I've had great success in taking a nut (say 5/16" ,1/2" wrench size), setting the wire feed on "stun", and welding through the center of the nut to the broken bolt/stud until you weld the nut flush. While the nut is still red hot, get a wrench or socket on it - I'll bet they'll come right out. This works great if it's broken off flush, and if you're decent with the wire feed, even recessed a ways.