Use of Nitrogen backing gas with P8 materials in wet H2S handling environment?

[JPearson84]

Hey all,

This is my first post here, long time lurker though.

I'm a project engineer currently working on a process package for handling wet vapour (approx 10 mol% CO2, 2% H2S). My client's product specifications require that nitrogen is not used as weld shield or backing gas for p8 materials, however I have a sub-supplier who has completed fabrication of their component - a heat exchanger - utilizing a manual GTAW weld procedure utilizing nitrogen as a backing gass. As I was not familiar with the effects of nitrogen on weld quality I've done some quick searching through what is publically available on the internet and it seems nitrogen leads to increased hardness & ferrite levels - undesirable for our system's service but ultimately things that we are able to test for easily.

I wish to propose a deviation to our client's specification allowing for acceptance of the product as is, provided additional testing is complete to satisfy that weld composition, ferrite levels, hardness, etc are all within acceptable levels per NACE stanadards; however I know I have only a very limited understanding of the issues at play. I'm hoping someone with more knowledge can confirm whether this is a reasonable proposition or if I should just reject my sub supplier's product & request a rework?

Thanks,
Jon

 

[metengr]

This is an obvious nonconformance with your client’s specification, and it is entirely up to them to decide otherwise, regardless of how much testing and information you provide them. I don't think it will fly, and you would be better off to cut your losses and have it re-worked and follow the client’s specification.

 

[metengr]

Just to correct your post, nitrogen dissolved in austenitic weld metal at the root region will tend to form more austenite versus ferrite, possibly causing solidification cracking issues due to less ferrite. Review the post below for additional information;

thread725-19716

 

[metengr]

I found this paper that might be worth trying to defend your position for the nonconformance;

http://www.aws.org/wj/supplement/WJ_1975_01_s12.pdf

 

[SJones]

A simple oxalic etch test, as per the above paper, wouldn't be a good indicator of corrosion performance. However, it does seem a curious restriction by the end user, but they may have the belief that nitrogen pick up could be detrimental to corrosion performance and toughness. If these properties have been examined during qualification, you may have a case to build a brass neck and engage the purchaser with a view to a concession. Clearly rework is not ideal, so a friendly get together about the problem could save a lot of grief.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[stanweld]

You have not provided all of the appropriate information to make a sound recommendation. What is the grade of SS, chloride content of the "Wet" fluid, AWS class of welding filler metal? As metengr stated, nitrogen is an austenite former and the weld may have a fully austenitic weld deposit in the root bead which could lead to underbead cracking, especially if the filler metal is ER316(L).

 

[JPearson84]

Thanks for the feedback so far guys. Steve Jones hit the nail on the head - rework is not ideal but as stated I don't have the expertise to be able to review the whole picture. NCR's are already being issued as our sub-supplier has completed work prior to receiving approval to proceed utilizing procedures which do not meet the project requirements. However we would like to minimize the negative impacts (especially to my schedule!) if possible & acceptable from a technical perspective.

@Stanweld, the material in question is 316L, the chloride content of the fluid is <145ppm, operating temperatures are below 75C, pressures below 200 kPag, solution pH is estimated between 5.2 & 5.5, filler material is ER316LSi; if there are any other parameters that may be useful I'll dig them up.

Thanks all!

 

[metengr]

Suggest you hire an outside consultant that can work with you and meet the client. You won't do this over the internet.

 

[EdStainless]

Odd restriction.
We actually use nitrogen in weld gas to deliberately raise the N level in the welds.
It can suppress all ferrite formation (this could be a problem in heavy sections) and it noticeably improves the pitting resistance.

Look at most modern grades of SS and you will see N levels of 0.25 up to 0.50 in the base alloys. These need N in weld to prevent depletion in the welds.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JPearson84]

@metengr, thanks but I'm not looking for definitive closure of the issue on this forum - that will only come through discussion with the client. I was just shooting my problem out to the universe to see if the general consensus was that a deviation of this nature would be reasonable or if it would just be a waste of time. Based on the input received I am proceeding with the deviation request with the client. Ultimately the decision to allow the use of the device is with them & their material experts.

thanks all,
Jon Pearson

 

[metengr]

JPearson84;
Did you see the paper I attached? You can try to forward it your client for consideration.

 

[JPearson84]

@metengr, I did yes and that is forming the basis of our deviation request.

 

[stanweld]

I would suggest that you propose to your customer to confirm delta ferrite in the root deposited weld metal and/or confirm freedom from underbead cracking if deposit is fully austenitic. An adverse effect on the corrosion resistance should not be of concern, assuming that the weld is sound.