Inspection / Testing of Materials:

• A major emphasize in any manufacturing operation is the production of high quality product and such a product implies the absence of defects that may cause poor performance or product failure.

• Care in product design, material selection, fabrication of the desired shape and as well as consideration of all possible service condition

• However it is important that we confirm that our efforts have been successful and the product is needed free from any harmful defects or flaws.

• Basically these are two basic objectives of inspection are:• The rejection of the products that fail to need the customer

requirements.• To serve as a means of maintaining the quality of workmanship

and products are used in the different machines or in the other field.

• In this regard a variety of tests can be conducted to evaluate product quality and ensure that free from any flaws, so far this there technique methods may be classified into two categories:

• Destructive Test. b. Non Destructive Test.

• Destructive Test:• Destructive test are those test by which we can check or inspect the

products by different mechanical test means we can directly calculate or measure the flaws of the products.

• All industries should have facilities to determine the mechanical properties of the products.

• Mechanical testing methods include certain procedure which required a standard type of equipments which are termite machine, hardness testing machine, impact testing machine and fatigue machine etc.

• OR In this test the material is destroyed and cannot any more be used after test. Examples are: Termite test, compression test and impact test.

Tensile test.• The tensile test is one of the most widely used of

mechanical tests.• There are many variations of this test to accommodate

the widely differing characteristics of materials such as metals and non metals.

• The tensile test on mild steel test piece is describe below.

• The tensile test carried out on a bar of uniform cross section through out the gauge length.

• The specimen is mounted in the jaws of a testing machine with which a gradually increasing load can be applied.

• The extension or elongation of the gauge length is recorded continuously and finally a graph is drawn between the load and extensions or between the stress and strain diagram.

• STRESS STRAIN DIAGRAM• In designing various parts of

machines, it is necessary to know how they material will function in service.

• For this certain characteristics or properties of the material should be known.

• The mechanical properties mostly used in material engineering practice are commonly determined.

• From a standard tensile specimen of a material and noting the corresponding values of load and elongation until the specimen fractured and also determine or calculate the values of the stress strain diagram of the material tested.

• Here a stress strain diagram for mild steel under tensile test is shown in the diagram.

• The various properties of the material are discussed.

• PROPTORIONAL LIMIT: -• It is defined as that stress at which the

stress-strain curve begins to deviate from straight line is called proportional limit.

• Now we see the diagram that from point ‘O’ to ‘A’ is a straight line, which represents that the stress is proportional to the strain.

• Beyond point ‘A’ the curve slightly deviates from the straight line. It is obvious that Hooks law hold good up to point ‘A’ and it is known as proportional limit.

• ELASTIC LIMIT: -• It is defined as the stress developed in the

material without any permanent set. It may be noted that even if the load is increased beyond point ‘A’ up to be point ‘B’ material will regain its shape a size when the load is removed.

• This means that the material has elastic properties as up to the point ‘B’. This point is known as elastic limit.

• YEILD POINT: -• It is define as on the removal of the load,

the material will not able to recover its original size and shape is called yield point.

• If the material is stressed beyond the point ‘B’, the plastic stage will reach a little consideration will show that beyond point ‘b’ the strain is increased at a faster rate with any increase in the stress until the point ‘C’ is reached at this point the material yield before the load and their appreciable strain with any increase in stress. In case of mild steel it will be seen that a small load drop to ‘D’ immediately after yielding commence. Hence there are two yield points ‘C’ and ‘D’ are called the upper and lower yield point respectively. The stress corresponding to yield point is known as yield point stress or yield limit.

• ULTIMATE STRESS POINT: -• It is defined as the largest stress

obtained by dividing the largest value of the load reached in a test to the original cross sectional area of the test specimen.

• At point ‘D’ the specimen regain some strength and higher values of stresses are required for higher strain then those b/w ‘A’ and ‘D’. The stress goes on increasing till the point ‘E’ is reached. The gradual increase in strain length of the specimen is the followed with information reduction of its cross sectional area. The work done during stretching the specimen is transform largely into heat and the specimen becomes hot. At ‘E’ the stress, which attain its maximum value, is known as ultimate stress point.

• BREAKING STRESS POINT OR NECKING: -

• After the specimen has reached the ultimate stress, an neck is form, which decrease the cross-sectional area of specimen as shown in fig:. A little consideration shows that the stress necessary to break away the specimen is less then maximum stress. The stress is therefore reducing until the specimen breaks away at point ‘F’. The stress corresponding to point ‘F’ is known as breaking stress or necking.

• PERCENTAGE REDUCTION IN AREA: -

• It is the difference b/w the original cross-sectional area and cross-sectional area at neck. This difference is expressed as percentage of original cross-sectional area so let.

• A = Original cross sectional.• Af = Cross sectional area at neck.• A = a - A/A *100• A = A0 – Af/A0 *100

• Percentage elongation: -• It is the percentage

increase in the standard gauge length either original length obtained by measuring he fractured specimen after bring the broken parts together.

• Lo = Original length of the specimen.

• Lf = Final length of specimen after fracture.

• L = Lf – Lo/Lo *100.

Hardness test. • Definition. It is defined as it is the ability of the

materials which resist to easy penetration, wear, scratch and cutting.

• The hardness can be determined by following tests.

• 01, Brinell tester 02, Rockwell tester.• Brinell hardness tester. In this test a standard

hardened steel ball is pressed in to the surface of the specimen by a gradually applied load which is maintained on the specimen for definite time.

• The impression so obtained is measured by a microscope and the Brinell hardness number (B.H.N) is found out by the following equation.

• B.H.N = load/area of impression/indentation.

• = 2P/pi D (D- under root D2-d2)

• Where, P = load (Kg)., D = diameter of ball (mm) and

• d = diameter of indentation (mm).

• The test is carried with a hardened steel ball of 10mm diameter.

• A 3000Kg load is used for testing of ferrous metals and their alloys, when non ferrous metals and their alloys are tested a 500/700Kg load with 5mm diameter steel ball.

• The time of load is specified between 10 and 30 seconds after applying the load, depending upon the metals and alloys being examined.

• Hardness tester. A Brinell hardness tester is shown in the figure, the hardness test is carried out as follows.

• The test piece is placed on the top of the elevating screw and the screw is raised.

• As the screw moves up the piece touch with the ball.

• At this time apply the required load required time and wait till the required time completed.

• this period indenter moves to During the position of the test piece and makes indentation/ impression.

• After the specified time finished remove the load and take off the specimen from the top of elevating screw.

• Now measure the diameter of impression with the help of microscope and note that.

• Lastly applying the equation and calculate the B.H.N.

• The unit of hardness is number.• Precautions. The test should be

performed on smooth, flat specimen from which dirt and rust have been cleaned.

• The test should not be made on the specimens so thin that the impression shows through the metal nor should impression be made too close to the edge of specimen.

• Limitations. It can not be used on very soft metals/materials.

• It should not be used on materials less than 2.54mm thick.

• The test should be carried out/conducted on a location far enough removed from the edge of the material so no bulging results.

• Rockwell hardness test. • The Rockwell hardness test is probably the most

widely used method of hardness testing.• Rockwell testers use much smaller penetrators

/indenters and loads than does the brinell tester.• Different sizes of hard ball in diameter are available

as well as a diamond cone shape.• For metallic material testing the 1/16 inch ball and

diamond penetrator are used most commonly.• Working principle/construction of tester. The

penetrator chuck is mechanically connected to dial indicator which responds to vertical motion of the penetrator.

• Since the pentrators are small the specimen should be ground smoothly and well cleane.

• The specimen is placed on the anvil of the machine and the penetrator seated by mean of minor 10Kg load.

• The dial indicator is zeroed and then a major load of 100, 150Kg applied. Forcing the penetrator in to the specimen.

• Upon removal of a major load, the indented specimen recovers slightly and the final depth of penetration is registered directly on the dial indicator as a hardness number.

• In this way get at least three reading and find mean.

• In this test the two commonest scales are the HRB and the HRC scales respectively standing for the 1/16 inch with 100Kg load and diamond cone indenter with 150 as a major load. In general very hard materials are tested with diamond cone indenter.

• For example mild steel might have a HRB reading of 90, hardened alloy steel might have a HRC of 55.

• These are stated as HRB 90 and HRC 55.

• Precautions. • Successive impressions should not be superimposed on

one an other nor be made too close together when making hardness determination.

• Nor should a measurement be make too close to the edges of specimen or on a specimen so thin the impression comes through the other side.

• The care required in preparing the surface is greater for Rockwell than Brinell test because of smaller impressions.

• The surface of specimen should flat and free from rust and dust also from un even.

• Since impression is small it is desirable to take several readings in order to get a representative value of hardness.

• Advantages. • It is more flexible than the Brinell a large number of

combinations of indenters and load make it more use full to test a wider range of materials.

• Rockwell hardness measurement can be made quickly as because they are read directly from the instrument scale.

• The test considered to be non destructive for most applications because of small size of the impression.

• Limitations. The Rockwell test is limited by greater care required in preparation of sample.

Impact Test.• Significance of impact test. An impact test

signifies toughness of material that is ability of material to absorb energy during plastic deformation.

• This important factor is determined by impact test. Toughness takes in to account both strength and ductility of a material.

• several engineering materials have to with stand impact or suddenly applied load while in service.

• Impact strength are generally lower as compared to strength achieved under slowly applied load.

• All types of impact tests the notched bar tests are extensively used.

• Therefore the impact test measure the energy necessary to fracture of a standard notch bar by applying the load.

• Still it is important to note that it provides a good way of comparing toughness of various materials or toughness of same material under different conditions.

• Impact test. It is a pendulum type impact testing machine is generally used for conducting notched bar impact. The following type of impact test are performed on this machine.

• IZod test. Charpy test.• Charpy test. This test is more common

than IZod test and it uses simply supported test piece of 10*10mm section.

• The specimen is placed on support or anvil so that the blow striker is opposite to notch.

• The energy used in rupturing the specimen in both charpy and Izod test is calculated as follows.

• E = PDL (Cos Beta - Cos Alpha).

• Where p is weight of pendulum , D is distance from striker to specimen., L is a length of pendulum, alpha is angle of fall and beta is a angle of rise respectively. Actually the values of PDL are constant which are 26.72Kg, 0.684m, 0.75m respectively.

• FATIGUE STRENGTH: -• It is the behavior of material under repeated

load condition is called fatigue. • If the material becomes fail under such a load

is called fatigue failure. • The maximum load that a material can with

stand without failure during a large number of reversible of load is called fatigue strength.

• For example:- A rotating shaft which support a weight a tensile force (shear) on the top portion of the shaft and compressive force on the bottom as the shaft is rotated there is a repeated change in tensile and compressive stresses.

• This property is desire in the aircrafts wings and other structures subjected to rapidly fluctuating loads.

• Fatigue is an important form (property) of behavior in all materials including metal, plastics and concert.

• All rotating parts e.g. axes, Crain shafts are subjected to alternating stresses under repeated load are said to under fatigue load conditions.

• MECHANISM OF FATIGUE:-• A fatigue fracture always start as a small crack which

under repeated stresses, grows in sides, as the cracks expends the load carrying cross section of the metal component is reduce with the result that the section rises. Ultimately a point is reach where the remaining cross section is not longer enough to carrying the load and finally the result is the fracture. Most cracks that are responsible for fatigue failure starts at visible discontinuity such as design and other details for example: Holes, inclusions, below holes, fabrication cracks etc.

• The ‘SN CURV’ may determine the fatigue strength of the materials. In a fatigue test the value of stress at which the metal failed is called the fatigue strength. But in practice the component are never design to develop the value of stress which is equal to fatigue strength, by employing factor of safety in design. A limiting stress calculated from the endurance limit is always taken in to account. The endurance is determinant for particular number of cycles of stress.

• A suitable basis for design involving static loads is the yield strength of the material but if it is a fatigue loading in which load is repeatedly applied and removed, the design criterion is the endurance limit.

• The endurance strength is the stress, as that will cause a metal to fail, a large number of application of that stress. The fatigue test helps to estimating endurance strength/endurance limit.

• As we know that the maximum value of tensile, compressive stress is usually lower then the yield strength of the material. The test is continued either the cycles of stress are applied until the specimen fails are until limiting number of cycles has been reached. For example: if a soft steel specimen is able bear five lake cycles with out failure the test may be stopped because it has been proved experimentally that the specimen will be able to with stand under same stress is very large number of cycles a number of specimen of the same material are fatigue tested under a different stress levels and the results are plastid on a graph paper with the stress ‘S’ on the y-axis and the number of cycles ‘N’ to cause failure of specimen on X- axis. Finally the result is a S-N curve (fatigue curve).

• CREEP: -• Creep may be defined as “the process by which

plastic flow occurs when a constant stress is applied to a metal for a prolong period of time. A viscous flow in metal involving applied stress, time and temperature is called creep. Creep can takes place and need to fracture at static stress much smaller then those, which will break the specimen when loaded quickly.

• Probably the first reorganization given to industrial importance of creek was by ‘Dickinson’ in 1922. When published his researches on the creep resistance of structural steels and alloys steel members in furnace.

• Under many service condition, material are required to sustain steady load for long periods of time an under different temperature conditions.

• For example: Furnace parts, blade of turbine rotors and filament in vacuum tubes.

• Under these conditions material may continue to deform until its usefulness is seriously impaired. Such time dependent deformations can grow large and resulting fracture of the member without any increasing the load. This time dependent strain occurring under stress is known as creep. The creep test result is useful in the design of machine part, which is exposed, to elevated temperature.

• CREEP TEST: -• The creep test determents the continuing change

in the deformation of a material as elevated temperature. When stressed below the yield point. The objective of creep test is to determinant the creep limit or the limiting creep stress, defined as the stress that will just not break the specimen when applied for an infinite period of time at a specific constant temperature. This value of stress may be obtained from strain/ time observations. A creep test is simply a tension test run at constant load and constant temperature. A creep test is required to measure and record stress, strain, temperature and time for the entire duration of the test and result is a creep curve.

• The above fig: shows a system for testing of creep.

• 1: Specimen for creep test is usually the same as for conventional tension test.

• 2: The specimens are spot-welded, one platinum wire and one platinum tube. The wire slides inside the tube therefore marks on both are absorbed through a single telescope at the middle, elongation can be measured on a scale inside the telescope.

• 3: The specimen is subjected to constant loading a constant stress through a system of dead weight and fit around the specimen; elongated ones of the specimen may have a thermo couple in each end for temperature measurement purposes.

• Non-Destructive Test:• A component does not break in nondestructive and even

after being tested so, it can be used for the purpose which it was made; it is an examination of a component in any manner which will not impair its future use. Although non destructive tests do not provide direct measurement like mechanical properties, yet then are extremely useful in revealing defects, the ability to detect the invisible subsurface defects in components that could impair their performance when put in service, to give aids in maintaining high quality standards, but provides a valuable help to the development of manufacturing methods. That is why nondestructive tests make components more reliable, safe and economical.Various non-destructive tests commonly used are given as follows:

• i. Radiography inspection / test ii. Magnetic particle inspection • iii. Penetrate testing iv. Ultrasonic inspection

• i. Radiography:• Radiography is a shadow picture

of a metal more or less transparent to radiation such as x-rays or gamma rays.

• The process of obtaining a radio graphic is known as radiography or radio graphic inspection. Radio graphical inspection techniques depend upon the use of x-rays and gamma rays.

• Various metals posses different absorption values of radiation absorption of radiation by a metal (material) increases with its atomic number, its density and also the ware length of the x-ray beam used consequently a sound metal will absorb more radiation than an unsound metal.

• It is also possible to locate regions in a particular metal which differ appreciably in density from the surrounding regions.

• These differences are the basis for radio graphic inspection techniques.

• The use of x-ray and gamma ray radiography in inspecting the defects such as blow holes, cracks, shrinkage cavities and slag inclusion, basically these types of defects may occur in the different manufacturing process like welding forging but mostly they occur in castings.

• These defects are of special importance in components designed to with stand high temperatures and pressures are employed in power plants, atomic reactors, chemical and pressure vessels and oil refining equipments because they (i.e. defects) cause stress concentration which may frequently lead to part failure.

• Principle of test:• Radiography technique is based

upon exposing the components to short wave length radiations in the form of x-rays wave length for this x-ray machine is used to generate x-rays.

• Various types and sizes of machine are commercially available in these days, but generally are not used for site work, but are widely used under laboratory or test department conditions.

• Procedure:• x-rays are produced when a target

is bombarded by a fast moving stream of electrons, for the production of electron high voltage discharge tubes are used, when the electrons are suddenly stopped by the target, their kinetic energy is converted to energy of radiation known as x-rays.

• The portion of the component of which defects are suspected is exposed to x-rays emitted from the x-ray tube; a cassette containing x-ray film is placed behind and in contact with the component (being tester) perpendicular to the rays.

• During exposure, x-rays penetrate the casting and thus affect the x-ray film, since most defects (such as blow holes, porosity and cracks) posses lesser density than the sound metal of the component, they transmit x-rays better than the sound metal does, there fore the film appears to be more darks, where defects are in line of x-ray beam.

• The exposed and developed x-ray film showing light and dark areas is termed as Radio graph or precisely known as an x-graph.

• Precautions in Radiography:

• Safety precautions and adequate precautions are extremely important while working with x-rays or radioactive materials over exposure of the local area of the human body to the rays may result in skin burns and ulcer over exposure of the entire human body will cause severe and mine, leukemia, and sterility.

• Thus proper shielding of all radio actives.

• ii. Magnetic Particle inspection:• This method of NDT tends to

supplement rather than displace radiography.

• For example: radiography ordinarily cannot detect small cracks, especially when they are too small to be seen with the human eye. Mostly this method of inspection used and magnetic ferrous components for detecting invisible surface or slightly sub surface defects principally of iron and steel.

• The defects commonly revealed by magnetic particle inspection are quenching cracks, thermal cracks, grinding cracks, non metallic inclusions, fatigue cracks, and hot tears etc. magnetic particle inspection (MPI) is a relatively simple and easy technique it is almost free from any restriction of size, shape, composition and heat treatment of a ferromagnetic specimen.

• Procedure to detect the cracks:

• This method is used for iron and steel and their alloys which show magnetic properties.

• The components (like made from casting, welding and other processes) are first magnetized and them iron particles are sprinkled all over the path of the magnetic field.

• The particles align themselves in the direction of the lines of force their distribution is also in proportion to the strength of the magnetic field in the case of a faultless component, particles will be distributed uniformly all over surface where as if a defect exists the iron particles will jumble round the defect.

• The reason is that a discontinuity in the component causes the lines of force to by pass the discontinuity and to concentrate around the extremities of the defects.

• In this way the magnetically held collection of particles from an out line of the discontinuity and generally indicates its location, size, shape and extent.

• Generally a component being tested can be magnetized by passing an electric current through it for producing lines of flux.

• The current may be either alternates current or direct. An alternating current is used when high surface sensitively is designed and the direct current is preferred where defects are to be located beneath the surface.

• Other methods for magnetizing casting include positioning the casting between two magnetic poles or placing the casting components in a coil carrying a direct current.

• From particles may be applied either dry with a hand shaker a bulb blower or in wet from by spraying or pouring over the surface when wet, the particles are carried in suspension from in liquid, for instance kerosene, carbon tetra chloride.

• Dry particles are most sensitive fro use on very rough surfaces such as weldments and casting and for detecting sub surface defects in this method large casting and forgings long tubular parts such as tubing, pipes, hollow shafts, long solid parts such as billets, bars, shafts, cracks of welds, inclusions etc are examined.

• Requirement of magnetic particle inspection:

– The flaw must be perpendicular to the lines of magnetic flux.

– The flaw must be near the surface.

– The flaw must have a lower magnetic permeability than the metal.

– Only ferromagnetic material can be tested, ferromagnetic materials include most of the iron, nicked, and cobalt alloys.

– Non ferrous magnetic materials cannot be inspected by this method.

– Prior to making the test part should be free oil, grease, dirt etc.

– After inspection the part should be demagnetized.

• iii. Liquid penetrant inspection:• This type of testing helps to detect small

cracks, seams, porosity, cold shut the surfaces of the components etc. it can be effectively used not only in inspection of ferrous metals but specially useful for non ferrous metals as well non porous, non metallic materials such as ceramics, plastics and glass etc. this method is simple and can be applied to all metals which are manufactured or fabricated by different methods, this inspection involves the following five steps.

• 1. Surface preparation:• The success of any dye-penetrant

inspection is greatly dependent upon the cleanliness of the surface being examines.

• All surfaces of a work piece must be thoroughly cleaned a completely dried before it is subjected to liquid penetrant inspection.

• It should be dry and free from rust, welding flux, grease paint, oily film and dirt , this surface will free for at least 1” beyond the area being inspected for this detergent cleaners, solvent, abrasive blasting, ultrasonic cleaning may be employ for cleaning this parts prior to examination.

• 2. Penetration:• After the work piece has been cleaned,

liquid penetrant is drawn into flaw by capillary action or may be applied in a suitable manner so as to form a film of the penetrant over the surface for at least ½” beyond the areas being inspected. The penetrant may be applied by spar, brushing or duping the penetrant should cover the part completely with a thin coating of liquid penetration time usually ranges from 5-60 minutes depending upon the type of penetrant type of material being inspected (Note: mostly the oil dye type penetrant and fluorescent dye type penetrant (liquid are used)).

• 3. Removal of excess penetrant: • The excess penetrant is removing from the

surface it can be simply wiped of for washable with water or some suitable solvent may be used but area must be avoided.

• 4. Development:• A developing agent is sprayed on to the

surface so, that it forms a film over the surface. The developer acts as a blotter to assist the natural sea page of the penetrant out of surface and to spread it at the edges and in this way it also magnifies the apparent width of the flaw.

• 5. Inspection:• After being sufficiently developed, the

surface is visually examined for indication of penetrant.

• Actually the penetrant migrates from the discontinuity such as cracks, reacts with the developer and causes a color change which identifies the crack location.

• Visible dye penetrant inspection is performed in a good white light. When specially developed fluorescent penetrant used, inspection is performed in a suitable darkened area using black (ultra violet) light which causes the penetrant to emit visible light.

• Applications: • Besides locating cracks and shrinkage in

ferrous and especially non ferrous castings.• Fluorescent penetrant inspection is used to

determine cracks in the fabrication and regrinding of carbide tools, cracks and pits is welded structures.

• Cracks in steam and gas turbine balding and cracks in ceramics insulators for spark plug and electronic application also method is useful for plastics and glass etc.

• Heat treatment of material:• It is the process of changing the structure of a metal by heating it to

a pre determined temperature holding it at this temperature for a prescribe period to time and cooling it at a prescribe rate in the solid state to induce certain desired properties into them.

• It is an important operation in the manufacturing process of machine ports and tools. In any heat treatment operation the rate of heating is important while heating the steel.

• The flows from the exterior to interior of the steel at a definite rate. • If the steel is heated too fast, the exterior becomes hotter then the

interior and uniform structure cannot be obtained.• If the piece is irregular in shape it should be heated at a slow rate to

eliminate wrapping and cracking too. • Heavy section should be heated for a longer time to achieve a

uniform result.

• Objectives of heat treatment:• Heat treatment is generally employed fro the following

purposes.– To produce hard surface on a ductile interior.– To improve machine ability.– To relive interior stress.– To increase resistance to wear, heat and corrosion.– To improve mechanical properties like tensile strength, hardness, and

toughness and shock resistance.– To change the chemical composition.

However the heat treatment is the controlled heating and cooling of metals for the purpose of uttering their properties and can perform this function without change in product shape, because all the properties can be ultras by this method, it is one of the most important and widely used in manufacturing process.

Many of the processing heat treatment are applied to plane C-steel of low alloy steel and also may be applied for heavy machine cold working, and also after cold working.

• Heat Treatment operation:• There are so many operations of heat treatment

but only main are discussed below.• Annealing:• It is one of the most widely used processes in the

heat treatment of iron and steel. According to ASTM annealing is defined as a softening process consisting of heating the steel to a temperature at or near the critical pout holding these for a proper time and then alloying it to cool slowly in the furnace it self. The temperature required for annealing various with different steel.

• For Example: Suppose we have a steel which contain less than 0.77% of carbon its heat 200C above the upper critical temperature (actually the critical temperature for set steel is 6500C it means this may be heated up to 6700C while the hyper eutectoid steel greater then 0.77% is heated to 200C above the critical temperature means approximately at 6300C.)

• If the high C-steel which contains more the 0.9% carbon is heated to temperature with in critical range means up to 6500C.

• Mostly the articles for annealing are kept in the furnace for half to one an half an hour and allow them to cool in the furnace it self the cooling rate may be 30-1500C per hour.

• Merits:• The purpose of annealing may be inducing softness to

improve the ductility and to improve the machine ability.• Demerits:• Actually this process is costly generally it is applied on

small articles made of plane-carbon steel.

• Normalizing:• It is one of the super processes of heat

treatment. According to ASTM it is defined as the process consisting of heating the steel at 40-500C above the critical point holding therefore a special period and then alloying it to cool in air at cool temperature this process is principally used with low and medium carbon as well as alloy steel to make the great structure more uniform to relief more internal stresses and to induced certain desired physical properties most commercial steels are normalized after being rolled or casting, mostly low-c-steel is usually normalized to refined grain size and the toughness, tensile strength and ductility.

• Normalizing is cheap process; it is generally carried out on large casting and forging. It is frequently applied as the final heat treatment process an item which are subject some stresses. In normalizing the normal weight of cooling in air are about 50 – 1200C / minimum depending upon the section, sized and air condition. The main purpose in case of welded steel it improved the structure of weld.

• In some cases due to this process the ductility was reduce mostly this process is applied for low and medium c-steel to increase their strength.

• Hardening:• According to ASTM hardening is defined as heat treatment process

in which the steel is heated to a temperature with in or above its critical range, held at this temperature for a considerable time to insure through penetration of the temperature inside he component and then allowed to cool by quenching, in water, or oil.

• Mostly this process is applied on the parts which are subjected to hardening or brittle poor in ductility and toughness. Poor impact strength etc, as the hardness of steel is due to carbon content only the hardening process is carried out only high c-steel. It is also applied on tools and structural steel, it also depends upon the quenching rate and work size but steel with low-c-content will not response appreciably.

• Main objectives of hardening:• To harden the steel to resist wear.• To an able the steel to cut other steel

• Tempering:• The harden steel is brittle and unsuitable for most uses the

hardness and brittleness may be reduced by tempering. According to ASTM the tempering defined as the process of reheating the harden steel to same temperature below the critical range followed by any rate of cooling. In general over the brought range of tempering temperature the hardness decreases and toughness is increases as the tempering temperature is increased. The temping temperatures are determined by the specification of steel and the final hardness and toughness desired. The tempering is an essential operation after hardening to modify the properties of the harden steel for the purpose of increasing its usefulness these properties are modified by the different temperature to the different type of steel and also due to the quenching. In this regard tempering is dividing into three classes according to the usefulness of steel required.

• The following brief details will acts as a guide to the normal tempering temperature to which it may be desirable to work are given below.

• Objectives of tempering:• To reduce the hardness, brittleness and to

increase the tensile strength.• To increase the ductility and toughness etc.• To release the internal stresses.• Classification of tempering:• 1. Low Temperature Tempering:• In this process the harden steel is heated

up to 2000C. The process is generally applied on the cutting and measuring tools of low carbon and low alloy steels and the parts which are surface harden and ease carburized.

• 2. Medium Temperature Tempering:• In this process the steel is heated from 200-

3500C. This process gives the highest elastic limit with toughness it is applied an oils and laminated springs.

• 3. High Temperature Tempering:• In this process the steel is heated from 530-

5500C. This process eliminates completely the internal Stress; also it provides the most favorable ratio of strength and toughness it is applied on structural steels.

• THE END