One of the most important things to consider when welding cast iron is weld prep. How you prepare the surface to be welded will have a great impact on weldability. Rule #1:
Never touch cast iron with an abrasive. Most people break this rule, using a grinder to cut a nice groove in a cracked piece of cast iron (assuming a repair here.) All that does is smear graphite all over the surface to be welded, & the resulting weld looks like swiss cheese. Those same guys grind out the weld & do it again, & they get baby swiss cheese, & they do it again, & smaller holes, but still porous!
Simply use either a carbide burr (in a die grinder) to
cut the cracked casting instead of abrading it. Or better yet -- if you know how to use a cutting electrode (gouge rod) -- gouge the crack to be welded. Now, don't use a carbon arc to gouge it, use a
gouging electrode. That way you won't introduce more carbon to the casting, & you'll preheat it in the process.
Speaking of preheat, 400 deg. F is a good preheat for cast iron. Peening & annealing are also good practices, as mentioned in other responses.
Filler rod: Nickel-based alloy made for cast iron, as mentioned in other responses, or go here to see some excellent filler rods for cast iron (warning they're not cheap.)
http://weldit.com/product_category/stic ... cast-iron/
What alloy (A/AAA or B/BBB) you need is based on whether you need the weld to be super strong, or super machinable. (the B's are the more machinable, the A's are stronger.)