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Up Topic Welding Industry / Technical Discussions / Use of Nitrogen as a Backing Gas
- - By apiguy (*) Date 03-28-2002 22:21
Has anyone ever qualified a procedure for the use of Nitrogen as a backing gas when welding SA358 type 254 SMO material ? The filler is ER-NiCrMo-3. I am an inspector and all of the welds I have inspected have failed. I have a person telling me that the procedure was qualified, but I have yet to see the PQR. The WPS specifies High Purity Argon. I realize the alloying effects of N2 on the stainless material, I would just like to know has anyone else tried to qualify the same set-up with any luck?
Parent - - By John T. (*) Date 03-29-2002 21:53
Sound like this "person" needs to qualify a new procedure.
Parent - - By apiguy (*) Date 03-30-2002 13:19
John T.,

I am having quite a bit of difficulty convincing the contractor that he has to requalify his procedure. Their position is that a change in backing gas composition is a nonessential variable. In my opinion, this is incorrect. The current edition of ASME IX section QW 408.9 gives a list of materials that apply. The base material is a P8 group 4 but the filler material is a P43.

The gentleman that wrote the contractors' procedures is a fellow CWI / CWE as well as an engineer, but yet every one of the procedures has contained multiple errors. I am rather confused as to what course of action I should follow.....
Parent - - By CHGuilford (****) Date 03-30-2002 14:56
Can you go to a higher level? client? boss? whoever? If you can then I would do that and I would bring anything I could find to support my position. Life is too short for a urination contest, especially when you never know when the wind direction will change. :)
Parent - By apiguy (*) Date 03-30-2002 15:07
My options are pretty limited...I on the other hand, am not an engineer. Therefore, it is very difficult to have any authority over someone with a higher level of "professionalism". I have tried to schedule several meetings with the powers that be.

It seems like this is turning into a classic engineering vs inspection "urination event".
Parent - By MBSims (****) Date 03-30-2002 20:23
A change in backing gas composition for P-8 base metals is a non-essential variable for the GTAW process. But if the WPS specifies argon as the backing gas, then the WPS must be revised to add nitrogen as an option, otherwise they are not following the WPS. A nonessential variable only means that the WPS may be revised without requalification, not that changes can be made by the welder without revising the WPS.

Parent - - By apiguy (*) Date 04-01-2002 12:41
This is one of the pitfalls of the ASME code in my opinion. By definition a nonessential variable is " A change in a welding condition which will not affect the mechanical properties of the weld." There are numerous references that deal with the alloying effects of nitrogen on fully austenitic stainless steels. There are many grey areas of the codes that are left up to interpretation. If you read QW-408.9 it reads that "For groove welds in P41 through P47 and all welds of P10I, P10J, P10K, P51-P53 and P61, P62 metals, the deletion of a backing gas or change in the nominal composition of the backing gas from an inert gas to a mixture including non-inert gas(es)."

My conditions are as follows:

P8 group 4 SA358 type 254 SMO base metal (20Cr-18Ni-6Mo)
P43 ERNiCrMo-3 filler metal (20Cr-58Ni min.-8Mo) basically

Original procedure qualifiled with High Purity Argon on the torch
and as backing gas.

At some point the welders switched to Nitrogen for backing gas.

Due to the weld material being a P43, it is my opinion the WPS should be requalified. The effects of the nitrogen will produce a "harder" weld as I am told by several engineers. Their position is that since the nitrogen is not as dense as argon, hence the weld will cool faster, producing a harder weld that is not fully austenitic.

Any information or suggestions are always welcome..........
Parent - - By Niekie3 (***) Date 04-01-2002 18:38
I have never used anything other than Ar for a backing gas on S/Steels. I do however know that some people use N2 mixed into the Ar for materials such as Duplex S/Steels because these alloys relly quite heavily on N2 as a strengthener and Ferrite stabilizer.

Your explanation of why the N2 will result in a harder weld (less dense etc) is not correct. It is a purely metallurgical alloying effect. It has nothing to do with cooling rates.

You stated that all the welds have failed. What were the failure mechanisms? Did they crack, show porosity on the radiographs or were they visually unsuitable?

While it is true that one can change non-essential variables without re-qualifying the procedure, it does not mean that everything that is allowed will work. Theoretically, you are allowed to do a lot of things according to ASME IX, but they will not all be suitable for your particular application. If you are finding problems, then as the inspector on the job, you should get all the relevant people together and resolve the issues. At the end of the day, the client wants something that will give him/her a good service life.

I believe your approach should not be that the gas is the problem. Rather state that you have a problem that needs resolving. (The welds are failing!) At that stage you need to go through some sort of a problem solving meeting where everyone is happy they understand the problem, and how they are going to try and resolve it. If that means going to the Ar backing gas then that is good. If it means that the welder needs to weld while standing on his head, then that is also good. If the resolution does not deliver the desired results, then go through the same ritual, untill the problem is solved.

Regarding the problem with qualifications, I can not comment on the American situation, merely on the South African one and in particular the environment in which I work. Each professional has his own job. Even though another professional has a higher qualification, it does not negate or in any way make the job of the lesser qualified professional inferior. It is merely different. You job is to inspect and ensure that system requirements are met and that the end product also meets certain requirements. If the product does not meet these requirements it is your job to reject the work. No fancy qualification can "talk the product right". It may be the engineer's job to solve the problem, but it is jour job to find the problem!

As a welding engineer myself, I always listen to what the inspector has to say, and also take his advice regarding what would be acceptable or not. At the end of the day, this is his job.

Niekie Jooste
Parent - By CHGuilford (****) Date 04-02-2002 14:16
Niekie, I like your approach and feel it is a good one. By bringing out only that there is a weld quality problem without going into the why's and wherefore's no one should feel they are being "attacked". They will be more open to resolving the problem.
As far as supporting the weld inspectors, you are familiar with welding and inspection. All too often the engineer-of -record is not familiar with welding other than what most product literature says. He/she may feel the pressure of deadlines and may seem to side with contractors regarding where the problems are. Usually they will listen to reason though, especially if what the contractor is saying does not produce satisfactory results. So you are right that the best that an inspector can do is to be "part of the solution, not part of the problem".
Parent - By MBSims (****) Date 04-01-2002 20:24
The weld material is F43, not P43. So you can't apply QW-408.9 to the weld metal. I see your point though - if the variable is important for P43 base metals why not F43 weld metals since they are essentially the same?

I think they are way out in left field on the cooling rate vs. gas density vs. weld hardness explanation. That makes no sense at all. Formation of chromium nitrides due to chemical reaction with the nitrogen at high temps would be a more likely explanation for the higher hardness.

As far as requalifying the WPS goes, they are correct that the words in ASME IX don't require the WPS to be requalified. But, they do need to address the issue of welds not meeting acceptance criteria and welders using a backing gas not addressed on the WPS.

Up Topic Welding Industry / Technical Discussions / Use of Nitrogen as a Backing Gas

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