Purging is a term used when utilizing a gas on the backside of a weld. Generally to keep it from oxidizing, but in retrospect it is to keep the microstructure of the entire weld the same. An inert gas must be used to keep the materials from reacting. What gases are inert? The most common inert gases are Argon and Helium, followed by Neon, Krypton, Xenon.
Argon is the most commonly used purging gas due to its low cost compared to the other gases that are as much as 1000X the cost. The best thing about Argon is it displaces the gases you do not want near your weld pool, such as nitrogen, carbon dioxide, and oxygen. When purging a positive pressure is preferred, no where near even a single PSI but a positive pressure will displace and remove remnants of other gases. Stainless Steel has the best tell tale of a good purge, colors from silver to a bright purple (silver being the best) show a successful purge.
Contrary to popular belief Carbon dioxide and Nitrogen are not inert gases. When used as a backing gas for stainless steels nitrogen will form nitrides. These nitrides can go very deep into the weld, depending on how long they were exposed to the molten pool. Nitrides increase the hardness of the steel and remove its corrosion resistant properties. Carbon dioxide will also show the same results but will often oxidize faster, as when it mixes with water or moisture it produces an acid referred to as carbonic acid. The tell tale of a bad purge is a dull colored weld, also known as sugaring.
The properties of nitrogen in stainless steels could be seen as both bad and good. Certain amounts of nitrogen (generally <.4%) can increase strength, ductility, and wear resistance without jeopardizing corrosion resistance. These alloys are now referred to as nitronic 30’s, 40’s, 50’s, and 60’s. However they are engineered to evenly distribute the nitrogen into the grain structures, they are not easily produced. If you attempted to purge a stainless steel weld with nitrogen you would not see this effect, it would supersede the .4% and remove its corrosion resistance properties.
Mild steels are much less susceptible to these hardening and corrosion loss properties. A slight hardness increase will be seen if purged with nitrogen or carbon dioxide, but not a devastating change. This is also seen in common shielding gases for mild steel. The lowest cost shielding gas is carbon dioxide, which can only be utilized in the GMAW process due to its reactive nature.
For more information on the Process of Purging or if you’re looking for supplies for your next welding job, check out bakersgas.com for all of your welding supplies needs.