To be clear, superheated is a technical term which means a fluid at boiling temperature at such pressure that it remains a liquid. I will disregard this here.
Heat is energy. Adding energy to metal causes it to do two things:
-Changing the intensity and spectrum of em-radiation.
In simple terms, hot metal starts to glow well before it becomes fluid. It starts with a faint reddish glow at $500°C$, turning to bright white at around $1300°C$
-Changes the vibration intensity of the atoms.
Atoms, like more complex molecules, vibrate, and the harder they vibrate, the easier the material is to manipulate. Steel at $700°C$ and above will be much more easy to bend than cold steel. At $1000°C$ (this is roughly the temperature jet fuel burns at) it might even bend if you swing it around too hard. This means that the blade cannot be heated at such temperatures that it glows visibly, and still retain usefulness as a combat weapon.
At a $100°C$ it is unlikely to bend easily, but at this point it will just cauterise any wounds it causes, and is unlikely to do any kind of damage that a cold blade will not. It will burn the skin, which can cause infections in the long run, but the long run is not what a knife is for.
It unlikely to provide a useful amputation tool, as blistering caused by burns in a combat situation can be dangerous, even deadly.
A tourniquet and sterilized blade are a better idea.