Bessemer Steel

BESSEMER STEEL. In 1856, in a paper before the British Association, Bessemer outlined the method associated with his name which, with modifications in detail, gave to the world the power to make steel cheaply. After several revisions in the design of the blowing vessel, the present shape was de termined as the most satisfactory and a works was established in Sheffield in 186o to operate the process. Almost concurrent with Bessemer's experiments trials on similar lines were being made by William Kelly in the United States. Kelly's efforts were never carried to a commercial stage, al though he was adjudged to have certain rights under the American patent laws. A settlement was made, however, between Besse mer and Kelly under which the latter retired and the claims of the English inventor were in future unchallenged. The appli cation of the process spread rap idly both in Great Britain and abroad, but the later develop ment of the Open Hearth method of steel manufacture checked the growth of the Bessemer process. Whilst the output of Bessemer steel especially on the Continent, for reasons which will be indi cated later, is still very large, the quantity tends to decline in re lation to the amount made in Open Hearth furnaces.

Theory of

Process.—Pig-iron contains a number of constituents in addition to iron. These include carbon, silicon, sulphur, phos phorus and manganese. Steel con tains similar constituents, but in different proportions. Steel mak ing therefore consists in eliminating, or adding, the necessary con stituents to pig-iron under conditions which enable the ultimate result to be controlled with certainty. This is an important point since the character of steel alters quickly with a relatively small difference in its constituents.

All the elements named, including iron, have a strong affinity

for oxygen with the exception of sulphur and phosphorus, which are not affected by it. Further, the reactions of iron, carbon, silicon and manganese with oxygen are each heat producing. The essential principle of the Bessemer process consists of blowing air under pressure through a body of molten pig-iron thus oxidizing rapidly by atmospheric oxygen a portion of the iron, practically all the silicon and manganese and, finally, the carbon. The heat generated is sufficient to maintain the metal in a liquid state.

At the end of the blow the vessel contains liquid metal con sisting of iron, sulphur and phosphorus covered with a layer of slag. In order to convert the metal into steel the requisite amounts of manganese, silicon and carbon have to be added. This is done by using ferromanganese, ferro-silicon or anthra cite coal. These additions also act as deoxidizers and serve to remove any excess oxygen present in the metal after blowing.

As already indicated, sulphur and phosphorus are not affected by the process as described, and Bessemer was compelled to use pig-iron as free of these elements as possible. A large proportion of the pig-iron made, however, contains more phosphorus than is permissible in steel, and this iron was useless until Thomas and Gilchrist conceived the idea of lining the vessel with basic material, such as magnesian limestone, and adding lime to the metal while blowing. A reaction takes place between the phos phorus and the lime, forming calcium phosphate, which again makes a slag on the metal. The discovery of this Basic Bessemer process in 1878 was of the utmost importance, as it made possible the utilization of the vast resources of phosphoric iron ore in Lorraine and in America.

Description of Plant.

The principal component of a Besse mer plant is naturally the vessel or converter. This is illus trated in fig. 2 which shows a pear-shaped vessel of steel lined with a suitable refractory material, siliceous in the case of the Acid Bessemer and dolomite in the Basic Bessemer. The capacity of the vessel varies with the conditions of the plant, many of those in Great Britain being capable of holding i o tons, whilst in the U.S.A. vessels of 25 tons capacity are frequently used. The bottom of the vessel is made easily removable, a development introduced by Holley in America. In the bottom are carried the tuyere blocks in which are formed the holes for the introduction of the pressure air to the liquid metal.

The vessel is carried on substantial trunnions, and is capable of being rotated through an angle of about zoo° by hydraulic cylinders operated through controlling valves arranged on a plat form located at a safe distance. The pig-iron in some cases is brought cold to the plant and melted in a cupola, but in more modern practice the iron is carried in ladles, whilst still liquid, from the blast furnace and poured into a large cylindrical vessel known as a metal mixer. The mixer in some works has a capacity of i,000 tons of metal, and therefore with such a large store any variation in the quality of the pig-iron is rectified by mixing. The three positions shown in fig. i indicate how the converter is held whilst filling, blowing and pouring respectively. As the converter is rotated from the filling to the blowing position the liquid metal flows over the tuyeres, but as air at a pressure of 25 lb. per sq. in. is being blown through them this prevents any choking. The action during blowing is vigorous. A shower of sparks is ejected continuously, and after a few moments a yellow ish red flame appears which persists for four or five minutes. The temperature of the metal rises and boiling commences. The flame increases in size and becomes bright, intense; and of charac teristic colour. After about eight minutes the flame changes to a bluish colour, becomes almost transparent and "drops," showing that the reactions are over. The blow occupies about zomins. in all. The converter is now turned horizontal and the ferro manganese added. The steel is then ready and is poured into a ladle from which it is tapped into ingot moulds.

Thus a charge of pig-iron of a weight in some cases up to 25 tons can be converted into steel in about 25mins. The vessel is turned with spout downwards and completely emptied ; any repairs are then done to the lining and a new charge is at once introduced. A diagrammatic arrangement of a Bessemer plant is shown in figure 3, above.

A typical example of the changes which take place during the blowing of an acid Bessemer charge is as follows :—