ASME IX Essential Variables by Soudage Process
ASME Section IX defines three categories of welding variables: essential, supplementary essential, and nonessential. Essential variables — if changed beyond the qualified range — require a new Qualification de Mode Opératoire de Soudage (PQR). Supplementary essential variables become additional essential variables when impact Essai is required by the construction Code.
Key distinction: AWS D1.1 uses a single list of essential variables in Tableau 6.6 that applies across all processes. ASME IX assigns different variable classifications per Procédé de Soudage — a variable that is essential for SMAW (QW-253) may be nonessential for GTAW (QW-256). Always check the correct QW-250 table for your specific process.
The Three Variable Categories
API 1104 references on this page are to the 2021 edition; Addendum 1 (published 2026-09-03) is not incorporated.
ASME IX Article IV (QW-400 series) defines all welding data variables. The QW-250 series tables then classify each variable as essential, supplementary essential, or nonessential for each welding process. Understanding this classification is fundamental to managing procedure qualifications efficiently.
Essential variables (QW-251.2) are parameters that, if changed beyond the range qualified by the PQR, require re-qualification with a new Éprouvette. Examples include Métal de Base P-number, Métal d'Apport F-number, welding process, and Plage d'épaisseur. These apply to every procedure regardless of the application.
Supplementary essential variables (QW-251.3) are parameters that become essential only when the referencing construction code requires impact testing. When impact testing is not required, these variables are treated as nonessential. This dual classification is unique to ASME IX — neither D1.1 nor API 1104 has an equivalent concept.
Nonessential variables (QW-251.4) can be changed in the WPS without re-qualification. The WPS must still document them, but changing them does not invalidate the PQR. Examples include Conception de joint changes, stringer-to-Cordon balayé changes, and Diamètre d'électrode changes within a group.
Essential Variable Tables by Process
Select a welding process to view its variable classifications per the QW-250 series. Each table shows the most important variables that fabricators encounter during Qualification de mode opératoire. The complete tables are in ASME BPVC IX Article IV.
| Category | Paragraph | Brief of Variables | Essential | Supp. Essential | Nonessential |
|---|---|---|---|---|---|
| Joints | QW-402.1 | Groove design change | — | — | ✓ |
| Joints | QW-402.4 | Deletion of backing | — | — | ✓ |
| Joints | QW-402.10 | Root spacing change | — | — | ✓ |
| Base Metals | QW-403.5 | Group Number change | — | ✓ | — |
| Base Metals | QW-403.8 | Thickness qualified change | ✓ | — | — |
| Base Metals | QW-403.9 | Pass > 1/2 in. (13 mm) | ✓ | — | — |
| Base Metals | QW-403.11 | P-Number change | ✓ | — | — |
| Filler Metals | QW-404.4 | F-Number change | ✓ | — | — |
| Filler Metals | QW-404.5 | A-Number change | ✓ | — | — |
| Filler Metals | QW-404.12 | Classification change | — | ✓ | — |
| Filler Metals | QW-404.30 | Weld deposit Épaisseur change | ✓ | — | — |
| Filler Metals | QW-404.33 | Classification within SFA spec | — | — | ✓ |
| Preheat | QW-406.1 | Decrease > 100°F (55°C) | ✓ | — | — |
| Preheat | QW-406.2 | Preheat maintenance change | — | — | ✓ |
| Preheat | QW-406.3 | Increase > 100°F interpass | — | ✓ | — |
| PWHT | QW-407.1 | PWHT change | ✓ | — | — |
| PWHT | QW-407.2 | PWHT time and Température range change | — | ✓ | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.4 | Current or Polarité change | — | — | ✓ |
| Electrical | QW-409.8 | I&E range change | — | — | ✓ |
| Electrical | QW-409.30 | Current or polarity change | — | ✓ | — |
| Technique | QW-410.1 | String/weave change | — | — | ✓ |
| Technique | QW-410.5 | Method of cleaning change | — | — | ✓ |
| Technique | QW-410.6 | Method of back gouge change | — | — | ✓ |
| Technique | QW-410.9 | Multi to single pass per side | — | — | ✓ |
| Technique | QW-410.25 | Manual or automatic change | — | — | ✓ |
| Technique | QW-410.26 | Peening | — | — | ✓ |
| Technique | QW-410.64 | Use of thermal processes | ✓ | — | — |
| Technique | QW-410.87 | Multi to single pass per side | — | ✓ | — |
| Technique | QW-410.92 | Bead width | — | ✓ | — |
| Category | Paragraph | Brief of Variables | Essential | Supp. Essential | Nonessential |
|---|---|---|---|---|---|
| Joints | QW-402.1 | Groove design change | — | — | ✓ |
| Joints | QW-402.4 | Deletion of backing | — | — | ✓ |
| Joints | QW-402.10 | Root spacing change | — | — | ✓ |
| Base Metals | QW-403.5 | Group Number change | — | ✓ | — |
| Base Metals | QW-403.6 | T Limites (toughness) | — | ✓ | — |
| Base Metals | QW-403.8 | Thickness qualified change | ✓ | — | — |
| Base Metals | QW-403.9 | Pass > 1/2 in. (13 mm) | ✓ | — | — |
| Base Metals | QW-403.11 | P-Number change | ✓ | — | — |
| Filler Metals | QW-404.4 | F-Number change | ✓ | — | — |
| Filler Metals | QW-404.5 | A-Number change | ✓ | — | — |
| Filler Metals | QW-404.6 | Diameter change | — | — | ✓ |
| Filler Metals | QW-404.9 | Flux-wire classification change | ✓ | — | — |
| Filler Metals | QW-404.10 | Alloy flux change | ✓ | — | — |
| Filler Metals | QW-404.24 | Supplemental filler change | ✓ | — | — |
| Filler Metals | QW-404.27 | Alloy elements change | ✓ | — | — |
| Filler Metals | QW-404.29 | Flux designation change | — | — | ✓ |
| Filler Metals | QW-404.30 | Weld deposit thickness change | ✓ | — | — |
| Filler Metals | QW-404.33 | Classification within SFA spec | — | — | ✓ |
| Filler Metals | QW-404.34 | Flux type change | ✓ | — | — |
| Filler Metals | QW-404.35 | Flux-wire classification | — | ✓ | ✓ |
| Filler Metals | QW-404.36 | Recrushed slag addition | ✓ | — | — |
| Preheat | QW-406.1 | Decrease > 100°F (55°C) | ✓ | — | — |
| Preheat | QW-406.2 | Preheat maintenance change | — | — | ✓ |
| Preheat | QW-406.3 | Increase > 100°F interpass | — | ✓ | — |
| PWHT | QW-407.1 | PWHT change | ✓ | — | — |
| PWHT | QW-407.2 | PWHT time and temperature range change | — | ✓ | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.4 | Current or polarity change | — | — | ✓ |
| Electrical | QW-409.8 | I&E range change | — | — | ✓ |
| Electrical | QW-409.30 | Current or polarity change | — | ✓ | — |
| Technique | QW-410.1 | String/weave change | — | — | ✓ |
| Technique | QW-410.5 | Method of cleaning change | — | — | ✓ |
| Technique | QW-410.6 | Method of back gouge change | — | — | ✓ |
| Technique | QW-410.7 | Oscillation change | — | — | ✓ |
| Technique | QW-410.8 | Tube-work distance change | — | — | ✓ |
| Technique | QW-410.9 | Multi to single pass per side | — | — | ✓ |
| Technique | QW-410.10 | Single to multi electrodes | — | ✓ | ✓ |
| Technique | QW-410.15 | Electrode spacing change | — | — | ✓ |
| Technique | QW-410.25 | Manual or automatic change | — | — | ✓ |
| Technique | QW-410.26 | Peening | — | — | ✓ |
| Technique | QW-410.64 | Use of thermal processes | ✓ | — | — |
| Technique | QW-410.87 | Multi to single pass per side | — | ✓ | — |
| Technique | QW-410.92 | Bead width | — | ✓ | — |
| Category | Paragraph | Brief of Variables | Essential | Supp. Essential | Nonessential |
|---|---|---|---|---|---|
| Joints | QW-402.1 | Groove design change | — | — | ✓ |
| Joints | QW-402.4 | Deletion of backing | — | — | ✓ |
| Joints | QW-402.10 | Root spacing change | — | — | ✓ |
| Base Metals | QW-403.5 | Group Number change | — | ✓ | — |
| Base Metals | QW-403.6 | T Limits (toughness) | — | ✓ | — |
| Base Metals | QW-403.8 | Thickness qualified change | ✓ | — | — |
| Base Metals | QW-403.9 | Pass > 1/2 in. (13 mm) | ✓ | — | — |
| Base Metals | QW-403.11 | P-Number change | ✓ | — | — |
| Filler Metals | QW-404.4 | F-Number change | ✓ | — | — |
| Filler Metals | QW-404.5 | A-Number change | ✓ | — | — |
| Filler Metals | QW-404.6 | Diameter change | — | — | ✓ |
| Filler Metals | QW-404.12 | Classification change | — | ✓ | — |
| Filler Metals | QW-404.23 | Filler metal product form change | ✓ | — | — |
| Filler Metals | QW-404.24 | Supplemental filler change | ✓ | — | — |
| Filler Metals | QW-404.27 | Alloy elements change | ✓ | — | — |
| Filler Metals | QW-404.30 | Weld deposit thickness change | ✓ | — | — |
| Filler Metals | QW-404.33 | Classification within SFA spec | — | — | ✓ |
| Preheat | QW-406.1 | Decrease > 100°F (55°C) | ✓ | — | — |
| Preheat | QW-406.2 | Preheat maintenance change | — | — | ✓ |
| Preheat | QW-406.3 | Increase > 100°F interpass | — | ✓ | — |
| PWHT | QW-407.1 | PWHT change | ✓ | — | — |
| PWHT | QW-407.2 | PWHT time and temperature range change | — | ✓ | — |
| Gas | QW-408.1 | Addition or deletion of trailing gas | — | — | ✓ |
| Gas | QW-408.2 | Single, mixture, or percentage change | ✓ | — | — |
| Gas | QW-408.3 | Flow rate change | — | — | ✓ |
| Gas | QW-408.5 | Backing flow change | — | — | ✓ |
| Gas | QW-408.9 | Deletion of Support envers gas | ✓ | — | — |
| Gas | QW-408.10 | Deletion of trailing gas | ✓ | — | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.4 | Current or polarity change | — | — | ✓ |
| Electrical | QW-409.8 | I&E range change | — | — | ✓ |
| Electrical | QW-409.30 | Current or polarity change | — | ✓ | — |
| Electrical | QW-409.32 | Transfer mode change | — | — | ✓ |
| Technique | QW-410.1 | String/weave change | — | — | ✓ |
| Technique | QW-410.3 | Orifice, cup, or nozzle size | — | — | ✓ |
| Technique | QW-410.5 | Method of cleaning change | — | — | ✓ |
| Technique | QW-410.6 | Method of back gouge change | — | — | ✓ |
| Technique | QW-410.7 | Oscillation change | — | — | ✓ |
| Technique | QW-410.8 | Tube-work distance change | — | — | ✓ |
| Technique | QW-410.9 | Multi to single pass per side | — | — | ✓ |
| Technique | QW-410.25 | Manual or automatic change | — | — | ✓ |
| Technique | QW-410.26 | Peening | — | — | ✓ |
| Technique | QW-410.64 | Use of thermal processes | ✓ | — | — |
| Technique | QW-410.87 | Multi to single pass per side | — | ✓ | — |
| Technique | QW-410.92 | Bead width | — | ✓ | — |
| Category | Paragraph | Brief of Variables | Essential | Supp. Essential | Nonessential |
|---|---|---|---|---|---|
| Joints | QW-402.1 | Groove design change | — | — | ✓ |
| Joints | QW-402.5 | Addition of backing | — | — | ✓ |
| Joints | QW-402.10 | Root spacing change | — | — | ✓ |
| Base Metals | QW-403.5 | Group Number change | — | ✓ | — |
| Base Metals | QW-403.6 | T Limits (toughness) | — | ✓ | — |
| Base Metals | QW-403.8 | Thickness qualified change | ✓ | — | — |
| Base Metals | QW-403.11 | P-Number change | ✓ | — | — |
| Filler Metals | QW-404.3 | Size change | — | — | ✓ |
| Filler Metals | QW-404.4 | F-Number change | ✓ | — | — |
| Filler Metals | QW-404.5 | A-Number change | ✓ | — | — |
| Filler Metals | QW-404.12 | Classification change | — | ✓ | — |
| Filler Metals | QW-404.14 | Addition or deletion of filler metal | ✓ | — | — |
| Filler Metals | QW-404.22 | Addition or deletion of insert | — | — | ✓ |
| Filler Metals | QW-404.23 | Filler metal product form change | ✓ | — | — |
| Filler Metals | QW-404.30 | Weld deposit thickness change | ✓ | — | — |
| Filler Metals | QW-404.33 | Classification within SFA spec | — | — | ✓ |
| Filler Metals | QW-404.50 | Flux addition or deletion | — | — | ✓ |
| Preheat | QW-406.1 | Decrease > 100°F (55°C) | ✓ | — | — |
| Preheat | QW-406.3 | Increase > 100°F interpass | — | ✓ | — |
| PWHT | QW-407.1 | PWHT change | ✓ | — | — |
| PWHT | QW-407.2 | PWHT time and temperature range change | — | ✓ | — |
| Gas | QW-408.1 | Addition or deletion of trailing gas | — | — | ✓ |
| Gas | QW-408.2 | Single, mixture, or percentage change | ✓ | — | — |
| Gas | QW-408.3 | Flow rate change | — | — | ✓ |
| Gas | QW-408.5 | Backing flow change | — | — | ✓ |
| Gas | QW-408.9 | Deletion of backing gas | ✓ | — | — |
| Gas | QW-408.10 | Deletion of trailing gas | ✓ | — | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.3 | Pulsing current change | — | — | ✓ |
| Electrical | QW-409.4 | Current or polarity change | — | — | ✓ |
| Electrical | QW-409.8 | I&E range change | — | — | ✓ |
| Electrical | QW-409.12 | Tungsten Électrode change | — | — | ✓ |
| Electrical | QW-409.30 | Current or polarity change | — | ✓ | — |
| Technique | QW-410.1 | String/weave change | — | — | ✓ |
| Technique | QW-410.3 | Orifice, cup, or nozzle size | — | — | ✓ |
| Technique | QW-410.5 | Method of cleaning change | — | — | ✓ |
| Technique | QW-410.6 | Method of back gouge change | — | — | ✓ |
| Technique | QW-410.7 | Oscillation change | — | — | ✓ |
| Technique | QW-410.9 | Multi to single pass per side | — | — | ✓ |
| Technique | QW-410.10 | Single to multi electrodes | — | ✓ | ✓ |
| Technique | QW-410.25 | Manual or automatic change | — | — | ✓ |
| Technique | QW-410.26 | Peening | — | — | ✓ |
| Technique | QW-410.64 | Use of thermal processes | ✓ | — | — |
| Technique | QW-410.87 | Multi to single pass per side | — | ✓ | — |
| Technique | QW-410.92 | Bead width | — | ✓ | — |
Key Differences Between Processes
Même si de nombreuses variables ont la même classification pour les quatre procédés de soudage, plusieurs différences importantes déterminent la gestion des qualifications de mode opératoire par les fabricants :
Variables de flux en SAW : Le SAW introduit trois variables de métal d’apport absentes des autres procédés : la classification flux-fil (QW-404.9), le type de flux (QW-404.34) et le laitier rebroyé (QW-404.36). Toutes trois sont essentielles. Un changement de marque du flux (QW-404.35) est essentiel supplémentaire. Cela traduit le rôle important du flux dans la composition chimique du métal fondu en SAW.
Mode de transfert et gaz en GMAW/FCAW : Le tableau QW-255 classe le mode de transfert (QW-409.32) comme variable non essentielle de la WPS. La WPS doit préciser au moins un des modes suivants : court-circuit, globulaire, pulvérisation ou pulvérisation pulsée. Le changement de mode de transfert (QW-409.2) est une variable essentielle du soudeur dans le tableau QW-355. La composition du gaz de protection (QW-408.2) est essentielle pour GMAW et GTAW. La limite d’épaisseur de métal fondu en court-circuit (QW-404.32) est une variable de qualification du soudeur figurant au tableau QW-355 mais absente du tableau QW-255 ; ce n’est donc pas une variable essentielle de WPS. Si moins de 1/2 in. (13 mm) est déposé sur l’éprouvette en GMAW à faible tension et court-circuit, le soudeur est qualifié jusqu’à 1.1 fois cette épaisseur déposée. L’épaisseur de métal fondu déposé pour la procédure relève de QW-404.30 et la plage d’épaisseur du métal de base de QW-403.8, avec les limites de qualification de QW-451.
Courant et polarité en GTAW : Dans le tableau QW-256, le changement de courant ou de polarité (QW-409.4) est non essentiel pour GTAW, comme pour SMAW et SAW. Les inserts consommables (QW-404.22) sont également non essentiels. L’ajout ou la suppression de métal d’apport (QW-404.14) est essentiel. Le type de courant (AC ou DC) et la polarité (DCEN ou DCEP) font l’objet d’un traitement essentiel supplémentaire distinct lorsque des essais de ténacité sont exigés, sous réserve des exceptions P-No. ci-dessous.
Pour GTAW, le tableau QW-256 classe aussi QW-409.30 comme essentiel supplémentaire lorsque le code, la norme ou la spécification de référence exige des essais de ténacité. Il couvre les changements entre AC et DC ou entre DCEN et DCEP, dans les deux sens, mais ne s’applique pas à une WPS qualifiée pour des métaux de base attribués aux P-Nos. 8, 21 à 26 et 41 à 49.
Apport de chaleur : L’augmentation de l’apport de chaleur (QW-409.1) est essentielle supplémentaire pour SMAW, SAW, GMAW/FCAW et GTAW selon les tableaux QW-253 à QW-256. Elle ne devient pleinement essentielle que lorsque le code de construction de référence exige des essais de résilience ou de ténacité.
Practical Implications for Fabricators
Understanding variable classifications directly impacts how many PQRs a shop must maintain. A Fabricant working with SMAW on P-Number 1 carbon steels may need only a handful of PQRs to cover their typical work. The same shop adding SAW capability must also qualify the specific flux-wire combination, since changing flux type or adding recrushed slag requires a new PQR.
For shops performing impact-tested work (low-temperature vessels, cryogenic piping), the supplementary essential variables become the controlling factor. Heat input, Température entre Passes, and PWHT parameters that would normally be freely adjustable now lock the PQR to a specific range. A fabricator building ASME Section VIII vessels for both ambient and low-temperature service often maintains separate PQR sets for impact-tested and non-impact-tested applications.
"The most expensive essential variable change in a typical pressure vessel shop is not a P-number change — it is discovering at the last minute that the construction code requires impact testing, which retroactively promotes every Variable essentielle supplémentaire to essential status and may invalidate existing PQRs that were qualified without Essai de résilience coupons."
— Field observation, pressure vessel Fabrication practice
CWI Exam Tip: The CWI exam frequently tests the concept of supplementary essential variables. The key phrase to remember: supplementary essential variables become essential only when impact testing is required by the referencing construction code. ASME IX itself does not require impact testing — the construction code (Section VIII, B31.3, etc.) makes that determination. If you see a question about when re-qualification is required, check whether impact testing is specified before evaluating supplementary essential variables.
Related Standards Guides
Frequently Asked Questions
Essential variables are welding parameters that, if changed beyond the qualified range, always require a new procedure qualification record (PQR) regardless of the application. Supplementary essential variables only become essential when the referencing construction code requires impact (toughness) testing — for example, when ASME Section VIII Division 1 invokes UCS-66 for low-temperature service. When impact testing is not required, supplementary essential variables are treated as nonessential and can be changed without re-qualification. This dual classification is unique to ASME IX and does not exist in AWS D1.1 or API 1104.
When an essential variable is changed beyond the range established by the original PQR, the existing procedure qualification is no longer valid for the new conditions. The fabricator must either qualify a new procedure by welding and testing a new test coupon under the revised parameters, or reference a different existing PQR that covers the new variable range. There is no provision in ASME IX to extend or amend an existing PQR — it documents a single test event. Common essential variable changes that trigger re-qualification include changing the base metal P-number, changing the filler metal F-number, and exceeding the qualified thickness range.
Supplementary essential variables become essential when the referencing construction code requires impact testing (Charpy V-notch toughness testing) on the procedure qualification test coupon. This is determined by the construction code, not by Section IX itself. For example, ASME Section VIII Division 1 requires impact testing per UCS-66 for vessels in low-temperature service, and B31.3 requires it for certain process piping applications. When impact testing is required, variables such as heat input increase, preheat decrease, and PWHT changes become essential and require re-qualification if changed.
Yes. A single PQR establishes a qualification range based on the actual test conditions and results. Any WPS whose essential variable ranges fall within the qualification range of that PQR can reference it. For example, a PQR qualified with SMAW using E7018 (F-Number 4) on P-Number 1 material at 1 inch thickness in the 3G position establishes ranges that could support WPSs covering multiple thicknesses, positions (1G, 2G, 3G), and joint configurations. Conversely, a single WPS can reference multiple PQRs if the combined qualification ranges cover all the essential variables specified.
Base metal thickness range (QW-403.8) is the most frequently encountered essential variable change in fabrication. When production work requires welding material thicker than what the original PQR test coupon covered, the fabricator needs a new PQR at the greater thickness. ASME IX generally qualifies a thickness range from the test coupon thickness down to some minimum, and up to 2T (twice the test coupon thickness) for test coupons under 3/4 inch. P-number changes and filler metal F-number changes are also common triggers, particularly in shops that work across carbon steel and stainless steel applications.