Use of nitrogen as a purge, (nitriding?)

Date 07-14-2004 12:49

I've been questioned about the use of nitrogen for a purging agent in natural gas piping. Not being a metallurgist I refer the question to those who may be able to find out.

I have no other information than what I have provided. Skimpy to say the least. To err on the safe side I recommended argon.

The question, does the use of nitrogen for a purge while welding create nitrides and is this beneficial, detrimental, or does not make a difference to the weld?

Forgive the wording because I really don't know how to ask the question. Hopefully, it will give you an idea of what I'm trying to say.

By Bill A

Date 07-14-2004 19:19

we have no restrictions against welding on nitrogen purged natural gas lines and have never heard of anyone that has seen any evidence of nitriding on those kinds of welds (at least not to the extent that it influenced weld performance). Parts that are intentionally nitrided to improve surface properties are heated for a lot longer than weld is. I suspect that any evidence of nitriding during welding would be found to be very superficial and inconsequential, but I have not done the testing to verify that.

I suspect that a grossly contaminated weld would have lower toughness caused by aluminum nitrides and similar precipitates in the steel.

By Niekie3 (***)

Date 07-18-2004 19:18

Hi Steve

I assume you are welding on austenitic stainless steel piping? This is a topic that has been debated often. I prefer to use Argon, but many companies use N, especially on large pipes where Ar may be too costly. While it does lead to nitrides on the surface, it appears to not be so bad as to lead to unacceptable welds. At any rate, you should qualify a procedure using this gas. This will give you a good idea of what the results are.

Regards
Niekie Jooste
Fabristruct Solutions

By stever

Date 07-19-2004 15:29

I appreciate the information that has been given.

The natural gas lines are carbon steel. No, I've not been given any ID numbers for the pipe that would give more info.

I have been reading up on nitriding and and thought that maybe the weld puddle could absorb the nitrogen which could cause cracking due to hardening.

What I've gathered so far from the answers to this question is that the welding does not get hot enough to absorb the nitrogen. If it does happen to create nitrides it will only be on the surface of the exposed weld.

Thanks guys. I'll pass along what you've posted here.

By Niekie3 (***)

Date 07-19-2004 15:45

Hi Steve

If you are working with C/Steel, then I can only imagine that the purging is required to make sure that there are no longer any flammable (explosive) gasses present in the pipe. As such, N2 should not be any problem, because C/Steel is normally welded without any backing gas. It is usually welded with air in contact with the root of the weld. As air is 80% N2, changing this to 100% N2 will really not make a difference. You will only reduce the surface oxidation slightly, which can only be a good thing.

Regards
Niekie Jooste
Fabristruct Solutions