Chip the edges from major breaks of a casting with a hammer and chisel. Drill and tap holes over the surface of the cast iron with the tap and die set.
Screw steel bolts into the threaded holes so that 5 to 6 mm of each stud is above the surface. These bolts will eventually fill the gaps between each piece of the casting and will also help hold the weld together.
Align the pieces of the casting and mark the locations where the studs will fit into each piece. Grind out slots with an angle grinder in these locations so that the studs will fit into place.
Clamp the pieces of cast iron into place. Preheat the pieces to about 60 degrees Celsius with a blowtorch before you weld them together. Weld the pieces using metal inert gas welding with stainless steel filler wire. Use a low current and restrict your welds to one-inch-long segments if necessary to prevent cracking. Cover the studs with the filler wire and grind down any raised areas of the weld.
Wrap the part in an insulating blanket or bury it in sand to slow its cooling rate. Finish and paint the casting as needed.