Oxy-acetylene welding manual

by Lorn Campbell

Fig. 45.—Starting a Cast-iron Weld.

Fig. 46.—Reinforcing a Cast-iron Weld.

(73) No set rule can be given as to the sized tip to be used on various kinds of metal. It will largely depend upon the welder’s ability and judgment. When the metal is brought to red-heat, the neutral flame or cone is brought into contact with the lowest portion of the “V” and held there until it is seen that the metal is melted on both sides. The filler-rod, which has previously been heated at one end and dipped into the flux so that an amount adheres to the end of the rod, then carries this flux to that portion of the weld which is under way. Enough flux is blown off the rod into the weld to clean up the surface and permit the metal flowing together. The crack should be melted together all along before any additional metal is added, for the elimination of the crack is extremely important. It might be noted that as soon as the metal begins to flow freely the neutral flame should be raised a short distance from the work in order to better control the molten metal. In order to build up the metal to the original state along the line of weld or perhaps reinforce it, the sides and bottom of this “V-ed” out part are then brought to a molten state and held there while the filler-rod which brings up more flux is stirred into this metal and the end melted off. In this way the flame does not come in direct contact with the filler-rod and is used only to keep the metal in a molten condition. As much of the filler-rod can be melted off as is thought necessary to bring the weld to the normal condition of the metal or an additional reinforcement can be built up, if it is thought advisable. If care is taken in the above procedure, many of the blow holes and hard spots in the weld will be eliminated, for any impurities that might gather will be displaced by the melted metal and will float to the top. In cooling a weld of this kind, care should be taken not to permit any sudden chilling for this will tend to harden the weld. It is best to cool it slowly by burying it in slack lime, ashes, or wrap it with asbestos paper to keep the air from it as much as possible.

Fig. 47.—This Problem does not Require Preheating to Care for Contraction, as the Ends of A and B are not Confined.

(74) There may be a great many causes for blow holes and hard spots in the weld, but probably they can all be traced directly to the lack of heat. It must be remembered that welding is a fusing process and heat is absolutely essential. Therefore it should not be used sparingly. The application of heat always causes expansion. There are no exceptions to this rule, likewise upon cooling the metal there will be a contraction. Outside of the actual welding, that is, the fusing of the metal into a homogeneous mass, perhaps the greatest problem that the welder has to confront is the expansion and contraction of his metals. Whenever the ends of two pieces of metal which are to be welded are free to move, or even one end, there will be no difficulty encountered with contraction and expansion, but if these ends are confined, it is an entirely different problem.

Fig. 48.—Preheating Problem. Ends of Bars A′ and B′ are Confined.

(75) To illustrate this point more clearly, the following very simple example will be given. In [Fig. 47] we have two bars of metal A and B which have been beveled off or “V-ed” out as shown at the point C. Now as soon as the heat is introduced at C there is bound to be an expansion of the metal at that point. Naturally if the pieces were heated slowly and for a considerable distance, the cool ends of these bars would be forced outward. We will assume that the heat is introduced very rapidly and the metal is brought to a molten state; that instead of the contraction forcing the cool ends outward, whatever expansion there is, is taken care of, at the weld, for the metal when melted will readily push together. It is also assumed that the bars are heavy enough to overcome what slight force might be in evidence from the expansion. A weld is then made and allowed to cool. As it cools, there is bound to be a contraction along the line of the weld and the welded piece will be slightly shorter than the work before the weld, for it will draw in the pieces A and B. As can be seen, there is no particular force preventing the contraction of such a weld for the ends are free to move. However, let us turn to [Fig. 48], which constitutes an entirely different problem. It might seem that the ends A′ and B′ appear the same as A and B in [Fig. 47], but such is not the case. The ends farthest from the weld are confined, held in place by a heavy frame which does not permit their free movement. When heat is introduced at the point of welding C′, about the same action takes place as in the previous problem, but as soon as the weld commences to cool let us see what happens. The bar A′B′ must be shortened so there is an inward pull on the bars D′ and E′. If this work were cast iron or aluminum it would certainly be broken by the strains set to working and would naturally break at C′, where the metal is still hot. If it were steel or one of the ductile metals, it might twist and warp in its endeavor to overcome these internal strains. This illustrates in a very simple manner the difference between what is known as a “cold” and a “preheating” job. In the first no provision is made for expansion and contraction. In the second means are taken to overcome these important factors. In order to provide for the successful welding of the second problem, it is only necessary to heat up the bars X and Y about the same distance as the center will be heated, and keep them in that condition while executing the weld at C′, then allowing the whole to cool gradually.