cast iron, and contains little or no graphitic or uncombined carbon, we
have the very useful compound known as steel. In the earlier half of the
century it was customary to distinguish steel from malleable iron on the
one hand, and cast iron on the other. If the compound contained from 0·5
to 1·5 per cent. of carbon, it was called steel by some authorities,
while others extended these limits a little on either side. Later it was
found that the presence of elements other than carbon can confer steely
properties on iron, and indeed it is possible to have a metal containing
no carbon, but possessing the characteristic properties of steel. Sir
Joseph Whitworth proposed to classify a piece of metal according to its
tensile strength, without any regard to either its chemical composition
or its mode of manufacture: if it could not bear more than 30 tons per
square inch it should be considered iron, but if it had a higher tensile
strength, it should then be regarded as steel. To estimate the
engineering value a figure depending upon the elongation or stretching
of the specimen before breaking was to be added to the number of tons of
the breaking load. This stretching power of steel is in some cases of as
much importance as the tensile strength: the ordnance maker, for
instance, considers a steel with a breaking strength of 53 tons under an
elongation of 5 per cent. as _for his purposes_ to be rejected: while a
specimen showing a breaking strain of only 30 tons along with an
elongation of 35 per cent., on 2 inches of length, he will regard as
good. The tensile strength of steel depends in part on its composition,
in part on the mode of manufacture, and in part on the subsequent
treatment. The _average_ tensile strength of a wrought iron bar per
square inch of section is about 25 tons (30 is the maximum); while the
like average for steel is 43 tons, and some kinds of cast steel will
bear nearly 60 tons. Steel bars of a certain temper subjected by Sir
Joseph Whitworth to a process of hardening in oil showed a tensile
strength of even 90 tons per square inch. These figures will suffice to
show the great utility of steel in structures and machines. But steel
has besides a characteristic property which makes it extremely valuable
in a great variety of applications, namely, its capability of being
_tempered_. If a piece of steel is heated to dull redness and suddenly
cooled by plunging it into cold water, it becomes so extremely hard that
it cannot be acted on by a file; nay, its hardness may be made to rival
that of the diamond, which is the hardest substance known. Now by a
second operation this hardness can be reduced to any required degree:
this is done by re-heating the metal to a certain moderate degree
between 430° F. and 630° F. and again cooling it by immersion in some
cooling medium. In this “letting down” process, it is the highest
temperature that produces the greatest softening, and the properties of
the tempered steel will depend upon the precise degree to which the
metal has been reheated. For example, if the product be required for
making into sword blades, or watch-springs, and to possess much
elasticity, the proper temperature is between 550° F. and 570° F.; but
if the steel is to be suitable for saws the temperature must range
within a few degrees of 600° F., according to the fineness of the tool
intended; a lower temperature would give a metal too hard for them to be
sharpened with a file. On the other hand, sharp cutting instruments and
tools for working metals are obtained hard by tempering at lower degrees
than springs. In practice the index of the temperature is taken from the
colour of the film of oxide that gradually forms on a polished surface
of the metal as the heat is raised, and begins by a very pale yellow (at
430° F.), passing through deeper shades into brown, then through purple
into deep blue (at 570° F.), etc. The reader will now see why watch and
clock springs have their deep blue colour, and he can observe for
himself the whole series of colours by very gradually heating a piece of
polished steel over a small flame.
If we compare the chemical composition of wrought iron and of cast iron
with that of steel as regards the content of carbon, we see at once that
steel holds an intermediate position, so that if in the puddling furnace
we could arrest the decarbonization at a certain point we should obtain
steel; or if, on the other hand, we could put back into chemical
combination with the decarbonized wrought iron a due percentage of
carbon we should in that way also obtain steel. And it will be observed
that the oldest primitive furnaces could not have failed sometimes to
have produced steel as the net or final result of such actions. In fact,
steel always has been and still is produced on one or other of these two
principles, applied in divers ways, but severally and distinctly
directed to that end. Of the many more or less modified processes of
steel-making that have been in use, we need here but briefly mention a
few which were _the_ processes of the first sixty years of our century,
and are to a considerable extent still in operation, although eclipsed
in importance by two other processes that, since the date referred to,
have been supplying the metal in enormously increased quantities, and
which will have to be particularly described.
The most usual of the older processes of steel-making, still carried on
at Sheffield and elsewhere, is known as the _cementation process_: it
consists in heating bars of the best wrought iron in contact with
charcoal, at a high temperature, for three or four weeks. At Sheffield
the iron bars and charcoal are packed in alternate layers into troughs
14 ft. long by 3½ ft. deep and wide, constructed of slabs of siliceous
sandstone 6 in. thick. The last layer of charcoal at the top is covered
to a certain depth with a layer of refractory matter, and the flames
from a furnace beneath are made to envelop the stone troughs or _pots_,
as they are technically called, for a period of a week or more according
to the thickness of the bars operated upon. These are generally 3 in.
broad and from five- to six-eighths of an inch thick. When it is found
by withdrawing a test bar for examination that the operation is
complete, the fire is gradually diminished and the whole allowed to cool
slowly, which requires about a fortnight. Instead of only charcoal, a
mixture of powdered charcoal or soot with a little salt has been used by
some makers—which mixture, technically called _cement powder_, has given
its name to the process. In some works 16 tons or more of iron are
treated in one operation. The bars are found unchanged in form, but
increased in weight by perhaps 27 lbs. per ton, for carbon has combined
with the iron, being apparently transferred in the iron from one
particle to another. The surface of the bars becomes rough and uneven
from a multitude of blebs or blisters, and hence they are called
_blister bars_, and the steel of which they now consist is named
_blister steel_. In this conversion we may suppose that the iron at its
outer surface first enters into combination with carbon taken from the
carbonic oxide gas, which would be produced by combustion of the
charcoal with the limited quantity of air in its interstices, and the
oxygen thus set free would immediately seize again on the surrounding
charcoal, and by repeated changes of this kind in which the oxygen acts
as a carrier of carbon to the iron, in which it is transferred inwards
from particle to particle. The cause of the blisters has been much
discussed: probably the cause is the formation and escape of a volatile
compound of carbon and sulphur at the surface of the soft metal; for it
is known that nearly the whole of the little sulphur in the wrought iron
disappears in the cementation process. Blister steel is never
homogeneous, for near the surface it always contains more carbon than
within; the bars are therefore broken up into short lengths which are
carefully assorted, bound together with wire, heated, welded together
under a hammer or by rolling, and finally formed into a bar, which is
stamped with the outline of a pair of shears, and is then known as
_shear steel_, because this product was generally found the most
suitable for making the shears used in dressing cloth.
Another method of dealing with the blister steel is to charge crucibles
or pots having covers with 50 or 100 lbs. weight of the broken-up bars,
and subject the crucibles to a strong heat in a reverberatory furnace,
when the metal melts, and at the proper moment the contents of a great
number of pots are almost simultaneously poured into a mould to form an
_ingot_. The result is a very uniform steel of the finest texture, known
and highly esteemed as _cast steel_ or _crucible steel_. This steel is
much more fusible than iron, but less so than cast iron.
The production of steel by arresting at a certain stage the
decarbonizing of cast iron in the puddling furnace requires much
experience on the part of the workman, who has to learn when the desired
point has been reached by certain indications, such as the appearance of
the flame, or by the examination of a small sample of the fluid metal
withdrawn and rapidly cooled. Various additions to the charge in
definite proportions are generally made, such as scales of iron oxide,
or a quantity of an oxide ore (hæmatite, etc.) or other materials, the
most essential for a good product consisting of a little manganese in
some form. The result is _puddled steel_; and this, like blister steel,
can be converted into cast steel by fusion in crucibles, running into
ingot moulds, and subsequent treatment by hammering, pressing, rolling,
etc. In 1864 puddled steel was described as an article of great
commercial importance, but this it soon lost by the introduction of
simpler, cheaper, and more reliable processes. The methods and
improvements proposed for the production of steel have been exceedingly
numerous, as is shown by the records of the English Patent Office alone,
which contain up to the end of 1856 specifications of ninety-two patents
for different steel-manufacturing processes, while from 1857 to 1865,
the epoch-marking period of steel making, seventy-four more patents were
obtained for this purpose. It would be quite beyond our limits to make
special reference to these, and to the numerous patents which have since
been granted, but there is one of great importance in steel-making which
must be mentioned, and that is the patent for the employment in the
cementation process of carbide of manganese, taken out by J. M. Heath in
1839. This made England almost independent of the former large
importations of Swedish and Russian iron, and it caused an immediate
reduction of £40 in the price per ton of good steel, effecting a saving
which up to 1855 is calculated at not less than £2,000,000. Heath was
one of those who fail to benefit by their inventions, for his was boldly
appropriated by another person who took advantage of a verbal flaw in
the specification, and Heath did not obtain any redress from the law
courts until, after ten years’ litigation, a majority of Exchequer
judges reversed all the previous decisions against him (1853). In the
meantime the man had died, but as the patent was about to expire his
widow was on petition granted an extension of it for seven years. The
nature of the influence of manganese on steel-making has not been fully
explained, and there is some diversity of opinion on the subject, as it
is said—on the one hand, merely to remove or counteract the injurious
effects of sulphur or phosphorus; on the other, to impart to the steel
greater ductility, strength, and power of welding, tempering, etc.
The manufacture of _crucible_ or _cast steel_ has been carried on at
Essen in Prussia by the firm of A. Krupp & Co., on a scale surpassing
anything attempted elsewhere,—theirs being the largest steel-works in
the world, and remarkable for the variety and excellence of its
products. It began in so small a way that it is said only a single
workman was employed. To the Great Exhibition of 1851, at London,
Krupp’s firm sent a block of crucible cast steel weighing 2¼ tons, a
larger mass of the metal than had ever been shown before, and looked
upon with no little astonishment, for at that time steel was a precious
commodity, the price of refined steel ranging from £45 to £60 per ton.
At the next London Exhibition, in 1862, the Essen Works showed a block
of cast steel 20 tons in weight, and at the Vienna Exhibition of 1873,
one of 52 tons. This casting, which was first made of a cylindrical
shape, was forged into an octagonal form under an immense steam-hammer,
larger than the Woolwich hammer described on a previous page, for the
weight of the moving part is no less than 50 tons. This huge mass of
cast steel was of the finest quality; the forging into the prismatic
form was to show its malleability, for it was intended for the body of a
gun to have a bore of 14 inches. Since the period referred to, ingots of
more than 100 tons have been cast. That shown at Vienna was the product
of some 1,800 crucibles, each containing 65 lbs. of melted steel, which
had to be poured into the mould in a regular and continuous stream, so
that the metal might solidify into a perfectly uniform mass. Such work
can be done only by trained men, who act in regular ranks with military
precision, and in pairs emptying their crucibles into channels
previously assigned, then filing off to the other end of the rank to
receive another crucible, while the pair of men who were behind are
pouring out theirs, and so on in succession. The crucibles are emptied
into a number of channels formed of iron lined with fire-clay, and
leading down into the mould. Many precautions have to be taken to ensure
the regular progress of the operations, and all the time required to
fill the huge moulds may be counted by minutes.
The headpiece to our chapter on Fire-Arms gives but a very inadequate
idea of the magnitude of the Essen Works about 1870. A better notion
will be obtained from a few figures which we select from a list giving
some of the contents of the Essen Works in 1876. There were 1,109
furnaces of various kinds, of which 250 were for smelting; 77 steam
hammers, 294 steam engines, 18 rolling mills, 365 turning lathes, and
700 other machine tools; 24 miles of ordinary gauge railway for traffic
within the works; together with 10 miles of narrow gauge railway; 38
miles of telegraph lines, with 45 Morse apparatus, etc. (J. S. Jeans’
_Steel: its History, etc._, 1880). These figures belong, be it observed,
to the state of things in 1876; but we learn from a later authority that
in 1894 these works employed 15,000 men, and we must suppose that the
plant has been proportionately increased since the earlier period, when
10,000 men were employed.
In the year 1854 a regular system of records began to be kept of the
amounts of coal and ores raised in Great Britain, and also of the
quantities of the various metals produced. These show that in 1894 very
nearly three times as much coal was raised as in 1854, and that in the
same period the quantity of British pig iron smelted annually had
increased four-fold; these increases look small when compared with the
expansion of the steel production in Britain within the same period of
forty years, for this had enlarged _thirty-fold_. This extraordinary
development is attributable to the introduction of two processes by
either of which various steels of excellent quality, and adapted to a
great range of applications, can be produced cheaply and with certainty.
These processes are respectively known as the Bessemer and the Open
Hearth, and the reader should observe that with the main principles
involved in these he has already been made acquainted.
Henry Bessemer, who first saw the light in England in 1813, may be said
to have been born an inventor, for his father was one before him—a
Frenchman employed in the royal mint at Paris, afterwards appointed by
the Revolutionary authorities to superintend a public bakery; on an
accusation of giving short weight, thrown into prison, from which, and
probably from the guillotine, he escaped, and found employment in the
English mint. Subsequently he devised some notable improvements in the
art of producing letterpress type, and for many years carried on a
prosperous business as a typefounder. The son developed inventive
faculties at a very early age: in lathe engraving, dies, dating stamps,
etc. His name became familiar to everyone by his production of the
metallic powder long known as “Bessemer’s Gold Paint.” It became known
to Bessemer that the raw material of this substance, which was then sold
at £5, 10_s._ per lb., really cost only about one shilling per lb., and
he set himself to discover its composition and mode of manufacture. He
succeeded in this so well that he could produce the article at the
insignificant cost of four shillings a pound, and his first order for a
supply of it was at the rate of £4 per lb., and the business was
continued, realising profits of something like 1,000 per cent. at first.
For this article no patent was taken out, but Bessemer himself, assisted
by two trustworthy workmen, carried on the manufacture in secret, and he
some time afterwards rewarded the fidelity of his men by handing over
the business to them as a free gift. Then he took out patents for
improvements in the manufacture of oils, varnishes, sugar, plate glass,
etc. Several of his machines for these purposes were shown at the London
Exhibition of 1851. Bessemer is said to have obtained altogether some
150 patents, including those granted for inventions connected with our
subject. He may be regarded as the type of the very fortunate inventor,
since on the patents of the one process we are going to describe he
ultimately obtained royalties to the value of more than £1,057,000, and
this irrespective of profits derived from commercially working it
himself.
At the time of the Crimean War, Bessemer had some experiments made at
Vincennes with cylindrical projectiles he had devised for firing from
smooth-bore guns, yet so as to impart to the projectile at the same time
rotation about its axis. The experiments were successful, but it was
pointed out that the guns of cast iron then in use would not bear heavy
projectiles, and he was induced, at the suggestion of the Emperor
Napoleon III., to undertake some researches with the view of finding
metal more suitable for artillery. Bessemer, having then little
knowledge of the metallurgy of iron, applied himself on his return to
England to the study of the best books on the subject, visited the
principal iron-working districts, and began a series of experiments at a
small experimental installation he set up in London. There, after
repeated failures, he did at length succeed in producing a metal much
tougher than the cast iron then used, and a small model gun was
submitted to the Emperor, who encouraged Bessemer to persevere with his
experiments; which he did, though the expense was a great tax on his
capital, continued as the experiments were for two years and a half. But
by this time he had acquired a knowledge of many important facts, and
these gradually led him to the experimental realization of the idea he
had conceived, but only after many trials in which several thousand
pounds were expended. At length the agenda of the British Association
for the Cheltenham meeting of 1856 announced that a paper would be read
by H. Bessemer, entitled “The Manufacture of Iron and Steel without
Fuel.” It will be easily understood that a title in such terms would
give rise to much derisive incredulity; and we may imagine the
iron-masters on that occasion crowding into Section G, while asking each
other in the spirit of certain philosophers of old, “What will this
babbler say?” Some of what he did say may here be quoted, as at once
explanatory and historically memorable.
“I set out with the assumption that crude iron contains about 5 per
cent. of carbon; that carbon cannot exist at a white heat in the
presence of oxygen without uniting therewith and producing combustion;
that such combustion would proceed with a rapidity dependent on the
amount of surface of carbon exposed; and lastly, that the temperature
which the metal would acquire would be also dependent on the rapidity
with which the oxygen and carbon were made to combine; and consequently,
that it was only necessary to bring the oxygen and carbon together in
such a manner that a vast surface should be exposed to their mutual
action, in order to produce a temperature hitherto unattainable in our
largest furnaces.
[Illustration:
FIG. 23.—_Experiments at Baxter House._
]
“With a view of testing practically this theory, I constructed a
cylindrical vessel of 3 ft. in diameter and 5 ft. in height, somewhat
like an ordinary cupola furnace (see Fig. 23). The interior is lined
with firebricks, and at about 2 in. from the bottom of it I inserted
five _tuyère_ pipes, the nozzles of which are formed of well-burned
fire-clay, the orifice of each _tuyère_ being about three-eighths of an
inch in diameter; they are so put into the brick lining (from the outer
side) as to admit of their removal and renewal in a few minutes, when
they are worn out. At one side of the vessel, about half-way up from the
bottom, there is a hole made for running-in the crude metal, and on the
opposite side there is a tap-hole, stopped with loam, by means of which
the iron is run out at the end of the process. In practice this
converting vessel may be made of any convenient size, but I prefer that
it should not hold less than one nor more than five tons of fluid iron
at each charge; the vessel should be placed so near to the discharge
hole of the blast furnace as to allow the iron to flow along a gutter
into it. A small blast cylinder is required capable of compressing air
to about 8 lbs. or 10 lbs. to the square inch. A communication having
been made between it and the _tuyères_ before named, the converting
vessel will be in a condition to commence work; it will however on the
occasion of its first being used after re-lining with firebricks be
necessary to make a fire in the interior with a few baskets of coke, so
as to dry the brickwork and heat up the vessel for the first operation,
after which the fire is to be all carefully raked out at the
tapping-hole, which is again to be made good with loam: the vessel will
then be in readiness to commence work, and may be so continued without
any use of fuel until the brick lining, in the course of time, becomes
worn away, and a new lining is required. I have before mentioned that
the _tuyères_ are situated nearly close to the bottom of the vessel, the
fluid metal will therefore rise some 18 in. or 2 ft. above them; it is
therefore necessary, in order to prevent the metal from entering the
_tuyère_ holes, to turn on the blast before allowing the fluid crude
iron to run into the vessel from the blast furnace. This having been
done, and the metal run in, a rapid boiling up of the metal will be
heard going on within the vessel, the metal being tossed violently about
and dashed from side to side, shaking the vessel by the force with which
it moves; from the throat of the converting vessel flame will
immediately issue, accompanied by a few bright sparks such as are always
seen rising from the metal when running into the pig-beds. This state of
things will continue for about fifteen minutes, during which time the
oxygen in the atmospheric air combines with the carbon contained in the
iron, producing carbonic oxide, or carbonic acid gas, and at the same
time evolving a powerful heat. Now, as this heat is generated in the
interior of, and is diffused in innumerable fiery bubbles through, the
whole fluid mass, the metal absorbs the greater part of it, and its
temperature becomes immensely increased, and by the expiration of the
fifteen minutes before named that part of the carbon which appears
mechanically mixed and diffused throughout the crude iron has been
entirely consumed: the temperature however is so high that the
chemically combined carbon now begins to separate from the metal, as is
at once indicated by an immense increase in the volume of flame rushing
out of the throat of the vessel. The metal in the vessel now rises
several inches above its natural level, and a light frothy slag makes
its appearance and is thrown out in large foam-like masses. This violent
eruption of cinder generally lasts about five or six minutes, when all
further appearance of it ceases, a steady and powerful flame replacing
the shower of sparks and cinder which always accompanies the boil. The
rapid union of carbon and oxygen which thus takes place adds still
further to the temperature of the metal, while the diminished quantity
of carbon present allows a part of the oxygen to combine with the iron,
which undergoes combustion and is converted into an oxide. At the
excessive temperature that the metal has now acquired, the oxide as soon
as formed undergoes fusion, and forms a powerful solvent of those earthy
bases that are associated with the iron; the violent ebullition which is
going on mixes most intimately the scoria and metal, every part of which
is thus brought in contact with the fluid oxide, which will thus wash
and cleanse the metal most thoroughly from the silicon and other earthy
bases which are combined with the crude iron, while the sulphur and
other volatile matters which cling so tenaciously to iron at ordinary
temperatures are driven off, the sulphur combining with the oxygen and
forming sulphurous acid gas.
“The loss in weight of crude iron during its conversion into an ingot of
malleable iron was found, on a mean of four experiments, to be 12½ per
cent., to which will have to be added the loss of metal in the finishing
rolls. This will make the entire loss probably not less than 18 per
cent. instead of about 28 per cent., which is the loss on the present
system. A large portion of this metal is however recoverable by heating
with carbonaceous gases the rich oxides thrown out of the furnace during
the boil. These slags are found to contain innumerable small grains of
metallic iron, which are mechanically held in suspension in the slags
and may be easily recovered.