CHAPTER V
PREHEATING AGENCIES
(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.
(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.