There was a brief discussion of this recently on one of the groups I read, can't remember where, someone proclaiming that MIG was the answer to everything etc etc... I needed to bend about 50 pieces of 40 x 8 flat bar to a consistent radius (about 9" I think) through about 85 degrees, and was looking for something to use as a former. I lighted on the cast iron formers for an old manual 2" pipe bender, decided to give them a try. I started with the 1 1/2" former, and to avoid making fancy fixtures tacked it to the steel bench and tacked a steel bracket to it using the MIG welder with ordinary steel wire. It worked a treat. The welds were showing a tendency to crack down the centre as they cooled, but had more than enough strength for the job with no sign of failure in the HAZ or the actual bond between the weld metal and the casting. Trouble was, the finished radius was too small :-( Next step, try the 2" former, part of the same set, with exactly the same arrangement. This time, the weld had no strength at all, either in tension or shear, and almost literally fell off the casting every time. I ended up clamping the former to the bench and using weld beads to stop it moving. Good news was that the radius was spot on and the job is done.
Goes to show that you *can* get a useful weld with the MIG onto *some* cast iron, but I wouldn't stake too much on being able to do it every time. To reassure those of nervous disposition, no permanent harm was done to either former, they can still perform their normal duties perfectly
Tim Leech Dutton Dry-Dock
Traditional & Modern canal craft repairs