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The Three Variable Categories

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.

Key Differences Between Processes

While many variables 共有 the same classification across all four processes, several critical differences determine how fabricators manage their qualifications:

SAW flux variables: SAW introduces three filler metal variables not found in other processes — flux-wire classification (QW-404.9), flux type (QW-404.34), and recrushed slag (QW-404.36) — all classified as essential. A change in flux brand name (QW-404.35) is supplementary essential. These reflect the significant role flux plays in SAW 溶接金属 chemistry.

GMAW/FCAW 溶滴転移方式 and gas: GMAW adds transfer mode change (QW-409.2) as essential — switching from spray to short-circuit or グロビュラー転移 requires re-qualification. シールドガス composition (QW-408.2) is essential for both GMAW and GTAW. The short-circuit weld-metal thickness limit (QW-404.32) is an essential variable specific to short-circuiting GMAW; base-metal thickness range is governed by QW-403.8 with the qualification limits in QW-451.

GTAW current and polarity: For GTAW, current or polarity change (QW-409.4) is an essential variable, unlike SMAW and SAW where it is nonessential. This is because GTAW 溶接 quality is highly sensitive to 電流の種類 (AC vs DC) and polarity (DCEN vs DCEP). GTAW also adds filler metal variables for 溶接材料 inserts (QW-404.22) and addition or deletion of filler metal (QW-404.14) as essential.

Heat input: 入熱 increase (QW-409.1) is classified as supplementary essential across SMAW, SAW, GMAW/FCAW, and GTAW per Tables QW-253 through QW-256. It becomes a full essential variable only when the referencing construction code requires impact (toughness) testing.

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 重要変数 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

Related Standards Guides

• ASME Section IX — 溶接資格 Standard
• ASME IX P-Numbers — Base Metal Grouping
• ASME IX F-Numbers — Filler Metal Grouping
• PQR — Procedure 資格記録 Guide
• ASME IX vs D1.1 — Side-by-Side Comparison
• NDE 要求事項 — D1.1 条項 8
• Post-Weld 熱処理 (PWHT)
• WPS — 溶接施工法 Specification