Iron and Aluminum. [Volume: 8, Issue: 2, 1888, pp. 116-122]

CHARCOAL IRON WORKERS.

119

a perfectly homogeneous and solid casting, but the color is darker
and the grain shows that some of the carbon has taken the graphitic
form. The thin casting shows this even more than the heavier
bar, showing that the change occurred suddenly and that time
had but little effect. Each similar addition of aluminium produces a corresponding effect until, at the third addition, or with
three-fourths of one per cent.,.the casting is gray, with no sign of
white, either in the square or in the thin bar.
The set of tests with the gray iron base, containing 11 per cent.
of silicon, shows that silicon and aluminium work together in the
same direction, and that a slight addition of aluminium takes the
white out of the casting at once, giving the same grain in a thin
as in a thick casting. This effect increases as the aluminium increases, and the indications arc that at least up to 4 per cent., the
limit of our experiments, the more the aluminium, the softer and
grayer the castings.
If cast iron be cooled very suddenly, the carbon, which the melted
metal holds in combination, will not have time to separate, and
will be retained in the combined state. Such castings are called
chilled castings. Chill is caused by molten iron running against
a body which rapidly withdraws its heat, causing it to retain its
carbon in the combined form. Back from the chill, where this
instantaneous cooling could not exert its full effect, a portion of
the carbon takes the graphitic form. This property is made use
of when it is desirable to obtain hard wearing surfaces, and, in the
same casting, tough and soft central portions, as in car wheels.
While this chilling effect is exceedingly valuable for many purposes, yet, generally speaking, the founder desires exactly the reverse.
We have said that aluminium causes the carbon to assume the
graphitic form on the instant of solidifying, and therefore the sudden
abstraction of heat does not imprison the combined carbon and
cause chill. This effect of aluminium is to give a uniform grain
for thick and thin castings, and not allow the coldness of the
mold to affect the grain. * * * * *
In castings having sufficient aluminium to cause the separation
of graphite, there will be no sand clinging to the face, and the
surface will be as soft as the interior of the casting. * *