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Full text of "Modern Mechanical Engineering Vol-I"

153                              FOUNDRY  WORK
down on the charge. Or electrodes are inserted perpendicularly, and the
arc is drawn between these and the bath of metal, or the slag or carbon in
the trough.
The furnaces that operate by electrical induction must be so designed
as to counteract what is termed the " pinch effect". When the molten
metal lies in an open channel in a horizontal plane, a break occurs in the
current at an early stage, interrupting the circuit at the point of smallest
cross-section, and checking the melting. This pinch effect, which does not
occur in furnaces melting steel, has to be counteracted by producing a violent
circulation of the liquid metal in secondary channels or loops situated below
the charge. This is effected in different ways, in which the electric energy
is converted into heat, with rapid movements, sufficient to prevent inter-
ruption of the circuit.
Furnaces for Malleable Cast Iron.—Frequently, these are air
furnaces of the reverberatory design. To a very small extent, cupolas and
open-hearth furnaces are used. As the white iron used has to be melted
very hot, the reverberatory furnaces are built of great length, and the metal
is tapped where it is hottest. This occurs near the fire bridge, and the bed
is sloped towards this part. The fire grate is located at one end, and the
chimney at the end opposite, or to one side. The flame passes over a bridge
next the hearth, and is deflected on the metal by the low roof, which is
usually arched. To facilitate the charging of the metal, the roof is generally
made in separate sections, " bungs ", each consisting of an iron framing,
enclosing fire bricks. The sides of the furnace are built of steel plates,
reinforced with binders, and the foundation is concrete. The lining is of
brick, enclosing fire brick, also used for the roof. The working bed is of
siliceous sand, and is relined when it becomes burned away.
The annealing of the castings is done after they have been fettled, with
the result that the combined carbon is nearly all changed to graphite, and
the castings, instead of being intensely hard and brittle, have their strength
and ductility greatly increased, so that they have acquired the general pro-
perties of iron forgings. The castings are packed in boxes, " saggers ",
with hammer scale or haematite ore, piled in furnaces, and subjected to a
prolonged temperature of from 800° to 900° F. in annealing ovens. The
designs of these are numerous, though the principle is simple. The boxes
of castings, luted to exclude all air and piled in the oven furnace, are sub-
jected to the heat from solid fuel burnt in a grate at one end, or from gaseous
fuel. Flues are arranged beneath the floor, frequently also at the sides and
roof, designed with the object of delaying the escape of the hot gases until
they have rendered up all their useful heat.