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.
Regards
Niekie Jooste
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.
Marty