metallurgical test

7/27/2019 Metallurgical Test

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Chapter 6

Metallurgical Tests

NON-DESTRUCTIVE TESTING (N.D.T)

Non-destructive test are carried out on components, not test pieces. They are used todetect flaws or imperfections during manufacture or those that develop during service.The test give no indication of mechanical properties.

Visual inspection for surface defects is assisted of hairline cracks. Where internalflaw suspected, use is made of X-rays or ultrasonic testing. There are special devicesfor examination of machine finish.

LIQUID PENETRANT METHODS

One type of test uses a low viscosity liquid containing a fluorescent dye. The area to be

tested is sprayed or soaked and after a time lapse, to allow for penetration by capillary

action, is wiped dry. When view under ultraviolet light, any faults will be shown up by the

glow of the penetration in them.

Fig. 57 Crack Test with Liquid Penetrant


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Another test uses a penetrant containing a powerful dye. This is sprayed on thesuspect area with an aerosol. After allowing time for penetration, the area is wiped cleanand cover with a liquid which dries to leave a chalky sediment(developer). Thepenetrant stains the developer along the line of the crack.

These methods are based on the old chalk and paraffin tests but the penetrantcan have a hydrocarbon or alcohol based. Some are emulsiable for removal by waterspray, others can be cleaned off with solvents to reduce possible fire risk.

MAGNETIC CRACK DETECTIONThis type of test is suitable only for materials which can be magnetized (can not be usedfor austenitic steels or non-ferrous materials). After the test the component is normallyde-magnetized.

Fig. 58 Magnetic Crack Test

A magnetic field is produced in the component by means of an electric current orpermanent magnet (Fig. 58) and magnetic particles are spread on the surface. Cracksare revealed by a line of magnetic particles.

The powder used, may be black iron oxide held in suspension in thin oil. It ispoured on the surface, the surplus being collected in a tray beneath. Coloured magneticinks in aerosols are also available and the dry method makes use of powder only andthis is dusted on the surface. Powder tends to collect at a crack in the same way asiron fillings will stick to the junction of two bar magnets, placed end to end with oppositepoles together.

RADIO GRAPHIC INSPECTION

X-rays and gamma rays are used for inspection of welds, casting, forgings etc. faults inthe material affect the intensity of rays passing through the material. Film exposed bythe rays gives a shadow photograph when developed.

There is a requirement for radiographic examination of many welds, particularlythose in pressure vessels. The sketch shows the method of examining longitudinal andcircumferential welds. (Fig. 59).

Defects such as porosity, slag inclusions, lack of fusion, poor penetration,


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Fig. 59 Radiographic Examinations of Welds

Fig. 60 X-ray Machine

cracks and undercutting are shown on the film. Limits are placed on the extent of defect,by the different inspecting bodies. The requirements ma be give in booklets containing

diagrams, charts and typical films.An X-ray machine works on similar principle to the thermionic valve. The

attracted by the positive anode (Fig. 60). The energy of electrons striking the tungstentarget as they are accelerated across by the g=high voltage, is mostly converted to heat,which is absorbed by the chopper and removed by the coolant. About 1 percent ofelectrons are defected as X-rays through the side of the tube.

A transformer is used to obtain the high voltage. In general, voltage ranges form200 kV up to 400 kV. Increases of voltage produces rays of shorter wavelength whichhave greater penetration. Wavelengths are below those of light, radio or heat in theorder of 10 to 10cm. they may be referred to in angstrom units (1A= 10cm).


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intensity of rays is measured in roentgen (r). This unit is defined as the quantity of rayenergy which is passing through 1cc of dry air at 0C and pressure of 1 atmosphere,releases by ionization a quantity of electricity equal to one electrostatic unit.

The alternating current supplied by the transformer is rectified because the

electrons will only move to the tungsten target when it is anodic to the filament. Thecurrent flow is measured is mA and gives an indication of ray intensity.Gamma rays are produced by a radioactive source such as cobalt 60, iridium 192

or caesium 137. They are an alternative to x-rays. They have a shorter

Fig. 61 Gamma Ray Source

Wavelength and are spontaneously emitted by decay of the source. Each source has anassociated wavelength and a different rate of decay which gives a loss of intensity withage. Rate of decay is given as the time to halve the radioactivity and is known as thehalf life. Loss of intensity must be considered when calculating exposure time, whichvaries also with the different thicknesses and densities of materials.

The sealed capsule for the gamma ray source (Fig. 61), protects personal fromharmful radiation and the radioactive source from damage in transit. The gamma raysource is easily portable for use on site. Neither an electrical supply nor cooling arerequired.

Films for radiographic examination provide a permanent record of quality of weldetc., and must be indentified by the serial numbers or other locating marks. Imagequality indicators are placed on or adjacent to welds. There are various types and thewire type (Fig.59) consists of a number of two wires mounted in a material of low rayabsorption. It is placed with the wires across the weld. Sensitivity is given by thepercentage ratio of diameter of thinnest wire visible, to maximum weld thickness. Thelower the figure, the higher the higher the sensitivity. 2 percent is required by the D.Tp.Obviously faults of the size of the thinnest wire that can be seen on the film will also bevisible.


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Radiographs are viewed by radiologist on a uniformly illuminated diffusingscreen. Training is necessary for interpretations of film both with regard to the faults inthe part being examined and misleading marks that sometimes appear on film.

A skilled radiographer is required for obtaining photographs. Exposure times forgamma rays vary with the type of material, its thickness and intensity of the rays. X-ray

machine voltage and exposure time are also varied to suit the material and itsthickness. Distances between ray source, faults and film are important for imagedefinition.

Rays are harmful either in a large dose or a series of small ones where theeffects are cumulative. Monitoring against overdose is necessary with film badges,medical examination and blood counts. Direct exposure is avoided by the use ofprotective barriers but there is a danger that objects in the ray path will scatter radiation.

ULTRASONIC TESTINGInternal flaw detection by ultrasonic means in is principle similar to radar. The probeemits high frequency sound waves which are reflected back by any flaws in the object.

Reflections are also received back from the opposite surface. The probe is connected toa cathode ray oscilloscope which shows the results in a simple way.A single probe can be used, which combines both transmitting and receiving

functions. Alternatively separate devices for transmitting and receiving the soundsignals, are available.

The probe contains a slice of quartz which is cut in a particular plane from aquartz crystal. The quartz has a special property which is that it will pulse if analternating current is applied to it. Opposite faces of crystal are coated with a thinmetallic film for connection to the electrical supply. The quartz will expand and contractat the rate of the applied frequency. The amplitude is greatest at resonant frequency(i.e. natural frequency of the quartz). The action is reversible in that mechanical pulsesreceived by the quartz will produce a small current.

The site of crystal is protected by a thin steel plate which carries the pulses.When the probe is in use, a smear of oil or grease acts as the contact between thematerial under inspection and the probe. The vibration would not be carried satisfactoryacross an air gap.

The cathode ray tube contains a hot cathode and tubular anodes whichaccelerate s stream of electrons so that they strike the oscilloscope screen, the inside ofwhich is coated with zinc sulphide that fluoresces whether it is touched by them. If theelectrons are deflected slowly across the screen, the effect is to produce a line becausethe fluorescence lingers. The electrons are deflected by means of the metal plates (X X and YY) built into the tube. The X plates will produce horizontal deflection when one

is made positive to the other. The Y plates will produce vertical deflection also due topositive charging. The positive plate attracts the negative electrons.The oscilloscope contains a device which regulates the positive potential on the

X plats in such a way that electrons sweep slowly from X to X and then rapidly back toX. The slow sweep is the time base.

The quartz crystal (pulse generator) is triggered to give a short pulse of vibrationssimultaneously with the start of electron movement from X to X. The pulse is fed to theY plates causing a peak at the start of the horizontal line. Then the pulse of vibrations isreflected back by the opposite surface it generates an electrical signal in the quartzwhich is amplified and rectified when fed as d.c. signal to Y plates. This produces a


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peak at the end of the horizontal line. The distances part of these peaks measures thethickness of the material, taking into account the different in scale.

Any flaw in the material being inspected will also produce a peak. If the defect is

large enough, it will show as a large peak at the expense of the peak at the right.

Fig. 62 Cathode Ray Oscilloscope

DESTRUCTIVE TESTING

Special test pieces are used which are damaged during the process.

TENSILE TESTING

The test pieces are machined to standard sizes depending on the thickness of the metalin question. When material is tested under a tensile load, it changes shape by

elongating. Initially the extension is in proportion to the increasing tensile load. If agraph is plotted showing extension for various loads, then a graph straight line isobtained at first (O to A in Fig.63). if the loading is continued, the graph deviates asshown.

Within the limit of straight line, if the load is removed the material will return to itsoriginal length. The graph can be plotted as load and extension or as stress and strain.Stress is load per unit area. Strain is extension divided by original length.

STRESS

STRAIN


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Hookes law state that within the elastic limit, stress is proportional to strain.

STRESS STRAIN

Thus STRESS = STRAIN + CONSTANT

Or STRESS = CONSTANT E

STARIN

The constant termedYoungs Modulus of Elasticity and given the symbol E.

STIFFNESS

This is the property of resisting deformation within the elastic range and for a ductilematerial is measured by the Modulus of Elasticity (E). A high E value means that thereis a small deformation for any given stress.

BEHAVIOUR OF THE MATERIAL

During the initial stretching of the test piece and until the elastic limit is reached, theacross sectional area reduces. it can be shown by experiment that a bar with the sameelastic properties in all directions will have a constant relationship between axial strain.This is termed Positions Ratio and given the symbol .

Thus POSITION RATIO

At point Y the material yields (i.e. there is a failure of the crystalline structure of the

material, not long grain boundaries as has been happening, but through the grain

themselves. This is known as slip. A partial recovery is made at the lower yield point,then the extension starts to increase out of proportion to the load increases.

Fig. 63 Load/Extension Testing


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If the load is removed at any stage along the curved from Y to U, the material willbe found to be permanently deformed by amount OP. this is termed permanent set.

ROUTINE TEST

A full test is carried out of materials investigation. Acceptance test for steel to be used ina pressure vessel or for weld test pieces etc., are based on ultimate tensile stress andpercentage elongation. Sometimes reduction in area is required.

PROOF STRESS

For material that do not have a marked yield point such as aluminum, there is a

substitute stress specified. This is Proof Stress.

Fig. 64 Graph for Proof Stress

Proof stress is determined from a load/extension or stress/strain graph.(Fig. 64). It is

obtained by drawing a line parallel to the straight portion and distant from it on the

horizontal scale, by an amount representing a particular non-proportional elongation

e.g. 0.1 per cent proof stress is found from a line through 0.1 per cent non-proportional

elongation as shown.

VALUES OBTAINED

If stress is plotted, then figures for Ultimate Tensile Stress, Yield Stress, Proof Stress

and Breaking Stress can be read directly. If load is plotted then the loads at these points

have to be divided by the cross-sectional area of the test piece to find the stress.


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Percentage elongation is found form:

Percentage reduction in area is found from:

FACTOR FOR SAFETY

If a material is stressed beyond the elastic limit it will be permanently deformed. Toprevent this, a factor of safety is used when calculating the working stress to which thematerial may be subjected.

SAFE WORKING STRESS =

For steady loads, a safety factor of 4 may be used. Where there are shock loads,as in the drive chain for a camshaft, the factor of safety may be 25.

CREEEP (HOT FAILURE)

Plain carbon steels when used at temperature of 400C and above tend to deformunder stress. The rate of deformation is very slow and often is the result of loadings wellbelow stress limits. In many instances the deformation takes the form of extensionunder tensile stress. This gradual change of shape due to steady stress is termedcreep. It occurs in materials such as lead and tin, at room temperatures.

Steel used a high working temperatures in boilers, turbines and steam pipes etc.,may have to be designed for lower stress than normal. Even in diesel engine cylinderheads, creep is thought to cause a gradual bulging upwards of the bottom of thecylinder head (the combined effects of heat and gas pressure being responsible).

Creep temperatures coincide with recrystallization temperatures of variousmetals when changes in the crystal structure occur.

Alloys or steel have been developed which have creep resistance. Molybdenumis the essential alloying element and make up about 0.5 per cent of the alloy steel but

additions of small quantities of chrome and sometimes vanadium will improve creepstrength.

SHORT DEFINATION

Creep is the slow plastic deformations of metals under constant stress and eventualfailure at stress well below the normal failure stress.

Creep is the example of hot failure because fracture will ultimately follow theextension.


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CREEP TESTING

Creep test are carried out at controlled temperature over an extended period of time inthe order of 10,000 hours. The test piece is similar to the type used for tensile testscreep is usually through of as being responsible for extensions of metals only. In fact

creep can cause compression or other forms of deformations.Temperature of the test is around that of recrystallization which for steels starts

at about 400C. for other metals the recrystallization temperature is different beingabout 200C for copper and room temperature for tin and lead.

At the start of the test the initial load must be applied without shock. This load,normally well below strength limit of the material, will extend the test

Fig. 65 Creep Test

piece slowly. The load is kept steady through the test and the temperature is maintainedaccurately.

Extension is plotted and the extension due to creep is seen to proceed in threestages. Initial and final extension periods are separated by prolonged secondary stageof extension which follows a straight line law.

HARDENESS TESTING

The basis of Brenill hardness test is resistance of the material under test to deformationby steel ball.

The equipment used for the test (Fig.66) consists of cylinder into which oil isforced by a pump. The oil pressure is raised until the top piston, which supports aweight, is floating. The bottom piston holds the hardened steel ball which is pressed intothe metal beneath it. The loading for steel and metals of similar hardness is 3000kg.The load is allowed to act for 15 seconds to ensure that plastic flow occurs.


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Surface diameter of the indentation is measured with the aid of microscope whichis traversed over the test piece on a graduated slide with the vernier. Cross wires inmicroscope enable it to be accurately lined up above the edge of the indent. Knowingthe ball diameter to be 10 mm, the surface area of the indentation can be calculated andthe Brinell hardness number is found from the loading (3000 kg) Divided by the surface

area indentation. Hardness can be usually be read from a chart once the indentdiameter is known.When the test is used on softer materials, the load is reduced. For copper it is

1000 kg, and for aluminum 500 kg. depth of penetration must be less than half of thediameter of the ball. Thickness of the specimen must be not less than 10 x depth ofimpression. The edge of the impression will tend to sink with the ball if

Fig. 66 Brinell Hardness Test

the surface being tested has worked hardened; otherwise the local deformation will tendto cause pilling up of the metal around the indent.

If the above hardness test is used on very hard materials, the steel ball willflatten. This method is not reliable for readings over 600. It is used in preference toother methods where the material has large crystals, e.g. cast iron.

IMPACT TESTING

Toughness of materials is compared by impact testing, not by tensile or other tests.Metals have the same tensile strengths but different impact strengths.

A beam type test piece is used in the Charpy test (Fig. 67). The specimens forclass 1 pressure vessel tests are of dimensions laid down and taken across the weldfrom the middle of the test plate. The test piece is laid across supports with the notch onthe opposite side from the impact point of the striker.

The striker is released and the swing of the pendulum after striking the testpieces is used as an indication of impact strength. The tougher the material, the greaterthe amount of energy absorbed in fracturing it, and the smaller will be the extent of theswing after it has been fractured. The scale is kg.m, and the extent of the swing isshown by the small pointer.


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Temperature is carefully controlled during impact test and for materials which areto operate at low metal temperatures; the test would be carried out at temperature inquestion.

MACRO-SCOPIC EXAMINATION

The macro-specimen is taken from across the weld or any section that needsexamination. It is polished and acid etched so that penetration and fusion arehighlighted. Other defects may also become evident. A magnifying glass is used forcloser security.

Fig. 67 Impact Test (Charpy)

MICRO-SCOPIC EXAMINATION

It is possible to examine the crystals of metal through a microscopic if the surface hasbeen finely polished and etched. The results of heat treatment and of metal failures areshown up in this way.

TEST FORE CLASS 1 PRESSURE VESSELS

Boilers and air receivers are termed pressure vessels and manufactured by approvedfirms to the requirements of the various regulating bodies. The materials themselvesmust be from approved manufacturers and tested.

Weld test. Test pieces are cut from the same material used for the shell plating. Theseare attached to the shell plate (Fig. 68) so that the longitudinal welds run through thetest plates. The test pieces are treated in the same ways as the pressure used vesselmaterial. However they may be strengthened before being subjected to the same heat


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treatment as the pressure vessel. Usually test plates are not required for thecircumferential seams.

Fig. 68 Class 1 Pressure Vessel Test

A test plate is cut into the different specimens with a piece in reverse for retests. Testpiece (A) is of all weld metal. Gauge length, diameter and radius being fixed proportionsto each other. Diameter is limited by the thickness of the metal. Results for tensilestrength and elongation are required. Bend test specimens (B) are required for the innerand outer surfaces of the weld. The weld joined tested by specimen (C) which a tensiletest piece is taken across the weld. A macro specimen is taken at (D) and notch impactspecimens from (E).

Radiographic examination of welds is required with lead type used for

identification and image quality indicators. An Ultrasonic examination is sometimesaccepted as an alternative.A hydraulic test is required on pressure vessels. A number of failures occur

during pressures tests due to brittle fracture. These failures are frequently in pressurevessels of thick walled construction and occur in low temperatures conditions.

FRETTING

A small relative movement between two metallic parts in close contract can cause aform of mechanical wear, termed fretting. It is sometimes found on the backs of shellbearings and is due to slight movement of the bearing shell in its housing. Propeller

shafts with a shrunk on liner and taper fitted propeller are sometimes prone to frettingtroubles. The actions occur under the outboard end of the liner and under the forwardend of the propeller hub. Fretting can occur in any area where there is a chafing action.

The wearing action produces, in turn, wear particles. These acts as abrasives. Insome cases the wear particles are found to be hard materials such as the chromecompounds from stainless steels.

Fretting Corrosion. Metals are normally protected by an oxide film from corrosion. Afretting action exposes bare metal which then tends to oxidize. The wear particles


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themselves tend to further oxidize. The appearance of the oxide like the red FeO fromsteels is sometimes an indication that fretting corrosion is in progress.

Cracks occur in areas where there is fretting and extend due to fatigue. In theinitial stages they can be removed by grinding. Areas of trouble are improved byrelieving in the case of the fretting around shaft liners.

Brinelling is a form of fretting in ball races caused by vibration of an otherwisestationary race. A gradual indentation results in the race. The name is taken from theBrinell hardness test.

metallurgical test

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