one other trick i found is to weld a nut over the shaft. Then while it is still hot from the weld try and unturn the bolt with a socket or wrench. I have had luck with this method. The trick is to get the weld strong enough, get the nut somewhat centred and also not try and weld the nut to anything else but the shaft.
I've had a few shafts keep breaking off more and more with every new nut i welded on till it was flush down to the surrounding plate and i still was able to get it out this way!
