2. Structural steel to compete with Siemens or Bessemer steel can not be economically produced in electric furnaces, and such furnaces can be used commercially only for the production of exceptionally high-class steel for special purposes.

LATER DEVELOPMENT OF THE ELECTRIC STEEL FURNACE.

THE GIROD ARC FURNACE.

In connection with the development of the Girod ferro-alloys works at Ugine, France, Girod had devised an arc furnace with a noncarbon

FIGURE 22.—Elevation of 2.5-ton, single-phase Girod steel furnace showing arrangement of conductors.

conducting hearth that was

used for the manufacture of low-carbon alloys. As the ferro-alloy industry became well established, Girod turned his attention to a steel furnace, and in 1905 constructed a furnace based upon the principles of the earlier ferro-alloy furnace. The furnace (fig. 22) has a conducting hearth consisting of magnesite, in which iron poles are embedded to serve as conductors. The poles are water-cooled at the lower ends. There may be one or more upper carbon electrodes. Thus the chief feature of the Girod furnace may be traced back to the lower water-cooled iron electrode of Siemens.

Girod was the first to apply the noncarbon conducting hearth to the steel furnace.

OTHER ARC FURNACES.

After the introduction of the noncarbon conducting hearth in 1905 other furnaces similar to the Girod were developed. The Keller furnace (fig. 23) is similar to the Girod furnace, except that there are numerous iron rods embedded in the hearth of magnesite about 1 inches apart, instead of about six poles, as in the Girod furnace. The whole mass of magnesite and iron acts as a conductor when hot.

Grönwall constructed a furnace (figs. 24, 25) having a conducting hearth composed of a mixture of dolomite and tar, and two upper car

FIGURE 23.-Elevation of 8-ton, three-phase Keller steel furnace, Unieux, France.

bon electrodes which were each connected to a phase of a two-phase system, the hearth forming the neutral point of the system.

FIGURE 24.-Elevation of 2.5-ton, two-phase Grönwall steel furnace, Sheffield, England.

Nathusius has built a furnace (fig. 26) combining the principles of the arc and resistance furnaces, as the hearth contains three poles

of iron, each connected to a phase of a three-phase system, while there are also three upper carbon electrodes, each connected to a phase. This is the latest development of the arc-resistance furnace.

Other furnaces of the arc type which have been designed and built since 1904 are the Anderson, Chaplet, and Soderburg furnaces.

FIGURE 25.-Longitudinal elevation of 2.5-ton, two-phase Grönwall steel furnace, Sheffield, England.

The Röchling-Rodenhauser electric steel furnace (fig. 27) was developed from the original Kjellin design in 1906 to meet the requirements for refining molten basic Bessemer steel. It combines two methods of electric heating-the ordinary heating by electric induction currents and heating by induction currents that are introduced into the molten bath through metal plates. It is the most widely adopted induction furnace of the present time, and the design is practically the same as the original. The Röchling-Rodenhauser

furnace was the first induction furnace that could be heated with a current of ordinary frequency without having too low a power factor, as had the old design.

which has not passed beyond FIGURE 26.-Elevation of Nathusius three-phase the experimental stage.

steel furnace showing principle of operation.

GENERAL LINES OF DEVELOPMENT OF ELECTRIC STEEL

FURNACES.

With the natural increase in size of units constructed there has been a marked tendency to use three-phase electric current in the

FIGURE 27.-Plan and elevation of 2-ton, single-phase, Röchling-Rodenhauser steel furnace, Volklingen, Germany. For explanation of lettering see p. 86.

larger furnaces of all types, except the induction furnace. Many of the recent furnaces have been designed especially with this in view Greene, A. E., Electric steel processes as competitors of the Bessemer and open-hearth; Trans. Am. Electrochem. Soc., vol. 19, 1911, p. 233.

to save the expense of installing motor generator sets for transformation from three or two phase current to single-phase current. At present new generating units are invariably three-phase if the energy is to be transmitted a considerable distance. It is probable that a more even distribution of heat is obtained in a large furnace by the use of three-phase current. In induction furnaces it has been proved by experience that for units above 3 tons three-phase current does not give as good satisfaction as single-phase current, owing to the complicated nature of the electrical transforming part of the furnace. Electrode consumption has been reduced considerably as a result of the production abroad of amorphous carbon electrodes in sizes up to 24 inches (61 cm.) in diameter, that screw into each other so that the butts are not wasted. Graphite electrodes of this design have been made in this country, but up to a recent date no domestic amorphous carbon electrodes threaded for continuous feeding were made. The strength and density of the carbon electrode are considerably greater than they were several years ago. The carbon electrode is used almost entirely by foreign electric-steel manufacturers, because of its greater cheapness.

Generally speaking electric steel furnaces were first operated by men not familiar with the steel industry, so that linings and roofs were not always properly constructed. The rather high cost item that resulted has been reduced considerably with the adoption of the electric furnace by established steel firms.

Under the hand of the steel manufacturer, the electric furnace seems to be gradually taking the form of the open-hearth furnace. The hearth proper of the Héroult furnace is like an open hearth, but the furnace is charged from the sides and front and is generally set back a short distance from the edge of the working floor. In the open-hearth furnace the charging door is on one side and the pouring spout on the opposite side, the furnace being level with the edge of the working floor. The electrode supports of the Héroult furnace as at present arranged prevent the use of a back charging door. The Girod furnaces at Ugine, however, are similar to the open-hearth furnace in these respects, as the electrodes are supported from the sides. A three-phase Héroult furnace now being constructed in England is to have the three electrodes suspended from the sides as in the Girod furnace, with a back charging door opposite the spout and close to the edge of the working floor. The Kjellin furnace is simply an annular crucible. The Röchling-Rodenhauser furnace has a rather large hearth in the center, and a charging door opposite the pouring spout.

The wider use of the electric process, together with the improvements mentioned, has resulted in a considerable increase in heating efficiency over the results obtained in 1904.