ASME BPVC · Section IX · QW-432

ASME IX F-Numbers — Filler Metal Grouping for Welding Qualification

F-numbers are ASME Section IX filler metal classifications that group electrodes, wires, and rods by usability characteristics. They determine welder performance qualification ranges and are essential variables for procedure qualification, reducing the number of separate qualifications a fabricator must maintain across filler metal products.

Practical impact: A welder qualified with E7018 (F-Number 4) is also qualified to use E7016 and E7028 because all three share F-Number 4 usability characteristics. But switching to E6010 (F-Number 3) requires re-qualification — the handling characteristics are fundamentally different even though both are SMAW electrodes.

What Are F-Numbers?

F-numbers are the filler metal classification system in ASME Section IX that groups electrodes, wires, and rods by how they handle during welding — their usability characteristics. The system is listed in QW-432 of Section IX. Unlike P-numbers which group base metals by composition and weldability, F-numbers group filler metals by the skill required to use them.

The logic behind F-number grouping is that filler metals within the same F-number handle similarly enough that a welder who can successfully use one can use any other in the group without additional qualification testing. An E7018 and an E7016 both use a low-hydrogen coating with iron powder, producing similar arc characteristics, slag behavior, and deposition patterns. An E6010, by contrast, produces a deeply penetrating, forceful arc with a fast-freezing cellulosic slag — a completely different welding experience that requires different technique.

F-numbers apply to both procedure qualification and welder performance qualification. Changing F-number is an essential variable for both. This means a new PQR is required for procedure qualification, and a new test weld is required for welder qualification.

Carbon Steel F-Number Assignments (SMAW)

F-Number 1 — E6020, E7020, E7027: High iron oxide electrodes with a fluid slag and high deposition rate. Primarily used in the flat and horizontal positions. The heavy, fluid slag makes these electrodes unsuitable for vertical or overhead welding. Less common in modern fabrication but still used for certain flat-position production applications.
F-Number 2 — E6012, E6013, E6014, E7014: Rutile-type electrodes with a medium-penetration arc and easy slag removal. E6013 is the most common in this group. These electrodes are popular for general-purpose fabrication and sheet metal work. They produce a smooth, quiet arc with minimal spatter but are not classified as low-hydrogen — they should not be used for preheat-critical or high-restraint applications under D1.1 or similar codes.
F-Number 3 — E6010, E6011: Cellulosic-covered electrodes with a deeply penetrating, forceful arc and fast-freezing slag. E6010 (DC only) and E6011 (AC/DC) are the standard electrodes for pipeline root passes under API 1104 and for open-root groove welds in structural work. The aggressive arc requires specific welder technique. These electrodes are NOT low-hydrogen — they produce the highest diffusible hydrogen levels of any SMAW electrode group.
F-Number 4 — E7018, E7016, E7028, E7048: Low-hydrogen iron powder electrodes. This is the most commonly specified F-number in pressure vessel and structural fabrication. E7018 is the dominant electrode in this group. Low-hydrogen characteristics are critical for avoiding hydrogen-assisted cracking in higher-carbon and alloy steels. D1.1 and ASME IX both impose storage requirements (heated rod ovens) for these electrodes because moisture absorption degrades their low-hydrogen performance.

Wire F-Number Assignments (GMAW, GTAW, FCAW, SAW)

F-Number 6 — Carbon Steel Solid Wires: ER70S-2, ER70S-3, ER70S-6, and other carbon steel solid wires per AWS A5.18. Used in GMAW (MIG) and GTAW (TIG) processes. ER70S-6 is the most widely used GMAW wire in structural and pressure vessel fabrication. All F-Number 6 wires produce inherently low-hydrogen weld deposits. A welder qualified with ER70S-6 (F-6) can also use ER70S-3 without re-qualification.
F-Number 36 — Carbon Steel Flux-Cored Wires: E71T-1, E71T-8, E71T-9, and other carbon steel flux-cored wires per AWS A5.20. FCAW wires are widely used in structural fabrication for higher deposition rates than SMAW. E71T-1 (gas-shielded) and E71T-8 (self-shielded) are the most common. Despite sharing an F-number, gas-shielded and self-shielded FCAW wires require different equipment and technique. The F-number grouping reflects the similar wire-feeding and arc characteristics within the FCAW process family.
F-Number 23 — Aluminum Alloy Filler Metals: ER4043, ER5356, ER5183, and other aluminum filler metals per AWS A5.10. F-Numbers 21 through 25 cover aluminum and aluminum alloy filler metals in QW-432.2. ER4043 (F-23) is the most widely used aluminum GTAW/GMAW wire, suitable for most 6xxx-series base metals. ER5356 (F-22) is preferred for 5xxx-series alloys and higher-strength applications.
F-Number 43 — Nickel Alloy Filler Metals: ERNiCr-3, ENiCrFe-2, ENiCrFe-3, and other nickel-based filler metals per AWS A5.14 and A5.11. F-Numbers 41 through 45 cover nickel and nickel alloy filler metals in QW-432.4. Used for dissimilar metal joints, overlay welding, and high-temperature service. Common in power generation and petrochemical applications where carbon steel transitions to stainless steel or where high-temperature corrosion resistance is required.
F-Number 6 — Austenitic Stainless Steel Solid Wires: ER308L, ER309L, ER316L, and other 300-series stainless steel wires per AWS A5.9. Stainless steel bare wires share F-Number 6 with carbon steel solid wires in QW-432.1. ER308L is the most common for 304 stainless steel, ER316L for 316 stainless steel, and ER309L for dissimilar joints between carbon steel and stainless steel. Note that F-Number 6 covers both carbon steel and stainless steel solid wires — a welder qualified with ER70S-6 (carbon steel) is also qualified for ER308L (stainless) since both are F-6.

A-Numbers (Weld Metal Classification)

While F-numbers classify filler metals by usability, A-numbers classify the deposited weld metal by chemical composition. A-numbers are listed in QW-442 of ASME IX. Both F-number and A-number are independent essential variables for procedure qualification. A-Number 1 covers carbon steel weld deposits, A-Number 2 covers carbon-molybdenum deposits, A-Number 4 covers chromium-molybdenum deposits, and A-Number 8 covers austenitic stainless steel deposits.

The distinction matters because two filler metals with the same F-number can produce different A-number deposits. The F-number determines welder qualification, while the A-number determines whether the weld metal properties match the application requirements. A complete procedure qualification evaluates both classifications.

How F-Numbers Compare to D1.1 Filler Metal Rules

D1.1 does not use F-numbers. Instead, D1.1 specifies acceptable filler metals through Table 5.7 (now split into process-specific tables in the 2025 edition), which lists approved electrode/wire classifications for each base metal group. D1.1 treats each AWS classification as a separate essential variable — changing from E7018 to E71T-1 requires re-qualification even though both produce 70 ksi weld metal. This is more restrictive than the ASME IX approach where the F-number grouping may allow changes within a group.

Fabricators working under both D1.1 and ASME IX should maintain separate qualification documentation. A procedure qualified under D1.1 using E7018 does not automatically qualify under ASME IX, and an ASME IX F-Number 4 qualification does not satisfy D1.1 requirements for specific electrode classification approval.

Aspect	ASME IX F-Numbers	D1.1 Table 5.7 / A5.x	CSA W48
Grouping basis	Usability characteristics	AWS A5.x classification	CSA designation
E7018 group	F-No. 4 (low-hydrogen)	A5.1 (Table 5.7 Group I–II)	E49018
ER70S-6 group	F-No. 6	A5.18 (Table 5.7 Group I–II)	ER49S-6
E71T-1 group	F-No. 6	A5.20 (Table 5.7 Group I–II)	E49XT-1
Cross-qualification	Within same F-number	Per specific classification	Per W48 classification
Interchangeable?	No — D1.1 and ASME IX use different filler metal grouping systems

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Frequently Asked Questions

What are F-numbers in welding?

F-numbers are ASME Section IX filler metal groupings that classify electrodes, wires, and rods by usability characteristics. Filler metals within the same F-number handle similarly during welding, which means a welder qualified with one filler metal in the group is qualified for all filler metals in that group. F-Number 4 covers low-hydrogen SMAW electrodes (E7018, E7016), F-Number 6 covers carbon steel GMAW/GTAW wires (ER70S-6, ER70S-3), and F-Number 36 covers carbon steel flux-cored wires (E71T-1). Changing F-number is an essential variable for both procedure and performance qualification.

What is the difference between F-numbers and A-numbers?

F-numbers classify filler metals by usability (how they handle during welding), while A-numbers classify the deposited weld metal by chemical composition. F-numbers affect welder qualification because they reflect the skill required to use the filler metal. A-numbers affect procedure qualification because they reflect the mechanical and chemical properties of the completed weld. For example, E7018 is F-Number 4 (low-hydrogen usability) and A-Number 1 (carbon steel weld deposit). Both F-number and A-number changes are essential variables, but they serve different purposes in the qualification framework.

What F-number is E7018?

E7018 is classified as F-Number 4 in ASME Section IX. F-Number 4 covers low-hydrogen SMAW electrodes with iron powder coverings, including E7018, E7016, E7028, and E7048. All F-Number 4 electrodes share low-hydrogen characteristics and similar usability. A welder qualified with E7018 (F-4) is also qualified to use E7016 and E7028 because they share the same F-number. However, switching to E6010 (F-Number 3) or E6013 (F-Number 2) requires re-qualification because those are different F-numbers.

What F-number is ER70S-6?

ER70S-6 is classified as F-Number 6 in ASME Section IX. F-Number 6 covers carbon steel solid wires used in GMAW and GTAW processes, including ER70S-2, ER70S-3, and ER70S-6. All F-Number 6 wires are solid carbon steel wires with similar handling characteristics. A welder qualified with ER70S-6 (F-6) can also use ER70S-3 without re-qualification. However, switching to a flux-cored wire like E71T-1 (F-Number 36) requires re-qualification because the usability characteristics are fundamentally different.

Does changing F-number require a new PQR?

Yes. Changing F-number is an essential variable for both procedure qualification and performance qualification in ASME Section IX. A procedure qualified with F-Number 4 (E7018) does not qualify for F-Number 6 (ER70S-6). A welder qualified with F-Number 6 (GMAW wire) does not qualify for F-Number 4 (SMAW electrode). Each F-number change requires a new PQR for procedure qualification and a new test for performance qualification. The A-number (weld metal composition) must also be considered — both F-number and A-number are independent essential variables.