Oxy-Acetylene Welding of Cast Iron
Oxy-acetylene welding of cast iron is comparatively easy, if attention is paid to some simple rules. The cast iron in common use is known as “gray;” it is quite soft, easily machined and has a lower melting point than wrought iron or steel. It contains two kinds of carbon, combined and graphitic. If we change the graphitic into combined, we have white cast iron, which is extremely hard, very brittle and practically impossible to machine. Because of certain chemical actions which take place when cast iron is melted, we may obtain a very brittle weld in this manner unless we pay strict attention to ALL of the following requirements:
- Heat or cool slowly
- Use a proper welding rod
- Employ a good flux
- Handle the torch carefully
We have already learned the value of preheating to overcome the ill effects of expansion and contraction. Preheating is also an economical factor, since oxygen and acetylene are more costly than ordinary means of heating, and with cast iron it is essential that it be carefully heated and as carefully cooled to prevent hardening.
The casting to be welded should be beveled.
If entirely broken through and the ends are free to move, we should slightly separate the edges before we start welding to allow for contraction. This separation, if we have properly chamfered the break, will be quite short-about l/32nd of an inch will be found about correct.
The utmost care should be exercised in preparation and in setting the article up to prevent its moving while it is being welded. Since we have a comparatively light edge from beveling, it is advisable to flow the metal together at this spot, “Tacking” it, as it is called in the welding shop.
Especial care should be used to heat slowly, remembering that in the construction often employed, “we will find heavy sections adjoining light and the heavy section requires more heat than the light.
Whatever means we may employ to preheat then, arrange to have the casting heated evenly and this can only be done by heating slowly.
The size of the welding tip will be about the same as we would use for the same thickness of steel. While cast iron melts at a lower temperature than steel, its ability to absorb heat is slightly greater than steel and this will offset the lower melting point.
The welding rod is furnished in three sizes; we will choose the one best adapted to the work that size which is somewhere near the same thickness of the metal-up to one-half inch in thickness. Beyond this thickness, we will use a rod of about 5/8ths and on extreme thickness we may tack two or three together, though this is rarely necessary. Now, with the welding rod in one hand and the torch, with the flame properly adjusted in the other, and the flux can in convenient reach, we are ready to start welding. Presumably the casting is at a red heat in the furnace and we may protect ourselves from any excessive heat by using heat resistant material to cover such portions of it as may be necessary.
Bring the welding flame down to the metal until the end of the cone is almost, but not quite, touching the edges. The welding rod is placed near the flame to slightly heat it, then it is dipped in the flux can and the· flux picked up by the hot rod is placed in the spot the flame is playing upon. Usually this is sufficient to break the film of oxide and to cause the metal to flow together.
Note that we have added no material from the welding rod as YET. Now, we melt the SIDES of the break and flow them towards the bottom, until the molten pool of metal is well formed. Then we are ready to use the material from the welding rod, which should be kept in contact with the weld at all times to avoid loss of heat.