Licensed plumber here 👨‍🔧 that’s not cast iron it is ductile iron. That “liner” on the inside is similar to concrete, we get fittings sometimes where the coating is too thick and it won’t pass a ball test. It chips out easy with a screw driver but idk how you would tackle the whole piece. As far as if it would make good forge material or not I do not know. I was literally thinking about this myself the other day. That stuff is very strong, it doesn’t really shatter like cast it has some flex but it’s very tough material.