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Cast Iron is misnamed.
Cast steel would be a better term. In my old job as a T.I.G. welder and welding all kinds of cast steels I can tell you the carbon content varies quite a bit, but if the cast iron is running .50 to .70% it will forge just fine and many old tools were made of cast steel in that range. Higher carbon is the stuff you can't forge and if you break one of those high carbon castings note how large the grain is on some of them. No kidding some castings were 4% carbon, impossible to weld.
I have welded pure cast iron and it welds like regular low carbon steel. It's the doggone carbon that causes problems. Your typical exhaust manifold is 2% carbon and welding it is nigh on next to impossible, so we'd use Ni-rod or nickel or silicon bronze to braze cracks. If you preheat it to 1200 degrees and slow cool it you can weld it, but almost nobody does that.
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