What kind of rod and welding process is best for welding cast iron and avoiding grain structure distortion and progressive cracking?

Gas welding has a more concentrated heat zone not so?

Interesting website that might assist you. Good Luck!
http://www.locknstitch.com/CastIronWelding.htm

I heard need to preheat and post heat to avoid cracking after cooling.

Yes, most likely a rather high temperature preheat will be required during welding, and a post weld heat treatment may also be needed.

Cast Irons are nasty to weld. Your best bet is to contact an experienced welding engineer (different cast iron grades need to be treated differently, and some you're better off just replacing rather than trying to weld).

Once you find a welding engineer who can specify your repair method, you need to make sure you qualify the procedure, and use a highly qualified welder to do the work.

Kim Williams

V the crack out by pneumatic chipping. Preheat casting to 900*F, wrap casting in heat blankets, braze weld area. Post heat in an oven to 900*F for one hour, reduce temperature in 100*F increments over the next nine hours down to 100*F, turn oven off and allow to cool to ambient temperature. The most important step is to say a prayer before hand and keep your fingers crossed in the process. No one in the right mind will ever guarantee a cast iron weld repair.