BESSEMER STEEL Early Steel-making Processes.—Prior to 1870 the principal metallic materials of construction were cast iron and wrought iron. The former could be made quite hard, but then was brittle; the latter resisted shock excellently, but was comparatively soft. There was a great and growing demand for metal which was both strong, wear resisting and tough, but there was no method of producing it cheaply enough to make, for example, railroad rails. Consequently, the discovery of the Bessemer process for making cheap steel was a godsend—an eager market was waiting; and in turn the satisfied market created other demands. Steel had been known for centuries. While accurate statistics are lacking one may reasonably guess that 250,000 tons a year of blister steel and crucible steel were being made in Europe in 1865. Blister steel was, and still is, made by packing wrought iron bars in a long stone box full of charcoal, and heating the container for several days at a full red (see BLISTER STEEL; CEMENTATION). Carbon gradually is absorbed by the iron under these conditions, and it becomes steel. In order to improve the quality by equalizing the carbon content in this steel, and also eliminating the slag con tained in the original wrought iron, blister steel is cut in short lengths and melted in a clay crucible (see CRUCIBLE CAST STEEL). For the manufacture of fine tools and cutlery such processes survive even yet, principally in the Sheffield region where the industry thrived a half-century ago. But obviously this very in direct method of getting steel, bit by bit, was totally incapable of supplying the needs of the coming mechanical age.
As has been pointed out the alloyed impurities in pig iron can be eliminated by oxidizing them. In all the old refining processes, iron oxide in the form of ore, mill scale, or iron silicate slag (basic cinder) is the means whereby the necessary oxygen is carried into the pig iron under purifica tion. While experimenting on some methods for improving metal for cannon in 1856, it occurred to the English inventor Henry Bessemer that this oxygen for refining could be taken directly from the air if a blast were blown on or through the melted pig iron. On trial it not only purified the iron, but instead of blowing cold and freezing, the metal became hot enough to remain molten even when all the carbon was gone and hitherto infusible pure iron remained. The whole reaction required only a few thinutes; at the end an ingot of mild steel could be cast, ready for the forge or rolling mill.
It took years of experimentation before the new process be came a commercial success. The conservatism of engineers and constructors retarded the adoption of cheap steel and the useful mechanical auxiliaries, largely perfected by the American, Alex ander L. Holley, which enables a modern plant with two con verters to send forth a continuous procession of ingots, without halt, day and night (see BESSEMER, SIR HENRY; BESSEMER STEEL).
(See fig. 2.) Molten pig iron comes from the blast furnaces in ladle cars, and an overhead crane lifts the ladles and pours the contents into the mixer. By joining a reser voir of ceveral lnindred tnnc of metal wide variatinnc in chemical composition in a single cast are lost by dilution. A reservoir of uniform and promptly available raw material is thus established. Uniformity and continuity are foundations of rapid production of Bessemer steel; neither of these results when converting metal direct from the blast furnace or remelted pig iron from the cupola —although the latter sources may be used in emergency. At the proper time a ladle of iron, 10 to 25 tons depending upon the size of the converter, is poured from the mixer; it tilts by rolling on a circular track under positive control of a screw mechanism or hydraulic ram. A small locomotive pushes this full ladle car to the converter house, and by means of a launder the contents are run into the hot converter, with mouth tilted over to receive the charge. The blast is turned on, the converter tilted up to vertical position, and the blow is on (see Plate I).
The converter itself (q.v.) is a cylin drical steel pot perhaps 10 ft. diameter by 20 ft. high mounted on elevated trunnions and geared into a rack and pinion so it can be turned completely over. It is lined a foot thick with a refractory mixture. The bottom bricks are perforated by many small open ings, tuyeres, through which air is sprayed into the metal at high enough pressure to keep it from trickling down into the wind box attached to the bottom of the converter shell, yet not so high as to throw too many splashings out the open mouth. Oxygen in the air blast burns the pig iron it first strikes. This product of reaction may be represented by FeO, and it is swiftly distributed throughout the churning mass. At the temperature of operation silicon and manganese atoms have a greater affinity for oxygen than iron has, so these two reactions occur: Si+2Fe0----2Fe+Si02
Mn+Fe0------Fe+Mn0 These oxides are insoluble in metal and accumulate into droplets of slag—a mixture of iron oxide, manganese oxide and silica. When the metals silicon, manganese and iron combine with oxygen, or burn, much heat is released—enough to heat the incoming air and escaping nitrogen, to provide for radiation losses, and to in crease the temperature of the remaining metal and slag so it is always quite liquid. During this early stage of the blow the red hot nitrogen coming from the mouth glows faintly. Gradually the carbon beeins to burn. thus: Fe0+Fe3C = 4Fe A roaring boil then takes place in the vessel; the carbon monoxide burns in a big, yellow, luminous flame at the converter mouth, and countless flying sparks of metal and slag add to the spectacle.
In only a few minutes the carbon is gime; the flame flickers, and suddenly contracts. This is the signal for turning the vessel over on its side, and stopping the blast. Inside the hot converter is now a seething mass of fluid iron, practically free of silicon, manganese and carbon, covered with a thin layer of slag. The metal, however, contains much gas in solution and if cast im mediately would solidify in a spongy mass. Consequently some cupola-melted spiegel-a pure pig iron containing 20% manganese -is run into the converter. A rapid reaction ensues, wherein the steel is freed of oxygen by combination with the excess of manganese and carbon in the spiegel. The converter then tilts further over, discharging its contents into a waiting ladle. The blow has taken perhaps 12 min.; no greater time is necessary to add the spiegel, pour out the steel, turn the converter upside down to discharge any loose slag chunks, and back up to receive another ladle of liquid pig iron. A second converter alongside will be blowing while the first is dumping. In fact the whole equipment of the converter department is co-ordinated for a complete cycle every 12 to 15 min., and is built of the most rugged proportions, so delays from breakdowns will seldom occur.
The violent reactions at high tempera tures cause considerable scour on the lining of the vessel; espe cially the tuyere brick at the bottom must be replaced after 20 to 25 blows. Holley is responsible for the plan of sectionalizing the converter shell ; when a new bottom is required a car carrying a stout jack is placed under the upright converter, a series of bolts unloosed, and the jack lowered away, carrying with it the entire bottom part of the shell. This car is immediately replaced by another carrying a newly made bottom, already warm, r.mdy for lifting and bolting into place. It is therefore unnecessary to leave a shell in the stand while it cools and the lining is re paired, dried and reheated-a matter of several days-but these sectional lining operations are done in a separate department and replacement of bottom or mouth portion requires less than 3o min. In American practice the lining material is a highly siliceous rock, ground with a little fire clay, moistened and tamped in place with pneumatic rammers. Such a lining is known as an acid lining, and steel made therein is called acid Bessemer steel.
Since the reactions described above leave un touched any sulphur or phosphorus, it is necessary that the pig iron entering the converter be quite low in these two elements. Furthermore silicon is limited by the rate of driving. It is the chief heat-producing element ; if silicon in the pig is high the con verter shell and its contents get superheated during continuous blowings and must be cooled either by dropping a quantity of cold scrap into the shell, by blowing steam in with the blast (which dissociates and absorbs heat), or by delays for radiation. On the other hand if the converter with a new bottom is a little cold, it is tilted partly over so some blast does not penetrate the iron, but burns the CO to CO2 thus liberating more heat within the vessel. Maximum production may be had by blowing a pig iron with higher silicon and lower carbon ; reducing the blowing time by charging mill scale or ore at the beginning of the blow; and charging scrap to absorb the excess heat generated. But in general the pig iron used for acid Bessemer practice, and the product therefrom will be about as follows: Growth of Uses.—Obviously such material as represented by the last analysis is immeasurably superior to wrought iron for railroad rails. The carbon content is responsible for hardness, wear resistance and strength which wrought iron could never acquire. Furthermore, the amount of carbon, and the correspond ing degree of improvement in properties, is under control, for it is only necessary to place some charcoal in the steel ladle to make a higher carbon steel, or to use a spiegel or a ferromanganese containing higher manganese and lower carbon when a milder or softer steel is desired. It is not surprising, then, that when the process was finally established on a producing basis in
own plant in Sheffield, the product was rapidly absorbed for rails and tires. In 1868 there were i io,000 tons produced; in 1888 Bessemer steel reached its peak of production in Great Britain of 1,700,000 tons. Puddling had meanwhile declined ; 70% of British pig iron went into wrought iron in 1883 ; only 5% in 1887. A similar story may be told of America. In 1867 460,000 tons of iron rails were made and sold for $83 per ton; that same year 2,55o tons of Bessemer steel rails were produced and some purchasers paid $170 a ton for them. By 1884, how ever, the last iron rails were made, steel had replaced them to an annual production of 1,500,000 tons a year, and the selling price was $32 per ton. In 1938 rails declined to a minor commodity in America and those were made of open-hearth steel, for reasons to be given later. Much Bessemer steel is still made into skelp for welded pipe, into wire and free machining steel (low in carbon, but high in sulphur and phosphorus).