ELECTRIC STEEL Electric furnaces (q.v.) an usually round in plan, with low side walls, flat domed roofs and invert bottoms, built inside a stout bowl of steel plates, which in turn is attached to some tilting device. Side walls are pierced by one or more doors and a pouring spout. The side wall and bottom linings may be either acid—silica brick and fused siliceous material—or basic magnesite. (The acid furnace is confined generally to small installations in foundries, and is merely a melting and super-heating device, since the unique carbide slags to be described later cannot be formed inside a siliceous lining.) Roofs are of silica brick, and are pierced for carbon or graphite electrodes, one for each phase of current, of proper size to carry the electricity without undue resistance. Fur nace capacities range from ton for small foundries, to 5o tons for super-refining liquid steel. Size is limited by the size of electrodes available ; of the latter, a carbon 4o in. in diameter is the maximum ; a 7-ton furnace would be about 12 ft. in outer diam eter, with 13 in. side walls, and hearth 51 ft. inside height. In operation, the electrodes are automatically adjusted so the lower ends are constantly a correct distance above the slag. Electric currents of large amperage arc cross this gap, flow through the slag to the metal and there neutralize equal currents of opposite phase led in through the other electrodes. Heat is generated principally by the resistance offered by the air gap and the slag layer to the flow of this current. The electrodes are consumed gradually, combining with oxygen of the air within the furnace, producing a carbon monoxide atmosphere, desirable for steel refining.
No virtue is given to electric steel by the electricity. It is merely a convenient source of heat, under close control. The advantages of electric furnaces for steel making are: (a) ability to generate a high temperature and thus cause certain desirable reactions to occur ; (b) virtual ab sence of products of fuel combustion and their unwanted
pounds of carbon, hydrogen, sulphur and oxygen; (c) the ability to remove a slag and make up new ones, thus controlling refining reactions; (d) the metal is hotter than the refractory, a prime advantage in cost over crucible melting; (e) less slagging of alloy ing elements, thus making for economy and control of chemical composition within narrow limits. Such advantages are illustrated in the process of super-refining molten metal from the open-hearth furnace; in such operations the basic electric furnace is a unit of a duplex or even a triplex process—the pig iron may be de siliconized in an acid converter, de-phosphorized in a basic open hearth, and de-oxidized and de-sulphurized in a basic electric furnace.
Steel.—Open-hearth steel is taken from the furnace when the carbon has dropped to the required figure, and a little ferro-silicon or aluminium added to the ladle to prevent further decarbonization during the transfer. In such condition the phosphorus and silicon are both very low—o•oio% or less—while the sulphur depends upon the purity of the metal at the beginning, none being lost in the process so far, in fact the metal picks up a little from the furnace gases. If a nickel steel is to be made, the correct amount of shot nickel is added when this liquid metal is poured into the electric furnace. After the current has played long enough to remelt any crust of metal which may have chilled, a highly basic white slag is made up and melted, consisting mostly of burned lime with only enough silica sand and fluorspar to give it the proper fluidity. In quantity this would amount to about 3% of the metal. An excess of ground carbon is then shovelled in, the furnace closed, left under current for from i to 5 hours, depending upon the quality of steel re quired, and the doors opened only for sampling or adjusting the slag. Under these conditions of a reducing atmosphere, a high lime slag, and intense heat, calcium carbide is formed by reaction between the lime and crushed carbon Slag samples quenched in water give off much acetylene (q.v.) easily distinguished by its characteristic odour. Such a carbide slag not only reduces oxygen in the steel but is capable of abstract ing the sulphur. The reaction for the latter may be something like this: 3 MnS+2Ca0+CaC2= 3 Mn+3CaS+2C0 By this means ordinary open-hearth charges which usually melt to 0•05% sulphur are reduced to one-half that amount in a four hour cycle, while selected charges melting to 0.015% sulphur can be driven to less than o•oio% in a heat taking 8 hours. It will be observed from this reaction that some manganese is necessary. This will be residual in the open-hearth metal, for it has been found that if say 2% manganese is in the original pig iron, the resulting steel, containing 0.30 to 0.50 manganese will be of much better quality, sounder and cleaner, than from a low-manganese charge.
When the soapy slag stops its slight foaming, it is a sign that desulphurizing and deoxidizing are about completed, and the various alloys necessary to make the required analysis are added at intervals in order of their oxidizing power, namely, ferro-silicon first, then ferro-manganese, ferro-chromium, and finally ferro vanadium. Carbon is picked up from the carbide slag and from
these alloys. After all these alloys are melted and diffused the current is shut off and the metal lies quiet for 3o min. or more, to allow entrapped particles of slag to arise and the superheated metal to cool and liberate dissolved carbon monoxide gas. Slag is then most carefully skimmed, and the steel poured into a ladle which may contain a little carbon, ferro-silicon and ferro-zirco nium. After 3o min. further cooling the metal is teemed into warm, scrupulously clean moulds with hot tops. A sectioned ingot will show no cavities as large as a pin head.
Quality of such deoxidized and desulphurized steels rests in a number of characteristics, some not capable of evaluation, such as better response to heat treat ment, less non-metallic inclusions, as shown under the microscope and after deep pickling in strong acid. A better gauge is the trans verse properties. If a well made heat of basic open hearth steel is forged into a big gun, tension test pieces cut out of this forging will show high ductility if the piece is cut in a directon parallel to the axis of the gun. On the other hand if these test pieces are cut tangentially to the bore, the ultimate strength is satisfactory but the ductility is not much more than half the longitudinal piece. Unfortunately this transverse weakness is in the very direction which must bear the heaviest stress when the gun is fired. An ac cepted explanation of this phenomenon is that the work of forging ranges the non-metallic impurities into layers parallel to the faces of compression, and interrupts the metallic continuity. Obviously, the cleaner the steel the less would be the transverse weakness. For that reason important forgings, such as hollow tubes to resist internal pressure, are commonly made of acid open-hearth steel. Super-refined basic electric steel is even better, as shown by the following typical tests from big gun forgings of nickel steel : Melting Cold Scrap.—By far the larger number of electric furnaces are of small capacity for melting cold scrap. While it is possible to refine impure pig iron in either the acid or basic electric furnace, it has been generally found more economical to start with clean, relatively heavy, high manganese scrap and melt without additions of iron ore. In a basic furnace considerable lime would be charged into the empty furnace ; melting is done under the heaviest power input practicable. The slag formed on the steel is always black from the rust and oxide formed during melt ing. This slag is removed as soon as the charge is melted, and some ground carbon electrode butts are shovelled into the furnace and flapped under the metal. As the carbon dissolves, it first tends to deoxidize the steel Fe0+C=Fe-1--00 and the carbon monoxide gas comes up in bubbles. Some surplus carbon helps bring the carbon to the required analysis. Then a white or carbide slag is made up, and the refining and alloying is done with current of lower voltage and as described above.
The 5939 substitute for the crucible melting hole is the high frequency induction furnace. The fur nace case is a small cubical box which can be tilted about one upper edge. It contains a cylindrical spiral coil of water cooled copper tubing which carries alternating current at about 2,500 cycles per second. The coil is electrically and thermally in sulated, and inside it is rammed a magnesite refractory to form a crucible, into which is charged selected steel and ferroalloys for simple melting and mixing without refining. The intense al ternating magnetic field set up inside the coil induces eddy or Foucault currents (q.v.) in this metal to heat and melt it. Elec trodynamic movement of the bath keeps the metal well stirred, which is of advantage in making high alloys of heavy metals, such as high speed tool steel (q.v.), but tends to stir in particles of incidental oxide slag. As a simple melting furnace of rapid action the induction furnace has no equal.
Statistics show that electric steel has practically supplanted crucible steel in America, but has not been so successful in Great Britain. Italy being rich in water power, but very poor in fuel, has at least 200 electric furnaces with annual capacity of one million tons, but nearly all of them produce tonnage steels. Much controversy exists as to the relative quality produced by the two processes. It is probably true that there is little difference when selected raw material is charged, an expert furnaceman is em ployed, and equally careful pouring, forging and annealing practice is adhered to—in other words, when equal care is given to the respective heats. But steel from either process can be disappoint ing when conditions get out of control. The crucible is more convenient for smaller casts of plain high-carbon steels ; the electric is more convenient for heavy ingots of lower carbon, higher alloy steels.