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Full text of "Modern Mechanical Engineering Vol-I"

CASTINGS  MADE   IN  METALLIC  MOULDS         139

CHAPTER  VI
Castings  made in  Metallic Moulds

Castings made in metallic moulds are embraced in three groups: (a) chill
casting, (£) die-casting, (c) casting in permanent moulds. These have nothing
in common beyond the fact that cast iron forms the whole or a portion of
the moulds. The conditions which control the pouring of liquid metal into
moulds of porous sand and those of iron are so different that the foundries
using metallic moulds are entirely separated from the sand foundries.

Chill Casting.—The fact is familiar that the effect of pouring liquid
metal in contact with a cold metallic surface is to harden—"chill"—the por-
tion that comes in immediate proximity with it. This is utilized in portions

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Fig. 47.—Chill Mould for Trolley Wheel
of numerous castings that are subjected to severe wear, as the treads of trolly
wheels of all kinds (figs. 47 and 48), in the rolls (fig. 49) for the iron and steel
works, for plough points, for mining stamps, stone breakers, balls and rollers
used in crushing and grinding mills, the bores of some wheel boxes, &c.
In all these cases the mould is of a composite character, being composed of
metal over the areas that have to be chilled, and of sand elsewhere.
Composition of the Metal to Chill.—The grey iron used for ordinary cast-
ings, in which the carbon is nearly all in the graphitic condition, will not chill
beyond a surface hardness of the thickness of stout paper. This is of no
value for service. An average thickness is generally required of from -J in.
to | in., extended in massive articles to i in. To produce this, it is necessary
to select a highly mottled iron, in other words, one in which a considerable
proportion of the carbon is in the combined condition, and the total carbon
Content high. And as silicon tends to throw out carbon in solution into the
graphitic state, the proportion of this element must be kept low. Sulphur
and phosphorus should be higher than for grey iron castings, since they
intensify the chilling effect. Manganese, below one per cent, is beneficial.