SOME Ferro-alloys are highly important and include those metallic substances used for finishing carbon steel and manufacturing alloy steel. Most of them are electric furnace alloys of iron and carbon plus the third distinguishing metal, manganese, silicon, chromium or some other, although for some purposes the carbon content must be as low as possible. For such specifications the alloys are made using either metallic aluminium or silicon for the reduc ing agent.
It was early discovered that manganese must be added to make sound steel by Bessemer's process. As already noted, the usual ferroalloy for this purpose is spiegel-eisen, a blast furnace product made from iron ore sufficiently high in manganese to smelt into a pig iron containing 10 to 3o% of that element, with carbon about 5%. Spiegel cannot be used for low carbon steels because the required amount of manganese carries with it too much carbon. For such purposes, ferro-manganese is used, con taining nominally 8o% manganese and 61% carbon. This is also a product of the blast furnace when smelting an ore containing 40 to 50% manganese, low in iron. Since manganese and iron are so alike chemically, the actions in the furnace are very similar to those previously described. The output in ferromanganese from so% ore is only about one third as much as though common pig iron were produced; the blast is 250° F hotter and the coke consumption twice as much ; about of the manganese is lost in the slag. While ferro-manganese can be made in an electric furnace of the pit type (see ferro-silicon) it cannot compete with the blast furnace product except under special economic condi tions, as in Norway, or where carbon below 0.25% is required, as for high manganese alloy steels.
Ferro-silicon is the next most important alloy. A 12% silicon pig iron may be made in the blast furnace, but alloys higher in silicon are now used to deoxidize nearly all steel for castings, to quiet most tonnage steel from basic open-hearth furnaces, and to make several silicon alloy steels. Ferro-silicons are exclusively an electric furnace product and are made in several grades—so, 75, 8o, and 95%. Carbon is about 0.5%, phosphorus and sulphur less than 040%—the former particularly must be kept to the minimum to prevent the alloy from absorbing moisture from the air and generating poisonous phosgene gas A pit-type electric furnace is employed, rectangular in cross section, with bottom built of carbon blocks. Into this shaft are suspended three carbon electrodes, one for each phase of current ; a granular mix ture of pure quartzite, clean iron turnings and coke is shovelled into the open top, filling the entire furnace. Passage of current from electrodes to furnace bottom gives the necessary heat for the coke to reduce silicon metal from the rock. Carbon monoxide produced by the reaction rises and burns at the top of the charge column, and a molten alloy of iron and silicon collects on the hearth, from which it is tapped periodically.
Ferro-chromiutn.—This is one of the earliest ferroalloys pro duced in an electric furnace. At present the commercial alloys contain about 67% chromium. The so-called high carbon alloys range from 4 to 8% carbon by units, whereas the low carbon al loys necessary to make stainless irons range 'from 0•20 to 2.0% carbon by steps. Chromium metal up to 98% pure is also com mercial. Ferro-chromiums are ordinarily made in closed furnaces of the Heroult type, with bottoms laid in carbon blocks. A charge of chromium ore, iron or iron ore, anthracite coal for reducing, and flux for slagging impurities, is mixed and shovelled in, the doors closed and electric current turned on. Carbon reduces the metallic oxides in the intense localized heat ; at the finish the molten charge is poured into slabs, broken, analyzed and graded for shipment. To reduce high carbon, these varieties are remelted with concentrated chromium oxide, under a lime-fluorspar slag. Nickel is added to molten steel as shot metal, or a nickel-steel scrap may be used to make alloy steel—nickel having no tendency to enter the furnace slag.
Ferro-vanadium is one of the most difficult alloys to make, on account of the activity of the element. The pure metal is not com mercial. B. D. Saklatwalla developed various means of commer cial alloy production, using a mixture of vanadium oxide, iron oxide and powdered aluminium, either in crucibles by means of the thermit reaction, or in shaft furnaces with the assistance of the electric arc. At present the ore is smelted in a reverberatory
furnace, and the vanadium concentrated in the slag. This slag is ground, mixed with carbon, iron scale and fluxes (glass cullet, lime and fluorspar) and fed in a fine stream into a covered fur nace, directly through the arc between graphite electrodes, thus ef fecting the reduction almost instantly in the intense heat. Reduced alloy and slag collect in the hearth and are tapped three or four times a day. A common analysis contains 35% vanadium.
Ferro-tungsten.—Tungsten ore is widely distributed, and the concentrate comes to the refiner in such complex mixtures of min erals that the first step is frequently the production of the oxide by chemical means (see TUNGSTEN). This oxide is easily re duced to metallic powder without melting by mixing with fine car bon, sealing in a crucible and heating to 1,3oo° C for several hours. Ferro-tungsten, which has become more popular for making high speed steel, may contain from 6o to 85% tungsten, and is made in single-phase electric shaft furnaces. The bottom is one ter minal; the electrode set above is surrounded by a shaft of loose brickwork, and a mixture of oxide ore, iron and carbon is filled in. As the reduced alloy builds up on the hearth it chills into a thicker and thicker button, because it has an unusually high melting point ; when this solid metallic mass has grown to a point where its electrical resistance interferes with operations, the temporary fur nace is torn down and the sow crushed, analyzed and classified.
General Structure may best be exhibited on a roughly polished surface by etching in freshly prepared ammonium persulphate solution (so% (NH4)2S208). It shows immediately differences in grain size caused by differences in heat treatment or other cause.
Deep etching is much favoured in America for acceptance tests on high grade steel. It shows any non-uniformity in chemical composition or general bad quality. The smooth sample is held in hot hydrochloric acid (diluted 5o%) for about 3o min. Impure spots, blowholes or cracks are eaten more deeply than elsewhere, and the result is an irregularly pitted surface (Plate III., fig. 8). During solidification of a steel ingot, excess of sulphur compounds will collect toward the centre and top part, where they are trapped and remain during further rolling and manufacture. The distri bution of sulphur compounds in a smooth section may be shown by pressing it firmly for about a minute against a sheet of photo graphic paper moistened with 2% sulphuric acid, then washing and fixing. The dark areas indicate the location of higher sulphur in the steel (Plate III., fig. 7). Phosphorus also accumulates around the borders of the large metallic crystals which form in the ingot, and during rolling or forging are pressed out into stringers or sheets in ruling directions. Ordinary plate or forging steel will al ways be found thus marked. In forging, these flow lines are ar ranged more or less parallel to the finished surface, and the dies are so constructed that the flow lines take an easy curve around corners. Any crinkling is associated with a weak brittle region. To show phosphorous segregations a polished surface is washed in alcohol, and immersed in so% copper-ammonium chlo ride. A thin copper plate will coat out on those portions low in phosphorus, leaving high-phosphorous regions bright (Plate III., fig. so).