I just posted this on another forum. It is kind of long, sorry.
WELDING CAST IRON
I have been welding cast iron on antique tractors, will try to put down what I have learned. First, the parts I have done are 5/16 inch or less thick. Preheat helps a lot, but it can be done without it. I have been using a Nickle 99 rod on AC. Most of the manufacturers have a rod like this. The Ni 99 has more nickle and less iron than the more popular Ni 55. The Ni 99 has less strength than the Ni 55 also. As I remember, the Ni 55 has yield around 37,000 psi, the Ni 55 is around 60,000 psi. I don't have the reference material handy, these are approximate figures. The benefit of the lower strength material is that the peening you do after welding is more effective, and the stress the weld material can exert on the casting is less. The Ni 99 grinds well, the Ni 55 is harder. I doubt that these old castings have yield above 30 or 40,000 psi. Since yield and tensile strength are almost the same on the more brittle cast materials, you have to be careful about the stresses that are induced, cracking can result. My procedure is to grind a U, not a V. The reason for this is that as the weld cools, it contracts and stresses the casting. The U tends to pull the crack together, the V tends to try to bend the casting, therefore more likely to crack. Without preheating, you must weld short sections at a time. On very thin castings, I weld no more than 1/2 inch, lay the electrode holder down, and begin peening immediately. By peening, I mean hammer the filler material gently with a small peening hammer or the dull point end of a chipping hammer. Weld each end of the crack first. I have not tried drilling holes at the ends, but that is supposed to be a good procedure. Without preheating, you will need to wait for the casting to cool so that you can lay your hand on it between welding periods. If you prepare and do this correctly, after you have welded the ends and one spot in the middle of the crack, the crack should be pulled together so tight that it almost disappears. The same contraction that can cause cracking does this, a good indicator of the forces exerted by the cooling filler material. I hope this is helpful, if I can explain better, post questions. There are some very premium rods available, also some MIG wire for cast. I have not tried these, they are quite expensive.
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