After the steel is tapped into the ladle and from that into ingot molds (Fig. 8) they are stripped therefrom when the metal has solidi fied and the ingots are reheated and rolled out into various shapes.
Basic Practice.— To successfully conduct the basic open-hearth process it is necessary to have a hearth lined with such material that will withstand the action of slags highly charged with lime. The presence of lime in moderate or excessive quantities in an acid furnace would be fatal to the hearth because of the strong fluxing action between silica and lime when brought together at high temperature. A basic lining in general practice consists of calcined magnesite, a substance carrying a high percent age of magnesia. In furnace construction it is ground and mixed with about 10 per cent of anhydrous tar, then rammed in and shaped to the sloping form. At the beginning of the heating of the furnace the tar is carbonized into a solid cinder. It can also be mixed with pulverized basic slag which assists the sintering and hardening effect. A hearth of magnesite so formed is practically inert under the action of a limey slag, but will fuse or cut if siliceous matter should be present excessively. The life of a magnesite hearth is practically indefinite. With a hearth of basic and refractory material, such as magnesite, there is offered an oppor tunity to treat a charge of such stock carrying a high content of elements not removed in an acid hearth; namely, pig-iron with 1.00 per cent of phosphorus and 0.07 per cent of sulphur is easily treated, so that 98 per cent of the phosphorus and 75 per cent of the initial sul phur can be removed by the action of the slag, consisting largely of lime and iron oxide. The only element that is objectionable is silicon and basic stock or raw material is selected with amounts of silicon not exceeding 1 per cent. To get a slag'of the necessary dephosphorizing and desulphurizing activity 10 to 15 per cent of the charge is made up of limestone of a pure variety with ore added from time to time. Sili con under the oxidizing action of flame and additions of iron ore to the bath of lipoid metal, will be converted to silica and combine with the lime of the slag and if excessive rob it of its power to absorb phosphorus and sul phur. The details of charging are the same as
01 acid melting excepting that a charge is best made up of 50 per cent of pig and 50 per cent of scrap, when scrap is used, or the charge may be all direct molten metal. It is desirable to use scrap moderately because it is easier to promote liquation with a higher carbon content initially.
The layer of supernatant slag is thick in a basic heat and more heat units are required to penetrate it than an acid slag. For that reason the fuel consumption per ton is greater in the basic process.
The flame manipulation is the same as in acid melting as is also the carbon oxidization by the action of iron ore. The details of re carburizing are followed as in acid practice. During the progress of the heat samples arc frequently taken which, in addition to being studied for the carbon content, are also tested to determine how far the dephosphorizing ac tion has gone. The heat is not tapped until its removal is complete or only a few thousandths of 1 per cent remain. There is some tendency on the part of the phosphorus to revert to the metal after it has been tapped into the ladle. The amount given tip by the slag at that stage varies directly with the quantity of phos phorus charged initially. But with proper slag conditions and care in stock selection the re absorption of phosphorus is not excessive. Be ginning with pig-iron carrying 1 per cent of phosphorus and an equal amount of scrap with not more than 0.1 per cent a heat of basic steel can be produced to equal in analysis acid finished material.
When the furnace is emptied after an op eration some unavoidable cutting of the hearth at the slag line is patched with basic material of a cheap kind, usually dolomite, a double car bonate of lime and magnesia. There is not, under normal conditions, any cutting of the hearth below the slag line since the function of the hearth is only a refractory one playing no part in the purification of stock, that function being effected only by the calcareous.