Brief introduction on Melting practices in foundries

2. Solidification
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3.  Embriyo… Nuclei…Dendrite…Cell….Grain….Growth…
Solidification
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4. Solidification
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Solid (GS)
Liquid (GL)
Tm T →
G→
T
G
Liquid stableSolid stable
T - Undercooling
↑ t
 On cooling just below Tm solid becomes stable
 But solidification does not start
 E.g. liquid Ni can be undercooled 250 K below Tm
G → ve
G → +ve

5. Homogenous Heterogenous
Nucleation
NucleationSolidification + Growth=
Nucleation
 The probability of nucleation occurring at point in the parent phase is same
throughout the parent phase
 In heterogeneous nucleation there are some preferred sites in the parent
phase where nucleation can occur
 Liquid → solid
walls of container, inclusions
 Solid → solid
inclusions, grain boundaries,
dislocations, stacking faults

6. Homogenous nucleation
)((Surface).)(Volume).(ΔG  G
  ).(4).(
3
4
ΔG 23
 rGr v 






r2
r3
1
Neglected in L → S
transformations
)( TfGv 
energystraininincreaseenergysurfaceinincreaseenergyfreebulkinReduction
nucleationonchangeenergyFree


Nucleation

7. Solidification
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8. Heterogeneous nucleation
Consider the nucleation of  from  on a planar surface of inclusion 







  )()()(A)(VΔG lenslens circlecirclev AAG 
Alens 
Acircle 
Acircle 
Created
Created
Lost
Interfacial Energies






Cos

9. 0
0.25
0.5
0.75
1
0 30 60 90 120 150 180
 (degrees) →
G*
hetero/G*
homo→
G*
hetero (0o) = 0
no barrier to nucleation
G*
hetero (90o) = G*
homo/2
G*
hetero (180o) = G*
homo
no benefit
Complete wetting No wettingPartial wetting






Cos
Rate of nucleation
Nucleation

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Growth
Solidification
 Growth process decides crystallographic structure of solid
 Rate of growth depends on the constraint by neighbouring nuclei

11. Solidification
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Pure metal Alloy Nucleating agent

12. Solidification
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13. ◦ The quality of casting depends on the method of melting.
◦ Molten metal is prevented from oxidation by covering the
molten metal with fluxes
◦ Before pouring into the mould the metal has to be in
liquid state.
◦ A furnace is used to melt the metal.
◦ Different furnaces are employed to melt ferrous and non-
ferrous metals.
◦ Heat in the furnace is created by combustion of fuel,
electric arc etc.
◦ A furnace contains high temperature zone where the
metal to be melted is placed
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14. Furnace selection depends on,
• The type of metal,
• The maximum temperature required
• Rate and the mode of molten metal delivery
• Initial cost of the furnace
• Fuel cost
• Melting and pouring temperatures
• Quantity of metal to be melted
• Method of pouring required
• Cost of furnace repair and maintenance
• Cost of operation
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• Cupola furnace
• Air furnace
• Rotary furnace
• Electric furnace
• Open hearth furnace
Cupola Furnace:
• Cupola is used for melting scrap metal or pig iron used in
the production of iron casting
• Cupola is available in different sizes
• Cupola can be employed for as long as needed for
producing a given amount of iron
• Fuel used is generally a good grade low sulphur coke,
anthracite coal or carbon briquettes

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1.Preparation of cupola
2. Lighting the fire in coke bed
- Through tap hole with electric or
dry wood pieces
3. Charging the cupola
- C-F-M…
- 4% Fluxes- CaCO3, NaCO3, CaC2
- M:C  4:1 to 12:1
4. Melting
5. Slagging and molt tapping
6. Dropping down the cupola

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1. Well - molten metal collector
2. Combustion zone (Superheating/ oxidizing
zone)
A temperature of about 1540°C to 1870°C
is achieved in this zone. Few exothermic
reactions takes place in this zone these are
represented as:
C + O2 → CO2 + Heat
Si + O2 → SiO2 + Heat
2Mn + O2 → 2MnO + Heat
3. Reducing zone
The temperature falls to about 1200°C
CO2 + C (coke) → 2CO + Heat
4. Melting zone
3Fe + 2CO → Fe3C + CO2

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5. Preheating zone
The main objective of this zone is to
preheat the charges from room
temperature to about 1090°C before
entering the metal charge to the melting
zone.
CO2, CO, N2 expelled gases maintain this
temperature
6. Stack
The empty portion of cupola above the
preheating zone is called as stack. It
provides the passage to hot gases to go to
atmosphere from the cupola furnace.

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 It is simple and economical to operate.
 A cupola is capable of accepting a wide range of materials
without reducing melt quality. They therefore play an important role
in the metal recycling industry
 Cupolas can refine the metal charge, removing impurities out of the
slag.
 Cupolas are more efficient and less harmful to the environment
than electric furnaces.
 The continuous rather than batch process suits the demands of a
repetition foundry.
 High melt rates
 Ease of operation
 Chemical composition control
 Efficiency of cupola varies from 30 to 50%.
 Less floor space requirements comparing with those furnaces with
same capacity.

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 Since molten iron and coke are in contact with each other, certain
elements like Si, Mn are lost and others like Sulphur are picked up.
This changes the final analysis of molten metal.
 Close temperature control is difficult to maintain

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• Combustion carried out using ‘preheated blast of air’
• Preheating by ‘cupola stack air’ or ‘external air pre heater’
• Combustion improvement and lesser coke consumption
• Efficient but maintenance problems occur

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4.