I welded alot of CI. Nickel rod is the best. Pre-heat a little then weld it, and let it cool down slow or you might get a chill in the weld. V it out with a die grinder so you can get a bit more penitration. When you hit CI with a grinder it throws off sparks that look like a sparkler you probably played with when you were a kid.An AC buzz box will do a fine job. Best to weld a little from both ends,alternating from end to end of the break.