if one where to (hypothetically) take a cathode ray tube from a television set, remove the front, put it in the necessary vacuum etc etc and put a piece of metal in front of it, would this kind of cathode ray have the power to melt the metal, in the same way the electron beam welders do?

Only if it was a very small bit of metal, or one that was very easy to melt.

Having said that, because the metal is in a vacuum there's no way for it to lose heat by convection or conduction (throughh the vacuum) so it may take less heat to melt than you would normally expect.

The EBW power supply pulls a low current (usually less than 1 A), but provides a voltage as high as 60 kV in low-voltage machines, or 200 kV in high-voltage machines.

 

http://en.wikipedia.org/wiki/Electron_beam_welding#Equipment

 

TVs are generally lower energy, and probably don't focus the spot to as small of an area.