ASME IX Essential Variables by Schweißen 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 Verfahrensprüfung (PQR). Supplementary essential variables become additional essential variables when impact Prüfung is required by the construction Regelwerk.
Key distinction: AWS D1.1 uses a single list of essential variables in Tabelle 6.6 that applies across all processes. ASME IX assigns different variable classifications per Schweißverfahren — 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 Prüfstück. Examples include Grundwerkstoff P-number, Zusatzwerkstoff F-number, welding process, and Dickenbereich. 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 Nahtausführung changes, stringer-to-Pendelraupe changes, and Elektrodendurchmesser 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 Verfahrensprüfung. 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 Dicke 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 Temperatur range change | — | ✓ | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.4 | Current or Polarität 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 Grenzwerte (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 Badsicherung 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 Elektrode 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
Obwohl viele Variablen bei allen vier Schweißverfahren gleich eingestuft sind, bestimmen einige wichtige Unterschiede, wie Hersteller ihre Verfahrensqualifikationen verwalten:
SAW-Flussmittelvariablen: SAW führt drei Zusatzwerkstoffvariablen ein, die bei anderen Schweißverfahren nicht vorkommen: die Klassifikation der Draht-Flussmittel-Kombination (QW-404.9), die Flussmittelart (QW-404.34) und wiederaufbereitete Schlacke (QW-404.36). Alle drei sind wesentlich. Eine Änderung des Flussmittel-Markennamens (QW-404.35) ist zusätzlich wesentlich. Dies zeigt den großen Einfluss des Flussmittels auf die Chemie des SAW-Schweißguts.
GMAW/FCAW-Werkstoffübergang und Schutzgas: Tabelle QW-255 führt die Art des Werkstoffübergangs (QW-409.32) als nichtwesentliche WPS-Variable. Die WPS muss mindestens eine der Übergangsarten Kurzschluss-, Grobtropfen-, Sprüh- oder Impulssprühübergang angeben. Eine Änderung der Übergangsart (QW-409.2) ist in Tabelle QW-355 eine wesentliche Schweißer-Variable. Die Zusammensetzung des Schutzgases (QW-408.2) ist sowohl für GMAW als auch für GTAW wesentlich. Die Grenze für die Schweißgutdicke beim Kurzschlussübergang (QW-404.32) ist eine Schweißerprüfungsvariable in Tabelle QW-355 und fehlt in Tabelle QW-255; sie ist daher keine wesentliche WPS-Variable. Wenn beim GMAW mit Kurzschlussübergang und niedriger Spannung weniger als 1/2 in. (13 mm) im Prüfstück abgeschieden wurden, ist der Schweißer bis zum 1.1-Fachen dieser Schweißgutdicke qualifiziert. Für die Schweißgutdicke der Verfahrensprüfung gilt QW-404.30, für den Grundwerkstoff-Dickenbereich QW-403.8; die Qualifikationsgrenzen stehen in QW-451.
GTAW-Stromart und Polarität: In Tabelle QW-256 ist eine Änderung von Stromart oder Polarität (QW-409.4) für GTAW nichtwesentlich, ebenso wie für SMAW und SAW. Abschmelzende Einlegeteile (consumable inserts) (QW-404.22) sind ebenfalls nichtwesentlich. Das Hinzufügen oder Weglassen von Zusatzwerkstoff (QW-404.14) ist wesentlich. Stromart (AC gegenüber DC) und Polarität (DCEN gegenüber DCEP) werden gesondert zusätzlich wesentlich behandelt, wenn Zähigkeitsprüfungen verlangt sind; dabei gelten die unten genannten P-No.-Ausnahmen.
Für GTAW führt Tabelle QW-256 außerdem QW-409.30 als zusätzlich wesentlich, wenn das Bezug nehmende Regelwerk, die Norm oder die Spezifikation Zähigkeitsprüfungen verlangt. Erfasst sind Änderungen zwischen AC und DC oder zwischen DCEN und DCEP in beiden Richtungen. Dies gilt jedoch nicht für eine WPS, die für Grundwerkstoffe der P-Nos. 8, 21 bis 26 und 41 bis 49 qualifiziert ist.
Wärmeeinbringung: Eine Erhöhung der Wärmeeinbringung (QW-409.1) ist gemäß den Tabellen QW-253 bis QW-256 bei SMAW, SAW, GMAW/FCAW und GTAW zusätzlich wesentlich. Sie wird erst dann voll wesentlich, wenn das Bezug nehmende Konstruktionsregelwerk eine Kerbschlag- oder Zähigkeitsprüfung verlangt.
Practical Implications for Fabricators
Understanding variable classifications directly impacts how many PQRs a shop must maintain. A Hersteller 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, Zwischenlagentemperatur, 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 Zusätzliche wesentliche Variable to essential status and may invalidate existing PQRs that were qualified without Kerbschlagbiegeversuch coupons."
— Field observation, pressure vessel Fertigung 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.