It is a significant fact that the above elements - to which may be added nickel and tungsten - make excellent alloys with iron. They all are elements of high melting point but whether they are oxidized easily at high temperatures, or not, makes little difference, if they are dissolved in iron. Their behavior is absolutely opposite to many low-melting elements which also alloy easily with iron and ruin it for all practical purposes - such elements are sulphur, phosphorus, arsenic, and aluminum.
Cobalt is reduced easily from its oxide to metal, but most of the others require more energetic means. Silicon and manganese commonly are reduced with varying amounts of iron in the usual iron blast furnace, and their iron alloys are on the market in considerable variety and are used very much in steel making and in foundry work.
All of these elements can be reduced with iron in electric-arc furnaces; some, indeed, can be reduced quite pure, except for carbon, which it is difficult to prevent combining under such circumstances.
But a method is available which is used considerably now since aluminum has become reasonably cheap; this is to reduce the purified oxide of the metal in question with metallic aluminum. This is called aluminothermics. A crucible like that seen in Fig. 49 commonly is used. The oxide, or the mixture of oxides, if an alloy is to be made, is mixed with fine granular aluminum and is placed in the lined steel cavity of the crucible. If the mixture is brought to reacting temperature at any one point, the activity spreads quickly throughout the mass, and in a few seconds the newly formed metal can be tapped out into suitable molds. A bit of ignition powder is used to start the reaction. The temperatures produced are extremely high, for the heat has no chance to dissipate in the few seconds of the reaction, and alumina has by far the highest heat of formation of any of the common oxides whose metals are used. The alumina formed by the reaction floats (it fuses at 2000° C.) on the metal reduced and'can be recovered as a by-product.
Fig. 48. Typical Thermit Crucible.
Courtesy 0f the Goldschmidi Thermit Company, New York City.
New alloys continually are being developed by this process. Besides quite pure cobalt, chromium, and manganese, as well as their iron alloys, there are on the market ferrotitanium, ferrovanadium, and ferromolybdenum, and numerous other alloys produced by aluminothermics.