Solidification of Castings

Bhaskara P AcharAsst. Professor,

NMAM Institute of Technology, NitteKarnataka

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SolidificationSolidification

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Embriyo… Nuclei…Dendrite…Cell….Grain….Growth…

SolidificationSolidification

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SolidificationSolidification

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

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

Homogenous nucleation

)((Surface). )(Volume).( ΔG G

).(4 ).(34 ΔG 23 rGr v

r2r3

1

Neglected in L → Stransformations

)( TfGv

energystrain in increase energy surfacein increase energy freebulk in Reduction nucleationon changeenergy Free

Nucleation

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SolidificationSolidification

Heterogeneous nucleation

Consider the nucleation of from on a planar surface of inclusion

)( )()(A )(V ΔG lenslens circlecirclev AAG

Alens

Acircle

Acircle

Created

Created

Lost

Interfacial Energies

Cos

0

0.25

0.5

0.75

1

0 30 60 90 120 150 180 (degrees) →

G* he

tero /

G* ho

mo → G*

hetero (0o) = 0no barrier to nucleation

G*hetero (90o) = G*

homo/2

G*hetero (180o) = G*

homo no benefit

Complete wetting No wettingPartial wetting

Cos

Rate of nucleationNucleation

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GrowthSolidification

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

nuclei

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Cast structuresSolidification

Pure metal Alloy Nucleating agent

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Dendritic GrowthSolidification

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◦ 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

Melting Furnaces

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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

Furnace selection

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Melting of Metal

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Filling the Material into the mould

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Type of Furnaces• Cupola furnace• Air furnace• Rotary furnace• Electric furnace• Open hearth furnaceCupola 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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Cupola Furnaces

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Cupola operations1.Preparation of cupola2. Lighting the fire in coke bed

-Through tap hole with electric or dry wood pieces3. Charging the cupola- C-F-M…- 4% Fluxes- CaCO3, NaCO3, CaC2- M:C 4:1 to 12:14. Melting5. Slagging and molt tapping6. Dropping down the cupola

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Zones of Cupola Furnaces1.Well - molten metal collector2. 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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Zones of Cupola Furnaces5. 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 temperature6. 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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Advantages of Cupola 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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Limitations of Cupola 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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Hot blast Cupola • 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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Cokeless Cupola4.