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.
REMEMBER THAT THE ARTICLE AND THE WELDING ROD ARE THE SAME METALS, MELTING AT THE SAME TEMPERATURE. We must, then, keep the article and the rod in fusion at all times to effect a bond. Be very careful, that the metal is actually melting while the rod is being added.
Avoid the habit of pulling the torch away from the weld, rather use a slow circular movement which insures fusion and does away with loss of heat. Use the flux sparingly-never throwing it in with the hands-the amount picked up by the hot welding rod is sufficient at all times. At times it may be necessary to break the oxide by stirring the molten iron with the rod and if the metal is very dirty, by pulling it out of the line of welding by means of the rod.
Do not move away from the section being worked upon until the weld on that section is complete. Never re-weld without first grinding out the old material. Don’t bring the cone in direct contact with the metal-hold it just a little distance away.
As we progress with the welding, we note that the metal always does not flow where we want it to; i. e., where we are holding the flame. The force of the flame usually prevents this and we add the metal from the welding rod at a point a little distance away from where we actually want it to flow, and when we are ready to have it join the casting, we remove the flame: from that point and swing the metal to the section desired by the circular motion described. Cast iron does not immediately solidify the moment the flame is removed, it remains liquid for some little time and this condition presents two difficulties; one the danger of allowing this fluid metal to flow over, without bonding to other metal, and the other is the collapsing of the weld. The first difficulty may be offset by watching the weld carefully and bringing all parts into fusion. The second one is usually experienced by beginners and is caused by their lack of knowledge of the metal and the force of the flame with the metal in a liquid condition is sufficient to cause the metal to collapse and create a hole. When we get this condition, it is sometimes discouraging, as our efforts to fill up the hole usually result in making it larger.
Remember that one of the reasons for the collapse is the force or velocity of the flame-the metal is fluid all the way through and this force is sufficient to let it drop. We must, therefore, have a solid base at all times, which may be secured by the circular motion of the welding torch, not keeping the flame in one spot too long.
To fill a hole, work down the sides the same as we have done in starting the weld, then tip the torch on an angle, being careful, however, to keep the metal in fusion all the time. The idea is, as may readily be seen, to divert the direction of the force of the flame. This same method is applicable where small sections may be missing.
Where large sections are gone, we may make a rough pattern by the use of plaster-of-Paris and cast the desired design, or from useless castings of approximately the same thickness, break several pieces and by properly tacking them, form the general shape required. The user of the oxy-acetylene torch should realize that he has at his command a very powerful agent capable of replacing missing parts, a true “putting on” tool.
If we have carefully followed out the directions given for cast iron welding, we have a joint which is even stronger than the original article, and we can make it considerable stronger by adding additional metal to the line of welding, as we may desire, to strongly reinforce it. If we have had trouble, if the metal is hard, or if we get contraction cracks, it is because we have not properly heeded the suggestions given.
Since the welding of cast iron is usually repair work, the welder does not have a choice of conditions. They must take the article as it is, study carefully its construction with a view of determining the effects of expansion and contraction and plan to overcome them; they must use a welding rod which will prevent hardness; a flux which will make the metal fluid and remove the oxide; and use judgment in slow and careful cooling. These are not difficult conditions, they simply call for that judgment which is sometimes called a knack.
A little study means a good weld, the lack of knowledge means a failure, and since foresight is a great deal more valuable than second guesses, let us understand these conditions so we may intelligently use the welding torch.