Purpose of nonessential variables? 1

[trottiey]

Why does Section IX require nonessential variables to be recorded on the WPS, if they can be revised arbitrarily without requalification? I'm looking at WPS's where the nonessential variable ranges have been extended beyond what can realistically allow a good weld. That would seem to reduce the nonessential variables to meaningless bureaucracy. It feels like there's a QA step that I'm missing.

 

[metengr]

Nonessential variables are variables that are recorded for information purposes and will not influence the metallurgical and mechanical properties of the weld joint in production.

Now back to one particular statement you made

I'm looking at WPS's where the nonessential variable ranges have been extended beyond what can realistically allow a good weld.

What do you mean by a "good" weld?????? What is good????

 

[weldtek]

trot,
I'm sure you'd agree that generally speaking, one could expect better results when specific requirements pertaining to the job at hand are clearly communicated to the welder.
Think of the WPS as the vehicle for that purpose.
While the term ' non-essential ' may seem to denote unimportant, this is not the case. I think if you reflect on some of those variables you'll see the danger in having a welder arbitrarily make changes to those variables at his or her whim. Consider proper use of the WPS as a part of controlling the process.

 

[trottiey]

A good weld would be one that conforms to the drawings, meets the code of construction, and passes NDE. I do agree with weldtek that some of those non-essential variables can influence the mechanical properties of the joint. I'm trying to understand where that control is supposed to happen if not through re-qualification.

For example, root spacing is nonessential for GTAW. But if the root spacing gets too big, the root reinforcement will exceed code limits, and root defects will become more common. Beyond that, excessive root spacing can make the weld just plain impossible to make. If an impossible root spacing maximum is recorded on the WPS "for information only," that would be what I call "meaningless bureaucracy."

It seems to me that there should be some mechanism that prevents revision of these nonessential variables to silly extremes, especially if the WPS will be used on a pressure boundary. I can't figure out what that control mechanism is. So my question, put another way, is where do these nonessential variables get checked?

 

[metengr]

trottiey;
Maybe another way to look at nonessential variables are those variables that allow the welder more flexibility in technique for welding production joints. Lets be realistic, welders want to produce quality work otherwise they become unemployed. By having nonessential variables, this allow the welder on the production floor some flexibility to produce a sound weld.

I look at nonessential variables as good design practices to increase the chances for the welder to make sound welds under adverse conditions. Does it mean if a welder has to go out of range for a nonessential variable that a sound weld is not produced? No. This is what I was trying to explain above.

 

[stanweld]

For the most part, essential variables affect strength and ductility of the weld as demonstrated through the procedure qualification. Non-essential variables affect the welders' performance to make visually and radiographically or ultrasonically sound welds.

 

[weldtek]

trot,
To answer your question, the non essential variables get checked by you if you're the originator of, or person responsible for revisions to the WPS. In the example you use, you are communicating to the welder the acceptable range for root openings. I'd agree, it would be a complete waste of ink and paper to record bogus information, but, why would you do so?
It sounds as though you're looking for some outside governing restrictions on these non essential variables. Why?
Establish what make sense for your welders for a particular application and use the WPS to communicate this info to the welder.

 

[trottiey]

Thanks for your answers. I would agree that when revisions to nonessential variables are checked by technical personnel based on real experience, then yes, they are useful communication tools. However, I have found nothing in code to prevent unqualified personnel from stepping in and opening up the nonessential ranges. In this situation, the communication value of the nonessential variables is lost.

I was hoping that I had missed something - maybe that WPS's needed to be signed by an engineer or somehow backed up by something real, but I am so far disappointed. The backup position is to hope that stanweld's explanation is accurate; basically that any problem caused by nonessential excursions would be picked up by NDE.

 

[Guest102023]

Don't look at non-essential variables as specific how-to-weld instructions for the welders, but outer bounds they may not exceed. Skill and workmanship are brought to the table by the welder, not the procedure. As always, 'sound engineering practice' is assumed/expected by the Code.

 

[Kiwi2671]

Trottiey,

From ASME IX Foreword

The Code does not fully address tolerances. When
dimensions, sizes, or other parameters are not specified
with tolerances, the values of these parameters are considered nominal and allowable tolerances or local variances may be considered acceptable when based on engineering judgment and standard practices as determined by the designer.

As Stanweld has said, non essential variables generally affect the welders performance, not the metallurgical properties of the weld.
Take root opening as an example - WPS states 2 - 5 mm but the actual production joint is 7 mm.
The mechanical properties of a weld with a 4 mm root opening (as per the PQR)and the production joint of 7 mm will likely be the same if not very similar.
What will be different is the increased chance of the welder getting excess penetration due to the increased root opening.
Regards,
BB

 

[gtaw]

Your question pertains to ASME Section IX, so my comments will be restricted to that code section. Bear in mind that Section IX is not a standalone document. The construction code, i.e., Section I, Section III, Section VIII, etc, may modify the requirements of Section IX.

As noted by others, essential variables have an effect on the mechanical properties of the weld. A change in an essential variable beyond the ranges permitted by Section IX will affect the mechanical properties, so the procedure has to be requalified.

The nonessential variable does not have a profound influence on the mechanical properties if the weld is sound, i.e., free of unacceptable discontinuities. The nonessential variables provide specific direction to the welder.

An example will help illustrate the point. The F number is an essential variable. If the manufacturer decides to switch to a filler metal that falls into a different F number grouping, the welding procedure has to be requalified. The filler metal classification is a nonessential variable, so if the manufacturer decides to switch from DCEP using an E6010 electrode to AC and an E6011 electrode, an editorial change to the WPS is all that is required. Changing the electrode does not materially affect the mechanical properties in the example provided. However, the change in the electrical current, i.e., DC to AC, requires a change in the electrode classification. That is information that is useful to the welder to prevent the wrong electrode from being used.

A basic assumption of ASME is that the individual assigned the task of designing a system or in this case writing a WPS is assumed to have the training and experience needed to fulfill the function. As you may have noted, that is not always the case when dealing with issue relating to welding. ASME's code sections are not "how too" manuals. There are certain requirements that must be met; certain restrictions that must be observed, but the code section does not tell the reader how to design a system or how to write a WPS. Scant direction is provided on how to weld. The selection of the proper filler metal is left to the manufacturer. The manufacturer determines proper joint designs with appropriate tolerances. The ranges of the nonessential variables are left to the discretion of the manufacturer. Again, little, if any direction is provided by the various code sections relative to how to do anything. It is incumbent on the manufacturer to ensure the individual assigned to developing welding procedures understand the technology as well as the code requirements.

Many of us have reviewed WPSs that are supposed to meet ASME Section IX, but fail in the practical aspect of being a viable WPS. The result is that welders simply ignore the WPS because they have learned from past experience the WPS is useless from the welder's viewpoint.

A thorough review of the WPS provides us with some insight about the technical competence of the vendor. A well-written WPS is an indication there is someone on staff that is technically capable or the vendor has access to someone, i.e. a consultant, who is competent to provide the needed direction. The listing of reasonable ranges for the nonessential variables requires a certain level of technical expertise that is can be missing from the WPS. A poorly written WPS or a purchased Standard Welding Procedure is a reasonable indication the vendor does not have the infrastructure needed to support their welding operations.


Best regards - Al

 

[stanweld]

trottiey,
Control of your WPS's should be defined in your Quality Manual. If you have ASME Code Certificates, it must be defined in your ASME Code Quality Manual. This will prevent unauthorized revisions to a WPS.