9.2 Filler Metal F-Numbers and As-Deposited Chemistry A-Numbers

9.2 Filler Metal F-Numbers and As-Deposited Chemistry A-Numbers

Core Principle: In ASME Section IX, filler metals are governed by two distinct classification systems that serve entirely different engineering functions: F-Numbers (Table QW-432) classify electrodes by operator handling and usability characteristics, whereas A-Numbers (Table QW-442) classify ferrous weld deposits by their final as-deposited chemical composition. An inspector must never confuse usability (how an electrode runs) with metallurgy (the chemical composition of the deposited weld metal).

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1. Engineering Basis of Table QW-432 F-Numbers

Under paragraph QW-430, filler metals are assigned F-Numbers to reduce the number of welding procedure and welder performance qualifications. The fundamental basis of F-Number assignment is usability characteristics—the operational behavior of the flux coating, slag viscosity, arc force, wetting characteristics, and operator dexterity required to deposit sound weld metal in various positions.

Table QW-432 organizes filler metals across ASME Section II Part C (SFA specifications) into distinct F-Number categories. For steel welding, the first six F-Numbers represent the cornerstone of the qualification system.

+-----------------------------------------------------------------------------------------+
|                   THE STEEL SMAW COATING SPECTRUM (F-1 THROUGH F-4)                     |
+-----------------------------------------------------------------------------------------+
| F-No. 1 | Heavy Rutile / Iron Powder | High deposition, thick slag, flat/horizontal     |
| F-No. 2 | Titania / Rutile           | Medium penetration, smooth arc, sheet metal      |
| F-No. 3 | Cellulosic                 | Deep penetration, fast freezing, high arc force  |
| F-No. 4 | Low-Hydrogen Basic         | Clean weld metal, high impact toughness, crack-resistant|
+-----------------------------------------------------------------------------------------+

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2. In-Depth Breakdown of Steel and Alloy F-Numbers

Nonferrous and Specialty F-Numbers

Beyond the steel classifications, Table QW-432 assigns specialized blocks for nonferrous alloys:

• F-No. 21 through F-No. 25: Aluminum and aluminum-base alloys (SFA-5.10, e.g., ER4043, ER5356).
• F-No. 31 through F-No. 37: Copper and copper-base alloys (SFA-5.6, SFA-5.7, SFA-5.8).
• F-No. 41 through F-No. 46: Nickel and nickel-base alloys (SFA-5.11 covered electrodes, SFA-5.14 bare wire). Notably, F-No. 43 encompasses the premier nickel-chromium welding alloys: Alloy 625 (ERNiCrMo-3 / ENiCrMo-3) and Alloy 82 (ERNiCr-3).
• F-No. 51 through F-No. 56: Titanium and titanium-base alloys.
• F-No. 61: Zirconium and zirconium-base alloys.
• F-No. 71 through F-No. 72: Hard-facing weld metal overlay alloys.

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3. Procedure Qualification vs. Welder Qualification Rules for F-Numbers

One of the greatest points of divergence in ASME Section IX is how F-Numbers are treated for Procedure Qualification (WPS/PQR) versus Welder Performance Qualification (WPQ).

The Strict Procedure Qualification Rule: Paragraph QW-404.4

For procedure qualification under Article II, paragraph QW-404.4 designates a change from one F-Number to any other F-Number as an essential variable (or to any filler metal not assigned an F-Number).

There is zero cross-qualification between F-Numbers for welding procedures! A PQR qualified using an E7018 (F-No. 4) electrode cannot support a WPS specifying E6010 (F-No. 3), nor can it support an ER70S-6 (F-No. 6) GTAW wire. Each F-Number represents a unique metallurgical flux formulation and deposit dynamic that must be independently qualified by mechanical testing.

The Flexible Welder Qualification Hierarchy: Table QW-433

For welder performance qualification under Article III, Section IX recognizes that welders who master more difficult electrode coatings possess the manual dexterity to handle easier coatings. Thus, Table QW-433 establishes a progressive qualification hierarchy for groove welds:

+-----------------------------------------------------------------------------------------+
|                    THE F-4 WELDER QUALIFICATION SUPERIORITY RULE                        |
+-----------------------------------------------------------------------------------------+
| Welder qualifies on pipe with E7018 (F-4) on a backing ring:                            |
|  - Can they weld E7024 (F-1) with backing? YES!                                         |
|  - Can they weld E6013 (F-2) with backing? YES!                                         |
|  - Can they weld E6010 (F-3) with backing? YES!                                         |
|  - Can they weld E7018 (F-4) with backing? YES!                                         |
|  - Can they weld E6010 (F-3) open-root (without backing)? NO! (Tested with backing).     |
+-----------------------------------------------------------------------------------------+

[!IMPORTANT] The Open-Root Welder Trap: A welder who tests with an F-No. 4 electrode without backing qualifies to weld F-No. 4 without backing, but for F-No. 1, F-No. 2, and F-No. 3, they are qualified with backing only! Qualifying open-root with low-hydrogen basic electrodes (which requires dragging or weaving a tight arc) does not prove competence in whipping and manipulating an open-root cellulosic keyhole (E6010).

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4. Table QW-442 A-Numbers: As-Deposited Weld Metal Chemical Composition

While F-Numbers define usability, A-Numbers (Table QW-442) classify the final chemical composition of the deposited weld metal. Understanding A-Numbers is critical because mechanical properties, corrosion resistance, and postweld heat treatment responses depend entirely on weld chemistry.

The Scope Boundary: Ferrous Metals Only!

Table QW-442 applies strictly to ferrous (iron-base) weld metals. Nonferrous alloys—including aluminum, copper, nickel, and titanium filler metals—do not have A-Numbers. One of the most common CWI exam tricks asks candidates to find the A-Number for an Inconel 625 (ERNiCrMo-3) or aluminum (ER4043) deposit; the correct answer is always that no A-Number exists.

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5. Establishing A-Numbers under QW-404.5 & Requalification Rules

Paragraph QW-404.5 governs how a fabricator determines and verifies the A-Number of a weld deposit. A change in the weld metal A-Number is an essential variable for procedure qualification. If a WPS was qualified using an A-No. 1 deposit (carbon steel), changing production welding to an A-No. 2 (C-Mo) or A-No. 3 (Cr-Mo) deposit requires qualifying an entirely new PQR.

Methods for Establishing A-Number Compliance (QW-404.5)

Section IX provides four distinct methods for an organization to determine the A-Number on a PQR:

1. Chemical Analysis of the Actual PQR Weld Deposit: Taking chips or drillings directly from the qualification coupon weld metal and conducting a laboratory spectrometer analysis.
2. Chemical Analysis from an All-Weld-Metal Coupon: Using certified chemistry data from a separate coupon welded in accordance with the applicable SFA specification.
3. Certified Material Test Report (CMTR) / Manufacturer's Certification: Relying on the filler metal manufacturer's certified chemical analysis for that specific heat/lot of electrode or flux-wire combination.
4. AWS Classification Chemistry in Section II Part C: Assuming the weld deposit matches the chemical composition limits published in the SFA specification for that AWS classification.

[!CAUTION] Special Warning for SAW and FCAW: Under QW-404.5, for Submerged Arc Welding (SAW) and Gas-Shielded Flux-Cored Arc Welding (FCAW), the alloy content of the deposit depends heavily on flux alloy transfer, wire dilution, and shielding gas interaction. Fabricators cannot simply assume wire chemistry equals deposit chemistry; the flux-wire combination or lot CMTR must verify the final deposited A-Number.

Essential Variable Contrast: Welder Qualification Immunity

Does a change in A-Number affect a welder's qualification? No! A-Numbers are never an essential variable for welder performance qualification. A welder who qualifies on carbon steel using E7018 (F-No. 4, A-No. 1) is fully qualified to weld P-No. 4 chrome-moly steel using E8018-B2 (F-No. 4, A-No. 3) in production! The welder's manual manipulation of the basic flux puddle is identical, even though the chemical composition differs.

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6. Practical Exam Traps & Code Navigation Strategies

• Trap 1: Confusing F-Number Welder Rules with Procedure Rules: A question states: "A manufacturer qualifies a PQR using SMAW with an AWS E7018 (F-No. 4) electrode. Can the manufacturer issue a WPS specifying AWS E6010 (F-No. 3) based on this PQR?" Candidates often remember that an F-4 welder can weld F-3 with backing, and mistakenly answer YES. For procedures (WPS/PQR), F-Number changes are strictly essential variables under QW-404.4 with zero cross-qualification. The answer is an emphatic NO.
• Trap 2: Assigning A-Numbers to Nonferrous Alloys: If an exam item asks for the A-Number of an ERNiCr-3 (Inconel 82) or ERCuSi-A (silicon bronze) filler metal, look for the option stating: "No A-Number is assigned because Table QW-442 applies only to ferrous weld metals."
• Trap 3: F-6 Process Boundaries for Welders: Under Table QW-433, testing with an F-No. 6 filler metal qualifies only F-No. 6 filler metals. An operator qualified on GTAW using ER70S-6 (F-6) is never qualified to weld SMAW with E7018 (F-4) or E6010 (F-3) without a separate performance test.