The amount of thermit required to make a given weld will be twice the amount necessary to fill the space formed by the wax collar, because one-half of the weight of the original powder will rise in the form of aluminum slag, as already stated. On the other hand, the cubical area of riser and gate must be twice as great as the collar because the volume of the slag will be two-thirds of the total volume of the casting. It has been determined by experience that the weight of thermit necessary for a given job will be 32 times the weight of the wax required to form the collar for the mold; so the wax should be weighed after melting out of the mold in order to know how much thermit is required for the job. The size and shape of the mold and riser and gate will vary somewhat for different jobs and the relation between weight of wax and mixture will vary accordingly, but the ratio of 32 to 1 is a good average. It is necessary to preheat the article at the joint until it is red-hot before starting to pour the metal, and this is done with the gasoline torch through the heating gate at the bottom of the mold before plugging it.
When more than 10 pounds of thermit are required for the weld, it is necessary to moderate the heat of the reaction slightly, and this is done by adding small pieces of clean steel to the powder. These may be punchings, rivets, or any other soft steel pieces but must be free from grease to keep carbon out of the mixture, and from 10 to 15 per cent of the weight of the thermit may be added in this way. About 2 per cent of pure metallic manganese should also be added in order to increase the strength of the weld. If the manganese is not obtainable, 3 per cent of ferromanganese may be added instead, although it increases the violence of the reaction and hardens the metal.
Fig. 130. Typical Thermit Weld Showing Riser and Pounug Gnu Casting Still Attached.
Courtesy of Goldschmids Thermit Company.
Fig. 131. Hulling Milt Housing Welded by Thermit Process Courtesy of Goldechmidl Thermit Company.
The average tensile strength of the welds made with thermit are 30 tons per square inch of cross section and, if the joint is properly made, it will be hard to see where it is.
If a reinforcement can be left around the weld, it will give a higher strength than the original material, but where it is machined off the strength will average 80 per cent of that of the piece welded unless it be of unusually high tensile strength. When preparing the joint for welding, it is best to leave an opening for the metal to flow into, and this should be at least ¾ inch wide, preferably more.
Fig. 131. Broken Locomotive link Ready for the Molds Courtesy of Goldschmidl Thermit Company.
The applications of thermit welding are numerous, although the process is better suited to large jobs where the saving in cost of new pieces will justify the cost of the work, Fig. 131. It will be evident also that the process lends itself better to welding large articles than small ones and experience up to the present shows that most thermit welding has been done on such large articles as engine and machine tool frames, locomotive side frames, and motor cases. Fig. 132 shows a typical thermit weld at a rail joint. This is a modified Clark joint. Another useful application of the method is in welding stern posts and rudder posts of vessels, Fig. 133. The widest application seems to be in steam railroad shops and, while it is true that the electric arc-welding process is rapidly superseding all others for that service, some of the work done is worthy of description. Considerable saving has been made by doing the work without dismantling the engines in order to get at the break. The process is to form the mold about the break, as described, and set the crucible above it ready for pouring. Where it is possible to lay the article on the floor, as when welding crank shafts or a broken link, Figs. 134 and 135, the job is much easier and quicker to perform.
Fig. 135. Welded Locomotive Link with Molds Removed Courtesy of Goldschmidl Thermit Company.
If the break is in the upper part of a locomotive frame, for example, the break should be cut out about an inch and the frame jacked apart another quarter of an inch. The inch space is for filling and the ¼ inch is to allow for shrinkage; so the jacks should be removed as soon as the mold is filled. Breaks in other parts of locomotive frames are treated in the same way. For welding driving-wheel spokes, it is best to heat the adjacent spokes with a torch to expand before welding the broken ones, and then allow them all to cool at once. Rail welding for street railways is another application of thermit and is clearly shown by the cuts herewith, In all cases it is necessary to clean the metal thoroughly around the joint to remove grease and scale, and this is best done with a sand blast so as to insure bright clean metal to fill against. For work of this nature it pays to provide the fullest equipment in order that there may be no failures, because it is a very expensive operation and very hard to do over again.
Other uses of thermit are in foundries for improving the quality of the castings and in metallurgical work to produce metals and alloys free from carbon. In foundries it has been found that, by placing a can of thermit in the ladle before pouring, the temperature of the metal will be raised and, by using thermit of the proper composition, the strength of the metal can be increased or its composition varied to suit different jobs. It is also used in steel mills to reduce losses from "piping" of the ingots. A pipe is a hole formed in the top of the ingot when cooling, due to shrinkage and the presence of slag, and it may extend a considerable distance down into the ingot and reduce its value for rolling. So a can of thermit is thrust down into the top of the ingot at a certain point in the cooling, and this ignites and fuses the steel down and forces the slag out. The mold can then be filled the rest of the way with good steel.
The use of thermit, for the production of alloys, etc., has been successful with such metals as titanium, chromium, manganese, vanadium, etc., and alloys of the following com-positions have been made.
Ferrotitanium......................20-25 % Ti.
Chromium........................97-98 % Cr.
Chromium Manganese...............30-70 parts.
Chromium Copper.................. 10 % Cr.
Chromium Molybdenum.............50-50 parts.
Manganese.........................97-98 % Mn.
Manganese Copper..................30-70 parts.
Manganese Titanium................30-35 % Ti.
Manganese Tin.....................50-50 parts.
Manganese Zinc....................20-80 parts.
Manganese Boron...................30-35 % Bo.
Ferrovanadium.....................30-35 % Va.
Ferroboron.........................20-25 % Bo.
Ferrotitanium is used as a purifying agent for steel; chromium is used as an alloy with steel to produce crucible steel, etc.; manganese is used to produce very hard steel, bronze alloys, etc.; molybdenum is used in making tool steels; vanadium is used to add to the strength of iron and steel.