Illustration: BESSEMER’S PROCESS.
Steel is usually made by a process called “cementation.” Bars of the
best Swedish or Russian iron, about six feet long, are placed in an
iron box, the bottom of which is covered with a layer of charcoal
powder; over the first row of iron bars some more charcoal is put,
and then another row of iron bars, and so on till the box is full,
when it is carefully closed and kept at a white heat for four or
five days. When cold, the bars are found to be converted into steel,
and, being rough and blistered on the surface, are called “blistered
steel;” this is broken up, and the bars laid side by side and made
hot in a forge, where they are welded together by the blows of a
heavy hammer, and drawn or rolled out by machinery into bars of
“fine steel.” Steel differs from iron in the closeness of its grain,
in being very much “tougher,” and in having that very useful and
peculiar property called “temper,” which is the power of hardening
when suddenly cooled while red-hot. If a bar of steel as soft as
iron be made of a bright red heat, and then suddenly plunged into
cold water, it will be found to have become harder than any other
metal (so hard, indeed, that it will scratch glass), and is as
brittle and readily broken as flint or glass. If now a gentle heat
be applied to it, this extreme hardness of temper gives way. For
instance, if a piece of bright hard steel is held for a moment in
the hollow of a clear fire, a pale straw color appears on its surface,
it is now still very hard (but not so hard as before), and is fit
for razors, surgeons’ instruments, &c; but if held in the fire a
moment or two longer, it becomes of a bright golden yellow, and is
fit for penknives, and other cutting instruments; held longer still,
it becomes bright blue, and is fit for watch-springs, swords, and
other purposes requiring great elasticity but no great hardness; if
the heat be carried still further a brown tinge is seen, and it is
now rather soft, but greatly harder than iron, and is still elastic;
saws, coach-springs, and many other articles are made from steel at
this temper. If the heat be carried on to redness, the steel would be
quite soft when it had slowly cooled, but if suddenly cooled (as by
being plunged into water) the original hard temper comes back again.
Steel, like iron, may be cast, and cast-steel is one of its most
useful forms, and much resembles “fine steel.” The mode of preparing
cast-steel is to melt the “blistered steel” in a crucible, or earthen
pot, and then run it into a mould: this forms an “ingot” of steel,
which may be afterwards rolled or welded as the case may require.
Steel may be drawn into very fine wire, or wrought into the most
minute articles, as the springs and other parts of watches. It
bears a very fine and bright polish, and does not rust or tarnish so
easily as iron. It has lately been proposed to make heavy cannon of
cast-steel, which is much tougher than either cast-iron or gun-metal.
Illustration: (‡ FIG. 1. and FIG. 2.)
PERPENDICULAR SECTIONS OF CONVERTING VESSEL.
Illustration: FIG. 3. OUTSIDE VIEW OF CONVERTING VESSEL.
A process has lately been invented by Bessemer to supersede the
long and laborious process of “puddling.” It consists essentially of
transferring the melted iron into a vessel in which there are tubes
inserted at the lower part, and through which air is forced at a
great pressure, which bubbles up in streams through the melted metal,
and, as it does so, unites with the carbon and sulphur of the iron,
converting them into carbonic and sulphurous acids, and at the same
time producing an increased heat, which is quite necessary to the
success of the process, for as the iron becomes purer it also becomes
more difficultly fusible, and would set into a solid mass, but that
this greatly increased temperature keeps it fluid. This rising of
temperature is similar to what takes place upon blowing a common fire
with bellows; for the more air that is admitted to carbon raised to
a very high temperature, the more rapidly does it combine with its
oxygen, or, in other words, the more rapidly does it burn. So that,
in this process of Bessemer’s contriving, the carbon of the iron acts
as fuel to keep up the heat necessary to maintain its fusion, and
at the same time, by being converted into carbonic acid, escapes in
bubbles (like the bubbles which escape from soda-water), and this
rapid production of gas in every part of the fused iron also assists
in bringing about a thorough stirring-up and mixing together of all
its parts. Reference to the accompanying diagrams will illustrate the
working of the process. In figs. 1 and 2, A B are lower chambers,
C is the melted iron, D is an upper chamber for melting scrap-iron,
&c., E in fig. 1 and D in fig. 2 are openings for the escape of gas
and flame, F F is an air-passage running all round and communicating
by G with the tuyere-holes shown at E in fig. 4, H is the fire-brick
lining to the furnace, and I the tapping-hole through which the
fluid iron is discharged. In fig. 3, I is the tapping-hole, K the
main air-tube leading from the blast-engine, L L perpendicular tubes
(marked G in figs. 1 and 2) leading from circular air-passage to
tuyere-holes, M is an opening for the insertion of iron into the
upper chamber, opposite to opening D in fig. 2, and N the tap for
regulating the blast; O indicates the outer casing of wrought iron.
When the process is carried only to a certain point, the result is a
sort of semi-steel, which the inventor of the process expects will be
a very useful article of commerce.
Illustration: FIG. 4.
HORIZONTAL SECTION OR PLAN OF CONVERTING VESSEL.
During the bubbling up of the whole mass of iron and the extreme
elevation of temperature caused by the union of the carbon of the
impure iron with the oxygen of the air, the oxide of iron as fast as
it forms fuses into a sort of glass, and this unites with the earthy
matters of the impure iron and floats on the upper part as a flux,
thus ridding the cast-iron of all its impurities, with no other fuel
than that contained in the iron itself and the air.
The accompanying illustration represents a horizontal section or
plan of the converting vessel; A the central chamber, B a lining
of fire-bricks, C the main air-tube, D the tapping-hole, E the
tuyere-holes through which the air is forced into the melted iron
to be purified, F is the outside casing of wrought-iron.