LECTURE - 07.04.2014l - Refractories.pdf

Refractory metals

1. Lecture- 6: Refractories
Few major references are included here. Other references may be found in individual
chapters.
Materials Science and Engineering, V. Raghavan,
Fifth Edition, Prentice Hall of India Pvt. Ltd., New Delhi, 2004.
Materials Science and Engineering: An Introduction, William D. Callister
John Wiley & Sons, 2010.
Materials: Engineering, Science, Processing and Design, M.F. Ashby, H.R.
Shercliff and D. Cebon, Butterworth Heinemann, Oxford 2007.
 Materials Selection in Mechanical Design, 3
rd
edition, M.F. Ashby, Butterworth
Heinemann, Oxford, 2006.
REFERENCES
Materials engineer make things. They make them out of materials, using
processes.

2. What are the common types of refractory materials?
Materials selection and design
Some important refractory materials: Carbides, Nitrides, Aluminides, Silicides etc.
Recent developments in Refractory materials

What will you learn in this lecture?

5. Previous Class Problem ?

10. CERAMICS
Ceramic is an inorganic, nonmetallic solid prepared by the action of heat and
subsequent cooling. Example: clay, Mixed oxides like alumina, zirconia etc.

Ceramic materials are rather promising. Due to their chemical passivity, ceramics
are stable in corrosive media and have long service life under high-temperature
conditions.

Properties

1.Ceramics are Strong solid inert materials.

2.They withstand chemical erosion due to Acid and Caustic.

3.These can withstand high temperature of about 1000 °C to1600°C.


Types:

1.Whitewares:
clays
2.Refaracotories: Have high Silicon or Aluminium oxide content.
3.Abrasives:
Natural garnet, diamond, Silicon carbide.

11. Applications of CERAMIC MATERIALS
WHITE WARES are used in including
tableware, wall tiles, pottery products and
sanitary ware
REFRACTORIES are used in making fire bricks silica crucible and ovens. Due to there low thermal conductivity and high strength to temperature
Sandpaper is a very common coated abrasive.

12. GLASSES
Glass
is an amorphous (non- crystalline) solid material and
typically brittle and optically transparent.

Silica(SiO
2) is a common fundamental constitute of glass.
Synthesis of glass
1.Mixture of soda ash ,limestone, sand
and broken glass in dry condition.
2.
Send to furnace and heat to 1600°C
3.Molding.
4.Annealing.
5.Shaping.
6.Marketing .

13. Application of GLASSES
Borosilicate glasses formerly called Pyrex are
often used laboratory reagents due to their
resistance to chemical corrosion and heat
fancy glass started to
become significant branches
of the decorative arts.

14. Refractories
In general usage, products applied at temperatures > 600 °C are referred
to as refractories.
A refractory material is one that retains its strength at high temperatures.
ASTM C71 defines refractories as:
“Non-metallic materials having those chemical and physical properties
that made them applicable for structures, or as components of systems,
that are exposed to environments above 1000
°F (800K, 500 °C)”
Refractory materials must be chemically and physically stable at high
temperatures.
Depending on the operating environment, they need to be resistant to thermal
shock, be chemically inert, and/or have specific ranges of thermal conductivity
and of the coefficient of thermal expansion.
The oxides of aluminum (alumina), silicon (silica) and magnesium (magnesia)
are the most important materials used in the manufacturing of refractories

15. •Linings put in equipment to withstand high temperatures,
•Many devices are made of metal, but the temperatures inside the vessels
would melt the metal
•They must be lined with something that takes a lot of heat without breaking
up or melting.
•Base brick ingredient in clay, Flint clays or Kaolin with fusion temp over 425
CLow alkali content helps with temp resistance

16. Types of Refractories
• Refractories can be classified on the basis of chemical
composition, method of manufacture, physical form or according
to their applications.
• There are four basic types of refractories:
•Acidic Refractories
•Basic Refractories
•Neutral Refractories
•Super Refractories

17. Acidic Refractory
•A refractory that is composed mainly of silica and reacts at high
temperatures with bases such as lime, alkalies, and basic oxides.
•These are used in areas where slag and atmosphere are acidic.
They are stable to acids but attacked by alkalis.
•The main components of these refractories are silica along with
alumina (Al
2O
3).
•The steel industries are the largest consumer of acidic refractories.
•These are used on areas where slags and atmosphere are basic,
stable to alkaline materials but reacts with acids.
•The main raw materials is magnesia (MgO) is a very common
example. Other examples includes dolomite (MgCO
3 + CaCO
3) and
chrome-magnesia (Cr
2O
3 + MgO).
Basic Refractories

18. Neutral Refractories
•These are used in areas where slags and atmosphere are either
acidic or basic and are chemically stable to both acids and bases.
The common examples of these materials are alumina (Al
2O
3),
chrome ( Cr
2O
3) and carbon.
•Normally we have to use acidic and basic refractories combined but
we use neutral bricks to avoid the reaction.
•The neutral bricks are made of graphite and chromites.

19. Super Refractories
•These refractories are manufactured for exceptional high temperature
application higher than 1800
o
C.
•For example zirconia (ZrO
2) its melting point 2340
o
C – 2550
o
C, Thoria
(ThO
2), its melting point is 3200
o
C.

20. Alumina Refractories
•Alumina refractories hold shape at higher pressure and take more heat.
–Mix incorporates
•Bauxite
•Kyanite
•Sillimanite
•Andalusite
–Raw materials are calcined at high temperature
–Action forms mixtures of silica and mullite glass
–Glass is ground and dry pressed into form of bricks

21. Bauxite
gibbsite, Al(OH)3
boehmite, AlO(OH)

Really a combination of oxides and
Hydroxides

S.G. 2-2.5
Hardness 1-3

22. Kyanite
Al
2SiO
5
S.G. 3.61
Hardness 4-7
Streak - White

23. Sillimanite
Same composition-
Different crystal form than
Kyanite

24. Andalusite
Same composition as
Kyanite and Sillimanite
Different crystal structure.

25. Magnesia Refractories
•They are not easily eaten by molten slags
•Raw materials
–Magnesite
–Brucite
–Dolomite
–Olivine
–Serpentine (with extra step)
•Calcine them to make MgO magnesia
–Can be used as a chemical but
–Fired to make a very resistant refractory

26. Magnesite
MgCO
3
S.G. 3-3.3
Hardness 3.5-4

27. Brucite
Mg(OH)
2
S.G. 2.39
Hardness 2.5-3

28. Dolomite
CaMg(CO
3)
2
S.G. 2.84
Hardness 3.5-4

Limestone with a partial replacement of
Calcium with magnesium.

29. Olivine
Peridot
(gem quality
Olivine)
Forsterite = Mg
2SiO
4
Olivine (Chrysolite) = (Mg,Fe)
2SiO
4
Fayalite = Fe
2SiO
4
Olivine is a solid solution with variable amounts
Of iron and magnesium
S.G. 3.2 – 4.2
Hardness 6.5-7

30. Serpentine
((Mg, Fe)
3Si
2O
5(OH)
4)
Hydrous magnesium iron silicate
One mineral variation is Asbestos
S.G. 2.59
Hardness 2.5-3

31. Refractory Blades
Diamond Refractory Blades are designed for high-performance chopping on
masonry saws for even the toughest refractory brick and block.
From difficult, dense refractories such as solid alumina and zircon to the less
difficult to cut materials such as basic brick.

34. Other Refractories
•Zircon based
–Fine ground pressed into bricks often used in borosilicate glass
manufacture
•Zirconia
–Ultra high melting point 2710 C
•Artificial minerals – Silicon Carbide (also an abrasive)
–Made from coke (a carbon source) and sand (a silicon source) with
sawdust and salt
–Cook the mix at about 2200 C
–Refractory very good at resisting oxidation

35. Intermetallics

36. 36

38. Classification

40. Aluminides

41. Nitrides - Boron Nitride (BN)
•Giant covalent molecule.
•Exist in two forms:
•H-boron nitride,
•C-boron nitride.

The hexagonal form corresponding to graphite is the most stable and softest
among BN polymorphs, and is therefore used as a lubricant and an additive to
cosmetic products. The cubic (sphalerite structure) variety analogous
to diamond is called c-BN.

42. H-Boron Nitride
•Often referred to as “white graphite”;
•lubricious material;
•semi-conductor;
•very high thermal conductivity;
•good thermal shock resistance;
•chemically unreactive but oxidises at 850° C .

C-Boron Nitride
•Higher strength;
•wear resistant;
•high thermal conductivity;
•chemically inert;
•can be used as a catalyst.

43. Applications
•It is a good lubricant at both low and high temperatures;
•cosmetic products;
•widely used as an abrasive.

44. •Almost all metals in periodic table react with Si to silicides with general
formula ‘MxSiy’.
•Silicides have ‘metal-like’ properties.

•Early use of silicides to make contact for Schottky diodes.
•Interconnects made from WSi2 silicides had reduced line resistance.
•Gate length, junction depth and contact surface are important parameters for
CMOS transistors.
•Miniaturization below 0.5 µm would require technological changes in the
transistor itself.
•Self-Aligned silicides (SALICIDE) play an important role for contact
formation
.
Silicides

46. High-power electric molybdenum disilicide (MoSi
2) heating elements
for element temperatures up to 1850° C (3360° F). Kanthal Super
MoSi
2 heating elements are available as straight or bent elements in
a wide range of shapes and sizes, all characterized by long life and
consistent performance.

48. Carbides - Silicon carbide
48
1893: Henri Moissan discovered silicon carbide

The mineral was named moissanite in his honor

Synthetic silicon carbide: Acheson graphite electric resistance furnace with high
temperature 1700 - 2500° C

49. Silicon carbide is made today in much the same way as it was when invented in 1891:

High purity quartz is mixed with a high quality coke or anthracite in large electric resistance
furnaces at temperatures of over 2 000°C according to the following reaction:

SiO
2 +3C=SiC +2CO













Production Process
The process is an endothermic reaction requiring between
8 000 – 10 000kWh per tonne of product.

The product is removed from the furnace when cool and
separated into different grades. Sorting is aimed at separating
high-grade crystalline silicon carbide from metallurgical grade
silicon carbide.

Lower grades of silicon carbide are recycled in the production
system. The sorted silicon carbide is then crushed and
screened to a saleable product.

15% 85%
Furnace
Silica Coke Coal
Crushing
&
Screening
Met Grade SiC Crystalline Grade SiC

50. Examples
50
SiC ceramic.
SiC tubes.
SiC ring.

51. Properties
Silicon Carbide is also called carborundum, including black and green silicon carbide both with a
shape of hex crystal. The black silicon carbide is classified into coke-made and coal-made black
silicon carbide depending on different raw materials. The material is extremely hard and sharp,
with excellent chemical properties. The hardness is between diamond and fused alumina, but
the mechanism hardness is higher than fused alumina. The micro hardness is in the range of
2840-3320kg/mm
².

Silicon Carbide is sharp but fragile with good heat-resistance, heat- conductibility, can be antacid
and antalkali, lower dilatability and can be aseismatic.

SiC has:
high hardness
high thermal consistency
very good resistance at high temperatures
low thermal expansion
electrical conductivity
is a semiconductor
non linear electrical resistance
Si and C as alloying additive

52. Properties cont.
Appearance:
When removed from the furnace, the Silicon Carbide is a mass of interlocking iridescent crystals,
the crystals themselves being largely twinned and often coalescent. Their iridescence is due to a
thin surface layer of silica resulting from superficial oxidation of the carbide. Washing in
hydrochloric acid will remove this layer. The crystals vary in colour from very pale green to black
depending on the amount of included impurities.

Hardness:
Silicon Carbide was the first material entering the range of hardness between corundum and
diamond. It is given the relative position of 9.5 on Mohs scale and diamond at 10.

Properties:
SiC is quite stable chemically. It is stable to acids, not reacting with fuming nitric acid, nor with
boiling sulphuric hydrochloric or hydrofluoric acid. Sodium silicate attacks it above 1300ºC,
calcium and magnesium oxides attack above 1000ºC. Copper oxide reacts at 800ºC to form the
metal silicide. It oxidizes slowly in air above 1000ºC.

Silicon Carbide dissociates in molten iron and the silicon reacts with the metal oxides in the melt.
This reaction is of use in the metallurgy of iron and steel.

53. Silicon carbide forms natural crystals, which are very hard, very abrasive and dissociate or sublimate
at high temperatures. It is for these reasons that silicon carbide is used in the following applications:

Abrasive Industry
With a good hardness, silicon carbide is the first choice raw material for manufacturing abrasive pipe,
impeller, pumping chamber etc. Its abrasiveness is 5-20 times than that of cast iron and rubber.

Macrogrits are used to make items like sandpaper, grinding wheels, disks, wire saws and a number of
other abrasive products.




Applications for SiC

54. Refractory Industry
Because of its high temperature and abrasive resistance it is used to make
refractories for furnaces and other high temperature components. The ceramic
industry is one of the largest users of SiC refractory products.

Metallurgical Industry

Silicon carbide is used for the deoxidation and recarburation of cast iron
and steel in foundries. Metallurgical grade Silicon Carbide grain is a unique material for use in the
production of iron and steel. It is used in the foundry industry for the electric furnace production of
gray, ductile, and malleable iron. It is an excellent source of carbon and silicon, promotes nucleation
and renders the iron more responsive to inoculation, and deoxidizes the iron, which enhances furnace
lining life.

Silicon carbide can also be used to enhance efficiency in ferroalloy production using the patented
Maxred process developed by Sublime Technologies.

Other Industries
Silicon carbide is used in several specific electro-technical applications such as autovalves
andresistance. It is also used in traditional mechanical fields such as non-slip floors.
Silicon carbide is also being used for a number of new technologies and advanced
materials such as special ceramics that are used to line the space shuttle to protect it
during its re-entry into the earth’s environment. It is also used to make revolutionary new materials for
Components such as brake disks with an indefinite lifespan.

55. Applications
55
• Schottky diodes, tryristors, MOSFETs, etc.
• Characteristics:
- Ultrafast.
- High Thermal properties.
- High Voltage work condition.
- Better system efficiency.




First Commercial Silicon Carbide Power MOSFET

56. Pelletising:
Silicon Carbide can be used in the production of Ferrochrome.

Currently Silicon Carbide is added directly into Ferrochrome furnaces but new research by the company is
suggesting that by adding the product into the Chrome ore pellets which are used to charge Ferrochrome
furnaces, significant improvements in Ferrochrome production recoveries and efficiencies may be possible.

To this end improved furnace recovery techniques are being trialed in conjunction with local major steel
producers, which are showing encouraging results.

New Developments

57. Tungsten Carbide
•Its the hardest known jewelery material
•Its affordable compared to gold
•Its maintenance free
•The above make it an awesome wedding
band material

58. As you can see diamonds are harder and it is diamond jewelry making tools that have
to be used to cut, style and craft the tungsten carbide rings.
Normal jeweler's tools won’t cut it and would most likely break if they came in contact
with tungsten carbide which is why there are specialist who work with this substance.
•The extreme hardness of the tungsten carbide creates a high resistance to
scratching.
•This means it is resistant to typical wear and tear but its not scratching proof.
Harder non-metallic materials like diamonds, sapphire and hard crystals can
scratch it.

•Tungsten Carbide rings are engineered to be maintenance free. They don’t need
expensive jewelry cleaning products or getting sent off for re-plating.

60. Next Lecture
Amorphous Materials

- Glass ceramics, Bulk metallic glasses etc.
- Recent developments

61. EXAM 1: 28
th
April, 2014
There will be a report of 6-8 questions;
and 4 to 6 to be attempted.

You can submit the Answers upto May 12, 2014