Here is my brief experience with welding DI castings......
I have welded on DI castings before, but was told the weld would not be considered a structural/load-bearing weld. You mentioned this in earlier, regarding the carbon. We welded on castings to rework cosmetic issues, to fill voids and/or other flaws.
The filler was expensive, high nickel, as well as the weld procedure. We had to spend a lot of time cleaning! Preheating and PWHT was also exhausting, we had to preheat the entire component to 650 F and sustain this temperature for1 hr per inch of thickness. We then welded, and immediately began the post weld heat treat process at 650 F while decreasing the temperature at a rate of 50 F per hr until ambient temperatures were reached. We obviously had to use an oven.
In my humble opinion, I would stay away from this material, regarding welding, and use an alternate material with better weldability. There is a lot of money and effort spent on making a non-structural weld.
Good Luck!