8.1 Shielded Metal Arc Welding (SMAW): Electrode Flux Chemistries & Variables

8.1 Shielded Metal Arc Welding (SMAW): Electrode Flux Chemistries & Variables

Quick Answer: Shielded Metal Arc Welding (SMAW) relies on a consumable core wire coated in an engineered flux that vaporizes under arc temperatures (5,000–20,000 K) to establish a shielding gas canopy, deoxidize the molten puddle, and generate a protective slag cover. Electrode performance is dictated by coating formulations classified per AWS A5.1/A5.5: cellulosic coatings (EXX10/EXX11) deliver deep penetration via CO/H₂ gas generation; rutile types (EXX12/EXX13) provide smooth arc stability with low spatter; and basic low-hydrogen types (EXX15/EXX16/EXX18) utilize CaCO₃ and CaF₂ to eliminate atmospheric moisture, yielding diffusible hydrogen levels below $4\text{ mL}/100\text{g}$ and exceptional low-temperature Charpy V-notch toughness. Constant-current (CC) power sources are mandatory to maintain stable energy delivery across variable manual arc lengths.

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SMAW Process Physics & Electrical Arc Characteristics

Shielded Metal Arc Welding is an electric arc process in which coalescence is produced by heating the workpiece and a flux-coated consumable electrode with an electric arc maintained between the tip of the electrode and the weld puddle.

                    Core Wire (Conducts Current & Adds Metal)
                   ===========================================
                 [ ================= Flux Coating ============ ]
                                     |         |  Gaseous Shield
                                     | Arc Gap |  (CO, CO2, H2, H2O)
                                     V         V
       Direction of Travel <---    ( ( ( ARC ) ) )
       ===========================[ M-PUDDLE ]========== Base Metal
       [ Solidified Slag Blanket ][ Weld Metal]

Arc Electrical Dynamics and Power Source Requirements

SMAW requires a Constant Current (CC) power supply, commonly referred to as a "drooper." In a CC power source, the volt-ampere output curve displays a steep downward slope:

$$\frac{dV}{dI} \approx -0.05 \text{ to } -0.15 \text{ V/A}$$

Because the welder manually manipulates the arc length, minor fluctuations in arc gap produce instantaneous changes in arc voltage ($V_{\text{arc}}$). On a CC machine, these voltage shifts cause only negligible changes in welding current ($I$), maintaining a constant thermal energy input to the base metal.

The relationship between arc length ($L_{\text{arc}}$, in mm) and arc voltage ($V_{\text{arc}}$) is modeled empirically as:

$$V_{\text{arc}} = V_0 + \beta I + k_a L_{\text{arc}}$$

where $V_0$ is the cathode/anode fall voltage sum (typically $14\text{ to }18\text{ V}$), $\beta$ accounts for column resistance, and $k_a$ is the arc electric field gradient (approximately $1.0\text{ to }2.2\text{ V/mm}$). If a welder pulls a long arc ($L_{\text{arc}}$ increases by $3\text{ mm}$), $V_{\text{arc}}$ surges by $4\text{ to }7\text{ V}$. On a constant voltage (CV) machine, this would cause current to drop precipitously, destabilizing the puddle and causing arc extinction; on a CC power source, current remains stable within $\pm 5\%$.

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AWS Electrode Classification Systems (AWS A5.1 & A5.5)

Carbon steel and low-alloy steel covered electrodes are classified under AWS A5.1 and AWS A5.5, respectively. The alphanumeric designators encode mechanical properties, operational capabilities, coating metallurgy, and chemical integrity.

   E   70   1   8   -  1   M   H4   R
   |   |    |   |      |   |   |    |
   |   |    |   |      |   |   |    +-- Moisture Resistant (optional)
   |   |    |   |      |   |   +------- Diffusible Hydrogen (<= 4 mL/100g)
   |   |    |   |      |   +----------- Military-grade / Ductility designator
   |   |    |   |      +--------------- Improved Impact Toughness (-45°C)
   |   |    |   +---------------------- Coating Type & Electrical Polarity
   |   |    +-------------------------- Welding Position (1 = All Position)
   |   +------------------------------- Min Tensile Strength (x 1000 psi)
   +----------------------------------- Electrode (Covered)

Breakdown of Classification Digits

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Flux Coating Systems & High-Temperature Slag Reactions

The electrode coating is an inorganic-organic composite engineered to perform five metallurgical functions simultaneously: gas shielding, slag creation, deoxidation, arc stabilization, and alloying/iron powder addition.

Comparative Metallurgy of Major SMAW Coating Types

1. Cellulosic Flux Chemistry (EXX10, EXX11)

Cellulosic coatings contain large fractions of organic cellulose ($(\text{C}_6\text{H}_{10}\text{O}_5)_n$). In the arc column ($T > 3000\text{ K}$), cellulose combusts and thermal-cracks into a massive volume of reducing gas:

$$\text{C}_6\text{H}_{10}\text{O}_5 + \frac{1}{2} \text{O}_2 \longrightarrow 6\text{CO} + 5\text{H}_2$$

Because the arc gas consists of over $40\%\text{ H}_2$, which possesses high thermal conductivity at elevated temperatures, the arc voltage gradient increases significantly. This produces a stiff, high-velocity plasma jet that scours away the liquid puddle and drives deep penetration into the joint root. However, the high hydrogen partial pressure ($P_{\text{H}_2}$) saturates the liquid puddle with atomic hydrogen ($[\text{H}]$), yielding $30\text{ to }50\text{ mL}$ of diffusible hydrogen per $100\text{ g}$ of deposit. Cellulosic electrodes are therefore strictly prohibited on high-strength steels ($R_m > 450\text{ MPa}$) prone to Hydrogen-Induced Cracking (HIC), but remain the standard for cross-country downhill oil/gas transmission pipelines (API 5L X52/X60) where productivity and rapid root-pass progression dominate.

2. Rutile Flux Chemistry (EXX12, EXX13, EXX14)

Rutile electrodes are dominated by titanium dioxide ($\text{TiO}_2$). Because titanium and potassium have low ionization potentials ($E_i(\text{Ti}) = 6.82\text{ eV}$, $E_i(\text{K}) = 4.34\text{ eV}$), they readily ionize in the arc column, promoting easy arc ignition, ultra-smooth droplet transfer, and stable operation on low open-circuit voltage (OCV) alternating current (AC) power supplies. The slag is governed by the $\text{TiO}_2-\text{SiO}_2-\text{FeO}$ ternary system, producing a fluid slag with low surface tension that spreads evenly across the bead profile. However, rutile flux deposits exhibit moderate-to-high oxygen contents ($600–900\text{ ppm}$), resulting in coarse upper-bainite microstructures with modest Charpy impact toughness.

3. Basic / Low-Hydrogen Flux Chemistry (EXX15, EXX16, EXX18)

Basic coatings are formulated with limestone / calcium carbonate ($\text{CaCO}_3$) and fluorspar / calcium fluoride ($\text{CaF}_2$). The shielding mechanism operates entirely without organic matter. At arc temperatures, calcium carbonate calcines endothermically:

$$\text{CaCO}_3 \longrightarrow \text{CaO} + \text{CO}_2$$ $$\text{CO}_2 + \text{Fe} \rightleftharpoons \text{FeO} + \text{CO}$$

The high partial pressure of carbon monoxide ($\text{CO}$) and carbon dioxide ($\text{CO}_2$) sweeps atmospheric air away from the puddle. Concurrently, calcium fluoride lowers the melting point of the lime slag and actively scavenges atomic hydrogen in the arc column via gas-phase halide reactions:

$$2[\text{H}] + \text{CaF}_2 \longrightarrow \text{Ca} + 2\text{HF} \uparrow$$ $$\text{SiO}_2 + 2\text{CaF}_2 \longrightarrow \text{SiF}_4 \uparrow + 2\text{CaO}$$

The generation of volatile hydrogen fluoride ($\text{HF}$) gas traps hydrogen before it can dissolve into the molten weld metal. The resulting slag is highly basic, with a Basicity Index ($BI > 1.5$). Basic slags exhibit low oxygen potential, deoxidizing the weld metal through high ferromanganese ($\text{FeMn}$) and ferrosilicon ($\text{FeSi}$) recoveries:

$$[\text{Fe}] + [\text{O}] + [\text{Mn}] \longrightarrow (\text{MnO}) + [\text{Fe}]$$ $$[\text{Si}] + 2[\text{O}] \longrightarrow (\text{SiO}_2)$$

Consequently, weld metal deposited by basic electrodes has ultra-clean interstitial chemistry: total oxygen is constrained to $250–350\text{ ppm}$, and diffusible hydrogen is reduced to under $4\text{ mL}/100\text{g}$ ($-H4$). This promotes fine acicular ferrite (AF) microstructures, guaranteeing high Charpy V-notch fracture toughness down to $-50^\circ\text{C}$ and exceptional resistance to cold cracking.

4. Iron Powder Additions and Deposition Economics

Iron powder added to the coating acts as a supplementary metallic source. In standard electrodes (e.g., E6010), the deposition efficiency (ratio of deposited metal weight to core wire consumed) is approximately $75–80\%$ due to spatter and vaporization losses. In high iron powder electrodes (e.g., E7018 contains $\sim 25–30\%$ iron powder; E7024 contains $\sim 50\%$ iron powder):

$$\eta_{\text{recovery}} = \frac{M_{\text{weld deposited}}}{M_{\text{core wire consumed}}} \times 100\%$$

Deposition recovery surges to $115\%–150\%$. The iron powder melts directly into the pool via resistive and conduction heating from the slag layer without passing entirely through the arc plasma droplet stream. This permits higher operating currents without overheating the core wire stub, dramatically increasing deposition rates:

$$\text{Deposition Rate: } DR_{\text{E7018}} \approx 1.8 \text{ to } 2.8 \text{ kg/h} \quad \text{vs.} \quad DR_{\text{E7024}} \approx 3.5 \text{ to } 5.5 \text{ kg/h}$$

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Critical Welding Variables and Arc Manipulation

          DRAG (BACKHAND) TECHNIQUE                 LEAD (FOREHAND) TECHNIQUE
            Torch / Rod Leans Back                    Torch / Rod Leans Forward
                 Travel: --->                              Travel: --->

                    /  (10° - 20°)                            \  (10° - 20°)
                   /                                           \
                  /  Electrode                                  \  Electrode
                 /                                               \
            +---v-----------+                         +-----------v---+ 
       ====[ M-Pool ->- Slag ]=====              ====[ Slag -<- M-Pool ]=====
         Deep Penetration, High Convexity          Shallow Penetration, Wide Flat Cap

Lead vs. Drag Electrode Angles

• Drag (Backhand) Angle ($10^\circ–20^\circ$ from perpendicular): The electrode points backward toward the solidifying weld pool. Arc force pushes the liquid puddle away from the joint root, allowing the direct arc plasma to impinge upon the solid base metal. This maximizes penetration depth and creates a narrower, higher-crowned bead profile. Drag technique is standard for basic electrodes (E7018) in flat/horizontal positions to prevent liquid slag from running ahead of the arc.
• Lead / Push (Forehand) Angle ($5^\circ–15^\circ$ from perpendicular): The electrode points forward in the direction of travel. Arc force pushes molten metal over the unmelted base metal ahead of the arc. This dampens arc penetration, producing a wide, shallow, flat weld bead. It is used primarily for thin gauge sheet metal and vertical-up root progression.

Arc Length and Atmospheric Contamination

Maintaining a tight arc length ($L_{\text{arc}} \approx 1.0 \times d_{\text{core}}$) is essential. When an operator pulls a long arc:

1. Arc voltage increases proportionally ($+4\text{ to }8\text{ V}$).
2. The protective shielding gas cone expands and aspirates atmospheric air.
3. Nitrogen and oxygen dissolve catastrophically into the molten puddle: $$\frac{1}{2}\text{N}_{2(\text{gas})} \rightleftharpoons [\text{N}]_{\text{dissolved}}$$
4. Dissolved nitrogen precipitates as brittle grain-boundary iron nitrides ($\text{Fe}_4\text{N}$) or forms gas porosity during rapid solidification.
5. Manganese and silicon deoxidizers in the slag are consumed by oxidation, depleting matrix solid-solution strength and ruining notch toughness.

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Electrode Conditioning, Handling, and Rebaking (AWS D1.1 Clause 7.3)

Low-hydrogen covered electrodes are hygroscopic: potassium/sodium silicate binders absorb moisture from ambient air. When a wet electrode is struck, water vapor dissociates in the arc plasma ($2\text{H}_2\text{O} \rightarrow 4[\text{H}] + 2[\text{O}]$), driving atomic hydrogen into the heat-affected zone (HAZ) and causing catastrophic Hydrogen-Induced Delayed Cracking (HIC).

Mandatory AWS D1.1 Storage and Exposure Requirements

Strict Engineering Rule: Under AWS D1.1 Clause 7.3, electrodes may be rebaked only once. Electrodes that have been wet (immersed in water) or whose coatings display flaking, cracking, or binder crystallization must be discarded immediately.

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Comprehensive Worked Engineering Example: Deposition Economics & Heat Input

Problem Statement

A structural fabrication shop is qualifying a heavy column splice on ASTM A992 structural steel ($50\text{ mm}$ thick). The engineer must calculate the heat input and consumable procurement requirements for an SMAW pass running E7018 (diameter $d = 4.0\text{ mm}$) at:

• Current $I = 175\text{ A}$ (DCEP)
• Arc Voltage $V = 24.5\text{ V}$
• Travel Speed $v = 140\text{ mm/min}$
• Arc thermal efficiency $\eta = 0.80$
• Joint groove length $L = 6.0\text{ m}$
• Total required deposited weld metal mass $M_{\text{dep}} = 8.50\text{ kg}$
• Electrode stub loss $L_{\text{stub}} = 15\%$ (based on a $50\text{ mm}$ stub discarded per $350\text{ mm}$ rod)
• Deposition efficiency $\eta_{\text{dep}} = 68\%$

Calculate: (1) the net heat input per unit length ($H_{\text{net}}$), (2) the total gross mass of E7018 electrodes that must be checked out from the conditioning oven, and (3) the arc-on time required to deposit the pass.

Step-by-Step Engineering Solution

Step 1: Calculate Net Heat Input ($H_{\text{net}}$) Using the standard thermodynamic heat input formulation:

$$H_{\text{net}} = \frac{\eta \cdot V \cdot I \cdot 60}{1000 \cdot v} \quad (\text{kJ/mm})$$ $$H_{\text{net}} = \frac{0.80 \cdot 24.5\text{ V} \cdot 175\text{ A} \cdot 60}{1000 \cdot 140\text{ mm/min}} = \frac{205,800}{140,000} = 1.47\text{ kJ/mm}$$

Evaluation: The heat input of $1.47\text{ kJ/mm}$ falls safely within the typical qualification window ($1.2–1.8\text{ kJ/mm}$) required to prevent excessive HAZ grain coarsening while ensuring sufficient heat to avoid martensite formation.

Step 2: Calculate Required Electrode Checkout Mass ($M_{\text{purchased}}$) The net deposition efficiency accounts for flux weight and spatter, but stub ends are discarded mechanically. The total consumable mass required is:

$$M_{\text{purchased}} = \frac{M_{\text{dep}}}{\eta_{\text{dep}} \cdot (1 - L_{\text{stub}})} = \frac{8.50\text{ kg}}{0.68 \cdot (1 - 0.15)} = \frac{8.50}{0.68 \cdot 0.85} = \frac{8.50}{0.578} = 14.71\text{ kg}$$

Result: The engineer must requisition at least $14.71\text{ kg}$ (approximately three $5\text{ kg}$ hermetically sealed tins) of E7018 electrodes.

Step 3: Calculate Arc-On Time ($t_{\text{arc}}$) for the 6-Meter Pass

$$t_{\text{arc}} = \frac{L}{v} = \frac{6000\text{ mm}}{140\text{ mm/min}} = 42.86\text{ minutes} \approx 42\text{ min } 51\text{ s}$$

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Industrial Scenarios & Certified Welding Engineer Exam Pitfalls

Real-World Field Failure Scenario

During cross-country pipeline construction on API 5L X70 pipe ($14.2\text{ mm}$ wall thickness), an inexperienced contractor attempted to replace E6010 root-pass cellulosic electrodes with E7018-1 low-hydrogen electrodes to "improve toughness." The welders were unable to maintain the fast vertical-down travel speed required on downhill pipe seams. The fluid slag of the E7018 ran ahead of the arc, resulting in severe cold lap (lack of fusion) and trapped slag pockets across $85\%$ of the root girth welds. Furthermore, when the contractor ran out of hot holding ovens, welders left opened E7018 tins on the muddy right-of-way for 18 hours in humid conditions. Within 48 hours post-weld, non-destructive shear-wave ultrasonic testing revealed extensive transverse hydrogen cracks in the high-hardness HAZ. The pipeline operator halted fabrication, issued a stop-work notice, and mandated the complete gouging and replacement of 140 joint seams at massive contractor expense.

Common Exam Traps

Exam Trap 1: Rebaking Cellulosic Electrodes (E6010 / E6011) Never place cellulosic electrodes in a holding or rebaking oven! Cellulosic coatings require $3\%–7\%$ moisture content to generate the hydrogen and steam vapor essential for arc force and gas shielding. Baking an E6010 electrode destroys the organic cellulose and dries out the sodium silicate, causing the coating to disintegrate, the arc to wander violently, and the weld to suffer severe porosity.

Exam Trap 2: Believing a Long Arc Increases Melting Rate on CC Power Sources When an operator draws a long arc on a constant-current power supply, the voltage rises, but because the machine is a "drooper," the current drops slightly or stays constant. The extended arc does not increase weld deposit rate; instead, it superheats the arc column, widens the plasma cone, vaporizes deoxidizers, causes undercut, and draws in ambient nitrogen, creating wormhole porosity and embrittling the weld deposit.

Exam Trap 3: Position Code '4' Misinterpretation Most engineers memorize '1' (all-position) and '2' (flat/horizontal fillet). On the CWEng exam, questions occasionally test code '4' (e.g., E7048). Code '4' indicates an electrode specifically engineered with a fast-freezing, low-viscosity slag for vertical-down progression, not overhead.