The Romance of War Inventions Part 6

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 acc.u.mulates 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 quant.i.ties 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 quant.i.ties 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.

[Ill.u.s.tration: 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 sh.e.l.l ready for placing in the bore. No. 5 adjusts the fuse and hands the sh.e.l.l 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 pa.s.s over the metal, being deflected downwards upon it by the roof as they pa.s.s.

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 b.a.l.l.s 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 h.o.m.ogeneous, 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 ma.s.s 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 h.o.m.ogeneous 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 quant.i.ties 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 quant.i.ty 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 quant.i.ties 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 c.o.ke 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 pa.s.ses 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 pa.s.s 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 pa.s.s 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 quant.i.ty 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 quant.i.ties 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 a.s.sure you, a great invention in war-time, when a nation is straining every nerve to turn out guns and sh.e.l.ls 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.