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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 test coupon. Examples include base metal P-number, filler metal F-number, welding process, and thickness range. 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 joint design changes, stringer-to-weave bead changes, and electrode diameter 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 procedure qualification. The complete tables are in ASME BPVC IX Article IV.

Key Differences Between Processes

While many variables share 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 weld metal chemistry.

GMAW/FCAW transfer mode and gas: Table QW-255 lists transfer mode (QW-409.32) as a nonessential WPS variable: the WPS must specify at least one of the short-circuiting, globular, spray, or pulsed-spray transfer modes. Transfer mode change (QW-409.2) is a welder essential variable in Table QW-355. Shielding gas composition (QW-408.2) is essential for both GMAW and GTAW. The short-circuit weld-metal thickness limit (QW-404.32) is a welder performance variable listed in Table QW-355 and absent from Table QW-255, so it is not a WPS essential variable: for low-voltage short-circuiting GMAW with less than 1/2 in. (13 mm) deposited in the test coupon, the welder is qualified up to 1.1 times the test deposit. Procedure deposited weld-metal thickness is governed by QW-404.30 and base-metal thickness range by QW-403.8, both with the qualification limits in QW-451.

GTAW current and polarity: In Table QW-256, current or polarity change (QW-409.4) is nonessential for GTAW, as it is for SMAW and SAW. Consumable inserts (QW-404.22) are also nonessential. Addition or deletion of filler metal (QW-404.14) is essential. Current type (AC vs DC) and polarity (DCEN vs DCEP) receive separate supplementary essential treatment when toughness testing is required, subject to the P-No. exceptions below.

For GTAW, Table QW-256 also lists QW-409.30 as supplementary essential when toughness testing is required by the referencing code, standard, or specification: it covers changes between AC and DC or between DCEN and DCEP, in either direction, but does not apply to a WPS qualified for base metals assigned to P-Nos. 8, 21 through 26, and 41 through 49.

Heat input: Heat input 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 fabricator 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, interpass temperature, 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 supplementary essential variable to essential status and may invalidate existing PQRs that were qualified without impact test coupons."

— Field observation, pressure vessel fabrication practice

Related Standards Guides

• ASME Section IX — Welding Qualification Standard
• API 1104 — Pipeline Welding Qualification
• ASME IX P-Numbers — Base Metal Grouping
• ASME IX F-Numbers — Filler Metal Grouping
• PQR — Procedure Qualification Record Guide
• ASME IX vs D1.1 — Side-by-Side Comparison
• AWS D1.1 Quick Reference — Tables and Clauses
• Fillet Weld Size — D1.1 Table 7.7 Lookup
• NDE Requirements — D1.1 Clause 8
• Post-Weld Heat Treatment (PWHT)
• WPS — Welding Procedure Specification