Blue welding?

Welding uses/has a very high amount of energy. From E=hf, high energy implies a high frequency of light scattered/emitted. And blue light has a higher frequency than red, green yellow etc...

That's why yellow/orange flames are less hotter than blue flames.

 

Good question.
It depends on the molar density when comparing alcohol vs gasoline. You would need to use the ideal gas law to work out their temperatures.

My explanation above applies to one particular fuel.
ie. a blue alcohol flame is hotter than a yellow alcohol flame OR a blue gas flame is hotter than a yellow gas flame.

 

I'm not a materials engineering, but I believe that in welding (as opposed to complete vs. incomplete combustion, as alluded to by n0_3sc) you have a few different things happening simultaneously. The hot sparks flying off (which radiate black body depending on the temperature of the sparks), and the electric arcing--breaking down (and ionizing) either the air or whatever process gas is being used, and the material in the electrode.

Black body radiation (what happens when you heat something up to a certain temperature):
http://en.wikipedia.org/wiki/Black_body

Electric arc discharge:
http://en.wikipedia.org/wiki/Electric_arc

Flame testing (happens with various metal ions, including trace ones):
http://en.wikipedia.org/wiki/Flame_test

Hope this helps!

 

Your question needs to be more specific. What kind of welding are you talking about, MIG, TIG, Arc? Welding usually requires a shielding gas (typically CO2 or Argon) so nitrogen isn't a factor.

 

I think the reason that arc discharge in air looks blue (just like lightning), is because O2 molecules are easier to ionize (lower ionization energy) and therefore, ionized O2 emits blue color.
Am I right?