Bessemer Process of Making Steel. — Steels made either in a Bessemer or an open-hearth furnace are designated as either acid or basic steels, depending on the nature of the refractory linings of the furnace. Silica is an acid lining, while dolomite and magnesite are basic linings. The nature of the linings controls the slag since a basic slag could rapidly dissolve an acid lining and an acid slag would have the same effect on a basic lining. With a basic lining, a large percentage of the phosphorus and some of the sulphur can be removed, but the greater amount of iron oxide left in the steel renders basic steel inferior to acid steel. The basic Bessemer process is used extensively in Europe. In the U.S.A., where iron sufficiently low in phosphorus for acid Bessemer is still available, that process is used instead.

In making Bessemer steel, molten iron direct from the blast furnace is poured into the converter. In the bottom of this vessel (converter) are a number of holes through which air is blown. The air first oxidizes the silicon and manganese, which, together with some iron oxide, rise to the top and form a slag. The carbon then begins to burn, and the blowing is continued until all but about 0.05% of the carbon has been eliminated. The progress of the blow can be determined from the flame coming from the vessel. The oxidation of the impurities has raised the temperature of the metal to the point where it can be cast conveniently. When the blow is completed, the amount of carbon necessary to bring the carbon content to the specific percentage, together with manganese to counteract the influence of sulphur, and silicon to degassify, are added to the molten metal. The finished steel is then poured into a ladle by tipping the vessel, and from the ladle it is poured into ingot molds for subsequent rolling or forging.

The Bessemer Converter. Bessemer steel is likely to be highly oxidized and dirty, and, although it is usually considered inferior to steel produced by other methods, it can compete with steels made by these other methods on jobs calling for low-carbon steel with not very high requirements for strength, ductility, and toughness. The acid process does not remove sulphur and phosphorus, with the result that these elements are excessive for a good grade of steel. Because no extraneous fuel is used in Bessemer process, and a heat of 15 to 25 tons can be made in 10 or 15 minutes, the cost of the process is low. However, the process is losing its prominence and is increasing in operational costs, due to the exhaustion of low-phosphorus iron ores. Bessemer steels find extensive use in low-grade sheets, wire, pipe, skelp for the manufacturing of pipe, and screw stock; also wherever steel is required which is not subjected to severe loadings, and where easy machining is desirable.

A duplexing process where acid Bessemer steel is additionally refined in the basic open-hearth results in a low-carbon steel inferior to that of the basic open hearth, but one which finds much use because of its cheapness. Another duplexing process takes steel produced by either Bessemer or the open-hearth process and refines it in the electric furnace. Such steel is superior, usually, to open-hearth steel. A triplexing process starts with steel from the Bessemer process, removes part of the phosphorus in the basic open hearth and further refines it in the electric furnace.

The Bessemer process in brief outline consists of introducing a quantity of molten metal of proper composition and temperature into a vessel through the bottom of which air is forced upward into the iron. The oxigen thus sent through the metal in a finally divided state at once sets up a violent reaction and oxidizes the silicon, manganese, carbon and some iron. Very low carbon steel corresponding to wrought iron is the result. Ferromanganese or spiegeleisen is now added to deoxidize the metal and give the required carbon to make steel.

It consists of three separate parts, known as the nose, the body and the bottom. All of these are made of heavy steel plates. The nose section is bolted to the body, but the bottom is held in place against the body by linked key bolts, making it easy to key the two parts firmly together and just as easy to replace an old bottom by quickly knocking out the wedges and removing it. The whole converter body is carried on two trunnions which in turn rest on heavy bearings. On the end of one of the trunnions, both of which are hollow, an air connection is made through a packed joint to the air lines. From this a copper gooseneck leads down to the bottom of the vessel, forming a continuous passage for the blast from the air main, which is stationary, to the wind box on the converter. The converter must change its position as the blast progresses for charging of molten metal and addition agents and for skimming of slag. To the other trunnion is attached a pinion which meshes with the two racks that slide horizontally, actuated by a double-acting hydraulic piston. By this means the vessel may be easily rotated through an arc of 270 degrees, the pinion and rack being geared so that the vessel can be completely inverted for dumping slag or for relining, if necessary.

Essentially the Bessemer process consists of blowing air, under pressure, through a bath of molten metal, whereby a portion of the iron, all of the silicon and manganese, and finally the carbon, are oxidized. Unlike all other steel-making processes, no extraneous fuels are required in the Bessemer process; consequently, the charge must be held within fairly narrow chemical limits, and the blowing time for a 10 to 25 ton converter is limited to 10 to 20 minutes. The charge consists of molten pig-iron and cold pig-iron or steel scrap in amounts sufficient to furnish the heat requirements of the blow. In the course of the blow the only source of heat (apart from the intrinsic heat of the molten iron) is the oxidation of iron, silicon, manganese, carbon, and the formation of the ferrous silicate slag; consequently, the composition of the pig-iron is very important. Usually the amount of heat generated by the oxidation of these elements is more than sufficient to keep the contents of the converter at a proper temperature if the charge consists entirely of molten metal from the mixer. The charge, weighing 15 to 18 tons, must be weighed at the mixer as it is poured into the ladle, covered  with a thin layer of coke breeze in order to reduce oxidation, brought to the converter, where the latter is turned down to a horizontal position so as to bring the tuyeres well above the bath. The molten metal poured into the mouth of the vessel will collect in the body of the converter, and before the vessel is turned up the required amount of scrap will be added. When the charge is in the converter, the air blast is turned on, the converter is brought to an upright position and the blow started.