Recarburization is not done to the same extent as it is in the Bessemer process. As the open-hearth elimination of carbon is slower and under so much better control, the furnace usually is tapped when the carbon has been reduced to the percentage desired in the finished steel. When it is necessary to add carbon it is done sometimes by adding pig iron to the bath and sometimes by throwing a weighed amount of coal or coke in the ladle as the steel is going in. Molten iron and steel have strong appetites for carbon and dissolve it very readily. Ferro-manganese is used to prevent red-shortness and to deoxidize the metal. This also is usually put into the ladle as too much loss would occur were it added in the furnace.
While the furnace is again being charged through the charging doors at the rear, the steel is teemed through the nozzle of the big ladle into the waiting ingot molds. These go to the stripper, to the soaking pits, and then to the rolls of the blooming mills just as did the Bessemer ingots.
At the “Stripper”
In the acid-lined furnace no attempt is made to reduce the phosphorus. It would be futile. Therefore the materials charged must be very low in phosphorus and sulphur. No lime additions are made, the flame simply melts down the pig iron and scrap, the iron oxide later is added from time to time to keep up the boil until the test bars show that the carbon as well as the silicon and manganese have been eliminated as fully as is desired. The metal is then tapped as described above.
Three or more hours are usually required to melt down cold charges. The elimination of the remainder of the silicon, manganese, and the carbon requires about four or five hours more. So for each heat the open-hearth furnace requires from eight to twelve hours, depending largely upon the speed of charging and melting.
Of late years the difficulties attending the use of molten metal from the blast furnace in place of cold pig iron have been largely surmounted. The use of uniform metal from the “mixer,” which was described in the article on the Bessemer process, has aided the open-hearth process also. Of course, when molten metal is added none of its silicon and manganese is reduced by the flame as occurred with the cold metals during the melting down, so the molten metal charged is usually low in these elements to compensate. By use of “hot” (molten) metal the time necessary to produce a “heat” of steel is considerably shortened.
The first and perhaps the majority of furnaces yet building are “stationary.” Some have found it advantageous to construct furnaces that can be tipped to pour the metal into the ladle. Such are known as “tilting” furnaces. One furnace designer has even gone so far in a smaller type used for steel castings as to make the furnace removable, thus doing away with a ladle entirely. The big crane simply lifts the whole furnace out from between the housings which contain the ports. It is taken bodily to the molds which are poured directly.
Open-hearth furnaces have been built of larger and larger capacity. A great many fifty-ton furnaces have been built and furnaces which produce eighty or more tons at a heat are now not uncommon.
Furnaces of the Talbot type are built for as much as 200 and even 300 tons of metal, but from these only part of the finished steel is tapped at a time, the remainder being left to help work down the additions of new material which is added to replace the steel tapped out.