The Iron and Steel Manufactures

In the manufacture of iron numerous improvements have been introduced in recent years. New processes almost without number have been devised, whereby a more or less considerable saving in time and expense is effected, or by which the quality of the material produced is improved. But of all these improvements none can compare in importance with the process invented by Mr. Bessemer, and bearing his name. " The discoveries and experiments of Bessemer," says Sir W. Fairbairn, "have effected a total change in the manufacture of iron." To convert iron from the cast into the malleable state, either by the Bloomeney furnace or the puddling process, was a work which could not be done without a large outlay of time and expense, in both of which the Bessemer process has effected a very large saving. There is also another advantage secured by Mr. Bessemer's invention. The quality of the metal produced can be controlled with an ease and accuracy previously unattainable. From the most perfect cast steel down to wrought iron with the faintest character of steel about it, any variety may by this process be obtained without difficulty.

Mr. Bessemer first made known his process at the meeting of the British Association for the Advancement of Science, at Cheltenham, in 1856; and the importance of the invention, which was at once communicated through the newspapers to the whole country, was immediately recognised. Great expectations as to the value of the new method were formed by scientific and practical men. So eagerly, indeed, did the practical men receive the new invention, that, within twenty-five days of the announcement of it at Cheltenham, licences were purchased to the extent of £25,000 by ironmasters authorising them to manufacture malleable iron under Mr. Bessemer's patent. A rapid reaction, however, in the feeling of the iron trade and public generally soon followed. Several rough trials were made by ironmasters, and turned out so badly, that the invention was pronounced valueless. The press; teemed with letters on the subject, their general burden being that the Bessemer process was impracticable, and the hopes with which it had been regarded fallacious. Certain imperfections had been, no doubt, discovered in the process; and these the inventor, instead of arguing against the fallacious reasoning and false representations of his detractors, set himself to remedy. Confident of the soundness of the principle his method of operation was founded on, he proceeded to prosecute his experiments, and resolved to observe silence until he had made the process a commercial success. After three years of incessant labour and an expenditure of £10,000, he once more brought the invention before the public. But the iron trade now manifested no interest in it, and the public had almost forgotten it; and Mr. Bessemer, now really on the eve of a vast success, everywhere found his scheme regarded as a total failure. " One of two things," says Mr. Bessemer, " became imperative; either the invention must be abandoned or the inventor must become a steel manufacturer." And a steel manufacturer accordingly he became. Sheffield was the centre of the steel manufacture in England, and it was there that the new firm of Henry Bessemer and Co. established their first steel works. From the first moment the enterprise was a success. The works at Sheffield became the resort of practical steel-makers anxious to initiate themselves into the mysteries of the new art, and from the place of its origin it soon spread into every state of Europe, and was even carried to India and America. In the course of a few years the Bessemer process had risen again from undeserved discredit to the highest popularity, the inventor reaping the just reward of his ingenuity and indomitable perseverance by rapidly amassing a large fortune.

The process, briefly described, is as follows. Pig-iron, which contains generally about five per cent, of carbon, is melted in a reverberatory furnace and then poured into a vessel lined with refractory clay. This vessel, which is capable of holding from five tons and upwards, is called the converter. The converter is fixed on a pivot, and through the latter passes a tube leading from a powerful blowing apparatus. The air passing along the tube is blown into the converter through tuyeres or blow-holes fixed in the bottom of the vessel, and is thus forced to pass through the body or mass of the molten metal. In its passage through the metal the air burns off the carbon, as well as silicon and small quantities of other bodies. At a certain point the blast of air is stopped, and the operation is finished, the whole process occupying about twenty minutes.

After the crude molten iron has been run into the converter, the blast is turned on, "when the process instantly commences, the jets of air rushing upwards, expanding in volume, and dividing into an infinite number of globules, which become dispersed throughout the fluid mass. The silicium is first attacked, neither the iron nor carbon being operated upon to any extent while any silicium remains. When the crude iron contains about one and a quarter per cent, of silicium, it requires about twelve minutes' blowing to remove it, during which time only a few sparks make their appearance; but as soon as the silicium is nearly all eliminated, the carbon and iron are more and more acted upon. At about this period, two minutes suffice to change entirely the outward indications of the process; for in that short space of time the bright sparks previously seen issuing from the vessel have almost wholly disappeared, and a voluminous flame rushes out of the mouth of the vessel, gradually passing from orange colour to a brilliant white. The light is so intense as to project shadows of every object on the wall of the building, even at midday. In about twenty-five minutes from the commencement of the process, the flame is observed to drop off suddenly, thus indicating the complete decarbonisation of the metal. Combustion can therefore no longer go on. The vessel is then immediately turned again into the horizontal position, and a small quantity of carburet of manganese, mixed with carburet of iron and silicium, added, when the vessel is again turned up, and the blast driven through it as before; the manganese almost wholly disappearing in a few seconds, whilst the carbon is retained. The steel may thus be carbonised to any desired extent, entirely depending on the known quantity of carbon thus added to the converted metal; while the carburet of manganese effects precisely the same chemical change as it does in the thousand other steel-pots in which it is daily employed in Sheffield - that is, it confers on it the property of welding and working more soundly under the hammer."

" By means of the various mechanical appliances which Mr. Bessemer has engrafted on his original process, the amount of labour and the exposure of the workmen to heat, when dealing with five tons of fluid steel, is found to be far less than has to be encountered in the manipulations of an 80-lb. puddle ball, or the removal from the furnace of a set of 30-lb. crucibles of cast-steel; but while the exposure of the workman to severe temperatures has been diminished, and the reduction of manual labour, by a series of almost self-acting hydraulic apparatus, has been effected, improvements of equal importance have been made in the converting process, by which the degree of carburation and toughness of the metal are put under the most perfect control of the workman, who, by weight and measure, can insure a thousand consecutive charges of precisely the same quality, or he can vary it by almost imperceptible gradations, from the hardest steel to the softest malleable iron."

In the manufacture of Bessemer steel Professor Roscoe has utilised the powers of the spectroscope, an instrument which is, beyond dispute, one of the most wonderful which the progress of science has bestowed upon mankind in the recent times. This ingenious instrument has revealed the fact that bodies in an incandescent or burning state have definite spectra - a fact which has not only led to the discovery of a number of elementary substances whose existence on the earth had previously escaped the researches of chemists, but which has also enabled astronomical chemists to elucidate to a considerable extent the composition of the sun and the planets, and even that of nebulae, comets, and fixed stars. In order to explain the use of the spectroscope in the Bessemer process, it is important to remember that steel differs from cast-iron in containing a smaller quantity of carbon, and that in the process of Mr. Bessemer, the carbon of cast-iron is burnt out of the molten white-hot metal by a blast of atmospheric air. The chemical changes occurring during the process are, speaking generally, as follows: - " In the first place, the graphite which is contained in the pig- iron is converted into combined carbon; and in the second place, we find that the silicon begins to burn off, and that afterwards the combined carbon is oxidised." The oxygen of the air blown through the molten metal combines with the carbon and silicon contained in it - in other words, burns them off, and the burning gases issue in the form of a flame from the mouth of the converter. The appearance of this flame is changed in the course of the process; and to the success of the operation it is absolutely necessary that, at a certain stage, the blast of air should be stopped. If for ten seconds after this stage has been reached the blast is continued, or if it be stopped ten seconds before the proper point has arrived, then the operation will have failed, and Bessemer steel will not be produced. The metal contained in the converter is either too viscid to admit of being poured off, or it contains too much carbon, and will crumble up under the hammer like cast-iron. Experience had enabled those conducting the operation to tell, by the appearance of the flame, with tolerable accuracy, when the proper time had arrived for turning off the blast of air; but they naturally often made mistakes. Indeed, to an inexperienced eye no difference can be detected in the flame when the critical moment has arrived. By help of the spectrum analysis, the point can now be determined with the greatest certainty and accuracy; and that which formerly depended on the quickness of vision of a skilled eye has now become a matter of exact scientific observation. " By a simultaneous comparison of the lines in the Bessemer spectrum with those of well-known substances," writes Professor Roscoe, " I was able, in the year 1863, to detect the following' substances in the Bessemer flame - sodium, potassium, lithium, iron, carbon, hydrogen, and nitrogen. At a certain stage of the operation I found that all at once the lines supposed to be due to carbon disappeared, and we got a continuous spectrum. The workmen, by experience, had learned that this is the moment at which the air must be shut off; but it is only by means of the spectroscope that this point can be exactly determined."

In the Bessemer process the molten metal attains a temperature or heat far more intense than had ever before been attained in metallurgical operations, and this it does without the addition of a particle of fuel, but simply from the combustion of the carbon in the crude iron by the blast of air forced through the molten metal. The metal thus decarbonised retains its fluidity long enough to admit of its being cast in ingots, capable of extension by being passed through rollers or under the hammer.

The iron and steel produced by this process are not only obtained at a very great economy of time and expense, but they possess the further advantage of being of better quality than iron or steel produced by the old methods. This has been conclusively established by numerous experiments. The tensile strength of Bessemer unhammered cast-iron is more than twice as great as that of common cast-iron, and about twenty per cent, greater than the Swedish, which is the best of all the old varieties of cast-iron. The weight required to break a rod of ordinary cast-iron, of a square inch, was found to be 18,500 lbs.; for Swedish cast-iron it is 33,000 lbs.; while in the case of Bessemer cast-iron it is 41,000 lbs. The breaking weight of Yorkshire wrought-iron plates, the best of all English wrought-iron, is 59,500 lbs. per square inch; while that of Bessemer wrought-iron plates is, for soft iron, 68,000 lbs., and that for Bessemer soft cast-steel is 110,000 lbs. per square inch. The breaking weight of the best Sheffield cast-steel is 130,000 lbs. to the square inch, while that of the best Bessemer cast-steel is 152,000 lbs. to the square inch.

A practical illustration of the strength and powers of endurance of Bessemer steel was given in 1862, when one of the earliest experiments with steel rails was made. Near the bridge at the Chalk Farm station on the London and North-Western Railway - a point where it was believed that there was more traffic than on any other rails in the world - two new rails of Bessemer steel were laid down, and opposite to them two new rails of iron, so that no engine or carriage which passed over the one could fail to go over the other also. As soon as the iron rails were too much worn to be safe any longer they were, as usual, reversed, and the other surface exposed to the traffic. When worn out, the iron rails were taken up, and another pair laid down in their place. These, also, after a time, had to be reversed, and afterwards to give place to a third pair. This process went on until eight pairs of iron rails, or sixteen faces, had been worn out, while the steel rails had not yet even been reversed. When the steel rails were taken up for examination, the ninth pair of iron rails had already been worn out on their upper surface. The steel rails were found to have been worn much thinner, but it was the opinion of the platelayers that their first surfaces would still have worn as long as six more surfaces of iron rails, showing that the strength or endurance of the steel rail was equal to that of twenty- two or twenty-three iron rails. The success of the experiment led to the rapid adoption of steel rails, particularly for those parts of railway lines where the traffic is greater than the average. On an average, 8,000 goods trucks passed daily over the steel rails at Chalk Farm station; and it is estimated that between the time of their being laid down and their examination in September, 1864, no fewer than 7,000,000 wagons had passed over them. The earliest trial with rails of Bessemer steel, however, was at the railway station at Crewe, where the traffic is also very great. In November, 1864, or three years after the rails had been laid down, they had not even been reversed, and the upper face still showed very little of the effects of wear and tear.