STEEL PROCESS. In recent years two distinct modifications of steel making process have been included under this common title : the Siemens process, in which pig iron and ore are the materials used, and (2) the Siemens-Martin process, which uses a mixture of pig-iron and scrap steel. A com bination of the two is also in general use.
Steel-making by this process was not a success until the brothers Siemens perfected their system of regenerative gas heating. The principle involved is simple and consists in utilizing the waste heat of the products of combustion to raise the temperature of the gas and air employed in heating the furnace, and so increas ing very considerably the temperature obtained. The furnace is of the reverberatory type but instead of having a fire-box at one end and a chimney flue at the other both ends are built with ports for gas and air; the latter are generally arranged above the former in order to promote mixing of the gas and air and at the same time to afford some protection for the roof against the in tense heat of the furnace. The ports are connected by flues with chambers filled with brickwork—called regenerators—which in turn are connected with the gas and air supply through a system of valves ; these last named allow the direction of the gas and air through the furnace to be reversed from time to time. The roof of the furnace must be built with very refractory bricks, generally silica, and the hearth made of sand or dolomite according to the process for which the furnace is to be used.
A diagrammatic sketch of the furnace is given above. Gas from a producer and atmospheric air are admitted to the furnace through the valves A and B, from which they pass to the re versing valves C and D and from them through the regenerators E and F into the hearth where combustion takes place ; from here through the hearth of the furnace the waste gases pass into the regenerators G and H and then through flues to the chimney via the reversing valves. The gas and air are reversed in direction every 20 to 3o min., by which means it is possible to maintain the high temperature required in the furnace.
Two methods of working the process are in general use—acid and basic. In the former the hearth is made of silica sand mixed with a small quantity of oxide of iron, in the latter it is of dolo mite mixed with a small amount of tar; in both cases the hearth is fritted together under the intense heat of the furnace before any melting is done.
In the acid process no elimination of sulphur and phosphorus is possible during the working of the charge, and it is necessary to start with pig-iron and scrap which are low in these elements. The
amount of silicon admissible depends upon the amount of scrap in the charge; when the amount is high, pig-iron containing a higher percentage of silicon can be used than when the quantity of scrap is small, but the total amount of silicon present should be about 1.5%. Assuming the furnace already hot from previous working, the pig-iron and scrap are charged into it in the order given ; during the melting down the greater part of the silicon and manganese is oxidised and unites with the oxide of iron derived from the scrap to form a slag consisting mainly of silicates of iron and manganese. From time to time additions of iron ore are made to the slag in order to render it oxidising and so capable of removing by oxidation the impurities in the charge. Of these, silicon and manganese are the first to go, and carbon in the form of gaseous carbon monoxide follows, the gas liberated causing the bath to boil vigorously. Towards the close of the operation great care must be taken to avoid the addition of more ore than is necessary to make the carbon sufficiently low, otherwise steel of poor quality will be produced. Ferro-manganese and ferro-silicon are now added to deoxidise the metal and the charge tapped into a ladle and then cast into ingots.
In the basic process the procedure is somewhat different. In this case the elimination of the phosphorus and sulphur is possible on account of the nature of the slag, which consists of phosphate and silicate of calcium containing a large quantity of lime in suspen sion ; additions of iron ore are made as in the acid process, and small quantities of lime must also be added from time to time to maintain the basicity of the slag; excess, however, must be pre vented, otherwise the slag will become pasty and incapable of carrying on its work. Precautions must also be taken to secure the removal of the phosphorus and carbon in the order mentioned; otherwise the bath will go off the boil, and great difficulty will be experienced in getting rid of the phosphorus. The nature of the slag also necessitates the postponement of the deoxidation of the bath until the charge is in the ladle ; should the ferro-manganese necessary be added in the furnace, there is great risk of the phos phorus being reduced from the slag and passing back into the metal. Pig-iron for use in the basic process may contain from I to 2% of phosphorus and should be low in both sulphur and silicon but fairly high in manganese. (See also IRON AND STEEL.)