THE GENESIS OF THE BESSEMER PROCESS

Experiments with Reverberatory Furnaces -- Early Experiments on the Bessemer Process -- Early forms of Bessemer Convertors -- The Tilting Convertor -- The Bessemer Steel Works, Sheffield

It will, perhaps, assist the non-technical reader to understand what follows if I explain, in a few words, the forms in which iron and steel existed at the time when I commenced the experiments which resulted in the creation of the Bessemer process. At that date there was no steel suitable for structural purposes. Ships, bridges, railway rails, tyres and axles were constructed of wrought iron, while the use of steel was confined to cutlery, tools, springs, and the smaller parts of machinery. This steel was manufactured by heating bars of Swedish wrought iron for a period of some six weeks in contact with charcoal, during which period a part of the carbon was transferred to the iron. The bars were then broken into small pieces, and melted in crucibles holding not more than 60 lb. each. The process was long and costly, and the maximum size of ingot which could be produced was determined by the number of crucibles a given works could deal with simultaneously. Such steel when rolled into bars was sold at £50 to £60 a ton. The wrought iron bars from which the steel was made were manufactured from pig-iron, as was all wrought iron, by the process known as "puddling." Naturally, such a process was costly; puddling demands great strength and endurance on the part of the workmen, combined with much skill.

Practically, all objects in iron, except such as were simply castings, were at that time made from wrought iron manufactured by puddling. The object I set before myself was to produce a metal having characteristics comparable with those of wrought iron or steel, and yet capable of being run into a mould or ingot in a fluid condition. I was aware that Fairbairn and others had sought to improve cast iron by the fusion of some malleable scrap, along with the pig iron, in the cupola furnace. This fusion of scrap-iron, intermixed with a mass of coke, was found to convert the malleable iron into white cast iron, which was at the same time much contaminated with sulphur. Therefore, to a great extent, this system had failed in its object. In my experiments I avoided the difficulties inseparable from Fairbairn's method, by employing a reverberatory furnace in which the pig-iron was fused. Into the bath so formed I put broken-up bars of blister-steel, made from Swedish or other charcoal-iron, its fusion taking place without its being further carburised by contact with the solid fuel, or contaminated by the absorption of sulphur. The high temperature necessary for the fusion of a large proportion of steel in the bath was obtained by constructing the fire-grate much wider than the bath, by contracting the width of the furnace considerably at the bridge, and also by continuing to taper slightly the furnace all the way from the fore-bridge to the downcast flue, which was connected with a tall chimney-shaft. Many alterations and modifications of this furnace were made from time to time, but it was found that the large volume of flame sweeping over the open hearth of the furnace was mixed with a considerable quantity of combustible gas. To consume this gas a hollow fire-bridge was employed, having numerous perforations made in the clay lumps of which it was composed, and so arranged as to allow jets of hot atmospheric air to mingle with these combustible gases, and produce an intense heat close down to the surface of the bath. It was also found that this admission of hot air all along the back of the fire-bridge produced a decarbonising action on the bath; hence the state of carburation of the metal might be altered by regulating the admission of air. This passage of air through the hollow fire-bridge served also to keep down the temperature of the latter and render it more durable.

Some of the samples of metal which I produced were, when annealed, of an extremely fine grain, and of great strength. At this stage of my experiments I cast a small model gun, which in the lathe gave shavings slightly curled, and closely resembling the turnings from a steel ingot; the metal, when polished, also looked white and close-grained like steel. I was so well pleased with this little model gun that I took it over to Paris, obtained an audience with, and showed it to, the Emperor, who had encouraged this attempt to improve the iron employed in founding heavy ordnance. His Majesty, who had desired me to report progress, accepted this experimental gun, remarking that some day it might have an historical interest. It was in recognition of this circumstance that His Majesty, later on, intimated, through Colonel Belleville, his desire to confer on me the decoration of the Legion of Honour, provided I could obtain permission to wear it, a privilege which our Ambassador twice refused. His Majesty also sanctioned the erection of my furnace at the Government Cannon Foundry at Ruelle, near Angouleme, to which place I went with proper introductions for the purpose of arranging all the necessary details. I also sent over from England several thousand special firebricks, etc., for the erection of the furnaces.

But, on resuming my further researches, after my return to London, an incident occurred which suddenly put a stop to the intended works at the Ruelle gun-foundry, and in fact altered all my future plans and investigations.

The furnace, as then arranged, is shown in vertical section in Fig. 35, and in horizontal section, on the line passing above the fire-bridge, in Fig.36, Plate XII., the bath being empty and showing the tapping-hole, and the way in which the furnace narrows at the fire-bridge. Fig. 37, on the same Plate, is also a horizontal section, taken on a line passing through the openings in the perforated hollow fire-bridge, and clearly shows how the jets of air were directed so as to produce an intense ignition of the combustible gases, mingled with, and passing over with, the large volume of flame from the overcharged fire-grate.

The small scale on which this experimental furnace was built (a capacity of 3 cwt. only) was much against my obtaining the high temperature necessary to melt a large proportion of steel in a pig-iron bath. I was, of course, fully aware that a furnace of sufficient capacity to cast a 5-ton or a 10-ton gun would acquire a much higher temperature than was possible in my small furnace. I knew also that forced draught, obtained by closing in the ashpit and forcing air into it, would still further increase the temperature. That this forced draught was in my mind at the time is shown by the fact that I took out a patent for the manufacture of cast steel, dated October 17th, 1855; that is, about two months after the casting of the model gun, in which specification I fully described the forcing of air by a fan into the closed ashpits of the furnaces employed in the manufacture of cast steel. It has since often occurred to me that, with these additional resources still untried, I did not act wisely in so suddenly abandoning these open-hearth experiments, in favour of an entirely different system, suggested to my mind by the incident to be presently referred to. But with my impulsive nature, and intense desire to follow up every new problem that presented itself, I at once threw myself unreservedly into this new study, which seemed to open the way to the rapid production of bars, rails, and plates of malleable metal direct from the blast-furnace.

Before dismissing this subject, it may be interesting, even at this distant period, to speculate on what would have been the natural outcome of my open-hearth furnace experiments, had I not been so suddenly diverted from their further pursuit.

Such a furnace, with forced draught and a capacity of 10 tons, would undoubtedly have melted malleable iron or steel in a bath of pig iron, and have decarburised the latter to the desired extent; for I had, in fact, already fused steel, in a bath of pig iron, on the open hearth of this small reverberatory furnace; and as far back as January, 1855, I had claimed in my patent, "The fusion of steel in a bath of melted pig or cast iron in a reverberatory furnace, as herein described."

This was about ten years prior to the patent taken out by M. Emile Martin, and now generally known as the "Siemens-Martin process." This latter patent was obtained in England in the name of A. Brooman, the patent agent of Emile Martin, and is dated August 18th, 1865, or more than ten years after my patent of January 10th, 1855. M. Emile Martin in his patent says: "The manufacture is effected upon the principle of fusion of iron or natural steel in a bath of cast iron, maintained at a white heat in a reverberatory furnace, such as Siemens gas furnace."

I, however, desire to say that I make no claim to the prior invention of the Siemens-Martin process, nor do I assume that my patent of 1855 furnished any information which either of these gentlemen had availed themselves of, although my patent for melting steel in a bath of cast iron on the hearth of a reverberatory furnace had been granted, and the specification published, some nine years prior to M. Martin's application for his patent. But seeing how many years I was in advance of M. Martin, I feel perfectly justified in saying that the fusion of steel in a bath of pig iron on the open hearth of a reverberatory furnace, which I had patented and accomplished ten years prior to the Siemens-Martin patent, was, to use a favourite expression of Mr. Gladstone, "approaching within measurable distance" of that successful process known as the open-hearth manufacture of mild steel.

On my return from the Ruelle gun-foundry I resumed my experiments with the open-hearth furnace, when the remarkable incident, mentioned above, occurred in this way. Some pieces of pig iron on one side of the bath attracted my attention by remaining unmelted in the great heat of the furnace, and I turned on a little more air through the fire-bridge with the intention of increasing the combustion. On again opening the furnace door, after an interval of half an hour, these two pieces of pig still remained unfused. I then took an iron bar, with the intention of pushing them into the bath, when I discovered that they were merely thin shells of decarburised iron, as represented at A, Fig. 37, Plate XII, showing that atmospheric air alone was capable of wholly decarburising grey pig iron, and converting it into malleable iron without puddling or any other manipulation. Thus a new direction was given to my thoughts, and after due deliberation I became convinced that if air could be brought into contact with a sufficiently extensive surface of molten crude iron, it would rapidly convert it into malleable iron. This, like all new problems, had a special interest for me, and I became impatient to test it by a laboratory experiment. Without loss of time I had some fire-clay crucibles made with dome-shaped perforated covers, and also with some fire-clay blowpipes, which I joined on to a 3 ft. length of 1-in. gas-pipe, the opposite end of which was attached by a piece of rubber tubing to a fixed blast-pipe. This elastic connection permitted of the blow-pipe being easily introduced into and withdrawn from the crucible, as shown at Fig. 38, Plate XIII., which represents a vertical section of an air furnace containing a crucible that, in this case, forms the "converter."

About 10 lb. of molten grey pig iron half filled the crucible, and thirty minutes' blowing was found to convert 10 lb. of grey pig into soft malleable iron. Here at least one great fact was demonstrated, viz., the absolute decarburisation of molten crude iron without any manipulation, but not without fuel, for had not a very high temperature been kept up in the air furnace all the the time this quiet blowing for thirty minutes was going on, it would have resulted in the solidification of the metal in the crucible long before complete decarburisation had been effected. Hence arose the all-important question: can sufficient internal heat be produced by the introduction of atmospheric air to retain the fluidity of the metal until it is wholly decarburised in a vessel not externally heated? This I determined to try without delay, and I fitted up a larger blast-cylinder in connection with a 20 horse-power engine which I had daily at work. I also erected an ordinary founder's cupola, capable of melting half a ton of pig iron. Then came the question of the best form and size for the experimental "converter." I had very little data to guide me in this, as the crucible converter was hidden from view in the furnace during the blow. I found, however, that slag was produced during the process, and escaped through the holes to the lid. Owing to this, I determined on constructing a very simple form of cylindrical converter, about 4 ft. in height in the interior, which was sufficiently tall and capacious, as I believed, to prevent anything but a few sparks and heated gases from escaping through a central hole made in the flat top of the vessel for that purpose, as shown in the vertical section at Fig. 39, Plate XIII.

The converter had six horizontal tuyéres arranged around the lower part of it; these were connected by six adjustable branch pipes, deriving their supply of air from an annular rectangular chamber, extending around the converter, as shown.

All being thus arranged, and a blast of 10 or 15 lb. pressure turned on, about 7 cwt. of molten pig iron was run into the hopper provided on one side of the converter for that purpose. All went on quietly for about ten minutes; sparks such as are commonly seen when tapping a cupola, accompanied by hot gases, ascended through the opening on the top of the converter, just as I supposed would be the case. But soon after a rapid change took place; in fact, the silicon had been quietly consumed, and the oxygen, next uniting with the carbon, sent up an ever-increasing stream of sparks and a voluminous white flame. Then followed a succession of mild explosions, throwing molten slags and splashes of metal high up into the air, the apparatus becoming a veritable volcano in a state of active eruption. No one could approach the converter to turn off the blast, and some low, flat, zinc-covered roofs, close at hand were in danger of being set on fire by the shower of red-hot matter falling on them. All this was a revelation to me, as I had in no way anticipated such violent results. However, in ten minutes more the eruption had ceased, the flame died down, and the process was complete. On tapping the converter into a shallow pan or ladle, and forming the metal into an ingot, it was found to be wholly decarburised malleable iron.

Such were the conditions under which the first charge of pig iron was converted in a vessel neither internally nor externally heated by fire.

I, however, desired to convert a second charge of pig Iron which had been put into the cupola; and in order to prevent this dangerous projection upwards of sparks and molten slags, a temporary expedient was resorted to, which, however, failed in its object.

I procured one of those circular, chequered cast-iron plates so much used in the London pavements to allow coals to be put into the cellars below the pavement. This plate, which was about a foot in diameter, was suspended by a chain at a distance of about 18 in. above the central opening in the top of the converter, as shown in Fig. 39, Plate XIII.

This, as a mere temporary device, was deemed sufficient to allow the conversion of another 7 cwt. charge to be effected, without any danger of setting fire to the premises. The converting operation went on quietly as before, but when the eruption commenced, I saw the suspended plate get rapidly red-hot, and in a few minutes more it melted and fell away, leaving the chain dangling over the opening, and allowing the slags and splashes of metal to shoot upwards as before. Thus it happened that the first converter that I constructed was at once condemned as commercially impracticable, owing to this vertical eruption of cinder, and for this reason only.

All attempts to lessen the violence of the process by the reduction of the number of tuyères,or by lessening their diameter, or by diminishing the pressure of the blast, only resulted in a reduction of the necessary temperature, and in preventing the conversion of the molten pig into malleable iron. In one case the trial of a diminished area of tuyére openings resulted in nearly the whole charge of metal, after more than an hour's blowing, being converted into a solid mass of brittle white iron, similar to ordinary refiner's plate metal. Indeed, I may say the result of all my early investigations proved to me, beyond the possibility of a doubt, a fact which has since been confirmed in every Bessemer steel works throughout Europe and America, viz.: that rapidity of action, ending in a violent eruption, is an absolutely necessary condition of success. Not only must the converted metal acquire an enormously high temperature, so that it may not be chilled when pouring it out of the converter, or when a relatively large quantity of much cooler metal be added to deoxidise it, but it must not chill and form a shell in the ladle during the comparatively long time required for casting the ingots. Hence, to carry out the Bessemer process successfully, a temperature must be obtained very considerably above the mere melting temperature of malleable iron; and in order to secure this it is necessary to drive powerful streams of air into the metal, so as to divide it into innumerable tiny globules diffused through out the whole body of iron under treatment which, for the time being, may be likened to a fluid sponge with the active combustion of carbon with oxygen going on in every one of its myriads of ever-changing cavities.

It has been found that the union of carbon and oxygen takes place so rapidly at this high temperature as to produce a series of mild explosions. In the large converters in common use, a space some 8 ft. or 10 ft. in height above the normal level of the metal is provided, in which this violent action expends itself unseen, and is only partially recognised by a small quantity of slags leaping out of the mouth of the converter.

With these facts before us, it must be self evident that all attempts to produce malleable iron in a plain cylindrical vessel that has no top to it, and in which the metal normally rises to within 6 in. of the open mouth, must utterly fail from two causes: first, because heat would fly off so freely that the temperature of molten malleable iron could never be reached; and secondly, because nearly all the metal contained in such a shallow, open-topped vessel would have leaped out of it, and have been scattered in all directions on the occurrence of the explosive eruption, without which no charge of molten pig iron has, or can be, converted into fluid malleable iron by a blast of air.

I had no sooner condemned my first cylindrical converter than I commenced to remedy its defects. The most obvious and ready way of doing this would have been simply to make an opening on one side of it near the top, and thus allow the escape of the ejected matter to take place horizontally, directing it against a wall, or allowing it to fall into a pit. But I desired to prevent this discharge of metal splashes as much as possible. Hence I determined on constructing a new converter with an upper chamber, having an arched roof and a conical sloping floor.

This converter is represented in Figs. 40 and 41, on Plate XIII., the last-named view being a horizontal section through the tuyéres. When a converter is so constructed, the ejected fluid, that would otherwise pass vertically upwards into the air, is thrown against the arched roof, and any metal that may be emitted falls again on the sloping floor of the upper chamber, and returns to the lower one. The flame and a portion of the slags find their way out of the two square lateral openings provided for that purpose. This upper chamber also served as a receptacle for heating up any metal intended to recarburise, or alloy with, the steel in course of being converted. The sectional plan, Fig. 41, shows six well-burned fire-clay or plumbago tuyére pipes fitted to openings left in the lining for that purpose. Their outer ends were made conical to facilitate the ramming in of loam around them, which effectually held them in position, and at the same time admitted of their easy removal when worn out; a jointed piece of iron tube, with a catch to hold it in place, conveyed the blast to each tuyére.

Another view, Fig. 42, Plate XIV., of this converter, taken at right angles to Fig. 40, shows on one side the hopper by which the molten iron was run into it by a movable spout direct from the cupola. This view also shows the tapping-hole open, and the spout which conducted the converted metal into a movable shallow pan or receiver, supported by a long handle (not shown). A fire-brick plug attached to a long handle was fitted to a fire-brick ring or opening in the bottom of the pan, and prevented any débris from the tapping-hole being carried into the mould. As this apparatus was intended to exhibit the process, it was essential that an easy way should be provided for getting away the ingots and quickly repeating the operation. This casting apparatus, constructed precisely as represented in Fig. 42, was erected at my Bronze Manufactory in London, about two months prior to my reading the "Cheltenham" paper, in August, 1856, to which I shall refer later. The mould was 10 in. square, and about 3 ft. in length inside; it was made in two pieces planed quite parallel, and then permanently bolted together. The base was a massive square flange, resting on four dwarf columns, which stood on the square upper flange of an hydraulic cylinder; bolts passed through these dwarf columns, and through the square flanges, thus uniting the ingot mould and hydraulic cylinder. To the latter a ram or plunger was fitted, having a movable square head, which accurately fitted the mould, and formed a movable bottom to it. Both the ram and the external surface of the mould were kept cool by a water-jacket, provided with supply and waste pipes. Matters being thus arranged, the converted metal was allowed to fall in a vertical stream from the receiver on to the head of the ram. The receiver was then removed, and as soon as the steel was solidified, water under pressure was turned on to the hydraulic cylinder, when a beautiful ingot, 10 in. square, and weighing about 7 cwt, steadily rose and stood on end ready for removal, the head of the ram rising one or two inches above the top of the mould. There are, no doubt, many persons still living who witnessed this combined converting and casting apparatus in successful operation.

Two 10-in. square ingots, made with this apparatus, were sent to the Dowlais Iron Works in Wales, and, without hammering, were rolled into two flat-footed rails on the 6th September, 1856; that is, twenty-four days after the reading of the "Cheltenham" paper. They were rolled under the personal superintendence of Mr. Edward Williams, Past President of the Iron and Steel Institute. Two pieces of these rails are still kept at the Institute in a large glass case containing many other examples of the early working of my process in London and in Sheffield.

Before concluding this brief sketch of the earliest forms of apparatus designed by me to facilitate or improve the process, I must revert to the difficulties inseparable from a fixed converter. In this form of apparatus much heat is dissipated by the blowing which takes place during the running in of the metal, and by the continuation of the blast after the metal is converted, and during the whole time of its discharge, which is a period of uncertain length. There is also the difficulty of stopping the process if anything goes wrong with the blast engine, or if a tuyére gives way. I searched diligently for a remedy for these and other grave defects, which at that time appeared impossible to remove, until the happy idea occurred to me of mounting the converter on axes, so as to be able to keep the tuyéres above the metal until the charge of molten iron was run in, thus permitting the blowing of the whole charge to be commenced at one and the same time, and admitting also of the cessation of blowing during the discharge. This movement of the converter permitted a stoppage of the process to take place at any time for the removal of a damaged tuyére if necessary, and afforded great facilities for working.

The special form of the movable converter was also a matter of great importance, and there were several requirements to provide for. First, in order to make the heavy lining secure when turned upside down, a more or less arched shape in all directions was necessary. A long oval form seemed best adapted to the purpose, as it allowed some eight or nine feet in height for the metal to throw itself about in without leaving the converter. Then the large mouth or outlet pointing to one side was desirable, so that the sparks could be discharged away from the casting pit.

After much study, I arrived at the form shown at A, Fig. 43, Plate XV., which is an external elevation; B is a vertical section showing the position in which the vessel is retained during the running-in of the metal; C shows it during the blow, and D the position it assumes when the converted metal is poured into a loamed-up casting ladle. This ladle is shown at E and F: it is provided with a discharge valve at the bottom, so that it can be moved from mould to mould by closing the valve during such movement, and then permit a vertical stream to descend into the mould perfectly free from any mixture of slags. The advantage of this mode of filling the moulds will be understood when it is borne in mind that they are necessarily narrow upright vessels. It is well known that a stream of molten metal, poured from the lip of a ladle, will describe a parabolic curve in its descent, tending to strike the further side of the mould before reaching the bottom. The surface of the cast-iron mould so struck is instantly melted by the incandescent stream of steel, and the ingot and the mould thus become united, causing great inconvenience. Nor is it easy, in pouring the steel from the lip of the open ladle, to prevent some of the fluid slag floating on its surface from flowing over with the steel and spoiling the ingot. All of these difficulties are avoided by the ladle fitted with a bottom valve discharging a vertical stream down the centre of the mould, the quantity and flow being regulated with great facility by the hand-lever on the side of the ladle. At G and H, Fig. 43, are shown the bottom of the converter and the form of tuyéres.

Many other mechanical contrivances were necessary to perfect the process, such, for instance, as my patent blast engine, with its noiseless self-acting valves; the hydraulic crane carrying the pouring ladle over every mould in the semi-circular casting pit, and designed to rise and fall in accordance with the movement of the converter when filling the ladle for casting; the direct-acting ingot cranes, which clear the pit and refill it with another set of moulds rapidly, and with very little manual labour; the elevated "valve-stand," from which safe position a single workman can overlook the whole converting apparatus, and control all their movements, govern the blast, and work the hydraulic cranes, etc.

The mode of transmitting semi-rotating motion to the converter was another important problem which I had to solve. I was of opinion that ordinary shafting and straps were inapplicable to this fiery monster. Five or ten tons of fluid metal had to be lifted in one direction, this load diminishing until the fluid running to the opposite end of the converter tended to reverse the driving gear. If anything went wrong, or slipped, the converter might swing itself round and discharge the incandescent metal on to the floor or among the workpeople. These considerations led me to adopt the hydraulic apparatus now universally employed for governing the motions of the converter: for, with this simple and reliable means, a lad at a safe distance can start or stop it instantly, can alter its speed and motion, and control the pouring of a 10-ton charge with ease and certainty. The first movable converter was erected at my steel works at Sheffield, and was moved by hand-gearing, because at that early date I had not invented the hydraulic apparatus just described.

This early converting plant did good work at Sheffield, and was constructed precisely as represented in Fig. 44, Plate XVI., which shows also the first modification of the hydraulic casting crane, and its ladle with valve, afterwards elaborated by me and rendered suitable for casting heavy charges of steel. The development of this earliest form of plant is shown in Figs. 45 and 46, Plates XVII. and XVIII., and Fig.47, annexed.

The early experiments at Baxter House were so far successful, as to justify myself and some of my friends in entering into partnership, and erecting in the town of Sheffield, a steel works which still remains in active operation under the style of "Henry Bessemer and Company, Limited." These works were established both for commercial purposes, and also to serve as a pioneer works or school, where the process was for several years exhibited to any iron or steel manufacturers who desired to take a license to work under my patents. All of these were allowed, either personally or by their managers, to see their own iron converted prior to their taking a licence.