Nitrogen Purging of CS pipe for TIG

admin

Anybody ever use Nitrogen to purge carbon steel pipe?Weldtek
Reply:Why?You do not need to purge the inside of carbon steel pipe when tig welding your root with 70S-2 filler.It is not like tig welding stainless pipe where purging is required to prevent sugaring/porosity issues.Purging carbon steel pipe just adds to the time and cost of the welding job. IMO.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:Originally Posted by snoeproeWhy?You do not need to purge the inside of carbon steel pipe when tig welding your root with 70S-2 filler.It is not like tig welding stainless pipe where purging is required to prevent sugaring/porosity issues.Purging carbon steel pipe just adds to the time and cost of the welding job. IMO.
Reply:Hello Snoeproe, the nuclear industry used to purge a lot of carbon steel piping systems on their superheated high-pressure steam lines in some places, as I recall. Nitrogen for carbon is less expensive than argon so that might be an additional reason for it's use. Additionally, there are cases where stainless steel piping is purged with nitrogen, apparently the addition of nitrogen in some cases has a beneficial result for the weld deposit. I am sorry I can't remember the specifics of that benefit in order to spell it out here. Best regards, Allanaevald
Reply:I work in a pulp/paper mill with a high pressure power plant that produces super heated steam (900psi). We dont purge any of our carbon steel pipe when tig ing the roots. The nuke industry must be different. Good info.JasonLincoln Idealarc 250 stick/tigThermal Dynamics Cutmaster 52Miller Bobcat 250Torchmate CNC tableThermal Arc Hefty 2Ironworkers Local 720
Reply:Originally Posted by snoeproeI work in a pulp/paper mill with a high pressure power plant that produces super heated steam (900psi). We dont purge any of our carbon steel pipe when tig ing the roots. The nuke industry must be different. Good info.
Reply:Hello again Snoeproe, I did a bit more investigating into the "nitrogen" purge application. Some rather interesting stuff that I apparently didn't pay attention to very closely when I had first heard of it. Hope that I can relay this correctly. Obviously argon is an innert gas and thus it works well for arc protection when GTAW is utilized and also when a purge/protective gas is needed. Nitrogen on the other hand does not possess the ability to fully remain neutral as a shielding gas, yet as a purge gas, as long as it is not in direct contact with the welding arc it will serve a similar purpose. For open root welds though, nitrides will be generated and can cause cracking issues or possible hardening of exposed materials in carbon steel applications. Stainless steels are another issue altogether. There are certain metallurgical results when using a nitrogen purge that can be beneficial and desirable, thus the nitrogen is both a cost savings and to a certain degree a requirement. Good topic for discussion. Best regards, Allanaevald
Reply:Thanks for the info. I have a customer in the refrigeration business who uses nitrogen to purge its copper brazing work.  They are developing a CO2 refrigerant system, which has to be pressure tested to about 1500 to 2000 psi.  I believe the operating pressure is between 900 -1000 psi.  They want to use nitrogen for the obvious reason, they already have it manifolded into the building.Aevald, could you elaborate or give sources?Thanks a million!!WeldtekWeldtek
Reply:Hello Weldtek, I am a member of a couple of other forums, I believe I have found some posts from a number of very savvy individuals which might explain some of this much better. Following are some quotes from posts regarding Nitrogen for purging purposes. One of the essential resounding comments has to do with whether an arc will be exposed to the nitrogen purge gas, if that happens there can be changes in the basemetal/weldmetal where the exposure occurs. Hope some of this can be of interest or use to you. Best regards, Allan" The NFPA has a brazing standard for medical gases used in hospitals, etc.. BCuP filler metal, no flux, with a nitrogen purge is often used when brazing copper pipe."  "Nitrogen is fine for a backpurge on SS, but one needs to realize that when doing a root pass using the TIG process, the pure nitrogen will cause possible breakdown of the tungsten causing very likely tungsten inclusions. 100% nitrogen is also not recommended as a back purge for the Duplex SS. Nitrogen, being a strong austenite former, will lower the ferrite levels to possible unsatisfactory levels. It's just hard to beat good ole argon for a backpurge for SS, even the Duplex grades.""Nitrogen typically a diatomic, "two atoms" per molecule. Nitrogen in the diatomic form is usually insoluble in molten stainless. However if the nitrogen gets into the weld arc, the plasma arc energy can seperate the diatomic molecules and create monatomic molecules. The monatomic molecules are soluble in the weld. The nitrogen, monatomic (seperated molecules) become an alloying element and can reduce the ferrite in a stainless weld. A reduction in ferrite in some alloys can cause the weld to be more austenitic and sensitive to hot cracking. If nitrogen enters a weld or the welding arc, it can have a negative and sometimes a positive influence." "Our local Union gives classes in med gas every year and recerts people every year. I have not attended the class but know many who have. From what I understand there is no specified brazing rod that is required I have seen 5% and 15% silver rods used. I do know that the lines need to be purged with nitrogen and full penitration is required.""With closed root, austenitic stainless welds, as found in tanks, corner, edge welds, or thin gage, partial penetration tube welds, nitrogen is the logical, economical, purge gas choice for all austenitic, duplex, martensitic and precipitation hardening stainless steel applications. The only concern would be a few specific, ferritic alloys in which nitrogen could cause severe weld mechanical issues".aevald
Reply:Allan,Thanks a million.  This is the kind of info I was looking for.Weldtek
Reply:Purging for carbon steel is unnecessary.Superheated high pressure lines are probably not out of carbon steel but low alloy chromium steel, these do require purging.Concerning duplex steel (UNS S 31803/1.4462) nitrogen as a backing gas gives slightly better pitting resistance than pure argon. The best is to use Ar+1.5-2%N2 as a shielding gas and either Ar or N2 for backing gas. For super duplex Ar+1.5-2%N2 for both shielding and backing is preferable.The reason nitrogen is beneficial to duplex steels is that it replaces some of the nitrogen of the metal lost during tig welding.
Reply:I asked this question one time about purging SS because I had three bottles of Nitrogen  hanging around and figured I might save a few bucks on Argon by using it for my SS welding.........Learned that Nitrogen can make problems for SS and stayed with Argon.......Ended up using the Nitrogen for my air shifter on the drag bike....  Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock