Thermit Welding-Two
From LoveToKnow 1911
"THERMIT WELDING.-TWO methods are employed, known as the plastic and the fusion. The first is used for welding pipe and the latter for solid or large sections. In the plastic method, in which the FIG. r. - Showing the action of Thermit when poured into a pipe welding mould.
thermit is used merely for heating purposes, the ends of two pieces of pipe are machined square and clamped in a cast-iron mould with the ends butted together. This mould is in two parts, so arranged that the pipe ends may he forced together when heated. The thermit is placed in an open-top crucible lined with magnesia-tar, and ignited. After the reaction takes place the slag rises to the top of the molten metal, and is first poured into the mould as shown at the, FIG. 2. - Tapping a crucible showing a partial sectional view.
left in fig. i. This slag forms a protective coating on the pipe and on the inside of the mould, and keeps the thermit from melting or burning through. At the right the thermit is shown flowing into the mould and forcing out the bulk of the slag, but leaving a coating as mentioned. When the pipe ends become plastic they are forced together, completing the weld. After cooling the mould is easily knocked off, since the slag coating prevents adhesion. To weld a pipe takes from; to 12 minutes. In fusion welding on solid sections, in which the thermit mixture forms a casting holding the parts together, a special V-shaped magnesia-tar-lined crucible, open at the bottom, is used. The hole in the bottom is closed by a headed plug, covered with refractory sand, which may be pushed upward. This crucible is placed over the mould, as shown in fig. 2, the proper amount of thermit put in and ignited. After the reaction, which takes about 35 seconds, the plug in the bottom is pushed up and the molten thermit allowed to run into the mould. In this method great care is taken to keep the slag from contact with the surfaces to be welded, and consequently enough thermit must be used to fill the mould before the slag on top of the melted metal can enter.
FIG. 3. - Sectional view of typical Thermit mould.
A typical mould for heavy sections is shown in fig. 3. In preparing to weld, the surfaces to be joined should be cut or set so as to be in. or more apart. Yellow wax is then built up around the joint in the same shape as the weld is to be. Next the mould box is placed and sand rammed up round the wax, wooden patterns being placed for the pouring gate, riser and preheating gate. The mould is vented, the patterns withdrawn, and a gas or oil flame used to melt out the wax. The heating is continued until the sections to be welded are red-hot. This prevents chilling of the thermit, which is poured in after the preheating gate has been plugged. The steps in making a typical thermit weld on a locomotive frame are graphically shown in FIG. 4. - Steps in making a Thermit locomotive frame weld.
fig. 4. Thermit welding is largely used in repairing broken rudder frames, propeller shafts, locomotive frames, steel rolling-mill pinions and other heavy sections, but it cannot be economically used for welding thin sheet metal sections. Welds have been made where from 3,000 to 4,000 lb. of thermit were used.
For commercial purposes there are now produced three varieties of thermit, known as plain thermit, railroad thermit and cast-iron thermit. The plain thermit is simply a mixture of aluminium and iron oxide, as already given. Railroad thermit is plain thermit with the addition of % nickel, r % manganese and 15% mild steel punchings. Cast-iron thermit is plain thermit with the addition of 3% ferrosilicon and 20% mild steel punchings. The names of these mixtures indicate their principal uses. (E. Vt.)
