Can I substitute E7018-H4 for E7018 on an ASME IX qualified WPS without requalifying?

Both are F-4, so QW-404.4 is satisfied. However, verify whether the H4 diffusible-hydrogen designator affects any other essential variable on your WPS, and confirm the A-number remains unchanged. If all other essential and supplementary essential variables are unaffected, no new PQR is required.

Does F-number apply to procedure qualification, welder qualification, or both?

Both, but under different clauses. QW-404.4 governs procedure (WPS/PQR) qualification as an essential variable. QW-404.15 governs welder performance qualification — a change in F-number there can invalidate a welder's WPQ. Always check both when changing filler metal.

What F-number is ER70S-6 (GTAW/GMAW solid wire)?

ER70S-6 is F-6 per ASME IX QW-432. F-6 covers solid and metal-cored GTAW and GMAW filler metals for ferrous applications. It is not interchangeable with F-3 or F-4 SMAW electrodes under QW-404.4.

If a PQR was run with two processes (GTAW root, SMAW fill), do both F-numbers get qualified on that one PQR?

Yes — a combination-process PQR qualifies each process with its respective F-number, provided the PQR documents both fillers and the test results are acceptable for the combined joint. The WPS referencing it can then list both processes and their respective fillers.

Does changing filler diameter affect the F-number or trigger QW-404.4?

No. Filler diameter is covered by QW-404.22 (an essential variable for certain processes) but does not affect F-number. A diameter change may still require a WPS revision — evaluate QW-404.22 and heat input implications separately.

ASME IX QW-404.4 F-Number Explained for Pressure Vessel Shops

QW-404.4 F-numbers control filler-metal substitution on qualified WPSs. Here's what pressure vessel shops need to know to stay compliant.

F-numbers exist for one reason: to define which filler metals are interchangeable under a single procedure qualification without re-running a PQR coupon. Shops that swap electrodes without checking QW-404.4 routinely generate nonconformances — sometimes mid-production. Understanding the grouping logic and its limits is non-negotiable for any CWI signing off on ASME Section VIII or B31.3 work.

What QW-404.4 Actually Says

QW-404.4 classifies filler metals into F-numbers F-1 through F-6 (plus F-21 through F-45 for nonferrous and hard-facing materials defined in QW-432). A change in F-number is an essential variable for procedure qualification — full stop. If your qualified WPS lists an E7018 (F-4) and your purchasing team substitutes E6013 (F-3) to clear a material shortage, you have an unqualified procedure. The code doesn't care about tensile similarity.

The grouping logic loosely follows usability and slag-system characteristics: F-1 is high-cellulosic SMAW, F-2 is rutile, F-3 is low-hydrogen iron-powder with certain exceptions, F-4 is low-hydrogen, F-5 is austenitic stainless SMAW, F-6 covers solid and metal-cored GTAW/GMAW filler. The practical effect is that most common carbon-steel SMAW swaps live within F-3/F-4 territory — but those two are not interchangeable under QW-404.4.

QW-404.4 applies to both groove and fillet procedure qualifications. It is separate from the welder performance essential variable QW-404.15 (also F-number based), so don't conflate the two in your WPS vs PQR vs WPQ documentation.

F-Number vs A-Number: Don't Confuse Them

F-number governs usability/deposition characteristics. A-number (QW-404.5) governs deposited weld-metal chemical composition and is a separate essential variable for ferrous materials. A filler change can trigger both simultaneously — for example, switching from an E308 to an E309 stainless electrode changes both F-number (both F-5, so no impact) and A-number (A-8 vs A-9, which is an essential variable change requiring requalification).

Practical rule: when evaluating any filler substitution, pull QW-432 for the F-number assignment and QW-442 for the A-number. Run both checks before issuing a WPS revision. This is exactly the kind of dual-variable trap that shows up on common WPS mistakes that fail QC review.

Qualification Scope: What One F-Number Covers

QW-404.4 is not a range variable — it's a strict match. Qualifying with F-4 electrodes qualifies only F-4. There is no "qualified for F-4 and below" provision in ASME IX the way P-number groupings work for base-metal requalification under QW-403.8. Each F-number used in production must appear on a supporting PQR.

One important exception: QW-404.12 allows GTAW root passes qualified with a bare filler (F-6) to be used with or without filler, but that's a separate variable clause — it doesn't override QW-404.4's essential variable status.

For shops running multiple processes on a single joint (e.g., GTAW root / SMAW fill-and-cap), each filler must be independently traceable to a PQR that qualifies its F-number. A PQR run entirely with F-6 GTAW filler does not cover the F-4 SMAW cap on your production WPS. Both must appear, either on one combination-coupon PQR or two separate PQRs listed on the WPS.

Documentation and WPS Revision Triggers

Every filler metal listed on a production WPS must reference at least one PQR that qualifies that F-number under the applicable process, P-number, and thickness range. If your shop maintains multiple WPSs, a clean F-number matrix — cross-referencing WPS revision, supporting PQR number, filler AWS classification, and F-number — prevents the approval-chain gaps that auditors find instantly.

A filler substitution that changes F-number requires:

A new or existing PQR covering the new F-number (all other essential variables held).
A WPS revision listing the new filler and its supporting PQR — see WPS revision control best practices for revision-numbering discipline.
Updated welder qualification records if QW-404.15 is also triggered (check whether the welder's WPQ covers the new F-number).

If the substitution also changes A-number, thickness range, or P-number grouping, stack those essential variable checks before issuing the revision. Partial checks are how shops end up with technically unqualified procedures in active use.

Bottom Line

QW-404.4 F-number is a hard essential variable — no ranges, no grace, no engineering judgment override. Every filler metal on every process in every production WPS must trace back to a PQR that qualifies its F-number. Build the matrix, run the dual F/A-number check on every substitution, and make sure welder qualifications track any resulting QW-404.15 changes. The CWI signs it; the code enforces it.