1078 THE COLLIERY GUARDIAN. November 22, 1918. RECENT DEVELOPMENTS OF THE ELECTRIC STEEL FURNACE.* By F. J. Moffett, B.A., M.I.E E. (Continued from page 1032.J Electro Metals Furnace. (Fig*. 1.) There are two carbon electrodes passing through the roof of the furnace and capable of being adjusted at a suitable distance above the bath, the arrangement in this respect being similar to that used in the fieroult. The hearth itself is of special construction. The upper part consists of dolomite or similar refractory material, and underneath this is a course of magnesite bricks. These bricks rest on a layer of carbon mixture, in which the bottom electrical connections are embedded. The whole furnace is enclosed in a steel tank mounted on trunnions and tilted by means of gearing, usually operated through an electric motor. Two-phase current is supplied to the furnace, the two electrodes in the roof being connected to the two outers and the fixed common electrode under the hearth to the neutral. and the Fig. 1.—Elevation of Electro-Metals Furnace. Mt+f i iiiiim ■ ■1- SS IWliWSiWSt kssmsssa When the supply of current is on the two-phase system, this only needs to be transformed to the suitable pressure of about 80 volts. When the supply, as is more usual, is on the three-phase system this must be converted into two-phase current by the Scott system of connecting transformers. Recently with a view to using four carbon electrodes in large furnaces, Electro Metals Limited have devised a system of connection which provided four-phase secondary current. Greaves-Etchell Furnace. (Fig*; 2.) This furnace is, in many respects, similar in principle to the one just described, but is constructed to use three-phase current. It was designed by two Sheffield engineers, who have had a lengthy experience of various types of furnaces. There are two carbon electrodes passing through the roof, these being connected to two of the three phases of the secondary circuit of the transformers. The third phase of the secondary circuit is connected to the electrode under the hearth which is constructed of similar materials to those used by Electro Metals. The steel tank containing the furnace is fitted with two curved rails on the bottom. These rest on a system of rollers, to provide for tilting when necessary to pour the charge. These two furnaces (the Electro Metals and the Greaves-Etchell) embody two features not possessed by other arc furnaces—these features being due to the fact that the hearth is made of conductive material. In the first place, this consists of supplying beat to the bottom of the metal bath, and tends to maintain a more evenly distributed temperature throughout. Con- sequently there is less liability to overheating the roof, since a lesser proportion of the heat is reflected from it to the bath. In the second place, the fact that the resistance of the bath is always in circuit tends to reduce the violent fluctuations which occur when the arcs are * From paper read before the Staffordshire Iron and Steel Institute on November 9. short-circuited by touching the bath or by metal falling against them. Although the resistance of the hearth is small compared with that of the arc, nevertheless it has a considerable effect in limiting the rise of current and preventing excessive rushes, which are very objection- able from the point of view of the electric supply authority. Method of Operation of Steel Furnace. After the furnace has been fettled from the previous charge, the cold scrap is thrown into the hearth. When charging is completed, the current is switched on and kept as constant as possible, either by hand or by an automatic regulator. Melting soon begins, and during this period the flux is added. This consists of a mixture of lime, fluorspar, sand, and either iron ore or hammer scale. This flux floats on the top of the molten metal, and the chemical actions of refining take place. The carbon in the scrap is oxidised by means of the oxygen in the iron oxide of the flux, and is given off as carbon monoxide. This, rising through the bath, gives the appearance of boiling, and has the effect of thoroughly mixing the flux and the metal. The silicon manganese are oxidised, and these oxides dissolve in the flux. The phosphorus also is oxidised, and forms, with the lime, a phosphate which also enters into the slag. A portion of the sulphur oxidises and passes away in the gaseous state. When sufficient time has been allowed for these reactions to take place, the current is switched off and the slag run off by tilting the furnace—the surface of the metal being carefully cleared. An addition of carbon is then made to pro- vide the necessary carbon in the steel, and a second flux is thrown in—this also consisting of lime, sand and fluor- spar. During the subsequent refining only about half the current is required. The phosphorus has already been removed, but the bulk of the sulphur and a certain amount of iron oxide still remain. This iron oxide is soluble also in the second flux, and is removed by the addition of finely-powdered carbon, such as anthracite dust, which reduces the iron oxide, and at the same time forms a reducing atmosphere in the furnace. The sulphur is also gradually dissolved in the flux, and at the end of the second purification the metal is ready for pouring. Purposes for which Electric Steel Furnaces are Used. The most important field for the electric steel furnace is the production of a uniformly good quality of steel from cheap scrap material such as turnings and crop ends, which can be fed into the furnace without any,pre- paration. During the war a large amount of scrap steel has been produced, owing to the large output of guns, shell and other munitions. In the case of alloy steels, it is possible to make a steel of uniform composition, J since the amount of alloy to be added can be calculated exactly from the analysis of the steel in the furnace. Steels containing valuable alloys such as nickel, chromium, vanadium, etc., can be melted without any loss of these elements. Steel castings can be pro- duced which are dead mild and do not require any subsequent annealing. The properties of ordinary electric cast steel are— Tensile strength.......... 35 tons per sq. in. Yield point .............. 25 tons per sq. in. Elongation ............... 30 per cent. By the addition of elements such as nickel, a much higher tensile strength can be obtained. The electric steel casting is therefore capable of taking the place of a steel forging for many purposes. Owing to the high temperature at which the charge can be poured, the steel is very fluid and allows of castings of intricate shape being made xyithout difficulty. Electric steel castings will doubtless, in the near future, take.the place of malleable iron castings, which have to be treated by a tedious and expensive annealing process. Furnaces in Use in the United Kingdom. At the end of the year 1917 the furnaces installed were as follow:— Heroult .......... 46 varying from 1|—10 tons. Greaves-Etchell .... 31 ,, „ V—12| ,, Electro Metals..... 27 „ ,, 1|—10 ,, Rennerfelt........... 8 ,, ,, 2i ,, Stobi-).............. 6 „ „ 5—15 ,, Other makes........ 22 > It is of interest to point out that of the 140 furnaces mentioned above, nearly one-half are in the Sheffield and Rotherham district. The list indicates that the arc type of furnace has established its superiority from a commercial point of view; the induction furnace being now very rarely adopted in this country for steel refining. Transformer blze’ capacity. 25 cwt........... 300 kw. 2| tons ......... 500 ,, 5 ............. 800 Power, Required and Energy Consumption. The following is a list of standard sizes of furnaces, with the transformer capacity and the approximate units per ton of finished steel:— Units per ton. 850 800 750 It is evident that the units per ton are less in the larger furnaces; this decrease does not go on indefinitely ; when the capacity of 15 tons is reached, the units per ton appear to reach their minimum value. Sir Robert Hadfield, in a recent account of the fuel and electric power consumption in his works at Sheffield, stated that in 1917 the units per ton used for 31,850 tons of electric steel was 782. Supply of Energy for Electric Furnaces. The cost of electrical energy is the most important consideration in the operation of an electric furnace. When it is possible to use blastfurnace or coke oven gases for the generation of electricity, the cost can be brought very low. If a by-product recovery plant is employed, gas can be produced very cheaply, and used for driving electric generators by means of gas engines. With the urgent need for economy in the use of fuel, and the demand for by-products derived from coal, it is probable that in the period following the war very much more attention will be devoted to this method of power production. Electric energy has proved to be so essential for the output of munitions of war of every description that there is no doubt that the use of electricity will be enormously increased. The tendency will be for the cost per unit to diminish as the output increases. One of the committees of the Ministry of Recon- struction has already reported on the great advantage, from the point of view of efficiency in production, which can be gained by electric power, and has outlined a complete scheme of supply for the whole kingdom. When such a scheme has been put into operation, as it is to be hoped will be the case in the near future, the cost of electric power will be greatly reduced. Fig. 2.—Elevation and Plan of Four Electrode Greaves-Etchell Furnace. The supply of electricity to an electric furnace or furnaces can always be given at a lower rate than for most other purposes, owing to the good load factor. An electric furnace can be worked continuously through the 24 hours at a fairly uniform load. This is the condition which allows a generating station to generate at a minimum cost. We are therefore justified in looking forward to the possibility of obtaining in the near future electric energy for furnace work at a cost of some- thing like 0’2d. per unit. Lighting Economy Still Necessary.—The Coal Controller states that the modification of the regulations restricting the display of light in the streets appears to have led private consumers to think that they, too, may use more light. This, however, is not so. All restrictions under the Household Fuel and Lighting Order are still in force, and cannot be relaxed until adequate reserve stocks of coal for all purposes are in existence and the mines are producing sufficient for current requirements. This will not be the case this winter, and unless the public exercise self-control and maintain careful habits, there can only be trouble and hardship in the early months of next year. In particular, flares and illuminations, outside lighting and shop window lighting cannot be permitted. Imports and Exports of Mining Machinery.—The value of imports and exports of mining machinery during August is given below :— ^1917. 1918. 1917. 1918. <£ <£ «£ £ Imports ................. 8,989... 1,426 145,817 ...116,730 Exports ................ 53,531 ... 42,654 .. 582,623 ...416,222 These figures are not inclusive of prime movers or elec- trical machinery. The following shows the yalue of ex- ports of prime movers other than electrical :— October. Jan.-Oct. 1917. 1918. 1917. 1918. All prime movers £ £ £ £ (except electrical)... — ... — ... — ... — Rail locomotives ........... — ... —: ... — ... — Pumping ............... 220,371 292,146.3,238,777.. 3,130,260 Winding ................ 60,731 ... 49,851... 1,406,159.. 1,004,591