Without providing the forces that the cap is supposed to resist, comparing two materials is rather difficult. That being said, I am going to try to provide some information that might cause you to rethink your position.
You make some comments that don't seem to make sense.
Cost of fabrication will likely be a main consideration. Casing a ductile iron piece requires making a sand mold for each piece. This will be more expensive than the completely common welding and cutting required to manufacture the cap from steel plate and pipe. Any machine shop can weld this piece, only specialty foundries will cast pieces. If you are looking at very large number of parts, the unit price may eventually favor casting.
Strength of metal is also a consideration. You don't mention what material strength you require, but any typical strength of ductile iron will be matched by commonly available grades of plate steel.
Welding is not an issue. I don't know why you "don't trust welds and ... doubt the same strength can be achieved by welding." This is a very vague statement. Common welds use 70 ksi filler metal. This will certainly be adequate.
To answer your direct question, there probably isn't a reason why ductile iron couldn't work in your situation, but you haven't made very convincing arguments as to why it might be better. In a cost comparison, a ductile iron casting will probably be more expensive unless a very large number will be made. At that point you will need to get cost comparisons to know for sure.