H2 + O2 ----> 2 H2O
The chemical bond of water is polar covalent. Ionic dissociation of water occurs readily in solution to form the ions:
H2O ---> H+ + OH-
This occurs to the extent of one in 10 million molecules of water.
However, during welding, you are probably more concerned with the radical bond dissociation energy. The energy of an O-H bond is about 463 kJ/mol. Therefore, to break both bonds of water would require 926 kJ/mol.
Nothing would happen if you used water vapor for welding because water is the thermodynamic product of oxygen oxidation; therefore, you would not get any energy out if it were to react with oxygen. In fact, you'd probably have to supply an external energy to drive that reaction forwards.
I hope this helped.