when to use argon gas or liquid nitrogen during metal processing?

Argon is one of the "noble gasses" - Group 8 in the periodic table - helium, neon, argon, krypton, xenon & radon. These are, to all intents and purpose completey inert, not reacting with anything (in fact, they can be made to form a few compounds under very aggressive conditions, typically with fluorine).

Nitrogen is regarded as an "inert gas" as it is also very unreactive. However, it does react at high tremperatures with many metals (typically the transition metals used in steels, Ti,V,Mn,Fe,Cr,Fe,Co,Ni) to form nitrides. Most of these nitrides are not true compounds, but "interstital compounds" of no fixed composition, in which nitrogen atoms are trapped in the gaps between the metal atoms (the interstices). The small nitrogen atoms prevent the crystalline layers of metal from sliding over each other, thus producing a very tough, hard surface. You have probably heard of nitrided steel and case hardening - used in situations where wear (oops, sorry) is a big issue - gears, steering racks, camshafts, tool dies, etc.

I hope this is of some help.