The acid process is so-called from the char acter of the hearth lining which is formed of nearly pure silica sand, a substance, chemically an °acid.° It is fairly refractory but must carry a small percentage of foreign elements to very slightly lower its fusing point. In forming the hearth, as has been mentioned, it must be hard to resist wear and at the same time be inert under strong actions within the bath of molten metal at high temperature. It has no affinity for sulphur and phosphorus, but will fuse with oxides of iron or manganese. For the latter reason care is taken with heat manipulation to prevent scorification of hearth by such oxides. Since specifications for finished material, such as boiler plates, structural ma terial, etc., state that the sulphur and phos phorus be kept below certain limits, the choice of raw material in regard to those elements is also subject to limits, and because of non-re moval, in refining, whatever sulphur and phos phorus may be present in the initial charge will appear in the finished product in like propor tions.
Raw material for acid treatment is called °Acid Stock,I) and the following will represent average analysis: Scrap Steel.— Sulphur, 0.04% or less; phosphorus, 0.04% or less.
Pig Iron.— Carbon. 2.-3.5%; silicon. 0.50-1.5%; sulphur. 0.04% or less; phosphorus, 0.04% or less; manganese. 0.50-.73%.
The figures on carbon, silicon and man ganese for scrap are not specified, because the scrap is always the result of some previous con verting process and is, therefore, low in those elements. The shapes may be ingots, blooms, steel castings, tires, springs and rolling-mill waste. If scrap composes part of the charge it will be put in the furnace first followed by either molten pig-iron brought in a ladle direct from the blast furnace or iron in the forms of pigs. Before the iron addition some little time is allowed for the scrap to heat or partly melt. The chemical action is mainly a de carbonizing one or rather an oxidizing one. The first elements to oxidize are silicon and manganese of the charge forming respectively silica (SiO:) and manganese oxide (MnO) which fuse readily and unite to form part of the slag. The slag is mainly composed of SiO, and FeO. The FeO (ferrous oxide) being formed mainly in the melting of the scrap and the silica from sand carried in mechanically with the pig-iron and the oxidization of the silicon in the metals charged. The following analysis represents a normal acid open-hearth slag: SiO 52.00 per cent.
MnOt 17.50 per cent.
FeO. 28.50 per cent.
Under flame action the temperature of the bath is gradually increased mainly because the heat stored in the regenerator chambers and radiated to the air passing through them to support flame or fuel combustion increases the efficiency. The checkers nearly fill both sets of chambers and are formed of first quality fire bricks so placed that numerous passages three The reduced metallic iron entering the bath, adding to the yield of metal while the fraseous carbon bubbling through the bath keeps it mov ing and so promotes homogeneity, an advan tage in favor of any open-hearth process. When
the carbon removal has been completed or has proceeded to any desired point, as indicated by the fractured appearance of test pieces taken at intervals, the metal is tapped out of the furnace by breaking open with an iron bar the tapping hole previously closed, before charg ing with some suitable refractory material. While the metd1 is flowing into a preheated to three and one-half inches square are formed both vertically and horizontally, thus offering very large areas for the waste gases to pass through and give up their heat to the numerous bricks. The temperature at which the furnace is operated ranges from 1600° to 1700° C. (2912° to 3092° F.). The waste gases leaving the furnace body are about 1100° to 1650° C. and after passing through the chambers should normally measure about 500° to 600° C. at the stack. At regular periods the course of the flame and gases are reversed so that while one set of chambers are giving up heat to the inflowing air or gas (producer only) the other is absorbing the heat from waste products of combustion. Without a careful regulation of fuel there would be danger of melting the brick work in the furnace because with each reversal there is a constant increment of temperature carried into the furnace melting chamber.
The bath under increased temperature be comes very liquid both in regard to the metal and slag. There is a lively action caused by the burning of the carbon and silicon which draws upon the oxygen in the slag and oxides dis solved in the metal. To hasten the removal of carbon, iron ore is added from time to time which produces the following reaction: clay-lined ladle measured quantities of either spiegeleisen or ferromanganese are added which have a cleansing effect upon the steel and performs the function of recarburizing it. They are alloys of iron and manganese each carrying respectively 20 per cent and 80 per cent of manganese with about 5.5 to 6.0 per cent of carbon. It is possible to stop the re fining process at any stage, and bytising varying quantities of the substances just named known as deoxidizers or recarburizers, there is offered a possibility of producing numerous grades of steel. Without any additions of manganese at the end of an open-hearth operation the metal would be charged with oxides which would in terfere with its rolling properties. The action of manganese is to liberate such oxides by making them readily fusible, when they float upward and mingle with the slag above the metal. Metal not well deoxidized is likely ao crumble or crack when forged, rolled, ham mered or welded.