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116 Introduction to Basic Manufacturing Processes and Workshop Technology 7 CHAPTER PROPERTIES AND TESTING OF METALS 7.1 PROPERTIES OF METALS The important properties of an engineering material determine the utility of the material which influences quantitatively or qualitatively the response of a given material to imposed stimuli and constraints. The various engineering material properties are given as under. 1. Physical properties 2. Chemical properties 3. Thermal properties 4. Electrical properties 5. Magnetic properties 6. Optical properties, and 7. Mechanical properties These properties of the material are discussed as under. 7.1.1 Physical Properties The important physical properties of the metals are density, color, size and shape (dimensions), specific gravity, porosity, luster etc. Some of them are defined as under. 1. Density 3 Mass per unit volume is called as density. In metric system its unit is kg/mm . Because of very low density, aluminium and magnesium are preferred in aeronautic and transportation applications. 2. Color It deals the quality of light reflected from the surface of metal. 3. Size and shape Dimensions of any metal reflect the size and shape of the material. Length, width, height, depth, curvature diameter etc. determines the size. Shape specifies the rectangular, square, circular or any other section. 116Porperties and Testing of Metals 117 4. Specific Gravity Specific gravity of any metal is the ratio of the mass of a given volume of the metal to the mass of the same volume of water at a specified temperature. 5. Porosity A material is called as porous or permeable if it has pores within it. 7.1.2Chemical Properties The study of chemical properties of materials is necessary because most of the engineering materials, when they come in contact with other substances with which they can react, suffer from chemical deterioration of the surface of the metal. Some of the chemical properties of the metals are corrosion resistance, chemical composition and acidity or alkalinity. Corrosion is the gradual deterioration of material by chemical reaction with its environment. 7.1.3 Thermal Properties The study of thermal properties is essential in order to know the response of metal to thermal changes i.e. lowering or raising of temperature. Different thermal properties are thermal conductivity, thermal expansion, specific heat, melting point, thermal diffusivity. Some important properties are defined as under. Melting Point Melting point is the temperature at which a pure metal or compound changes its shape from solid to liquid. It is called as the temperature at which the liquid and solid are in equilibrium. It can also be said as the transition point between solid and liquid phases. Melting temperature depends on the nature of inter-atomic and intermolecular bonds. Therefore higher melting point is exhibited by those materials possessing stronger bonds. Covalent, ionic, metallic and molecular types of solids have decreasing order of bonding strength and melting point. Melting point of mild steel is 1500°C, of copper is 1080°C and of Aluminium is 650°C. 7.1.4Electrical Properties The various electrical properties of materials are conductivity, temperature coefficient of resistance, dielectric strength, resistivity, and thermoelectricity. These properties are defined as under. 1. Conductivity Conductivity is defined as the ability of the material to pass electric current through it easily i.e. the material which is conductive will provide an easy path for the flow of electricity through it. 2. Temperature Coefficient of Resistance It is generally termed as to specify the variation of resistivity with temperature. 3. Dielectric Strength It means insulating capacity of material at high voltage. A material having high dielectric strength can withstand for longer time for high voltage across it before it conducts the current through it.118 Introduction to Basic Manufacturing Processes and Workshop Technology 4. Resistivity It is the property of a material by which it resists the flow of electricity through it. 5. Thermoelectricity If two dissimilar metals are joined and then this junction is heated, a small voltage (in the milli-volt range) is produced, and this is known as thermoelectric effect. It is the base of the thermocouple. Thermo -couples are prepared using the properties of metals. 7.1.5Magnetic Properties Magnetic properties of materials arise from the spin of the electrons and the orbital motion of electrons around the atomic nuclei. In certain atoms, the opposite spins neutralize one another, but when there is an excess of electrons spinning in one direction, magnetic field is produced. Many materials except ferromagnetic material which can form permanent magnet, exhibit magnetic affects only when subjected to an external electro-magnetic field. Magnetic properties of materials specify many aspects of the structure and behavior of the matter. Various magnetic properties of the materials are magnetic hysteresis, coercive force and absolute permeability which are defined as under. 1. Magnetic Hysteresis Hysteresis is defined as the lagging of magnetization or induction flux density behind the magnetizing force or it is that quality of a magnetic substance due to energy is dissipated in it on reversal of its magnetism. Below Curie temperature, magnetic hysteresis is the rising temperature at which the given material ceases to be ferromagnetic, or the falling temperature at which it becomes magnetic. Almost all magnetic materials exhibit the phenomenon called hysteresis. 2. Coercive Force It is defined as the magnetizing force which is essential to neutralize completely the magnetism in an electromagnet after the value of magnetizing force becomes zero. 3. Absolute Permeability It is defined as the ratio of the flux density in a material to the magnetizing force producing that flux density. Paramagnetic materials possess permeability greater than one whereas di-magnetic materials have permeability less than one. 7.1.6 Optical Properties The main optical properties of engineering materials are refractive index, absorptivity, absorption co-efficient, reflectivity and transmissivity. Refractive index is an important optical property of metal which is defined as under. Refractive Index It is defined as the ratio of velocity of light in vacuum to the velocity of a material. It can also be termed as the ratio of sine of angle of incidence to the sine of refraction. 7.1.7Mechanical Properties Under the action of various kinds of forces, the behavior of the material is studied that measures the strength and lasting characteristic of a material in service. The mechanical properties of materials are of great industrial importance in the design of tools, machines andPorperties and Testing of Metals 119 structures. Theses properties are structure sensitive in the sense that they depend upon the crystal structure and its bonding forces, and especially upon the nature and behavior of the imperfections which- exist within the crystal itself or at the grain boundaries. The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. The main mechanical properties of the metal are strength, stiffness, elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, castability and weldability. These properties can be well understood with help of tensile test and stress strain diagram. The few important and useful mechanical properties are explained below. 1. Elasticity It is defined as the property of a material to regain its original shape after deformation when the external forces are removed. It can also be referred as the power of material to come back to its original position after deformation when the stress or load is removed. It is also called as the tensile property of the material. 2. Proportional limit It is defined as the maximum stress under which a material will maintain a perfectly uniform rate of strain to stress. Though its value is difficult to measure, yet it can be used as the important applications for building precision instruments, springs, etc. 3. Elastic limit Many metals can be put under stress slightly above the proportional limit without taking a permanent set. The greatest stress that a material can endure without taking up some permanent set is called elastic limit. Beyond this limit, the metal does not regain its original form and permanent set will occurs. 4. Yield point At a specific stress, ductile metals particularly ceases, offering resistance to tensile forces. This means, the metals flow and a relatively large permanent set takes place without a noticeable increase in load. This point is called yield point. Certain metals such as mild steel exhibit a definite yield point, in which case the yield stress is simply the stress at this point. 5. Strength Strength is defined as the ability of a material to resist the externally applied forces with breakdown or yielding. The internal resistance offered by a material to an externally applied force is called stress. The capacity of bearing load by metal and to withstand destruction under the action of external loads is known as strength. The stronger the material the greater the load it can withstand. This property of material therefore determines the ability to withstand stress without failure. Strength varies according to the type of loading. It is always possible to assess tensile, compressive, shearing and torsional strengths. The maximum stress that any material can withstand before destruction is called its ultimate strength. The tenacity of the material is its ultimate strength in tension. 6. Stiffness It is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity. A material that suffers slight or very less deformation under load has a high degree of stiffness or rigidity. For instance suspended beams of steel and aluminium may both be strong enough to carry the required load but the aluminium beam will “sag” or deflect further. That means, the steel120 Introduction to Basic Manufacturing Processes and Workshop Technology beam is stiffer or more rigid than aluminium beam. If the material behaves elastically with linear stress-strain relationship under Hooks law, its stiffness is measured by the Young’s modulus of elasticity (E). The higher is the value of the Young’s modulus, the stiffer is the material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of elasticity”; in shear, the modulus of rigidity, and this is usually 40% of the value of Young’s modulus for commonly used materials; in volumetric distortion, the bulk modulus. 7. Plasticity Plasticity is defined the mechanical property of a material which retains the deformation produced under load permanently. This property of the material is required in forging, in stamping images on coins and in ornamental work. It is the ability or tendency of material to undergo some degree of permanent deformation without its rupture or its failure. Plastic deformation takes place only after the elastic range of material has been exceeded. Such property of material is important in forming, shaping, extruding and many other hot or cold working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel is plastic at forging temperature. This property generally increases with increase in temperature of materials. 8. Ductility Ductility is termed as the property of a material enabling it to be drawn into wire with the application of tensile load. A ductile material must be strong and plastic. The ductility is usually measured by the terms, percentage elongation and percent reduction in area which is often used as empirical measures of ductility. The materials those possess more than 5% elongation are called as ductile materials. The ductile material commonly used in engineering practice in order of diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin and lead. 9. Malleability Malleability is the ability of the material to be flattened into thin sheets under applications of heavy compressive forces without cracking by hot or cold working means. 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 aluminium. Aluminium, copper, tin, lead, steel, etc. are recognized as highly malleable metals. 10. Hardness Hardness is defined as the ability of a metal to cut another metal. A harder metal can always cut or put impression to the softer metals by virtue of its 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. 11. Brittleness Brittleness is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. The materials having less than 5% elongation under loading behavior are said to be brittle materials. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron, brass and ceramics are considered as brittle material.Porperties and Testing of Metals 121 12. Creep When a metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep. 13. Formability It is the property of metals which denotes the ease in its forming in to various shapes and sizes. The different factors that affect the formability are crystal structure of metal, grain size of metal hot and cold working, alloying element present in the parent metal. Metals with smal1 grain size are suitable for shallow forming while metal with size are suitable for heavy forming. Hot working increases formability. Low carbon steel possesses good formability. 14. Castability Castability is defined as the property of metal, which indicates the ease with it can be casted into different shapes and sizes. Cast iron, aluminium and brass are possessing good castability. 15. Weldability Weldability is defined as the property of a metal which indicates the two similar or dissimilar metals are joined by fusion with or without the application of pressure and with or without the use of filler metal (welding) efficiently. Metals having weldability in the descending order are iron, steel, cast steels and stainless steels. 7.2 RECOVERY, RECRYSTALLISATION AND GRAIN GROWTH When metal is subjected to hot working and cold working processes, plastic deformation occurs which is an important phenomenon. Plastic deformation of metal distorts the crystal lattice. It breaks up the blocks of initial equiaxed grains to produce fibrous structure and increases the energy level of metal. Deformed metal, during comparison with its un-deformed state, is in non-equilibrium, thermodynamically unstable state. Therefore, spontaneous processes occur in strain-hardened metal, even at room temperature that brings it into a more stable condition. When the temperature of metal is increased, the metal attempts to approach equilibrium through three processes: (i) recovery, (ii) recrystallisation, and (iii) grain growth. Fig.7.1 reflects the recovery, recrystallisation and grain growth and the main property changes in each region. 7.2.1 Recovery When a strain-hardened metal is heated to a low temperature, the elastic distortions of the crystal lattice are reduced due to the increase in amplitude of thermal oscillation of the atoms. This heating will decrease the strength of the strain-hardened metal but there is an increase in the elastic limit and ductility of metal, though they will not react the values possessed by the initial material before strain-hardening. No changes in microstructure of metal are observed in this period. The partial restoration of the original characteristics, produced by reducing the distortion of the crystal lattice without remarkable changes in microstructure, is called recovery. At the initial state, the rate of the recovery is fastest and it drops off at longer times at given temperature. Hence the amount of recovery that occurs in a practical time increases with increasing temperature. The individual characteristic recover at different rates and gain various degrees of completion in a given cold worked metal.122 Introduction to Basic Manufacturing Processes and Workshop Technology 7.2.2 Recrystallisation Formation of new equiaxed grains in the heating process of metal, instead of the oriented fibrous structure of the deformed metal, is called recrystallisation. The process of recrystallisation is illustrated through Fig. 7.1. The first effect of heating of metal is to form new minute grains and these rapidly enlarge until further growth is restricted by grain meeting another. The original system of grains go out of the picture and the new crystallized structure is formed in the metal. Recrystallisation does not produce new structures however it produces new grains or crystals of the same structure in the metal. It consists in having the atoms of the deformed metal overcome the bonds of the distorted lattice, the formation of nuclei of equiaxed grains and subsequent growth of these grains due to transfer of atoms from deformed to un-deformed crystallites. Finer grains get refined and acquire a shape resembling fibres. The temperature at which crystallization starts, that is new grains are formed, is called recrystallisation temperature. Recrystallisation temperature is also defined as that temperature at which half of the cold worked material will recrystallise in 60 minutes. Ductility Strength Hardness Cold worked and reworked New grains Recovery Recrystallization Grain growth Fig. 7.1 Recovery, recrystalisation and grain growth 7.2.3 Grain Growth On recrystallisation of metal, the grains are smaller and somewhat regular in shape. The grains in metal will grow if the temperature is high enough or if the temperature is allowed to exceed the minimum required for recrystallisation and this growth of grain is the result of a tendency to return to more stable and larger state. It appears to depend primarily on the shape of the grain. For any temperature above the recrystallization temperature, normally there is practical maximum size at which the grains will reach equilibrium and cease to grow significantly. However, there are certain kinds of abnormal grains growth in metal that occur as a result of applied or residual gradients of strain due to non-uniform impurity distribution, and which permits growing very large single grain in metal. 7.3 TESTING OF METALS Metal testing is accomplished for the purpose of for estimating the behavior of metal under loading (tensile, compressive, shear, tortion and impact, cyclic loading etc.) of metal and for Strength Internal hardness residual Grain grow th du ctility stressPorperties and Testing of Metals 123 providing necessary data for the product designers, equipment designers, tool and die designers and system designers. The material behavior data under loading is used by designers for design calculations and determining weather a metal can meet the desired functional requirements of the designed product or part. Also, it is very important that the material shall be tested so that their mechanical properties especially their strength can be assessed and compared. Therefore the test procedure for developing standard specification of materials has to be evolved. This necessitates both destructive and non-destructive testing of materials. Destructive tests of metal include various mechanical tests such as tensile, compressive, hardness, impact, fatigue and creep testing. A standard test specimen for tensile test is shown in Fig. 7.2. Non-destructive testing includes visual examination, radiographic tests, ultrasound test, liquid penetrating test and magnetic particle testing. Shoulder Length Gauge Length R D = Diam eter R = Fillet Radius Fig. 7.2 Tensile test specimen 7.3.1 Tensile test A tensile test is carried out on standard tensile test specimen in universal testing machine. Fig. 7.3 shows a schematic set up of universal testing machine reflecting the test specimen griped between two cross heads. Fig. 7.4 shows the stress strain curve for ductile material. Fig. 7.5 shows the properties of a ductile material. Fig. 7.6 shows the stress strain curves for wrought iron and steels. Fig. 7.7 shows the stress strain curve for non ferrous material. Upper Cross Operating Hand Wheel Head D Test E Specimen B A C Strain Gauge or Extensionmeter Locking Liver Strain Column A – Lim it of proportionality Operating Hand B – Elastic limit Adjustable Wheel C – Yield point Cross Head D – Maximum stress point Square Threaded E – Breaking of fracture point Screws Fig. 7.3 Schematic universal testing machine Fig. 7.4 Stress strain curve for ductile material Stress124 Introduction to Basic Manufacturing Processes and Workshop Technology Strong Hard D E Brittle Tough Weak Soft Rigid Elastic Strain Fig. 7.5 Properties of a ductile material Tool Steel Alum inum Bronze Crucible Steel Medium Steel Hard Brass Annealed Brass Mild Steel Rolled Annealed Copper Wrought Iron Rolled Alum inium Strain Strain Fig. 7.6 Stress strain curves for wrought Fig. 7.7 Stress strain curves for non-ferrous material iron and steel 7.3.2 Compression Test Compression test is reverse of tensile test. This test can also be performed on a universal testing machine. In case of compression test, the specimen is placed bottom crossheads. After that, compressive load is applied on to the test specimen. This test is generally performed for testing brittle material such as cast iron and ceramics etc. Fig. 7.8 shows the schematic compression test set up on a universal testing machine. The following terms have been deduced using figures pertaining to tensile and compressive tests of standard test specimen. Hook’s Law Hook’s law states that when a material is loaded within elastic limit (up to proportional limit), stress is proportional to strain. Strain Strain is the ratio of change in dimension to the original dimension. Stess Stress StressPorperties and Testing of Metals 125 Column Lower Locking Cross- Lever Head Square Threaded Screw Hand Wheel Compression Plates Beds Test Piece Table Fig. 7.8 Schematic compression test set up on a universal testing machine Tensile Strain The ratio of increase in length to the original length is known as tensile strain. Compressive Strain The ratio of decrease in length to the original length is known as compressive strain. Modulus of Elasticity The ratio of tensile stress to tensile strain or compressive stress to compressive strain is called modulus of elasticity. It is denoted by E. It is also called as Young’s modulus of elasticity. E = Tensile Stress/Tensile Strain Modulus of Rigidity The ratio of sheer stress to shear strain is called modulus of rigidity. It is denoted by G. G = Shear Stress/Shear Strain Bulk Modulus The ratio of direct stress to the volumetric strain (ratio of change in volume to the original volume is known as volumetric strain) is called Bulk modulus (denoted by K). K = Direct stress/volumetric strain Linear and Lateral Strain When a body is subjected to tensile force its length increases and the diameter decreases. So when a test specimen of metal is stressed, one deformation is in the direction of force which is called linear strain and other deformation is perpendicular to the force called lateral strain.126 Introduction to Basic Manufacturing Processes and Workshop Technology Poisson’s Ratio The ratio of lateral strain to linear strain in metal is called poisson’s ratio. Its value is constant for a particular material but varies for different materials. Proof Resilience The maximum amount of energy which can be stored in an elastic limit is known as proof resilience. Modulus of Resilience The proof resilience per unit volume of a material is modulus of resilience or elastic toughness. 7.3.3 Testing of 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 of a metal may be determined by the following tests. (a) Brinell hardness test (b) Rockwell hardness test (c) Vickers hardness (also called Diamond Pyramid) test (d) Shore scleroscope Fig. 7.9 shows Rockwell hardness testing machine. 1 2 1. Indicator. 2. Indentor holder. 3 3. Indentor. 4 4. Screw. 5. Screw wheel. 7 5 6. Weight. 7. Load. 6 Fig. 7.9 Rockwell hardness testing machine 7.3.4 Testing of Impact Strength When metal is subjected to suddenly applied load or stress, it may fail. In order to assess the capacity of metal to stand sudden impacts, the impact test is employed. The impact test measures the energy necessary to fracture a standard notched bar by an impulse load and as such is an indication of the notch toughness of the material under shock loading. Izod test and the Charpy test are commonly performed for determining impact strength of materials. These methods employ same machine and yield a quantitative value of the energy required to fracture a special V notch shape metal. The most common kinds of impact test use notched specimens loaded as beams. V notch is generally used and it is get machined to standard specifications with a special milling cutter on milling machine in machine shop. The beams may be simply loaded (Charpy test) or loaded as cantilevers (Izod test). The function of thePorperties and Testing of Metals 127 V notch in metal is to ensure that the specimen will break as a result of the impact load to which it is subjected. Without the notch, many alloys would simply bend without breaking, and it would therefore be impossible to determine their ability to absorb energy. It is therefore important to observe that the blow in Charpy test is delivered at a point directly behind the notch and in the Izod test the blow is struck on the same side of the notch towards the end of the cantilever. Fig. 7.10 shows the impact testing set up arrangement for charpy test. The specimen is held in a rigid vice or support and is struck a blow by a traveling pendulum that fractures or severely deforms the notched specimen. The energy input in this case is a function of the height of fall and the weight of the pendulum used in the test setup. The energy remaining after fracture is determined from the height of rise of the pendulum Specimen due to inertia and its weight. The difference between the energy Fig. 7.10 Schematic impact input and the energy remaining represents the energy absorbed by testing machine setup the standard metal specimen. Advance testing setups of carrying out such experiments are generally equipped with scales and pendulum- actuated pointers, which provide direct readings of energy absorption. 7.3.5 Testing of Fatique Material subjected to static and cyclic loading, yield strength is the main criterion for product design. However for dynamic loading conditions, the fatigue strength or endurance limit of a material is used in main criteria used for designing of parts subjected to repeated alternating stresses over an extended period of time. Fig 7.11 shows a fatigue test set up determining the fatigue strength of material. The fatigue test determines the stresses which a sample of material of standard dimensions can safely endure for a given number of cycles. It is performed on a test specimen of standard metal having a round cross-section, loaded at two points as a rotating simple beam, and supported at its ends. The upper surface of such a standard test specimen is always in compression and the lower surface is always in tension. The maximum stress in metal always occurs at the surface, halfway along the length of the standard test specimen, where the cross section is minimum. For every full rotation of the specimen, a point in the surface originally at the top centre goes alternately from a maximum in compression to a maximum in tension and then back to the same maximum in compression. Standard test specimens are tested to failure using different loads, and the number of cycles before failure is noted for each load. The results of such tests are recorded on graphs of applied stress against the logarithm of the number of cycles to failure. The curve is known as S-N curve. Test Specim en Shut Off Switch Ball Flexible Coupling Revolution Shaft Bearing Counter Motor Shaft Weights Fig. 7.11 Schematic fatigue test setup128 Introduction to Basic Manufacturing Processes and Workshop Technology 7.3.6 Testing of Creep 2 Metal part when is subjected to a high 6 constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in 3 form of a crack which may further Gauge propagate further towards creep failure) Length 1 called creep. Creep is time dependent phenomena of metal failure at high 4 constant stress and at high temperature such subjecting of at steam turbine 2 blade. A schematic creep testing setup is shown in Fig. 7.12. Test is carried 7 out up to the failure of the test specimen. A creep curve for high temperature and long time creep is shown in Fig. 7.13. 5 The curve shows different portions of 1. Specim en 5. W eights 2. Grips 6. Therm ocouple the primary secondary and tertiary 3. Furnace 7. Instrument for creep which ends at fracture in metals. 4. Lever strain m easurem ent Fig. 7.12 Schematic creep testing setup Fracture Primary Creep Secondary Tertiary Creep Creep Instantaneous Elongation Time Fig. 7.13 Creep curve for a high temperature and long time creep test 7.4 CHOICE OF MATERIALS FOR THE ENGINEERING APPLICATONS The choice of materials for the engineering purposes depends upon the following factors: 1 Availability of the materials, 2 Properties needed for meeting the functional requirements, 3 Suitability of the materials for the working conditions in service, and 4 The cost of the materials. StrainPorperties and Testing of Metals 129 7.5 QUESTIONS 1 Classify the various properties of engineering materials. 2 Explain various physical properties of engineering materials. 3 Explain briefly thermal conductivity and thermal expansion. 4 Explain various mechanical properties of engineering materials. 5 Define various chemical properties of engineering materials. 6 Explain various electrical properties of engineering materials. 7 Define various optical properties of engineering materials. 8 Explain various magnetic, chemical and optical properties of engineering materials. 9 Write short notes on the following: (a) Elasticity (b) Plasticity (c) Fatigue (d) Creep (e) Toughness. 10 Write short notes on the following: (a) Malleability, (b) Brittleness, (c) Yield point, (d) Ductility, (e) Wear resistance and (f) Toughness. 11 Write short notes on the following: (a) Machinability, (b) Hardness, (c) Stiffness, (d) Weldabilty, (e) Formability, (f) Ductility and (g) Brittleness.130 Introduction to Basic Manufacturing Processes and Workshop Technology 8 CHAPTER HEAT TREATMENT 8.1 INTRODUCTION Heat treatment is a heating and cooling process of a metal or an alloy in the solid state with the purpose of changing their properties. It can also be said as a process of heating and cooling of ferrous metals especially various kinds of steels in which some special properties like softness, hardness, tensile-strength, toughness etc, are induced in these metals for achieving the special function objective. It consists of three main phases namely (i) heating of the metal (ii) soaking of the metal and (iii) cooling of the metal. The theory of heat treatment is based on the fact that a change takes place in the internal structure of metal by heating and cooling which induces desired properties in it. The rate of cooling is the major controlling factor. Rapid cooling the metal from above the critical range, results in hard structure. Whereas very slow cooling produces the opposite affect i.e. soft structure. In any heat treatment operation, the rate of heating and cooling is important. A hard material is difficult to shape by cutting, forming, etc. During machining in machine shop, one requires machineable properties in job piece hence the properties of the job piece may requires heat treatment such as annealing for inducing softness and machineability property in workpiece. Many types of furnaces are used for heating heat treatment purposes. The classification of such heat treatment furnaces is given as under. 8.2 HEAT TREATMENT FURNACES 8.2.1 Hearth Furnaces These furnaces are heated by fuel which may be coke, coal, gas (town, blast or natural) and fuel oil. They can also be operated electrically. They are generally of two types. (a) Stationary type It consists of four types (1) Direct fuel fired furnace (2) Indirect fuel fired furnace (3) Multiple furnace (4) Re-circulation furnace 130Heat Treatment 131 (b) Movable type It consists of two types (1) The car bottom type (2) The rotary type 8.2.2 Bath Furnaces In bath type furnaces, heating may be done using by gas, oil or electricity. These furnaces are further classified as: (1) Liquid bath type (2) Salt bath type (3) Lead bath type (4) Oil bath type 8.3 CONSTITUENTS OF IRON AND STEEL Fig. 8.1 shows micro structure of mild steel (0.2-0.3% C). White constituent in this figure is very pure iron or having very low free carbon in iron in form of ferrite and dark patches contain carbon in iron is chemically combined form known as carbide (Cementite). Cementite is very hard and brittle. Now if the dark patches of the above figure are further observed, a substance built up of alternate layer of light and dark patches is reflected in Fig. 8.2. These layers are alternatively of ferrite and cementite. This substance is called as pearlite and is made up of 87% ferrite and 13% cementite. But with increase of carbon content in steel portion of pearlite increases up to 0.8% C. The structure of steel at 0.8% C is entirely of pearlite. However if carbon content in steel is further increased as free constituent up to 1.5% C, such steel will be called as high carbon steel. The micro structure of high carbon steel is depicted in Fig. 8.3. Ferrite Cementite Crystals Areas Fig. 8.1 Micro structure of mild steel Fig. 8.2 Micro structure of Fig. 8.3 Micro structure of pearlitic eutectoid steel of high carbon steel 8.4 ALLOTROPY OF IRON In actual practice it is very difficult to trace the cooling of iron from 1600°C to ambient temperature because particular cooling rate is not known. Particular curve can be traced from temperature, time and transformation (TTT) curve. However allotropic changes observed during cooling of pure iron are depicted in Fig. 8.4. When iron is cooled from molten condition up to the solid state, the major allotropic changing occurs which are: 1539-1600°C Molten-Fe (Liquid state of iron) 1400-1539°C Delta-Fe (Body centered)132 Introduction to Basic Manufacturing Processes and Workshop Technology 910-1400°C Gamma-Fe (FCC atomic arrangement and austenite structure) 770- 910°C Beta-Fe (Body centered-nonmagnetic) Up to 770°C Alpha-Fe (BCC atomic arrangement and ferrite structure) 1700 1539°C Body Centered Delta 1500 Iron 1404°C A 4 1300 Face Centered Gamma Iron 1100 910°C A 3 900 Beta Iron 768°C A 2 700 Body Centered Alpha Iron 500 Time Fig. 8.4 Allotroic changes during cooling of pure iron (i) First changing occurs at l539°C at which formation of delta iron starts. (ii) Second changing takes place at 1404°C and where delta iron starts changes into gamma iron or austenite (FCC structure). (iii) Third changing occurs at 910°C and where gamma iron (FCC structure) starts changes into beta iron (BCC structure) in form of ferrite, leadaburite and austenite. (iv) Fourth changing takes place at 768°C and where beta iron (BCC structure) starts changes into alpha iron in form of ferrite, pearlite and cementite. Therefore, the temperature points at which such changing takes place into allotropic forms are called critical points. The critical points obtained during cooling are slightly lower than those obtained in heating. The most marked of these range commonly called the point of recalescence and point of decalescence. 8.5 TRANSFORMATION DURING HEATING AND COOLING OF STEEL When a steel specimen is heated, its temperature rises unless there is change of state or a change in structure. Fig. 8.5 shows heating and cooling curve of steel bearing different structures. Similarly, if heat is extracted, the temperature falls unless there is change in state or a change in structure. This change of structure does not occur at a constant temperature. It takes a sufficient time a range of temperature is required for the transformation. This range is known as transformation range. For example, the portion between the lower critical temperature line and the upper critical temperature line with hypo and hyper eutectoid Tem perature °CHeat Treatment 133 steels, in iron carbon equilibrium diagram. This range is also known as critical range. Over heating for too long at a high temperature may lead to excessive oxidation or decarburization of the surface. Oxidation may manifest itself in the form of piece of scale which may be driven into the surface at the work piece if it is going to be forged. If steel is heated, well above the upper critical temperature, large austenite grains form. In other words steel develops undesirable coarse grains structure if cooled slowly to room temperature and it lacks both in ductility and resistance to shock. Molten Iron Austenite Austenite 0.5% C 1505 E E 1340 D D Austenite Ferrite Pearlite Ferrite 800 C C A 721 A B B Time Time Fig. 8.5 Heating and cooling curve of steel 8.6 IRON-CARBON EQUILIBRIUM DIAGRAM Fig. 8.6 shows, the Fe-C equilibrium diagram in which various structure (obtained during heating and cooling), phases and microscopic constituents of various kinds of steel and cast iron are depicted. The main structures, significance of various lines and critical points are discussed as under. 8.6.1 Structures in Fe-C-diagram The main microscopic constituents of iron and steel are as follows: 1. Austenite 2. Ferrite 3. Cementite 4. Pearlite Temperature °C134 Introduction to Basic Manufacturing Processes and Workshop Technology δ Fe + liquid 1600 D δ iron A t 1 B 1500 Liquid J H t 2 γ Solid Solution 1400 δ-Iron crystals + Austenite t Liquidus 3 1300 Liquid + (Austenite) Solidus 1200 Cementite C t 4 F E 1130° 1100 Austenite Eutectic Solidus γ-iron 1000 Austenite Cementite Fe+Austenite G + + ( +αγ ) 900 Ledeburite Ledeburite Acm A 3 800 A 1 S P 723 K 0.025% 700 Austenite Carbon Eutectoid α-Iron + Fe Cementite 600 500 400 Cementite Cementite + + 300 Pearlite Ledeburite Fe + Pearlite Pear- + + 200 Ledeburite lite Cementite 100 Q0.8 0 12 3 44.35 6 6.7 Hypo- Hyper- eutectoid eutectoid Steel Cast Iron Carbon Percentage Fig. 8.6 Fe-C equilibrium diagram Austenite Austenite is a solid solution of free carbon (ferrite) and iron in gamma iron. On heating the steel, after upper critical temperature, the formation of structure completes into austenite which is hard, ductile and non-magnetic. It is able to dissolve large amount of carbon. It is in between the critical or transfer ranges during heating and cooling of steel. It is formed when steel contains carbon up to 1.8% at 1130°C. On cooling below 723°C, it starts transforming into pearlite and ferrite. Austenitic steels cannot be hardened by usual heat treatment methods and are non-magnetic. Temperature°CHeat Treatment 135 Ferrite Ferrite contains very little or no carbon in iron. It is the name given to pure iron crystals which are soft and ductile. The slow cooling of low carbon steel below the critical temperature produces ferrite structure. Ferrite does not harden when cooled rapidly. It is very soft and highly magnetic. Cementite Cementite is a chemical compound of carbon with iron and is known as iron carbide (Fe3C). Cast iron having 6.67% carbon is possessing complete structure of cementite. Free cementite is found in all steel containing more than 0.83% carbon. It increases with increase in carbon % as reflected in Fe-C Equilibrium diagram. It is extremely hard. The hardness and brittleness of cast iron is believed to be due to the presence of the cementite. It decreases tensile strength. This is formed when the carbon forms definite combinations with iron in form of iron carbides which are extremely hard in nature. The brittleness and hardness of cast iron is mainly controlled by the presence of cementite in it. It is magnetic below 200°C. Pearlite Pearlite is a eutectoid alloy of ferrite and cementite. It occurs particularly in medium and low carbon steels in the form of mechanical mixture of ferrite and cementite in the ratio of 87:13. Its hardness increases with the proportional of pearlite in ferrous material. Pearlite is relatively strong, hard and ductile, whilst ferrite is weak, soft and ductile. It is built up of alternate light and dark plates. These layers are alternately ferrite and cementite. When seen with the help of a microscope, the surface has appearance like pearl, hence it is called pearlite. Hard steels are mixtures of pearlite and cementite while soft steels are mixtures of ferrite and pearlite. As the carbon content increases beyond 0.2% in the temperature at which the ferrite is first rejected from austenite drop until, at or above 0.8% carbon, no free ferrite is rejected from the austenite. This steel is called eutectoid steel, and it is the pearlite structure in composition. As iron having various % of carbon (up to 6%) is heated and cooled, the following phases representing the lines will tell the about the structure of iron, how it charges. 8.6.2 Significance of Transformations Lines Line ABCD The line ABCD tells that above this line melting has been completed during heating the iron. The molten metal is purely in the liquidus form. Below this line and above line AHJECF the metal is partially solid and partially liquid. The solid metal is known as austenite. Thus the line ABCD represents temperatures at which melting is considered as completed. Beyond this line metal is totally in molten state. It is not a horizontal line the melting temperature will vary with carbon content. Line AHJECF This line tells us that metal starts melting at this temperature. This line is not horizontal and hence the melting temperatures will change with carbon content. Below this line and above line GSEC, the metal is in solid form and having austenite structure.

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