STEEL, THE BESSEMER PROCESS. The Bessemer process is employed to manu facture from the crude product of the blast furnace a comparatively low grade of steel, which is at the same time better suited for certain purposes than that manufactured by any other process. It does not seek to make high-grade tool steel or steel castings, but a cheaper grade, for which there is an im mense demand, for instance, steel rails and structural shapes. It occupies a field in which it is and probably always will be supreme, so long as the supply of suitable raw material lasts. Many modifications of the Bessemer process have been introduced, most of which have not met with approval, but one or two have become commercially recognized, such as the Clapp-Griffiths, the Robert and the Tropenas processes. These latter by variations in the method of applying the blast, or by modifications in the modus operandi, have made changes in the resulting product so that it may be successfully applied to the manufacture of castings, etc., but they are all essentially Besse mer converters. Further notes on these proc esses will appear later in this article.
To get a fair idea of the revolutionary nature of Bessemer's invention mention must be made of the fact that before his time steel, on amount of the difficulty of its manufacture, was in use for practically little else but cut lery, springs and the small parts of machines. It had to be made in crucibles by melting ce mented bar or ((blister steel'," itself the result of a long and costly operation on Swedish iron in a cementation furnace. A steel rail, build ing or bridge was unthought of. Everything was wrought iron, made at great expenditure of time and human energy in the puddling furnace or charcoal bloomery.
The story of Bessemer's discovery is of un usual interest. Experiments had been made from time to time to improve iron castings by the addition of wrought-iron scrap to pig iron in a cupola, and Bessemer had achieved more success than his contemporaries by the use of a reverberatory furnace instead of a cupola, into which he had introduced great improve ments such as a hollow bridge wall through which jets of air were forced, giving a higher temperature, enabling him to fuse more steel in the bath of cast iron. How he came to dis cover that great principle on which all air blast processes of steelmaking are based is best told in his own words. In connection with the description of some experiments with his forced draft reverberatory furnace he says: ((Some pieces of pig iron on one side of the furnace 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 iron 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 decarbur ized iron showing that atmospheric air alone was capable of wholly decarburizing gray pig iron and converting it into malleable iron with out puddling or any other manipulation. Thus another 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 be rapidly converted into malle able iron?) With great enthusiasm Bessemer proceeded to the next step, namely, the experimental test of his conviction. He procured a crucible hav
ing a perforated cover and a pipe of refractory material passing through the lid, connected to a blast supply. A small quantity of cast iron was melted in the crucible and while still in the furnace a current or air was passed through it. After 30 minutes blowing the contents of the crucible were found to be soft malleable iron. The next thing to be determined was whether the chemical reactions caused by the blowing would generate sufficient heat to keep the contents of the crucible liquid without any additional heat being applied from outside. This seemed a preposterous idea—that blow ing air into molten iron should only not solidify it in a short time, but actually increase its tem perature very materially. But Bessemer was confident it would be so and immediately set about the construction of a converter. The first converter was about four feet in height, cylin drical, having a flat top with a circular open ing, and six horizontal tuyeres around the bottom fed by a circular blast pipe.. Its capacity was about 700 weight of iron, which was introduced by a sort of funnel in the side after the blast was turned on. The first hlow was full of surprises, for what took place could be readily seen, whereas in the former experiments in the crucible the entire apparatus was out of sight. For the first few minutes sparks and smoke were emitted from the converter, but then a flame burst forth, which increased in violence so that no one could go near the converter. Then the flame died down and the metal was tapped out through a hole in the bottom of the converter, and found to be malle able. The possibilities of the process were now demonstrated beyond the slightest doubt, but plodigious labors were yet to be undertaken in order to make it mechanically perfect. The chief difficulties were that during the operation large quantities of slag and some drops of iron were blown out of the converter, and that the blast had to be kept on during the whole time of introducing the charge and also tapping it out when converted. Many changes were made in the interior shape to obviate the first. The height was increased and a double dome .shaped top added with the idea that any pro jections of slag and iron would strike the dome and drop back into the converter. This was fairly successful and was the type of con verter used in the first public exhibition of the process. In order to overcome the necessity of keeping up the blast while filling and emptying the converter the shape which is so well known to-day was evolved after a great many changes and experiments. It had to be a movable vessel, in order to bring the metal away from the tuyeres and permit of shutting off the blast, and so a shape somewhat oval, and with a mouth inclined to one side at an angle, the whole mounted on trunnions, became the stand ard Bessemer converter. Other vast improve ments came quickly, the hydraulically driven rack and pinion to rotate the converter, the hydraulic crane for handling the ladle and the casting pit for handling of ingots and molds. One of the difficulties encountered was the mix ing of the slag with the iron, and this was over come by providing a ladle having an opening at the bottom which was closed by a refrac tory stopper attached to a rod. The iron was run into the ladle and the stopper held in posi tion till it was full. Then as the slag floated the stopper was withdrawn, allowing perfectly clean metal to run into the ingot molds.