Inspection / Testing of
Materials:
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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.
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In this test the material is destroyed and cannot any more be used
after test. Examples are: Termite test, compression test and impact
test.
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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.
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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.
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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.
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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.
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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 crosssectional 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.
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L = Lf – Lo/Lo *100.
Hardness test.
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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
2
2)
D -d
Where, P = load (Kg)., D = diameter of ball
(mm) and
d = diameter of indentation (mm).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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:
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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
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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 xray 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.
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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.
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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.
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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:
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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.
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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:
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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.
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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.
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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.
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Requirement of magnetic
particle inspection:
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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.
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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.
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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.
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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.
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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 40500C 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 2003500C. 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 5305500C. 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