In the open-hearth process, a charge of material, consisting of wrought iron, cast iron, steel scrap, and other ores, is melted on the hearth of a reverberatory furnace and heated by gas, as in the Siemens-Martin or regenerative system. Sometimes the molten metal from the blast furnace is poured directly into the open-hearth steel furnace, as shown in Fig. 195. The carbon is thus partially burned out, much as in the wrought iron process, and its proportion is brought down to the desired point or somewhat below. A charge of spiegeleisen is then added, in order that the manganese may act on any oxide of iron slag which remains in the bath, and which would make the steel red-short (brittle when hot) if allowed to form a part of the charge. The manganese separates the iron from the oxide, returns it to the bath, and the carbon joins with that already present.
Fig. 195. - Pouring Molten Iron into Open-Hearth Steel Furnace.
In the open-hearth, as in the Bessemer process, there is no removal of sulphur or phosphorus, and only materials nearly free from these ingredients can be used. The product of this process is more reliable than that of the Bessemer process, because it is more homogeneous and is therefore less likely to show unexpected or inexplicable weaknesses. For these reasons it is used very much more than the Bessemer steel.
Fig. 196. - Drawing Steel from Open-Hearth Furnaces.