I wouldn't use ferroweld, everything I've read says its unmachinable. They make a couple of softweld rods that would do better. Preparing the piece for welding is the most important part of the job. This includes carefully grinding a groove that will be filled back in with weld material, drilling a small hole to stop the crack from further expansion, pre-heating to the black hot stage, then laying short beads to fill in the groove. Finally, cooling the piece down must be done slowly, as in over 8 to 10 hours.
Fusion