional ceramic bonded refractory materials lack strength due to their glassy matrix. This glassy matrix also reduces the RUL property of those materials . S o in order to obtain high refractoriness it is necessary to use very pure refractory ingredients and to eliminate the formation of glassy bond. The later condition is achieved in high grade refractories by adopting special methods of bonding the refractory ingredient. E.g. – pure-oxide refractories, carbide refractories, carbon and graphite refractories, cermet.

12. Pure-oxide Refractories To meet these demands, a group of special refractories, termed the pure oxides, has been developed. T he pure oxide refractories have superior qualities, due in great part to their freedom from fluxes. They are mono-crystalline and self-bonded, compared with the conventional glass-crystal-bonded refractories of the fire-clay or super refractory types. The refractory oxides of interest, in order of increasing cost per unit volume, are alumina, magnesia, zirconia, beryllia, and thoria. The number of oxides and also certain of their binary combinations which can be used at temperatures above 1900-2000 °C is sufficiently large. They can be produced by traditional slip casting method or by extrusion and pressing from suitable mixture of granular Grog materials and the fine particles of the same material. Grog materials and fine particles of oxide+ binder( PVA, PEG, Starch) Firing (sintering occurs causing surface reactions between individual particles) Finished shape Processing by extrusion technique The refractoriness of such a product is very high, approaching the melting point of the pure oxide.

13. Carbide Refractories 2 major carbides are used- 1)Silicon carbide (SiC) 2) Boron carbide (B 4 C) Silicon Carbide: mainly produced from crystalline silicon carbide or carborundum . Graded SiC particles + suitable bonding agent Dried S haped Fired Finished product Four major types of bonding have been used- Self-bonding, Bonding with refractory clay, silicon nitride and silicon. Self bonded SiC have superior RUL, high density, abrasion resistance, high chemical resistance. However it slowly oxidize to silica when heated in air at about 1000°C. Boron Carbide: mainly produced from high purity boron carbide powders. The method used is either hot pressing followed by firing to produce a self bonded material, or mixing with sodium silicate, boric oxide, and other silicates as a bonding agent and then firing to produce a ceramic bond.

14. Carbon and Graphite Refractories Carbon has by far the highest melting point (>3500 °C) of all elements. Carbon based refractories behave differently than the typical ceramic refractories, primarily because carbon based refractories are conductive rather than insulating. All carbon based refractory lining systems perform as a ‘conductive cooling system’ as opposed to a classic definition of a refractory lining that is typically an ‘insulating system’. Consequently, proper cooling must always be utilized with any carbon based refractory lining system to assist in maintaining refractory temperatures that are below the critical chemical attack temperature for mechanisms such as oxidation, alkali, CO degradation, or dissolution of the carbon by liquid metal. Crushed cokes in suitable size fractions+ Pitch as binder Setting Hardening Firing 1000°C 1) possesses high refractoriness under load. 2) high thermal conductivity. 3)high resistant to thermal shock.

15. Contd. Graphite Refractories:- Amorphous carbon is converted to crystalline graphite on prolonged heating at about 2200 °C- 2500 °C . Graphite has no melting point. It sublimes at a temperature of 4200 °C . Natural graphite is shaped by usual ceramic techniques using plastic fireclay or ball clay as binder. Properties- Possesses high degree of anisotropy. High electrical and thermal conductivity. Excellent resistant to acids(except oxidizing acids), alkalis, and solvent. Possesses high mechanical strength at higher service temperature. Uses: Due to its high refractoriness it is used for high temperature applications such as rocket nozzles and nozzle inserts Also used for making crucibles for steel industries. Specially used to make self-lubricating piston rings and molds for machine tool casting.

16. Contd. Pyrolytic graphite: Prepared by vapour deposition (by passing methane over a substrate or mandrel at 1000-3000 °C in a vacuum furnace). It is almost theoretically dense graphite (Sp. Gr.- 2.25) with highly preferred orientation. Possesses higher strength to weight ratio than commercial graphite. Its strength at 2200 °C may be as high as 4.1*10 7 to 1.4*10 8 N/m 2 Use: Pyrolytic graphite is used for making rocket nozzles.

17. Cermets A cermet is a composite material composed of ceramic ( cer ) and metallic (met) materials . A cermet is ideally designed to have the optimal properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation. The metal is used as a binder for an oxide, boride, or carbide. Generally, the metallic elements used are nickel, molybdenum, and cobalt. Depending on the physical structure of the material, cermets can also be metal matrix composites, but cermets are usually less than 20% metal by volume . Usually made by powder metallurgy techniques while some of them are also made by impregnating a porous ceramic with a metallic binder. E.g.- WC-(6-20%)Co , (30-70%)Al 2 O 3 -Cr etc. composites. Uses: Carbide based cermets are used gauge blocks, hot extrusion dies, gas-turbine nozzles. Oxide based cermets are used for high speed cutting tools.

18. References http:// ispatguru.com/ Wikipedia Refractories by : Dr. Hussein Alaa INTRODUCTION TO CERAMICS,GLASS AND REFRACTORIES by DR KASSIM AL-JOUBORY Science of Engineering Materials ( V ol . 2) by MANAS CHANDA

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