Welding by the thermit process is really "cast welding", because it is accomplished by pouring "thermit steel" around the parts to be joined. The main difference between this and other methods of cast welding lies in the method of producing the molten metal. The name for the process is derived from the Greek word therme, meaning "heat", and signifies that it is a heat process of welding or that the metal is produced by heat. The name was originally adopted as a sort of trade-mark but has come to be accepted as the name of the process.
The thermit welding process is based upon a long series of experiments carried on for a number of years by various physicists and metallurgists to find some method of reducing metals readily from their oxides and ores. It is the direct result of the work done by Dr. Goldschmidt, of Essen, Germany, in what is now the new field of aluminothermics, and is based on his discovery that if finely divided metallic oxides are mixed in certain proportions with finely divided aluminum they will, if ignited, fuse and produce a temperature of 5400 degrees Fahrenheit in less than 30 seconds without the use of heat or power from the outside. The high affinity of aluminum for oxygen will cause it to draw the oxygen from the metallic oxide, combine with it to form aluminum oxide, raise the temperature of the mass by the violent reaction, and set the metal free. The greater weight of the metal will cause it to flow down through the mass in the container and the aluminum slag will rise to the top.
For ordinary commercial welding purposes in machine shops and foundries, iron oxide is used and the reaction takes place according to the equation.
Fe203+2A1 = 2Fe+A120.
The liquid steel produced by this process represents one third of the original material by volume and one half of the original mixture by weight, the balance being lost as slag. This method of cast welding was developed about the year 1900 and the peculiar reaction used has also been applied to the production of numerous kinds of alloys and metals free from carbon. Further reference will be made to this.
According to data furnished by the makers of thermit welding apparatus the average analysis of thermit steel is as follows:
The balance of the mixture is iron.
During the experiments leading to the development of thermit welding, the mixture of metallic oxide and aluminum was heated from the outside to start the reaction, but finely divided aluminum will not melt at the temperature of cast iron and it was necessary to heat the mass so high that when action started it resulted in an explosion. So Dr. Goldschmidt used a storm match to ignite a fuse of barium peroxide, (BaO) which in turn ignited the mixture and started the reaction.
Fig. 127. Thermit Crudbla Pouring Charce into Mold Courtesy of Gotdachmidt Thtrmii Company.
The apparatus required for thermit welding consists of a crucible, tripod, preheater, yellow wax, and a spade, with which there must also be used perishable materials consisting of thermit, manganese, molding material, and ignition powder. The shell of the crucible is of sheet iron and it is lined with magnesia in order to stand the high temperature and has a magnesia stone thimble at the bottom through which the metal flows. The process of preparing the lining is rather elaborate and must be carefully done or the life of the crucible will be greatly reduced. The tripod is used to support the crucible above the work, and the pre-heater is a combination compressed air and gasoline outfit used to heat the article to be welded in order that it may not chill the filling material. The wax is for forming the space to be filled when welding and about which a mold is made. It is melted out of the mold before welding.
The process of preparing the crucible and the mold are the principal features of the entire operation of thermit welding, as. the mere act of casting the weld is comparatively simple. The crucible, which is shown in Fig. 127, suspended above the mold, is a sheet-iron shell with a hole below for the metal to pass through. It is to be lined with magnesia, carefully packed while hot enough to be plastic, and with a magnesia stone thimble at the bottom to form a hushed hole and protect the crucible. The magnesia lining should be put into place slowly and carefully and tamped tightly into place, for its value depends largely upon how hard it is packed. The lining is formed around a matrix to shape the hopper-like center and must be baked at a dull red heat for six hours before it is ready to use. A crucible will withstand about 20 reactions if well made, and must then be relined. The thimble must be placed in the bottom of the crucible so as to be removable.
The construction of the mold is really the most important part of the operation, because upon this depends the amount and application of the filling material. The container or flask is usually made of steel plates placed so as to form a box around the part to be welded, and then filled with the day, etc., of the mold, the plates being fastened with bolts, tie-rods, clips, or clamps of whatever sort may be available, Fig. 128. The first step in the formation of the mold is to build a collar of the yellow beeswax around the place to be welded, making this of the size and shape desired for the weld. After the collar is formed, the flask is placed around it and filled with a mixture of ground fire brick, fire clay, and fire sand in equal parts. There must be three channels in every mold, a pouring gate, a riser, and a heating gate. The pouring gate should run from the top of the mold down to the bottom of the wax collar to insure the metal filling the mold and to allow the good steel to reach the weld instead of being crowded out by the slag. The riser should be immediately above the wax collar, if possible, so that the slag and surplus metal can rise freely from the metal of the weld, and the heating gate should run from one side of the mold into the bottom of the collar in order that the wax can all run out of the mold when melted by the preheating torch. Am soon as the mold is completed, the torch is applied and the wax melted out, Fig. 129. The flame is allowed to play into the mold until it is entirely dry and then the heating gate must be plugged with clay to stop it up entirely. Fig. 130 shows a typical thermit weld, with pouring gate and riser still attached.