8.2 Submerged Arc Welding (SAW) Procedure Variables and Flux-Wire Combinations

8.2 Submerged Arc Welding (SAW) Procedure Variables and Flux-Wire Combinations

Core Principle: Submerged Arc Welding (SAW) is an automated, high-deposition process where an electric arc burns invisibly beneath a thick, protective blanket of granular, fusible flux. Unlike gas-shielded or manual processes where wire and shielding are evaluated independently, ASME Section IX Table QW-258 treats the SAW flux and consumable electrode as an indivisible chemical and metallurgical couple. Changes to flux classification (QW-404.9), slag recycling practices (QW-404.36), multi-electrode configurations (QW-410.9), or supplemental cold wire additions (QW-404.24) represent essential variables that alter weld pool chemistry and demand full procedure requalification.

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1. Table QW-258 Architecture and SAW Operational Physics

Submerged Arc Welding operates at extremely high electrical currents ($300\text{ to } 1500+\text{ A}$), delivering deposition rates of $10\text{ to } 60+\text{ lb/hr}$ with thermal efficiencies exceeding $85%\text{ to } 90%$. The molten flux blanket performs three indispensable roles:

1. Atmospheric Shielding: Melts into a conductive liquid slag barrier that completely excludes oxygen, nitrogen, and ambient humidity from the superheated molten pool.
2. Electrochemical Refining & Deoxidation: Reacts with dissolved oxides in the puddle, scavenging impurities into the floating slag via manganese ($Mn$), silicon ($Si$), and calcium fluoride ($CaF_2$) additions.
3. Thermal Insulation & Bead Shaping: Retards the cooling rate of the weld pool, promoting smooth bead transitions and facilitating the escape of entrapped gases.

+----------------------------------------------------------------------------------------------------+
|                                 SUBMERGED ARC WELDING (SAW) SCHEMATIC                              |
+----------------------------------------------------------------------------------------------------+
|                [Consumable Bare Wire Electrode (Continuous Spool)]                                 |
|                                    |                                                               |
|                                    v (Motorized Wire Feed Rolls)                                  |
|                          [Contact Tip / Jaw]                                                       |
|                                    |                                                               |
|         [Granular Flux Blanket]    v (Arc Current 300 - 1500 A)                                    |
|        ~~~~~~~~~~~~~~~~~~~~~~~~(Submerged Arc)~~~~~~~~~~~~~~~~~~~~~~~~                             |
|        [Solidified Slag] ->  (====== Molten Slag ======)                                           |
|        [Weld Metal Bead] ->  (=== Molten Weld Pool ===)                                            |
|        =====================[Base Metal Workpiece]=====================                            |
+----------------------------------------------------------------------------------------------------+

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2. Table QW-258 Variable Classification Matrix

Inspectors and engineers preparing for the CWI endorsement exam must know every variable in Table QW-258 and its exact classification:

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3. The Flux-Wire Combination: Inseparable Metallurgical Coupling (QW-404.9)

In SMAW or GTAW, an electrode specification (e.g., SFA-5.1 E7018 or SFA-5.18 ER70S-6) defines mechanical properties independently of any external flux brand. In SAW, an electrode wire alone has no certified tensile, yield, or impact properties!

AWS SFA-5.17 / SFA-5.23 Classification System

Mechanical integrity is certified exclusively for the flux-wire combination as a unified system under AWS A5.17 (carbon steel) or AWS A5.23 (low-alloy steel):

$\mathbf{F7A2-EM12K}$

• F: Submerged Arc Welding Flux
• 7: Minimum tensile strength in increments of $10\text{ ksi}$ ($70\text{ to } 95\text{ ksi}$, or $480\text{ to } 650\text{ MPa}$)
• A: As-welded condition (P indicates Postweld Heat Treated condition)
• 2: Minimum Charpy V-notch impact toughness of $20\text{ ft-lbf}$ at $-20^\circ\text{F}$ ($-29^\circ\text{C}$)
• E: Electrode (wire)
• M: Medium manganese content ($0.80%\text{ to } 1.25%\text{ Mn}$)
• 12: Nominal carbon content in hundredths of a percent ($0.12%\text{ C}$)
• K: Produced from killed (fully deoxidized) steel

The QW-404.9 Essential Variable Rule

Under paragraph QW-404.9, an essential variable is triggered by:

1. A change in the indicator for flux-wire classification (e.g., from F7A2-EM12K to F7A4-EM12K, or from F7A2-EM12K to F7P2-EM12K).
2. A change in the wire manufacturer or trade name when using unclassified or proprietary fluxes.
3. A change in flux manufacturer trade name, even when the AWS classification designation remains identical!

CRITICAL CODE DISTINCTION: Unlike bare wire in GMAW or GTAW (where an ER70S-6 wire from Manufacturer A can be substituted with ER70S-6 from Manufacturer B under the same WPS), you cannot swap flux brands in SAW without qualifying a new PQR! A Lincoln 761 flux combined with Lincoln L-61 wire cannot be replaced by an ESAB OK 10.71 flux and OK 12.22 wire without a new procedure qualification test coupon, even if both combinations are certified as F7A2-EM12K.

Neutral vs. Active Fluxes (QW-404.34)

• Neutral Fluxes: Formulated with a balanced chemical basicity index ($BI \approx 1.2\text{ to } 2.5$). They do not significantly alter the manganese or silicon content of the weld deposit, regardless of changes in arc voltage or pass count. Essential for heavy-section, multi-pass groove welds where deposit chemistry must remain constant from root to cap.
• Active Fluxes: Contain intentional additions of manganese and silicon deoxidizers. They resist porosity when welding over mill scale, light rust, and oil on single-pass or two-pass fillet/groove applications. However, in multi-pass welds, each subsequent pass introduces cumulative manganese and silicon buildup. This causes uncontrolled hardening ($> 300\text{ HB}$), severe embrittlement, and centerline cracking.
• Code Rule (QW-404.34): A change from a neutral flux to an active flux, or vice versa, is an essential variable.

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4. Crushed Slag and Recycled Flux Rules: Paragraph QW-404.36

In high-volume SAW operations (pipe mills, structural beam fabrication, shipyards), vast quantities of glassy, solidified slag are ejected as the weld bead cools. Historically, fabricators attempted to crush, regrind, and blend this waste slag back into virgin flux hoppers to cut consumable costs.

Metallurgical Degradation from Crushed Slag

When granular virgin flux is melted in the arc, vital thermodynamic reactions occur:

1. Deoxidizing elements (Mn, Si, Ti, Al) react with oxygen in the molten puddle and are consumed, forming stable oxides ($MnO$, $SiO_2$) that partition into the slag.
2. Volatile fluoride compounds ($CaF_2$) vaporize, altering arc stability and slag detachment characteristics.
3. Tramp impurities from the plate surface (sulfur, phosphorus, mill scale, moisture) are absorbed into the slag structure.

When this exhausted slag is pulverized and reused, the resulting flux mixture is depleted of active deoxidizers and enriched in oxides and contaminants. Re-welding with crushed slag yields coarse columnar microstructures, catastrophic loss of low-temperature Charpy V-notch toughness, and elevated porosity.

The QW-404.36 Statutory Mandate

+----------------------------------------------------------------------------------------------------+
|                           PARAGRAPH QW-404.36 REGROUND SLAG RESTRICTION                            |
+----------------------------------------------------------------------------------------------------+
| FABRICATION ACTION                                     | ASME SECTION IX CODE STATUS               |
+--------------------------------------------------------+-------------------------------------------+
| Addition of crushed / reground slag                    | ESSENTIAL VARIABLE (Requalification Req.) |
| Deletion of crushed / reground slag                    | ESSENTIAL VARIABLE (Requalification Req.) |
| Change in ratio of virgin flux to crushed slag         | ESSENTIAL VARIABLE (Requalification Req.) |
+--------------------------------------------------------+-------------------------------------------+

Exam Trap: An exam question might ask: "A manufacturer adds 20% reground crushed slag to virgin SAW flux. Is this acceptable?" The answer: It is an essential variable under QW-404.36. The manufacturer must weld a PQR coupon using that exact 20% blend, pass all tension and bend tests, and document the specific crushed slag ratio on the PQR and WPS. Crushed slag cannot be introduced into an existing virgin-flux WPS without requalification!

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5. Multi-Wire vs Single-Wire Arc Systems (QW-410.7 & QW-410.9)

To achieve massive deposition rates ($40\text{ to } 80+\text{ lb/hr}$) in spiral pipe mills and heavy pressure vessel girth seams, SAW systems frequently employ multiple wire electrodes operating in a single molten puddle or in closely spaced tandem puddles.

Arc Configurations

1. Single-Wire SAW: One continuous electrode feeding into the arc zone ($300\text{ to } 800\text{ A}$).
2. Twin-Wire SAW (Parallel Arc): Two wires fed simultaneously through a single contact tip powered by a single power source. Delivers higher deposition at lower current density.
3. Tandem-Arc SAW (Multi-Power): Two or more independent wire feeds, each energized by its own dedicated power supply and contact tip:
   • Lead Arc: Typically powered by DCEP to ensure deep penetration at the root of the joint.
   • Trail Arc(s): Powered by AC (Alternating Current). Utilizing AC on the trailing wire eliminates magnetic arc blow between adjacent arcs while filling the groove and flattening the bead profile.

+----------------------------------------------------------------------------------------------------+
|                                 TANDEM-ARC SAW CONFIGURATION (QW-410.9)                            |
+----------------------------------------------------------------------------------------------------+
|           [Lead Electrode: DCEP]                 [Trail Electrode: AC]                             |
|             (Deep Penetration)                   (Bead Shaping & Fill)                             |
|                     |                                      |                                       |
|                     v                                      v                                       |
|               (Lead Arc Zone)  <-- Spacing (S) -->   (Trail Arc Zone)                              |
|         ~~~~~~~~~~~~~~~~~~~~~~~~(Shared Flux Blanket)~~~~~~~~~~~~~~~~~~~~~~~~                      |
|         =====================[Heavy Plate Workpiece]=========================                      |
+----------------------------------------------------------------------------------------------------+

The QW-410.7 and QW-410.9 Rules

• QW-410.7: A change from a single electrode to multiple electrodes, or vice versa, is an essential variable.
• QW-410.9: In multiple-electrode systems, a change in electrode spacing, alignment angle, or electrical configuration (e.g., switching the trail arc from AC to DC) is an essential variable.

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6. Supplemental Filler Metal Additions: Paragraph QW-404.24

In ultra-high productivity SAW applications, fabricators introduce supplemental cold metal into the arc cavity without passing current through it:

1. Cold Wire Addition: An auxiliary solid wire fed into the molten puddle, melted solely by excess thermal energy from the primary arc.
2. Iron Powder / Cut Wire Addition: Granular metallic iron or chopped alloy wire metered into the groove bevel ahead of the flux blanket.

Metallurgical Impact

Supplemental filler metal acts as an internal heatsink, absorbing arc superheat to melt. This drastically increases deposition rate ($+30%\text{ to }+100%$), decreases base metal dilution, narrows the coarse-grained HAZ, and speeds up weld puddle solidification.

The QW-404.24 Rule

Under paragraph QW-404.24, the addition or deletion of supplemental filler metal is an essential variable. A PQR qualified with cold wire feed cannot support a WPS welded without cold wire, and vice versa.

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7. Heavy-Deposition SAW and Table QW-451.1 Note (1)

Because SAW deposits heavy beads, candidates must cross-reference Table QW-451.1 Note (1) rules:

• If a test coupon deposits weld metal $t \ge 3/4\text{ in.}$ ($19\text{ mm}$) using three or more layers, the maximum qualified weld metal thickness is $8.0\text{ in.}$ ($200\text{ mm}$).
• If high-amperage SAW deposits $t = 0.75\text{ in.}$ in only one or two massive passes, Note (1) does not apply! The maximum qualified weld metal thickness is restricted to standard $2t = 1.50\text{ in.}$

Single-Pass Grain Coarsening Hazard (QW-403.9 & QW-410.8)

When SAW deposits passes thicker than $1/2\text{ in.}$ ($13\text{ mm}$) per QW-403.9, or when single-pass welding is converted to multi-pass welding per QW-410.8, severe grain-size variations occur. In single-pass SAW, the entire weld nugget remains in the un-tempered as-cast dendritic condition, severely degrading impact toughness.