Popular Science Monthly/Volume 19/October 1881/Progress in the Manufacture of Steel
IMPROVEMENTS in the arts and sciences have gradually modified the methods of producing iron and steel, and, in their turn, the arts and sciences have felt the reaction; for all improvements in the manufacture of iron and steel have consisted, not so much in the production of a better quality of the article, as in the cheapening of production by the application of the principles indicated by the progress of science, and by the use of superior machinery. The direct result of this cheapening has been to extend the applications of the products in the arts.
The discovery of steel appears to have naturally followed that of the means of reducing iron from its ore. In all primitive methods of iron-smelting, steel, in more or less quantity, is inevitably produced. Such methods have been carried on in India and Africa from time immemorial to the present day. A furnace of a similar primitive character has, for several centuries, been employed in Catalonia, in Spain.
In working this furnace, the ore is crushed by the hammer, and divided by sifting into lumps (mine) and very coarse powder (greillade). The furnace being still red-hot from the last operation, it is filled with charcoal nearly to the tuyère, the hearth is then divided at a point about two thirds distance from the tuyère into two parts by a broad shovel; on the blast-side a further quantity of coal is added, while the coal on the other side having been rammed down firm, ore is added, so as to fill that part of the furnace; on this is placed moistened charcoal-dust, except at the top. A good blast is then turned on, and, if the whole is in proper order, jets of blue flame at once issue from the uncovered portion of the ore.
During the whole of the process, at short intervals, greillade and charcoal are added, and well moistened with water, to prevent too rapid combustion. After about two hours from the commencement, the wall of mine,—i. e. ore in lumps—is pushed well forward under the tuyère, and more mine is thrown into the space thus made; this part of the process is also subsequently repeated at intervals, until sufficient has been added to form a lump of iron or massé of the required size. From time to time slag is removed by opening the tap-hole. At the completion of the process, a mass of metal is obtained weighing about three hundred-weight, which invariably consists partly of soft iron, and partly of steely iron and steel.
The ore on one side of the furnace being in lumps, the hot carbonic oxide generated by the action of the blast on the charcoal is able to pass freely through its mass, reducing it, after the water has been driven off by heat, to metallic iron. At the same time the ore becomes impregnated with carbon, derived from the decomposition of the gases with which it is charged. The greillade on the other side is much richer in silica than the larger pieces, and from this it results that the quantity of slag will vary with the greillade added. It is always very rich in oxide of iron. It appears that in this process, carburized iron is produced by the gradual reduction and fusion of the lumps of ore, and this, coming in contact at the bottom of the furnace with slag, very rich in oxide of iron, the carbon of the one combines with the oxygen of the other, and the result is that iron containing more or less carbon is produced, according as much or little oxide was present.
In order that steel may be produced by this process, every precaution is taken to cause as much carburization as possible; the unavoidable presence of oxide of iron in the slag, and the low temperature, effectually preventing the formation of cast-iron; the former, indeed, making it very difficult to obtain steel.
Rightly looked at, this process explains how steel was first obtained, and what the essential conditions are in its production. When, owing to the increased size of blast-furnaces, and the consequent increase of temperature, cast-iron became the only product, it naturally followed that this substance should be treated with a view to the production of steel. This was first effected in the refining hearth, and formed an important industry in Styria, Carinthia, the Tyrol, and other places, in some of which it is still carried on. The operation was conducted in a finery, similar in construction to those employed in the production of iron—in fact, iron and steel are often produced alternately in the same finery. This furnace, in its simplest form, consists essentially of a shallow quadrangular hearth, formed of cast iron plates. In one side is a tuyère, inclined at an angle of 10º to 15º. The bottom is kept covered with a layer of charcoal. In the Siegen district, a piece of pig-iron, weighing fifty to sixty pounds, is placed on the hearth, having been previously heated; the hearth is then three parts filled with burning charcoal; on it is placed a portion of the cake produced in the last operation, which has been kept hot in burning charcoal, at the back of the furnace. The remainder of the hearth is then filled up with charcoal. The other six or seven pieces into which the last cake was divided are placed on the top. In this process, the production of steel and the reheating of that obtained in the last operation, preparatory to working it under the hammer, are conducted together. The blast is turned on. The piece of pig-iron forms into a pasty mass; cinder, rich in oxide of iron, produced in the latter part of the preceding operation, is then thrown in; a second piece of pig-iron, weighing about one hundred pounds, is added, and afterward four or five pieces of spiegeleisen (cast-iron, containing manganese), weighing each about a hundred pounds, are successively added. If the metal is found to be too much decarburized, more Spiegel is added. In this process, as in the Catalan, it is impossible to obtain a homogeneous product. The principle in both is essentially the same, viz., decarburization by oxide of iron. In this process, as in every other process for the production of steel, manganese is used with great advantage—an advantage which arises from its power of replacing iron in the slag and of forming a slag that is more liquid than one containing iron alone.
The essential difference between the finery and the puddling process consists in the use of a reverberatory furnace, the manipulation of the metal and the regulation of the temperature being thereby greatly facilitated. The decarburization is effected by the addition of oxide of iron produced during rolling, and partially by the air which enters the furnace as the metal melts slowly down; manganese is added during the process. It is important that the temperature should be kept low. It is difficult to weld this steel perfectly, because, probably, the temperature at which it has to be worked is too low to make the cinder sufficiently liquid to enable it to be squeezed out under the hammer to the same extent that it is in the case of malleable iron. This difficulty has, however, been got over by completely fusing the steel before working it, so as to enable the slag to separate completely. In this form metal manufactured by this process has been largely used by Krupp.
The principle which regulates the production of steel by these methods is taken advantage of in the Uchatius process, in which pig iron is first granulated by running it while molten into cold water. The granulated metal is then mixed with about twenty per cent, of roasted spathic ore, crushed fine; the mixture, to which a little flux has been added, if necessary, is then fused in clay crucibles. If very soft steel is required, some wrought-iron scrap is added.
Lastly, in this category we have a process which consists in heating cast-iron, but not so as to soften it, in oxide of iron, in the form of ore or iron-scale. In this way partial, or even total, decarburization of the metal can be produced at will.
So far the difference between iron and steel has seemed to be merely one of degree, depending on the amount of carburization. The methods we have considered are, in fact, only modifications of those practiced for the production of malleable iron. We will now consider the different processes that have for their object to impart a certain amount of carbon to malleable iron. The Hindoos have practiced one of them from time immemorial. They place in unbaked-clay crucibles, of the capacity of a pint, a piece of malleable iron, some chopped wood, and a few leaves of certain plants; the top of the crucible is then closed with clay, and the whole well dried near a fire. A number of these crucibles are then strongly heated for about four hours in a cavity in the ground, by means of charcoal and a blast of air forced in by a bellows. There is some reason to believe that an excess of carbon, over that required to produce the hardest steel, has to be added, in order to fuse the metal at the temperature which can be commanded in these furnaces. Before being drawn out into bars, the cakes of metal obtained in this way are exposed in a charcoal-fire during several hours to a temperature a little below their melting-point, the blast of air playing upon them during the time. The object of this is, doubtless, to remove the excess of carbon.
In 1800 a patent was taken out by David Mushet for a process in every respect analogous to that just referred to. He appears, however, to have applied it to the manufacture of a metal low in carbon, and therefore intermediate between iron and steel, partaking in a certain degree of the properties of both.
In another method referred to by Biringuccio, in 1540, steel was produced by keeping malleable iron in molten cast-iron until it became pasty, and on examination was found to possess the properties of steel. In connection with the theory of steel manufacture this process is of great interest. It shows that iron in a strongly heated condition is capable of absorbing carbon by direct contact, unless we suppose that the carburization is effected by dissolved gases, which is possible.
In the cementation process, which was well described by Réaumur, in 1722, bars of iron are kept at a glowing red heat, surrounded with charcoal in boxes, into which the air is prevented from entering. The operation lasts from seven to ten days, according to the quality of steel required. These bars are never uniformly carburized, and, besides, they contain cinder, as the metal has never been fused. The process had been a long time in use, however, before it occurred to any one to fuse the steel and make it homogeneous. This was done by Huntsman, about 1760.
By all the processes we have so far reviewed, good steel could be produced, but only in small quantity and at great expense. The applications of steel were, in consequence, very limited; in fact, practically, its use was confined to implements with a cutting edge.
In 1845 Heath patented a process which, had it been successful, would have given him the power of producing steel in quantity. He proposed to melt scrap-iron in a bath of molten pig iron in a reverberatory furnace heated by jets of gas. There were two conditions wanting in this method, which caused it to be a failure, viz., a sufficiently high temperature, and the power easily to regulate the character of the gases employed. Nevertheless, in this suggestion is to be found the germ of one of the two most important processes of the present day.
The dominant idea in treating cast-iron for steel had always been to refine the metal by the action of atmospheric air, and this was effected by causing a current of air to impinge upon the surface of the metal, by means either of a blowing apparatus or the drawing action of a chimney-stack. What more natural than that it should occur to some one to refine iron by blowing air into it, instead of merely on to its surface? We find that this idea did occur to several persons, widely separated, in the year 1855.
In this year a patent was taken out by John Gilbert Martien for refining iron, by forcing air through it as it flowed from the blast furnace, or cupola, along runners to the puddling-furnace. The process, as detailed in the patent, was impracticable, and showed internal evidence of not having been worked out on a manufacturing scale. Just after this patent was taken out, we find George Parry, of the Ebbw Yale Works, making the experiment of forcing air through molten cast-iron, on the bed of a reverberatory furnace, by means of perforated pipes imbedded in the fire-clay bottom. Vigorous action is said to have taken place; but the metal, through an accident, escaped from the furnace, and the further trial of the process was discouraged by the managing director. Two or three months after these experiments, Henry Bessemer took out his now celebrated patent for the production of cast-steel by blowing air through molten cast-iron; it should be clearly borne in mind that he had been, for a considerable time previously, engaged in experiments on the subject. He first carried out his process in crucibles, placed in furnaces, and so arranged that the contents could be tapped from the bottom into molds. Steam or air, either separately or together, and by preference raised to a high temperature, was forced down into the crucible through a pipe. The patent goes on to state that steam cools the metal, but air causes a rapid increase in its temperature, and it passes from a red to an intense white heat. Bessemer at first used extraneous heat to start the process, if not, indeed, during its progress, which shows that he was not then aware that the heat created by merely blowing in air would be sufficient. In his next patent he dispensed with the furnace around the crucible, and, instead of tapping the crucible from the bottom, he mounted it on trunnions, and, by tipping it up by machinery, poured the contents from the mouth. This apparatus is essentially the same as that used at the present day. It was soon found that, to produce steel by this process which would work properly, manganese, if not originally present, would have to be added. In the absence of manganese, sulphur and oxygen, in anything more than very minute quantities, make the steel crumble when worked at a red heat; it is said to be "red short." In the case of the oxygen, the manganese combines with it, and passes it into the slag; but with sulphur the reaction is different; its injurious effect is simply counteracted by the manganese: it is not removed from the steel. At first manganese was only employed in the form of spiegeleisen; but this use was liable to the difficulty that if enough spiegel was added to impart the requisite quantity of manganese, too much carbon would have been introduced, and alloys richer in manganese—known as ferro-manganese—have been sought and found.
By adding at the end of the process a known quantity of spiegel or ferro-manganese, containing a known quantity of carbon, steel of any required hardness could be obtained.
The year which saw the birth of the Bessemer process was doubly remarkable, for it was at that time that the regenerative system of heating was first introduced by Dr. Siemens. Nothing can be simpler than the principle involved in this method, yet it was destined to play a most important part in the progress of the arts. The idea was to store up the heat escaping in the waste gases from furnaces, and to employ it to raise the temperature of the gas and air previous to their combustion in the furnace. This was accomplished by causing the spent gases to pass through two chambers filled with loose brickwork. When these chambers have become heated to a high temperature, the waste gases are made to pass through two other similar chambers, and the air and gas necessary for combustion in the furnace are caused to pass through the highly heated regenerators. By causing the ingoing gases to pass alternately, at suitable intervals of time, through each pair of regenerators, a very high and, at the same time, uniform temperature can be obtained in the furnace, without any greater consumption of fuel than in the older methods. The success of this process depended entirely on the fuel being first converted into a combustible gas. This was done in a chamber to which only sufficient air is admitted to convert the carbon into carbonic oxide, which is then conducted by tubes to one of the regenerators to be heated, and thence to the furnace, where, coming in contact with air which has been passed through the other regenerator, it burns, giving out intense heat.
There are two methods now in use for the production of steel in the reverberatory furnace, or open-hearth, as it is called. In France, pig-iron and scrap-steel are fused together; in England, pig-iron is decarburized by means of iron-ore, some scrap, however, being generally added for the sake of utilizing it. As in the Bessemer process, the necessary amount of carbon is imparted to the metal by the means of spiegeleisen or ferro-manganese. This process has been largely employed for the production of ship and boiler plates. It has the great advantage that the metal can be kept fluid on the hearth, and its composition adjusted until it is exactly that required.
In 1876 a patent was taken out by M. Pernot, in which it was proposed to produce steel on an open-hearth furnace with a revolving bed, inclined at an angle of 5º or 6º to the vertical. Pig-iron previously heated to redness is placed in the bed of the furnace and covered with scrap-steel. The bed of the furnace is then made to revolve slowly, the pig gradually melts, and the scrap is alternately exposed to the strong heat of the flame, and then dipped under the molten pig iron. In this way the fusion is very rapid, comparatively, the whole mass becoming fluid in about two hours. The process is then completed in the ordinary way. M. Pernot informs me that he has just taken out a patent for an arrangement of his furnace by means of which he can employ gas under pressure, and that within the last few months he has obtained by this means results which have never been equaled before.
The Ponsard furnace aims at combining the advantages of the Bessemer and open-hearth processes. The furnace is so arranged that, by giving it a half-revolution on its oblique axis, the tuyères with which it is supplied may be brought either beneath or above the surface of the bath of metal. By these means the metal can be rapidly decarburized nearly entirely, as in the Bessemer converter, and then, by removing the tuyères from beneath the metal, the final adjustment of the carbon can be made as in the Siemens process. The rapid destruction of the tuyères which is effected is a formidable obstacle to the practical success of this process.
The one important drawback to the Bessemer process was that phosphorus was not in any degree eliminated by it. Notwithstanding this, enormous quantities of steel were made by it; and, within the last three years, means have been devised in the Thomas-Gilchrist, or "basic" process, by which this difficulty has been overcome. In the ordinary Bessemer converter the lining was formed of ganister, a siliceous material, the chemical effect of which was to prevent the elimination of phosphoric acid. Messrs. Thomas and Gilchrist sought a basic material which they could substitute for the ganister, and found a magnesian limestone which worked very satisfactorily. The result of the application has been, that phosphorus has been converted from an enemy into a friend, and aids in producing and maintaining the temperature that is needed. Silicon is also useful as a combustible, and in preventing the metal from becoming honeycombed by escaping gases while solidifying. This it does by combining with oxygen and preventing the latter substance from combining with carbon and forming a gaseous product.
In consequence of the extremely high temperature which we can command, either in the Bessemer or open-hearth process, it is possible to obtain in a molten state a metal practically free from carbon, or containing carbon to any required amount. All of the products have been called steel, although they constitute in effect a new metal, having qualities considerably different from those of steel.
It thus has resulted that we speak of steel ships, steel boilers, and steel rails. The metal of which ship-plates are made contains about 100 per cent. of carbon, that for boilers about 100 while rails usually have about 10. The first and the second could not be appreciably hardened, and the third is considerably below what would formerly have been considered steel.
At present there is but one sound reason why steel should not universally replace iron with advantage, and that is, that in some cases it is cheaper to employ iron. Statistics show us that the enormous quantities of steel now manufactured have but little, if at all, affected the production of wrought-iron. It is, however, I am convinced, but a question of time. When the day comes—and every day brings us nearer to it—when steel will be manufactured as cheaply as iron, then will wrought-iron be a thing of the past among the great civilized nations.
One word as regards the employment of steel made by these modern methods for cutlery. Cutlery-manufacturers would tell you that it is useless for the purpose; nevertheless, on the Continent, it is very largely used, and in this country to a considerable extent. I do not hesitate to assert that, with suitable ores and proper care in the manufacture, steel well suited for cutlery can be made both in the open hearth and the converter. The essential in the ore is that it should not contain phosphorus; with but a trace of phosphorus present, a good cutting edge could never be obtained.
If we glance back for a moment to review our history, we shall see that the open-hearth processes embody the same principle as the first process by which steel was produced, viz., the mutual action of carburized iron and oxide of iron on one another, and the Bessemer process is, after all, though a splendid offspring, only the natural descendant of the finery process, the origin of which, as we have seen, was due to modifications in the primitive blast-furnaces. There is perfect continuity throughout, and, after all, what more natural?
- Abridged from an address delivered before the London Society of Arts.