Inventors at Work, with Chapters on Discovery

by George Iles

Bessemer’s Steel Process.

In the supreme effort of his life Bessemer once more held himself a debtor to his ignorance, to the fact that his mind was unworn by routine and ruttiness. Referring to his attempt to make a cheap metal stronger than cast iron for guns, he says: “My knowledge of iron metallurgy was at that time very limited, and consisted only of such facts as an engineer must necessarily observe in the foundry or smith’s shop; but this was in one sense an advantage to me, for I had nothing to unlearn. My mind was open and free to receive any new impressions, without having to struggle against the bias which a life-long practice of routine cannot fail more or less to create.”

Now appears the genius of the man, showing that if his brain was unoccupied by rules-of-thumb it was full to overflowing with original and sound ideas. He goes on to say: “A little reflection, assisted by a good deal of practical knowledge of copper and its alloys, made me reject all these from the first, and look to iron or some of its combinations, as the only material suitable for heavy ordnance.” Of fascinating interest is the great inventor’s story of how step by step he arrived at his final success. After reciting his preliminary experiments, in an endeavor to remove carbon from pig iron so as to make malleable iron and steel, he says:

“On my return from the Ruelle gun-foundry I resumed my experiments with the open-hearth furnace, when 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 shells of decarburized iron, showing that atmospheric air alone was capable of wholly decarburizing 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. Without loss of time I had some fire-clay crucibles made with dome-shaped perforated covers, and also with some fire-clay blow-pipes, which I joined on to a three-foot length of one-inch 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 which, in effect, formed a converter. About ten pounds of molten grey pig iron half filled the crucible, and thirty minutes’ blowing was found to convert this metal into soft malleable iron. Here at least one great fact was demonstrated, namely, the absolute decarburization 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 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 carburization 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 carburized 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 few 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 holes in the lid. Owing to this, I constructed a very simple form of cylindrical converter, about four feet in interior height, sufficiently tall and capacious, 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. This converter had six horizontal tuyères 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.

“All being thus arranged, and a blast of 10 or 15 pounds’ pressure turned on, about seven hundred-weight 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 an opening on the top of the converter, just as I had 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 decarburized 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.”

First Bessemer Converter and Ladle.
A, external elevation. B, vertical section during an in-pour of metal. C, during a blow. F, E, ladle with discharge valve at bottom. H, tuyères. G, bottom with tuyères.
From “Sir Henry Bessemer: an Autobiography,” by permission of Engineering, London.

The narrative continues with details of further masterly experiments until the new process was turning out steels of excellent quality, containing any desired fraction of carbon, at a cost of but six to seven pounds sterling per ton as against fifty to sixty pounds by the methods which Bessemer laid upon the shelf. His predecessors had made forty to fifty pounds of steel at a time in small crucibles, he made five tons in twenty minutes. In his magnificent simplification Bessemer at a stroke dismissed a long series of troublesome processes long believed to be as unavoidable as winter’s cold. He did away with the smelting of pig iron, the rolling, shearing and piling of bars, and the heating furnace. From the beginning of the Bessemer manufacture to the present hour, its main output has been rails for railroads. In this single service the debt due to Bessemer surpasses computation, for his steel has as least six-fold the durability of the iron it has replaced. A rail laid at Crewe Station in 1863, weighing twenty pounds to the yard, was turned in 1866 and taken up in 1875; it was estimated that 72,000,000 tons had passed over it, while the greatest wear of its tables was but .85 inch.

Bessemer did not at once enter upon success in the practical application of his process. British pig iron, with which he dealt, abounded in phosphorus, an element which he could not drive out, and which made his steels faulty. It was only when, at length, he obtained pure pig iron from Sweden that he was able to supply the market with pure, soft malleable iron, and with steels of various degrees of hardness. In a sequel, full of interest, he sketches the shrewd means by which he secured a handsome fortune from his great invention, for Bessemer had remarkable business ability as well as inventive genius. His labors in steel-making obliged him to neglect his devices in the plate-glass manufacture which, despite their merit, were also neglected by the producers of plate-glass. He remarks: “The simple fact is that an invention must be nursed and tended as a mother nurses her baby, or it inevitably perishes.”