Fisher tops are 3/4 of an inch thick so you shouldnt be welding on any cast iron with that small repair. If i was to repair it which i would do some serious thinking before i did but i would only preheat the hardie area with a torch and be quick to do really short welds and dont focus the welds in one area, kinda jump back and forth from one edge to the other. Try not to get the surrounding area over say...500°f to not effect the heat treat of the face.