Weld Question - Qualification vs. Testing 3

[AggieYank]

Please offer your opinion on this. I have a situation that I need to resolve. Does the following seem right to you?

As a general rule, welds are either pre-qualified or qualified by test to ensure that it is possible to achieve a solid weld using that specific joint configuration. If every joint on the jobsite that is welded is prequalified, very little or no non destructive inspection may be necessary. If a joint is welded up, but it is not a pre-qualified joint and has not been qualified by testing, then it would be necessary and would be acceptable to inspect by means of non destructive testing such as UT every similar welded joint to ensure that the weld is acceptable per AWS. Meaning that because the weld doesn’t have any discontinuities greater than the max acceptable per AWS, that the weld is essentially a solid weld, despite not having been a qualified weld.

 

[evelrod]

Pre qualified welds by definition are generally arrived at by test ( in some time frame, present or past).
From practical application, in my experience, NDT is just part and parcel of every critical structural weld regardless of test or qualification. There are exceptions, of course, but like I said, virtually every structural weld I made in my 38 years of practice was UT'd or X-rayed.

Rod

 

[metengr]

If every joint on the jobsite that is welded is prequalified, very little or no non destructive inspection may be necessary.

Why? All prequalified means is that the welding procedure used to produce the welds has been qualified for production, this has nothing to do with NDT of actual production welds. One can always specify NDT of welds prequalified or otherwise because most Codes and Standards provide only minimum requirements, one can go beyond this.

 

[AggieYank]

metengr: To answer your question about why less testing may be required for pre-qualified welds made by a qualified welder. Both the joint and welder have been shown to be succesfully used in the past, meaning it's likely the joints will be successfully made in the future.

For an unqualified joint, it has no history of successfull use in the past, so you'd want to examine every joint, regardless of whether the welder was qualified or not.

Same thing would hold true for an unqualified welder making a qualified joint. You'd want to test every joint based on the welder not being qualified to make those welds.

 

[metengr]

AggieYank;
I understand the concept behind pre-qualified weld joints. I look at things from a workmanship standpoint, as well. You can have the best weld procedure qualification around, but welder execution is critical and that was why I mentioned NDT. Trust but verify.

 

[SJones]

It's not the joint that is 'pre-qualified' - it is the welding procedure specification. There is more to a weld than joint design plus NDT won't tell you about the mechanical properties or the toughness of the weld zone.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[metengr]

SJones;
Correct, I was referring to the prequalified WPS not the joint. I am concerned with weld quality in addition to mechanicals. However, in the context of welding carbon or low alloy steel for structural applications, the NDT plays a bigger role in my opinion.

 

[Dinosaur]

AggieYank,

You don't have a complete understanding of the QC process for welds.

Prequalified joints are those that though a long use history have proven they have a reasonable chance of being sound if the welder takes all the appropriate steps in preparing the joint, selecting good base metals, proper consumables, uses proper preheating and so on. If a joint is not prequalified, there is a process for qualifying the joint spelled out in the AWS code (I presume this work is controlled by AWS codes). All this has to do with prequalification before production work begins.

Actual production welding is subject to 100% VT and varying amounts of RT, UT, or MT in most situations. This is necessary to verify the welder is doing all his stuff correctly and that you don't have some unknown variable causing problems. For example, (and this did happen on one of my jobs) you could have a WPS that you have used for years with a qualified operator and everything, but one of your consumables is contaminated. As a result there is unacceptable porosity. This is one example why you need weld inspection including NDT on the job, regardless of whether you are using prequalified joints or not.

Good luck.

 

[dbooker630]

I found these words of wisdom in a welding journal about Qualification vs. Quality Control. I posted them on the wall in my office.

"The aim of Qualification is to demonstrate that a welder is capable of making satisfactory welds."

"The aim of Quality Control is to ensure that the welder's resolve to make satisfactory welds doesn't lapse!"

 

[BUTCHCWIQAM]

Dinosaur
Hit it on the head... perfect answer
could have said it better myself

 

[seaduck]

You can't expect someone to do a good job but you can inspect them to do a good job

 

[HgTX]

You can't inspect them to do a good job either. The good job is either there or it isn't; quality control may or may not catch a good job.

Hg

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