How to Weld Cast Iron. Cast iron usually has a carbon content of 2 to 4 percent which is much greater than in most steels. This causes most cast iron to be brittle and difficult to weld. Most cast iron welding involves repairs to existing castings instead of forming new castings.
Use cooling or heating when welding cast iron. The single most important consideration in welding cast iron is to keep it out of the 150- to 500-degree F range. Preheating is generally preferred, but cooling also is used. Do not change methods in the middle of a weld.
Preheat the cast iron part. If possible, heat the entire casting slowly and uniformly in the 500- to 1,200-degree F range. Do not overheat; most cast iron starts to crack above 1,400 degrees F. Use a low current to minimize admixture and stress.
Keep the cast iron cool but not cold. If the part is on some type of powered machinery, it may be possible to run it for a short time to achieve the desired temperature. The casting should never get too hot to touch with your bare hand. Make short welds approximately 1-inch long to avoid overheating.
Expect small cracks when welding cast iron, even when performed correctly. A sealing compound normally must be applied to joints that need to be watertight.
Repair major cracks in cast iron with studding. Screw steel studs into holes that have been drilled and tapped into the surface to be welded. The studs should have 5 to 6 mm above the surface. The studs are then welded into place.