Heat is a huge help, if you can apply it properly, as Kutter advised. I have a friend who is a master welder, gets called by nuclear power plants to do things that "can't be done". On my farm I have wrung off exhaust manifold bolts more an once, mostly because I just didn't have the time to wait for weeks while penetrant did it's work. He has welded a "handle" onto the remaining piece of bolt, way down in a blind hole, and screwed it out every time. It's the heat that breaks it loose.

SRH

May God bless America and those who defend her.