Increasing carbon content leads to a loss of weldability. I think more than 1.7% carbon it becomes "cast irons".

There are many diverse grades of cast iron.  Some will weld with care, some with more care, some not at all.  Gotta know what you are up against if there is any chance lives, health, safety, could be considerations.

Even without care for safety, not recommended unless you know the exact grade and what you are doing.

He Is In Control, Have a Great Day,  Brent