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, 3rd 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 Refractory Materials
• Refractory 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(Al2O3), Zirconia(ZrO2)
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 (Al2O3)
• Silica (SiO2)
• Zirconia (ZrO2)
• 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 Tm. The Tm 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) Due 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=kσ(1-μ)/αE.
– 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 Refractory Materials
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.
• RUL 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/cm2 for
dense refractories and 1 kg/cm2 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 cones
• Orton cones
• Staffordshire
Cones

9. Basic 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 Grade 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.
• So 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. The 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 (B4C)
• Silicon Carbide: mainly produced from crystalline silicon carbide or
carborundum.
Graded SiC particles +
suitable bonding agent
DriedShaped 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-
I. Possesses high degree of anisotropy.
II. High electrical and thermal conductivity.
III.Excellent resistant to acids(except oxidizing acids), alkalis, and solvent.
IV. Possesses high mechanical strength at higher service temperature.
• Uses:
I. Due to its high refractoriness it is used for high temperature applications such
as rocket nozzles and nozzle inserts
II. Also used for making crucibles for steel industries.
III.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*107 to
1.4*108N/m2
• 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%)Al2O3-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 (Vol. 2) by
MANAS CHANDA

19. Thank You