UNIT - IV
ENGINEERING MATERIALS
Refractories – classification – acidic, basic, and neutral refractories – properties (refractoriness, refractoriness under load, dimensional stability, porosity, thermal spalling) – manufacture of alumina, magnesite and zirconia bricks, Abrasives – natural and synthetic abrasives – quartz, corundum, emery, garnet, diamond, silicon carbide and boron carbide. Lubricants – mechanism of lubrication, liquid lubricants – properties – viscosity index, flash and fire points, cloud and pour points, oilyness) – solid lubricants – graphite and molybdenum sulphide. Nanomaterials – introduction to nanochemistry – carbon nanotubes and their applications.
Introduction:
The natural raw materials and minerals have been exploited since early civilization for personal benefits, utilization and to lead quality life using scientific knowledge. The production of metals, alloys, abrasives, refractories, adhesives, polymers, ceramics, lubricants, etc has largely been the result of the hard work of the chemists. These materials are called engineering materials.
ABRASIVES: The substances used for cutting, grinding, polishing, sharpening, drilling and shaping,… processes are called abrasives. Example: Diamond, emery, topaz, carborundum, etc.
Properties:
Ø They have high hardness and high refractoriness
Ø They show excellent resistance to mechanical shocks.
Ø They have high melting point.
HARDNESS: It is an ability of an abrasive material to grind away other materials. It can be measured by Moh’s / Vicker’s scale. It is comparative scale in which abrasive materials are arranged in the increasing order of their hardness. It varies from 1 to 10.
ABRASIVE / FORMULA / HARDNESSTalc / 3 MgO. 4SiO2. H2O / 1
Gypsum / CaSO4.2H2O / 2
Calcite / CaCO3 / 3
Fluorite / CaF2 / 4
Apatite / CaF2.3Ca3(PO4)2 / 5
Feldspar / K2O.Al2O3.6H2O / 6
Garnet / SiO2 / 7
Topaz / AlF3.SiO2 / 8
Corundum / Al2O3 / 9
Diamond / C / 10
CLASSIFICATION: Abrasives are classified into natural and synthetic abrasives.
Example for natural abrasives: Diamond, Corundum and Emery.
Example for artificial abrasives: Carborundum and boron carbide.
DIAMOND: It is a crystalline form of carbon. It has highest hardness among all abrasives. It has hardness rank 10 on Moh scale. It is chemically inactive and not affected by acids, alkalis and fused KClO3. It is also available as Borts and Carbonade.
Uses: It is used as drill points, saw teeth for cutting rocks, stones or grinding wheels, dressers, etc.
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QUARTZ: It is a crystalline form of silica. Impure form of quartz is used as sand paper. It is almost as hard as garnet.
Uses: It is used for grinding – flour, pigments and ores, etc . and also used for making grinding wheels, grinding dies & for cutting, sharpening, hard high speed tools.
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GARNETS: It is available as a mixture of trisilicates of alumina, magnesia & ferrous oxide. It is a complex of Ca –Al – Fe silicates of formula, Ca3Al2(SiO4)3 . Ca3Fe3(SiO4)3 . Fe3Al2(SiO4)3 . It has hardness rank 6 to 7.5 on Moh scale.
Uses: It is used in bearing pivots in watches, glass grinding & polishing metals, etc.
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EMERY: It is a fine grained, opaque, massive mineral, dark grey to black in colour. It has hardness rank 8 on Moh scale. It consists of 55 – 75 % crystalline alumina (Al2O3) , 20 – 40 % magnesite Fe3O4 and about 12 % other minerals like tourmaline.
Uses: It is used as tips of bits of cutting & drilling tools, etc.
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CORUNDUM: It is a crystalline form of alumina. It has hardness rank 9 on Moh scale.
Uses: It is used for grinding glasses, gems, lenses, metal & metal – cutting, etc.
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CARBORUNDUM: It is a bluish – black crystalline mineral. It has hardness rank 9.3 on Moh scale. It is prepared by heating a mixture, of silica and coke in an electric furnace to 1650 – 2200oC.
SiO2 + 3 C SiC + 2 CO
It is chemically inert and can withstand high temperature.
Uses: It is used in cutting – wheels, abrasive papers & cloths. It is used for grinding of materials of low tensile strength like cast iron, brass, bronze, porcelain, marble, finishing of leather, glass & optical grinding of lenses, etc.
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BORON CARBIDE: It is also called Norbide. It is chemically inert material. It has hardness rank 9 on Moh scale. It is one of the hardest abrasive of all other synthetic abrasives. It is prepared by heating a mixture of Boron oxide and coke in an electric furnace to 2700oC.
2 B2O3 + 7 C B4C + 6 CO
Uses: It is used on hard materials for making grinding dies & for cutting & sharpening high speed tools.
ALUNDUM: It is consists of alumina. It is prepared by heating a mixture of calcined bauxite, coke and Fe in an electric furnace to 4000oC.
Uses: It is used for grind hard wheels & other materials of high tensile strength and an abrasive paper & cloth are used for finishing wood works.
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ABRASIVE POWER: It is an ability of an abrasive to grind away another surface.
MANUFACTURE OF ABRASIVE PAPER:
The roll of paper / cloth is passed through a series of rollers on which glue is sprayed to get thin coating of glue on paper / cloth. Then, it is passed under a hopper from which abrasive powder is allowed to fall & spread evenly on the glued paper. Then, it is dried in a warm drying chamber. Then, it is cut in abrasive paper / cloth. Finally, abrasive paper / cloth sheets are allowed to set for few days so that glue sets firmly on the paper / cloth.
MANUFACTURE OF GRINDING WHEELS: Grinding wheels are manufactured by mixing thoroughly abrasive grains with binder. Then mixture is moulded in the form of wheels, heated and cured to get a grinding wheel.
Uses: It is used for removing scales from surfaces of metals and also used for sharpening of cutting tools.
REFRACTORIES: Refractory is any inorganic material that can withstand high temperature without softening, melting or deformation in shape.
Properties:
Ø They have high resistance to abrasion action of flue gases & flames, etc.
Ø They have high resistance to loss of heat.
Ø They are chemically inert.
Ø They can withstand the overlying load of structure, at operating temperatures.
Ø They have uniform expansion & contraction with rise & fall of temperature respectively.
Uses: It is used for the construction of lining of furnaces, tanks, converters, kilns, crucibles, ladles, etc. employed in various metallurgical processes.
CLASSIFICATION: They are classified into acidic, basic and neutral refractories, based on chemical properties of their constituent substances.
Acid refractories: They contain acidic materials and are not attacked by acidic materials, but easily attacked by basic materials. Ex: Alumina & silica
Basic refractories: They contain basic materials and are not attacked by basic materials, but easily attacked by acidic materials. Ex: Magnesite & dolomite.
Neutral refractories: They contain weakly acidic / basic materials.
Ex: Chromite, Zirconia, Carborundum, etc.
CHARACTERISTICS OF REFRACTORIES:
· Refractoriness.
· Refractoriness under load.
· Dimensional stability.
· Porosity.
· Thermal spalling.
REFRACTORINESS:
It is the ability of a material to withstand high temperature, without appreciable deformation or softening under particular service conditions. It is measured in terms of softening or melting temperature of the material. Most of the commercial refractories have softening temperature over a wide range of temperature and do not have sharp melting point.
The softening temperature of a refractory material is determined by using “Pyrometric cones test”, also called Seger cones test. It is necessary that a material to be used as refractory should have a softening temperature much higher than the operating temperature of the furnace in which it is to be used.
MEASUREMENT:
Refractoriness is, usually, determined by comparing the behaviour of heat on cone of material to be tested with that of a series of Seger cones of standard dimensions. The refractoriness is expressed in terms of Pyrometric Cone Equivalent (PCE). These cones are small pyramid-shaped, 38 mm high and have a triangular base, with 19 mm long sides. They melt or fuse at definite temperatures, when heated under standard condition of 10oC per minute. The temperature at which the fusion of softening of the test-cone occurs is indicated by its apex touching the base. The PCE value of the given refractory is taken as the number of the standard cone, which fuses along with the test-cone. If the test-cone softens earlier than one standard cone, but latter than the next cone, the PCE value of the test sample is approximately measured as the average value of the two.
Seger cone test
Objectives of PCE test:
· To determine the softening temperature of a refractory material.
· To classify and test the purity of refractory material.
· To check whether a given refractory material can be used at the particular servicing temperature or not.
· To check the uniformity of composition of refractory raw material and finished products.
REFRACTORINESS UNDER LOAD: The refractory materials must possess high mechanical strength to bear the maximum load at the operating temperature conditions without softening or deformation. Some refractories like fire clay, soften gradually over a range of temperatures, but under appreciable load they collapse far below their true fusion point. Other refractories like silica bricks, soften over a relatively narrow range of temperatures and exert good load bearing characteristics close to their fusion points. The high temperature load bearing characteristics of a refractory material is determined by refractoriness under load (RUL) test.
RUL test: It is conducted by applying a constant load of 1.75 kg/Sq.cm on test specimen of size 5 Sq.cm and height 75 cm, heating in a furnace at a standard heating rate of 10oC per minute. It is expressed in terms of temperature at which 10 % deformation occurs on the test specimen.
DIMENSIONAL STABILITY: It is the resistance of a refractory material to change in volume due to its prolonged exposure to high temperature. These dimensional changes may be reversible or irreversible. Irreversible changes may lead to contraction in magnesite bricks and expansion in silica bricks. These changes take place because of transformation of one crystalline form into another having different specific gravity.
POROSITY: All refractories contain pores (may opened / closed) due to the manufacturing methods / defects. The porosity of a refractory material is the ratio of its pores volume to the bulk volume.
Where, W = Weight of saturated specimen in air, D = Weight of dry specimen in air &
A = Weight of saturated specimen in water.
Advantages of high porosity:
· It reduces thermal spalling.
· It is used for lining in ovens, furnaces, etc.
Disadvantages of high porosity: It reduces strength, abrasion resistance, corrosion resistance and resistance to penetration by slags, gases, etc.
THERMAL SPALLING: It is the breaking, cracking, peeling off or fracturing of a refractory material under high temperature. A good refractory should have high resistance to thermal spalling.
Spallation is generally due to,
· Rapid changes in temperature, which causes uneven expansion and contraction within the mass of refractory, leading to development of internal stresses and strains.
· Slag penetration, which causes variation in the co-efficient of expansion.
Spallation can be decreased by,
· Using high pososity, low co-efficient of expansion and good thermal conductivity refractory materials.
· Avoiding sudden temperature changes.
· By over-firing the refractories at high temperatures for sufficiently long time.
· By modifying the furnace design.
· Preventing slag penetration.
MANUFACTURE OF REFRACTORIES:
Manufacture of Alumina bricks: It contain 50% or more of alumina and it can be prepared by mixing calcined bauxite (Al2O3) with Clay bind.
Properties: They have,
· Very low co-efficient of expansion and high porosity.
· Great resistance to slag.
· High temperature load-bearing capacity and less spallation.
· Excellent wear-resistance and stability.
· Inertness towards gases like CO2, H2 and natural gases, etc.
Uses: Medium-duty bricks are used in vertical shaft kilns for burning limes, linings of Portland cement rotary kilns, re-heating furnaces, etc. and High-duty bricks are used in hottest zones of cement rotary kilns, brass melting reverberatories, Pb-dressing reverberatories, furnaces, etc.
Manufacture of Magnesite bricks: Calcined magnesite (MgO) is powdered to a proper size, and then mixed with caustic magnesia (binder), and then ground with water, and moulded into bricks. The bricks are then slowly heated to 1500oC, and kept for 8 hours at this temperature and then, cooled slowly.
Properties:
· RUL is 1500oC under the load of 3.5 kg/sq.cm and porosity is 25%.
· Softening temperature is 2000oC.
· They possess good crushing strength, good resistance to basic slags, less shrinkage & more spallation, etc.
Uses:
· It is used in lining of Basic converters, Open-hearth furnaces, etc.
· It is used in Copper converters, reverbertory furnace, rotary kilns, refining furnaces for Au, Pt, Ag, etc.
Manufacture of Zirconia bricks:
It is prepared by heating the mixture of Zirconite (ZrO2), colloidal zirconite and alumina at 1700oC.
Properties:
· RUL is 1900oC under the load of 3.5 kg/sq.cm.
· Softening temperature is 2000oC.
· Quite resistance to thermal shock and acidic slags.
Uses: It is used in high-frequency electric furnaces.
LUBRICANTS: In machineries, due to mutual rubbing of one part against another, a resistance is offered to their movement. This resistance is called friction, which causes a lot of wear & tear of surface of moving parts and a large amount of energy are dissipated in the form of heat, thereby causing loss in the efficiency of machine. These ill-effects of frictional resistance can be minimized by using a suitable substance, which is called lubricant. The substance which is introduced between two moving / sliding surfaces in order to reduce the frictional resistance between them is called lubricant.