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Argon back purging

Argon back purging

Argon back purging

(OP)
Hi,
Recently I came across a very old WPS for welding alloy steel (ASTM A 335P22 to A 335P22). The WPS recommended use of argon back purge? Is that common to have back purge and what is the advantage of having it while welding. I am aware of back purge in stainless steel and risk of oxide formation due to temp.  

RE: Argon back purging

mthakur;
For the lower chromium content (5% or less) Cr-Mo steels an argon back purge is not necessary. I have on occasion seen it used for extra cleanliness requirements but these are rare. The back purge simply protects the weld root from exposure to pick-up of oxygen and nitrogen, while protecting the surrounding base material on the ID side of the weld joint from local oxidation.
 

RE: Argon back purging

In the early '70s, the largest Energy firm required backpurging on P22 piping. Those requirements spread to other Owners and were "adopted" by Engineering firms. While there is a benefit for the use of the argon purge, it is not necessary to provide sound welds in most services.  

RE: Argon back purging

I understand the "Lower chromium content steels" is the key word and tricky phrase above:  An Ar purge is needed for TIG welding most stainless steels (if you are not going to back grind and clean up the inside weld passes) but not for regular carbon steels.    

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