STEEL, OPEN HEARTH MANUFAC TURE. The process of producing steel of this class derives its name from the type of furnace used open hearth. In European practice the process is known as the Siemens Martin, receiving its name because of being originated, developed and perfected by two eminent and early metallurgists (1864-67). The process is conducted either in an acid or basic furnace depending upon the character of the raw material treated.
There are several divisions of the process. The original plan was to treat a charge of all pig-iron with the assistance of iron ore to con vert into steel. Later a modification was intro duced wherein pig-iron and steel or wrought iron scrap were melted together and subse quently refined to steel. The first named con stituted the Siemens process and the second the Siemens-Martin. To-day the essentials of both processes remain, but modifications have been introduced from time to time. The Monell process, conducted in an open hearth furnace with a basic lining, consists of a charge of molten pig-iron to which certain quantities of iron ore and lime are added which fuse to form a slag and which slag flows out of the furnace at a point above the surface of the molten metal, thus carrying with it certain impurities in the charge of metal it is desired to remove. The ore and lime are added at certain intervals to make the overflow of slag continuous until the desired stage of purification is reached.
Next in importance is the Talbot process, also conducted in a basic lined open hearth steel furnace. The furnace for this process is of movable type and is so constructed that the body can be tilted forward to discharge the bath of metal through an opening placed at the level of the bath, or it can be moved in the opposite direction to pour out of suitable openings placed at a higher _point greater or less quanti ties of the slag. The process is a continuous one. A full charge of metal — about 200 tons which may consist of molten pig-iron direct from the blast furnace, or a mixture of pig iron and steel scrap is treated by additions of iron ore and lime until proper pre-determined conditions are reached by slag action. At that stage the bulk of the slag is poured off and about one-third of the refined charge is with drawn into a ladle allowing the balance to re main in the furnace. The steel in the ladle is recarburized and teemed into the ingot molds and an additional charge of impure molten pig-iron is poured into the furnace in sufficient quantity to bring the contents up to the initial charge. The remaining slag is then removed by tilting the furnace, a fresh slag made by ore and lime and the refining operation repeated.
In the second treatment, however, the interval of time is considerably reduced because the residue of refined metal from the initial charge greatly dilutes the succeeding addition of molten impure stock and so making a smaller total of objectionable elements to remove by slag action. Under normal conditions the furnace is never empty excepting at week ends when incidental repairs are made. In Siemens-Martin open hearth practice the furnace is emptied com pletely at the end of each heat. It is then patched up, as may be necessary and recharged.
Another process of recognized importance is the Bertrand-Thiel process. It is used in Eu rope to some extent. Two open-hearth furnaces are used, one acid lined and another basic. One furnace may be placed at a higher level than the other so that the charge treated in the upper furnace can be tapped directly by gravity into the one below. Or the furnaces may be placed at the same level and the charge transference from one to the other be accom purification by the basic process, by which the metal is treated and fully refined in the sec ond stage of the process. It is a combined acid and basic process whereby pig-iron is con verted to steel and allows the use of stock so high in phosphorus and sulphur that it could not be successfully refined by either process singly. In the acid-lined furnace the charge of molten metal is desiliconized and partly de carburized, but not desulphurized or dephos plished by ladles. In the Hoesch modification of this process only one furnace is employed, the metal being temporarily transferred to a ladle while the basic lining is being spread, when it is returned for final treatment The principle of the process is that a charge of impure stock is first treated in an acid-lined furnace to greatly lessen by nearly full re moval certain elements objectionable to further phorized, but when transferred after such treat ment to a basic-lined furnace, either by gravity or ladle, the decarbonization, desulphurization and dephosphorization are completed to meet any requirements. In the United States and Canada the Bertrand-Thiel process has, to some extent, been modified by the introduction of the Duplex process, in which an acid Bessemer converter replaces the acid open-hearth furnace. the Bessemerizing effecting the same action practically in regard to silicon and carbon re moval from the charge which is subsequently transferred to a basic open-hearth furnace for removal of the phosphorus and sulphur and further purification.