Unit 4 by Jg Notes
-
Upload
arunkumarnoola -
Category
Documents
-
view
217 -
download
0
Transcript of Unit 4 by Jg Notes
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 1/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 70
Unit – 4
Fettling, Cleaning and Inspection of Castings
• Need for fettling, stages in fettling, equipments used in fettling and cleaning of castings
• Common important defects in castings• Inspection procedure
Need For Fettling
After a casting has solidified and cooled down sufficiently in an expendable mould, the
first step is to make casting free from the mould. This operation is called as the " Shake out
operation". Since a great deal of heat and dust are involved in this operation, the
operation is usually mechanized. Shake out is usually done by means of vibratory
knockouts, jolt ing grids and vibraters. The mould is intensively jolted and broken up.
After shaking the casting out of the mould, it is conveyed to the fettling (dressing)
shop for cleaning and finishing. The fettling process (cleaning and finishing) consists
of the following operations:
De coring or core removal, cleaning of surfaces, removal of gates, risers and fins,
repair of defective castings if possible and heat treatment.
Before starting the fettling process, the castings are examined for defects such as misrun,
drop, cold laps and cold shuts. Defective castings are set aside and are not cleaned.
Stages in Fettling:
1. Core removal or Core knockout: -
Due to the reasons mentioned under "shake out operation", this operation is also donemechanically. Hammering and vibrating will loosen and break up cores. Stationary or
portable vibrators are employed for this purpose. To knockout cores from heavy castings, it
is advantageous to use air drills. Removal of cores by hydro-blasting is more clean process
keeping in view the dust problems. The operation consists of breaking up and washing out
the cores with a jet of water delivered at a pressure of 25 to 100 atm. A recent method for
core removal is hydro-sand blasting in which sand is mixed with the water jet. This method
results in a sufficiently clean surface.
2. Cleaning of Surfaces:-
This operation involves the removal of adhering sand and oxide scale and produces a
uniformly smooth surface. Some Mechanical methods are employed for this as:
a. Tumbling:
This method is used for cleaning small and light castings. The castings are loaded into a
tumbler or a barrel along with white iron picks (jack stars) (in an amount of 20 to 35% of
the mass of castings). Rotation of the barrel causes the castings and jack stars to tumble.
Jack stars remove the unwanted sand from the surfaces of the castings and also the
castings slides on one another. This operation removes the adhered sand and oxide
scale from the surface of the castings. The rotational speed of the barrel is 30 rev/min.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 2/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 71
b. Sand blasting and Shot-blasting:
These methods are widely used to clean surfaces of light, medium and heavy castings. In
these machines, dry sand or shots (white C.I. shot, steel shot) or grit of white C.I. or
steel (grit is made by crushing shot) is blown by a stream of compressed air against the
surfaces of the casting. The impact of the abrasive particles traveling at a high speed, onthe surface removes the adhering sand and oxide scale. Velocity of the abrasive
particles leaving the nozzle of the machine is in the range of 35 to 75 m/s and the air
pressure is in the range of 0.7 MPa.
c. Airless shot blasting:
In airless shot blasting or mechanical impact cleaning, shots are hurled on the surface of a
casting by a fast rotating paddle wheel. For harder castings, the shots are made of white
iron, malleable iron or steel, whereas for softer non-ferrous castings, these are made of
copper, bronze, glass or mild steel. The wheel rotates at 1800 to 2500 rev/min. The
velocity of shots striking the casting surface is about 60 to 72 m/s.
This method offers the following advantages: a high output (10 times the output of a
pneumatic shot blasting machine), low power consumption, shot jet speed regulation by
changing the speed of the rotor, and better working conditions. The drawbacks of the
method are: rapid wear of rotor blades and poor cleaning of shaped castings with intricate
cavities.
d. Hydro blasting:
This is the most effective surface cleaning method. Here two operations are
accomplished simultaneously: Core knockout and surface cleaning. Castings are placed on a
rotary or stationary table and high velocity jets containing about 15% sand and 85% water,
under a pressure of 10 to 20 MPa, are directed at the casting surface. The jet velocity can beupto 100 m/s. The method produces no dust but consumes lot of water.
3. Removal of Gates and Risers:
Gates, runners, risers and sprue can be removed before or after cleaning operations. In brittle
materials, these are simply broken off from the castings. In more ductile materials, the
following methods are used to remove them: power hacksaws, band saws, disk type
cutting benches, Abrasive cut off wheels, flame cutting with an oxyacetylene cutting
torch and arc cutting for heat-resistant and acid-resistant steels which are not amenable to
gas cutting. The surface of cut becomes rough and needs additional treatment.
Fins or flash (that forms when melt flows into gaps between two mould halves or at cores),
ends of nails and other unwanted projections are removed by: chipping, sawing, flame
cutting, flame gouging and grinding.
4. Power Cutting:
Power cutting is a process by which large risers and gates can be rapidly removed from
castings. Preheated iron powder is introduced into an oxygen stream. This burning iron
then attacks the metal riser or gate by a process of fluxing and oxidation.
5. Some minor defects detected may sometimes be repaired by welding without affecting the
function of the finished casting.
6. The finished castings are sometimes subjected to various heat treatments to modifymechanical properties or to reduce residual stresses.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 3/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 72
Common Important Defects in Castings:
Casting Defects and Remedies:
The defects in a casting may be due to pattern and moulding box equipment, moulding sand,
cores, gating system or molten metal. Some of the defects and their reasons are discussed
below:
1. Mould shift:
It results in a mismatching of the top and bottom parts of a casting, usually at the parting
line.
It occurs due to following reasons
a) Misalignment of pattern parts, due to worn or damaged patterns, and
b) Misalignment of moulding box or flask equipment.
This defect can be prevented by ensuring proper alignment of the pattern, moulding boxes,
correct mounting of pattern on pattern plates etc.
2. Core shift:It is an abnormal variation of the dimensions which are dependent on core position.
Figure: Mismatch
It is caused by
a) Misalignment of cores in assembling cored-moulds,
b) Undersized or oversized coreprints, and
c) By using incorrect size of chaplet.
This defect can be eliminated by providing the core at the proper place and must be
gripped firmly in the sand.
3. Swell:It is an enlargement of the mould cavity by molten metal pressure resulting in localised
or general enlargement of the casting.
Figure: Swell
It is due to the following reasons
a) Insufficient ramming of sand
b) Insufficient weighting of the mould during casting
c) Pouring of molten metal too rapidly or too hard
The swells are avoided by the proper ramming of sand and uniform flow of molten metal into
the mould.
4. Fins and flash:
These are thin projections of metal not intended as a part of casting. These I usually occur atthe parting line of the mould or core sections.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 4/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 73
These are caused by
a) Excessive rapping of the pattern before it is withdrawn from the mould,
b) Insufficient weight on the top part of the mould, and
c) Loose clamping of the mould.In order to avoid this defect, sufficient weight should be placed on the top part of the mould
so that the two parts fit tightly together.
5. Sand wash:
It usually occurs near the ingates as rough lumps on the surface of a casting. The sand that
has been washed away appears on the upper surfaces of the casting as rough holes or
depressions.
Figure: Sand wash
This is due to the following reasons
a) Soft ramming of sand,
b) Weak sand,
c) Poor pattern, and
d) Insufficient draft.
This defect is avoided by the proper ramming of sand.
6. Shrinkage:
It is a crack in the casting or dishing on the surface of a casting which results from unequalcontraction of the metal during solidification. This is due to the following reasons :
a) Improper location and size of gates and runners,
b) Inadequate risers,
c) Lack of directional solidification,
d) Incorrect metal composition, and
e) Incorrect pouring temperatures.
This defect can be eliminated by the use of feeders and chills at proper locations to promote
directional solidification.
7. Hot tear:
It is an internal or external ragged discontinuity in the metal casting resulting from
hindred contraction occurring just after the metal has solidified.
Figure: Hot tears
This defect is due to the following reasons
a) Abrupt changes in section, inadequate filleting of inside corners, and improper
placement of chills. b) Poor collapsibility of mould and core materials which will place extra stress on
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 5/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 74
certain details.
c) Improper pouring temperature.
In order to eliminate this defect, abrupt changes in section should be avoided. The pouring
temperature should be correct and there should be even rate of cooling.
8. Sand blow or blow hole:It is an excessively smooth depression on the outer surface of a casting. This defect is also
called blow holes.
Figure: Blow Holes
This defect is due to the following reasons:
a) High moisture content in moulding sand,
b) Low permeability of sand,
c) Hard ramming of sand,
d) Defective gating system, and
e) Improper venting of sand.
This defect can be removed by proper venting, completely drying up the mould, selecting
proper sand with required permeability and proper in-gate system for the flow of molten
metal.
9. Core blow:
It is an excessively smooth depression on the inner surface of a cored cavity or a gas pocket
immediately above a cored cavity. This defect is caused by using insufficient baked cores. Thusthe cores should be sufficiently baked before using.
10. Honeycombing or slag holes:
These are smooth depressions on the upper surfaces of the casting. They usually occur near
the ingates. This defect is due to imperfect skimming of the metal or due to poor metal.
This defect can be avoided by preventing the slag from entering along with the molten metal.
11. Scab:
These are patches (i.e., slightly raised areas) of sand on the upper surface of casting.
Figure: Scab
This defect is due to the following reasons
a) Uneven ramming of sand, and
b) Slow or intermittent running of metal.
The proper ramming of sand and uniform flow of the molten metal into the mould can
eliminate this defect. Another method to remove this defect is to mix additives such as wood
flour, sea coal or dextrine into the sand.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 6/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 75
12. Cold shuts and misruns:
These occur when the mould cavity is not completely filled and incomplete casting results.
Figure: Misrun Figure: Cold ShutThis defect is due to the following reasons
a) Too small gates,
b) Too many restrictions in the gating system,
c) Pouring head is too low,
d) Faulty venting of the moulds, and
e) Metal lacking in fluidity
In order to eliminate these defects, the casting should be designed keeping in mind the
fundamental principles of gating and risering. The thin sections should be preheated and
the molten metal should be poured at the correct temperature.
13. Pour short:
It occurs when the mould cavity is not completely f illed because of insufficient metal.
It is due to the following reasons
a) Interruptions during pouring operation, and
b) Insufficient metal in the ladles being used to pour the metal.
In order to avoid this defect, the ladle should have sufficient molten metal at the correct
temperature.
14. Metal penetration:
It occurs when the alloy being cast tends to penetrate into sand grains and causes a fused
aggregate of metal and sand on the surface of the casting. It is due to
a) Soft rammed sand,
b) Moulding sand and core sand being too coarse,
c) Improper use of mould and core washes will cause penetration
d) Excessive metal temperature or increased fluidity of metal.
This defect can be eliminated by removing the above mentioned reasons.
15. Run-outs and bust-outs:
These permit drainage of the metal from the cavity and result in incomplete castings. Theseare due to the following reasons
a) A pattern that is too large for a given flask or pattern placed too close to the flask
edge results in a weak spot and cause run-out
b) The match plate surfaces that are out of parallel or uneven results in a poorly
formed parting line and cause a run-out
c) Inadequate mould weights or clamps will permit the cope to lift which results a
run-out d) Improper sealing of mould joints causes run-outs
e) Excessive pouring pressures may cause run-out
f) Misalignment of cope and drag may promote a run-out
The corrective measures taken in respect of the above reasons will prevent this defect.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 7/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 76
16. Rough surface finish:
It is merely a lack of sufficient smoothness in the casting. It is due to the following reasons
a) Soft ramming of sand,
b) Coarser sand,
c) Hard pouring or too high metal fluidity, and
d) Improper use of mould and core washes often promotes rough casting surfaces.
This defect can be avoided by using a proper mould and ramming of sand.
17. Crush:
It is an irregularly shaped cavity or projection on the castings caused by the displacement of
the sand at the mould joints or core prints, which usually occurs when the mould is being
closed. It occurs due to the following reasons
a) Badly made mould joints,
b) Excessive pressure on the sand surface c) Too large cores or too small core prints.
This defect can be eliminated by taking proper care in placing the cope over the drag.
The sand in the cope should be rammed properly.
18. Warpage:
It is unintentional and undesirable deformation that occurs during or after solidification. It is
due to the following reasons:
a) Continuous large flat surface on castings, indicating a poor design, and
b) No directional solidification of casting.
This defect can be eliminated by modifying the casting design and proper directional
solidification.19. Drop:
This defect appears as an irregular deformation of the casting. It occurs on
account of a portion of the sand breaking away from the mould and dropping into the
molten metal. The above breaking takes place due to low green strength in the sand,
too soft ramming, insufficient reinforcement of the cope or other sand projections.
Increase in green strength of the sand by suitable modification in its composition, hard
ramming and adequate reinforcing of cope and other sand projections by means of
bars, mails and gaggers etc„ are the principal remedies of this defect.
Figure: Drop
20. Rat Tails or Buckles:
When molten metal, poured into the mould, is at excessively high temperature, it
causes the thin outer layer of moulding sand to expand appreciably. When this layer
fails to compress back and gets, separated from the sand behind it, it remains over the
surface of the casting and finally appears over it as an irregular line, called a Rat tail
Figure: Buckle Figure: Rat tail
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 8/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 77
Inspection Procedure and Testing of castings:The aims of inspection and testing of castings (that is quality control) are to prevent
defective castings being supplied from the foundry and to reduce the percentage of
reprocessing.
The various inspection and testing procedures may be classed as follows:1. Visual Inspection:
Visual inspection of castings can reveal many of the common surface defects such as
misrun, cracks and warping etc. This method is very common and is applicable both in
piece and mass production of castings.
The inspection is carried out in two steps: prior to cleaning and annealing and then after the
final finishing operation.
2. Dimensional Inspection:
Geometric dimensions of castings are checked by means of measuring tools such as plug and
snap gauges, template gauges, marked-out plates and special alliances, to establish whether
the dimensions of the casting conform to the drawing or not and to make sure that the
pattern and core boxes are correct. The deviations of dimensions should not exceed the
permissible limits.
3. Metallurgical Control:
Under this the chemical composition and the mechanical and other properties are determined
in a laboratory.
The chemical composition of castings is checked by the methods o f chemical and spectral
analysis. For this, the test pieces are commonly cast-on test bars, that is, cast integral with
the casting, or separately cast test specimens prepared for checking strength properties.
The strength or mechanical tests include test in: bending, tension, hardness, compression,shear and creep.
4. Pressure Testing:
This test is carried out on those castings to be used for conveying liquids or gases. The
castings are checked for pressure tightness or impermeability and leakage. The tests
include:
Water or air-pressure test: In water pressure test, the casting is held tube under a
certain pressure of water; the test pressure depends on the conditions under which the
casting has to function. The outer surface of the casting must be dry; otherwise it will not
be possible to detect the traces of leakage, if any.
Air-pressure test: A soap solution is applied to the surface of the casting. When thecasting is subjected to air-pressure testing, bubbles will appear on the surface showing the
place of leakage, if any.
5. Radio-graphical Testing:
Internal defects in a casting such as cracks, voids, cavities and porosity etc., as well as
surface cracks can be observed by radiographic inspection using x-rays and y-rays.
In x-ray testing short wave length rays from an x-ray tube are passed through a
casting and recorded on a special film held against the opposite face of the casting. If
the casting-has an internal defect, the density of the material at that spot will be less as
compared to the surrounding material. This area will allow more penetration of the rays,
that is, the sections of the casting with crack cavities will absorb a smaller amount of x-
\
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 9/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 78
rays as compared to fully dense material. This will result in the appearance of a dark
shadow on the x-ray film reproducing the contour of the defect. The power source used
for x-ray tube is a high-voltage source: 200 kV for casting thickness upto 50 mm and 1
million volts for thickness form 50 to 180 mm.
Gamma-ray testing is used for checking heavy-walled castings since these rays are
more penetrating and less scattering as compared to x-rays. Y-ray radiates from Radium or
its salts contained in a capsule.
Figure: Radiographic Testing
.
6. Magnetic Testing:
In this method, the casting to be tested is magnetized and then placed between the poles
of an electromagnet or in the magnetic field of a solenoid coil. The energized coil is now
moved along the casting. If the coil comes across a defect on its way, the magnetic flux
changes its direction and induces an emf in the coil turns, the value of which shows up on the
galvanometer.
The method can detect defects (cracks) on the surface or slightly below the surface of a
casting. Thus, it supplements the radio graphical methods which ordinarily cannot detect
small cracks. However, the method can be applied to castings made from ferromagnetic
metals.
7. Magnetic Particle testing:
This method of inspection is a procedure used to determine the presence of defects at or
near the surface of ferromagnetic castings.
The method is based on the principle that, if an object is magnetized, surface cracks and
voids in the material, which are at an angle to the magnetic lines of force, interrupts the
magnetic field which gets distorted. That is, there is an abrupt change in the path of a
magnetic flux flowing through the casting normal to the surface defect. This results in a
local flux leakage field and hence interference with the magnetic lines of force. The
magnetic lines spread out in order to detour around the interruptions as shown in Figure.This interference is detected and hence the shape and size of the crack or void is revealed,
by the application of a fine powder of magnetic material, which tends to pile up around and
bridge over the discontinuities. A surface crack is indicated by a line of the fine particles
following the outline of the crack.
The magnetic powder may consist of fine iron filings, but Fe 2O3 is preferred which is
ground to pass a 100-mesh sieve. A variation of the method is that the magnetic particles
are prepared with a fluorescent coating. Inspection will be carried out under U.V. light to
intensify the effect. Every crack will be marked by a glowing indication.
When the plain magnetic powder is used, the trade name of the method is "Magna-flux",
but when magnet ic part icles with a fluorescent coat ing are used, the method is
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 10/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 79
called "Magnaflow- or "Magnaglo".
The powder may be applied dry or wet. For the dry method, the powder is applied in the
form of a cloud or spray. In the wet method, the powder is suspended in a low
viscosity, non-corrosive fluid such as kerosene oil (100 g of magnetic powder in about 5 1
of K.oil). This liquid (supra flux paste) is sprayed over the surface to be tested, by
hydraulically operated machine or the casting is immersed in the liquid. Then the casting
is allowed to dry. Now, when the casting is magnetized, the magnetic particles will
gather around the crack and in the "Magnaglo" method, they will also glow. The
magnetic fields can be generated either with D.C. or AC., using yokes, bars or coils.
Dry powder method is better for locating near surface defects and is also less messy than
the wet method. The wet method is superior for detecting fine surface defects. Another
big advantage of wet method is that all surfaces of the casting can be reached, including
vertical. Surfaces and the underside of the horizontal surfaces, by housing or by immersion
in the liquid.
Figure: Magnetic particle Testing
From the above discussion, it is apparent that cracks that are in a direction parallel to the
magnetic field would not be detected. The cracks which are perpendicular to the
direction of magnetic field are the easiest to detect.
8. Eddy current Inspection:
In this method, the material of the casting need not be ferromagnetic. The test
includes a probe which is supplied with a high frequency current. It induces an
electric field in the casting. The field changes in the presence of surface or near-
surface defects. These changes show up on instruments.
9. Liquid - penetrant Inspection:
This method can reveal surface defects only but can be used for any material. The surface
of the casting is thoroughly cleaned and dried. Then the liquid penetrants are applied as
sprays or by immersion. The penetrant liquid contains either a material which will
fluoresce under black light or a dye that can be visually detected. The liquid penetrant will be readily drawn into extremely small surface cracks. The surface is cleaned and dried.
Then, a powder material called a "developer" is sprayed on the surface. The penetrant
trapped in defects bleeds out due to blotting action and delineate defects during development.
The extent of the discontinuity in the casting surface will be proportional to the amount of
penetrant bleeding out. If a fluorescent penetrant is used, defects show up as glowing
yellow green dots or lines against a dark back ground. In dye penetrant, defects are
revealed as red dots or lines against a white background.
10. Ultrasonic Testing:
This test is based on the fact that a beam of ultrasonic waves (frequency 20,000 Hz) passes
through a solid (dense) material with little loss but is partially reflected from surfaces.
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 11/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 80
Therefore, this method can detect voids, cracks and porosity within a casting.
The ultrasonic waves are produced by the application of reverse Piezo-electric effect. That
is, if an electric potential is applied across the flat ends of a crystal (quartz crystal), it will
either contract or elongate in the normal direction. The crystal is held against a smooth
surface of the casting with the help of a coupling fluid. A high frequency A.C. (I
million c/s) is impressed across the faces of the crystal with the help of an oscillator.
The sound waves produced travel through the casting. These will get reflected from the
other end of the casting and the signals are measured with a C.R.O. If the casting has same
flaws within it, some of the sound waves will be reflected back and will return to the
instrument earlier. The location of the defect from the testing surface may be readily
obtained by measuring the relative position of the flaw "pip" between the two "pips"
representing the metal thickness. The method is not very suitable for a material with
high damping capacity, e.g. C.I., because in such a case, the signal gets considerably
weakened over some distance.
Figure: Ultrasonic Testing
Repairing and Salvaging defective castings:
Minor defects on the unimportant surfaces of a casting can be repaired and the casting
salvaged. The methods used for repairing the defects of castings are :(1) Cold welding (2) Hot welding (3) Liquid metal welding (4) Metal spraying (5) Luting and
impregnation
7/27/2019 Unit 4 by Jg Notes
http://slidepdf.com/reader/full/unit-4-by-jg-notes 12/12
FABTECH College of Engineering and Research, Sangola UNIT 4
Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade Page 81
1. Cold Welding:
This method is employed to rectify cracks and cavities only on surfaces which are not to be
subsequently machined. Both gas welding or electric arc welding can be used for this.
Various filler materials used are Steel, copper, steel-sheathed copper, copper nickel.
These are used in the form of long rods 5 to 6 mm in diameter. Defective areas to be repaired
are grooved with pneumatic chippers or drilled out. Cracks should be cut out to the entire
depth.
2. Hot Welding:
Hot welding is used to repair large cavities, holes and cracks. Before welding, a casting is
preheated to 5000 to 600°C to preclude the appearance of cracks, stresses and chilling of
the casting metal. The casting is held at this temperature for 45 to 60 minutes. After
this, the defects can be repaired either with an OAW flame or electric arc welding. OAW
with preheating is a suitable method for rectifying the defects in gray iron castings of
complex configuration, whose sections sharply vary in thickness. The filler rods used are ofthe following composition: - C : 3.2 to 3.5%, Si : 3.5 to 4%, Mn : 0.5 to 0.6%. The flux to
be used is a mixture of : Borax : 50%, soda : 47% and silica sand : 3%. After welding, the
casting is annealed at 5000 to 600°C and is removed from the furnace at 50° to 60°C and left
to cool slowly to exclude chilling in the well.
Blow holes (a) Excess moisture content
in moulding sand.
(b) Rust and moisture on
chills, chapletsand inserts used.
(c) Cores not sufficiently baked.
(d) Excessive use of organic
binders. (e) Moulds notadequately vented.
(f) Cores not adequately
vented. (g) Moulds rammed very
hard.
(a) Control moisture
content.
(b) Use clean andchaplets and metal inserts.
(c) Bake cores properly.
(d) Use organic binders
with restraint.
(e) Provide adequate
venting in moulds and cores.
(f) Ram the moulds less
hard.