lect-ns-casting6 Melting Furnaces

8/7/2019 lect-ns-casting6 Melting Furnaces

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Melting is an equally important parameter for obtaining a quality

castings.

Different furnaces are employed for melting and re-melting

ferrous and non-ferrous materials

Some of the furnaces used in metal casting are as following:

Cupola

Reverberatory or air furnaceCrucible furnaces

Electric furnace

Melting Furnaces


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Factors :

(i)considering of initial cost and cost its operation

(ii)relative average cost of repair and maintenance

(iii)availability and relative cost of various fuels in the particular locality

(iv)melting efficiency in particular speed of melting

(v) composition and melting temperature of the metal

(vi)degree of quality control required in respect of metal purification and

refining

(vii)cleanliness and noise level in operation

(viii)personnel choice or sales influence

Furnace Selection


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-A cupola is a vertical cylindrical furnace equipped with a

tapping spout near its base

-Cupolas are used only formelting cast irons, and although other

furnaces are also used, the largest tonnage of cast iron melted in

cupola.

-Cupolas - advantages

are capable of operating continuously,

have high melting rate,

are available in various sizes and capacities,

and are easy and economical to operate

Cupola Furnace


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It consists of a large shell of steel lined with

refractory.

The charge consisting of iron, coke, limestone

(flux) and possible alloying elements is loaded

through a charging door provided at the top

(alternate layers). The iron is usually a mixture of

pig iron and scrap.

Coke is the fuel used to heat the furnace.

Air blast is introduced through tuyeres (openings

near the bottom of shell) forcombustion of the coke

- metal melt

Limestone acts like a flux - that reacts with coke

ash and other impurities to form slag - will be

removed through a slag hole

The slag serves to cover the melt, protecting it

from reaction with the environment (oxidation)

inside the cupola and reducing heat loss.


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The furnace is periodically

tapped to provide liquid metal

for the pour.

The exhaust gases emerge from

the top of the cupola. Emission

control technology is used to

treat the emissions to meetenvironmental standards.

Well -is the space between the

bottom of the tuyeres - to

collect the molten metal before

tapping out


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In the reverberatory furnace the

material to be melted is separated fromthe fuel - the heat from the combustion

flame is transferred to the metal by

radiation and convection.

(It suffers from low energy efficiency

which is normally in the range of 20-

25%)

The basic chemical conversion is the

combination of carbon and hydrogen

from the fuel with oxygen from the

intake air to form carbon dioxide andwater vapor.

A schematic of Reverberatory furnace

Reverberatory /air Furnace

The flames are directed across and at least 12 in above the surface of the

metal bath in horizontal paths.


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A schematic of Reverberatory furnace

-This furnace type is more robust and less sophisticated that others.

-It suffers from low energy efficiency which is normally in the range of 20-25%

-Is used for melting in one heat large

quantities of metal,

-those most suited being all grades of

cast-iron and the alloys of brasses

and bronzes.

-this type of furnace is also used for

the production of wrought iron

-is often the choice for die casting

aluminum.


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The advantages

-high volume processing rate

-low operating and maintenance costs.

The disadvantages

-high metal oxidation rates,

-low efficiencies,

-large floor space requirements.


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-Is the most suited forsmall foundries and can be designed

for melting any of the metals

-Non ferrous metals are melted in numerous types of

furnaces, the crucible furnace is the most common and the

lowest in initial cost

-The furnace is essentially a refractory-lined cylinder with

a refractory cover, equipped with burner and blower for

the intense combustion ofcoke, oil or gas.

-The metal is melted in crucible (pot) made of clay and

graphite or silica carbide which is placed in the furnace.

-When the melting is completed, the furnace is turned off,

the furnace cover is opened, and the crucible removed with

tongs and placed in a pouring shank. Then the liquid metal is

poured into prepared molds

Crucible Furnace


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-Advantages

it gives high rate of production,

much less pollution

and the ability to hold molten metal at constant temperature for a

long time for alloying purposes.

-It suitable for all types of metals and alloys(preferred for high meltingpoint alloys e.g. steel)

-Unlike cupola or air furnace, electric furnace possess greater

adaptability and provide precise control over the temperature of

molten metal.

-The high cost of electric power is a limitation.

These may be

(a)direct-arc type (b)indirect arc type and (c)induction type

Electric Furnace


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Direct-arc furnaces develop temperature up to 1925OC.

Indirect-arc furnaces, often used for melting copper and its alloys, develop

lower temperatures.

Induction type furnaces with still lower temperature are used for melting

nonferrous alloys


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- Used chiefly for the melting of non-ferrous metals Induction heating is a

heating method.

-An induction furnace uses alternating current passing through a coil to

develop a magnetic field in the metal and the resulting induced current

causes rapid heating and melting of the metal

-Once the metal is in its molten state the magnetic field yields a stirring

motion

-The stirring rate is determined by

the applicable power and

frequency. This rate is controlled

to lower the temperature gradients

in the charge and to assure entire

melting of charge and adequate

mixing of alloy and fluxing

materials.

Induction Furnace


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The heating system in an induction furnace includes:

1. Induction heating power supply,

2. Induction heating coil,

3.Water-cooling source, which cools the coil and several

internal components inside the power supply.


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Advantages of Induction Furnace

Induction heating is a clean form of heating

High rate of melting or high melting efficiency

Alloyed steels can be melted without any loss of alloying elements

Controllable and localized heating

Disadvantages of Induction Furnace

High capital cost of the equipment

High operating cost

lect-ns-casting6 Melting Furnaces

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