There is only one type of welding that will work on cast iron.  It
is very expensive, and involves heating the whole piece in a furnace.
It is then torch-welded, while still hot, using iron rod.  It then
must be cooled over a period of hours at a controlled rate.  This
allows the internal stresses to equalize and makes the weld a part of
the piece.  If done properly, it is difficult to see where the repair
was done.  Arc-welding cast iron usually results in a crack next to
the weld.

Larry Mayo