welding a fireplace grate?

I have been MIG welding for a long time. I had always heard that you can't weld Cast Iron. A while back I saw a welding segment on a car show on TV. They showed that with a MIG welder, the weld will sort-of stick, but you don't get the same 'Bond' you get with MIG welding steel. They heated the area up with a torch and then used a high nickel content rod.