and powerful flame succeeds, which continues without any change for
about ten or twelve minutes, when it rapidly falls off. As soon as this
diminution of flame is apparent the workman will know that the process
is completed, and that the crude iron has been converted into pure
malleable iron, which he will form into ingots of any suitable size and
shape by simply opening the tap-hole of the converting vessel and
allowing the fluid malleable iron to flow into the iron ingot moulds
placed there to receive it. The masses of iron thus formed will be free
from any admixture of cinder, oxide, or other extraneous matters, and
will be far more pure and in a forwarder state of manufacture than a
pile formed of ordinary puddle bars. And thus it will be seen that by a
single process, requiring no manipulation or particular skill, and with
only one workman, from three to five tons of crude iron pass into the
condition of several piles of malleable iron in from thirty to
thirty-five minutes, with the expenditure of about a third part the
blast now used in a finery furnace, with an equal charge of iron, and
with the consumption of no other fuel than is contained in the crude
iron.
“To those who are best acquainted with the nature of fluid iron, it may
be a matter of surprise that a blast of cold air forced into melted
crude iron is capable of raising its temperature to such a degree as to
retain it in a perfect state of fluidity after it has lost all its
carbon and is in the condition of malleable iron, which, in the highest
heat of our forges, only becomes softened into a pasty mass. But such is
the excessive temperature that I am enabled to arrive at with a properly
shaped converting vessel and a judicious distribution of the blast, that
I am enabled not only to retain the fluidity of the metal, but to create
so much surplus heat as to remelt all the crop-ends, ingot-runners, and
other scrap that is made throughout the process, and thus bring them,
without labour or fuel, into ingots of a quality equal to the rest of
the charge of new metal....
“To persons conversant with the manufacture of iron, it will be at once
apparent that the ingots of the malleable metal which I have described
will have no hard or steely parts, such as are found in puddled iron,
requiring a great amount of rolling to blend them with the general mass,
nor will such ingots require an excess of rolling to expel cinder from
the interior of the mass, since none can exist in the ingot, which is
pure and perfectly homogeneous throughout, and hence requires only as
much rolling as is necessary for the development of fibre; it therefore
follows that, instead of forming a merchant bar, or rail, by the union
of a number of separate pieces welded together, it will be far more
simple and less expensive to make several bars or rails from a single
ingot. Doubtless this would have been done long ago had not the whole
process been limited by the size of the ball which the puddler could
make.
“The facility which the new process affords of making large masses will
enable the manufacturer to produce bars that, in the old mode of
working, it was impossible to obtain; while at the same time it admits
of the use of more powerful machinery, whereby a great deal of labour
will be saved and the process be greatly expedited.... I wish to call
the attention of the meeting to some of the peculiarities which
distinguish cast steel from all other forms of iron, viz., the perfectly
homogeneous character of the metal, the entire absence of sand-cracks or
flaws, and its greater cohesive force and elasticity, as compared with
the blister steel from which it is made,—qualities which it derives
solely from its fusion and formation into ingots, all of which
properties malleable iron acquires in like manner by its fusion and
formation into ingots in the new process; nor must it be forgotten that
no amount of rolling will give the blister steel, although formed of
rolled bars, the same homogeneous character that cast steel acquires by
a mere extension of the ingot to some ten or twelve times its original
length....
“I beg to call your attention to an important fact connected with the
new process which affords peculiar facilities for the manufacture of
cast steel. At that stage of the process immediately following the boil
the whole of the crude iron has passed into the condition of cast steel
of ordinary quality. By the continuation of the process the steel so
produced gradually loses its small remaining portion of carbon, and
passes successively from hard to soft steel, and from soft steel to
steely iron, and eventually to very soft iron; hence, at a certain
period of the process, any quality of metal may be obtained. There is
one in particular which by way of distinction I call semi-steel, being
in hardness about midway between ordinary cast steel and soft malleable
iron. This metal possesses the advantage of much greater tensile
strength than soft iron; it is also more elastic, and does not readily
take a permanent set, while it is much harder and is not worn or
indented so easily as soft iron; at the same time it is not so brittle
or hard to work as ordinary cast steel. These qualities render it
eminently well adapted to purposes where lightness and strength are
specially required, or where there is much wear, as in the case of
railway bars, which from their softness and lamellar texture soon become
destroyed. The cost of semi-steel will be a fraction less than iron,
because the loss of metal that takes place by oxidation in the
converting vessel is about 2½ per cent. less than it is with iron; but
as it is a little more difficult to roll, its cost per ton may fairly be
considered to be the same as iron; but as its tensile strength is some
30 or 40 per cent. greater than bar iron, it follows that for most
purposes a much less weight of metal may be used than that so taken. The
semi-steel will form a much cheaper metal than any we are at present
acquainted with. These facts have not been elicited from mere laboratory
experiments, but have been the result of working on a scale nearly twice
as great as is pursued in our largest iron works, the experimental
apparatus doing 7 cwt. in thirty minutes, while the ordinary puddling
furnace makes only 4½ cwt. in two hours, which is made into six separate
balls, while the ingots or blooms are smooth, even prisms, 10 in. square
by 30 in. in length, weighing about equal to ten ordinary puddle balls.”
The startling novelty of the methods and results described in this paper
had the effect of paralyzing discussion at the time. But soon the voice
of detraction was heard; many iron-masters ridiculed the idea of
producing iron and steel without fuel, and indeed it may have been
observed, the title of the paper notwithstanding, that first the silicon
and carbon, and then the iron itself, really supplied the fuel. And we
must remember that malleable iron in a molten state was then deemed an
impossibility, for the hottest furnaces then known could not effect the
fusion, however prolonged their action might be, yet Bessemer was to
obtain five tons in this condition in the short space of half an hour
with no other aid than cold air. Then it was said that Bessemer’s
process of forcing air into melted cast iron had no claim of novelty,
for it had been tried before and found valueless. Some iron-masters on
trying experiments on a small scale and with imperfect appliances met
with failures, and discredited the process at once; but five large
establishments paid for licences sums amounting to £26,500 within three
weeks of the reading of the paper. At the works of the Dowlais Iron Co.,
in South Wales, who were the first licensees, the first converter was
set up under Bessemer’s personal superintendence, and at the first
operation five tons of iron were produced direct from the blast furnace
pig. This apparently satisfactory result proved quite otherwise when
this iron came to be practically tested; for it was found quite useless!
It was both “_cold-short_” and “red-short,” to use the technical
terms,—the former of which means that although the sample may be welded,
it is when cold brittle and rotten; the latter means that at a low red
heat it breaks and crumbles under the hammer. Further trials were made,
new experiments instituted, but the success that attended Bessemer’s
early experiments could not be repeated, and as yet no one knew the
reason why. Now it so happened that in the preliminary experiments an
exceptionally pure pig iron had been made use of containing little or no
phosphorus or sulphur, substances very deleterious in iron, and still
more so in steel. With the capital obtained by the sale of his licences
Bessemer quietly set to work to investigate the cause of his
non-success, making daily experiments with a ton or two of metal at a
time. These experiments extended over a period of two and a half years,
and upon them Bessemer and his partner spent about £16,000, besides the
£4,000 the preliminary researches had cost. But all difficulties were at
length overcome, and the process was now found capable of turning out
pure iron and steel when the pure pig iron of Sweden was used in the
converter. In the meantime the licensees had made no attempts
practically to carry out the process, which began to be denounced as
visionary: it was “a mare’s nest”; it was “a meteor that had passed
through the metallurgical world, but had gone out with all its sparks.”
When Bessemer again brought the subject before the public, he found that
no one believed in it; everyone said, “Oh, this is the thing that made
such a blaze two or three years ago, and which was a failure.” Neither
iron-makers nor steel-makers would now take it up. Bessemer and his
partner thereupon joined with three other gentlemen to establish at
Sheffield a steel-works of their own, where the invention should be
carried into full practice. In due time works were erected, and they
commenced to sell steel, receiving at first very paltry orders, for such
quantities as 28 lbs. or 56 lbs.; but the orders soon became larger, and
afterwards very much larger, for they were underselling the Sheffield
manufacturers by £20 a ton, and their steel was undistinguishable from
the higher priced article. Bessemer had now bought his licences back
again, and in the course of his second set of experiments had patented
each improvement as it occurred to him, finally bringing the mechanical
apparatus to the degree of efficiency requisite for practical working,
without which his primary idea would have been valueless. Before
directing the reader’s attention to the form the apparatus had assumed,
we may transcribe what Mr. Jeans, in the work above referred to, has
told about the commercial success of the Bessemer steel-making firm:—
“On the expiration of the fourteen years’ term of partnership of this
firm the works, which had been greatly increased from time to time out
of revenues, were sold by private contract for exactly twenty-four times
the amount of the whole subscribed capital, notwithstanding that the
firm had divided in profits during the partnership a sum equal to
fifty-seven times the gross capital, so that by the mere commercial
working of the process, apart from the patent, each of the five partners
retired after fourteen years from the Sheffield works with eighty-one
times the amount of his subscribed capital, or an average of nearly
cent. per cent., every two months,—a result probably unprecedented in
the annals of commerce.”
[Illustration:
FIG. 24.—_Bessemer Converter._
A, Front view, showing the mouth, _c_; B, Section.
]
The form of the Bessemer apparatus as it finally left the inventor’s
hands may now be considered: but in certain details and arrangements
some modifications, dictated by the experience and requirements of
individual establishments, have been made, leaving the principles of the
apparatus unchanged. Thus instead of making the converting vessel turn
on trunnions, it is sometimes constructed fixed, the fluid metal after
conversion being let out at a tap-hole; the number and size of the
_tuyères_ are varied; and so with the disposition of the air chamber or
_tuyère_ box, the pressure of the blast, the capacity of the converter
itself, etc. In capacity converters vary between 2½ tons and 10 tons;
one of medium size is shown in elevation and section in Fig. 24, and may
be described as an egg-shaped vessel about 15 ft. high and 6 ft.
diameter inside. It is strongly made of wrought iron in two parts bolted
together, and is lined inside with some thick infusible coating, of
which more is to be said presently. The converter swings on trunnions,
one of which is hollow, and admits the blast by the pipe _b_ to the base
of the vessel, whence it passes through the passages shown at _e_. The
thickness of the lining at _e_ may perhaps be 20 in., and passages for
the air are perforated in fire-clay _tuyères_, of which there may be
seven, each with seven perforations of half an inch diameter. To the
other trunnion is attached a toothed wheel which engages the teeth of a
rack receiving motion from hydraulic pressure. The iron for the
operation is melted in a furnace having its hearth above the level of
the converter; and to receive its charge the latter is turned so that
the molten cast iron may be poured in from a trough until its surface is
nearly on a level with the lowest of the _tuyères_. The blast having
been turned on, the hydraulic power is set to work and the converter is
slowly brought to an upright position. The pressure of the current of
air prevents any of the fluid metal from entering the blow-holes. The
blast of cold air is continued until all the silicon and carbon have
been removed by oxidation. If the production is to be steel, there is
then added to the contents of the converter, placed in position to
receive it, a certain weight of melted cast iron of a special
constitution, and the blow is resumed for a few minutes; or in more
recent practice this special metal is added to the fluid metal run out
of the converter into a spacious ladle in known quantity. On this
addition an intense action takes place, attended by an extremely
brilliant flame and a throwing out of cinder or slag. The metal thus
added to the decarbonized iron is a carbonized alloy of iron and
manganese obtained from an ore naturally containing the latter metal,
and scarcely any phosphorus or sulphur. The charcoal pig from this ore
is called _spiegeleisen_ (German = mirror-iron) from its brilliant
reflecting facets; it contains from 12 to 20 per cent. of manganese,
with about 5 per cent. of carbon, and a considerable proportion of
silicon. An exact chemical analysis of the particular spiegeleisen
having been previously made, it is known what proportion of it is to be
added to the decarbonized iron in order to convert this into a steel
with any required content of carbon. The manganese probably acts by
combining with oxide of iron diffused through the mass, and together
with the silicon forming the very easily separated slag which is
ejected.
The whole series of operations connected with the Bessemer process may
be easily followed by the help of Fig. 25, which is taken from a
beautiful model in the Museum of Practical Geology. This model, which
was presented to the museum by Mr. Bessemer himself, represents every
part of the machinery and appliances of the true relative sizes. C is
the trough, lined with infusible clay, by which the liquid pig iron is
conveyed to the converters, A. The hydraulic apparatus by which the
vessels are turned over is here below the pavement, but the rack which
turns the pinion on the axis of the converter is shown at B. The vessel
into which the molten steel is poured from the converter is marked E,
and this vessel is swung round on a crane, D, so as to bring it exactly
over the moulds, placed in a circle ready to receive the liquid steel,
which on cooling is turned out in the form of solid ingots. The valves
which control the blast, and those which regulate the movements of the
converter through the hydraulic apparatus, are worked by the handle seen
at H. The crane, or revolving table, D, is also under perfect control,
so that the crude pig iron is converted into steel, and the moulds are
filled with a rapidity and ease that are positively marvellous to a
spectator.
[Illustration:
FIG. 25.—_Model of Bessemer Steel Apparatus._
]
The development of the Bessemer process soon had the effect of so
reducing the price of steel that this material came into use for almost
every purpose for which iron had previously been employed, such as
railway bars, girders, etc., for bridges, boiler plates, etc., for all
which “steely iron” containing only 0·12 to 0·40 per cent. carbon proved
admirably adapted. The practical success of the Bessemer process had not
long been demonstrated commercially by the inventor and his partners at
Sheffield before other firms began the manufacture: so that in 1878
there were in Great Britain alone twenty-seven establishments making
Bessemer steel and using 111 converters. It may give an idea of the
magnitude the Bessemer steel manufacture had attained even at that time
if we quote the cost of erecting a complete plant for two 5–ton
converters: it was £44,400, as given in a detailed estimate. In all
these cases pig iron from ores free from phosphorus and sulphur had to
be used, for as we have seen the converter failed to eliminate these
vitiating elements. Imported pig ores had in general to be used, or pig
from the limited supply of British hæmatite ores in West Cumberland. The
Barrow Hæmatite Steel Company engaged in the production of Bessemer
steel on a very large scale, having by 1878 erected no fewer than
sixteen converters of the capacity of 6 tons each. In the meanwhile many
efforts were made to discover some method of eliminating phosphorus, so
that the ordinary qualities of British pig iron, and iron derived in any
part of the world from the coarse phosphorized ores, might be available
for the converter. Many of the methods then devised proved correct in
principle and feasible in practice; but as, for sundry reasons, none of
them came extensively into use, we need not here allude to them further.
The solution of the problem was announced in 1879. Some years before, G.
J. Snelus had come to the conclusion that with a siliceous lining it
would be impossible to eliminate phosphorus in the Bessemer converter,
and that some refractory substance of a basic character must be sought
for in order that the slag produced should be in a condition to absorb
the phosphoric acid as fast as it is produced. He patented in 1872 the
use of magnesian limestone as a material for the lining; as that
substance when intensely heated became very hard and stony, being in
that condition quite unaffected by water. Two young chemists, Messrs.
Thomas and Gilchrist, apparently without being aware of Mr. Snelus’s
conclusions, had also convinced themselves that the chief deficiency in
the Bessemer process was due to the excess of silica in the slag, and in
1874 they began to try the effect of basic linings, and also of basic
additions, such as lime, etc., to the charge in the converter, so that
the lining itself should not be worn out by entering into the slag.
Their results proved that phosphorus could be eliminated when the slag
contained excess of a strong base. An example of an operation at
Bolckow, Vaughan, & Company’s Eston works with the highly phosphorized
Cleveland pig iron may be quoted. The basic-lined converter received
first 9 cwt. of lime, then 6 tons of metal. When the blast at 25 lbs.
pressure was turned on, the silicon began at once to burn; for three
minutes the carbon was not affected, but for fourteen minutes longer it
regularly diminished, the silicon keeping pace with it. After the blow
had been continued for thirteen minutes from the commencement, the
converter was turned down to allow of the further introduction of 19½
cwt. of a mixture of two parts of lime with one of oxide of iron. So
long as 1·5 per cent. of carbon remained in the metal the phosphorus was
untouched, and at the end of the blow, _i.e._ when the flame dropped,
only one-third of it had been eliminated; it still formed 1 per cent. of
the metal. The blast continued for another two minutes brought it down
to ¼ per cent., and in one more minute only a trace was left. Most of
the sulphur was got rid of at the same time. From Cleveland pig, thus
de-phosphorized in the Bessemer converter, large quantities of steel
rails were rolled for the North Eastern Railway Company, and were found
entirely satisfactory, being as good as those made from the Cumberland
hæmatite steel. This de-phosphorized process has been brought into
operation wherever phosphoric ores are dealt with, and it has been
applied with equal success in the “open hearth” furnaces, of which we
have now to speak.
All discoveries and all inventions may be traced back to preceding
discoveries and inventions in an endless series, and it is only by its
precursors that each in its turn has been made possible. If we take one
of the greatest marvels brought into existence at nearly the close of
our epoch, namely, “wireless telegraphy,” we may follow up links of a
chain connecting it with the recorded observations of an ancient Greek
(Thales) who flourished seven centuries before our era, and even these
may not have been original discoveries of his. And it will have been
gathered from what has already been said that steel must have been
produced, however unwittingly, at the earliest period at which man began
to reduce iron from its ores. So the very latest, and for many purposes
the most extensively practised, process of modern steel-making, brought
indeed to working perfection mainly by the perseverance and scientific
insight of two individuals, is the result of the observation and the
accumulated experience of former generations. The observations and
experience here alluded to are chiefly those that follow two lines: one
concerning the properties of the metal itself, the other relating to the
means of commanding very high temperatures on a great scale. On this
occasion we are able almost to lay a finger on some proximate links of
the chain. Réaumur, the French naturalist, made steel in the early part
of the eighteenth century by melting cast iron in a crucible, and in
this liquid metal he dissolved wrought iron, the product being, as the
reader will now easily understand, the intermediate substance, steel;
and this was obtained of course at a temperature which was incapable of
fusing wrought iron by itself. He published in 1722 a treatise on “The
Art of converting Iron into Steel, and of softening Cast Iron.” For
this, and certain other metallurgical discoveries, Réaumur received a
life-pension equivalent to about £500 per annum,—a treatment very
different from that dealt out by the British to Henry Cort. The action
in Réaumur’s crucible is precisely that used on the large scale in
Siemens’ open hearth. But this last became possible only when Siemens
had worked out his “regenerative stove” or heat accumulator, the
development of an idea suggested by a Dundee clergyman in 1817.
A general notion of the Siemens’ regenerative stove will have been
already gained from the account given before of its application to the
modern type of blast furnace. Of the inventor himself, C. William
Siemens, it may be observed that he was one of a family of brothers, all
remarkable for their scientific attainments, and in many of his
researches and processes he was aided by his brothers Frederick and
Otto. In our article on “Electric Power and Lighting” there will be
found some notice of a few of Siemens’ inventions pertaining to those
subjects. A still more admirable invention of his is the electric
pyrometer, an instrument of the utmost utility for measuring, with an
accuracy previously unapproachable, the high temperature of furnaces,
etc. Indeed there are few departments of science, pure or applied, which
have not been enriched by the researches and contrivances of this
distinguished man, whose merits were acknowledged by the bestowal upon
him of the highest scientific and academical honours, and also of a
title, for he became Sir William Siemens.
[Illustration:
FIG. 26.—_Section of Regenerative Stoves and Open Hearth._
]