ALUMINO-THERMICS, a term designed to embrace the processes of reduction of metals by taking advantage of the intense affinity for earthy lustre and fracture, and is soft and light. temperatures.
The principal industrial applications of these processes are: (1) The welding of iron and steel in place as in the uniting of railroad rails, and in repairing broken shafting and the like; (2) the production of pure metals and alloys free from carbon, nitrogen and other common impurities; (3) in foundry practice for re viving or increasing the temperature of molten iron or steel in the ladle before pouring; (4) in the manufacture of small castings and repairing flaws in large ones; (5) in surface softening of hardened steel to permit tooling or boring and the cutting of large openings.
In the welding process the aluminum is re duced to a powder or granulated and mixed with a metallic oxide, in most cases the oxide of iron. This mixture has been given the com mercial name 'xtherinit.)) The chemical reaction, which consists in oxidizing the finely divided aluminum, is started by producing the high temperature necessary at only one point in the thermit. This is ac complished by a priming powder composed preferably of metallic magnesium and barium peroxide. The primer is ignited by a fiercely burning storm match or by touching it with a red-hot iron rod. The reaction then spreads spontaneously and with great rapidity and with considerable violence. In a few seconds the entire mass is a white-hot fluid at a temperature estimated to be about 5400° F.
In the crucible after the reaction there are two layers. The bottom one is pure metal of equal weight to, but occupying only one-third of the space of, the top layer, which is now oxide of aluminum, so-called corundum.
The most commonly employed of the weld ing processes is the one by which a continuous rail — a desideratum in modern trolley-road construction—is simply, cheaply and effectively obtained. The marked advantage enjoyed by this system is the freedom from bulky equip ment; a crucible, a mold box and, in instances where a complete butt weld of the head of the rail is desired, a rail-clamp is all that is re quired. All these materials, including the neces sary quantity of thermit, can easily be moved on a hand truck. Each weld, according to the section, requires from 15 to 20 pounds of ther mit, and the metal welded around the joint will only weigh, therefore, from 8 to 10 pounds.
The thermit reaction takes place in a crucible which rests on a simple iron stand that can be attached to the rails or rail-clamps where such are used. The crucible consists of a sheet-iron mantle lined with magnesia or corundum slag, which is tamped round a sheet-iron conus sus pended in its middle. The bottom is formed by a hard magnesia stone provided with a re placeable outlet which will stand 9 or 10 runs.
The life of the crucible itself is about 25 reactions. The crucible is plugged by two as bestos washers covered by a metal disk over the outflow or thimble. In the latter is sus pended a piece of iron wire, the lower end of which projects below the base of the crucible. This is driven up by a blow and so °taps° the crucible. The heat of the thermic reaction might in spite of the asbestos washers burst through the plugging material. To prevent this the metal disk is further covered by a layer of magnesia sand.
The mold is made according to a special design for each section. Its two parts, one on each side, exactly fit and firmly enclose the rail. It must be dry and porous. On a large scale molds are made by manufacturers of refractory earthenware or by railway lines, according to their requirements, in their own shops, by tamping an ordinary mixture of loam and sand in equal parts into a sheet-iron case placed over the model. This sand mold must be dried during a couple of hours at a tem perature of about 212° F. Before placing the mold round the joint, the rail ends are cleaned of dirt and rust with a wire brush and warmed with a gasoline torch. In case the tops of the rails are to be butt-welded the sections must be filed. The thermit is then poured into the crucible from bags containing the necessary amount for each section. After the charge in the crucible has been ignited in the usual way the reaction takes its course and the crucible is tapped to allow the liquid steel to flow into the mold. The great heat of the liquid thermit steel literally melts and amalgamates the ends of the rails projecting into the molds, making them as one, so that when cool it leaves a continuous rail. The joint is stronger than an equal section of the rail, from the fact that a shoe or collar of metal is cast around the rail at the joint.