What filler material are you using? We had good luck with a combo weld with mild steel tacks and then 309 rod to fill in on top of it.

 

i just hit mine up w/grinder and went slow speed high heat flux-co *****.
although i was welding to a tube, u was welding a flange on.

 

Heat it and use nickel rod. Then cool it by burying it in sand.

 

DONT do this, because its NOT cast IRON.

definately clean it, and definately preheat it. and then run 1" long beads alternating back and forth on opposite sides of the pipe, 180* apart untill they meet. Keep going untill its all done, use the highest amperage setting, and focus most of the heat on the cast portion, allowing the puddle to flow into the pipe, as was said above. If possible, put the ****** in a preheated oven when done and slowly cool it down to room temp.