ASME IX Essential Variables by 溶接 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 溶接施工法確認試験記録 (PQR). Supplementary essential variables become additional essential variables when impact 試験 is required by the construction 基準.
Key distinction: AWS D1.1 uses a single list of essential variables in 表 6.6 that applies across all processes. ASME IX assigns different variable classifications per 溶接工程 — 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 試験片. Examples include 母材 P-number, 溶加材 F-number, welding process, and 厚さ範囲. 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 継手設計 changes, stringer-to-ウィーブビード changes, and 電極径 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 手順資格. 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 板厚 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 温度 range change | — | ✓ | — |
| Electrical | QW-409.1 | Heat input increase | — | ✓ | — |
| Electrical | QW-409.4 | Current or 極性 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 限界値 (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 バッキング 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 電極 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
多くの変数は四つの溶接方法で同じ区分ですが、いくつかの重要な違いが製作者による施工法の認定管理を左右します。
SAWのフラックス変数: SAWには他の溶接方法にない三つの溶加材変数があります。フラックスとワイヤの組合せ区分 (QW-404.9)、フラックスの種類 (QW-404.34)、再粉砕したスラグ (QW-404.36) で、いずれも重要変数です。フラックスの銘柄変更 (QW-404.35) は追加重要変数です。これはSAWの溶接金属の化学成分にフラックスが大きく影響するためです。
GMAW/FCAWの溶滴移行形態とガス: 表QW-255では、溶滴移行形態 (QW-409.32) をWPSの非重要変数としています。WPSには短絡、グロビュール、スプレー、パルススプレーのうち少なくとも一つを指定する必要があります。移行形態の変更 (QW-409.2) は表QW-355で溶接士の重要変数です。シールドガスの組成 (QW-408.2) はGMAWとGTAWの両方で重要です。短絡移行での溶接金属厚さ限界 (QW-404.32) は表QW-355にある溶接士技能変数で、表QW-255にはありません。したがってWPSの重要変数ではありません。低電圧の短絡移行GMAW試験片での溶着厚さが1/2 in. (13 mm) 未満なら、溶接士は試験時の溶着厚さの1.1倍まで認定されます。施工法の溶着金属厚さはQW-404.30、母材の厚さ範囲はQW-403.8が規定し、両方の認定限界はQW-451に従います。
GTAWの電流と極性: 表QW-256では、電流または極性の変更 (QW-409.4) は、SMAWやSAWと同様にGTAWでも非重要変数です。消耗インサート (consumable inserts) (QW-404.22) も非重要変数です。溶加材の追加または削除 (QW-404.14) は重要変数です。電流の種類 (ACまたはDC) と極性 (DCENまたはDCEP) は、靭性試験が要求されるとき、以下のP-No.の例外を条件として別個の追加重要変数として扱われます。
GTAWでは、参照する規格、基準または仕様書が靭性試験を要求するとき、表QW-256はQW-409.30も追加重要変数とします。ACとDCの間、またはDCENとDCEPの間の双方向の変更が対象ですが、P-Nos. 8、21から26、41から49に割り当てられた母材で認定されたWPSには適用されません。
入熱: 入熱の増加 (QW-409.1) は、表QW-253からQW-256により、SMAW、SAW、GMAW/FCAW、GTAWのすべてで追加重要変数です。参照する建設規格が衝撃試験または靭性試験を要求する場合に限り、重要変数になります。
Practical Implications for Fabricators
Understanding variable classifications directly impacts how many PQRs a shop must maintain. A 製作者 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, パス間温度, 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 補助重要変数 to essential status and may invalidate existing PQRs that were qualified without 衝撃試験 coupons."
— Field observation, pressure vessel 製作 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.