17.2 SAW Wire-Flux Systems, Alloy Pickup & PWHT Property Effects

AWS A5.17 & AWS A5.23: Submerged Arc Wire-Flux Systems

Submerged Arc Welding (SAW) differs fundamentally from gas-shielded or coated processes: the electrode and the flux are classified together as a complete functional system. An electrode classified with Flux A will produce radically different weld metal mechanical properties and chemistry when paired with Flux B.

                                AWS A5.17 SAW SYSTEM NOMENCLATURE

             F         7         A         2     -     E         M12       K
             |         |         |         |           |          |        |
           Flux     Tensile   Condition   CVN      Electrode   Manganese  Killed
                    (x10 ksi) A = As-Weld 20 ft·lbf Solid Wire M = Med Mn (Silicon
                              P = PWHT    @-20°F               (0.8-1.25%) Deoxidized)

Low-Alloy SAW Systems (AWS A5.23)

AWS A5.23 incorporates two compositional suffixes: the first designates the solid or composite wire, while the second designates the chemical composition of the deposited weld metal:

$$\mathbf{F8A4-EA2-A2}$$

• F8: 80–100 ksi tensile strength.
• A: As-welded condition.
• 4: 20 ft·lbf CVN at -40°F (-40°C).
• EA2: Solid wire containing 0.5% Mo.
• -A2: Deposited weld metal chemical composition meets the A2 class (0.5% Mo).

Active Flux vs. Neutral Flux Dynamics

Understanding flux behavior is one of the most heavily tested areas on the CWEng exam:

+-----------------------------------------------------------------------------------------+
|                              ACTIVE VS. NEUTRAL SAW FLUXES                              |
+------------------------------------+----------------------------------------------------+
| Characteristic                     | Active Flux             | Neutral Flux             |
+------------------------------------+-------------------------+--------------------------+
| Alloying Additions (Mn, Si)        | High (deliberately      | Negligible (maintains    |
|                                    | added to flux matrix)   | balance without alloying)|
| Voltage Sensitivity                | Extreme: higher voltage | Negligible: chemistry    |
|                                    | burns more flux, adding | remains stable over      |
|                                    | excessive Mn and Si     | wide voltage window      |
| Maximum Plate Thickness / Passes   | Single-pass or limited  | Unlimited multi-pass     |
|                                    | two-pass welds only     | groove welds on heavy    |
|                                    | (t <= 3/16" to 1/2")    | plates (unlimited t)     |
| Primary Failure Mode if Misused    | High hardness (>350 HV),| Oxidation or wormhole    |
|                                    | microcracking, and low  | porosity if applied over |
|                                    | CVN in multi-pass welds | uncleaned, heavy scale   |
+------------------------------------+-------------------------+--------------------------+

In an active flux, the recovery of manganese and silicon is directly proportional to the quantity of flux melted per unit length of weld. Because arc voltage governs arc length, increasing the arc voltage increases the flux consumption ratio phi = M_flux / M_wire:

$$\Delta [\%Mn] \propto V_{\text{arc}} \cdot \left(\frac{M_{\text{flux}}}{M_{\text{wire}}}\right)$$

If an operator increases arc voltage from 28 V to 35 V on a multi-pass joint using an active flux, weld metal manganese can spike from 1.20% to over 2.50%, and silicon can exceed 1.00%. The resulting weld metal transforms into brittle, untempered martensite, causing catastrophic cold cracking under restraint.

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Mechanical Property Requirements & PWHT Effects

AWS A5 specifications mandate rigid tensile, yield, elongation, and Charpy V-notch toughness thresholds:

Influence of Postweld Heat Treatment (PWHT)

Postweld heat treatment relieves residual stresses and tempers hard microstructures. However, for plain carbon-manganese weld metals, extended PWHT holding times can result in significant loss of tensile and yield strength (often dropping 5–10 ksi due to subcritical carbide coarsening and dislocation recovery) and may degrade CVN toughness if step-cooling induces temper embrittlement.

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Comprehensive Worked Engineering Example: SAW Flux Selection & Alloy Pickup Calculation

Problem Statement

A pressure vessel manufacturer is welding a 38 mm thick longitudinal shell seam in ASTM A516 Grade 70 normalized plate. The joint is a symmetrical double-V groove requiring 14 passes. The welding supervisor proposes using an existing shop inventory of Submerged Arc flux designated as an active flux paired with an AWS A5.17 EM12K wire (0.10% C, 1.05% Mn, 0.22% Si). The qualified welding parameters specify 32 V, 550 A, and 450 mm/min.

From empirical flux calibration curves, the active flux transfers manganese and silicon to the weld metal according to the flux-to-wire consumption ratio phi = M_flux / M_wire:

$$\phi = 0.035 \cdot V_{\text{arc}} - 0.22$$

The incremental alloy recovery into the deposit is:

$$\Delta \%Mn = 0.85 \cdot \phi$$ $$\Delta \%Si = 0.60 \cdot \phi$$

Assuming base metal dilution is D = 0.30 (30%), with base metal chemistry of 0.20% C, 1.20% Mn, and 0.25% Si:

1. Compute the flux consumption ratio phi at 32 V.
2. Calculate the resulting weld metal Mn and Si content for Pass 1 (diluted by base metal).
3. Calculate the steady-state weld metal Mn and Si content in Pass 8 (fill pass where dilution comes entirely from prior weld passes, D_weld = 0.30).
4. Evaluate compliance with AWS D1.1 Clause 5.6 / ASME Section VIII rules regarding active fluxes.

                              DOUBLE-V GROOVE PASS PROFILE

               Plate Surface  ================================ Plate Surface
                                |   Pass 14       Pass 13   /
                                 |   Pass 8  (Diluted from /  <-- Fill Passes:
                                  |   Pass 7   Pass 6)    /       Mn & Si accumulate
                                   |  Pass 1 (Root Pass) /        from active flux!
                                    +-------------------+
                                    |   ASTM A516 Gr 70 |

Step-by-Step Engineering Solution

Step 1: Compute Flux Consumption Ratio (phi)

$$\phi = 0.035(32) - 0.22 = 1.12 - 0.22 = 0.90$$

For every 1.00 kg of wire consumed, 0.90 kg of active flux is melted.

Step 2: Calculate Alloy Transfer Increments

$$\Delta \%Mn = 0.85 \times 0.90 = 0.765\%\text{ Mn}$$ $$\Delta \%Si = 0.60 \times 0.90 = 0.540\%\text{ Si}$$

Step 3: Calculate Pass 1 Composition (Root Pass) In Pass 1, dilution from the base metal is D = 0.30, and consumable contribution is (1 - D) = 0.70:

$$\%Mn_{\text{deposit, pass 1}} = D(\%Mn_{\text{base}}) + (1 - D)(\%Mn_{\text{wire}} + \Delta \%Mn)$$ $$\%Mn_{\text{deposit, pass 1}} = 0.30(1.20) + 0.70(1.05 + 0.765) = 0.360 + 0.70(1.815) = 0.360 + 1.2705 = 1.63\%\text{ Mn}$$ $$\%Si_{\text{deposit, pass 1}} = D(\%Si_{\text{base}}) + (1 - D)(\%Si_{\text{wire}} + \Delta \%Si)$$ $$\%Si_{\text{deposit, pass 1}} = 0.30(0.25) + 0.70(0.22 + 0.540) = 0.075 + 0.70(0.760) = 0.075 + 0.532 = 0.61\%\text{ Si}$$

Step 4: Calculate Pass 8 Composition (Multi-pass Steady-State Accumulation) In multi-pass welding, subsequent passes dilute prior weld passes. At steady state, dilution D from preceding passes with identical consumable inputs leads to the undiluted weld metal composition:

$$\%Mn_{\text{steady}} = \%Mn_{\text{wire}} + \Delta \%Mn = 1.05\% + 0.765\% = 1.815\%\text{ Mn}$$ $$\%Si_{\text{steady}} = \%Si_{\text{wire}} + \Delta \%Si = 0.22\% + 0.540\% = 0.760\%\text{ Si}$$

If the operator allows arc voltage to wander up to 36 V to widen the cap pass:

$$\phi_{36V} = 0.035(36) - 0.22 = 1.26 - 0.22 = 1.04$$ $$\%Mn_{\text{cap}} = 1.05 + 0.85(1.04) = 1.05 + 0.884 = 1.934\%\text{ Mn}$$ $$\%Si_{\text{cap}} = 0.22 + 0.60(1.04) = 0.22 + 0.624 = 0.844\%\text{ Si}$$

Step 5: Metallurgical & Code Evaluation

• The steady-state silicon content reaches 0.76–0.84%. Under AWS A5.17 and ASME Section VIII Div 1, silicon in structural weld deposits is typically restricted to <= 0.80% because excess silicon forms coarse silicate films and embrittles the matrix.
• Manganese exceeds 1.80–1.93%, significantly increasing the Carbon Equivalent (CE) and hardenability: $$CE = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} = 0.12 + \frac{1.93}{6} = 0.12 + 0.322 = 0.442$$
• AWS D1.1 Clause 5.3 explicitly prohibits the use of active fluxes in multi-pass welds exceeding 3/16 in (5 mm) thickness. The proposed procedure must be REJECTED. The engineer must mandate a neutral flux conforming to AWS A5.17 (e.g., F7A2-EM12K with neutral bonded or fused flux).

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Industrial Scenarios & Certified Welding Engineer Exam Traps

Real-World Field Disaster Scenario

A fabricator constructing heavy box columns for a 45-story commercial tower switched from an AWS A5.20 E71T-1C gas-shielded FCAW wire to an AWS A5.20 E71T-11 self-shielded wire on a night shift to eliminate gas cylinder handling. The column flange thickness was 2.5 in (63 mm). Within 48 hours, catastrophic full-length centerline cracking occurred down the core of every column seam. The failure investigation revealed two fatal engineering errors:

1. AWS A5.20 Table 1 Limitation: E71T-11 is explicitly restricted to a maximum plate thickness of 3/8 in (9.5 mm) due to high residual aluminum deoxidizers and lack of notch toughness.
2. Applying E71T-11 to 2.5 in highly restrained plate generated a coarse columnar microstructure with zero ductility, triggering immediate solidification and delayed cold cracking under transverse shrinkage stresses.

Certified Welding Engineer Exam Traps

Exam Trap 1: Active vs. Neutral Flux Voltage Sensitivity Exam writers frequently ask: "An operator notices slag sticking during SAW multi-pass groove welding and increases the arc voltage from 30 V to 37 V while using an active flux. What is the metallurgical impact on the deposited weld metal?" Candidates mistakenly think higher voltage merely widens the bead. The true engineering answer: Active flux consumption increases, driving excessive manganese and silicon into the weld puddle, causing extreme hardness (> 300 HV) and brittle notch sensitivity.

Exam Trap 2: Suffix Designation in AWS A5.23 vs. A5.5 In AWS A5.5 (SMAW), -B2 indicates the chemical composition of the deposited weld metal. In AWS A5.23 (SAW), a classification like F8A4-EB2-B2 has two -B2 suffixes: the first (EB2) specifies the solid wire composition, while the second (-B2) specifies the chemical composition of the deposited weld metal produced by that wire-flux combination. If an unalloyed wire (EM12K) is used with an alloy flux, the classification becomes F8A4-EM12K-B2.

Exam Trap 3: Triple-Deoxidized ER70S-2 in Multi-Pass GMAW Spray Arc ER70S-2 contains Ti, Zr, and Al deoxidizers. While unmatched for single-pass GTAW root passes, using ER70S-2 in high-current, multi-pass spray-transfer GMAW causes non-metallic oxide inclusion accumulation (TiO2, Al2O3), lowering Charpy V-notch toughness compared to an ER70S-6 wire.