WHAT GUNS ARE MADE OF
No longer ago than the days of the Crimea, the largest guns were made of
the cheapest and commonest kind of iron, that known as cast iron.
This material has the advantage of being cheap and easily worked, but is
comparatively weak and liable to crack, so that the guns of that time
were comparatively small compared with those of to-day; they could only
withstand a feeble explosion and their range was therefore limited. Had
the energetic explosives of the present time been employed in them they
would inevitably have burst, killing their gunners instead of the enemy.
Attempts were made to strengthen them with bands made of wrought iron, a
form of the metal which is tough and elastic and therefore better able
to withstand sudden shocks than the more brittle cast iron, but it was
not a real success.
At first sight one naturally wonders why the whole gun was not made of
the stronger wrought iron. The reason was that while cast iron can be
melted and poured in a liquid form into a mould, so as to produce the
shape of the gun, wrought iron will not melt. It will soften with heat,
in which condition it can be hammered into shape and, moreover, when in
a very soft state two pieces can be joined by simply forcing them
closely together, which operation is called welding.
With the machinery available now it would be possible to make a gun of
wrought iron, but even a few years ago it would have been quite
impossible. There was an obvious need therefore of a metal which could
be melted and cast in moulds like cast iron, yet tough and strong to
resist shock like wrought iron. Fortunately this problem excited the
interest of a certain Mr. Henry Bessemer, a gentleman who, having made a
considerable fortune through an ingenious method of manufacturing bronze
powder, had sufficient leisure and money to devote himself to its
solution.
The vast steel industries of Great Britain and the United States are the
direct results of this gentleman's labours, and in the latter country
there are quite a number of towns which, being the home of steelworks,
are called by his name.
Iron is one of the most plentiful things in the world. Deposits running
into millions of tons are to be found in many parts, but it is
practically always in the form of ore, that is to say, in combination
with something else generally oxygen and sometimes oxygen and carbon.
The former sort of ore is called oxide of iron and the latter carbonate
of iron, and both of them bear not the slightest resemblance to the
metal. They are just rocks which form part of the earth's crust, and it
is only the metallurgist who can tell what they consist of.
In order that the iron may be obtained from the ore it is necessary for
the oxygen to be separated from it, an operation which requires the
intervention of heat, and the heat must be obtained from a fuel which
consists mainly of carbon. Wood fulfils these requirements, but there is
not enough wood in the whole world to smelt all the iron which we need.
It was not until "pit-cole" displaced "char-cole" (to use the spelling
of the period) that the iron industry began to assume its present
importance.
To produce iron cheaply, therefore, ore and coal should for preference
lie side by side, and in some few favoured localities that state of
things exists. Generally speaking, however, the ore and the coal are not
found together, with the result that one has to be taken to the other,
and in practice it is usually the ore which is taken to the coal. Hence,
the iron and steelworks are generally to be found on the coalfields,
while the ore comes by rail or ship from, it may be, remote parts of the
world.
The method by which the metal is obtained from the ore is in principle
very simple. Coal and ore are mixed together in a furnace, the fire
being fanned by a powerful blast of air. The result is that the bonds
uniting iron and oxygen are relaxed by the heat, when the oxygen, having
a preference for union with carbon rather than with iron, leaves the
latter to join up with some of the carbon of the coal.
The furnace in which this operation is carried out is a tall, vertical
cylinder of iron, lined with firebrick. The fire is at the bottom and
the fresh fuel and ore are thrown in at the top. As the ore is
"reduced" (the chemist's term for removing oxygen from anything) the
liquid iron accumulates in the lowest part of the furnace, whence it is
drawn off at intervals, being allowed to run into grooves or gutters in
a bed of sand, where it solidifies into what is called "pig iron."
Along with the coal and ore, there is thrown into the furnace from time
to time quantities of limestone which combines with the earthy
impurities with which the ore is contaminated. Together these form what
is called "slag," which also exists, while in the furnace, as a liquid,
but is so much lighter than the molten iron that it keeps quite separate
and can periodically be drawn off through a hole higher up than that
through which the iron is obtained. The slag solidifies into a hard
stone which is broken up and used for making concrete and tar-paving,
also for road metal.
The kind of furnace just described is, owing to the strong blast of air
needed for its operation, called a "blast-furnace." One would be
inclined to think that a fire so well supplied with oxygen, both from
the blast and from the ore itself, would cause the fuel to be completely
burnt up, yet such is not the case. The gases which ascend from the fire
consist largely of "carbon-monoxide," a burnable gas with lots of heat
still left in it. Years ago, and one may still see instances of it, this
gas was allowed to escape at the top of the furnace, where it burnt in
the form of a huge flame. In most modern furnaces, however, there is a
kind of plug in the orifice at the top which, while it can be lowered in
order to admit the ore and fuel, normally prevents the escape of the
gases, which are led away through pipes. In some cases the gases are
burnt under boilers to provide the works with steam, in other cases they
heat other furnaces for metallurgical purposes, while in yet others they
are employed to drive large gas-engines to generate electricity. It is
sometimes a difficulty to find useful employment for the vast quantities
of this "blast-furnace gas" which are produced at a large works.
We see, then, how is obtained the pig iron from which the other kinds of
iron and steel are made. It is not pure iron by any means; indeed, it is
not sought to make iron pure, as is the case with most other metals,
since, in its pure state, it is too soft to be of much use. All the
familiar forms of iron and steel are really alloys of iron and carbon, a
fact which tends to give iron its unique position among the metals,
since by exceedingly slight variations in the percentage of carbon we
can vary the properties of the iron to an amazing extent, thereby
producing in effect a wide range of different substances each
particularly suitable for a particular purpose.
To make cast iron, such as the guns of the Crimea were made of, it is
only necessary to melt up some pig iron and to pour it into a mould.
There is scarcely a town in which there is not an iron foundry, either
large or small, and that is the work carried on there. A smaller form of
the blast-furnace, known as a "cupola," melts the pig iron, and the
moulds are generally made of sand. The process of pouring the melted
metal into the moulds is called "casting" and the things so produced are
"castings," and are said to be made of "cast" iron.
[Illustration: AN 18-POUNDER IN ACTION.
The crew consists of six men. No. 1 (the sergeant) gives instructions.
No. 2 stands at the right of the breech. No. 3 fires the gun. No. 4
holds the shell ready for placing in the bore. No. 5 adjusts the fuse
and hands the shell to No. 4. No. 6 prepares the ammunition and hands it
to No. 5. In this picture only three of the crew are left.]
Wrought iron is made by working the molten pig iron instead of casting
it. The work is done in a different type of furnace altogether from the
blast furnace and the cupola. It is more like an oven, in the floor of
which is a depression wherein the molten metal lies. The fire-place is
so arranged that the flames pass over the metal, being deflected
downwards upon it by the roof as they pass.
It should be understood that in casting pig iron one does little more
than form it into some desired shape, the nature of the metal undergoing
little or no change. In working it, however, into wrought iron, we
change its nature.
The pig iron contains from 2 to 5 per cent of carbon, which it obtains
from the coal in the blast-furnace, and it is this particular proportion
of carbon which gives it its own peculiar properties. To convert it into
wrought iron a workman puts a long iron rod into the furnace and stirs
the metal about, thereby exposing it to the air and permitting the
carbon to be burnt out. As it loses carbon the iron becomes less and
less fluid until it reaches a sticky stage. Thus the workman, who is
known by the name of puddler, as the process is called puddling, works
up a ball of decarbonized and therefore sticky iron upon the end of his
rod. Having thus produced a rough ball or lump he draws it out of the
furnace and leaves it to cool.
Thus the result of the puddling process is to produce a number of rough
lumps or balls of iron with only about one-tenth per cent of carbon.
They are next reheated, in another furnace, and a number of them are
hammered together under a mechanical hammer into larger lumps called
blooms or billets. The hammering process has the effect of driving out
impurities and also of improving the texture of the metal.
Iron sheets, bars, rods and so on are formed by heating the billets and
rolling them out in powerful rolling mills, machines which in principle
are precisely similar to the domestic mangle, wherein two iron rollers
with properly shaped grooves in them squeeze out the billet into the
desired form.
Wrought iron, owing to the method by which it is produced, is not
homogeneous, that is to say, it is nor quite the same all through, with
the result that when it is rolled it develops a grain somewhat similar
to the grain in wood, so that if bent across the grain it is somewhat
liable to crack. On the other hand, it has the advantage over steel that
it rusts much less readily. Hence, for outdoor purposes it is still
sometimes preferred to the otherwise more popular steel.
Now the problem which Bessemer set before himself was to find out how to
make a metal which could be cast like cast iron yet should be as strong
and tough as wrought iron. After a little experimenting, by a happy
inspiration, he hit upon the idea of blowing air through a mass of
molten pig iron, thereby burning out the carbon, just as is done in the
puddling process, only much quicker and with less labour. By this means
he produced a metal with less carbon than cast iron and more than
wrought iron, a sort of intermediate state between the two, and to his
joy he found that this "Bessemer steel" could be cast like cast iron yet
had strength and toughness equal to if not superior to that of wrought
iron. Moreover, it was homogeneous and when rolled did not possess the
troublesome grain characteristic of wrought iron.
Having thus found the way to make this new and desirable metal, Bessemer
encountered a great disappointment, so great that it would have entirely
beaten many men. He made samples of steel and submitted them to experts
in iron manufacture. Everyone thought them admirable and many large iron
works were induced by them to make arrangements with Bessemer for the
right to use his process. His name was already famous and it seemed as
if a new fortune was made, when, to his alarm, he learned that wherever
it was tried except in his own works, the process was a miserable
failure. Instead of being at the end of his labours he was just at the
beginning.
It turned out that the particular iron which he happened to buy and use
at his own works was particularly free from an impurity which is,
generally speaking, a great nuisance in iron, namely, phosphorus. It was
pure accident which had led him to use this iron: it happened to be the
kind he could purchase most easily in the small quantities needed for
his experiments but it led him into a great difficulty, for other
people, after paying him for the right to use his process and after
spending large sums on the requisite plant, found themselves unable to
make the steel because of the phosphorus in their iron and finding
themselves unable to make a success were inclined to write him down a
fraud. As it turned out, after much labour on Bessemer's part, it was
due to the presence of tiny percentages of phosphorus in most of the
iron that is produced.
After much trouble he was able to induce certain owners of
blast-furnaces to make, by special methods, a kind of pig iron
practically free from phosphorus and therefore suitable for his process.
This special pig iron was known as Bessemer Pig Iron.
A little later a new inventor, a Welshman, Thomas by name, overcame the
difficulty in another way, but to explain that I must first describe the
Bessemer Converter, the special apparatus designed by Bessemer for
making his steel.
It can best be likened to a huge iron kettle with a big spout at the top
and with two projecting pins, one on each side. These pins rest in
supports, so that it is easy to tilt the whole thing over on to its
side. This is lined with fire-clay or some suitable heat-resisting
material.
Through one of the "pins" (trunnions is their proper name) there runs a
hole, communicating to what we might call a grating in the bottom of the
converter. To this hollow trunnion there is connected the pipe from a
powerful blowing engine, so that air can be driven in at will.
To load or charge the converter it is tilted over somewhat to one side
so that molten pig iron can be poured into it. The blast is then turned
on after which it is raised to an upright position with the air
bubbling up from below through the iron. Thus by being brought into
close contact with air, the carbon is burnt out of the metal until none
is left. That, however, is not desired, so, as soon as the carbon is
known to have all gone, a fresh quantity of molten iron is added of a
special kind, the amount of carbon in which is known very exactly. Thus
all the carbon is first removed and then exactly the right amount is
added, and so the desired result is attained with certainty.
Now Thomas's improvement was this. He discovered that the converter
could be lined with certain substances which have a great attraction for
phosphorus and under those conditions any phosphorus which may be in the
ore goes readily from the iron into the lining, or forms, with material
from the lining, a slag which floats upon the surface of the metal.
When the process is completed the converter is tipped over once more and
the metal, now steel, is poured into rectangular moulds from which the
steel can be lifted after cooling in the form of ingots.
Steel produced by Bessemer's process as improved by Thomas is called
Basic Bessemer Steel.
Incidentally Thomas, by this invention, laid the foundation of much of
the steel industry of Germany and Belgium, for there are enormous
deposits of ore in the neighbourhood of Luxemburg which because of the
presence of phosphorus were useless until Thomas showed how it could be
dealt with.
And there is another interesting feature of this "basic" process.
Phosphorus is a valuable fertilizer, so that the "slag" makes a very
fine chemical manure. It is ground up into a fine powder and is sold to
farmers under the name of Thomas's Phosphate Powder. It owes its
fertilizing virtues to the presence of the phosphorus which it has
stolen from the molten iron.
Bessemer derived a huge fortune from his process after he had fought and
overcome his difficulties, in addition to which he received the honour
of knighthood and became Sir Henry Bessemer.
It will be noticed that one of the virtues of the process is its economy
in fuel. During the whole time that the metal is in the converter, from
twenty to thirty minutes, no fuel is used to keep it hot. The reason for
that is that the carbon which is being got rid of is acting as fuel. It
is burning with the air which is driven through, thereby generating
heat.
In Bessemer's early days, it was arranged that he should attend a
meeting of ironmasters at Birmingham to explain his new process. On the
morning of his lecture two eminent ironmasters were breakfasting
together in a Birmingham hotel when one exclaimed to the other, "What do
you think, there is a fellow coming here to-day to tell us how to make
steel without fuel." To this eminent South Wales ironmaster the proposal
seemed preposterous but it was true all the same.
Although vast quantities of steel are made by the Bessemer process there
is another one of equal importance known as the Siemens-Martin
Open-hearth process. In this the molten metal is kept in a huge bath
practically boiling until the carbon has been reduced to the required
amount. Perhaps the most interesting feature about it is the way in
which fuel is saved by what is called the "regenerative" method due to
that versatile genius Sir William Siemens.
The open-hearth, as it is termed, is a huge rectangular chamber of
firebrick with a firebrick roof, and doors along one side just under the
roof through which the process can be watched and new materials be added
from time to time.
The fire is some way away and not underneath as one might perhaps
expect. Now if a deep coke fire is fed with insufficient air it does not
give off carbonic acid such as usually arises from a fire, and which as
everyone knows will not burn, but a gas called carbon monoxide which
will burn very well. So the fire-place for these furnaces is constructed
in such a manner as to produce carbon monoxide, which then passes
through a huge flue to one end of the open-hearth. Here it meets air
coming through another flue and the two combining burst into flame over
the metal.
The hot gases resulting from this burning pass out through a flue at the
other end of the hearth to a tall chimney which causes the necessary
draught, but on their way they pass through a chamber loosely filled
with bricks. Consequently the hot gases only reach the open air after
having given up much of their heat to these bricks.
After that operation has been going on for a time certain valves are
operated and the gas and air then come in at the other end of the
hearth, travelling through it in the opposite direction. And the air
comes through the chamber which has the hot bricks in it, bringing back
into the furnace a large quantity of that heat which otherwise would
have gone up the chimney but which the bricks intercepted. Thus all day
long does this reversal take place at intervals, the fresh air all the
time picking up and bringing back some of the heat which just previously
had escaped towards but not into the chimney. This arrangement enables
the process to compete, so far as economy is concerned, with the
Bessemer process.
At intervals the steel is tapped off from the furnace and run into
ingot-moulds, the same as with the other process. On the whole it is
regarded as producing a slightly better steel, the operation being under
slightly better control.
However the steel is made the ingots are reheated and either hammered
under a powerful steam hammer or pressed in an enormous hydraulic press.
This greatly improves the quality.
The steel can then be rolled into plates, bars or whatever form may be
required.
The finer qualities of steel such as are used for making sharp tools are
made in quite another way. Instead of being made from crude iron by
taking out the carbon, the materials are the finest qualities of wrought
iron and charcoal which are mixed together in the correct quantities and
melted in a crucible. This cast steel is very hard, so that it will
carry a very fine, sharp edge. It is also capable of being tempered by
heating and cooling, so that the exact degrees of hardness and
toughness can be attained.
Of recent years a special quality of steel for tools called "high-speed"
steel has been produced, mainly by the addition to ordinary cast steel
of a small percentage of tungsten. The advantage of this is that, within
certain limits, this does not soften with heat, and it is, I can assure
you, a great invention in war-time, when a nation is straining every
nerve to turn out guns and shells as fast as possible.
For all these things need to be turned in lathes and if you have ever
watched a metal-turning lathe at work you will have noticed that the
tool which actually takes a shaving off the article being turned tends
to get hot. For this reason lathes are usually fitted with pumps which
pump cold soap-suds on to the tool as it works. What you see there is
the energy employed in shaving the metal being turned into heat in the
tool. If left uncooled by the water it would soon be red-hot. And the
faster the machine works the hotter will the tool get.
Now with the old steel a very little heat will suffice to make it soft,
when its cutting power is lost. So with the old steel, no matter how
much cooling water you might use, there was a distinct limit to the
speed of the lathe and the speed at which the work was finished, for if
that speed were once exceeded a stop became necessary to regrind the
tool or to put in a fresh one.
But with high-speed steel that limit is much higher, for it can get
almost red-hot before it loses its hardness and consequently machines
can be run and jobs finished at a speed which would have been out of
the question only a few years ago. If one belligerent knew how to make
high-speed steel while the other did not the former would have an
enormous advantage in war-time.
Speaking generally, steel such as is used for tools is called hard
steel, while that made by the Bessemer and Siemens-Martin processes is
called mild steel. Leaving out of account for the moment fancy steels
such as that just described, where other metals are added to the
mixture, the essential difference between all the varieties of steel is
simply a slight difference in the percentage of carbon. This is so
remarkable that it is worth while to tabulate these percentages again.
Cast iron has from 2 to 5 per cent.
Steel from one-fifth to one per cent.
Wrought iron less than one-fifth per cent.
Mild steel, which has least carbon of all the varieties of steel and in
this respect is therefore nearest to wrought iron, is used for the same
purposes as wrought iron, such as shipbuilding, bridges and roofs,
tanks, gas-holders, etc. When the Admiralty want a specially fast ship
such as a torpedo-boat destroyer with a hull as light as possible
consistent with strength they have it made of steel with a slightly
larger percentage of carbon so that the steel is stronger and the
vessel's frame can be made lighter. The steel for shells, too, needs to
be of a certain strength to give the best results, so the percentage of
carbon is adjusted accordingly.
For guns themselves, again, special properties are needed, and so not
only is the carbon regulated to a nicety but other things such as nickel
and chromium are added. Altogether, steel is one of the most marvellous
substances known, certainly the most marvellous metal. Copper is just
copper and no more, zinc is just zinc, and the same with lead, but iron
(which really includes steel) can be adapted to so many purposes, can be
endowed at will with so many different properties, that without doubt
iron, common, plentiful iron, is the king of all the metals.