What does an A-number classify in ASME Section IX?

An A-number classifies the chemical composition of the deposited weld metal — what the weld bead contains after solidification. This is separate from the F-number, which groups filler metals by the kind of welding they require. Two electrodes with the same F-number can deposit weld metal with different A-numbers if their alloying chemistry differs.

When does an A-number change trigger WPS requalification in ASME IX?

For the major arc welding processes (SMAW, SAW, GTAW, GMAW, FCAW), a change to a different A-number is an essential variable in QW-404. This means that a WPS qualified with A-1 weld metal does not cover production welding with A-2 weld metal, even if the F-number is the same. A new PQR with the different A-number weld metal is required.

How do you determine the A-number of a filler metal?

The electrode manufacturer's published data sheet or the AWS A5-series classification for that electrode type typically identifies the expected weld metal chemical composition. Comparing that composition against the ranges in QW-442 of ASME Section IX gives the A-number. When the data sheet doesn't clearly establish it, a deposited weld metal chemical analysis (sample tested in a lab) is the definitive method.

Does A-number apply to all ASME IX welding processes?

A-number is an essential variable for the major arc processes including SMAW, SAW, GMAW, GTAW, and FCAW. For processes where A-number is not listed as an essential variable, the weld metal composition classification doesn't drive requalification in the same way — check the applicable QW-250 series table for the specific process.

Jun 13, 20267 min read

ASME IX A-numbers: weld metal chemical classification

ASME IX A-numbers classify weld metal composition, not electrode brand. Understand when an A-number change forces WPS requalification and how to read QW-442.

ASME IX WPS PQR Filler Metal Weld Metal Qualification

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

ASME Section IX organizes weld procedure qualification around three grouping systems: P-numbers for base metals, F-numbers for filler metals, and A-numbers for weld metal chemical composition. P-numbers and F-numbers appear on almost every PQR record; A-numbers receive less attention in shop training. But for the major arc welding processes, a change in A-number is an essential variable — and shops that miss it during procedure reviews end up with WPS coverage gaps that surface during NBIC audits, ASME witness inspections, or third-party quality assessments.

What A-numbers classify and why they exist

F-numbers group filler metals based on their usability characteristics — the type of electrode and the welding technique it requires. An E7018 (F-4) and an E6010 (F-3) are in different F-number groups because they require fundamentally different welding techniques. F-numbers are assigned based on how the filler metal behaves in the hand of the welder.

A-numbers are different. They classify the deposited weld metal by chemical analysis — what the weld bead actually contains after the electrode burns, the flux reacts, and the puddle solidifies. This matters because two electrodes can be identical F-numbers (same technique requirements) while depositing chemically different weld metal. The F-number doesn't tell you what alloy you just put into the joint; the A-number does.

QW-442 in ASME Section IX lists the A-number groups with chemical composition ranges for each group. A-1 covers carbon steel weld metal — manganese-silicon systems without significant alloy additions — and is by far the most common group in structural and pressure vessel work with carbon steel base metals. A-2 covers carbon-molybdenum weld metal. A-3 through A-5 cover progressively higher chromium and molybdenum alloy steels (used in high-temperature pressure vessel and piping work). A-8 through A-12 cover austenitic stainless steel and high-nickel alloy weld metals.

The practical point: when you burn an E7018-A1 SMAW electrode, the deposited weld metal contains approximately 0.5% molybdenum — that's an A-2 weld metal, not A-1, despite the "7018" designation suggesting plain carbon steel. Two E7018-classified electrodes from different manufacturers may deposit A-1 or A-2 weld metal depending on their specific wire chemistry. The F-number is the same for both; the A-number may not be.

A-number as an essential variable in QW-404

ASME IX QW-404 addresses filler metal essential variables for arc welding processes. For SMAW, SAW, GTAW, GMAW, and FCAW, a change to a different A-number is listed as an essential variable in QW-404. This means:

If a PQR was run with A-1 weld metal and the shop wants to substitute a filler that deposits A-2 weld metal, a new PQR is required. The A-1 PQR does not cover A-2 production welding.
If a shop runs two electrodes of the same F-number classification — say, two different brands of E7018 — but one is A-1 and the other is A-2, switching between them on a production weld is an essential variable change even though the F-number and electrode designation are identical.
The WPS must identify the A-number of the qualified weld metal, and that A-number must match (or be supported by) the PQR record.

This is distinct from an F-number change. Both an F-number change and an A-number change can independently trigger requalification. A production change that happens to change both (e.g., switching from SMAW to GMAW with a different alloy filler) triggers both essential variables simultaneously.

The A-1 assumption and where it breaks down

For plain carbon steel work — structural steel, carbon steel pressure vessels, pipework in A-106 or A-53 material — almost all qualified SMAW, GMAW, and FCAW procedures use A-1 weld metal. The filler metals in routine use (E7018, ER70S-6, E71T-1C) all deposit A-1 weld metal chemistry, so shops often build procedures without ever consciously considering the A-number. The A-number field on the PQR is filled in as "A-1" and no one thinks about it again.

The assumption breaks down in three situations:

Low-alloy or chrome-moly filler substitution. A shop qualified on carbon steel fillers (A-1) wants to use a low-hydrogen chrome-moly electrode (such as E8018-B2, which deposits A-3 or A-4 weld metal) for a repair or upgrade. The existing A-1 PQR does not support this. A new PQR with the chrome-moly electrode is required — and the PQR must include the appropriate tensile and bend tests, and possibly impact tests if applicable, on deposited A-3 or A-4 weld metal.

AWS A5 specification revisions. When a filler metal manufacturer revises the wire chemistry for an existing classification — changing alloy additions, adjusting trace element limits — the deposited weld metal chemistry may shift across an A-number boundary even though the electrode classification designation is unchanged. This happened when the A5.36 specification was introduced for flux-cored electrodes with expanded designator systems. Shops that assume a classification is always A-1 without verifying against QW-442 can end up with a coverage gap.

Stainless steel and high-alloy work. For shops doing ASME IX work on austenitic stainless steel or nickel alloys, A-numbers become more differentiated. The transition from 308L filler (A-8) to 309L filler (A-9) crosses an A-number boundary even though both are austenitic stainless. A shop that qualifies a WPS for a 308L stainless filler and then substitutes 309L in production has made an essential variable change without a supporting PQR.

For an overview of how P-numbers and F-numbers interact with base metal and filler metal selection, see ASME IX P-numbers and F-numbers explained.

Finding A-number on the PQR record

Every PQR for arc welding processes should record the A-number of the filler metal used during the qualification test weld. This is a required entry — the PQR form should include a dedicated A-number field. When reviewing a PQR for adequacy, confirm that:

The A-number field is populated, not blank.
The A-number is consistent with the electrode classification listed on the PQR — you can cross-check this against QW-442 and the electrode's published chemistry.
The A-number on the WPS derived from that PQR matches the A-number on the PQR. A WPS that lists a filler metal depositing A-2 weld metal, when the supporting PQR only qualifies A-1, is not covered.

An auditor from an ASME-authorized inspection agency (AI) or a third-party quality survey will check these fields. A blank A-number is a documentation deficiency; a mismatched A-number is a qualification coverage deficiency.

When A-number fields are missing from old PQRs

Older PQRs — particularly those produced before shops implemented formal quality management systems, or those produced under earlier editions of ASME IX — sometimes lack the A-number entry entirely. The record may show electrode classification, heat number, and cert number but nothing for A-number.

The correction approach: go to the original filler metal's AWS A5-series data sheet for the product actually used (or the closest current equivalent of that classification) and identify the expected weld metal chemistry. Compare against QW-442. If the chemistry clearly falls in A-1 (the most common outcome for carbon steel work), document that determination in a record amendment and attach it to the PQR. If the chemistry can't be confirmed from available documentation, a new PQR with current materials and complete documentation is the clean solution.

This kind of gap is routine in PQR review for shops trying to update legacy procedure libraries to ASME standards. For how the review process fits into a qualification audit, see reading a PQR test report and common WPS deficiencies in third-party audits.

A-numbers vs. F-numbers: the combined check

Both F-numbers and A-numbers must be checked when reviewing whether a WPS change requires new qualification. The two essential variable checks are independent:

F-number check: Is the filler metal in the same F-number group as what was qualified? If not, requalification is required regardless of A-number.
A-number check: Does the filler metal deposit the same A-number weld metal as what was qualified? If not, requalification is required regardless of F-number.

A change that triggers one but not the other is still a requalification trigger. A change that triggers both requires the new PQR to address both variables.

For shops managing multiple welding processes (SMAW for root passes, GMAW for fill) on a single WPS, the A-number check applies to each process independently. The root pass and fill pass may use filler metals with different A-numbers, and each must be supported by a PQR that qualifies that A-number for that process.

For the broader picture of how essential variables are categorized in ASME IX — essential, supplementary essential, and nonessential — see ASME IX essential variable types in QW-250.

Documenting A-number on the WPS

The WPS should identify the A-number for each process and filler metal it qualifies. This doesn't require listing every electrode brand — the A-number groups filler metals at the chemical composition level, so the WPS can state "A-1 weld metal (per QW-442)" rather than listing individual product names. When a shop substitutes a different brand of the same classification, verifying that the new product's deposited chemistry still falls in A-1 (or the specified group) is the check that keeps the WPS current without requiring a new PQR.

If your shop manages multiple WPS/PQR sets across SMAW, SAW, GMAW, and FCAW processes with varying filler metals, tracking A-number coverage across those procedures manually is error-prone. A purpose-built procedure management tool that cross-references PQR A-numbers against WPS filler metal selections helps catch coverage gaps before an audit does. See how WPS tracking software handles this.

Verify against the current edition of ASME BPVC Section IX applicable to your project — code requirements and table references are edition-specific.

FAQ

Frequently asked.

What does an A-number classify in ASME Section IX?: An A-number classifies the chemical composition of the deposited weld metal — what the weld bead contains after solidification. This is separate from the F-number, which groups filler metals by the kind of welding they require. Two electrodes with the same F-number can deposit weld metal with different A-numbers if their alloying chemistry differs.
When does an A-number change trigger WPS requalification in ASME IX?: For the major arc welding processes (SMAW, SAW, GTAW, GMAW, FCAW), a change to a different A-number is an essential variable in QW-404. This means that a WPS qualified with A-1 weld metal does not cover production welding with A-2 weld metal, even if the F-number is the same. A new PQR with the different A-number weld metal is required.
How do you determine the A-number of a filler metal?: The electrode manufacturer's published data sheet or the AWS A5-series classification for that electrode type typically identifies the expected weld metal chemical composition. Comparing that composition against the ranges in QW-442 of ASME Section IX gives the A-number. When the data sheet doesn't clearly establish it, a deposited weld metal chemical analysis (sample tested in a lab) is the definitive method.
Does A-number apply to all ASME IX welding processes?: A-number is an essential variable for the major arc processes including SMAW, SAW, GMAW, GTAW, and FCAW. For processes where A-number is not listed as an essential variable, the weld metal composition classification doesn't drive requalification in the same way — check the applicable QW-250 series table for the specific process.