There is also a product made by a company called X-ERGON that is specifically designed for this purpose called X-TRACTALLOY. It has a flux that will not stick to threads. I have used it many time to pull broken head bolts and others from industrial equipment. It runs at about 70 amps for 3/32 diamater rods. Take a flat washer bigger than the bolt by about one size, place it over the broken bolt (I usually offset it a bit like an eccentric tack it to the top of the bolt, then weld the nut to the washer in a couple spots. Let it cool for just a couple seconds and carefully turn it out with and end wrench. If it doesen't come the first time dress it a bit, get another washer and bolt and try again.