Iron foundry basics
Figure 4: Cast iron fire bar
Practical Action
As melting proceeds, the material in the prolong slowly sinks into the charge already melted in the
crucible itself. The rapid melting of the charge should be encouraged by working the furnace nearly
"flat out". There is a tendency for the hot, but not yet melted charge in the prolong, to stick, and by
poking and prodding with a long steel bar (about 10/15 m/m in diameter), the pieces will drop into
the crucible.
It is advisable to keep the furnace very full of charcoal, and it does not matter (indeed it is
desirable) if some pieces of charcoal fall into the crucible with the iron, as this tends to reduce
oxidation. At quite an early stage it is good practice to have a lid sitting on the top of the prolong;
and as soon as the charge has melted down, and is contained entirely in the crucible, the prolong
itself should be removed, and the sliding cover on the furnace should be closed. The charcoal must
be replenished as it burns - the speed of the stoking has a significant effect on the speed of
melting. Normal melting time is approximately two hours, but this may differ considerably with the
type of fuel used. Approximately 56Ibs (25 kg) of iron can be melted in the crucible furnace shown
in Figs. 1 or 2 at each melt.
Although the object of this profile is to concentrate mainly on the melting of cast iron, it is worth
mentioning that non-ferrous alloys, such as aluminium, can also be used with this system. The fact
that these types of alloy become molten at a much lower temperature than cast iron does not
diminish the need to take very great care in carrying out the melting operation (see Safety).
Carrier for Crucible
To be purchased. Recommended to be of safe construction and correct fit for crucible, can be
made by local blacksmith.
The Cupolette Iron Furnace
Although the cupolette has greater melting capacity, its output could prove to be more than is
required. For economy reasons, the cupolette should operate for at least an 8 hour period, after
which it will require re-lininq, or repairs to the lining. Estimating an output of approximately one
ton per hour, this gives 8 tons of molten metal, available in one cycle of operation. It is essential,
therefore, that sufficient moulds are available to take a pour of this quantity. Any surplus molten
metal can be poured into open moulds, or sand trenches, and can then be fed back into the
cupolette in future melts. This surplus should be
kept to a minimum, as the greater amount of metal re-melted, the greater the cost of the product.
The cupolette consists of a steel shell "A" (Figs. 5, 6 & 7) in an upright position on a base-plate,
which is usually supported on four steel joists or tubular columns "J". The shell is lined with good
quality refractory brick "R". At the base of the steel shell are dropped doors, "S". These are hinged
doors, which after the furnace has completed operation, are opened to allow the debris to be
discharged.
The steel shell is 1/4 (6 m/m) thick plate of riveted or welded construction. A wind belt "G" is
provided at a height of 2 or 3 feet (610 to 915 m/m) above the base plate. The air is supplied to
the wind belt from a fan or blower "N" and the blast is conveyed to the interior of the furnace by
tuyeres (nozzles) which may be situated in, or under, the wind belt. The number of tuyeres used
depends on the size of the furnace. It is usual to allow one tuver o for each 6" (150 m/m) of
internal diameter. The height at which the tuyeres are set above the working bottom of the furnace
depends on the capacity of molten metal required.
The working bottom in the cupolette is made up with moulding and a fettling hole "0", provided at
the bottom of the shell. The fettling hole is usually about 18" (450 m/m) square, and is covered by
a plate held in position by a bar during the operation of the furnace.
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