Types of Casting Furnaces, Inspection and cleaning of casting

UNIT NO 2
Technology of Melting and
Casting
By- Prof P.B. Borakhede

CONTENTS
 Melting Furnaces
1. Crucible Furnaces
2. Cupola Furnace
3. Open Hearth Furnace
4. Electric Arc Furnace
 Inspection of Casting
 Cleaning of Casting

INTRODUCTION
 An effective melting of various metals in suitable
furnaces is always carried out in a foundry shop.
 The various melting operations carried out in foundries
are actually remelting processes in that the ores found
in mines are not directly used.
 Remelting is done in different types of furnaces and
poured at suitable temperature into mould to give the
casting.
 The large number of furnaces are have been
developed.
 The furnaces are based on following factors:
a) Rate of melting desired, depending upon quantity of
metal required to be melted per hour.
Prof. P.B. Borakhede, MGI-COET, Shegaon

b) Type of metal to be melted.
c) Temperature required.
d) Capability of melting medium for absorbing impurities.
e) Method of pouring the molten metal
f) Economical considerations; ie initial investment to be
made as cost of equipment and its installation,
maintenance cost and cost of fuel to be consumed.

TYPES OF FURNACES
1. Crucible Furnaces
o These are simplest of all the furnaces used in
foundries.
o They are used in most of the small foundries where
melting is not continuous and a large variety of metals
is to be melted in small quantities.
o In these furnaces the entire melting of metal takes
place inside a melting pot, called crucible which is
made of clay and graphite.
These furnaces can be classified in two groups
a) Coke-fired furnace b) Oil and gas fired Furnace.

a) Coke Fired Furnace
 These furnaces are generally
installed in a formed Pit and are
used in a small quantities of ferrous
Metals( pig iron) for producing iron
Castings and also for non ferrous
metals and alloys.
 They are provided with Refractory Linings inside a
Chimney at the top.
 Coke is used as a fuel. Both natural as well as artificial
droughts can be used.
 The air flow is provided to coke for burning.
 Broken pieces of metal are placed in the crucible.

 Bed coke is fired in the furnace and crucible placed into
it.
 When coke is burnt the crucible in pit gets heated and
metal inside the crucible is melted.
 This melted metal is used for casting purpose.
Advantages:
 Faster Heating
 Saves Floor Space
 Easy to Load
 Lower installation and startup cost.

b) Oil and Gas Fired Furnaces
 These furnaces utilize oil or gas as a
Fuel.
 A mixture of gas and air or oil and air
is fed into the furnace which burns inside
to produce the desired temperature.
 The mixture usually enters tangentially and encircles
the crucible while burning.
 The furnaces essentially consists of a cylindrical steel
shell, provided with refractory lining inside and proper
passage for entry of the fuel mixture.
 The crucible is seated on pad formed at the bottom.
 A cover is produced at the top to prevent heat losses.

 These furnaces may be of stationary type or tilting type.
Advantages:
 Low investment costs
 easy operation and maintenance ability
 capable of melting small batches of various alloys
 the melt can be treated directly in the crucible and the
alloy can be quickly and easily replaced as necessary.

2. Cupola
Furnace

 For melting of cast iron in foundry the Cupola Furnace
is used.
 Figure shows cross sectional view of cupola.
 It has a construction in the form of hollow vertical
cylinder made of strong mild steel plates and riveted or
welded at the seams.
 In cupola the stress in the whole structure is distributed
uniformly.
 The bottom door of shell can be in one piece or two
piece hinged to supporting leg.
 The bottom door is supported by a prop so that it may
not be collapsed.
 When prop is removed door drops down providing a
clear space for the coke fire, residue of the molten
metal with slag and sand bed.

 A wind chamber is connected to furnace blower by
means of blast pipe.
 The amount of air required is forced into the chamber by
blower, which enters in the furnace through openings
called tuyeres.
 The charging door is located at a suitable height above
the charging platform.
 This platform is of robust mild steel supported by four
legs.
 Weighted quantities of metal, coke, scrap and flux are
collected on this platform which are charged into cupola
when required.
 The top of cupola is provided with a mesh screen and
spark arrester. It is a cone shaped construction.

 This attachment facilitates a free escape of waste gases
at the same time deflects spark and dust back to
furnace.
Working:
 Soft and dry pieces of wood is first placed over the sand
bed followed by a small amount of coke charge known
as coke bed.
 Coke is put gradually in furnace through charging door.
 Cover plates are opened to allow free entry of air for
combustion and they are open till entire bed is fully
ignited.
 A weighed proportionate amount of metal (pig iron,
scrap), flux and coke is then fixed over the bed charge.

 They are repeated in alternate layers of each until the
cupola is full to the charging door.
 The furnace is preheated before charge in the furnace.
 As the charge enters in furnace blower starts, tuyers
directs the air flow.
 The amount of flux to be added in metal charge
depends upon quality of the charged metal and scrap
and composition of coke.
 Usually limestone is used as flux.
 When the coke burns, the metal melts.
 Molten metal is drained out through well and slag is
removed through slag hole.
 This molten metal is used for casting purpose.

Zones in Cupola
 Various zones of cupola are shown in figure.
 Number of chemical reactions are takes place in these
zones which are explained below.
1. Well
• It is space between bottom of tuyers and sand bed.
The metal after melting, trickles down and collects in
the space before it is tapped out.
2. Combustion Zone
• It is also known as oxidising zone. It is located above
the top of the tuyeres.
• Total height of this zone is 15 cm to 30 cm.
• The actual combustion is takes place in this zone,
consuming all free oxygen from air blast and

producing a lot of heat, which is sufficient enough to meet
the requirements of other zones of cupola.
• A temperature of about 1540˚C to 1870˚C is produced in
this zone.
• Reactions takes place in this zone can be represented
C + O2 CO2 + Heat
Si + O2 SiO2 + Heat
2Mn + O2 2MnO+ Heat
3. Reducing Zone
• It is also known as the protective zone. It is located
between the top of the combustion zone and the top
level of coke bed.
• CO2 is reduced to CO in this zone through an
endothermic zone temperature to about 1200 ˚C.

• The reaction is as follows
CO2 + C ( of Coke) 2CO – Heat
• Nitrogen and other main constituents of the upward
moving hot gases does not participate in reaction.
4. Melting Zone
• The first layer of metal charge over the coke bed
constitutes this zone.
• The solid metal changes to molten state in metal picks
up sufficient carbon content in this zone as represented
by following reaction:
3Fe + 2CO Fe3C + CO
5. Preheating Zone
• It extends from above the melting zone to the bottom
level of charging door and contains a number alternate
layers of coke and metal charge.

• The function of this zone is to preheat the charges from
atmospheric temperature to about 1093 ˚C before they
settle downwards to enter the melting zone.
• This preheating takes place due to upward advancing
hot gases.
6. Stack
The empty portion of cupola above the preheating zone,
which provides the passage to hot gases to go to
atmosphere, is known as stack.

Advantages of Cupola Furnace
• Initial cost is comparatively lower than other furnaces.
• Operation and maintenance of this furnace does not
involve too many complications.
• Cost of operation and maintenance is lower.
• Floor area required is very small.
• It can be operated for a number of hours continuously.
• It does not involve very complicated problems in its
design which is comparatively simpler.

3. OPEN HEARTH FURNACE
 Open hearth process is also known as Siemens
process, who was the first to introduce the idea of using
a regenerator for preheating the air for combustion
before entering the open hearth furnace.

 These furnaces are used to melt pig iron which is used
in steel making.
 The furnace is heated by burning of gas.
 It operates on preheat fuel and air.
 There is a combustion chamber in which iron ore is
melted.
 The charging doors are attached to combustion
chambers through which charge inserts into chamber.
 Refractory chambers are connected combustion
chamber .
 On the other side there are regenerative chambers
which are preheated by flue gases.
 The gas and air pipelines are connected to these
chambers.

 At first the gas and air flows through pipelines and
refractory chambers to combustion chamber.
 When they enters in combustion chamber they mixed
together and burning takes place.
 They creates a flame which increase the temperature of
combustion chamber and the metal charge with flux
melts.
 There is generation of flue gases because of melting of
flux and metal.
 These flue gases flows through pipelines to
regenerative chambers, which preheats the chambers
and exits through chimneys.
 Next time the gases and air flows through regenerative
chambers which are already preheated.

 When burning gases and air passes through regenerative
chamber they gets heated, when they enters in
combustion chamber the efficiency of burning increases.
Advantages
 Scrap Iron, low grade pig and cast iron and the iron ore
(haematite) can he directly converted to steel.
 The temperature can be controlled more effectively since
external source of heat Is used.
 The composition of steel is uniform and accurate and can
be controlled easily as the product is analysed from time
to time.
 Iron is not lost as slag since blast of hot air is not passed
through the molten mass.
 Steel obtained is of high grade and good quality.

Electric furnaces are widely used in steel-making.
Two types of electric furnaces are commonly used in steel
making. They are:
a) The direct Arc Furnace
b) High frequency induction furnace
a) Direct Arc Furnace
 It consist of a steel shell having spherical bottom.
 The complete furnace is mounted on rollers, so that it
can be tilted for pouring the melt into the laddle.
 The hearth has a bowl shape and it provided with
basic lining with magnesite or dolomite.
4. ELECTRIC ARC FURNACE

 Two spouts are provided on opposite side one for slag
and one for molten metal.
 The roof is detachable type and charge id feed through
it.
 Three vertical electrodes are suspended through the
top, through which a 3- phase current is led into the
furnace.

 These electrodes can be raised up or lowered as
desired.
 After charging, the furnace top is closed and the
electrodes lowered.
 The current is switched on to generate the arc, thereby
producing a high temperature of about 2000˚C or above.
 This intense heat melts the charge. As the level of
molten metal rises, the electrodes are also raised
automatically.
 The charge usually consist of light and heavy steel scrap
together with suitable amount of flux.

 This furnace consist of a crucible surrounded by water
cooled coil copper tubing.
 This coil also conducts the high frequency current and
acts as primary winding.
b) High Frequency Electric Furnace
( Indirect Electric Arc Furnace, Induction Furnace)

 The metal charge in the crucible serves as secondary
winding.
 Thus furnace works on the principle of a transformer.
 As high frequency current is passed into primary winding,
Eddy currents are produced in metal charge (secondary
winding) through induction.
 Thus charge is rapidly melted and agitated.
 The furnace is of tilting type.

3. Indirect Arc Furnace
 It is used for making carbon steel or ferrous steel from
scrap iron, iron ore.
 Pig iron, iron carbide is melted and converted it into high
quality steel.
 Lime stone is used as a flux.
 High power electric arc is used between electrodes.
 In this furnace the charge is heated indirectly by radiant
heat from electric arc.

 It consist of a horizontal barrel shape steel shell lined with
refractories.
 First metal charge and flux are inserted into the
combustion chamber.
 Then two graphite electrodes are inserted in combustion
chamber and placed in front of each other horizontally
near the charge.
 When electric supply is passed through both electrodes,
the arc is produced, which will give a heating effect.
 This heat melts the metal and flux, the slag will be at
upper side of the molten metal which removes impurities
in metal.
 The barrel shaped shell is designed to rotate and reverse
through approximately 180°C in order to avoid excessive
heating of the refractories above the melt level and to
increase the melting efficiency of the unit.

Advantages of Electric arc furnace
 It allow steel to be made from 100% scrap metal
feedstock.
 These furnaces are used when closed control of
temperature and exact amount of alloying elements are
important.
 These are good for making high carbon steel, steel
alloyed with metals, stainless steel.
 Higher temperature can be reached other steel making
furnaces.

INSPECTION OF CASTING
 Inspection is done to detect the internal and external
defects in them.
 Since a large number of defects are not visible from
outside, it is likely that casting may be defective inside
inspite though there perfectly clean and defect free
surface outside.
 This may ultimately result in the failure of casting to
provide the desired service.
 The inspection of casting is necessary to avoid failure.
 There two types of inspection
a) Destructive Testing
b) Non destructive Testing.

a) Destructive Methods
 This method includes picking few sample castings,
cutting them into pieces at the points where defects are
suspected and them examining their section surface to
find out internal defects.
 Few methods are described as follows:
1. Visual Inspection
• It comprises of inspecting the surface of the casting
either with naked eye or sometimes with the help of a
magnifying glass or a suitable microscope.
• Almost all the castings are subjected to this inspection.
• This method only detect defects which are on the
surface.

2. Inspection for Dimensional Accuracy
 Dimensional accuracy is important in those castings
which are to be machined.
 Their dimensions are checked after cleaning in the
foundry.
 Various precision measuring instruments are used to
check the dimensions.
 A proper checking of dimension also reveals as to
whether the pattern and core boxes used are correct.
 All undersized castings are rejected as any further
machining on them will effect more reduction in their
size.

3. Sound Test
 It consist of suitably suspending the casting, free of
floor and all other obstructions, and then gently striking
it with a hammer.
 The sound is carefully noted.
 Tapping by the hammer is done at different points and a
change in the pitch and quality of sound indicates a
discontinuity within the mass of casting.
 It is difficult to locate discontinuity and the extent to
which it is present.
4. Impact Test
 In this test a hammer of a suitable sixe is either struck
against or allowed to fall on those portions of castings
which are suspected to carry defects.

 A defective portion will break under this impact and a
sound portion is expected to be capable of withstanding
this blow.
 However, it cannot be said to be a reliable and sure test
always.
5. Pressure Test
 Pressure tests are always applied to those castings
which are to be used for containing or carrying gases or
liquids.
 Such castings are tested for leaks through their wall.
 The fluids used for applying pressure are water, steam
or air.
 Before applying the pressure, all openings of the
casting are closed.

 Water pressure may be given with the help of small
pump and the surface inspected to locate the leaks.
 When steam is used to provide pressure the leaks are
enlarged due to its heat and become more evident.
 When air pressure applied the casting is immensed in
water. If there are leaks, the air bubbles start coming up.
6. Radiography or X-Ray Tests
 Radiographic tests involve the use of light rays of
relatively shorter wave lengths.
 The rays used for this purpose are X rays and Gamma
rays.
 X rays are produced in a high vacuum glass tube
carrying a positive anode and a negative cathode.

 Electric current is used to heat the cathode, followed by
application of high voltage between anode and
cathode.
 As a result of this electrons starts flowing from cathode
to anode, which are obstructed by a tungsten disc to
convert them into a Beam of Rays.
 This beam is passed out of tube and is allowed to
penetrate through casting to fall on a photographic film
placed suitably behind the casting.
 This film is later developed in the usual way to reveal
internal defects.
 Cracks, Blow holes, cavities etc will show by light
portion.
 A better penetration through metal is obtained by using
Gamma rays instead of X rays on account of their
shorter wave lengths.

 This enables application of these rays even at those
places where X rays cannot be used.
 Gamma rays are emitted by both Radium and Cobalt 60
and both are commonly used for the purpose.
7. Magnetic Particle Testing
 This method can be used only for those metals and
alloys which can develop magnetic properties eg iron
and steel.
 The principle involved in this test is that in a magnetized
metal if its magnetic field is interrupted by a crack its
continuity is broken.
 Due to low magnetic permeability of air some magnetic
flux lines leak out of the metal.

 If a magnetic material is spreads over that portion some of
it is held there by the flux lines to show the presence of a
crack or void there.
 So for this test, casting is first magnetized and then fine
particles of iron or steel are spread over its surface.
 The presence of cracks is revealed by the held up
particles on the surface.
8. Ultrasonic Testing
 The technique is using of sound as a basis of judging the
soundness of an article.
 Ultrasonic inspection is used to detect very small internal
defects.
 Ultrasonic means a sound wave of a small wavelength
which can not be hear by human being.

 This technique involves sending of very high frequency
vibrations into the material being tested, which is
reflected partially or fully after striking the flaw or the
surface of component.
 The noted signal is noted and interpreted to get the
result.
 A pulse oscillator is used with transducer to convert
electrical energy into mechanical vibrations.
 Transducer carries piezoelectric crystal which changes
electric oscillations to mechanical vibrations.
 The transducer is placed at the top surface of component
and Ultrasonic beam of vibrations sent into material.
 Another transducer called as reciever transducer is used
to receive reflected signals which are then amplified,
filtered, processed.

 These signals are displayed on oscilloscope which are
finally interpreted to get the result.
 This is highly sensitive technique through which internal
defects, cracks, voids can be detected.
 The test is performed speedily and it needs access to
only one side of the component.
 It can also be used for measuring thickness and
detecting flaws in joints or between adjoining surface
between materials.
9. Liquid Penetrant Test
 It is a simple test for detecting such flaws or defects
which extend up to surface of the material.
 At first component is cleaned and dried.

 Then liquid material called penetrant is applied to
surface.
 The penetrant used is such that is can be drawn into the
surface discontinuity by capillary action.
 It can be applied to surface by spraying, brushing or
dipping.
 A fluorescent material is added into it. This material will
radiate in ultra violate light to make penetrant traces
more evident.
 The penetrant is saturates in flaws, cracks voids etc.
 Excess penetrant is then wiped out from surface.
 Then absorbant material called as developer is added on
the surface which absorbs penetrant.
 The cracks, defects etc shines due to flouroscent
material. The cracks are repaired then.

CLEANING OF CASTINGS
 When casting is manufactured it goes through many
processes.
 When casting is removed from mold it is not completely
finished as it carries risers, runners, gates, chills etc
attached to it. Also a lot of sand remains on its surface in
form of core.
 The cleaning and finishing is necessary before it can be
brought to usable form.
 The various operation are as follows
a) Removal of dry sand cores
b) Removal of gates and risers
c) Removal of unwanted metal projections fins, nails.
d) Surface Cleaning

1. Removal of Cores
 For removal of dry sand core from the casting the latter
is first suitably rapped to loosen and break the core
sand.
 The core sand is then removed through a poking
action by means of a metallic bar.
2. Removal of Gates and risers
 Gates and risers attached to castings can be removed
through various means.
a) They may be broken away by hammering.
b) They may be sawn by means of metal cutting saw.
c) May be sheared off by means of suitable punches or
sprue cutters.
d) May be chipped off by chipping hammers.

e) May be cut off by oxy acetylene flame.
f) May be removed by means of Abrasive cut off wheels.
3. Removal of unwanted Metal projections, Fins & Nails
1The operation of removal of these unwanted metal parts
attached to casting called snagging.
1. Chipping with hand or pneumatic chisels.
2. Cutting with oxy-acetylene flame.
3. Grinding by means of grinders.
4. Machining
5. Filing

4. Surface Cleaning
Surface of most of castings are required to be cleaned to
remove adhering sand and oxide scale.
a) Use of wire brush to clean surface.
b) Tumbling :
 In this process casting is placed inside large barrel
with number of cast iron pieces.
 Both ends of barrel are closed and the same rotated
for some time. The casting is cleaned.
c) Sand blasting:
 In this method a stream of high velocity air, carrying
large grain size sand particles is thrown on to surface
of casting.
 Abrasive sand particles are introduced into air blast by
means of suction, gravity feed or direct pressure.

 The abrasive action of sand particles, striking against
casting surface at very high velocity provides cleaning
action.
d) Shot Blasting:
 This method is similar to sand blasting, but in this
metallic abrasive are fed into air blast instead of sand
grains.
 They may be in form of shots or cut wires.
 It provides a very fast rate of cleaning.

IMPORTANT QUESTIONS
 Discuss direct arc and indirect arc electric furnaces with
neat sketch.
 Describe 'Pit Furnace' with figure.
 What is 'CUPOLA'? Explain preparation or operations of
Cupola for melting.
 Explain 'lnduction furnace' with its importance.
 What do you mean cleaning of casting? Explain any one
method.
 Explain chemical reactions in different zones of cupola
furnace.
 Draw a sectional view of cupola furnace showing details.
Also, explain various cupola operations.

 Explain the following inspection methods.
Magnetic particle testing.
Radiography
 What are the advantages of Electric Furnace? Explain
direct arc furnace with neat sketch.
 What do you understand by Non destructive testing of
castings ? Explain any one method of NDT in detail.
 What factors governs the selection of type of furnace in
foundry ?

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