Cast iron is not easy to weld, but that doesn’t mean you should avoid it. There are a lot of things in life that are hard but also enjoyable and worthwhile. Cast iron welding is no different. The hard thing about welding cast iron is that there are good days and bad days. Sometimes you weld some cast iron and think “Well that wasn’t so bad, I could do that again,” and other days you just want to throw everything out of the window. This inconsistency can be frustrating. But why does this inconsistency exist?
Cast iron comes in a few different grades, and you can’t necessarily tell which grade you are working with just by looking at it. Some professionals, like many welding engineering consulting experts, can tell the grade of cast iron by looking at the sparks it makes or the surface fractures of the material, but that is atypical.
Cast iron has many different applications. If it is being used as an exhaust manifold, it has been absolutely soaked in carbon from its time served heating, cooling, and being in the presence of fuel exhaust. This results in a higher hardenability and consequently a lower weldability. A piece of cast iron furniture, however, is incredibly easy to weld. So if you go into welding an exhaust manifold after welding a pot belly stove, you will be taken off guard by how different, and how much more difficult it is. So what is the best way to weld cast iron?
Mild steel has a very small amount of carbon, at about two tenths of a percent. Medium carbon steel has a bit more than that and will harden a bit when heated and then quenched. Cast iron, on the other hand, has anywhere from 3-8% carbon content. If you are not careful, it will become hard and brittle when you weld it. When you hear a crack when welding cast iron, it is a result of the weld metal shrinking and stressing the brittle cast iron, resulting in a physical crack.
For a successful cast iron weld, start by cleaning it. You will need to make a groove in the crack and grind it so it is shiny, to about a half an inch past where the welding will occur. Then, preheat the cast iron until it is between 400-500 degrees Fahrenheit. If that isn’t possible because it is a large part, or for other reasons, you will want to limit your heat input during the welding process. Most parts can reasonably be preheated, so start with that. If you find it impossible then feel to use the other method. Now, it is time to weld. With cast iron it is best to tig weld with alternating current, with the balance control all the way up, and use aluminum bronze for your filler. The advantage of aluminum bronze is clear. It is strong, it works well on alternating current, it is not very diluted, and it doesn’t cause much shrinkage stress which means it won’t crack your adjacent cast iron.