Brazing cast iron is not too difficult with practice but there are a few gotcha’ s that will cause problems if you don’t know about them.
Prepare the area to be brazed. It must be very clean. Avoid using grinding wheels and other methods that leave abrasive particles behind that will later contaminate the weld. If you are fixing a crack, grind it out to a U-shaped profile.
Preheat the cast iron. A large torch or oven may be needed. Some people use a propane weed burning torch.
Get the metal red hot. Add flux if not using a coated rod and feed the filler rod into the joint – it should flow readily.
If you get the metal too hot, it will “burn”, creating a carbon layer that the filler metal will not stick to. You need to go back and grind it clean.
Finally, allow the item to cool as slowly as possible.