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Seminar on
FELTTING OF CASTING, INSPECTIONS & CASTING DEFECTS
Submitted In partial fulfillment of Bachelor of Engineering Technology
Degree of the Jodhpur National University, jodhpur.
Guide By: Submitted By:
Mr. Pawan Gupta Patel Chintan
Department of Mechanical Engineering
Faculty of Engineering & technology
Jodhpur National University
Jodhpur(Raj.)
2011-12
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FACULTY OF ENGINEERING & TECHNOLOGY,
JODHPUR NATIONAL UNIVERSITY, BORANADA. JODHPUR (RAJ.): 342001
CERTIFICATE
This is to certify that the seminar report summited by Mr.
PATEL CHINTAN s/o DEVENDRABHAI (roll no:- 08ET405036) towards
the partial fulfillment of the requirements for the Degree of Bachelor of
Technology in Mechanical Engineering (Fourth year) of Jodhpur National
University is record of work of carried out by him under my supervision and
guidance. The work submitted has in my opinion reached a level required for
being accepted for examination.
APPROVED BY: GUIDED BY:
HOD of Mechanical
Prof. S.N.Garg Mr. Pawan Gupta
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Acknowledgement
Apart from the efforts of me, the success of any seminar depends largely on the
encouragement and guidelines of many others. I take this opportunity to express
my gratitude to the people who have been instrumental in the successful
completion of this seminar.
I would like to show my greatest appreciation to Shri. Pawan Gupta(Asst.prof) I
cant say thank you enough for his tremendous support and help. I feel motivated
and encouraged every time I attend his meeting. Without his encouragement and
guidance this seminar would not have materialized.
The guidance and support received from all the members who contributed and
who are contributing to this seminar, was vital for the success of the seminar. I
am grateful for their constant support and help.
PATEL CHINTAN
B.Tech 4th Year
Mechanical Engineering
JNU. JodhpurDate :-
Place :- Jodhpur
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TABLE
TITAL PAGE NO
1.FETTLING & CLEANING OF CASTING 1
1.1 SHAKING OF MOULD
1.2 CLEANING OF CASTING
1.2.1 KNOCKING OF DRY SAND CORES
1.2.2 REMOVEAL OF GATES & RISERS
1.2.3 REMOVEAL OF FINS & UNWANTED
PROJECTIONS
1.2.4 CLEANING & SMOOTHEN CASTING
1.2.5 REPAIRING THE CASTING
1
1
2
2
2
3
5
2.CASTING DEFECT
2.1 INTRODUCTIONS
2.2SHRINKAGE DEFECT
2.3 GAS POROSITY
2.4 COLD SHOT DEFECT
2.5 HOT TEARING DEFECT
2.6 MISRUNS DEFECT
2.7 METAL PENETRATIONS
11
11
12
12
13
13
14
14
3.INSPECTIONS OF CASTING
3.1 VISUAL SURFACE
3.2 DIMENSIONAL INSPECTIONS
3.3 NON DESTRUCTIVE TESTING
3.4 DESTRUCTIVE TESTING
15
15
16
18
26
4. CONCLUSION
30
5. REFERENCE
31
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LIST OF FIGURES
FIGURE NAME PAGE NO
1.1 METAL ARC WELDING
1.2 TIG PROCESS
1.3 MIG PROCESS
1.4 SUBMERAGE ARC WELDING
1.5 ATOMIC HYDROGEN WELDING
1.6 TERMIT WELDING
1.7 BRAZE WELDING1.8 METAL SPRAYING
5
6
7
7
8
8
911
2.1 SHRINKAGE DEFECT
2.2 GAS POROSITY
2.3 COLD SHUT DEFECT
2.4 HOT TEARING
2.5 MISRUNS
2.6 METAL PENETRATIONS
12
13
13
14
14
14
3.1 VISUAL SURFACE
3.2 RADIOGRAPHY
3.3 MAGNETIC PATICAL
INSPECTIONS
3.4 LIQUID PENETRANT TESTING3.5 ULTRASONIC TESTING
3.6 TENSILE TESTING
15
19
21
2324
27
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INTRODUCTION
1.0: Abstract
Casting is a manufacturing process by which a liquid material is usually poured into a
mold, which contains a hollow cavity of the desired shape,and then allowed to solidify,
this solidify part is known as Casting.Casting is most often used for making complex
shapes that would be otherwise difficult or uneconomical to make by other
methods. Metal casting involves pouring molten metal into a mould containing a cavity
of the desired shape to produce a metal product. The casting is then removed from the
mould and excess metal is removed, often using shot blasting, grinding or welding
processes. The product may then undergo a range of processes such as heat treatment,
polishing and surface coating or finishing.
The different techniques have been designed to overcome specific casting problems or
to optimize the process for specific metals, product designs and scales or other
operational considerations such as automation. All casing processes use a mould, either
permanent or temporary, which is a negative of the desired shape. Once the metal is
poured and has solidified it forms the positive shape of the desired product. Processes
differ in the number of stages that are required to produce the final casting. The process
uses a permanent mould negative to produce the final casting positive. Processes, such
as sand mouldingand shell casting, use a temporary mould negative which is typically
produced using a permanent pattern or die positive. Investment casting and lost foam
casting techniques use a temporary mould negative that is build around a temporary
pattern positive. For repetitive work, patterns are often produced using a permanent
mould or die negative.
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1.FETTLING AND CLEANING OF CASTINGS[BOOK 1]
1.1 Shaking of Moulds[BOOK 1]
After the metal has solidified and cooled in the sand mould, the casting is knocked out
by breaking the mould. It is essential to ensure that the castings are removed from the
mouldas early as possible for economic reasons. Premature withdrawal may, however,
give rise to distortion, cracks and a chilling effect and cause rejections. It is, therefore,
advisable to establish temperatures at which castings of each type, alloy composition or
complexity are to be withdrawn from mould sand sent for shake out. Suggested
temperatures at which steel castings can be withdrawn from moulds are shown below:
(i) Simple castings of uncritical nature and uniform sections: 900C.
(ii) Parts within even wall thickness; cast with chills: 600C.
(iii) Castings with critical shapes, prone to warping or cold cracking; subjected to
variable impact loads: 300C.
(iv) Thin-walled casting shaving abrupt changes in sections: 100C.
The moulds may either be broken manually on the pouring floor itself or transferred
to a separate shakeout station. In the latter case the mould is dumped on the shakeout
where it is rapidly jarred so that the sand falls through a grate or screen either into a pit
or on a belt conveyor arranged below the floor. The casting and moulding boxes remain
on the grate and are removed from there. Shaking maybe done either manually or
mechanically, but generally. mechanical shake-outs are used for large-scale work. In
the manual type, a stationary grating is mounted and the moulds break when dropped
over the grating. The mechanical units consist of a perforated plate or heavy mesh
screen fixed to a vibrating frame. The screen is vibrated mechanically, producing a
jarring action and causing quick separation of sand from other parts.
1.2 Cleaning Of Castings [BOOK 1]After the casting is extracted from the mould, it is no longer fit for use as such, as it has
sprue, risers, etc. attached toit. Besides, It is not completely free of sand particles. This
operation of cutting off the unwanted parts, and cleaning and finishing the casting is
known as fettling. The fettling operation may be divined dinto different stages:
(1) knocking out of dry sand cores;
(2) removal of gates and risers;
(3) Extraction of fins and unwanted projections at places where the gates and risers
have been removed and also elsewhere;
(4) cleaning and smoothening the surface; and
1
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(5) repairing castings to fill up blowholes, straightening the warped or defomied
Casting.
1.1.1 Knocking Out of Dry Sand Cores[BOOK 1]
Dry and cores may be removed by rapping or knocking with an iron bar. For quick
knocking. pneumatic or hydraulic devices may be employed. These devices, besides
knocking the cores, also help in cleaning and smoothening die casting.
1.1.2 Removal of Gates and Risers [BOOK 1]
The choice of method for removing gates and risers from the castings depends upon the
size and the shape of the casting and the type of the metal. The options for such work
are:
(i) knocking off or breaking with a hammer. Which is particularly suited in case
of grey iron castings and other brittle metals.
(ii) sawing with a metal cutting saw. Which may be a band saw circular saw. or a
power hacksaw (a metal band saw of the "do-all type is considered
suitable for steel, malleable iron, and nonferrous castings);
(iii)flame cutting with oxyacetylene gas is generally adopted for ferrous metals,
specially for large-sized castings where the risers and the gates are very
heavy;
(iv)using a sprue cutter for shearing of the gates;
(v) employing abrasive cut-off machines, which can work with all metals but are
specially designed for hard metals. Which are difficult to saw or shear.
(vi)Plasma arc cutting is now being increasingly used to cut sprue sand risers of
plate-shaped castings with a view to eliminate the manual operation of
burning off and to make the work fast, clean and accurate, by using a
programmable robot for holding and manipulating the castings.
1.1.3 Removal of Fins and Unwanted Projections[BOOK 1]
The operation of removing unwanted metal fins, projections, etc. from the surface of
the casting is called snagging. While snagging, care must be exercised to see that a
proper casting contour is followed and too much metal is not removed.
'The methods for snagging include:(i) using grinders of pedestal, bench, flexible shaft, or swing-frame type;
(ii) chipping with hand or pneumatic tools;
(iii) gouging and flame-cutting;
(iv) removing metal by arc-air equipment; and
(v) filing.
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2
1.1.4 Cleaning and Smoothening Castings[BOOK 1]
In as-cast state, castings often have sand particles adhering to their surface in a fused
form. When the castings are heat treated, a scale is also formed on the surface. In order
that the casting surface be clean and smooth, the adhering sand particles and the scale
have to be removed. The various methods available for this purpose are now described
briefly.
1.Tumbling[BOOK 1]
The castings to be cleaned are put in a large steel shell or barrel. which is closed at its
ends by cast iron lids. The barrel is supported on horizontal trunnions and is rotated ata speed varying from 25-50 rpm. Along with the castings, small pieces of white iron
called "stars" are also charged to help complete the cleaning and polishing operations.
When the barrel is rotated, it causes the castings to tumble over and over again,
rubbing against each other. Thus, by continuous peening action, not only do the
castings get cleaned and polished but also the sharp edges and fins get eliminated and
the internal stresses in the castings arc relieved. When the band is charged, care should
be taken to ensure that the castings are packed tight enough to prevent any breakage.
At the same time these should not be so tight as to prevent the relative motion of the
adjacent pieces. The capacities of tumbling barrels may vary from 1-12 cum. The
limitation of this process is that heavy castings cannot be charged with small ones of
fragile nature. Generally, small sized castings. which are not fragile in nature, are best
suited for tumbling.
2.Tumbling with Hydroblast [BOOK 1]
In this method, the barrel is not horizontal but is arranged obliquely at an angle of
about 30. One end of the barrel, which is at a higher level than the other, may be kept
open to enable observation of the cleaning process. When the castings are tumbled., a
high velocity stream of water and sand is blasted on the castings at a velocity of about
6000 metres per minute. This action results in more efficient cleaning and polishing,
and the tumbling time is also considerably reduced. The method is better adapted tononferrous castings since ferrous ones tend to get corroded due to water treatment. The
base of the barrel is perforated to facilitate removal of the sand-and-water mixture. For
large castings, hydroblasting chambers are used. The castings are placed on a slowly
rotating table and a high velocity stream is emitted from an adjustable nozzle.
3.Cleaning with Compressed Air Impact (Sand Blasting)[BOOK 1]
A high velocity stream of compressed air along with abrasive panicles is directed by
means of a blast gun against the casting surface. The blast gun is designed to convey
air at high velocity into a mixing chamber. The abrasive is fed into this chamber
through a side tube by suction feed, gravity feed, or direct pressure.
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3
Generally, in the ease of small guns, the abrasive is drawn in the mixing chamber due tovacuum created by the passage of high velocity air. The abrasive used is either sand or
steel grit, From the mixing chamber. airborne sand particles are directed towards the
casting. Figure shows a compete sand blasting arrangement which has a manually
operated blast pipe.
The blasting operation is generally carried out in special cabinets or rooms where the
operator directs the blast against the castings to be cleaned. The discharged sand drops
through a perforated floor from where it is conveyed to the moulding hop for re use.
The small sized castings are cleaned in cabinets equipped with windows through which
the operator can manipulate the gun and direct the blast. While working, the operator
must be thoroughly protected against harmful dust He should wear large Faber gloves,protective clothing on the body, and an air pressurised helmet. Unlike tumbling, the
sand blasting method can be adopted for both fragile and large-sized castings. The
method is also more efficient and ensures good polish.
4.Cleaning with Mechanical Impact (Shot Blasting) [BOOK 1]
Instead of using air pressure for hurling the abrasive grit towards the casting,
centrifugal force may be exerted by means of an impeller wheel. The abrasive applied
in this case is steel shots. As die shots move from the hub of the impeller towards the
periphery, their velocity gets accelerated and they finally leave the impeller at a very
high velocity hitting the casting surface with enormous impact. Large cleaning units
may be equipped with one or more blasting impellers strategically positioned at
different places all around the casting. The casting may also be mounted on a rotating
table. In sonic units, the castings am tumbled and at the same time the abrasive is hurled
towards them. In a monorail type shot blast, the castings are carried by a power
conveyer into machine from one side and taken out from the otherside.
5. Arc-air Process[BOOK 1]
This method involves arc heating of the casting surface and blowing off the melted
metal with compressed air. The projections or surface imperfections are heated by the
arc so that they reach the molten state when the air simultaneously blows them away,leaving behind a clean and smooth surface. The process is used on large castings in
steel foundries. The equipment is portable and comprises a gun which is equipped for
producing the arc and blowing the air.
6.Pickling[BOOK 1]
It essentially involves cleaning of casting surface by dilute acid treatment. The castings
are suspended by means of nickel-plated steel or monel metal hooks, into a pickling
tank containing equal part of hydrofluoric acid and sulphuric acid, or only
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4
sulphuric acid for about four hours. The tank is made of mild steel plates but is linedwith lead sheets on the inner walls. The castings are then washed with plain water in a
washing tank and they arc further immersed in a neutralising tank, containing 10%
solution of washing soda, preferably maintained at about 75 c .The castings are once
again rinsed in plain water and dried. The pickling treatment is a cheap and yet
effective method of dislodging sand. scales or tentacles of metal and producing clean
and bright surface on iron and steel castings.
1.1.5Repairing the Castings[BOOK 1]
Defects such as blowholes, gas holes, cracks, etc. may often occur in castings.
Sometimes the castings get broken, bent, or deformed during shake-out or because of
rough handling. Often the castings gets warped during heat treatment or while they cool
down in the mould- Such defective castings cannot be rejected outright for reasons ofeconomy. They are therefore repairs by suitable means arid put to use unless the defects
ire such that they cannot be remedied. The common methods of repair are now dealt
with.
1.Metal Arc Welding [BOOK 1]
Large-sized cracks, blowholes. and other imperfections can be rectified by metal are
welding. The area to be welded must first be cleaned by chipping, filing, gouging. or
grinding, then the joint must be accurately prepared and, if necessary, widened before
welding is commenced. Metals that can be welded by this method include almost all
cast metals, except magnesium. A proper selection of welding electrode is vital. A.C.
metal arc welding is most often selected for welding steel castings. The electrodes used
should preferably be coated so that a dense and strong joint is produced. D.C. arc
welding is preferred for welding cast irons and nonferrous metals as the polarity can be
changed and more heat can he obtained on either the electrode or the workpiece. as
desired. D.C. welding can thus give the lower electrode consumption, higher metal
deposition rates and smoother welds. It is also less dangerous, lie arc voltage used
bieing lower than in the case of AC. welding.
FIG 1.1 METAL ARC WELDING[REF 9]
2.Oxy-acetylene Gas Welding [BOOK 1]
This method, which is the least expensive and easily portable.is suitable where the
sections to be melded are not too heavy and where slower cooling rates arc required. for
instance, to prevent hardenable steels from getting hardened. Gas welding can easily
allow the use of a broad flame, which can pre-heat the area ahead of the section being
welded.
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This is not possible in are welding. The flame temperature is also lower than that of the
arc. so cooling rates are slow. The flame can be adjusted so as to make it oxidising,
reducing. or neutral. An oxidising flame is used for welding brasses and bronzes,
reducing flame for high carbon and alloy steels, nickel alloys, and other hard-facing
materials, and a neutral flame for low carbon steels. By using the proper technique,
almost all cast metals and alloys, except magnesium. can be gas welded. Liquefied
petroleum gas (L.P.G.) or natural gas is also used in place of acetylene where a broad
flame is desired.
3.Inert Gas Tungsten Arc Welding (TIG Process)[BOOK 1]
This process uses a non-consumable type of tungsten electrode together with a shield of
Men gas. such as helium and argon for protection of the welding zone. It is most
suitable for metals that tend to gel quickly oxidised, for instance. magnesium and
inagnesium alloys. It is also widely used for welding thin aluminium castings as also for
stainless sleek and alloys of copper and nickel.
FIG 2.2 TIG PROCESS[REF 9]
4.Inert Gas Metal Arc Welding (MIG Welding) [BOOK 1]
The electrode is made of metal similar to the work metal and is of the consumable type.
The method is very fast as electrode wire is automatically fed and inert gas protects the
metal from oxidation. The gases used are argon. nitrogen, and carbon monoxide. The
method is suitable for the repair and joining of large-sized steel castings and is
economical where high speed of operation is required.
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FIG 1.3 MIG PROCESS[REF 9]
5. Submerged Arc Welding[BOOK 1]
In this case, the entire welding action takes place beneath a granular mineral material
which acts as flux (Fig. 8.4). The electrode used is in bare form. The flow of current
melts the flux, spreading it over the weld zone and keeping the arc and weld metal
submerged. The metal is has completely protected from oxidation; besides, there is no
visible arc, sparks, spatter, or smoke. This enables use of heavy welding currents, high
welding speeds, deeper penetration, and superior quality of welds. The method is
unsuitable for repair work as it is basically a production process. but it is adopted for
building up large pressure vessels or structures by welding together smaller steel
castings.FIG 1.4 SUBMERGED ARC WELDING[REF 9]
6.Atomic Hydrogen Welding [BOOK 1]
A continuous stream of hydrogen is passed through the arc produced between two
tungsten electrodes. Due to the heat of the am, the gas dissociates from molecular to
atomic form. When the atoms of hydrogen strike the cooler work surface, they again
re-unite and emit an enormous amount of heat, thus melting the base metals that
need to be jointed. The heat input thus available is very high; hydrogen also acts
as ashield the action of atmospheric oxygen and nitrogen. Filler metal is fed
separately from a wire. This process is ideal for the repair welding of metal
moulds and dies made of alloy steels and is used for welding of thin casting in
stainless steel, aluminium alloy etc. It produes a very homogenous and smooth
joint with strength that equals that in the parent metal.
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FIG 1.5 ATOMIC HYDROGEN WELDING[REF 9]
7.Thermit welding[BOOK 1]
The high temperature required for melting metal to fill up the joint is
attained by employing an exothermic reaction. The method is more like a casting
process. It entails igniting in a crucible mixture of iron oxide and finely divided
aluminium in the ratio 3:1 and a special powder is used to ignite the mixture.
Due to the heat of ignition the mixture explodes at a temperature of about 1540c,
and pure iron with aluminium oxide as slag is produced:
8AL+ 3FE3O4 = 9FE + 4AL2O3
FIG 1.6 TERMIT WELDING[REF 9]
The joint crack or cavity to be filled is arranged in a sand mould with a proper
gating and feeding system and the metal from the thermit crubical is poured into
the mould. Pure metal occupices the space between the pieces to be jointed and
slag floats at the top. To enable preparations of the gating system, also in wax,
is attached. The whole assembly is embedded in moulding sand and the mould
inverted and heated to cause the wax and flow out, leaving the cavity around
the metal parts to be jointed as shown in fig.
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Thermit welding is employed for repairing large and heavy steel casting such as
steel mill rolls, ship stern frames, and gears . It is also used for the fabrication of
heavy units by joining relatively simple casting. The process is simpler, less time
consuming and cheaper then other methods and produces good strength and
better quality. Also, no stress relief is necessary as the cooling is very slow and
the operations it self relieves the stresses.
8. Flow welding[BOOK 1]
This entails melting the metal, in the same way as for casting purposes,
then continuously pouring the molten metal directly into the crack or cavity tobe filled, till the surrounding area also starts melting. This method is not much
favoured now as easier and quicker methods of welding are available.
9. Braze welding[BOOK 1]
This process is applied for such parts that tend to get distorted or
cracked when welded by other means. A lower heat is required as the base
metal is not actually melted and the bond is obtained only by diffusion. A
nonferrous copper base or silver base alloy which melts at a temperature above
430c is employed as filler metal. It is method may be used to make casting
watertight and to repair pipe sand pipe fittings fine cracks, crevices , porosity
etc.
FIG 1.7 BRAZE WELDING[REF 9]
10.Slodering[BOOK 1]
This is similar to brazing, the difference being in the filler metal; a tin lead
alloy which at a much lower temperature(below 430c) is preferred for soldering.
The process serves to fill up surface imperfections when high strength is not
required and porous areas in copper base alloy casting are to be made pressure
tight.
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9
11. EPOXY FILLER[BOOK 1]
Certain epoxy plastic filler can be used to fill up pinholes, blowholes, crack, etc.
and to impart enough strength to the casting. For good mechanical properties,
filler are also duly charged with metal powders to suit different cast metal. These
fillers are of two types ,viz. general purpose and fastcuring. The latter takes
hardly two hours to harden whereas the former takes longer. Smooth on
cement , which is a pasty mixture of iron filings in hardening agent, is also
widely used to repair iron casting.
12. STAIGHTENING[BOOK 1]
Deformed or warped casting can be straightened in a press by applying
pressure. This operations is possible only in the case of ductile material ,such as
steel, aluminium ,copper, and bronze. Generally , a hydraulic press along with
formed dies. Small casting can be straightened by hammering manually. Both
cold and hot pressing are used according to size and material of casting.
13. Metal Spraying [BOOK 1]
When the casting becomes undersized. it can be built up by providing a coat of metal in
die desired thickness by a metal spraying process. This isa simple and relatively
inexpensive way of forming a layer of metal on the cast surface. The sprayed metal may
be e her the same as the base metal or a dissimilar one. The deposited metal is taken in
wire form. The spray gun uses oxygen and acetylene to melt the wire and compressed
air to atomise the molten metal in the form of spray. All types of metals and alloys can
be sprayed. The bond obtained is of the mechanical type with negligible diffusion. The
joint between the parent metal and the sprayed metal is not as strong as that obtained by
welding or brazing. This technique is also used for providing an anticorrosive metal
layer on iron and steel castings. Figure explains the principle of metal spraying and Fig.
shows the set-up required for the process.
10
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FIG 1.8 METAL SPRAYING[REF 9]
2 CASTING DEFECTS [REF 8]2.1 Introduction:
A casting defect is an irregularity in the metal casting process that is undesired. Some
defects can be tolerated while others can be repaired otherwise they must be eliminated
or by changing the casting process. The defects 17ccurring in casting process mainly
are as below,
Shrinkage defect
Gas porosity
Hot tearing defect
Misruns defects
Metal penetrations defect
2.2Shrinkage defects [REF 8]
Shrinkage defects occur when feed metal is not available to compensate
forshrinkage as the metal solidifies. Shrinkage defects can be split into two different
types: open shrinkage defects and closed shrinkage defects. Open shrinkage defects are
open to the atmosphere, therefore as the shrinkage cavity forms air compensates. There
are two types of open air defects: pipes and caved surfaces. Pipes form at the surface of
the casting and burrow into the casting, while caved surfaces are shallow cavities that
form across the surface of the casting.Closed shrinkage defects, also known
as shrinkage porosity, are defects that form within the casting. Isolated pools of liquid
form inside solidified metal, which are called hot spots. The shrinkage defect usually
forms at the top of the hot spots. They require a nucleation point, so impurities and
dissolved gas can induce closed shrinkage defects. The defects are broken up
into macro porosity and microporosity (or micro shrinkage), where macro porosity can
be seen by the naked eye and micro porosity cannot.
11
http://en.wikipedia.org/wiki/Metal_castinghttp://en.wikipedia.org/wiki/Shrinkage_(casting)http://en.wikipedia.org/wiki/Solidificationhttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Nucleationhttp://en.wikipedia.org/wiki/Metal_castinghttp://en.wikipedia.org/wiki/Shrinkage_(casting)http://en.wikipedia.org/wiki/Solidificationhttp://en.wikipedia.org/wiki/Atmospherehttp://en.wikipedia.org/wiki/Nucleation8/2/2019 Main Report Fettling 03
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FIG 2.1 SHRINKAGE POROSITY[REF 9]
2.3 Gas porosity [REF 8]
Gas porosity is the formation of bubbles within the casting after it has cooled. This
occurs because most liquid materials can hold a large amount of dissolved gas, but the
solid form of the same material cannot, so the gas forms bubbles within the material as
it cools. Gas porosity may present itself on the surface of the casting as porosity or the
pore may be trapped inside the metal, which reduces strength in that
vicinity.Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas
porosity .In aluminum castings, hydrogen is the only gas that dissolves in significant
quantity, which can result in hydrogen gas porosity. For casting that is a few kilograms
in weight the pores are usually 0.01 to 0.5 mm (0.00039 to 0.020 in) in size. In largercasting they can be up to a millimeter (0.040 in) in diameter.
Gas porosity can sometimes be difficult to distinguish from micro shrinkage because
micro shrinkage cavities can contain gases as well. In general, micro porosities will
form if the casting is not properly risered or if a material with a wide solidification
range is cast. If neither of these are the case then most likely the porosity is due to gas
formation To prevent gas porosity the material may be melted in a vacuum, in an
environment of low-solubility gases, such as argon orcarbon dioxide, or under a flux
that prevents contact with the air. To minimize gas solubility the superheat temperatures
can be kept low. Turbulence from pouring the liquid metal into the mold can introduce
gases, so the molds are often streamlined to minimize such turbulence. Other methods
include vacuum degassing,gas flushing, or precipitation. Precipitation involves reactingthe gas with another element to form a compound that will form a dross that floats to
the top. For instance, oxygen can be removed from copperby addingphosphorus, or
aluminum orsilicon can be added to steel to remove oxygen. A third source consists of
reactions of the molten metal with grease or other residues in the mold.
12
http://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Hydrogen_gas_porosityhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Superheatinghttp://en.wikipedia.org/wiki/Vacuum_degassinghttp://en.wikipedia.org/wiki/Gas_flushinghttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Phosphorushttp://en.wikipedia.org/wiki/Siliconhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Hydrogen_gas_porosityhttp://en.wikipedia.org/wiki/Argonhttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Superheatinghttp://en.wikipedia.org/wiki/Vacuum_degassinghttp://en.wikipedia.org/wiki/Gas_flushinghttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Phosphorushttp://en.wikipedia.org/wiki/Silicon8/2/2019 Main Report Fettling 03
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FIG 1.2 GAS POROSITY[REF 9]
2.4 COLD SHOT DEFECT[BOOK 3]
These are external defects caused by two streams of metals that are too cold to
fuse properly; these can occur due to slow pouring ; poor design and small gate; and
can be controlled by the use of hotter metal using streamlined splines to give
smoother flow. In this defect small shot like spheres of metal are almost distinct
from casting.
FIGURE 1.3 COLD SHUT[REF 9]
2.5 Hot tearing[Book 3 ]
There are the crack having ragged edags due to tensile stress during
solidification. It is due to the discontinuity in the metal casting resulting from
hindered contraction,occurring just after the metal has solidfied. It is caused by
excessive mould hardness of ramming, high dry and hot strength , improper
metallurgical and pouring temperature control , provision of insufficient fillets or
brackets at the junctions of sections.
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Figure 1.4 Hot tearing[REF 9]
2.6 Misruns[BOOK 3]
Misruns may be present in the from of improperly filled corners and mouldcavities. There occur because of low pouring temperature, lack of fluidity of the
metal too small gates, too many resterictions in gating system etc. Another defect
called the cold shot occur when two cold streams of molten metal meet at the
junctions of a mould cavity and do not fuse together and thus the mould is not
properly filled with metal.
Figure 1.5 MISRUNS[REF 9]
2.7 Metal penetrations[BOOK 3]
It is refer to the conditions of penetrations of metal in the interstices of the
sand grains. It causes a fused aggregate of metal and sand on the surface of
casting which results in rough surface finish. It is caused by soft ramming , too
coarse mould and core sand , and excessive metal penetrations.
Figure 1.6 metal penetrations[REF 9]
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3. INSPECTIONS OF CASTING[REF 1]Inspections of casting aims at finding both surface and subsurface defects in the
casting. Inspections will ascertain the quality of the casting and result in their
acceptance or rejections. Inspections procedures and tests may be classed as
follows.
1) Visual surface inspections for foundry defects
2) Dimensional Inspections
3) Non Destructive type
4) Destructive type
3.1 Visual surface:[REF 1]
Visual inspection refers to an NDT method which uses eyes, either aided or non-aided todetect, locate and assess discontinuities or defects that appear on the surface ofmaterial undertest (Fig).Ifis considered as the oldest and cheapest NDT method. It isalso considered as one of the most important NDT method and applicable at allstages of construction or manufacturing sequence. In inspection of any engineeringcomponent,
if visual inspectional on is found to be sufficient to reveal the required informationnecessary for decision making, then other NDT methods may no longer considerednecessary.
FIG 3.1 VISUAL SURFACE[REF 1]
Visual inspection is normally performed by using naked eyes. Its effectivenessmay be improved with the aid of special tools. Tools include fiberscopes, borescopes,magnifying glasses and mirrors. In both cases, inspections are limited only to areasthat can be directly seen by the eyes. However, with the availability of moresophisticated equipment known as borescope, visual inspection can be extended to coverremote areas that under normal circumstances cannot be reached by naked eyes.Defects such as corrosion in boiler tube, which cannot be seen with naked eyes caneasily be detected and recorded by using such equipment.
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15
Although considered as the simplest method of NDT, such an inspection must be carriedout bypersonnel with an adequate vision. Knowledge and experience related tocomponents are also necessary to allow him to make correct assessment regarding the
status of the components.
Advantages:
Cheapest NDT method
Applicable at all stages of construction or manufacturing Do not require extensive training Capable of giving instantaneous results
Limitation:
Limited to only surface inspection Require good lighting Require good eyesight
3.2Dimensional Inspections [REF 2]
Dimensional control is usually required for all types of
castings. Sometimes it is not so critical but at other times it may be vital. When
precision castings are produced by processes such as investment casting, shell
moulding and die casting, dimensions Trod to be closely checked. Initially, when the
castings are made from a new pattern, a few sample castings are first made which are
carefully checked with the drawings to ensure that the sizes obtained conform to those
specified and will be maintained within the prescribed tolerances in the lot under
production. On testing of the sample lot, deviations from the blueprint are rectified on
the pattern equipment. When the castings are found to be consistently within the
tolerances, spot checks, together with a regular check of the patterns and dies being
used, may be sufficient. In the case of the jobbing type of foundry, each casting
produced may be different and, therefore, according to the customer's requirements,
each one may have to be thoroughly inspected for dimensional variations. Dimensional
inspection of castings may be conducted by various methods:
3.2.1 Standard Measuring Instruments to Check the Sizes[REF 2]
Instruments such as rule, vernier callipers,vernier height gauge, vernier depth
gauge, micrometers, scribing block, combination set, straight edge, squares. spirit level,
and dial indicator are commonly used. For high precision castings or after machining,
more advanced measuring instruments, such as auto-collimator, comparator, ultrasonic
instruments tor measuring wall thickness and projection instruments are also required.
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3.2.2 Templates and Contour Gauges for the Checking of Profiles, Curves, and
Intricate Shapes [REF 2]
Templates act as time-saving aid in measurement and facilitate the
entire job. These can be easily prepared in mild steel or brass sheet by marking out, and
cutting and finishing the profile that is required to be checked on the castings.
3.2.3 Limit Gauges [REF 5]
For toleranced dimensions on casting produced on a repetitive basis, limit
gauges are usually. The type of limit gaugesthe plug, ring, snap, plate, etc.depends
on the shape of the parameter to be checked. Periodical checking and maintenance of
limit gauges is very important
3.2.4 Special Fixtures [REF 3]
Special fixtures are required to be designed and used where dimensionscannot be conveniently checked by using instruments, for instance, during the checking
of locations, relative dimensions, centre-to-centre distance, angularity of surfaces, and
so on.
3.2.5Coordinate Measuring and Marking Machine (CMM) [REF 5]
This machine is very useful for measurement and inspection of
uneven, undulated, irregular, or curved surfaces which cannot be conveniently or
accurately checked by other measuring tools or instruments. The accuracy of
measurement of these machine ranges from 0.01 mm to 0.05 mm. Besides measuring, it
can be used for marking purposes also in all three dimensions on metallic or non-metallic surfaces. Measurement and marking are accomplished easily without errors in
reading in all three dimensions. Once the machine is set, all measurements can be
carried out in a programmed sequence automatically. The machine in reality is a multi-
axial device providing measurement of output of position and displacement sequentially
without a need for changing tools.
The machine essentially consists of a touch probe, usually having a ruby tip, which is
mounted on a horizontally sliding arm, movable vertically along a column. The column,
is fixed to a base which in turn is held on a large accurately machined granite surface
plate and is movable in a direction perpendicular to the direction of be movement of
arm. Thus, the probe is capable of being moved along all three u for carrying out
measurement of different surfaces of a workpiece. The sliding movements of arm and
column are performed with great precision and are read on m electronic digital read out
unit, attached to the machine. When marking is to be lone on surfaces, a scriber is used
in place of a probe. A larger variety of probes, scribers and other accessories are
available to enable the machine to be highly flexible and accurate in operation. The
movements along the three axes may be manual or motorised. The machine can be
further equipped with a small computer system for processing the data obtained from
measurement and for storing and retrieving the same. A special software is also
available with the computer so that measurement and inspection of different types of
surfaces can be carried out automatically without the need for manual control.17
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The drawing data from CAD station an be also transmitted to this machine by inter-
linking the two systems with the actual value of dimensions. A printer can also be
provided with the computer for producing & hard copy of the inspection report.
The CMM machines are now getting increasingly popular in inspection departments
attached to tool rooms, pattern and die shops, foundry and forging shops, press shops,welding and structural shops and plastic and glass parts manufacturing units. The
appraisal of surface roughness or finish is required in addition to the dimensional
measurement. Surface roughness is expressed as a number (in microns), which is an
arithmetical average of the heights of the peaks and depths of the valleys on a casting
surface above and below a mean line within a specified sampling length. IS: 3073-1967
provides a method for assessing the surface roughness by this system. The approximate
values for different types of castings are specified
Surface roughness is evaluated approximately, as is usually
sufficient for castings, by surface roughness comparison standards, where the given cast
surface is compared visually or with the aid of a magnifying lens with a set of standards
duly marked with varying surface roughness values. For finished surfaces and moreprecise measurements, electrical type of direct reading, surface measuring instruments
or profilometers, such as Talysurf, are used.
3.3 Non destructive testing[REF 6]
Non-destructive testing (NDT) is a noninvasive technique for determining the integrityof a material, component or structure. Because it allows inspection without interferingwith a product's final use, NDT provides an excellent balance between quality controland cost-effectiveness.
The main goal of NDT is to predict or assess the performance and service life of acomponent or a system at various stages of manufacturing and service cycles. NDT isused for quality control of the facilities and products, and for fitness or purposeassessment (so-called plant life assessment) to evaluate remaining operation life of
plant components (processing lines, pipes and vessels).
NDT inspection of industrial equipment and engineering structures is important inpowergeneration plants, petroleum and chemical processing industries, and transportationsector. State-of-the-art methodology is applied to assess the current condition, fitness-for-service, and remaining lifeof equipment. NDT inspection provides basic datahelping to develop strategic plans for extending plant life.
NDT life extension and life assessment services include:
Equipment integrity analysis Corrosion monitoring of structures and equipment
Corrosion damage evaluation Fatigue and creep damage prediction Fitness-for-service evaluation
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The long list of NDT methods and techniques includes: radiographic testing (RT),ultrasonic testing (UT), liquid penetrant testing (PT), magnetic particle testing (MT),eddy current testing (ET), visual testing VT as well as leak testing LT, acousticemission AE, thermal and infrared testing, microwave testing, strain gauging,holography, acoustic microscopy, computer to mography, non-destructive analyticalmethods, non-destructive material characterization methods and many more.
The major four (or basic) NDT methods, which are largely used in routine services toindustryare:
Liquid penetrant testing
Magnetic particle testing
Radiography Ultrasonic testing
3.3.1 Radiography[REF 6]Radiography is an NDT method, which uses penetrating radiation. It is based ondifferential absorption of radiation by the part under inspection. In this inspection thesource of radiation can be from radioactive sources, typically Irridium-192, Cobalt-60,Caesium-137, which emit gamma rays orfrom a specially built machine that can emitX rays. The former is known as gamma radiography whereas the latter is referred asX ray radiography. Table I presents major radioisotope sealed sources largely used ingamma radiographic testing.
There are many methods of NDT, but only a few of them examine the volume ofa specimen; some only reveal surface-breaking defects. One of the best established andwidely used NDT methods is radiography the use of X rays and gamma rays to
produce a radiograph of a specimen, showing any changes in thickness, defects (internaland external), assembly details etc.
Radiographic testing (RT) method can be used in civil engineering equipmentsnotably to verify the integrity of pre-stressed wires in a pre-stressed concrete structure
by using radioisotope sealed sources, X ray machines or linear accelerator. Table Ipresents the main radioisotope sealed sources used for gamma radiography. Figure 9shows a typical set up in radiographic testing and figure 10 presents a radiographicimage of a metallic structure.
FIG 3.2 RADIOGRAPHY[REF 2]
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During the radiography X rays or gamma rays penetrate through material underinspection. While traversing through the material, these radiations experiencemodification by the internal structure of the material through absorption andscattering processes. If the internal structure is homogeneous, the absorption andscattering processes would be uniform throughout the material and radiations thatescape from the material would be of uniform intensities.
These radiations are then recorded by a suitable recording medium, typicallyradiographic film. When the film is processed, a uniform dark image will appearon the film that indicates the homogeneity of the material tested. The situation isdifferent for cases of materials containing discontinuities or different in thickness. Ingeneral, the absorption of radiation by a material depends on the effective thicknessthrough which the radiations penetrate.
Discontinuities such as cracks, slag inclusions, porosity, lack of penetration and lack offusion reduce the effective thickness of the material under test. Thus, the presence ofsuch discontinuities causes radiations to experience less absorption as compared with
those in areas with discontinuity. As a result, in areas containing discontinuities moreradiations escape, recorded by the film and forming a dark image that represents theinternal structure of the material.
The appearance of radiographic images depends on the type discontinuities encounteredby the radiation. Cracks for example will produce a fine, dark and irregular line, whereasporosities produce dark round images of different sizes.
Some discontinuities that presence in a material such as tungsten inclusion in steel has ahigher density than its surrounding. In this case, the effective thickness that needs to
be traversed by radiation is somewhat greater. In other words, more radiation isabsorbed in this area as compared with other areas. As a result the intensity of
radiation that escaped after traversing this area will be lesser than that for other areasgiving a lighter image bearing the shape of tungsten inclusion inside the material.
Radiography is widely used throughout the industry. Its capability to produce two-dimensional permanent images makes it as one of the most popular NDT methodsfor industrial application. However, radiation used for radiography presents a
potential hazard to radiographers as well as members of public. Due to itshazardous nature, the use of radiation, including for industrial radiography isstrictly controlled by Regulatory Authorities.
Almost all countries throughout the world have their own Regulatory Body thatregulates the use of radiation. Requirements imposed by the Authority upon the use of
this method make it as one of the most expensive NDT method.
Advantages and limitations of this method are as follows:
Advantages
Applicable to almost all materials Produce permanent images that are readily retrievable for future reference Capable of detecting surface, subsurface and internal discontinuities
Capable of exposing fabrication errors at different stages of fabrication Many equipment are portable
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Limitations Radiation used is hazardous to workers and members of public
Expensive method (cost of equipment and other accessories related toradiation safety are relatively expensive)
Incapable of detecting laminar discontinuities
Some equipment are bulky For X ray radiography, it needs electricity Require two sides accessibility (film side and source side)
Results are not instantaneous. It requires film processing, interpretation and evaluation Require highly trained personnel in the subject of radiography as well as radiationsafety.
Organizations applying this method need to be licensed and subjected tovarious rule and regulation.
3.3.2Magnetic partical inspections:[REF 4]
Magnetic particle testing (MT) is a NDT method that utilizes the principle of
magnetism. Material to be inspected is first magnetized through one of many waysof magnetization. Once magnetized, a magnetic field is established within and in thevicinity of the material. Finely milled iron particles coated with a dye pigment are thenapplied to the specimen. These magnetic particles are attracted to magnetic fluxleakage fields and will cluster to form an indication directly over the discontinuity.They provide a visual indication of the flaw.
The presence of surface breaking and subsurface discontinuity on the materialcauses the magnetic field to leak and travel through the air. Such a field is calledleakage field. When magnetic powder is sprayed on such a surface the leakage fieldwill attract the powder, forming a pattern that resembles the shape of the discontinuity.
This indication can be visually detected under proper lighting conditions
FIG.1.3 Magnetic field lines and magnetic particles influenced by a crack.[REF 4]
There are many methods of magnetizing materials. The use of permanent magnet is oneof the ways of magnetization. However, in many cases the use of electromagnet isconsidered as a more superior and effective way of magnetization. Another way ofcreating magnetic field in a material is by the use of coil carrying current.
21In this way, a longitudinal magnetic field would be able to be established in long items
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such as bars and cylinders. Circular magnetic field on the other hand is produced byallowing current flowing along the cylindrical material. Induction of magnetic fieldinto the material to be inspected can be achieved by the use of either alternating current(AC) or direct current (DC). In general the use of DC would produce magneticfield deeper below that surface that allow subsurface discontinuity to be detected.
Discontinuities can be best detected when the direction of magnetic field isperpendicular. The chance of detection reduces as the angle between the magnetic fieldand the plane of defect decreases.
When the angle between the magnetic field and the plane of defect is zero, i.e. themagnetic field is parallel with the plane of defect then the chance of detection becomeszero.
The application of MT involved the following sequence:
Pre-cleaning Magnetization Application of magnetic powder
Demagnetization
The advantages and limitations of using MT method are as follows:
Advantages Inexpensive
Equipment are portable Equipment easy to operate
Provide instantaneous results
Sensitive to surface and subsurface discontinuities
Limitations
Applicable only to ferromagnetic materials Insensitive to internal defects
Require magnetization and demagnetization of materials to be inspected Require power supply for magnetization
Coating may mask indication Material may be burned during magnetization
3.3.3 LIQUID PENETRANT TESTING [REF 3]
Liquid penetrant is an NDT method that utilizes the principle of capillaryaction in which liquid of suitable physical properties can penetrate deep intoextremely fine cracks or pitting that are opened to the surface without beingaffected by the gravitational force. Liquid penetrant testing (PT) methodconsists in depositing on the object surface of a special liquid, which will
be drawn into any surface defect by capillary action. A liquid with highsurface wetting characteristics is applied to the surface of the part andallowed time to seep into surface breaking defects (Fig.). Followingremoval of excess penetrant an
22
FIG. 3.4 Liquid penetrant testing principle[REF 3]
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application of a developer reverses the capillary action and reveals thepresence of the flaw so that it can be visually inspected and evaluated. ThePT method can be used on metallic parts of civil engineering equipments.Liquid penetrant inspection generally involved the following sequence:
Pre-cleaningAt this stage, surface of the inspected item is cleaned to avoid the presence of any dirt thatmay close the opening of discontinuity. Cleaning is accomplished by various
methods such as vapor cleaning, degreasing, ultrasonic cleaning etc.
Penetrant applicationOnce the surface is cleaned, penetrant either in the form of dye penetrant orfluorescence penetrant is then applied. The application of penetrant can be achievedeither by dipping, spraying or brushing depending on the nature or item to be inspected.This penetrant is then allowed to remain on the surface for some duration. Such durationis termed as a dwell time. During this period, if there is any discontinuity, penetrant will
penetrate deep into it.
Removal of excess penetrant
Excessive penetrant need to be removed from the surface to allow inspection to be made.Such removal can be achieved by applying water, proper solvent or emulsifierfollowed by water (depending on the type of penetrant used) on the surface. At thisstage, all unwanted penetrant will be removed from the surface, leaving only those trappedinside the discontinuity.
Developer applicationDeveloper is then applied to the surface of the inspected item. This developer either in theform of dry powder or wet developer acts as a blotting paper which draws penetrant out ofthe discontinuity.
In doing so, penetrant will bleed to form an indication whose shape depends uponthe type of the discontinuity presence in the material. Such an indication is recordedeither by the application of a special tape or by taking its photograph.
Post-cleaningApplication of penetrant and developer causes the surface to be contaminated.Thus, upon completion of the inspection, it is important for the item to be cleaned sothat no corrosive material remains on its surface that may affect its serviceability.
23
As for other NDT methods, liquid penetrant has its own advantages and limitations.
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Advantages: Simple to perform
Inexpensive Applicable to materials with complex geometry
Limitation Limited to detection of surface breaking discontinuity
Not applicable to porous material Require access for pre- and post-cleaning Irregular surface may cause the presence of non-relevant indication .
3.3.4ULTRASONIC TESTING [REF 6]
As the name implies, ultrasonic refers to an NDT method, which uses soundshaving frequencies beyond those audible by human ears. Sounds having frequencies
about 50 kHz to 100 kHz are commonly used for inspections of nonmetallic materials,whereas those with frequencies between 0.5 MHz up to 10 MHz are commonly used forinspections of metallic materials.
Ultrasonic testing (UT) method uses high frequency sound waves (ultrasounds) tomeasure geometric and physical properties in materials. Ultrasounds travel in differentmaterials at different velocities. The ultrasound wave will continue to travel throughthe material at a given velocity and does not return back unless it hits a
FIG.3.5. Principle of ultrasonic testing.[REF 1]
reflector. Reflector is considered any boundary between two different materials, or a
flaw. The ultrasound generator (transducer) emits waves and in the same positionreceives reflected sounds (if any). Comparing both signals (emitted and reflected) the
position of the defect and its size can be measured. The UT can be used on civilengineering equipments, outside metallic parts, to verify the granulation of roadcovering or of concrete.
High frequency sound waves are introduced into a material and they are reflected backfrom surfaces or flaws. Reflected sound energy is displayed versus time, and inspectorcan visualize a cross section of the specimen showing the depth of features that reflectsound.
24
As in the case of radiography, ultrasonic is an NDT method that is used for detectinginternal discontinuity. In ultrasonic inspection, sounds are generated by the usetransducers that are made of materials exhibiting piezoelectric effect. Materialsexhibiting piezoelectric effect are capable of converting electrical energy into sound
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energy and vise versa. Typical example of such a material is quartz. When a quartzcrystal is cut in certain orientation and thickness it is capable of generating soundsappropriates for ultrasonic inspections. Depending upon the orientation of crystalcutting, sounds generated by quartz can be of the longitudinal or transverse modes.Figure 10 shows ultrasonic testing in laboratory.
During the inspection, sound generated by a transducer is transmitted into thematerial to be inspected via couplant. This sound travels in the material with a speedthat depends on the type of material. For example, longitudinal waves travel atspeeds of 5960 m/s and 6400 m/s in steel and aluminum respectively. When there isno discontinuity in the material, sound continues to travel until it encounters the
backwall of the material.
At the backwall, sound is reflected and continues to travel until it reaches the transducer.At this transducer, piezoelectric converts sound energy into electrical pulse. The pulseis then amplified and presented on the screen as a backwall signal or backwall echo (Fig.12).
However, if there is a discontinuity in the material, a portion of sound energy is reflectedby this discontinuity whereas another portion continues to travel until it reaches backwalland reflected. Under these circumstances, a portion of sound that was reflected by thediscontinuity reaches the transducer first and followed by those reflected by the backwall. In both cases sound energies areconverted into electrical signals which then are displayed on the ultrasonic flawdetector screen as backwall signal and signal due to discontinuity. By properlycalibrating the equipment, both the position of discontinuity with respect to the
position of backwall and the size of discontinuity can be determined.
The fact that ultrasonic does not present any potential hazard to the operator makes this
method as a good competitor for radiography method. However, highly skillful andexperience operators are required to allow correct interpretation of the test results.Unlike in the case of radiography where the results are presented in the pictorial forms,results of ultrasonic inspections are purely in the form of electrical signal. Knowledgeabout the material, correct movement of the transducer and proper time base calibrationis absolutely necessary for correct assessment of the test results.
More sophisticated ultrasonic equipment is currently available which allowresults to be presented in 2D or 3D dimensions. This development provides greaterstrength to ultrasonic method in its rivalry against radiographic method.
The advantages and limitations of ultrasonic methods are as follows:
Advantages
Requires only one side accessibility Capable of detecting internal defect
Not hazardous Applicable for thickness measurement, detection of discontinuity, anddetermination of material properties
25 Can provide the size of discontinuity detected
Very sensitive to planar type discontinuity Suitable for automation Equipment are mostly portable and suitable for field inspection
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Applicable for thick materials
.Limitations
Not capable of detecting defect whose plane is parallel to the direction of sound beam Require the use of couplant to enhance sound transmission
Require calibration blocks and reference standards Require highly skillful and experience operator Not so reliable for surface and subsurface discontinuity due to interference betweeninitial pulse and signal due to discontinuity.
3.4 Destructive type[REF 7]
There are two type of the this part. It is also know as the mechanical
properties.
1) Tensile testing
2) Hardness testing
3.4.1Tensile testing [REF 7]
Most materials are generally supplied to a mechanical property specification. This
usually involves data on tensile strength and ductility. Tensile strength is a measure of thematerials ability to withstand a load under tension. Ductility is a measure of thematerials ability to be permanently stretched, again under tension.
The most common method used to determine tensile strength and ductility is thetensile test. This involves preparing a specially shaped standard test piece that has no
sudden changes in cross-sectional area and then pulling it carefully in one direction
with a continuously increasing load. The test-piece may be round or rectangular in
cross section, depending upon the shape of the bulk material; for
example, samples with rectangular cross sections are prepared from sheet material. In
both cases, the central portion of the test piece is reduced in section to form a gauge
length. The reduced section helps to ensure that fracture, when it occurs, does so
within the gauge length rather than within the grips where surface imperfections may
induce premature failure.
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FIG 3.6 TENSILE TESTING[REF 7]
The extension is measured and plotted against load producing a load / extension
curve, as illustrated in FIG.
The curve has several distinct sections.
0 A where the extension is linearly proportional to load. Point A is the limit ofproportionality.
A B extension non linearly proportional to load. The extension from
O B is elastic deformation, and point B is the elastic limit.
B C the extension is non linearly proportional to load, and is plasticdeformation uniformly distributed along the length.
C D extension is plastic but localised.
27
FIG 3.7 TENSILE TESTING
27
The point B is important as it marks the change from elastic to plastic behaviour. It can
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be difficult to locate on the curve, as the change can be gradual. To overcome this a point
is added to the curve at X. X is found by measuring a distance Y, along the extension
axis and drawing a line parallel to OA. The intersection of this straight line with the
curved line is not open to interpretation error. The generally used value for Y is 0.2% of
the original length under test.
The load L1 associated with X, divided by the original cross sectional area, gives the
0.2% proof stress for the material.
Similarly L2 divided by the original cross sectional area gives the tensile strength.
The elongation is given by the total extension divided by the original length (thegauge length) presented as a percentage.
It should be noted that stress is defined as the load per unit area(for example, expressed in units of MPa);
- strain is the extension of the gauge length divided by the original gauge length(expressed as a fraction).
In the linear elastic part of the load - extension curve, O A in there is negligible
change in the cross-sectional area of the sample, so we may say that the ratio of stress to
strain is a constant, that is :
stress / strain = a constant (E) , known asYoungs Modulus.
The springiness of a material (its stiffness) is indicated by its Youngs modulus. For
most aluminium alloys, irrespective of their metallurgical conditions, the value ofYoungs Modulus is close to 68 GPa ( for the special case of
lithium-containing alloys, where there is a significant increase in stiffness).
The part of the load-extension curve given by C D in represents
incipient fracture. Appreciable necking of the sample occurs, leading to fracture.
Note that a progressive reduction of cross-sectional area occurs in the necking region;
the stress (ie the load per unit area) continues to increase, even though the total load
decreases.
The ratio of the cross-sectional area of the fracture surface to that of the original
cross-sectional area is known as the reduction in area , usually expressed asa
percentage.
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3.2.2 Hardness Testing[REF 7]
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Hardness testing is a relatively quick and easy way to assess the strength of a material
without the need to prepare tensile test samples. For example, it may be a convenient way
of investigating the progress of precipitation hardening.
The majority of commercial hardness testers force a small hard metal or ceramic
sphere, diamond pyramid or diamond cone into the body of the metal under test bymeans of an applied load, and a definite hardness number is obtained from thedimensions of the indentation so formed. In practice, the dimension of the indent is
referred to a set of values defined in a hardness index chart. Hardness then may be
defined as resistance to permanent deformation, and a hardness test can often be
considered as a rapid non-destructive estimation of the plastic deformation behaviour
of metals.
Small indenters are used for microhardness testing, with a special instrumentequipped with an optical microscope to view the micro-indent. This provides a very
valuable technique for investigation of the relative hardnesses of phases within a
microstructure.
Although the term hardness is a comparative consideration of great engineering
importance, it is not considered to be a fundamental property of matter. The index of
hardness is a manifestation of several related properties of the metal, which may well
include a combined effect of yield point, tensile strength, ductility, work-hardening
characteristics and resistance to abrasion.
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4.0 CONCLUSIONS
The biggest issue that was raised in the simulation of foundry problems was the lack of
consistent process information which appears to be inherent in the foundry industry.
The response from the foundry, although delighted by their reduction of scrap, was
disappointment in the inability of the software to predict the occurrence of the defects
or to define the conditions which caused the defects.
The main conclusions that can be drawn from this study are:
By using simulation software intelligently it is possible to help foundries reduce
scrap rates even for defects which cannot be predicted.
The boundary conditions used to represent the process at the foundry are of
extreme
importance and must be assessed critically.
The difference between changing boundary conditions in reality and static
boundary conditions in the models gave rise to some discrepancies and an
inability to predict some defects.
More work should be performed in defining the mechanisms and or new modelsfor a wider range of defects than is currently possible.
MAGMAsoft has been validated to be able to produce reliable simulation
results that actually reflect the real castingphenomena. The results signify the
validity of using MAGMAsoft to perform mold design and casting
processsimulation. The results of mold filling and solidification would be of
high fidelity that can be relied upon to make decisions on designing the mold,
feeder, sprue, runner and gating system as well as setting the casting process
parameters to achieve the desired casting quality. The success of this validation
alsoserves as a milestone to further utilize and explore the application of
MAGMAsoft not only on sand casting, but also on gravity die casting, lowpressure die casting and high pressure die casting
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REFERENCE
1) INTERNATIONAL ATOMIC ENERGY AGENCY, Guidebook on Non-destructiveTesting of Concrete Structures, IAEA Training Course Series No. 17 (2002).
2) INTERNATIONAL ATOMIC ENERGY AGENCY, Training Guidelines in Non-
destructive Testing Techniques 2002 Edition, IAEA-TECDOC-628/Rev. 1, Vienna
(2002).
3) INTERNATIONAL ATOMIC ENERGY AGENCY, Development of Protocols for
Corrosion and Deposit Evaluation in Pipes by Radiography, IAEA-TECDOC-1445,
Vienna (2005).
4) MALHOTRA, V.M., 1984. In Situ Non-destructive Testing of Concrete."American Concrete Institute (ACI), Publication SP-82. 1984, 825 pp.
5) MALHOTRA, V.M., AND SIVASUNDARAM, V., 1991. CRC Handbook onNon-destructive Testing of Concrete: Resonance Frequency Methods. CRC press,
editors, V.M. Malhotra, N.J. Carino, pp. 147-168.6) MINDESS, SIDNEY. 1991. CRC Handbook on Non-destructive Testing ofConcrete: Acoustic Emission Methods.CRC press, editors, V.M. Malhotra, N.J. Carino,
pp. 317-334.
7) Prepare by M.H.Jacobs interdisplinary research centre in material the university of
Birmingham,UK.
8)http://www.espint.com/engineering/technical-reference-
guides/default.aspx by dr.jerry thiel
9) Googleimage.com
BOOKS
1) PRINCIPLES OF FOUNDRY TECHNOLOGY BY P. L. JAIN
2) A TEXT BOOK OF FOUNDRY TECHNOLOGY BY O.P.KHANNA
3) PRODUCTIONS TECHNOLOGY BY R.K .JAIN
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