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Full text of "Modern Mechanical Engineering Vol-I"

146                                 FOUNDRY  WORK
casting required. And even then the conditions of internal strain are so
severe that prolonged annealing of the castings is necessary for the sole
purpose of relieving these strains, and lessening the hardness of the metal.
Malleable Cast Iron.—This is a white iron, having the whole of its
carbon in the combined state. It is poured into sand moulds, and annealed
subsequently in ovens for about sixty hours. This changes the carbon
into the graphitic state, rendering the castings soft and extremely ductile.
White iron is necessary, because a grey iron would produce spongy castings
after annealing. The amount of combined carbon must never be lower
than 2-75 per cent. As the white irons, which are viscous when poured,
are used, the runners have to be large. The shrinkage allowance is also
greater than that for grey iron, and generally precautions similar to those
when making steel castings have to be taken, in the form of large shrinkage
heads, and the provision of fillets.
The Brasses and Bronzes.—In all these alloys the shrinkage is
large, being about J in. in 10 in. The metal is not so fluid as grey cast iron,
and it sets very quickly. Large runners and large shrinkage heads are there-
fore necessary, not with a view to prevent fracture, which rarely occurs, but
to avoid " draws " and hollow places in the more massive sections. Feeding
is necessary in almost all moulds, even more so than in iron, because the
shrinkage is greater. The metal in the pouring cup chills quickly, so that
fresh metal must be supplied if the mass of the casting is large. Special
care must be taken in making the dispositions of the runners. These must
be brought into the heavier sections. It is well in deep castings to pour
from the bottom. A very large volume of brass work is made with odd
sides, or is alternatively plated. In each case numerous small patterns,
which may be like or dissimilar, numbering, say, from half a dozen to twenty,
are moulded in one flask, and poured from a common ingate. In these
cases the runners must be of sufficient area to fill the moulds farthest from
the ingate before the metal has had time to congeal, and little or no feed-
ing can be done. Both green and dry sands are used for moulds, the
first, as with iron castings, predominating. Generally, the moulds should be
rammed harder than those for iron, and well vented.
Aluminium and its alloys are usually poured into moulds of green sand.
The shrinkage of the metal is about double that of brass, and large runners
are required. The melting-point is rather low, being about 1160° F. The
metal must be poured quickly, as it sets rapidly. The pouring basins are
large, to act as head metal. Metal rods are frequently inserted in moulds to
hasten the cooling of the thicker parts of castings. The alloys of aluminium
are numerous. The chief elements employed are copper, zinc, manganese,
and magnesium. Moulds of green sand are used, rammed more loosely
than those for iron or brass, to prevent shrinkage cracks. The sand may
be finer than that for brass, as relatively little gas is given off, and it need not
be very refractory. The moulds can be dusted with black lead or French
chalk. Green sand cores are desirable, but if dried, they must not be too
hard, or they will check shrinkage.