ASME Section IX QW-253: SMAW Essential Variables Guide

When a fabricator qualifies a SMAW welding procedure under ASME Section IX, the PQR test locks in a specific set of variables. Every subsequent production WPS written against that PQR must remain within the qualified range for each variable. QW-253 is the table that defines which SMAW variables are essential, which are supplementary essential (impact testing only), and which are nonessential.

Understanding which column a variable falls into — and what exactly triggers a change — is the daily practical work of QC managers and welding engineers who maintain ASME IX procedure libraries. This guide walks through the QW-253 structure and its most consequential entries.

The Three Variable Categories

ASME Section IX organizes welding variables into three categories, each with different compliance consequences:

Essential variables — any change from the qualified condition requires a new PQR and a new or revised WPS. You cannot simply revise the WPS document; the process must be re-tested with the new parameter range.

Supplementary essential variables — these additional variables become essential only when the WPS must meet Charpy V-notch (CVN) impact toughness requirements. They are triggered by the design specification, the code of construction, or the customer's purchase order specifying impact testing. When impact testing is not required, these variables can change with only a WPS revision.

Nonessential variables — changes require only a WPS revision. No new testing is needed, but the document must be updated before production welding begins under the changed condition.

This three-tier structure means the same physical change to a welding parameter can require full requalification on one job and only paperwork on another, depending solely on whether impact testing is specified.

QW-253 Essential Variables for SMAW

The essential variables in QW-253 that govern SMAW procedure qualification cover the process, filler metal, base metal, electrical characteristics, and weld joint geometry. Key entries include:

Filler Metal — F-Number (QW-404.4) A change to a different F-Number filler metal is an essential variable. SMAW electrodes are grouped into F-Numbers (F-1 through F-6 for carbon and low-alloy steels) based on usability characteristics. Moving from an F-4 electrode (E7018, E7016) to an F-3 electrode (E6011) requires requalification. Staying within F-4 — including changing between E7018, E7018-H4 R, and E7018-1 — does not trigger requalification for the essential variables (though the WPS must still accurately reflect what is used).

Filler Metal — A-Number (QW-404.5) A change in the weld metal chemical composition, defined by the A-Number classification in QW-442, is an essential variable. Most carbon steel SMAW electrodes deposit A-1 weld metal; low-alloy electrodes deposit A-2 through A-8 depending on the alloy content. Switching from a carbon steel E7018 (A-1) to an E8018-C3 low-nickel electrode (A-8) to weld the same A36 base metal would require requalification.

Base Metal — P-Number (QW-403.1 and QW-403.6) A change in the P-Number of the base metal — or, for some qualified ranges, a change in Group Number within the same P-Number — is an essential variable. Qualifying on P-1 carbon steel (A36, A572) covers a broad range of P-1 materials, but qualifying on P-8 austenitic stainless does not cover P-1, and vice versa. Qualifying on P-3 Cr-Mo low-alloy steels (A387 Gr 11, Gr 22) requires its own PQR even though these steels are common in pressure vessel work.

Electrical Characteristics — Current Type and Polarity (QW-409.1) Changing the current type (AC to DC or DC to AC) or polarity (DCEP to DCEN) is an essential variable. The arc behavior, deposition rate, and fusion characteristics differ meaningfully between AC and DCEP for SMAW, and the code requires the change to be validated by test.

Position — Vertical Welding Progression (QW-405.1) Changing from vertical-uphill to vertical-downhill progression, or the reverse, is an essential variable for SMAW. The heat input, bead geometry, and fusion behavior in vertical-down welding are different enough from vertical-up that a separate PQR test is required. Qualification on vertical-uphill does not qualify vertical-downhill on the same procedure.

Preheat — Decrease (QW-406.1) A decrease of more than 100°F (55°C) in the qualified minimum preheat temperature is an essential variable. If the PQR test was run at 200°F preheat, you may reduce preheat to 100°F without requalification; reducing below 100°F requires a new PQR. This variable is critical when shops attempt to reduce preheat for production speed on thicker P-1 sections.

PWHT — Addition or Deletion (QW-407.1) Adding post-weld heat treatment when the PQR was not tested with PWHT, or removing PWHT when it was included, is an essential variable. The mechanical properties of the weld metal and HAZ are changed by PWHT, and the code requires these changes to be tested. This is one of the most frequently encountered essential variable changes in repair welding, where PWHT may be specified on the repair WPS but was not used in the original procedure qualification.

Joint Design — Backing Removal (QW-402.1 and QW-402.4) Changing from a backed joint (steel backing bar) to an open-root joint, or vice versa, involves changes in root pass technique significant enough to require requalification. Adding or deleting backing is an essential variable.

QW-253 Supplementary Essential Variables (Impact Testing)

When the engineering specification requires Charpy V-notch impact testing, additional variables become essential. For SMAW, the supplementary essential variables in QW-253 include:

Increase in Heat Input (QW-409.1) A significant increase in heat input per unit length of weld, beyond the qualified range, is a supplementary essential variable when CVN testing is required. Higher heat input produces a larger and slower-cooling HAZ, which can reduce notch toughness — particularly in coarse-grained HAZ microstructures.

Increase in Preheat or Interpass Temperature (QW-406.3) Qualifying a procedure at 150°F preheat does not cover production welding at 350°F preheat when impact testing is required. Higher interpass temperatures extend the time in the elevated-temperature range, affecting HAZ toughness just as heat input does.

Change in Electrode Classification — Supplementary Designator (QW-404.26) When CVN testing is required, the supplementary designator on the electrode (such as the "-1" in E7018-1, indicating enhanced CVN performance at low temperature) is a supplementary essential variable. Changing from E7018-1 to E7018 when CVN testing is specified requires requalification.

Nonessential Variables — What CAN Change Without a New PQR

QW-253 nonessential variables for SMAW include:

  • Electrode diameter — changing from 3/32 to 5/32 in. requires only a WPS update
  • Pass sequence — changing from stringer beads to weave beads (within the electrode manufacturer's allowed travel width) is a nonessential variable for most SMAW procedures
  • Groove joint design details — within the qualified backing/no-backing category, changes to groove angle, root face, and root opening (within the fabrication tolerance ranges) are nonessential
  • Position — qualifying in the 3G and 4G positions covers all positions; changes between flat, horizontal, and overhead (for a procedure already qualified in those positions) don't trigger requalification

The practical implication: the WPS document must still be updated to reflect the actual production parameters, even for nonessential changes. An outdated WPS that shows 1/8 in. electrode when the welder is running 5/32 in. is a document control nonconformance, even though no new PQR is needed.

Comparing QW-253 to AWS D1.1 Table 6.6

QC managers who work under both ASME Section IX and AWS D1.1:2025 Table 6.6 often need to track which essential variables differ between the two codes for the same SMAW process.

Key differences:

  • ASME IX uses F-Numbers and A-Numbers to define filler metal scope; AWS D1.1 uses the electrode classification designation (E7018, E8018-C3, etc.) with specific brand-name latitude within classifications
  • ASME IX does not have a concept directly equivalent to AWS D1.1's "prequalified" WPS; all ASME IX WPS require PQR backing
  • AWS D1.1 Table 6.6 covers additional essential variables tied to specific structural connection types (e.g., backing bar removal for CJP welds is handled in D1.1 through the joint design qualification path)

For shops with dual-code WPS programs, a PQR run under ASME IX protocols can often also satisfy the AWS D1.1 Clause 6 tested-WPS qualification requirements — but the essential variable ranges need to be mapped against both tables before writing the WPS.

Keeping QW-253 Compliance Current

The most common QW-253 compliance lapse is an undocumented essential variable change in production. A welder who runs out of E7018 stock and borrows a box of E8018-C3 has just changed the A-Number — an essential variable — without authorization. The CWI's pre-weld inspection should verify that the electrode classification on the WPS matches what's at the joint.

For shops managing multiple ASME IX SMAW procedures across P-Number groups, a digital WPS library that links each WPS to its governing PQR and flags essential variable scope reduces the risk of an unauthorized change reaching production. The QW-253 table becomes a standing rule in the system, not a manual lookup.

Rule library based on ASME Section IX requirements; verify against the edition of Section IX specified in your code of construction. ASME IX is revised on a two-year cycle.