Supplementary Essential Variables As Per ASME Section IX

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This article addresses the subject of supplementary essential variables on a WPS, in accordance with ASME Section IX. Various supplementary essential variables, that appear under most processes under QW 250, have been discussed in some detail. At the end, a small portion is included to explain how to carry out requalification of a PQR when one or more of the supplementary essential variables is changed.

supplementary essential variables for procedure qualification

What Is A Supplementary Essential Variable On A WPS?

Supplementary essential variables on a WPS are those that become applicable only when toughness is a consideration. This signifies that a change in these variables affects the toughness properties of the weld metal.

When the referencing Code, standard or specification requires toughness testing, the supplementary essential variables become additional essential variables, besides those that are already designated as essential variables.

The referencing construction would specify toughness testing for base metal, weld metal, and sometimes heat affected zone of the weld too. When this happens, the variables that appear under the column ‘Supplementary essential variables’ in QW 250 must be observed, and actual values followed in the procedure qualification must be recorded.

Any WPS that is prepared with the support of such procedure qualification must then address all the supplementary essential variables.

The supplementary essential variables are applicable to both groove welds and fillet welds.

In addition to the essential variables, the supplementary variables further narrow down the qualified ranges of a procedure qualification, in accordance with the stipulations of QW 250. Any change beyond the qualified ranges will require re-qualification of the procedure.

When toughness has not been specified by the construction code, then the supplementary essential variables are not applicable, and need not be addressed on the WPS. It may also be noticed that supplementary essential variables exist only for procedure qualifications. For performance qualifications, no such thing exists.

A few of the prominent supplementary essential variables have been discussed below:

Filler Metals

QW 404.12 is a prominent supplementary essential variable, which imposes further restrictions (beyond QW 433) on the choice of filler metals that can be used in a production joint when the procedure qualification has been done with a certain filler metal.

This variable has been discussed in some detail in another article, which may be accessed here.

PWHT Temperature Range

QW 407.2 is an important variable that finds a place as a supplementary essential variable under most of the processes of QW 250. It basically says the following two things:

The term “essentially same” means that the temperature of PWHT of test coupon, and the soaking time spent by test coupon at that temperature, and the rate of cooling and heating of test coupon must all be same as the values of these parameters in the production weld.

  • The time spent by the procedure qualification test coupon at soaking temperature shall be that least 80% of the soaking time that the production joint would see. “The PWHT total times(s) at temperature(s) may be applied in one heating cycle.”

If the production joint has to undergo several rounds of PWHT at different stages of fabrication, then the aggregate soaking time of all those PWHT cycles must be calculated, and the soaking time given to the procedure qualification test coupon must be at least 80% of this time. Also, the PWHT given to the test coupon can be in a single cycle (and need not be simulated one after another).

Interpretation IX-18-33, is contextual to the above discussion.

Note that this 80% rule only appears in QW 407.2, and does not appear in QW 407.1. This means that this rule has been envisaged only when toughness is a requirement. In QW 407.1, there is no other stipulation with regard to soaking time. This implies that when toughness is not a requirement, the soaking time applied to the procedure qualification test coupon does not need to have any proportion to the soaking time that would be encountered in the fabrication of the production welds.

In other words, the PWHT soaking time given to a production joint can be any length of time, in either one cycle or multiple cycles. For example, a PQR qualified with a PWHT of half an hour can be used to support a WPS with a 4 hour long soaking (for a 8 inch thick P-No. 1 material production joint, say). Interpretation IX-83-130 contains a similar kind of question, and the reply given thereto affirms the above understanding too.

This, it must be said, does not sit too well with common sense. How can a ½ hour soaking in PQR support four hours long soaking for production weld?

Heat Input

Heat input is another prominent supplementary essential variable that comes up for consideration whenever toughness is a consideration.

This variable has been enshrined in Section IX under QW 409.1, and says that – “An increase in heat input, or an increase in volume of weld metal deposited per unit length of weld, over that qualified…” is a supplementary essential variable.

Simple mathematical formulas have been given to calculate heat input and volume of weld metal.

The rule is simple enough, and does not require dissection. Let us explore some other aspects of this variable.

QW 409.1 restricts increasing the heat input whenever toughness is a requirement; this signifies that increase in heat input has a deleterious effect on the toughness of the metal.

The reasoning behind this goes something like this: High heat input results in a lower cooling rate of the weld, thus leading to coarser grain sizes. Coarse grain sizes are not good for toughness of a metal. A fine grained structure makes for a more compact packing, and thus resists impact loads better.

Common Oversight In Applying QW 409.1

There occurs a common oversight while considering QW 409.1.

Consider this: Groove design (V-groove, U-groove etc.) is a non-essential variable as per QW 402.1. The volume of weld metal that gets accommodated in a U-groove is decidedly more than that in a V-groove. The layman reasoning goes that changing a V-groove to a U-groove violates QW 409.1, since U-groove entails higher volume of weld metal deposited and (therefore) a higher heat input in the joint.

The problem with this reasoning is that it forgets the formulas given in QW 409.1. The QW 409.1 does not intend volume of weld metal or heat input in the full joint, it only intends volume of weld metal deposited per bead; and heat input per bead. The formulas given in QW 409.1 indicate as much.

Commonly Asked Questions Regarding Heat Input On A WPS

There come to mind a few other questions when we apply heat input as a variable.

  • Does heat input of every pass deposited during the welding of test coupon need to be checked?

Yes, heat input of every pass needs to be checked and recorded. Checking the heat input of any one or a select few passes does not serve the intent of QW 409.1.

  • Does the heat input of each pass need to be equal?

No, Section IX does not say anything to this effect. Generally, in industry, it is preferred to use lower amperage for a few passes at the root to avoid burn-through, which is not restricted by Section IX in any way.

  • Should the maximum heat input value be calculated by taking the average of heat input in all passes?

The Section IX does not address how maximum heat input is to be calculated. A commonly accepted practice is that the maximum heat input observed among all passes becomes the ceiling (this finds affirmation in Interpretation IX-19-16 question 2), which gets recorded on the PQR, and which must not be exceeded in production welds.

How To Reckon Heat Input When Multiple Filler Metal Sizes Are Used?

Now, QW 409.1 gives rise to some interesting scenarios.

Suppose that a PQR has been qualified using a 40 mm thick test coupon by welding a succession of filler metal sizes, say dia 3.15 SMAW electrode for a few passes at the root, dia 4.0 SMAW electrode for half of the remaining thickness, and dia 5.0 for balance.

Multiple tension and bend tests (as permitted in QW 151 and QW 161) were done to cover the full thickness. The entire cross section, including the portion welded with dia. 5 mm got included in the test specimens.

The maximum heat input of the weld was of course noted during the welding with the dia. 5 mm electrode. Per QW 409.1, this maximum heat input must not be exceeded during production welding.

Can this PQR be used to weld a 80 mm thick production joint, fully with a dia. 5 mm SMAW electrode? Remember, in test coupon – dia. 5 mm has been used only for a small portion near the face of the coupon.

Let us stretch the argument a little further. Suppose that the dia. 5 mm electrode has been used for only one pass at the top in the test coupon mentioned above, while the remaining thickness has been welded using lower diameter electrode(s). Can this PQR be used for welding a 80 mm thick production joint fully with dia. 5.0 mm electrode?

There seems to be no violation of QW 409.1, or any other essential or supplementary essential variable. So, one would say, yes, the PQR described above can be used for welding the 80 mm thick production job fully with a dia. 5 mm electrode.

But then, how right would it be to proceed like this?

This situation has not been addressed in Section IX, nor does it find a mention in any Interpretation, to the best of my knowledge. In absence of the same, the best i can offer is my understanding of the issue.

The Section IX intends that the weld deposited with the higher heat input (say ‘Q’) must be subjected to the required tests. If the tests pass, that heat input ‘Q’ becomes a qualified parameter.

If multiple specimens are taken from a test coupon (in line with QW 151 and QW 161), and one or more of the specimens have only weld metal deposited with lower diameters (thus with a lower heat input, say ‘q’), then those specimens are only good enough for heat input ‘q’ (and not ‘Q’). The QW 409.1 therefore has not got satisfied.

If multiple specimens are not taken and the entire thickness is covered through single specimens, then perhaps it can be condoned that only a portion of the weld is deposited with ‘Q’, and remaining with ‘q’.

So, if you intend to fully weld your production joint with a diameter 5.0 mm electrode, it is good if you fill the bulk of the qualification coupon thickness with a dia. 5.0 mm electrode.

QW 409.1 came into existence in late 1970s. Before that, in its place – current and voltage used to be supplementary essential variables. Since then current and voltage have got relegated to being non-essential variables (QW 409.8).

This means that one can change the current and voltage to values outside the ranges noted on the procedure qualification record, without needing to re-qualify the procedure, as long as heat input is kept lower than the value noted on PQR. This finds affirmation in Interpretation IX-83-54 too.

Welding Position

Ordinarily, position is a non-essential variable for procedure qualifications. In other words, a procedure qualified in any position can be used for welding in any other position in production joints.

This means that a procedure qualification done in 1G position on a plate can be used to weld a pipe in 6G position too (provided you have a qualified welder to weld in 6G position, of course). This holds good for cases where toughness is not a requirement.

When toughness is a requirement, QW 405.2, which is a supplementary essential variable, comes into play. QW 405.2 says that “a change from any position to the vertical position” is not permitted. This means that if production joint is required to be welded in vertical position, the procedure qualification has to be done in vertical position too.

The QW 405.2 continues – “Vertical-uphill progression (e.g., 3G, 5G, 6G position) qualifies for all positions.” This is self explanatory.

This brings a question to mind.

If the procedure qualification is performed in 5G or 6G position, would it be necessary to remove impact specimens from the portion of the coupon that was welded in vertical-uphill position? QW 405.2 does not address this.

An Interpretation IX-80-34 comes close to addressing the question. The Interpretation is produced below for reference:

“Question: Supplementary essential variable QW 405.2 states: “Vertical-uphill qualifies for all positions.” Does a procedure qualification made in the 6G position qualify a welding procedure for all positions if the impact test specimens are taken from that area of the coupon which was welded in the vertical-uphill position?

Reply: Yes.”

Another Interpretation that is contextual is IX-81-43. These Interpretations indicates that taking specimens from the portion welded in vertical position is permitted. That does not tell us whether it is mandatory. This confusion can of course be circumvented by choosing a plate test coupon and welding it in 3G position.

Revision In 2021 Edition of Section IX

Note: An interesting development is that QW 405.2, since 2019 edition of Section IX onwards, has been deleted from all tables under QW 250. This means that the vertical-uphill business is no longer a restriction now. This means that a procedure qualified in any position can be used for welding in any other position in production joints, including when toughness is a consideration.

Group number

Table QW 422 in Section IX lists Group numbers for all ferrous materials (steels and steel alloys). The Group number is a supplementary essential variable (QW 403.5), so it comes up for consideration whenever toughness is a factor.

The QW 403.5 basically intends that the procedure qualification be done using the same base material grade that will be used in the production welding.

Alternatively, for ferrous materials, base metals having same P-No. and G-No. as the ones that will be used in production welding can be used for qualifying the procedure.

For non-ferrous materials, base metals having same P-No. and UNS No. as the ones that will be used in production welding can be used for qualifying the procedure.


For ferrous materials, a few further exemptions have been given:

  • If “two or more qualification records have the same essential and supplementary essential variables, except that the base metals are assigned to different Group Numbers within the same P‐Number, then the combination of base metals is also qualified.”

In order to understand this, suppose that you have two PQRs, one with a base metal combination P-No. 1 G-No. 1 + P-No. 1 G-No. 1, and another with the base metal combination P-No. 1 G-No. 2 + P-No. 1 G-No. 2.

If all other essential and supplementary essential variables of these two PQRs are same, then P-No. 1 G-No. 1 + P-No. 1 G-No. 2 combination is also considered qualified (in addition to the combination used in PQR), in accordance with QW 403.5.

Restating the case,

PQR 1: P1G1 + P1G1.

PQR 2: P1G2 + P1G2.

These two PQRs can support a WPS for P1G1 + P1G2 as well.

A couple of Interpretations in which the reply has been given making use of the above stipulation, are IX-79-32 and IX-86-15.

  • Also, “when base metals of two different Group Numbers within the same P-Number are qualified using a single test coupon, that coupon qualifies the welding of those two Group Numbers within the same P-Number to themselves as well as to each other using the variables qualified.”

Illustrating the above, consider a PQR with P-No. 1 G-No. 1 + P-No. 1 G-No. 2 base metal combination.

This PQR, in accordance with QW 403.5, can support WPS for P-No. 1 G-No. 1 + P-No. 1 G-No. 1 and P-No. 1 G-No. 2 + P-No. 1 G-No. 2 too (in addition to P-No. 1 G-No. 1 + P-No. 1 G-No. 2 of course). In simpler terms,

PQR: P1G1 + P1G2

This PQR can support WPSs for P1G1 + P1G1 and P1G2 + P1G2 base metal combinations as well, in addition to P1G1 + P1G2.

An Interpretation, in which the inquiry dealt with exactly the aforementioned matter, is IX-15-15. The reply given by ASME is in affirmation with the above understanding.

HAZ Toughness Testing On Both Sides Of Weld Metal

It is important to keep in mind that, when this exemption is availed – toughness test would need to be done on the weld metal, and both sides of the weld metal (so that HAZ of both Group numbers gets tested), if HAZ toughness is a requirement.

This is logical, and although does not find a direct mention in Section IX, becomes clear on perusal of Interpretation IX-79-94, which addresses precisely this subject.

There is another interesting thing to know about QW 403.5. Suppose that a production weld comprises of two dissimilar P-Number base metals, of which only one requires toughness testing, while the other does not. As an example, consider the following situation:

A production joint is made up of P-No. 5A Group 1 plus P-No 1. Toughness has been required for P-No. 5A base metal, and has not been required for the P-No. 1 base metal. A PQR qualified with P-No. 5A + P-No. 5A base metal, with impact testing, is available.

Can this PQR be used to support the WPS for the aforementioned production weld?

The answer is yes. Although, this has not been addressed explicitly in Section IX, it becomes abundantly apparent on perusing Interpretations on ASME’s website. Two such Interpretations that assert the understanding just described, are IX-16-18 and IX-89-23 (qs 2).

Not Applicable When HAZ Toughness Is Not Required

Notably, this variable QW 403.5 does not apply when toughness testing of the HAZ is not required by the referencing Code, standard, or specification.

Minimum base metal thickness

Ordinarily, the qualified base metal thickness (both minimum and maximum) are governed by QW 403.8, which sends us to QW 451 (with a few exemptions outlined in QW 202). However, when toughness is a factor, QW 403.6 places restriction on the minimum base metal thickness qualified.

QW 403.6 goes as follows:

“The minimum base metal thickness qualified is the thickness of the test coupon T or 5/8 in. (16 mm), whichever is less. However, where T is 1/4 in. (6 mm) or less, the minimum thickness qualified is 1/2T.”

The above restriction does not apply to following, as given in QW 403.6:

  1. WPS is qualified with a PWHT above the upper transformation temperature.
  2. WPS is for P-No. 10H material that is solution annealed after welding.
  3. WPS is for P-No. 8, P-Nos. 21 through 26, and P-No. 41 through 49.

The stipulation is simply worded, and easy enough to understand.

Code Trivia

A tiny bit of trivia here.

The 16 mm stipulation of QW 403.6 got firmly established only around 1978-1979 period. The Interpretations issued before 1978 reflect this. The Interpretations IX-78-04, IX-78-97 etc. tell us that before 1979, the QW 403.6 applied only when test coupon thickness is less than 16 mm.

For thicknesses above 16 mm, QW 451 would be applicable. A spurt of Interpretations around 1979, intending to reaffirm the 16 mm rule, reveals that that is when the 16 mm rule must have got introduced.

Applicable Only On Base Metal Thickness

An aspect worth noticing is that QW 403.6 only talks about base metal thickness, and does not say anything about weld metal.

If the referencing Section does not require toughness testing for the base metal, the QW 403.6 does not come into play, and the thickness limits calculated by QW 451 only would be applicable. This applies even when the referencing Section might require toughness testing of the weld metal (but not base metal).

In simpler terms, when toughness testing is not required for base metal, but is required for weld metal, the minimum qualified thickness should be calculated through QW 451 (and not QW 403.6).

This thinking of ASME is not stated explicitly in Section IX, but becomes apparent on perusal of Interpretation IX-86-48.

Also, regardless of whether toughness testing is applicable for weld metal or not, QW 403.6 does NOT have any bearing on weld metal thickness, which continues to be governed by QW 451. This has been repeatedly made clear by ASME through Interpretations, a few of them are IX-89-86, IX-89-96 etc.

Another aspect worth noticing about QW 403.6 is that, unlike QW 403.5, it remains applicable even when HAZ impact testing is not required by the referencing Code or Section. This gets reaffirmed through Interpretations IX-13-01 and IX-92-50.

Applicable For Fillet Welds Too

Another important thing to take note of, regarding QW 403.6, is that it is applicable to fillet welds too.

When a fillet weld WPS has been supported by a groove weld PQR, and toughness requirements are applicable, the ‘all thicknesses of base metal are qualified’ stipulation gets overridden by QW 403.6.

This has been firmly established through a number of Interpretations, a few of them are IX-17-56, IX-17-59, IX-80-62 etc. This has also been enforced through the ‘General Note’ at the bottom of QW 451.4. The QW 403.6 of course places no restriction on the maximum base metal thickness qualified, it only modifies the minimum base metal thickness qualified.

On similar lines, QW 202.3 rules that a WPS qualified on a groove weld can be used to do repair welding on fillet welds, without any limitation on base metal thickness. The stipulations of QW 403.6 are applicable to repair on fillet welds too. This gets established in the Interpretation IX-82-27 (qs 2).

Generally speaking, whenever the stipulations of both QW 202 and supplementary essential variables come into play simultaneously, the more stringent requirement among the two takes precedence.

Qualification Of A Procedure On Change/Introduction Of A Supplementary Essential Variable

When a change is made in production weld in one of the supplementary essential variables beyond the range permitted by the referencing PQR, it requires requalification of the PQR.

When re-qualification of a PQR is to be done for this purpose, Section IX does not intend that the whole procedure qualification that was originally done be repeated in entirety. There are some exemptions given, outlined in QW 401.1. In latest edition, these exemptions appear under QW 401.1.

Curiously, these rules keep getting moved around in different editions of Section IX. In 2013 edition, they appear under QW 401.3, in 1978 edition they appeared under QW 403.5, etc. [We come to know this about the 1978 edition through an Interpretation IX-78-19, which was issued in 1978, and which makes a mention of these rules, and indicates that these rules appear under QW 403.5].

Likewise, when an existing PQR (in which toughness tests were not done) is envisaged to be used for an application that requires toughness testing, then too the Section IX does not intend that all tests be repeated. Some exemptions appear in QW 401.1.

Following are the note-worthy points, of QW 401.1 (in 2021 edition), speaking of aforementioned exemptions:

  • Say, you already have a qualified procedure in which all tests other than toughness tests have been successfully completed. It is now required to apply this PQR for an application where toughness has been specified as a requirement by the referencing code of construction.

You would then need to prepare an additional test coupon, following same essential variables as the ones followed in original qualification, and the applicable supplementary essential variables (such as base metal of same P-No, G-No. as to be used in production joint, same PWHT as that to be given to production weld etc.).

It is then required to only perform the specified (weld metal, HAZ etc.) toughness tests on the additional coupon.

  • Another case is when you already have a qualified PQR, in which all tests including toughness tests have been completed. Suppose that the referencing Code has specified toughness tests from only the weld metal.

Now, suppose that one or more supplementary essential variables (such as change in Group number, PWHT time/temperature, change of trade name for filler metals having “G” suffix, etc.) have undergone a change.

In this case too, an additional test coupon needs to be prepared with same essential variables as the original qualification, and the new supplementary essential variable(s), and only toughness tests need to be done.

  • When HAZ toughness is specified by referencing Code: When the referencing Code specifies that HAZ toughness tests also need to be done (in addition to weld metal toughness tests), then the following is permitted:

When you have a qualified PQR in which all tests, including toughness (with or without HAZ toughness tests), have been completed, and a change has occurred in one of the supplementary essential variables, then it is only required to prepare an additional test coupon with same essential variables (as the original qualification) and the new supplementary essential variables, and do toughness tests only from the heat affected zone of the coupon.

So, this was all about the subject of supplementary essential variables. Would you like to add something? Please do leave your comments in the comments section below.

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