Essential Reading About Welding Procedure Variables … or is that nonessential?
Putting aside the facetious title for this article, let’s look at the importance of properly identifying all welding variables on welding procedure documents both for welding process control and code compliance.
As a young welding technology student, I listened to the Prof blather on about nonessential variables, Sect IX, PQRs, etc, etc and my reaction was … BORING! The payback for that was about a week into my post-grad career, I had a stack of ASME code books dumped on my desk and told to make sure the weld procedures I had to develop were in compliance with them. So, I learned the hard way … at the school of hard knocks. The goal of this article is to provide some guidance for newbies or the confused on how to document weld procedures with suitable reference to weld variables for code compliance.
Welding codes endeavour to assure weld quality by requiring all welding be done according to a documented and qualified procedure. These procedures list the exact or suitable range of variables to use in completing each weld so that they meet the required properties of the base metal. When welds are defective, it is most often due to process and/or technique variables not being appropriate for the application variables.
With all the reference to “variables” in that intro paragraph, it’s worth spending a bit of time explaining that. The word variable of course means that something is changeable, adjustable or can have values assigned to a particular item. Variable as it relates to welding is each discrete parameter that can have a value assigned to it. One method of categorizing welding variables is listed below.
Application Variables
This refers to the item to be welded and its characteristics including:
Process Variables
Which welding process to be used is a process variable along with items either unique to that process or common with other processes. Things to be selected or adjusted before welding begins to best match the application variables include:
Technique Variables
Once welding has begun, many variables can be controlled to influence quality including:
This is just one way of categorizing the confusing array of welding variables without identifying a specific weld process. This article will review and explain how welding variables must be identified in the procedures required for welding in accordance with the ASME BPVC, Section IX for pressure vessel, piping and nuclear welding applications. Codes for other sectors may have a similar system for identifying variables on welding procedures.
Table QW-255 below from Sect IX, lists a total of 51 variables in the 9 categories QW-402 to 410. In addition to the 9 categories in the left column, they are also classified as Essential or Nonessential in the columns to the right. Let’s ignore the middle Supplementary Essential column for the time being … I will explain that later. Another note is that QW-255 is for GMAW or FCAW only and there are similar tables for 14 other weld processes listed in Sect IX. The appropriate table must be reviewed in welding procedure development.
Essential vs Nonessential Variables
In the case of Procedure Qualification, essential variables are defined as “those which will affect the mechanical properties (e.g. tensile strength) of the weldment and shall require re-qualification of the WPS”. This means that the new or revised WPS will need to prove that it matches mechanical properties of the base metal by welding a test plate that is then subjected to destructive testing with results listed on a Procedure Qualification Record (PQR).
Nonessential variables must be listed on each Welding Procedure Specification (WPS) and are defined as “those in which a change may be made in the WPS without re-qualification”. In other words, a change significant enough to require revising or making a new WPS but would not require a new PQR as this change would not affect mechanical properties.
Welding Procedure Requirements
In requiring all fabricators to use only documented and qualified procedures for all production welding, it is appropriate for engineering, QA, and production personnel to spend time reviewing downstream welding for future projects. These discussions on how to best weld this new work is then followed by reviewing the existing WPS collection to see if documentation already exists to use for these applications (see earlier listing of Application Variables). If so, using existing WPSs is the most efficient route to code compliant welding.
Let's look at some scenarios with different requirements than an existing WPS with variables including the following:
There are many other WPS variables including gases and electrical parameters, but they will be within the same specified ranges for the new welding scenario.
Scenario 1
In the first new application, the only difference for the future project is that backing bars are not permitted with a complete joint penetration (CJP) weld required welding from one side only.
In reviewing QW-255, we see this change identified with item QW-402.4 as shown below. To expand on the symbol and Brief of Variables, see the legend at bottom of the table and/or additional information in Article IV. All variables for weld procedures are listed in Article II, all variables for qualifying welding personnel are listed in Article III and both articles reference the variables explained in Article IV – Data.
Seeing this new requirement identified as a nonessential variable means that the appropriate action will be to document a new WPS that does not require requalification. Instead, it just needs to reference the existing PQR to provide proof of meeting mechanical properties required.
Scenario 2
In the next future application being reviewed, the question is whether the existing backing bar WPS can be revised providing instructions for instead welding the same base metal but with a thickness of 7/8”. While the original WPS was for 3/8” thick base metal, in fact a range of thickness is qualified on a PQR test as specified in QW-451.1.
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In reviewing that table, we see that a PQR test on 3/8” would qualify for a range of thickness from 3/16 to 3/4" (2T). With the new application exceeding this range, that means this is an essential variable change and will require re-qualification with a new PQR. The logic here is that the thicker material will result in a faster cooling rate possibly influencing metallurgical properties of the completed weld. This requirement will mean additional time and expense for the required documentation and qualification testing.
Scenario 3
“OK, so what about those supplementary essential variables?” you ask. That column of variables can be ignored for fabrications where toughness is not a consideration. Think weldments subjected to only static loading conditions vs impact and cyclic loading conditions. Supplementary essential variables are defined in Sect IX as those for which other sections specify notch toughness testing and may be from Sect II - Materials or a construction sections such as Sect III - Nuclear or Sect VIII - Pressure Vessels. In these cases, any variable listed in the middle supplementary column gets included with other variables listed in the essential column. Both columns must be considered in reviewing essential variables when toughness is a code requirement. Let’s look at one example.
In going back to our original WPS, we are now considering a future application where it could be used with the exception being that as a large weldment, it cannot be rotated meaning some of the welding must be done in the 3G vertical position.
In reviewing the image above, in fact there are two paragraphs that could be used to identify this change from 1G flat to 3G vertical as a supplementary essential variable change requiring re-qualification with a new PQR. Of course, the position change per QW-405.2 is being made and that would probably also involve a change to a much slower travel speed with weaving technique thereby changing the heat input as per QW-409.1. This change in heat input would likely result in increased grain growth thereby influencing the toughness properties.
Summary of Variables for Procedure Qualification
Understanding all welding must be done in accordance with a WPS that is supported through PQR qualification testing (unless Standard Welding Procedure Specifications are used … explained later). Each of the variables that show up for the selected variables must be identified with a suitable value, range, or statement on the WPS.
In understanding that requirement, it is important to review the list of all variables for the chosen process and ensure it is identified on the WPS document. An example of an obscure, rarely used variable that is sometimes missed is peening as identified in paragraph QW-410.26. This mechanical stress relief technique involving repeated blunt hammering of the finished weld surface may not be used, but it does show as a nonessential variable in QW-255. This means you must state “no peening” or N/A on the WPS for code compliance.
Standard Welding Procedure Specification (SWPS) Option
There is an option to avoid the PQR in having WPSs qualified for production welding. QW-500 specifies an option for using SWPSs without needing a PQR but that comes with these cautions:
Performance Qualification Variables
With the above summary of variables, I bet you thought (or maybe hoped) this review of variables was finished. Sorry, but there is an additional listing of essential variables in Sect IX for the six processes that can be used in manual and/or semi-automatic modes.
The word “Performance” is used in reference to a person welding a qualification assembly that is then subjected to various tests to evaluate their ability to deposit sound weld metal. I hope you can hear me when I say that “sound” has nothing to do with imagining my voice boring you with talk about weld variables. Instead, this homonym of the word sound means that the weld is free from discontinuities and contains only pure weld metal.
Depending on the type and thickness of qualification assembly, the evaluation of the completed weld may be by radiographic, bend or macro-examination test methods. So, while the “procedure” essential variables influence mechanical properties, “performance” essential variables influence a Welder’s ability to make a clean weld.
Scenario 4
If you were paying attention earlier, you will remember that in scenario 1 there was a future application where the existing GMAW WPS with a backing bar joint design could not be used on a new fabrication that would require an open root joint design. This was identified as a nonessential variable for procedure qualification and while certainly needing a different welding technique, would not likely result in different mechanical properties of the completed weld (hint: no properties change if the Welder was able to make a “sound” weld).
So, the hint at the end of that last paragraph is the essence of the difference in essential variables for performance qualification. Any reasonable welder with a high heat input procedure and a steady hand should be able to make a sound weld in the flat position onto a backing bar. Remove that backing bar however and we now require a different skill set. Now the Welder needs to dial in a more exact set of electrical parameters to control heat input and read and react to what she/he sees while welding with a “keyhole” technique to make a sound weld from one side only. Thankfully, the listing of variables for the six processes are less confusing as there is a much shorter list of only essential variables without supplementary or nonessential variables. As you see in table QW-355, there are only 10 performance essential variables to be considered for the GMAW process.
Deletion of backing (as per paragraph QW-402.4) to weld instead with an open root technique is a performance essential variable and the Welder will need to re-qualify to use this technique.
That’s It - the Essentials About Weld Variables
Recycling some of this content from my earlier life as a welding technology educator, I will finish with three images to hopefully give understanding of variables and both procedure and performance qualification for ASME jurisdiction welding.
The first image below starts with production requirements for future work at the top and the steps to develop documentation, do qualification testing and test Welders to be ready for production welding. BTW, the reference to TSSA is the 3rd party jurisdiction here in Ontario, Canada that is responsible for ASME Code compliance.
The flowchart below shows the steps to having both documented and qualified welding procedures and qualified welding personnel to meet production demands. The sooner we get to the green OK boxes, the more efficient the whole process is.
And lastly, the table below shows the content and function of the three documents required for procedure and performance qualification.
Wait a sec … before I get off my ASME welding soapbox, I’ll take this opportunity to throw another dart at their awful WPS template. If you want Welders to actually use the WPS to guide welding, discard that confusing document and make your own WPS in a clear and concise format omitting all the clutter on that template. Here is a link to an earlier article on that topic.
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