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Material Science

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Published in: Civil
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Material Science

Divya V / Bangalore

3 years of teaching experience

Qualification: M.Tech (SJCIT - 2012)

Teaches: Mathematics, Mechanical, Kannada

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  1. The knowledge of engineering materials and their properties is of great importance for a design engineer. A design engineer must be familiar with the effects which the manufacturing processes and heat treatment have on the properties of the materials. The engineering materials are mainly classified as: 1. Metals and their alloys, such as iron, steel, copper, aluminum etc. 2. Non-metals, such as glass, rubber, plastic etc. The metals may further be classified as: (a) Ferrous metals; and (b) Non-ferrous metals. The ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel. The non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc etc. The important mechanical properties of metals are as follows: 1. Strength. It is the ability of a material to resist the externally applied forces without breaking or yielding. 2. Stiffness. It is the, ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness. 3. Elasticity. It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber. 4. Plasticity. It is property of a material which retains the deformation produced under load permanently. This property of material is necessary for forgings, in stamping images on coins, and in ornamental work. 5. Ductility. It is property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material commonly used in
  2. engineering practice (in order of diminishing ductility) are mild steel, copper, aluminum, nickel, zinc, tin and lead. 6. Brittleness. It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Cast iron is a brittle material. 7. Malleability. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminum. 8. Toughness. It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of a material decreases when it is heated. This property is desirable in parts subjected to shock and impact loads. 9. Resilience. It is property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials. 10. Creep. When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines. 11. Fatigue. When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of progressive crack formations which are usually fine and microscopic size. This property is considered in designing shafts, connecting rods, springs, gears etc. 12. Hardness. It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the tes
  3. Pig iron is the crude form of iron and is used as a raw material for the production of various other ferrous metals, such as cast iron, wrought iron and steel. The pig iron is obtained by smelting iron ores in a blast furnace. The iron ores are found in various forms as shown below: As oxides (a) Magnetite (Fes 04) (c) Limonite 03 MP) Iron ore As carbonates Siderite (FeC03) As sulphides Pyrite (Fe S2) The metallic contents of these iron ores are given in the following table: Table 13.1. Iron ore Magnetite Haematite Limonite Metallic content in iron ores Metallic contents in iron ores. Colour BIXk Brown Iron content (G) 72 70 60-65 48 The haematite is widely used for the production of pig iron. Since pyrite contains only 30 to 40% iron, therefore it is not used for manufacturing pig iron.
  4. The pig iron is obtained from the iron ores in the following steps: 1. Concentration. It is the process of removing the impurities like clay, sand etc. from the iron ore by washing with water. 2. Calcination or roasting. It is the process of expelling moisture, carbon dioxide, sulphur and arsenic from the iron ore by heating in shallow kilns. 3. Smelting. It is process of reducing the ore with carbon in the presence of a flux. The smelting is carried out in a large tower called blast furnace. The blast furnace is a chimney like structure made of heavy steel plates lined inside with fire bricks to a thickness of 1.2 to 1.5 metres. It is about 30 metres high with a maximum internal diameter of 9 meters as its widest cross-section. The portion of the furnace above its widest cross-section is called stack. The top most portion of the stack is called throat through which the charge is fed into the furnace. The charge of the blast furnace consists of calcined ore (8 parts), coke (4 parts) and lime stone (1 part). The portion of the furnace, below its widest cross-section is known as bosh or the burning zone (or zone of fusion). The bosh is provided with holes for a number of water jacketed iron blowing pipes known as tuyers. The tuyers are 12 to 15 in number and are connected to bustle pipe surrounding the furnace. In the lower part of the blast furnace (called zone of fusion), the temperature is 12000 C to 13000 C. In the middle part of the blast furnace (called zone of absorption), the temperature is 8000 C to 10000C. In the upper part of the blast furnace (called zone of reduction), the temperature is 4000 C to 7000 C. At the bottom of the blast furnace, the molten iron sinks down while above this floats the fusible stage which protects the molten iron from oxidation. The molten iron thus produced is known as pig iron. The slag from the blast furnace
  5. consists of calcium, aluminum and ferrous silicates. It is used as a ballast for rail roads, mixed with tar for road making and in the cement manufacture. The pig iron from the blast furnace contains 90 to 92% of iron. The various other elements present in pig iron are carbon (1 to 5%), silicon ( 1 to 2%), manganese (1 to 2%), sulphur and phosphorus (1 to 2%). Ote : Carbon la san im ortant role in iron. It exists in iron in two forms i.e ither in a free form as ra hite or in a combined form as cementite an a •te c a oi a o an oa r stalline structure to the metal while the combined carbon makes the meta ard and ives a fine rained cr stalline structure WHAT IS CAST IRON AND ITS TYPES. The cast iron is obtained by remelting pig iron with coke and lime stone in a furnace known as cupola. It is primarily an alloy of iron and carbon. The carbon content in cast iron varies from 1.7 to 4.5%. It may be present either as free carbon (or graphite) or combined carbon (or cementite). Since the cast iron is a brittle material, therefore, it cannot be used in those parts which are subjected to shocks. The properties of cast iron which makes it a valuable material for engineering purposes are its low cost, 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 amounts of impurities such as silicon, sulphur, manganese and phosphorus. The effect of these impurities on cast iron are as follows: 1. Silicon. It may be present in cast iron up to 4%. It provides the formation of free graphite which makes the iron soft and easily machinable.
  6. 2. Sulphur. It makes the cast iron hard and brittle. It must be kept well below 0.1% for most foundry purposes. 3. Manganese. It makes the cast iron white and hard. It is often kept below 0.75%. 4. Phosphorus. It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. he im ortantt es of cast iron are as follows a Gre cast iron It is an ordinary commercial iron having 3 to 3.5% carbon. The grey colour is due to the fact that carbon is present in the form of free graphite (When filing or machining cast iron makes our hands black, then it shows that free graphite is present in it). It has a low tensile strength, high compressive strength and no ductility. It can be easily machined. According to Indian standards, grey cast iron is designated by the alphabets FG followed by a figure indicating the minimum tensile strength in MPa or N/mm2. For example 'FG 150 means grey cast iron with 150 MPa or N/mm2 as minimum tensile strength. b White cast iron It is a particular variety of cast iron having 1.75 to 2.3% carbon. The white colour is due to the fact that the carbon is in the form of carbide (known as cementite), which is the hardest constituent of iron. The white cast iron has a high tensile strength and a low compressive strength. c Chilled cast iron It is a white cast iron produced by quick cooling of molten iron. The quick cooling is generally called chilling and the iron so produced is known as chilled cast iron. d Mottled cast iron It is a product in between grey and white cast iron in composition, colour and general properties. e Malleable cast iron It is obtained from white cast iron by a suitable heat treatment process (i.e. annealing). According to Indian standard specifications, the malleable cast iron may be either whiteheart, blackheart or pearlitic and are designated by the alphabets WM, BM and PM respectively. These designations are followed by a figure indicating the minimum tensile strength in MPa or N/mm2. For example 'WM3501 denotes white heat malleable cast iron with 350 MPa as minimum tensile strength.
  7. f Nodular ors heroidal ra hite cast iron It is also called ductile cast iron or high strength cast iron. This type of cast iron is obtained by adding small amounts of magnesium (0.1 to 0.8%) to the molten grey iron just after tapping. According to Indian standard specifications, the nodular or spheroidal graphite cast iron is designated by the alphabets 'SGI followed by the figures indicating the minimum tensile strength in MPa or N/mm2 and the percentage elongation. For example, SG 400/15 means spheroidal graphite cast iron with 400 MPa as minimum tensile strength and 15 percent elongation. Allo cast iron It is produced by adding alloying elements like nickel, chromium, molybdenum, copper and vanadium in sufficient quantities. The alloy cast iron has special properties like increased strength, high wear resistance, corrosion resistance or heat resistance. ROUGHT IRO Wrought iron is the purest iron which contains 99.5% iron but may contain upto 99.9% iron. The carbon content is about 0.02%. It is a tough, malleable and ductile material. It can not stand sudden and excessive shocks. It can be easily forged or welded. Steel is an alloy of iron and carbon, with carbon content upto a maximum of 1.5%. Most of the steel produced now-a-days is plain carbon steel or simply carbon steel. It is divided into the following types depending upon the carbon content: 1. Dead mild steel 2.Low carbon mild upto steel 3. Medium carbon 4.High carbon steel steel carbon 0.15% 0.45% 0.8% steels to to to are 0.15% 0.45% 0.8% 1.5% designated Carbon Carbon Carbon Carbon in the According to Indian standards, the following order:
  8. (a)Figure indicating 100 times the average percentage of carbon content, (b) Letter'C,and (c) Figure indicating 10 times the average percentage of manganese content. The figure after multiplying shall be rounded off to the nearest integer. For example 20C8 means carbon steel containing 0.15 to 0.25% (0.2% on an average) carbon and 0.60 to 0.90% (0.75% rounded off to 0.8% on an average) manganese. he rinci al methods of manufacturin steel are as follows 1. Cementation process. The steel made by this process is cement steel because ferrite in the wrought iron is converted into cementite (i.e. iron carbide). Since carbon combines with wrought iron and has its surface covered with blisters, therefore, the steel produced by this process is known as blister steel. 2. Crucible process. The steel produced by this method is very homogeneous, free from slag and dirt and much superior to cement steel. The steel so produced is known as crucible steel. 3. Bessemer process. In a bessemer process, following are the three distinctive used convert molten steel: pig iron stages to to (a) In the first stage (known as charging position), the molten pig iron is poured into the converter. (b) In the second stage (known as blowing position), the converter is tilted to the vertical position and the air blast turned on. In this stage, the silicon and manganese burns out which is indicated by the brown smoke rising up through the mouth of the converter. After this, the carbon is next to oxidise which is indicated by a white flame. (c) In the third stage (known as pouring position), the white flame of the burning carbon drops and the contents of the converter are poured in a ladle. Now a small quantity of some alloy rich in carbon and manganese (i.e. spiegeleisen or ferro-manganese) is added to produce steel of quite good strength and ductility. Ote : The bessemer process may be acidic or basic depending upon the linin f furnace. In the acidic bessemer process, the furnace is lined with silica ricks he slag produced in this process contains large amount of silica. Sinc hos horus a i •ro cannot b removed b this rocess therefore acidi
  9. essemer process is unsuita e or pro ucing stee rom pig iron containin In basic bessemer process, also known as Thomas process, the furnace is lined with a mixture of tar and burned dolomite. This process is applicable for making steel from pig iron which contains more than 1.5% phosphorus. 4. Open hearth process. The open hearth process of steel making is sometimes called 'Siemens-Martin Process'. This process is more suitable than Bessemer process when a large quantity of mild steel, with definite quality and composition, is required. 5. Duplex process. The duplex process of steel making is a combination of acidic bessemer process and basic open hearth process. This process is in (Bihar). Iron and Steel works, Jamshedpur operation at Tata 6. L-D process (Linz-Donawitz process). It is the latest development in steel making processes and is now adopted at Rourkela steel plant where three of 40 capacity converters tonnes are working. 7. Electric process. This process is mainly used for the preparation of high quality and special alloy steels of high melting point, The electric process may be acidic or basic, but basic process is mostly used because it permits extensive elimination of impurities. The basic lined furnace of the Heroult type is especially adopted to the production of best quality carbon and alloy steels. ote: The steel contains small amounts of impurities like silicon, sulphur anganese and phosphorus. The effect of these impurities are as follows ilicon in the finished steel usually ranges from 0.05 to 0.30%. It is added i ow carbon steels to prevent them from becoming porous. It removes th ase nd Xid s blow ol and e b ake h tee toughe Sulphur occurs in steel either as iron sulphide or manganese sulphide. Iron sulphide because of its low melting point produces red shortness whereas sulphide does effect not manganese so much. Manganese serves as a valuable deoxidising and purifying agent, in steel. When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. In high speed steels, it is used to tougher the
  10. metal and to increase its critical temperature. Phosphorus makes the steel brittle, It also produces cold shortness in steel. In low carbon steels, it raises the yield point and improves the resistance to atmospheric corrosion. The sum of carbon and phosphorus usually does not exceed 0.25%. LO STEEL ENGINEERING TERI L An alloy steel may be defined as a steel to which elements other than carbon are added in sufficient amount to produce an improvement in properties. The chief alloying elements used steel follows: in are as l. Nickel. The steels containing 2 to 5% nickel improves tensile strength, raises elastic limit, imparts hardness, toughness and reduces rust formation. It is largely used for boiler plates, automobile engine parts, large forgings, crankshafts, connecting rods etc. When 25% nickel is added to steel, it results in higher strength steels with improved shock and fatigue resistance. A nickel steel alloy containing about 36% nickel is known as Invar. It has nearly zero coefficient of expansion. So it is widely used for making pendulums of clocks, precision measuring instruments etc. 2. Chromium. The addition of chromium to steel increases strength, hardness and corrosion resistance. A chrome steel containing 0.5 to 2% chromium is used for balls, rollers and races for bearings, dies, permanent magnets etc. A steel containing 3.25% nickel, 1.5% chromium and 0.25% carbon is known as nickel-chrome steel. The combination of toughening effect of nickel and the hardening effect of chromium produces a steel of high tensile strength with great resistance to shock. It is extensively used for armour plates, motor car crankshafts, axles and gears etc. 3. Vanadium. It is added in low and medium carbon steels in order to increase their yield and tensile strength properties. In combination with chromium, it produces a marked effect on the properties of steel and makes the steel extremely tough and strong. These steels are largely used for making spring steels, high speed tool steels, crankshafts etc. 4. Tungsten. The addition of tungsten raises the critical temperature of steel and hence it is used in increasing the strength of the alloyed steels at high temperature. It imparts cutting hardness and abrasion resistance properties to
  11. steel. When added to the extent of 5 to 6%, it gives the steel good magnetic properties and as such it is commonly used for magnets, in electrical instruments etc. It is usually used in conjunction with other elements. Steel containing 18% tungsten, 4% chromium, 1% vanadium and 0.7% carbon is called as tool steel or high speed steel. Since the tools made with this steel have the ability to maintain its sharp cutting edge even at elevated temperature, therefore, it is used for making high speed cutting tools such as cutters, drills, dies, broaches, reamers etc. 5. Manganese. It is added to steel in order to reduce the formation of iron sulphide by combining with sulphur. It is usually added in the form of ferro- manganese or silico-mangenese. It makes the steel hard, tough and wear resisting. Steel containing manganese varying from 10 to 14% and carbon from 1 to 1.3% form an alloy steel which is extensively hard and tough and a high resistance to abrasion. It is largely used for mining, rock crushing and railway equipment. 6. Silicon. It increases the strength and hardness of steel without lowering its ductility. Silicon steels containing from I to 2% silicon and 0.1 to 0.4% carbon have good magnetic permeability and high electrical resistance. It can withstand impact and fatigue even at elevated temperature. These steels are principally used for generators and transformers in the form of laminated cores. 7. Cobalt. It is added to high speed steel from 1 to 12%, to give red hardness by retention of hard carbides at high temperatures. It tends to decarburise steel during heat treatment. It increases hardness and strength but too much of it decreases impact resistance of steel. It also increases residual magnetism and coercive force magnetic in steel for magnets. 8. Molybdenum. A very small quantity (0.15 to 0.30%) of molybdenum is generally used with chromium and manganese (0.5 to 0.8%) to make molybdenum steel. These steels possess extra tensile strength and is used for air plane fuselage and automobile parts. It can replace tungsten in high speed steels.
  12. WHAT IS FREE CUTTING STEEL The free cutting steels (sometimes known as free machining steels) contain sulphur and phosphorus. These steels have higher sulphur content than other carbon steels. These steels are used where rapid machining and high quality surface finish after machining is the prime requirement. It may be noted that free cutting steels have low dynamic strength and are more liable to corrosion. These steels are frequently supplied in the cold drawn form and have high tensile strength and hardness but less ductile when compared to ordinary carbon steels. STAINLESS STEEL I ENGINEERING MATERIALS Stainless Steel is defined as that steel which when correctly heat treated and finished, resists oxidation and corrosive attack from most-corrosive media. The different types of stainless steels are as follows: 1. Martensitic stainless steel. The chromium steels containing 12 to 14% chromium and 0.12 to 0.35% carbon is called martensitic stainless steel, as they possess martensitic structure. These steels are magnetic and may be hardened by suitable heat treatment and the hardness obtainable depends upon the carbon content. These steels can be easily welded and machined. 2. Ferritic stainless steel. The steels containing greater amount of chromium (from 16 to 18%) and about 0.12% carbon are called ferritic stainless steels. These steels have better corrosion resistant property than martensitic stainless steels. 3. Austenitic stainless steel. The steel containing high content of both chromium and nickel are called austenitic stainless steels. The most widely used steel contains 18% chromium and 8% nickel. Such a steel is commonly known as 18/8 steel. These steels are non-magnetic and possesses greatest resistance to corrosion and good mechanical properties at elevated temperature. TRUCTURE OF SOLID Structure of Solids - All solid substances are either amorphous solids or crystalline solids. In the amorphous solids, the atoms are arranged chaotically, i.e., the atoms are not arranged in a systematic order. The common amorphous solids are wood, plastics, glass, paper, rubber etc. In crystalline solids, the atoms making up the crystals arrange themselves in a definite and
  13. orderly manner and form. All solid metals such as iron, copper, aluminium etc. are crystalline solids. The definite and orderly manner and form of atoms producing a geometrical shape in the aggregate is called space lattice or crystal lattice. A crystal is composed of unit cells. A unit cell contains the smallest number of atoms, which when taken together have all the properties of the crystals of the particular metal. The unit cells are arranged like building blocks in a crystal, i.e. they have the same orientation and their similar faces are parallel. A unit cell may also be defined as the smallest parallelopiped which could be transposed in three coordinate directions to build the space lattice. The space lattice of various substances differ in size and shape of their unit cells. According to Bravais (a scientist), there are fourteen possible types of space lattices, but the following three types are usually found in most of the metals. 1. Body centred cubic (B.C.C.) space lattice. In a unit cell of body centred cubic space lattice, there are nine atoms. The eight atoms are located at the corners of the cube and one atom at its centre, as shown in Fig. 13.1 (a). This type of lattice is found in alpha iron, tungsten, chromium manganese, molybdenum, tantalum, barium, vanadium etc. 2. Face centred cubic (F.C.C.) space lattice. In a unit cell of face centred cubic space lattice, there are fourteen atoms. The eight atoms are located at the corner of the cube and six atoms at the centres of six faces, as shown in Fig. 13.1 (b). This type of lattice is found in gamma iron, aluminium, copper, lead, silver, nickel, gold, platinum, calcium etc. 3. Close packed hexagonal (C.P.H.) space lattice. In a unit cell of close packed hexagonal space lattice, there are seventeen atoms. The twelve atoms are located at the twelve corners of the hexagonal prism, one atom at the centre of each of the two hexagonal faces and three atoms are symmetrically arranged in the body of the cell as shown in Fig. 13.1 (c). This type of lattice is found in zinc, magnesium, cobalt, cadmium, antimony, bismuth, beryllium, titanium, zirconium etc.
  14. (a) Body centred cubic (B.C.C) space Ixtice. (b) Face centredcubie (F.C.C.) space Ftg- 13.1 (c) Ctosc packed hexagonal (CPR) space lattice. IA FFECT OF GRAIN SIZE O ME H ROPERTIES OF METAL What is the effect of grain size on mechanical properties of metals? All solid metals are crystalline and the crystals or grains are made up of several atoms. The grain size has an important effect on the mechanical properties of a metal. The size of the grains depends upon a number of factors, but the principal one is the heat treatment to which the metal has been subjected. When a low carbon steel is heated, there is no change in grain size upto the *lower critical point and it is same for all steels (7230 C). At this temperature, birth of new grains takes place. At the upper critical point, the average grain size is a minimum. Further heating of the steel causes an increase in the size of the grains, which in turn governs the final size of the grains when cooled. Some steels like medium carbon steel and many alloy steels when heated to a higher temperature, known as coarsening temperature, the grain size increases very rapidly. The coarsening temperature is not a fixed temperature and may be changed by prior hot or cold working and heat treatment. *The temperature point at which the change starts on heating is called lower critical point and the temperature point where this change ends in heating is called upper critical point. It varies according to the carbon content in steel. The quenching of steel from the upper critical point results in a fine grained structure, whereas slow cooling or quenching from a higher temperature yields a coarse grained structure. The coarse grained steels are less tough and have greater tendency for distortion than those having a fine grain. A fine grained steel, in addition to being tougher, are more ductile and have less tendency to distort or crack during heat treatment
  15. HAT IS METALLOGRAPH What is Metallography? The study of internal structure of a metal or alloy, in relation to its physical and mechanical properties, under a microscope is called metallography. When the structure of a metal is seen with the naked eye or by low power magnification, then it is said to be macrography and the observed structure is macrostructure. On the other hand, when the structure of the metal is seen at high magnification, then it is said to be micrography and the observed structure is called microstructure. LLOTROPIC FORM OF PURE IRO Allotropic Forms of Pure Iron - we know that pure substances may exist in more than one crystalline form. Each such crystalline form is stable over more or less well defined limits of temperature and pressure. This is known as allotropy or polymorphism. The pure iron exists in the following three allotropic forms: (a) Alpha iron which exists from the room temperature to 9100 C. The alpha iron is ferromagnetic at room temperature. It has a body centred cubic (B.C.C.) structure. (b) Gamma iron which exists between 9100 C to 14040 C. It has a face centred cubic (F.C.C.) structure. (c) Delta iron which exists between 14040 C to 15390 C (melting point of pure iron). It has a body centred cubic (B.C.C.) structure but has longer cube edge than B.C.C. structure of alpha iron.
  16. RON-CARBON E UILIBRIUM DIAGRA A modified iron-carbon diagram is shown in Fig. 13.2. The point A (15390 C) on the diagram is the melting point of pure iron. The point E shows the solubility limit of carbon in gamma iron at 11300 C (1.7%). The iron carbon alloys containing upto 1.7% carbon are called steels and those containing over 1.7% carbon are called cast irons. The iron carbon alloys containing 4.3% carbon are called eutectic cast irons, above 4.3% carbon are termed as hyper-eutectic cast iron and those in the range of 1.7 to 4.3% carbon are called hypo-eutectic cast irons. Au:te nits •c -e:iek 'lit' • Fig. Ptimar•; % menlltö erllié Lede i•ore. 5 4
  17. We have already discussed that the temperature point at which the change starts on heating is called lower critical point and the temperature point where this change ends in heating is called upper critical point. The range between these two critical points is known as critical range. The temperature at which the change starts (i.e. lower critical point) is same for all steels (i.e. 7230 C), but the ending point of transformation (i.e. upper critical point) varies according to the carbon content in steel. It will be seen that for a steel containing 0.8% (wholly pearlite), carbon there is only one critical point. otes . The steels which contain less than 0.8% carbon are known ash o-eutectoi teels onsists which consists of ferrite entirel and earlite earlite . The steels which contain above 0.8% carbon are known ash er-eutectoi teels a it which consists co si of ementi n ear it
  18. EAT TREATMENT OF ENGINEERING MATERIAL The process of heat treatment is carried out first by heating the metal and then cooling it in the caustic soda solution, brine, water, oil or air. The purpose of heat treatment is to soften the metal, to change the grain size, to modify the structure of the material and to relieve the stresses set up in the material after hot or cold working. The various heat treatment processes commonly employed in follows: engineering practice are as 4. Annealing._ It is one of the most important process of heat treatment of steel. Following are four of annealing: types (a) Full annealing. The purpose of full annealing is to soften the metal, to refine the grain structure, to relieve the stresses and to remove trapped gases in the metal. The process consists of heating the steel 30-50 degree celcius above the upper critical temperature for hypo-eutectoid steel and by the same temperature above the lower critical temperature for hyper-eutectoid steels. It is held at this temperature for sometime and then cooled slowly in the furnace. (b) Process annealing. It is also known as low temperature annealing or sub- critical annealing. This process is used for relieving the internal stresses previously set up in the metal and for increasing the machinability of the steel. In this process, steel is heated to a temperature below or close to the lower critical temperature (generally 5500 C - 6500 C), held at this temperature for sometime and then cooled slowly. (c) Spheroidise annealing (spheroidising). It is usually applied to high carbon tool steels which are difficult to machine. The operation consists of heating the steel to a temperature slightly above the lower critical temperature (7300 C to 7700 C). It is held at this temperature for sometime and then cooled slowly to a temperature of 6000 C. The spheroidising improves the machinability of steels, but tensile lowers the hardness and strength. (d) Diffusion annealing (Homogenization). This process is mainly used for ingots and large castings. After diffusion annealing, the castings undergo full annealing to improve their properties or to refine grain structure. The process consists of heating the steel to a high temperature (11000 C — 12000 C). It is held at this temnerature for 8 to 20 hours and then cooled to 8000 C — 8500 C
  19. inside the furnace for a period of about 6 to 8 hours. It is further cooled in the air to room temperature. Normalising._ The normalising is done for the following purposes: (a) To refine the grain structure of the steel to improve machinability, tensile strength and structure of weld. (b) To remove caused by cold working strains processes. (c) To remove dislocations caused in the internal structure of the steel due to hot working. (d) To improve certain mechanical and electrical properties. The process of normalising consists of heating the steel 300C — 500C above its upper critical temperature for hypo-eutectoid steels or Acm line for hyper- eutectoid steels. It is held at this temperature for about fifteen minutes and then allowed to cool down in still air. The process of normalising is frequently applied to 3.Hardening. The castings main and objects of forgings hardening etc. are (a) To increase the hardness of the metal so that it can resist wear. (b) To enable it to cut other metals, i.e. to make it suitable for cutting tools. The process of hardening consists of heating the metal to a temperature of 300C to 500C above the upper critical point for hypo-eutectoid steels and by the same temperature above the lower critical temperature for hyper- eutectoid steels. It is held at this temperature for a considerable time and then (cooled suddenly) quenched suitable cooling medium. in a 4. Austempering. The austempering is misnomer because it is not a tempering process, but a hardening process. It is also known as isothermal quenching. In this process, the steel is heated, above the upper critical temperature, at about 8750C where the structure consists entirely of austenite. It is then suddenly cooled by quenching it in a salt bath or lead bath maintained at a temperature of about 2500C to 5250C. S.ÅLLMÅc!empeüngÅThis process is also known as stepped quenching or interrupted quenching. It consists of heating steel above the upper critical point and then quenching it in a salt bath kept at a suitable temperature.
  20. 6. Tempering._ The tempering (also known as drawing) is done for the following reasons: (a) To reduce brittleness of the hardened steel and thus to increase ductility. (b) To remove internal stresses caused by rapid cooling of steel. (c) To make steel tough shock and fatigue. resist to The tempering process consists of reheating the hardened steel to some temperature below the lower critical temperature, followed by any desired rate of cooling. 7. Surface hardening or Case hardening. In many engineering applications, it is desirable that a steel being used should have a hardened surface to resist wear and tear. At the same time, it should have soft and tough interior or core so that it is able to absorb any shocks etc. This type of treatment is applied to gears, ball bearings, railway wheels etc. The various surface or case hardening processes (a)Carburising (b)Cyaniding (c)Nitriding are as follows: (d)lnductionhardeningand (e) Flame hardening NON-FERROUS METALS AND ALLOYS Non-ferrous metals and alloys - We have already discussed that the non- ferrous metals are those which contain a metal other than iron as their chief constituent. The various non-ferrous metals used in engineering practice are aluminum, copper, lead, tin, zinc, nickel etc. and their alloys. These non-ferrous metals and their alloys are discussed, in brief, follows: as 1. Aluminum and its allo s The chief source of aluminum is a clayey mineral called bauxite which is a hydrated aluminum oxide. It is extensively used in air craft and automobile components where saving of weight is an advantage. The main aluminum alloys are as follows: (a) Duralumin. The composition of this alloy is follows: as Copper = 3.5 0.70% — 4.5% ; Manganese = 0.40 — 0.70% ; Magnesium = 0.40 and the remaining is aluminum.
  21. This alloy possesses maximum strength (about 400 MPa) after heat treatment and age hardening. After working, if the metal is allowed to age for 3 or 4 days, it will be hardened. This phenomenon is known as age hardening. (b) Y-alloy. It is also called copper-aluminum alloy. The composition of this alloy is as follows: . —1.7% ; Nickel = 1.8 — 2.3% ; silicon, Copper = 3.5 4.5% ; Manganese = 12 0.6% each and the remaining is aluminum. magnesium, iron This alloy is heat treated and age hardened like duralumin. It has better strength than duralumin at high temperature. (c) Magnalium. It is made by melting the 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. also about contains 1.75% It copper. (d) Hindalium. It is an alloy of aluminum and magnesium with a small quantity of chromium. It is produced as a rolled product in 16 gauge, mainly for anodized utensil manufacture. copper is one of the most widely used non- ferrous metal in industry. It is not found in pure state form under the earth. It occurs in some minerals such as coper glance, copper pyrites, malachite and azurite. The copper alloys are broadly classified into the following two groups: (a) Copper-zinc alloys (Brasses), in which zinc is the principal alloying metal, and (b) Copper-tin alloys (Bronzes), in which tin is the principal alloying metal. The most widely used copper-zinc alloy is brass. This is fundamentally a binary alloy of copper with zinc each 50%. There are various types of brasses, depending upon the proportion of copper and zinc. Brasses are very resistant atmospheric and be easily soldered. corrosion to can The alloys of copper and tin are usually termed as bronzes. The useful range of composition is 75 to 95% copper and 5 to 25% tin. In corrosion resistant properties, bronzes are superior to brasses. Some of the common types of bronzes are as follows:
  22. (i) Phosphor bronze. When bronze contains phosphorus, it is called phosphor bronze. Phosphorus increases the strength, ductility and soundness of castings. — 90% copper, 9-10% tin and 0.1-0.3% phosphorus. The alloy It contains 87 possesses good wearing qualities and high elasticity. It is used for bearings, worm wheels, gears, nuts, linings. It is also suitable for making springs. (ii) Silicon bronze. It contains 96% copper, 3% silicon and 1% manganese or zinc. It has good general corrosion resistance of copper combined with higher strength. It is widely used for boilers, tanks, stoves or where high strength and good corrosion resistance is required. (iii) Beryllium bronze. It is a copper base alloy containing about 97.75% copper and 2.25% beryllium. It has high yield point, high fatigue limit and excellent cold and hot corrosion resistance. It is particularly suitable material for springs, heavy duty electrical switches, cams and bushings. It has a film forming and a soft lubricating property, which makes it more suitable as a bearing metal. (iv) Manganese bronze. It contains 60% copper, 35% zinc and 5% manganese. This metal is high resistant to corrosion. Worm gears are frequently made from this bronze. (v) Aluminum bronze. It is an alloy of copper and aluminum. The aluminum bronze with 6-8% aluminum has valuable cold working properties. The 6% aluminum alloy has a fine gold colour which is used for imitation jewellery and decorative purposes. . Gun metal It is an alloy of copper, tin and zinc. It usually contains 88% copper, 10% tin and 2% zinc. This metal is also known as Admirality gun metal. The zinc is added to clean the metal and to increase its fluidity. It is extensively used for casting boiler fittings, bushes, bearings, glands etc. 4. Babbit metal A tin base alloy containing 88% tin, 8% antimony and 4% copper is called babbit metal. It is a soft material with a low coefficient of friction and has little strength. . Nickel base allo s The most important nickel base alloys are as follows: (a) Monel metal. It is an important alloy of nickel and copper. It contains 68% nickel, 29% copper and 3% other constituents. It resembles nickel in
  23. appearance and is strong, ductile and tough. It is superior to brass and bronze in corrosion resisting properties. (b) K-alloy. It consists of 3% aluminum and 0.5% titanium, in addition to the composition of monel metal. It has better mechanical properties than monel metal. (c) Inconel. It consists of 80% nickel, 14% chromium and 6% iron. This alloy has excellent mechanical properties at ordinary and elevated temperatures. It is used for making springs which have to withstand high temperatures and are exposed to corrosive action. (d) Nichrome. It consists of 65% nickel, 15% chromium and 20% iron. It is used in making electrical resistance wire for electric furnaces and heating elements. (e) Nimonic. It consists of 80% nickel and 20% chromium. It is widely used in gas turbine engines. HIGH TEMPERATURE ALLOYS What are high temperature alloys? The high temperature alloys are those alloys which can withstand temperature in excess of 1100 degree celcius. These alloys are used in components of nuclear plants, jet and rocket engines. of the high alloys follows: Some temperature are as 1. Incoloy. It is a nickel base alloy. It consists of 42% nickel, 13% chromium, 6% molybdenum, 2.4% titanium, 0.4% carbon and the remaining is iron. 2. Hastelloy. It is also a nickel base alloy. It consists of 45% nickel, 22% chromium, 9% molybdenum, 1.5% cobalt, 0.5% tungsten, 0.15% carbon and the remaining is iron. 3. Vitallium. The main constituent of this alloy is cobalt. It consists of 62% cobalt, 28% chromium, 5.5% molybdenum, 2.5% nickel, 1.7% iron and 0.28% carbon. Ote : Inconel and Nimonic as discussed above are also hi h tem eratur Iloys. ETALS FOR NUCLEAR ENERG he various metals for producing nuclear energy are used as raw materials fo oderators reflectors fuel elements fuel cannin materials control element
  24. n pressure vesse materia s. T e important meta s use z or nuc LASTICS ENGINEERING MATERIAL The plastics are synthetic materials which are moulded into shape under pressure with or without the application of heat. These can also be cast, rolled, extruded, laminated and machined. Following are the two types of plastics: (a)Thermosettingplastics, (b)Thermoplastic. and The thermosetting plastics are those which are formed into shape under heat and pressure and results in a permanently hard product. The heat first softens the material, but as additional heat and pressure is applied, it becomes hard by a chemical change known as phenol-formaldehyde (Bakelite), phenol-furfural (Durite), urea-formaldehyde (Plaskon) etc. The thermoplastic materials do not become hard with the application of heat and pressure and no chemical change occurs. They remain soft at elevated temperatures until they are hardened by cooling. These can be remelted repeatedly by successive application of heat. Some of the common thermoplastics are cellulose nitrate (Celluloid), polythene, polyvinyl acetate, polyvinyl chloride (P.V.C.) etc

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