Carefully observe with stainless steel screws. When a trace of rust is found leaking from the fracture after tapping, the M18 nut can be placed over the surfacing stud and the two are welded together.
After welding, it is cool and hot. Use a plum wrench to wrap around the nut to twist it back and forth. You can also twist it back and forth with a small hand hammer to knock the end of the nut so that the broken bolt can be taken out.
After removing the broken bolt, use a suitable wire hammer to machine the thread in the frame to remove rust and other debris in the hole.