The silica brick roof of the furnace lasts for 60 to 70 heats. The walls are attacked most at the slag line. This is fettled between charges. After 120 heats it is necessary to reline the walls. The hearth bottom is also repaired then, as by that time it has sunk about 2 inches in the center. The hearth does not have to be pulled out when it is repaired.

The heat loss due to cooling of the bottom electrodes is 1.01 per cent of the energy consumption, and in producing 3.5 tons of steel the cooling of these electrodes consumes about 2.9 kilowatt-hours per ton of steel poured. The heat lost in cooling the silica roof around the carbon electrode is 3.65 per cent of the total energy supplied. With a production of 3.5 tons, 10.5 kilowatt-hours per ton would be lost in cooling the upper electrode.

PRODUCTS.

The analyses, tests, and uses for some of the products of this Girod furnace follow. Similar products can be made in any commercial electric steel furnace.

Analyses, tensile strength, and uses of various electric steels.

WORKS AT VOLKLINGEN, GERMANY.

Two Röchling-Rodenhauser furnaces at Volklingen, Germany, are in operation for the refining of molten basic Bessemer steel, with the production of steel varying in grade from rail steel to high-priced tool steel. At this plant the Röchling-Rodenhauser furnace was developed from the original Kjellin design.

One of the furnaces is of 8 to 10 tons capacity (fig. 27), taking 600 kw., supplied by a 25-cycle single-phase current at a voltage of 4,000 to 5,000 volts. The 2 to 3 ton three-phase furnace requires 200 to 250 kw. at a pressure of 400 volts on a 50-cycle circuit. The details of these furnaces have been previously described.

METHOD OF RELINING FURNACES.

In relining the furnaces the furnace castings are first protected by layers of fire brick, which serve as the principal protection against radiation losses and rarely require replacement. On the fire brick a layer of a mixture of calcined dolomite and tar is rammed to the level of the hearth bottom. Cast iron templates are then set and dolomite mixture is rammed behind them on the sides. After the templates are removed, the walls are finished with dolomite brick.

The lining is dried by placing cast-iron rings in the channels and heating the rings by the passage of current. When the tar in the mixture is burned out the roof is put on and molten pig iron is charged to complete the sintering of the hearth.

The time required for relining a 3-ton furnace is as follows: Removing old lining and ramming in new, 24 hours; burning out tar with hot rings, 6 hours; completing the sintering of the lining, 6 hours-total, 36 hours. During burning and sintering about 230 kilowatt-hours are consumed. Such a lining stands on the average 55 charges, or 360 to 500 tons of hot metal, according to the steel produced.

REFINING PRACTICE.

The molten metal charged into the furnaces at Volklingen from the basic Bessemer converters contains 0.1 per cent carbon, 0.5 per cent manganese, 0.08 per cent sulphur, and 0.08 per cent phosphorus. At times molten basic open-hearth steel that has been refined, containing about 1.22 per cent carbon, 0.38 per cent manganese, and 0.209 per cent silicon, is put in the electric furnace to allow removal of gases, to adjust the carbon content and to add alloys.

Dephosphorizing and desulphurizing are carried on as in the arc furnace, but not at so high a temperature, although the temperature is sufficient for the purpose, and is more evenly distributed through the charge. Also most of the desulphurizing is caused by the action of ferrosilicon and lime on ferrous sulphide, as the temperature is not

high enough to permit the formation of calcium carbide. Hence more ferrosilicon is charged in the induction furnace than in the arc furnace.

The furnace is started cold, rings of steel are placed in the channels, and molten metal is poured on them. The voltage of the secondary current, through the pole plates, is higher than usual at the start in order to warm the hearth more quickly and make it conducting. Formerly it was customary to pass current through the furnace on Sunday to keep it hot, but now the doors are closed and the current is shut off entirely. After the furnace has stood 30 hours part of the current is turned on and the amount is gradually increased until in 4 to 6 hours the furnace is at working temperature. After each run the furnace is practically emptied. In using cold scrap about two-thirds of the charge is molten steel and the rest is cold scrap.

With hot metal the length of a heat varies from 1 hour 15 minutes to 3 hours. With a cold charge the time required is 3 to 6 hours. To produce a rail steel containing 0.5 per cent carbon, 0.8 per cent manganese, 0.04 per cent sulphur, and 0.05 per cent phosphorus, about 100 to 125 kilowatt-hours per ton are necessary with a hot charge of Bessemer metal. For high-grade steel containing 0.05 per cent carbon, 0.25 per cent manganese, and trace of sulphur and phosphorus, the power consumption is about 300 kilowatt-hours per ton. The power factor averages about 0.60 with both the single and the 3-phase furnaces. With cold scrap the power consumption is claimed to be 580 kilowatt-hours per ton. The power cost at Volklingen is about 1.5 cents per kilowatt-hour.

PRODUCTS.

Steel of many grades has been made at Volklingen in the induction furnace, including high-grade tool, alloy, and rail steels. Some of the mild-steel products and the power consumption are as follows: Mild-steel products of Röchling-Rodenhauser furnace.

PLANT OF LA GALLAIS METZ & CO., DOMMELDINGEN, LUXEMBURG.

At Dommeldingen, Luxemburg, is the largest installation of Röchling-Rodenhauser furnaces in operation. There are three singlephase 33-ton furnaces, similar to the one shown in figure 27, and one three-phase 2.5-ton furnace. The single-phase furnaces take 350 to 400 kw. at 3,500 volts, 50 cycles. The three-phase furnace requires 275 kw. at 500 volts, 50 cycles. The power factor of the single-phase furnace averages about 0.6, and that of the three-phase furnace is sometimes as high as 0.7.

Pig iron is made into steel in a gas-heated 20-ton tilting furnace. When 3.5 tons of the steel is transferred to an electric furnace for further refining it is replaced by 3.5 tons of pig iron. The gas furnace acts as an oxidizing mixer. The steel in the gas furnace has 0.26 per cent carbon, 0.082 per cent phosphorus, and 0.03 per cent sulphur. For ordinary castings the steel is in the electric furnace about 2 hours, for the best machinery castings 3 to 33 hours.

The three-phase furnace was not in use at time of inspection, as faulty design had resulted in poor operation.

Some of the results obtained in practice at Dommeldingen were as follows:

Results obtained from furnace practice at Dommeldingen.

From what has been said it is clear that electric steel furnaces vary widely in design, but they may be divided into two general classes, arc furnaces and induction furnaces. The general features of the operation of the furnaces in a class are similar. Steel of about the same grade can be made in any of the commercial electric furnaces, but from their design some furnaces are better adapted to one purpose than to another. In general the power consumption and efficiencies of the various types are about the same. Hence the choice of the type of furnace for a specific purpose should be determined by the factors of the individual case.

Several general points should be considered in choosing the type of furnace. In comparing the arc and the induction furnace it should be remembered that the arc furnace is better adapted to melting cold scrap than is the induction furnace, because the latter must be charged with molten metal at the start to insure quick and regular heats. The power factor of the induction furnace is lower than that of the arc furnace and is much lower than that of most public power service lines. Hence, an induction furnace that might be a satisfactory load on a private plant carrying no other load, on a public line might lower the power factor of the whole system to an objectionable degree. If an induction furnace is built where power must be obtained from a public service line, the company operating the furnace may be required to put in a motor-generator set at its own expense. In general, the induction furnace makes a more steady demand on the central power plant than the arc furnace, because there is no constant breaking of the arc which may cause fluctuations.

In choosing an arc furnace for refining cold scrap it seems that the load on the power line is more satisfactory, if the furnace is of the conducting-hearth type with several upper electrodes in parallel rather than of the type with two arcs in series with the bath. This is because the arc breaks less frequently in the former than in the latter. For continuous steady operation on either cold scrap or molten steel, one furnace is probably about as good as the other as regards mechanical features, power consumption, and heat losses, although some consider the water-cooled electrodes of the conducting hearth and broken hearth to be objectionable. An arc furnace in which heating is by radiation of the arc has a higher power consumption than the other types. Electrode consumption and repair cost are also higher.

POWER CONSUMPTION.

The power consumption of furnaces depends so much upon the materials being treated, the process of operation, and the product desired that no satisfactory comparisons can be made. Also, the point in the circuit at which the reading is taken for figuring the basis of power consumption may be so variously selected that one furnace may show a much lower power consumption than the other when it should show about the same. With cold scrap the power consumption of the various arc furnaces varies from 459 to 1,200 kilowatt-hours per ton. Aside from the points mentioned, this wide variety of results may be due to the amount of refining done. Furnaces in which only one refining slag is used take about 600 kilowatt-hours per ton on an average, while if two slags are used 800 to 1,000 kilowatt-hours per ton are necessary. Power consumption per ton of metal is also influenced by the size of the furnace. For example, a 10 to 12 ton Girod