If you heat the knife you change the hardness and brittleness of the steel. Some heat is ok, but the bit which is easiest to effect is the tip, or the edge.
As I found out he hard way, you can overheat a fine edged tip on a buffing machine while trying to remove burrs.
But resharpenning can grind off the effected metal.

I wouldnt be using any of my knives as a torch, but I wouldnt have a Rambo type knife in the first place.