If you made the bolt head red hot you drew the hardness down and promoted the deformation. Next time try heating, a paint heat gun is convenient, and immediately spray with cold water. Repeat several times. The last time don't spray it with cold water, put a six point socket or wrench on it and wack that with a hammer to break it free.
Since its lock tighted, and deformed, bite the bullet and weld a much larger nut onto the deformed head and use method above.