It is not preferred to weld material in normal atmospheric gases, either we need to provide any shielding gas or produce it while welding; so that the weld can be done without any impurities. These impurities or oxides (formed in contamination with oxygen at high temp.) decreases the mechanical property of the metal or alloy which may effect the same in long run. There are many kind of shielding gases out of which few are inert and others are semi-inert gases, which basically doesn't produce any harm to the metal during welding moreover they protects the metal from contamination by other gases. Inert gases basically used are Argon and helium gases; whereas semi inert gases are nitrogen, carbon dioxide etc.

Basically a beam of non-reactive nitrogen gas is thrown at the welding to shield the weld so that no oxide formation takes place. This increases the weld penetration and arc stability. Moreover nitrogen helps in prevention of metal/alloy from pitting and nitrogen loss from metal. Hence increasing the metallic property of metal.