Bessemer Converter

Feature, Modern Era

Steel for All

Ready, Bessemer?”
“That is so”
“You must be dreaming, man. It’s only been in for, for half an hour”
“I am aware of that. Nevertheless, I assure you, the process is complete. It remains but to run the steel off. And, as you will note, in a very considerable quantity. Watch”
“It won’t be workable, Bessemer. It would have needed far longer than this. Particularly as you have chosen the crudest of crude pig iron for your raw material”
“Watch. You see it starting to tilt? Ah, there it goes.”
“Hmmm”
“You are surprised, then?”
“Somewhat, Bessemer. Such a white, white, heat”
“You are in for more surprise than that. Wait till it cools.”

And indeed, there were surprises ahead not only for the incredulous who watched this first demonstration of the process, but for men all over the world, as the amazing story was released. As Sir Henry Bessemer was to write, thirty-three years later in his autobiography, he was met on all sides with “The most stolid incredulity and distrust. Perhaps”, he went on, “I ought to make some allowance for this feeling, for I proposed to use as my raw material crude pig iron costing £3 per ton instead of the highly purified Swedish bar iron then used, costing £15 to £20 per ton. I proposed also to employ no fuel whatsoever in the converting process which in my case occupied only 25 to 30 minutes instead of the ten days and nights required by the process then in use: and I further proposed to make five tons of steel at a single operation, instead of the small separate batches of 40 to 50 pounds, in which all the Sheffield cast steel was at that time made. What, however, appeared still more incredible was the fact that I proposed to make steel bars at £5 to £6 per ton, instead of £50 or £60″.

To appreciate this tremendous step forward which Henry Bessemer took in 1856, we must consider briefly what steel is, what its uses are. For thousands of years men had won a hard, reasonably malleable substance from the earth. Occasionally it had been almost pure iron, but this was almost always when a meteorite, a thing from the great unknown, had survived entry into our atmosphere to crash into the surface of the earth. More usually it had been in the unworkable form of “iron ore” which, men soon discovered, could be treated with heat to remove its impurities. After this long, slow process, it could be hammered and bent into shape, used for tools, simple weapons.

The chief impurity in iron ore is oxygen and, as man had more or less accidentally discovered, if the ore is heated in the presence of some form of carbon, like coke or charcoal, this oxygen will combine with that, leaving more or less pure iron. The early processes of purification gave a pasty, malleable product in one operation. It was more workable than stone, harder than bronze, but it left a lot to be desired.

Then, halfway through the fourteenth century, as men experimented with bigger fires and hotter heat, the product changed. No longer was it a pasty iron which had changed slowly from one type of solid to another: now, suddenly it poured from the retort as a gleaming liquid.

Yet, disappointingly, it set, when it cooled, into an extremely hard and impossibly brittle metal, a stubborn substance which soon earned itself the nickname “pig iron”.

Yet here was something far harder than iron. If but a way could be found of working it, bending, casting, sharpening it, the metal or something like it would make swords and a host of other things of a strength and durability never before encountered.

Eventually the problem was solved, though in a roundabout way. The pig iron proved to be iron (separated from the thirty per cent or so oxygen content of its ore) but with the addition of about four per cent carbon which it had picked up in the process of reduction, mixed with coke or charcoal. Surprisingly, this small proportion of carbon had altered its character entirely; if only an even smaller proportion could be added to pure iron, it might be possible to make a metal of any degree of hardness required.

This proved to be so, and the new, carbon-transformed, iron came to be known as “steel”, “staal”, “stahl” and other similar sounds all over Europe, all related to the word “stay” or “strong support”. The pig iron could be made into almost pure iron, simply by heating it in a second furnace, away from any coke or charcoal. If, then, a minute proportion of carbon were added to it, steel could be made. Very soft, “mild” steel came from an admixture of about 0*04 per cent carbon; very hard steel from up to 1.5 per cent.

To Henry Bessemer, the restless genius whose life was devoted to making things better, more quickly and if possible more cheaply, whether it was gold paint or revenue stamps, telescopes or artificial flowers, this was a challenge. Surely one operation, carefully controlled, could make steel straight from pig iron?

He studied the processes then in use, a lengthy, expensive process in which, as he later wrote: “The costly bar iron of Sweden was chiefly employed as the raw material, costing from £15 to £20 per ton. The conversion of this expensive iron occupied about ten days, that is, about two days and nights for the gradual heating of the furnace, in which the cold iron bars had been carefully packed in large stone boxes with a layer of charcoal powder between each bar. In these boxes the metal was retained for six days at white heat, two days more being required to cool down the furnace and get out the converted bars. The steel so produced was broken down into small pieces and melted in crucibles holding not more than forty or fifty pounds each, and consuming from two to three tons of expensive oven coke for each ton of steel so melted. This steel was excellently adapted for the manufacture of knives, and for all other cutting instruments but its hard and brittle character, as well as its excessively high price, absolutely precluded its use for the thousands of purposes to which steel is now universally applied.”

And so the idea of Bessemer’s method of conversion, from pig iron to steel, occurred to him. He reasoned that if pig iron were melted, made into a liquid state (which was easy, absurdly so: it emerged from the blast furnace which had reduced it from the ore in just that state, and Bessemer would use it that way), and if this liquid then had jets of air forced up through it, the excess carbon would combine rapidly with the oxygen in the air and be removed. If he kept up his process, his jets of air, for long enough, all the carbon would have gone and he would be left with soft iron. If, though, he controlled the point at which the air was shut off, he could make anything from the softest iron to the hardest steel.

And perhaps the most miraculous part of the “trick” was that the air itself, combining with the carbon and other impurities like silicon, phosphorus, sulphur, generated enough heat to keep the process going, and at a tremendous rate. Bessemer in fact required only air and pig iron, and no fuel whatsoever. He could either take his molten pig iron from the blast furnace (a scheme which appealed to him) or, if that were inconvenient, heat it in a separate retort until it melted and he could pour it into his Bessemer Converter. Here, the chemical action of oxygen combining with carbon raised it quickly to a greater temperature, so that whereas the liquid metal entered the converter at about 1,350 degrees Centigrade, it soon attained 1,6oo degrees.

There were difficulties, but Bessemer solved them. The jets of air had to be forced right through to the core of the metal; the blowing must only take place when all the metal was in the converter; and it had to be possible to stop and restart blowing at will. Soon the process, with these difficulties solved, had been adopted all over the world, and its bulging, pear-shaped, converters could be seen in many countries. Surprisingly, perhaps, it was more popular at first abroad than in his native England, and it was years before many Bessemer converters were working at home. Some that were, were from his own factory in Sheffield, to which steel-men from all over the world came to watch, wonder and depart to make steel by his process, at a royalty of £2 a ton. It became a matter of pride, as Bessemer grew older, that so many towns had been named after him. By the time of his death there were thirteen of them in America alone, in states from Alabama to Wyoming.

The Bessemer Converter was an immense step forward, putting steel in the hands of countries which had never dreamed of affording it, setting it to tasks which no one had ever considered it cheap— or, indeed, efficient, enough to perform, spreading itself across the world as railway lines, and, less happily, arming that world, arming it with powerful, rifled cannon and high-explosive, fragmentation shells.

It is still extensively used, though over the years, with the development of other ways of producing high temperatures, it has not remained the only method of converting iron into steel. The “crucible” method, not unlike the one described, with dismay, by Bessemer at the start of this article, is still used, with much refinement, for making high-grade steels in small quantity. The “Siemens Martin” process (or “open hearth”) was developed ten years after Bessemer’s own and uses the discovery that very high temperatures can be reached (higher than in the Bessemer process) by pre-heating air and combustible gas before setting them alight. In this process the metal (usually a mixture of scrap steel and pig iron) is fed in cold. A further method is conversion in the tremendous, easily controlled, heat of an electric furnace, operated either by an arc or by induction.

The Bessemer process, then, revolutionized not only the manufacture of steel but the uses to which it was put, turning this highly priced, toolmaker’s metal, something without which the world as we know it, with its skyscrapers, its railways, its shipping, its motorcars, would hardly exist. Who was Henry Bessemer?

He was born on 19 January, 1813, in the village of Charlton in Hertfordshire, and of English parents (though he admitted that his name “does not sound like an English one”). His father could not stay idle, and though possessed of more than enough money, set himself to building and operating a type foundry on his estate. Henry helped him, did much experiment, and soon was famous from one end of the country to the other. He had produced a machine for franking stamps which saved the Exchequer thousands a year, he had invented a cheaper lead pencil and, the thing which made him a rich man long before he thought of his steel converter, he had found a method of making gold paint every bit as good as that manufactured by a secret process in Nuremberg and selling at seven shillings and sixpence an ounce, for little more than sixpence a pound.

Being no fool, he parted with it at eighty shillings a pound, which was considerably cheaper, even then, than the German equivalent, and within a few months he was a very rich man. He operated his secret factory with his three young brothers-in-law for thirty years, at which point he gave them the factory and the right to produce it themselves.

Among his other inventions were a new press for sugar-cane, many times more efficient than anything known, for which he was awarded a gold medal by Prince Albert; a new method of silvering mirrors; a new way of making glass; the first simultaneous braking system for a railway train (which came before its time and was rejected by the railway companies).

The knighthood, which raised him from plain Henry Bessemer to Sir Henry, was conferred on him, late in life, as a reward for having invented, forty-six years previously, the new way of printing the date indelibly on revenue stamps, which saved the Exchequer so many pounds over so many years. He had maintained all along that the idea had been his wife’s, a simple but effective scheme for printing the date in little perforations, and he was delighted, at the end of his life, to be able to share this belated honour with her.