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the furnace in a gas-tight ring, and to melt and cool the metal in vacuo. In this way all occluded gases are removed, and the magnetic permeability of the metal is greatly increased. With this exception the successful electric steel melting and refining furnaces are now all of the arc, or combined arc and resistance heating type. Detailed figures for all countries are not available, but the following com- parative figures for the United Kingdom are striking. In the year 1912, only nine electric furnaces were operating, or in course of erection, in the iron and steel works or foundries of Great Britain; Messrs. Edgar Allen & Co. of the Imperial Steel Works, Sheffield, being the pioneers in the introduction of electric steel refining into Sheffield. Six years later, in October 1918, when the Armistice was declared, the number had grown to 123 (see Table III).

TABLE III.

Electric Steel Furnaces in Operation or in Course of Erection in the United Kingdom in November 1918

Type of furnace

No.

Average capacity tons

Average monthly output

Average current consump- tion

I. Heroult. 2. Electrometals 3. Greaves-Etchells. 4. Stobie 5. Snyder 6. Rennerfelt 7. Stassano. 8. " Special "

50 26

24 8 6 5

2 2

2^ 2 10 I

ii

2 I

9595 3097 1109

1010

918 419 136 204

992 1031

1 122

880

787

1495 1770

769

I-M

4

16,488

1105

The average monthly output of these 123 furnaces was 16,500 tons of steel, in the form of ingot-metal, special alloys and castings. Further particulars of the types of furnaces installed, with figures for the average monthly output and current consumption, are given in the table. It must be pointed out here, however, that the figures for the average current consumption of each type of furnace cannot be used as the index of their respective thermal or electrical effi- ciencies, since, in the absence of information concerning the physical state and the chemical composition of the charge, and the amount of refining work carried out in the furnace, average figures of current consumption cannot be considered a safe guide to relative furnace efficiencies.

The largest number of furnaces at the end of 1918 were installed at the East Hecla Works, Sheffield, of Messrs. Hadfields, Ltd. These works were equipped with 1 1 Heroult arc-furnaces chiefly of 6 to 7 tons, capacity. The largest furnaces erected in England were the 12- and 15-ton furnaces of the Stobie type at Dunston-on-Tyne, and at the Openshaw works of Armstrong Whitworth & Co.

As regards the lines along which progress occurred during 1911-20, the tendency in the United Kingdom was to increase the power of the transformers, rather than to follow the American plan of reduc- ing costs by the erection of larger furnaces. When large tonnages of electrically refined steel have been required, it has been the custom to employ molten steel, which has been previously treated by the Bessemer or open-hearth process. This method of working is known technically as the " duplex " or " triplex " system, and it leads to a great reduction in the consumption of electric current per ton of metal refined. Three-phase current is also now generally employed for ehectric steel refining in place of single-phase current, since the use of three electrodes with the molten steel as the common neutral point of the system leads to much more uniform heating of the bath. As regards regulation of the load on the generators and transformers, the Thury system of automatic regulation of the arc gap is generally employed in the United Kingdom, and by an elec- trically operated device the total load of a battery of furnaces can be kept within any desired limits.

In the United States, for electric steel melting and refining, the tendency is to employ larger furnaces than in Great Britain, with improved mechanical equipment for charging and discharging the furnaces. Automatic apparatus for regulating the arc gap and power factor has also been coming into general use, the Thury system of current regulation being widely adopted. As regards electrode- holders and cooling-boxes, the tendency in American practice has been to substitute cast-steel holders and boxes for the bronze ones which have been hitherto used, since there is less risk of contamina- tion of the charge with impurities, should there be a stoppage of the flow of cooling water and fusion of the metal. The mechanism for tilting the furnaces is also receiving attention: controllers of the reversing type now being employed with a motor brake to prevent " overtravel " of the furnace, and to hold it stationary in any posi- tion. The largest electric furnaces erected up to 1921 in America for steel melting and refining are the 25-ton Heroult three-phase arc- type furnaces at the South Chicago works of the Illinois Steel Com- pany. In 1918 this company was stated to be producing electric steel at the rate of 16,500 tons per month, using the " triplex " system referred to above. The other types of electric furnace used in the American steel industry are: Booth-Hall, Girod, Greaves- Etchells, Gronwall-Dixon, Ludlum, Snyder and Von Baur.

Ferro- Alloys. Applications of ferro-alloys in the iron and steel industry have increased enormously since 191 1, and in 1971 the Sheffield tool-steel trade was dependent for some of its most valuable products upon the ferro-alloys obtained by aid of electric heat. The discovery made many years earlier, that small per- centages of chromium, nickel, manganese, vanadium and other rare metals, either separately or in combination, caused profound changes in the physical properties of steel, has in fact revolution- ized modern steel manufacture; and the production of a rustless or " stainless " steel was one of the most notable advances of the war period. Molybdenum is the latest rare metal to be added to the list of those employed now in steel manufacture; and Prof. Arnold's molybdenum-vanadium steel was expected by some authorities in 1921 to have a great future.

The special steels are called binary, ternary or quaternary, according to the number of elements (other than impurities) which are present, and it is the quaternary steels, which contain carbon and two other elements, that are now finding the widest applications in the arts and industries. The ferro-alloys used in their manu- facture have in the past been supplied chiefly from the large elec- trometallurgical works located in the French and Swiss Alps. The war showed, however, that dependence upon an overseas power for supplies of these essential raw materials for the steel industry was not a wise or safe arrangement. The production of ferro-alloys, therefore, was commenced in England at Widnes, Newcastle and other places, and has come to be regarded as a " key industry."

The furnaces used for the production of ferro-alloys are of the resistance type, and their design is based on that employed for the furnaces used in the manufacture of calcium carbide, but they have been modified considerably both in form and in other details of construction. The use of the electric arc for heating is not con- ducive to efficient working except in the steel and alloy refining processes, since the temperature required for reducing the ores lies between that of the ordinary metallurgical furnace and that pro- duced by the electric arc. Furnaces working on the resistance prin- ciple have, therefore, been most successful in the manufacture of ferro-alloys, and arc furnaces are used only for refining. The ferro- alloys made in these furnaces in the early days of the manufacture were very impure, and the first ferro-chrome placed upon the market by a French firm contained from 7 to 9 % carbon. Improvements in the design and method of working the furnaces, however, have led to the production of alloys containing a much lower percentage of this element ; the percentage of carbon in some of the ferro-chrome now produced having been reduced to under two per cent. By treatment in a refining furnace this percentage can be still further reduced to under '50%, or to any lower limit demanded by the steel- maker, but this of course adds considerably to the cost of the ferro- alloy. The use of pure raw materials, the avoidance of excessive heating of the charge, and the prevention of contact between the molten ferro-alloys and the electrodes or the walls of the containing vessel are the means by which this improvement in the purity of the product has been attained.

Electric Iron Smelting. Electric heat for smelting iron ore is now employed at various centres in Sweden, Norway, Italy, Japan and Brazil, where the local conditions favour the use of electrically generated heat for this purpose. The furnace em- ployed is an improved type of the shaft-furnace originally tried in the year 1911 at Domnarfvet and Trollhattan in Sweden, by the Swedish Iron and Steel Makers' Institute.

Magnesium and Magnesium Alloys. Magnesium and its alloys have come to the front as metals of considerable industrial value and importance in recent years, owing to the demands of the aeroplane and motor industries for a light metal which will combine with this quality strength, toughness and ability to resist the effects of vibration and shock. The alloys of magnesium and aluminium which contain from 5 to 30% of Al have ap- proximately the same mechanical properties as brass, and can be employed for the manufacture of screws, nuts, wire, tubes and sheets. The hardness of these Al Mg alloys increases with the proportion of the latter metal present in the alloy, and with 70% Mg the hardness is equal to that of mild steel. An alloy contain- ing 92% Mg and 8% Al has been patented by a German firm, and is stated to have a strength equal to that of gun metal, with a specific gravity of only 1-75. If its claims are substantiated, the demand from aeroplane and motor-car manufacturers ought to result in manufacture on a large scale.

In the manufacture of the Mg Al alloys it is of great importance that the metals should be pure. The aluminium is first melted in a raphite crucible, and a small amount of cryolite is added as a flux.

e magnesium in the required amount is then introduced and in

gra Th