Broken bolt removal

I was a Millwright for 33 years and what we usually tried to do was lets say for example it was a 3/8 broken bolt. We would take a smaller 5/16 ( grade 8 heavy duty ) flat washer and easily weld it to the broken bolt. then we would weld a 3/8 nut to the flatwasher. by then it was a little heat soaked and the broken bolt would easily come out.