CHAPTER V
PREHEATING AGENCIES
[Illustration:
(_Courtesy of the Messer Mfg. Co._)
FIG. 36.--A Large Job Prepared for Welding.]
(=58=) PREHEATING, as applied to oxy-acetylene welding, means the application of heat to the article to be welded in some manner which is usually different than by the welding flame itself. Charcoal, coke, kerosene, crude oil, coal and natural gas are used for this purpose. The principal reasons for pre-heating parts to be welded are: To take care of the effects of contraction and expansion on the confined ends; to save time, gas, and material; and to make a better weld by making it quicker and with less chance of burning up the metal.
[Illustration:
(_Courtesy of the Messer Mfg. Co._)
FIG. 37.--Showing how Large Work can be Covered with Asbestos Paper when Preheating.]
(=59=) On account of the ductile qualities of steel, there is not quite as much heat used in preheating, to take care of the contraction and expansion, as in cast iron. On brass work a very dull red heat is considered sufficient, or otherwise the alloys might burn out. When preheating aluminum, there will be no change in color as the heat is introduced, so other methods are used to determine the correct temperature. Three methods are used for this purpose by most welders. “Half-and-half” soldering wire will usually melt when applied to the surface of aluminum which is preheated to the proper state; the puddle stick when drawn smartly across the heated surface should scrape off the oxide and leave a clear blue streak if the work is in condition to be welded; and if a medium-sized tip is brought down so that the neutral flame just touches the surface for a second or two, the metal will sweat, if at the proper temperature, and small globules which have the appearance of mercury will stand out on the surface.
(=60=) The beginner must study contraction and expansion in order that he may know when and where to apply it in figuring out his work. Many welding jobs have turned out to be failures through lack of knowledge in this respect. Take, for example, a water-cooled cylinder block of the ordinary gas engine; the water jacket may be broken when the water is allowed to freeze in it. This problem has certainly confined ends, but some welders have attempted to weld such jobs cold, that is, without preheating, and possibly have succeeded in executing what they thought was a very fine weld, but upon examination, have discovered that the cylinder walls, which are very accurately machined, have been warped to such an extent that the block is rendered useless. This is strictly a “preheating” job, and the cylinder should be brought to a dull red heat if the best results are to follow.
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.
[Illustration: 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.
[Illustration: FIG. 39.--Temporary Preheating Oven, Built of Fire Brick.]
[Illustration: 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.
[Illustration: FIG. 41.--Showing How Oven is Built when Preheating Torch is to be Used. Torch is Shown at (_A_).]
(=64=) When using preheating torches, the ovens are built much closer to the work and do not have the openings along the bottom row of bricks. They are made as tight as possible, and in some cases it will be found advisable to build up the walls with two layers of bricks, with asbestos paper between them, in order to hold the heat and cause the work to heat up in a more uniform manner. A hole is left in one end of the oven, through which the flame of the preheating torch is introduced as shown in Fig. 41. It is not thought best to have the torch flame come in direct contact with the work which is being preheated, and a baffling plate of metal or brick is placed directly in front of the flame, in order to spread it around the oven. Judgment will have to be used in all such work.
(=65=) The setting up of the work, when preheating, is an important point overlooked by many welders, especially so in the case of aluminum. Care should be taken to see that the work has a good solid setting and is braced at a sufficient number of points, to prevent its sagging when in a preheated condition. Many times when working on rough surfaces, a few firebricks distributed around the bottom of the oven with a dab of putty, clay, or retort cement, placed upon them, will form an excellent cushion upon which the work can rest and the operator may feel confident that no sagging will occur.