A welding process that produces coalescence of metals by heating them with superheated liquid metal from a chemical reaction between a metal oxide and aluminum, with or without the application of pressure. Filler metal is obtained from the liquid metal.
Historical Background
The thermite process was developed at the end of the 19th century when Hans Goldschmidt of Gold Schmidt AG West Germany (Orgotheus Inc. USA) discovered that the exothermic reaction between a mixture of aluminum powder and a metal oxide can be
initiated by an external heat source. The reaction is highly exothermic, and therefore, once started, it is self-sustaining.
The thermite mixture can be ignited and brought to a high temperature in one spot, and when started, the reaction will continue throughout the rest of the mass. This reaction occurs when the aluminum combines with the oxygen of the iron oxide to form aluminum oxide (slag) in a super-heated molten state while the iron is set free and is produced as liquid steel, also super-heated. The temperature created by this reaction is about 3150°C (5650°F), but because of heat loss through the crucible, the molten metal actually reaches about 2400°C (4350°F). When steel at this high temperature is poured around and between two iron or steel sections which have been previously heated to red heat, they will become dissolved and will amalgamate with the thermite steel. When the entire mass cools down it forms a single homogeneous section.
The thermite reaction is not explosive, and no danger is incurred in storing and handling the material, since it requires the temperature of liquid steel to ignite it.
Applications
This process is not often used in production welding because other processes are more efficient, but thermite welding continues to be used for making butt welds between lengths of railroad rails, for joining very thick sections of cast iron and steel castings, and for joining very large size steel reinforcing bars embedded in concrete structures. Circumstances can arise where thermite welding is the best process to fill special needs. Repair of massive sections that have cracked in large machines is an application where thermite welding can be used to advantage.
The most common application of the process is the welding of rail sections into continuous lengths to minimize the number of bolted joints in the track structure. In coal mines, the main hauling track is often welded to minimize maintenance and to reduce coal spillage caused by uneven track. Crane rails are usually welded to minimize joint maintenance and vibration of the building as heavily loaded wheels pass over the joint.
Thermite welding is also used in the marine field for repair of heavy sections of ferrous metal, such as broken stem frames, rudder parts, shafts, and struts,
Worn wabblers on the ends of steel mill rolls may also be replaced with tough thermite metal deposit that is machinable. Thermite welding is particularly applicable for repairs involving large volumes of metal, where the heat of fusion cannot be raised satisfactorily or efficiently by other means, or where fractures or voids in large sections require a large quantity of weld metal.
Thermite welding can be used to repair ingot molds at significant savings over replacement. The blades of large dredge cutters may be thermite welded to a center ring. Quantities up to several thousand pounds are poured at one time. In this case, thermite welding is a production tool rather than a repair method.
Safety
The presence of moisture in the thermite mix, in the crucible, or on the workpieces can lead to rapid formation of steam when the thermite reaction takes place.
Steam pressure may cause violent ejection of molten metal from the crucible. Therefore, the thermite mix should be stored in a dry place; the crucible should be dry,and moisture should not be allowed to enter the system before or during welding.