Nice repair. It sounds and looks like a low expansion, high nickel content rod. Any nickel-iron allloy where the nickel reaches or exceeds mid 30's %, the low expansion "magic" starts to happen: http://en.wikipedia.org/wiki/Invar

I've had good results with it as well (a 55% nickel / 45% iron stick electrode, with the flux knocked off, and applied via TIG), but sometimes the cast iron can get a little cruddy. Interested in hearing how you prepped the cast iron because you got a nice result.

Did you weld both sides one pass each (2 passes total)? Or try to it is in 1 pass? Did you manage 1 continuous weld bead or did you beak it up? Did you skip around or "stitch weld"?