As has been stated elsewhere in this volume, the weld should not be considered successful unless the piece worked upon can be returned to a usable state.
(61) Several different fuels have been mentioned, all of which can be used for preheating purposes. Charcoal is considered the best agent for general welding, as it gives off a very steady heat which will gradually be absorbed by the article worked upon, bringing it to the heat desired and holding it there throughout the welding operation. It will then permit very gradual cooling, as this sort of fire takes a long time in dying—a desirable asset in work of this kind. On account of the scarcity of charcoal and its high price, other agencies are used and chief among them are torches using kerosene, crude oil, or city gas, as a fuel. These usually heat up the work more quickly, but care in their manipulation is necessary. A preheating torch to be used in conjunction with city gas can be very easily constructed, if the details of [Fig. 38] are observed. This proves to be a very efficient and cheaply constructed apparatus.
Fig. 38.—Preheating Torch, Constructed of Black Iron Pipe, for Burning City Gas.
(62) When work is being preheated, it is best to have it protected from all drafts, to prevent warping. Possibly the most extensively used material for building up temporary ovens to hold the heat and protect the work from the air currents is fire brick and with it asbestos paper. When setting up an ordinary casting for preheating, these bricks are built up in builder’s fashion, about four inches away from the piece itself, as shown in [Fig. 39], and practically level with the top of the piece. If charcoal is to be used, draft spaces are left in the first row of bricks as shown, and the charcoal ignited through the openings with the welding torch. The work to be welded should have the line of weld at the top if possible and be set up from the floor, or the surface upon which the oven is resting, on one or two fire bricks, in order that the full benefit of the heat will be received. Asbestos paper is then laid across the top, and the oven will appear as in [Fig. 40]. When starting the fire, a layer of charcoal, a matter of two or three inches thick, is at first used, but as the chill is taken off the piece the oven can be filled to the top, and usually this is enough to complete the work.
Fig. 39.—Temporary Preheating Oven, Built of Fire Brick.
Fig. 40.—Temporary Preheating Oven of Fire Bricks Covered with Asbestos Paper.
(63) In order to protect the operator, when working over hot fires, it is recommended that the asbestos covering be left on, and that only a small section immediately in the vicinity of the weld be removed, which can be accomplished by cutting a “U” in the paper as shown by the dotted lines in [Fig. 40]. This can be turned back, exposing the place which is to be welded, and at the same time protecting the operator, to a large extent, from the unnecessary heat. When the weld is finished, this lap can be turned back and the piece allowed to cool. On pieces which require turning and must be welded in several different positions, the preheating oven, as it is called, should be built considerably larger, to provide for handling the work. It must be remembered that during the entire operation, the piece should be left inside the oven and should not be removed to a welding table. Some beginners make the mistake of doing this. When welding with the charcoal in closed rooms, during the winter months, the fumes will be found to be very disagreeable and means should be taken to provide indirect ventilation, otherwise the welders will be troubled with headaches and smarting eyes.