unit 4 by jg notes

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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.





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.





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.





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





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.





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.




Manufacturing Processes Prepared by – Prof.S.C.Kulkarni & Prof. Jay Gavade 6

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





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-

\





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





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.





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





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.

unit 4 by jg notes

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