Skip to main content

Full text of "Modern Mechanical Engineering Vol-I"

See other formats

CASTING  THE   METALS  AND  ALLOYS             145

per minute.    Chilled car wheels, gear wheels, projectiles, and pipes are the
principal articles made in these moulds.

Casting the Metals and  Alloys
Although the principles and the general methods of making all sand
moulds are similar, yet some details have to be varied with the character of
the metal or alloy used. These are so important that the work of the different
foundries is carried out by different sets of men who have become specialists.
Each of these departments would admit of extended treatment, but the leading
facts only can be stated here.
The Iron Foundry.—This, which embraces the largest proportion of
cast work, is in a sense the standard to which the practice of the other depart-
ments is referred, and with which they are contrasted. The shrinkage of
iron is moderate, averaging -| in. in 15 in. The metal is poured mostly into
moulds made of green sand, the ingates and runners of which need not be
very large, since the metal flows freely, appearing when thoroughly melted
nearly as liquid as water. The thinnest pipes and plates can be poured,
no trouble arises from the segregation of the elements, and, generally, the
conditions under which the work of the iron foundry is performed are
satisfactory. The pouring of moulds has to be modified with the grade of
iron used. The grey irons, with say 3 per cent of graphitic carbon, remain
fluid longer than the mottled grades with about half the carbon in the com-
bined state, and therefore the runners for these have to be dimensioned
to fill the mould more rapidly. Another fact is, that the effects of shrink-
age are more severe, with liabilities to fracture if shrinkage is hindered.
The Steel Foundry.—The difficulties of the steel founder are those
consequent on the high temperature of casting, and the large amount of
shrinkage. While the temperature of molten grey iron is about 2250° F.,
that of steel is about 2800° F. As the melting-point of silica sand is in the
neighbourhood of 3200° F., partial fusion of the mould is liable to occur.
This is the reason of the rough skin seen on so many steel castings. Hence
these are seldom made in green sand, but in dried moulds with a sand mixture
high in silica. Only new sand which has not been damaged by heat is used
for facings. The chief trouble has always been the shrinkage. This, which
amounts to about TV in. per foot, coupled with the high temperature of pouring,
inevitably produces cracks, warps, hollow places, and fractures in castings
that are badly proportioned. Patterns designed for the iron founder cannot,
as a rule, be used for steel. Runners have to be much larger, feeding heads
and risers are necessary, large fillets are inserted to strengthen adjacent
parts, and thin sections must not be tied. In some cases these precautionary
provisions will add from 50 per cent to 100 per cent to the weight of the
VOL. I.                                                                                                                       10