Argon back purging

[mthakur]

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.

 

[metengr]

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.

 

[stanweld]

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.

 

[racookpe1978]

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.