Can an old millwright teach you a little trick. I’m assuming it’s a 3/8 bolt. If so take a 5/16 common heavy duty flat washer and plug weld it thoroughly to the ground flat broken off bolt. Then weld a nut to the welded on flat washer. The heat should make this easily removed with a wrench on the welded on nut. I’ve done 100’s of them in the mill. Much easier than trying to plug weld the nut on.