MATERIAL SCIENCE

MATERIAL SCIENCE IIIE PRILIMINARY

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MATERIAL SCIENCE

CAST IRON – Is obtained by re-melting pig iron with coke and limestone in a furnace known as cupola. It is primarily an alloy of iron and carbon. The carbon content in the cast iron varies form 2.1% to 4.5%. It may be present either as free carbon [ or graphite ] or combined carbon or cementite. Cast iron is brittle material, therefore it cannot be used in those parts which are subjected to shocks. It has very good casting characteristics, high compressive strength, wear resistance and excellent machinability. The compressive strength of cast iron is much greater than tensile strength. The cast iron also contains small amount of impurities such as silicon, sulphur, manganese and phosphor. Effect of the Impurities on CI :- Silicon – Maybe present upto 4% in CI . It provides the formation of free graphite which makes the iron soft and machinable. Sulphur – makes CI hard and brittle. It must be kept well below 1% for most foundry purposes Manganese – It makes CI white and hard. It is often kept below 0.75% Phosphor – It aids flexibility and fluidity in CI, but induces brittleness. It is rarely allowed to exceed 1%. The Important Types of CI –

1. Grey CI 2. White CI 3. Chilled CI 4. Molten CI 5. Malleable CI 6. Nodular or Spheroidal Graphite CI 7. Alloy CI

GRAY CAST IRON This is ordinary commercial iron having 3 to 3.5% carbon


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The grey color is due to presence of carbon in the form of free graphite IT has low tensile strength High compressive st. No ductility It can be easily machined WHITE CAST IRON This has 1.75 to 2.3% of carbon The white color is due to the presence of carbon in the form of carbide [ known as cementite] which is the hardest constituent of iron Has high tensile st. Low compressive st. CHILLED CAST IRON: Produced by quick cooling of molten iron The quick cooling is generally called chilling MOLTEN CAST IRON Is product in between grey and white CI in composition color and general properties MALLEABLE CAST IRON Obtained from white cast iron by suitable heat treatment process. [ annealing ] Nodular or Spheroidal graphite cast iron Called ductile cast iron or high st. cast iron Obtained by adding small amount of magnesium [ 0.1 to 0.8% ] to molten grey iron just after tapping. ALLOY CAST IRON Produced by adding alloying element like nickel, chromium, molybdenum, copper and vanadium in sufficient quantities. Properties – Increased strength High wear resistance Corrosion resistance Heat resistance.


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STEELS

Is an alloy of iron and carbon, with carbon content up to a maximum of 1.5% most of the steel produced now-a-days are plain carbon steel. Divided into following types based on carbon content – DEAD MILD STEEL – UPTO 0.15% CARBON LOW CARBON MILD STEEL – 0.15 – 0.45% MEDIUM CARBON STEEL – 0.45% TO 0.8% CARBON HIGH CARBON STEEL – 0.8% TO 1.5% CARBON PLAIN CARBON STEEL – [ UPTO 0.5% SILICON AND 1.5% MANGANESE ARE CALLED PLAIN CARBON STEEL ] These steels are strong, tough, ductile and used in expensive materials. They can be cast worked, machined and heat treated to a wide range of properties. Unfortunately, plain carbon steel has poor atmospheric corrosion resistance. But it can be protected easily by painting, enameling or galvanizing. The properties of plain carbon steel depend upon the presence of carbon content. Properties – Hardness and strength increases with increase in carbon content. Effect of Impurities on Steel – Impurities – Silicon, Sulphur, Manganese and Phosphor Silicon – [ 0.05% to 0.30% ]

• Prevents steel from becoming porous • Removes the gases and oxides • Prevents Blow holes and thereby makes steel tougher and harder.

Sulphur – [ as iron sulphide or manganese sulphide ]

• Iron sulphide has low melting point • Manganese sulphide – does not effect much

Manganese Serves as valuable deoxidizing and purifying agent in steel. When used in ordinary low carbon steel, manganese makes the metal ductile with good bending qualities.


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. Phosphor Makes steel- Brittle Cold shortness in steel In low carbon steel , raises the yield point and improves resistance to atmospheric corrosion. Alloy Steel – Maybe defined as a steel to which elements other than carbon are added in sufficient amount to produce an improvement in the properties. The chief alloying elements used in steel are – NICKLE – [ 2 to 5%] Improves – tensile strength Raises elastic limit Imparts hardness, toughness Reduces rust formation CHROMIUM – Increases strength Hardness Corrosion resistance VANADIUM [ LOW AND MEDIUM CARBON STEEL ] increases yield and tensile st. TUNGSTEN Raises critical temp of steel Imparts cutting hardness Abrasion resistance properties High speed steel [ 18:4:1] 18% tungsten, 4% chromium 1% vanadium and 0.7% carbon Manganese – Reduces the formation of iron sulphide by combing with sulphur. Makes steel, hard, tough and wear resistance. SILICON – Increases strength and hardness of steel without lowering its ductility


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COBALT It is added to HSS [ 1 to 12% ] Give red hardness by retention of hard carbide at high temperatures. MOLYBDENUM [ 0.15 to 0.30%] of molybdenum is used with chromium and manganese [0.5-0.8%] to make molybdenum steel. Possess extra tensile strength Used in airplane fuselage and automobile parts. Stainless Steel – Properly heat treated and finished steel that which resists oxidation and corrosive attack from most corrosive media. Different Types of Steel-

1. MARTENSITIC STAINLESS STEEL 2. FERRITIC STAINLESS STEEL 3. AUSTENITIC STAINLESS STEEL

MARTENSITIC STAINLESS STEEL Are Chromium steel, containing – Chromium – 12 to 14% Carbon 0.12 to 0.35% Properties – Magnetic in nature Maybe hardened by suitable heat treatment Hardness depends on Carbon content Can be easily welded and machined FERRITIC STAINLESS STEEL Chromium – 16 to 18% Carbon - about 0.12 % Properties – Good Corrosion Resistance than martensitic steels AUSTENITIC STAINLESS STEEL Chromium – 18% Nickel – 8%


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Known as 18/8 steel Properties – Non magnetic Greatest Corrosion Resistance and good mechanical properties at elevated temperatures. NON FERROUS METALS AND ALLOYS ARE THOSE WHICH CONTAIN METALS OTHER THAN IRON AS THEIR CHIEF CONSTITUENT. The non ferrous metals are usually employed in the industry due to the following characteristics

1. Ease of Fabrication, [ Casting, rolling, forging welding and machining ]

2. Resistance to corrosion 3. Electrical and thermal conductivity 4. weight

They are – I. Aluminium : It is a white metal prepared from bauxite. Its light in weight and has specific gravity 2.7 and melting point – 660deg cel. Aluminium alloys – Alloyed with other metals like copper, magnesium, manganese, silicon and nickel. Addition of these metals converts – Soft metal into hard and strong metal still retaining its light weight. The main aluminum alloys are – 1. Duralumin” Composition – Copper : 3.5 to 4.5% Manganese – 0.4 to 0.79% Magnesium – 0.4 to 0.70% and remaining is aluminium Possesses max.strength [ abt 400mpa] after heat treatment and age hardening. After working it is allowed to age for 3 to 4 days till it is hardened through the process of age –hardening


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2. Y-alloys – Called also – copper-aluminum alloy Copper – 3.5 to 4.5% Manganese – 1.2% to 1.7% Nickel – 1.8 to 2.3% Silicon, magnesium, iron – 0.6% each and Remaining is aluminum This is heat treated and age-hardened to get better strength than duralumin at high temperatures. 3. magnesium – It is made by melting aluminum with 2 to 10% magnesium in a vacuum and then cooling it in a vacuum or under a pressure of 100 to 200 atmospheres. It also contains about 1.75% copper. II. copper : Most widely used non-ferrous metals in industry. It is a soft, malleable and ductile material with a reddish brown appearance. Specific Gravity – 8.9 Melting Point – 1083deg Celcius It is Good conductor of Electricity Used – for making useful alloys with tin, zinc, nickel and aluminum. The main copper al loys are –

Copper zinc alloys [brass] in which zinc Is the principal alloying material

Copper tin alloys [bronze] in which tin Is the principal alloying material


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Brass: Most widely used copper-zinc alloy. This is fundamentally a binary alloy with each copper and zinc in 50% consistency. There are various types of brasses depending upon the proportion of copper and zinc. Brass is resistant to atmospheric corrosion and can be easily soldered. Types of Brass: CARTRIDGE BRASS: Copper – 70% zinc-30% Is a cold working brass used for cold rolled sheets, wire drawing, deep drawing pressing and tube manufacturing Yellow brass Copper – 60% zinc 40% Suitable for hot working by rolling and extrusion and stumping Leaded brass Admirality brass Copper – 62.5% 70% Zinc 36% 24% Lead 1.5% tin – 1% Used – plates, tubes, etc Naval brass Cu – 59% zinc – 40%, tin –1% Used morque castings Nickel brass [ german silver] Cu – 60.45% zinc –35.20% nickel – 5.35% Used – for valves, plumb fittings, automobile fittings, type writer parts and musical instruments.


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Bronze: Composition 75 to 95% copper and 5to25% tin Properties – corrosion resistant, Superior to brasses Types of Bronze: Phosphor bronze Copper – 87-90% tin – 9 to 10% Phosphor – 0.1 to 0.3% High strength, superb ductility and soundness of castings Possesses good wearing qualities and high elasticity. Silicon bronze Copper – 96% silicon – 3% manganese or zinc – 1% High strength and good corrosion resistant properties Used – boilers, stores etc., Berlliyum bronze Copper – 97.75% Beryllium – 2.25% High yield point, high fatigue limit and excellent cold and hot corrosion resistance. Used – Springs.. Manganese bronze Copper – 60% Zinc – 35% manganese-5% High resistant to corrosion Used - , worm gears… Aluminium bronze Alloy of copper and aluminium. Al – 6to 8%. Valuable cold working properties, used in imitation jewellery and decorative purposes because of its fine gold color


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III. lead : it is bluish grey metal having specific gravity – 1.36 and melting point – 326deg celcius this is soft and can be cut with a knife. It is used for making solders The lead base alloys are – Used where cheap and corrosion resistant material is required. Alloy containing - Lead – 83%, antimony 1.5% tin 1.5% and cu – 0.5% Used – large bearings iv. zinc : bluish white metal which in pure state has bright smooth crystals at its fracture. Specific gravity 7.1, mp – 420deg cel. Boils at 940deg cel anc can be ealiy distilledused – covering steel sheets to form gavanised ironhigh resistance to atmospheric corrosion. The zinc base alloys are – Die castings mostly produced by zinc based alloys. Can be casted easily with a good finish at fairly low templ Have considerable strength and low cost The usual alloying element for zinc are al, cu and magnesium v. cadmium: white metal with bluish tinge. Capable of taking high polish. Specific gravity 8.65, mp – 321deg cel. Harder than tin but softer than zinc Used – Artification alloys for bearings. Used as rust proof coating for iran and steel Bearing metals –

1. Copper base alloys 2. Lead base alloys 3. Tin base alloys 4. Cadmium base alloys.


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Selection of a particular type of bearing metal depends on the contions under which it is to be used. It involves factors relating to bearing pressure, rubbing speed, temps, lubrication etc., Properties to be possessed by bearing alloys : Have low co-efficient of friction Have good wearing qualities Have ability to withstand bearing pressures Have a sufficient melting point Have good casting qualities Have non –corrosive properties Have economies in cost The selection of a particular type of bearing material depends on the conditions under which it is to be used. It involves factors relating to rubbing speed, temperatures, lubrication etc.


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NON-METALLIC MATERIALS

PLASTICS Plastics are synthetic material. They are moulded into shape under pressure with or without the application of heat. They can also be cast rolled, extruded, laminated and machined The plastics may contain a number of constituents including binders, polymers, dyes or pigments, plasticisers, lubricants, solvents and catalysts. The plastics are usually classified on basis of the nature of binder used in their manufacture. The binder may be natural or synthetic polymers. Types of Synthetic Plastics: 1. Thermo setting Plastics 2. Thermo Plastics Thermo setting Plastics : These when heated under pressure to form shapes, they result in permanently hard products The heat first softens the material, but as additional heat and pressure are applied it becomes hard by a chemical change known as phenol-formaldehyde [ bakelite ] Thermo Plastics These plastics neither become hard nor do any chemical changes take place in them on application of pressure. They remain hard at elevated temperatures until they are cooled to harden. These can be remelted repeatedly by successive application of heat. Eg- celluloid polythene, pvc RUBBER – Is an organic polymer. This elongates on stretching and regains its original shaper after the removal of the stress. Major portion of the rubber is consumed in the field of manufacture of tyres and tubes for vehicles.


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A number of constituents like Sulphur, antioxidants, reinforcing agents, coloring agents are added to the polymer to obtain desirable properties in them. These properties include elasticity, tensile strength, resistance against chemical reagents etc., Structure: It consists of a long chain of molecules, which are interlocked with one another. When tensile stress is applied to a rubber molecule, the chain gets stretched and considerable elongation takes place. On removing the stress the thermal agitation will return the chain to the interlocked form and so the rubber to its original shape. TYPES OF RUBBER:

1. Natural Rubber 2. Synthetic rubber or elastomers

Natural Rubber: It is elastic material present in the latex of certain plants. It is a polymer of isoprene with the structure as shown in the figure The latex is treated in two ways to obtain the rubber. H H C H H H H C C C C H H N ISOPERENE First method: In the first method the latex is coagulated with organic acid to produce crude rubber. This rubber does not posses the desirable properties. Hence, its properties are improved by the addition of compounding materials such as sulphur, accelerators and coloring agents etc., It is then passed through the calendaring machine to produce sheets of desired thickness. Second method: The latex itself is mixed with the compounding materials and the precipitated directly from the solution in the final shape to be used.


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Types of Natural Rubber:

• Chlorinated rubber – used in protecting coatings and adhesives • Cyclized rubber – to manufacture paper in combination with parafine

wax • Rubber hydrochloride - used in packing and wrapping of delicate

equipments SYNTHETIC RUBBER: These are manufactured from raw materials such as coke, limestone, petroleum, natural gas, salt, alcohol, sulphur, ammonia, coal, tar, etc., They are not same in structure as natural rubber. They are rubber like materials having properties like natural rubber, infact some properties better than that of natural rubber. They are more resistant to sunlight than natural rubber. They have greater solvent resistance and elasticity also. Eg- Neoprene, Nitride rubber, silicon rubbers

1. Neoprene- Are produced by polymerizing chloroprene molecules. They are lcosely related to the natural rubber. But they have higher resistance to oils, greases, aging high temperatures etc., they are used in oil seals, gaskets, adhesives etc., Nitride rubbers- These are rubbers are produced by polymerization of crylonitride and out. They have excellent oil, grease and solvent resistance. They are widely used for tank lining, conveyor belts. Silicon rubber- These are produced by condensation polymeration of dimethyl silicon polymers, they are most stable rubber over a wide temperature ranging from 55 deg celc to 315deg ecl. These are used for wire and cable insulation, coatings, packaging, tubing gaskets etc.,

CERAMICS: These are made from inorganic materials and have non-metallic properties. Eg- Ionically bonded magnesia and covalently bonded silicon carbide The traditional ceramic materials – stone, brick, concrete, clay glass, vitreous enamel and refractories.


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They have low value of thermal conductivity Generally good thermal insulators They can withstand rapid fluctuations of temperature and pressure. They are not affected by oxygen.

GLASS It is an inorganic insulating material. Comprises of complex system of oxides Silica is the most essential constituent of many commercial glasses. It is fused with alkali [ potash, soda etc.,] and base like lime, lead oxide etc., Silica glass is the best insulating material This is extensively used in electrical bulbs, electronic valves, x-ray tubes, etc., COMPOSITE MATERIALS: Two or more materials combined together to produce a new material which possess much superior properties than any one of the constituent materials. Such a material is known as composite material. Eg – wood [ natural ], Artificial composite materials [ cement, concrete, glass reinforced plastics, plywood,etc.,] Types of composite materials:

1. Agglomerated materials 2. Laminated materials – Laminates 3. Reinforced materials

Agglomerated materials Here particles are condensed together to form an integral mass and are known as agglomerated materials. Eg – Cement concrete, grinding wheel Important terms – Particle size, packing factor, density and porosity Laminated materials – Laminates These are produced by completely bonding two or more layers of different materials Here the top most item provides the desired appearance and workability, which the lower layer provides the strength. Roll bending, co-extrusion, explosive welding and brazing


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Eg – Plywood, Tufnol, sunmica, linoleum etc. Plywood – It is produced by bonding together an odd number of thin layers [ called plies ] of fine wood and applying pressure Tufnol – is produced by combining laminated materials having layers of woven textiles with a thermosetting resin. In the composite material the resin provides the necessary appearance and rigidity. Reinforced materials These materials are produced by combining some suitable material to provide additional strength which do not exist in a single material. eg. Reinforced cement concrete [ ROC], Glass Fibre Reinforced Plastics, etc., Reinforced cement concrete [ ROC] Produced by placing the steel rods in a cement concrete mixture. The resulting product provides excellent resistance to both the compressive and tensile stresses Nylon Reinforced Rubber: Here the nylon thread [ sometimes steel wires, glass wires etc ] provides the necessary strength and the rubber provides necessary surfaces. Used in automobiles and cycle tyres Glass Fibre Reinforced Plastics: Produced by combining glass fibre with plastic. Here glass fibre provides the necessary strength and the plastic reduces the brittleness. The commonly used – resin, silicon These can be employed in the form of continuous lengths or whiskers.


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Insulating materials

Are materials in which electrical conduction cannot occur. They are also known as dielectric materials Conduction bond Wide energy gap valence bond Energy band diag of insulators The valence bond of these materials is separated from the conduction bond by a wide energy gap. This gap is so wide that at ordinary temperature, the electron cannot jump from valence bond to the conduction bond Eg. Rubber, bakelite, mica, glass, etc

REFRACTORIES Are materials which can withstand high temperatures and possessed sufficient mechanical strength, heat resistance and retain a constant volume. They also possess reversible thermal expansion and resistance to thermal shock. Used either in furnaces either to support heating element or to form the linings of the inner parts of the furnace. The materials used at temperatures greater than 900deg cel are – anudum [bonded aluminium oxide, alumina, silica, dolomite, mullite, clay, etc High temperature refractoy materials Graphite - 3200deg cel Thoria - 2700 deg cel Magnesia - 2000deg cel Alumina - 1900deg cel Quartz - 1700deg cel They are available in different sizes shapes. Some common shapes – refractory bricks, light weight bricks, refractory tubes, protection tubes and powders used as insulating materials


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ABRASIVES Are used for grinding wheels to produce better metal finish and removal of materials Abrasives are also used to improve product appearance. The processes include barrel finishing, viratory finishing, polishing and buffing. NATURAL ABRASIVE – Found in nature. Eg – emery, sand stone, corundum and diamonds. Both Turkish and American emery are natural mixture of aluminum oxide [ Al2O3 ] and magnetite [FeO4]. Both have milder cutting action than synthetic abrasives. Emery is often preferred for use on coated cloth and paper as well in many buffing compositions. It is no longer used in making grinding wheels Diamonds are another type of natural abrasive. They have the highest known hardness of any substance. Because of their nature, diamonds require a distinctive bond, which is more specialized than that of conventional grinding wheels. SYNTHETIC ABRASSIVE Most widely used in surface grining operations eg silicon carbide [ Sic] and Aluminium oxide. Silicon carbide is the harder and more brittle.

Adhesives

Adhesive material are those used to join two surfaces. Pressure sensitive adhesives cause adhesion simply by the application of pressure. Glues are widely used as adhesive materials. They are obtained from animal gelatins, vegetables and starches. Other adhesives are made from polymer. Eg – Cellulose, thermosetting and thermoplastic Cellulose adhesives – pyroxylin, cellulose acetate Thermosetting adhesives include epoxy cements, urea formaldehyde, polyester and silicon adhesives Thermoplastics – neoprene, poly vinyl alcohol and acrylic adhesives


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NON DESTRUCTIVE TESTS: [NDT] Are those tests in which the specimen is not destroyed and can be re used after test. They are performed to inspect the materials against any defects such as gas porosity, blow holes, etc., TYPES OF NDT :

1. Ultrasonic Test 2. Radio graphic Test

Ultrasonic Test: In this test, ultrasonic radiations are made to fall on the material to be tested. While passing through the material these ultrasonic radiations are scattered along different directions. The ultrasonic radiations are attenuated [ weakened] while passing through the defective area. And they remain unattenuated [stronger] while passing through the perfect area. This leads to the variation in the intensity of transmitted radiations. Now the received radiations are converted to an oscilloscope beam for interpretations. There are two methods for transmitting and receiving the ultrasonic radiation. First method- separate transducers are used for transmitting and receiving radiations. Second method. The same transducer is used both for transmitting and receiving radiations Used For : Detecting voids, cracks and other defects near or far below the surface of any material The air gaps over the order of 0.003mm can be detected Ultrasonic radiation frequency are more than 2 lakhs cycles/sec Radio graphic Test: Here high frequency [ short wavelength ] radiations of constant intensity are made to fall on the material to be tested. While passing through the mateials these radiations are also abosorbed and scalttered alon different directions. This also leads to the variation in the intensity of the transmtted radations. Which are usally recorded on a sensitive photographic film. It will be interesting to know that the darker region on the photographic filem, corresponds to a defect region.


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And less dark region corresponds to the perfect regions. It is due to the fact that more radiations pass through the defective area than the perfect one. The developed image of the photographic film is called radiograph and the various techniques to determine the flow in the material are known as radiographic techniques. They are classified into three categories – X-RADIOGRAPHY The x-rays are used as a high frequency radiations. This technique is useful to detect defects such as porosity in castings. But this technique has a draw back that it cannot be used to detect smaller defects. This is due to the fact that due to scattering of x-rau the small defects are not detected. GAMMA –RADIOGRAPHY The radio active elements like radium, cobalt, etc., are used as a source of producing radiations of very high frequency [even higher than x-rays] this technique is very useful for inspection of medium sized parts and also for the jobs which require stronger radiations and higher penetrations. The gamma radiography is less expensive as compared to x-radiography but it requires a longer period to obtrrain radiography as compared to that of x-ray radiography. Neutron Radiography – A neutron beam is used as source of high frequency radiations. This technique is very useful for inspection of very light materials like plastics, explosive, rubber components, etd. Whre x-radiography and gamma –radiography cannot be used. This may be used for detection of voids and detonators as well as inspection of rubber and paper products. MAGNETIC PARTICLE TEST OR CRACK DETECTION METHOD: In this test the material to be tested is magnetized and then fine magnetic particles are spread over the whole surface. The presence of crack causes some leakage in the magnetic field. The particles will not be able to spread in the crack area. But they adhere to the surrounding surface thus outlining the presence of a crack. In order to increase the visibility of a crack, fluorescent magnetic particles or ultra violet light may be used. The magnetic particle testing is useful for ferrous alloys or magnetic materials such as iron steel, nickel alloys etc. this is also use for detecting blow holes, grinding and fatigue cracks, and cracks on surfaces and just below it.


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TTT DIAGRAM FOR 0.8%CARBON – [ EUTECTIOID STEEL ] TTT Diagrams are drawn with temperature along y-axis Time along x-axis To study – effects of time on transformation of phases TTT diag – consists of 2 parallel curves which represent the beginning and ending of transformation of austenite respectively. Another set of parallel horizontal lines represent the martensite start and finish temperatures respectively. CONSTRUCTION OF TTT DIAGRAM- STEPS FOLLOWED- STEP 1 – A large number of identical samples are cut and prepared from the same bar. The cross section of the samples has to be small in order to react quickly to the changes in temperature STEP 2 – All the samples are placed in a furnace at the proper austenitizing temperature for sufficient duration until complete austenite phase in the micro-structure is ensured. Step 3 – A set of samples are then transferred to molten salt bath held at a constant sub-critical temperature [ i.e., below 727 deg cel and for example 700 deg cel ] STEP 4- Each sample in the set is held in the molten salt bath at this constant temperature for different intervals of time and then quenched. STEP 5 – After cooling each sample is checked for hardness and studied microscopically STEP 6 – the above steps are repeated for samples held at different sub-critical temperatures until sufficient points are determined to plot curves on the diagram The above procedure is carried out to draw the TTT diagram for one composition of iron and carbon. The entire effort is repeated to draw TTT diagram for reach combination of iron and carbon.


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Ref text – by Niranajan


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IRON CARBON SYSTEM- Most important binary system in engineering alloys. The alloys of iron-carbon system containing from 0 to 2.0% carbon are called steels And those containing 2.0% to 6.7% of carbon are called cast-iron In practice steels are manufactured with carbon content up to 1.4%. It is due to the fact that steels with carbon content more than 1.4% are brittle and hence are not useful. Similarly the cast irons that are manufactured in practice contain carbon from 2.0% to 4.5% only. It will be interesting to know that iron-carbon alloys exist in different phases in steel and cast irons. In steels, iron and carbon exists as two separate phases, ferrite and cementite. The ferrite is a solid solution of carbon in α-iron with negligible amount of carbon and cementite is an inter metallic compound called iron-carbide [fe3c]. cementite is a stable phase in steel only. But is its not stable in cast iron under all conditions and hence cementite is called a metastable phase. The following are 2 phase diagarams of iron –carbon system

1. IRON-IRON CARBON [ Fe-Fe3C] PHASE DIAGRAM 2. IRON –CARBON PHASE DIAGRAM.

PHASE DIAGRAM OF IRON CARBON SYSTEM FIG Shows the iron-iron carbide phase diagram. In this diag, the carbon composition [weight percent] is plotted along the horizontal axis and temperature along the vertical axis. The diag shows the phases present at various temperatures for very slowly cooled iron-carbon alloys with carbon content up to 6.7%. The diagram gives us information about the following important points.

1. Solid phases in the phase diagram 2. Invariant reaction in the phase diagram 3. Critical temperatures 4. Eutectoid, hypo ecutoctoid and hyper ecutectioid steel

PHASE DIAGRAM – TO BE DRAWN –text by Niranjan


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the low carbon region found above 1400 deg cel in the phase diagram in not of any practical importance. However the region lying in the 700-900 deg cel temp range and 0.1% carbon range is the most important region in the phase diagram. In this region an engineer can develop with in steel, those micro structures having different desired properties. SOLID PHASES IN IRON-IRON CARBIDE PHASE DIAGRAMS. 1. αααα FERRITE The solid solution of carbon in α iron is called α FERRITE or simply ferrite. This phase has BCC structure. Ant at 0% carbon it corresponds to α iron. 2. AUSTENITE The solid solution of carbon in γ iron is called austenite. This phase has FCC structure. And has much greater solubility for carbon than α FERRITE. The solubility of carbon in austenite reaches a max of 2.11% at 1148deg cel and then decreased to 0.8% at 723deg cel as shown by line CD in the phase diagram. 3. CEMENITE The inter metallic iron-carbon compound is called iron carbide or cementite. Its chemical formula is Fe3C. It containing 6.7% of carbon and 93.3% of iron. Cementite has an orthorhombic crystal structure with 12 iron atoms and 4 carbon atoms per unit cell. Its density is 7.6gm/cu cm. as compared austenite and ferrite, cementite is extremely hard and brittle. 4. δδδδ FERRITE The solid solution of carbon in δ iron is called δ FERRITE. This phase has BCC CRYSTAL STRUCTURE BUT WITH DIFFERENT LATTICE PARAMETER THAN α ferrite. The maximum solid solubility of carbon in δ FERRITE is0.09% at 1495deg cel.. The aim of Heat treatment is to bring about the desired changes in the form of carbon combination in steel. All metals are crystalline in structure. The crystals of pure iron are called ferrites. Ferrite may contain a very little amount of carbon in solid solution with iron. It is soft , ductile and strongly magnetic.


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A chemical compound of ferrite and carbon is called cementite. It remains as round particles in steel. Cementite is hard, brittle and magnetic substance. A mechanical mixture of ferrite and cementite is called pearlite which is hard, strong and machinable. A solid solution of cementite and ferrite is called austenite it is not stable at temperatures below 723deg cel. At lower temp austenite breaks into ferrite and cementite or pearlite. When high carbon solids are heated to temps above 850 deg cel and suddenly cooled down to room temps. The cementite in steel is forced to enter into the solid solution in ferrite. This forced solution of cementite in ferrite is called martensite. Which is very hard, Brittle and unstable substance.

MILLER INDICES: The Orinetation of a plane in every crystal system, is described in terms of coordinates through which they pass along x-x asix, y-y axis and Z-Z axis. This point is illustrated below: Consider a plane ABC, have intercepts of 1 axial unit [ along x-x axis], 2 axial units [ along y-y axis] and 3 axial units [ along z-z axis ]. The orientation of plane ABC is described a 1,2,3. Z C Y B A X Miller suggested that it is more useful to describe the orientation of a plane by the reciprocal of its numerical parameters, rather then by its linear parameters. These reciprocals when appropriately converted into whole numbers are known as miller indices. Thus the miller indices of the plane ABC in fig [a] are [ 1:1/2:1/3 ]or

6:3:2. It may be noted that for getting the whole numbers all the three reciprocals have multiplied by 6. The ratio sign[:] is omitted and it is due to the fact that multiplying all intercepts or reciprocal the same number does not change the orientation of the plane and numbers for the planes are enclosed in small bracket. Thus miller indices for plane ABC in fig [a] maybe written as [132]. The digits in the bracket are read as one three two and not six hundred and thirty two.


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PROCEDURE FOR FINDING MILLER INDICES OF CRYSTAL PLANES: ALONG THE THREE AXES: Find the intercepts x, y and z of the plane along the three reference axes.

Express the intercepts in terms of axial units [ i.e. x-pa, y-qb, and z=rc] Now find the ration fo their reciprocals [i.e 1/p, 1/q, 1/r ] Convert these reciprocals into whole number by multiplying each one of them with thir LCM.

Enclose these number in small brackets. This represent the indices of the given plane. Salient features of miller indices of crystal planes:

1. All the parallel planes have the same miller indices. Thus the miller indices define a set of parallel planes

2. A plane parallel to one of the co-ordinate axes has an intercept of infinity. 3. If the miller indices of the two planes have the same ration [ie. 841 and

422 or 211] then the planes are parallel to each other. Crystal defects or imperfections: The atoms do not have their full quota of electrons in the lowest energy level. But the atoms vibrate due to thermal effect and the electrons also change their positions. There are many other type of defects found in the structure of the crystals. We shall discuss only those defects, which are found in the arrangement of atoms. They are: 1. Point Defects 2. Line Defects 3. Surface Defects 4. Volume Defects

1.Point Defects: The defects which take place due to imperfect packing of atom during crystallization are known as point defects. The point defect also takes place due to vibrations of atoms at high temperatures. The important point defects are:

a. Vacancy Defect b. Interstitial defects c. Frenkel defect d. Substitutional Defect e. Shottky defect


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f. Photon Defect

Vacancy Defect: When ever one or more atoms are missing from a normally occupied positon as shown in the fig. The defect caused is known as vacancy. Such defects can be a result of imperct packing during formation of crystals or from thermal vibration of the atoms at high temperature.

Interstitial defects: When ever an extra atom occupies interstitial position [ ie., voids ] in the crystal system without dislodging the parent atom as shown in the fig. the defect caused is known as interstitial defects. This atom which occupies

the interstitial position is generally smaller than the parent atom

c. Frenkel defect: Whenever a missing atom [ responsible for vacancy ] occupies interstitial position [ responsible for interstitial defect ] as shown in fig. Noted that a frenkel defect is a combination of vacancy and interstitial defects d. Substitutional defect: Whenever a foreign [ atom [ ie., other than the parent atoms] occupies a position which was initially meant for a parent [ atom or replaces a parent atom]. The defect caused is known as substitution defect. Here atom which replaces the parent atom may be for the same size [ or slightly smaller or larger] than the parent atom e. Shottky defect: Whenever a pair of positive and negative ions is missing from a crystal, the fects caused is known as shottky defect. Here crystal is electrically neutral.

g. Photon: whenever a group of atoms is displaced from its ideal location. The defect is known as. The defect is caused by thermal vibration.


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2 . LINE DEFECTS. The defects which takes place due to dislocation of atoms along a line in same direction are called line defects. The line defects also takes place when a central portion of a crystal lattice slips without effecting the outer portion. There are two types of line defects;

a. Edge Dislocation b. Screw Dislocation.

a. Edge Dislocation:

Whenever a half plane of atom is inserted between the planes of atoms in a perfect crystals. This defect is produced Fig a: shows a cross-section of a crystal where dots represent atoms arranged in an orderly manner. Fig b: shows the displacement of atoms when an extra half plane is inserted from the top. It may be noted from this fig that top and bottom of the crystal above and below the line xy appears perfect. If the extra half plane is inserted from top the defect so produced is represented by ┴ [ inverted T ] as shown in fig. and if the extra half plane is inserted from the bottom, the defect so produced is represented by T Screw Dislocation. Whenever the atoms are displaced in two separate planes perpendicular to each other. The defect so produced is known as screw dislocation. Fig a: represents an isometric view of a perfect crystal. Fig b shows the displacement of atoms in the region are in screw dislocation the arrangement of atoms appear like that of a screw or a helical surface.


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s. surface defect: The defects which takes place on the surface of a material are known as surface defects or plane defects. It may be noted that surface defects take place either due to imperfect packing of atoms during crystallization or defective orientation of the surface. The types of surface defects

a. grain boundary when ever grains of different orientation separate the general pattern of atoms and exhibits a boundary as shown in fig a. the defects caused is known as grain boundary. This type of defect generally takes place during the solidification of the liquid metal. Twin Boundary: When the boundaries in which the atomic arrangement on one side of the boundary is a some what mirror image of the arrangement of atoms of the other side, as shown in fig. the defect caused is known as twin boundary The region in which a twin boundary defect occurs Is between the twinning panes as shown in fig.., Whenever the stacking of atoms is not in proper sequence through that crystal. The fault caused is known as stacking fault. Figa: . Shows the proper sequence of atomic planes if we read. From bottom to top is a-b-c-a-b-c-a-b-c fig b: shows the sequence of atomic planes a-b-c-a-b-a-b-a-b-c. The region in which the stacking fault occurs [a-b –a-b] forms a thin region of hexagonal closed packing in a FCC crystal


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HEAT TREATMENT PROCESSHEAT TREATMENT PROCESSHEAT TREATMENT PROCESSHEAT TREATMENT PROCESS The process of obtaining various desirable properties in components by the process of heating to certain temperature and holding at that temperature for considerable amount of time and subsequently cooling it by various methods. TYPES OF HEAT TREATTMENT PROCESSES:TYPES OF HEAT TREATTMENT PROCESSES:TYPES OF HEAT TREATTMENT PROCESSES:TYPES OF HEAT TREATTMENT PROCESSES:

1. NORMALISING Is the process of heating the steel 30deg to 50deg cel above its UCT hypereutectoid steel. It is held at this temperature for about fifteen minutes and then allowed to cool down in still air. ASLO known as Air QuenchingAir QuenchingAir QuenchingAir Quenching This process provides a homogeneous structure consisting of ferrite and pearlite for hypereutectoid steel and pearlite and cementite for hyper eutectoid steel The process of normalizing is frequently applied to castings and forgings. NOTE: UCT – UPPER CRITICAL TEMPERATURE LCT – LOWER CRITICAL TEMPERATURE OBJECT –

� To refine the grain structure of the steel and to improve machinability, tensile

strength and structure of the weld

� To remove strain caused by cold working process like hammering, rolling, bending etc., which makes the metal brittle and unreliable

� To remove dislocations caused in the internal structure of the steel due to hot working

� To improve certain mechanical and electrical properties.

ANNEALINGANNEALINGANNEALINGANNEALING The main objects of annealing in HT are:

1. Reducing hardness, refine grain structure, soften the steel 2. Improve machinability 3. Facilitate cold working by improving toughness.

The specimen is heated to prescribed temperature, and then cooled down to room temperature at a suitable slow rate.


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Types : Depends on the process adopted for annealing.

1. FULL ANNEALING 2. PROCESS ANNNEALING / SUB-CRITICAL ANNEALING

FULL ANNEALING Object – to soften the metal, to refine its grain structure, to relieve the stresses and to remove gases trapped in the metal.

PURPOSEPURPOSEPURPOSEPURPOSE ---- to increase ductility, to remove strain hardening effect, to remove to increase ductility, to remove strain hardening effect, to remove to increase ductility, to remove strain hardening effect, to remove to increase ductility, to remove strain hardening effect, to remove stressesstressesstressesstresses Temp – 940deg cel to 960 deg cel Holding Time – 2to3min per mm thickness of the material Rate of cooling – Very slow by switching of the furnace Applicable for – Hypo-eutectoid steel DISADVANTAGE / LIMITATION – very expensive and long cooling cycles

PROCESS ANNEALING -also known as sub critical / partial annealing Object – to remove internal stresses and to improve cold working Temp : step 1 – 940deg cel to 960 deg cel Holding Time – 2to3min per mm thickness of the material Rate of cooling – suitable rate In the atmosphere Applicable for – steel alloys


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DISADVANTAGE / LIMITATION not very expensive as full annealing, but not suitable for exhaustive large components. Approximate temperatures for annealing depending on composition of carbon in steel. Slno Carbon content in % Annealing temperature in deg

celcius 1 Less than 0.12

[ dead mild steel] 875 – 925

2 0.12 – 0.25 [ mild steel ]

840 – 970

3 0.25 – 0.55 [ medium carbon steel ]

815 – 840

4 0.55 – 0.80 [ high carbon steel ]

780 – 810

5 0.80 – 1.40 [ high carbon or tool steel ]

760 - 780

HARDENINGHARDENINGHARDENINGHARDENING

Object –

1. to increase the hardness of metal, to resist wear 2. to enable it to cut other metals, [ suitable for cutting tools ]

Process –

1. Heating the metal up to 30deg c to 50 deg c above UCT for Hypo eutectoid steels and by the same temperature above LCT for hypereutectoid steel.

2. The metal is held at this temperature for a considerable time depending upon its thickness and then quenched [ suddenly cooled ] in a suitable cooling medium.

3. The hardness obtained depends on rate of cooling, the carbon content and the work size

4. A very rapid cooling is necessary to harden low and medium plain carbon steels 5. As the carbon content goes on increasing the possible obtainable hardness also

increases.


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Types of hardening:

1. WORK HARDENING 2. AGE HARDENING 3. AIR HARDENING

WORK HARDENING WORK HARDENING WORK HARDENING WORK HARDENING –––– Process of hardening a metal while working on it AGE HARDENING – Also called precipitation hardening. Applicable to non-ferrous metals such as aluminum, magnesium and nickel alloys Effect of this shows a marked increase in strength and hardness for duralumin Process – Solution treatment : The alloy is heated into the single phase region held there for long and then rapidly quenched into the two phase region. This produces a super saturated solid solution, here the alloy is allowed to age at or above the room temperature for a specified time. This produces very fine precipitate particles, which increase the strength and hardness of the alloy Air Hardening – Process of hardening a metal, when it is cooled slowly in air blast. The effect of air hardening is usually seen in high speed steel and some of the tungsten alloys Hardening by heating and quenching It is common process of hardening, which generally employed for iron base alloy having a low carbon content TEMPERINGTEMPERINGTEMPERINGTEMPERING OBJECT –

1. To reduce brittleness of the hardened steel and thus to increase its ductility


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2. to remove the internal stresses caused by rapid cooling of steel 3. To make steel sufficiently tough to resist shock and fatigue

Process Process Process Process ––––

• Re-heating the hardened steel to some temperature below LCT, followed by any desired rate of cooling.

• The exact tempering temperature depends upon the purpose for which the article or tool is to be used

• The tempering temperature may be judged by the color formed on the surface of steel being tempered

• The colors are caused by surface oxidation of the steel with the formation of thin films of iron-oxide

• The baths using tempering oils may be employed for temperatures up to approx 230 deg cel

• The tempering oils are usually mineral oils having flash point to the order of 300 deg cel

Types of Tempering Process:

A. AUSTEMPERING

B. MARTEMPRING

AUSTEMPERING AUSTEMPERING AUSTEMPERING AUSTEMPERING –––– Steel is heated above UCT at about 875deg cel, where structure consists entirely of austenite It is suddenly cooled by quenching it in a salt bath or lead bath maintained at 250deg c – 525 deg cel This facilitate the transformation of austenite to binate After the transformation the steel is cooled in air. Result - Here a good impact strength is obtained and the degree of cracking is also reduced.

MARTEMPRINGMARTEMPRINGMARTEMPRINGMARTEMPRING---- Steel is heated above UCT and then cooled by quenching it in a bath kept at suitable temperature.


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So that it is in Upper Marten site range. After the temp becomes uniform throughout the steel structure, without the formation of binate, it is further cooled in air The steel is then tempered The mar-tempered steel is from internal stress It avoids cracks and wrapping , Usually caused by ordinary hardening. Used for - alloy steel NITRIDINGNITRIDINGNITRIDINGNITRIDING

Process of obtaining hard and wear resistant surface on components made from alloy steel which contain stable nitride forming elements such as aluminum, chromium, molybdenum, vanadium, tungsten. In this process the specimen is heated to a temperature of about 500deg c and held for considerable duration in an atmosphere of gaseous nitrogen. Nitrogen is produced when ammonia gas is passed through the furnace at 550deg cel., the reaction being 2NH3 2N + 3H2

• This nitrogen is readily absorbed into the surface of the steel. The holding time depends on the desired depth and size of specimens.

• Eg . the time required for case depth of 0.5-0.7mm is about 10hours

• Nitriding develops a very high hardness [ about HRC – 70 ] on the surface of steel.

• All machining operation are carried out earlier because no machining can be done after nitriding.

• Also before nitriding the components have to be heat treated properly in to order to develop core strength in them

Advantages –

1. Nitride parts are ready to be use becoz there is no further heat treatment like quenching required

2. a very high hardness and a good corrosion resistance is obtained 3. wear resistance is an outstanding characteristic of nitride cases 4. retaining hardness up to working temperature of 500deg cel where as a

carburized part starts losing the hardness at 200deg cel


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Disadvantages Long heat treatment cycles are required Specimen alloy steels to be used if maximum hardness is to be obtained Cost of ammonia atmosphere and the technical control required is very high Applications Nitriding is used extensively for aircraft parts such as cone, cylinder lines, shafts and piston rods.

CARBURISING Is the process of introducing carbon to low carbon steels in order to give it a hard surface. The surface is made hard only up to a certain depth Methods used-

1. Pack or solid carburising – Packing steel in an airtight box with carbon materials all around and heating it to a suitable temperature… holding it at that temperature for some time and cooling the same… the carbon film is formed on the surface. The time and temperature depends on the carbon film desired..

2. Gas carburising – Here the specimen is heated in an atmosphere of Methane [ CH4] gas

Major defects in a metal or alloy due to Faulty Heat Treatment:

1. Over heating 2. Burning 3. Oxidation 4. Decarburization 5. Cracks

Over heatingOver heatingOver heatingOver heating – prolong heating of a metal or alloy above a temperature marked by the DK in the iron-iron carbide phase diagrams [ called A3 line] leads to the formation of vary large actual grains, Called over heating.


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Burning Burning Burning Burning – heating a metal or an alloy to still higher temperatures near melting point for a longer time leads to burning. This leads to the formation of iron oxide inclusions along the grain boundaries. Burnt metal or alloy has a stoney fracture and such a metal or alloys is irredeemable and is rejected. Oxidation Oxidation Oxidation Oxidation –––– sometime a metal or alloy is oxidized dure to oxidizing atmosphere in the furnace. It is characterized bya thick layer of scale on the surface of a metal or alloy It can be prevented by using controlled atmosphere in the furnace or using molten salt baths. Decarburization Decarburization Decarburization Decarburization –––– it is the loss of carbon in the surface layers of the metal or alloy. Decarburization results in lower hardness and lower fatigue limit. It is caused by the oxidization of the furnace atmosphere Cracks Cracks Cracks Cracks –––– the cracks occurs in quenching when the internal tensile stresses exceed the resistance of metal or alloy to separation the tendency of a metal or alloy to crack formation increase and with carbon content, hardening temperature and cooling rate in the temperature interval of martensite transformation. It also increases with the harden ability of metal or ally another reason for crack formation is the concentration of local stresses.. Distortion and Warping Distortion and Warping Distortion and Warping Distortion and Warping ---- DistortionDistortionDistortionDistortion is the changes in the size and shape of heat treated work. This is due to the thermal and structural stresses. Warping Warping Warping Warping ---- Asymmetrical distortion of work in heat treating practice. It is usually observed in case of non –uniform heating or over heating for hardening, when the work is quenched in the wrong position and when cooling rate is too high in the temperature interval of martensite transformation results in warping. An elimination of these causes should substantially reduce warping


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INJECTION MOLDING:

The greatest quantity of plastic parts are made by injection molding.

Process:

1. the plastic compound is powdered or granular form is fed from a hopper

through melting stages and fed into the mold

2. after a brief cooling period, the mold is opened and the solidified part is

ejected.

3. in most cases it is ready for immediate use

Types;

1. reciprocating screw injection molding

2. Plunger injection molding

3. Two stage injection molding

1. Reciprocating screw injection molding:

• Plastic materials are fed from an overhead hopper through the heated

cylinder via the screw flights as the material becomes fluid

• The injection nozzle is blocked by the previous shot and this causes

the screw to pump itself backward through the cylinder

• During this step, material is being plasticated and accumulated for the

next shot.

• The volume of the plastic shot is controlled by a limit switch, which

shuts off the screw when it has reached its stroke, ending it s

backward movement

• When the press has locked. The injection phase takes place

• At this time the screw advances acting as a ram

• Simultaneously the non-return valve closes off the escape passages in

the screw making it a solid plunger moving the plastic ahead into the

mold

• When the injection stroke and holding cycle is completed the screw is

energized to return and the non-return valve opens

• Allowing the plastic to flow forward from the cylinder again, thus

repeating the cycle

• The advantages of the reciprocating screw over straight plunger

devices are that the reciprocating screw provides precise control of

filling pressure and the rotary shear effect of the screw melts and

mixes the materials homogenously.

PLUNGER INJECTION MOLDING

• This is the oldest type of molding

• It is ideally suited for molding of thermo set compounds.


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• The material in normal operation is forced by he ram over a spreader

or torpedo which causes heating and mixing and then on through the

nozzle and into the mold

• The plunger machine is modified for molding thermosetting

compounds by cooling the nozzle and barrel and eliminating the

spreader

• The changes are necessary to prevent build up which could cause the

compound to cure permanently in the barrel.

TWO STAGE INJECTION MOLDING

First stage:

The plastic is melted and mixed by using long rotating screw

When enough melt has filled the cylinder, the screw tops

The diverter valve shifts to create a flow path from the injection cylinder to

the mold

The second stage:

The melted plastic is forced out of the injection cylinder by hydraulically

driven piston plunger

The diverter valve shifts to connect the flow path from the rotating screw to

the injection cylinder, where it will be ready to transfer plastisized material

into the mold.

Advantage:

1. There is separation of two actions,

2. both the operations are performed by separate mechanisms hence there

is optimization of each action

Disadvantage

1. it is not suited to heat sensitive or shear sensitive materials, which may

tend to degrade rapidly upon reaching operation temperature.

FACTORS AFFECTING MECHANICAL PROPERTIES OF A METAL

� GRAIN SIZE � TEMPERATURE

� HEAT TREATMENT � ATMOSPHERIC EXPOSURE


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GRAIN SIZE – Metals are composed of crystal or grains. If the grain size is small, it is called

fine grained metal. Fi its large, it is called a coarse grained metal Fine grained metals –

Have a greater tensile and fatigue strength Coarse grained metals- Have greater hardenability and forgeability

Have better creep resistance. Less tough and have greater tendency to cause distortion

TEMPERATURE – This affects the mechanical properties of metals. With the decrease in

temperature the tensile and yield strength of a metal increases, But toughness and ductility decreases

In some metals copper, nickel, aluminum and austenite steel. The toughness and ductility show a gradual decrease in their values with the decrease in temperature. In some other metals such as iron there is a sudden decrease

in these properties.

HEAT TREATMENT This involves a combination of operation – heating and cooling of a metal or

an alloy in its solid state. This results in certain desirable properties in a metal

The heat treatment of a metal increases tensile strength, hardness, ductilitu and shock resistance. It improves the machinability of the metal

ATMOSPHERIC EXPOSURE At times metals are exposed to a normal atmosphere containing wet air or to

an industrial atmosphere for a considerable period. In such a case a thin oxide film is formed on the surface ot the metal.

This oxide film breaks down due to crack or discontinuity in it. As a result of this the condensed moisture on the surface of the metal absorbs carbon dioxide. Sulphur dioxide, hydrogen sulphide etc., from the atmosphere and

produces dilute acids. These acids act as electrolyte, this results in the formation of electro

chemical cell in which electron current starts flowing form the exposed metal through the electrolyte to the other regions of the metal which are still protected.

The atmospheric exposure of a metal decreases the tensile and fatigue strength of a metal

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