This is an old thread and there is another on the same sort of subject.
I have used the weld method for decades, never failed, not just into steel and cars, machinery cast and even Al and brass....and pipe fittings in valley covers.
I welded the nut on the bolt twisted and snapped. Good luck
If u break the weld that means u havint the penitration required for a good weld
When u weld a new bolt on...
taper the end of the bolt 1st...sort of the same way of welding heavy plate V ing out.
With a good punch, dimple the end of the broken stud...simply giving a few good wacks helps loosen
Turn the heat way up and feed right down..chances are the settings on the welder are for panel steel or something lighter.
One of the best 'tricks' told me years ago is spend the week before dismantling an old car or anyhting like that, use some good penitration oil and each night u come home from work go around and spray all bolts studs screws etc.
"Luck is directly related to the effort put in"
"Work hard at being lazy"
My Grandfather and Dad.