10.1 SMAW, GTAW & FCAW Welding Processes in Pressure Vessel Applications

Shielded Metal Arc Welding (SMAW) in Boiler and Pressure Vessel Fabrication

Shielded Metal Arc Welding (SMAW), commonly termed manual stick welding, remains a core joining process in industrial boiler construction, boiler tube replacement, and pressure vessel repair. The process relies on an electric arc struck between a flux-covered consumable electrode and the workpiece metal. As the arc burns, the core wire melts into the puddle while the flux coating decomposes, generating a protective gaseous envelope (carbon dioxide, carbon monoxide, water vapor, and hydrogen depending on formulation) and a molten slag blanket that shields the liquid metal from atmospheric oxygen and nitrogen.

                      ELECTRODE HOLDER (STINGER)
                                 │
                                 ▼
                    [ CORE WIRE + FLUX COATING ]
                                 │
                               (ARC)
        ───► TRAVEL              │              SLAG LAYER
  ══════════════════════╦════════▼════════╦══════════════════════
     BASE METAL         ║  MOLTEN PUDDLE  ║     SOLIDIFIED
    (BOILER SHELL)      ║  & SHIELDING    ║     WELD METAL
                        ║      GAS        ║
  ══════════════════════╩═════════════════╩══════════════════════

Power Source Characteristics: The Constant Current (CC) "Drooper"

Manual welding processes—both SMAW and GTAW—mandate a Constant Current (CC) power supply, colloquially referred to in the trade as a "drooper" power source due to its steep downward-sloping volt-ampere output curve.

• Operating Principle: In manual welding, a boilermaker cannot maintain an absolutely fixed arc length by hand. Minor hand tremors and body repositioning continuously vary the physical distance between the electrode tip and the weld puddle.
• Electrical Dynamics: In a CC machine, arc voltage is directly proportional to arc length (a longer arc increases voltage; a tighter arc decreases voltage). However, because of the steep drooper curve, large fluctuations in arc voltage produce only minute, negligible changes in welding amperage (current).
• Metallurgical Benefit: Because amperage governs heat input ($H = \frac{V \times A \times 60}{\text{Travel Speed}}$) and weld penetration, the constant-current characteristic ensures uniform penetration, stable puddle fluidity, and consistent bead profile despite natural manual variations in arc length.

In contrast, Constant Voltage (CV) power supplies are utilized exclusively for continuous wire-fed processes (GMAW and FCAW), where arc length is maintained automatically through self-adjusting burn-off rates.

Polarity Selection: DCEP vs. DCEN vs. AC

Welding current type and polarity profoundly affect penetration depth, electrode burn-off rate, and heat distribution across the arc:

1. Direct Current Electrode Positive (DCEP / Reverse Polarity):

   • Current flows from the workpiece (-) to the electrode (+).
   • Approximately 70% of arc heat is concentrated at the workpiece puddle, while 30% is generated at the electrode tip.
   • Delivers deep joint penetration, aggressive digging action, and superior oxide cleaning.
   • Primary Application: Standard polarity for boiler pressure boundary welding, including open-root cellulosic passes (E6010) and low-hydrogen fill and cap passes (E7018, E8018, E9018).

2. Direct Current Electrode Negative (DCEN / Straight Polarity):

   • Current flows from the electrode (-) to the workpiece (+).
   • Heat concentrates at the electrode tip, resulting in rapid electrode melt-off and shallow penetration.
   • Primary Application: High-speed thin sheet metal fabrication, non-pressure casing, seal welding of light-gauge boiler cladding, and specific automated hot-wire GTAW setups.

3. Alternating Current (AC):

   • Current reverses direction 120 times per second (60 Hz cycle), alternating equally between electrode positive and electrode negative.
   • Provides balanced heat distribution (50% workpiece / 50% electrode) with medium penetration.
   • Primary Application: Heavy-wall vessel fabrication where magnetic arc blow renders DC welding impossible.

Magnetic Arc Blow: Causes, Dynamics, and Field Elimination

Arc blow is the un-commanded, violent deflection of the electric arc away from its intended path along the weld joint centerline. It causes severe spatter, irregular bead contours, incomplete fusion, sidewall lack of penetration, and internal slag inclusions.

                  MAGNETIC FIELD CONCENTRATION
                             (((●)))
                                │
             ELECTRODE          ▼   DEFLECTED ARC (BLOW)
                 \             / ──►
                  \           /
     ══════════════\═════════/════════════════
       HEAVY PRESSURE VESSEL SHELL (FERROMAGNETIC)
     ═════════════════════════════════════════
                         ▲
                         │
                   GROUND CLAMP

Physical Causes of Arc Blow

Arc blow occurs exclusively when welding with Direct Current (DC) on ferromagnetic base metals (such as carbon and low-alloy chrome-moly steels). When direct current travels through the welding lead, electrode, arc column, and workpiece, it generates a concentric electromagnetic flux field around the current path. Arc deflection is driven by two magnetic imbalances:

1. Ground Current Asymmetry: Magnetic lines of force bunch up tightly between the arc and the workpiece ground connection, repelling the arc column away from the ground clamp (forward blow or back blow).
2. Geometric Mass Clustering (End Effect): In thick vessel shells, corner joints, and deep groove bevels, magnetic lines of force concentrate densely in the heavier steel mass. As the arc approaches the edge of a plate or the end of a seam, the magnetic field resists passing through air, forcing the arc backward into the joint mass.

Boilermaker Field Remedies for Arc Blow

When severe arc blow is encountered on pressure vessel joints, boilermakers apply the following systematic remedies:

| Remedy Technique | Operational Mechanism | Field Application Note | |---|---|---|| | Switch to Alternating Current (AC) | Rapid 60 Hz polarity reversal continuously breaks up and prevents steady-state magnetic field formation. | Most effective permanent solution; requires AC-compatible electrodes (e.g., E7018-AC or E6011). | | Relocate Ground Clamp | Moves magnetic concentration point away from the active welding puddle. | Place ground at the opposite end of the joint, or split the ground into two balanced leads attached at opposite ends. | | Shorten the Arc Length | Reduces arc voltage and stiffens the arc column, allowing arc velocity to overcome magnetic deflection forces. | Hold tight arc; drag technique with iron powder low-hydrogen rods. | | Wrap Ground Leads (Coiling) | Creates an opposing electromagnetic field (counter-flux) that neutralizes base metal magnetism. | Wrap work lead 3 to 6 turns around the pipe spool or vessel nozzle in the direction opposing the magnetic field. | | Weld Toward Heavy Tacks or Run-off Tabs | Provides a continuous ferromagnetic flux path, preventing flux lines from crowding at the plate edge. | Install heavy bridge tacks or weld toward previously deposited sound weld metal. | | Reduce Welding Current (Amperage) | Magnetic field strength is directly proportional to the square of current ($B \propto I^2$); lowering amps drastically reduces field strength. | Drop amperage 10–20 A within the qualified WPS operating range. |

Gas Tungsten Arc Welding (GTAW / TIG) in High-Pressure Boiler Systems

Gas Tungsten Arc Welding (GTAW), designated by AWS and known in the field as TIG welding, utilizes a non-consumable solid tungsten electrode to establish an intense electric arc within an inert gas atmosphere. Filler metal is added manually or mechanically into the leading edge of the molten weld puddle.

GTAW produces the cleanest, highest-integrity, and most cosmetically precise weld deposits of any manual arc process. In industrial power plants, utility boilers, and chemical refineries, GTAW is universally specified for:

• Open-root passes on small-bore high-pressure boiler tubing (superheater and reheater elements).
• Root and hot passes on main steam, hot reheat, and boiler feedwater headers.
• Full-penetration butt joints in Creep-Strength Enhanced Ferritic (CSEF) steels like Grade 91 (P91).
• Austenitic stainless steel (304H, 347H) and nickel-alloy (Inconel 625) cladding and pressure boundaries.

Shielding Gases and Gas Delivery

The molten GTAW puddle and hot tungsten electrode must be completely isolated from atmospheric air to prevent catastrophic tungsten oxidation and puddle porosity.

• 100% Argon (Ar): The universal standard shielding gas for boiler tube welding. Argon is chemically inert, monatomic, and heavier than air (density 1.78 g/L vs. air 1.29 g/L), providing an effective blanketing shield at moderate flow rates (15 to 25 CFH). Argon provides easy arc ignition, a smooth, quiet arc column, and low arc voltage characteristics.
• Argon / Helium Mixtures (75% Ar / 25% He or 50% Ar / 50% He): Helium is lighter than air and possesses higher thermal conductivity and higher arc ionization potential than argon. Adding helium significantly raises arc voltage and heat input, producing deeper penetration and flatter weld beads at higher travel speeds. Helium mixtures are utilized on heavy-wall chrome-moly headers, copper-nickel condenser tubes, and thick aluminum heat exchanger shells.

Tungsten Electrode Classifications and Preparation

AWS A5.12 governs tungsten and oxide-doped tungsten electrodes. In boiler tube welding on direct current straight polarity (DCEN), pure tungsten is never used because of poor current capacity and arc wandering. Doped tungstens containing rare-earth metal oxides are required for optimal electron emission and tip stability:

  LONGITUDINAL GRIND MARKS (CORRECT)         RADIAL / TRANSVERSE GRIND (PROHIBITED)
  ═══════════════════════════════╗           ═══════════════════════════════╗
  ───────────────────────────────►           ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░║
  ───────────────────────────────►           ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░║
  ═══════════════════════════════╝           ═══════════════════════════════╝
         ▲                                                  ▲
         │                                                  │
  Focused, stable arc;                               Wandering arc; ring-off
  reduced tungsten spitting                          contamination; inclusions

Tungsten Grinding Geometry

Tungsten electrodes must be ground strictly longitudinally (parallel to the axis) using a dedicated, fine-grit diamond grinding wheel. Transverse (radial) grinding leaves circular ridges that cause arc wandering, arc flare, and premature tungsten disintegration.

• Taper Angle: Ground to an included angle of $20^\circ\text{ to }30^\circ$ (taper length equal to 2 to 2.5 times the electrode diameter).
• Truncated Point (Land): The razor tip must be ground flat (creating a small blunt face approximately 0.010"–0.020" diameter) to prevent the sharp point from melting off and falling into the root puddle as a metallic inclusion.

Arc Starting Methods: High-Frequency vs. Lift-Arc vs. Scratch Start

1. High-Frequency (HF) Start: A high-frequency, high-voltage spark jumps the gap between the tungsten tip and the pipe bevel without physical contact, ionizing the argon gas path and igniting the welding arc. Preferred code method; completely eliminates contact contamination.
2. Lift-Arc: The welder touches the tungsten to the pipe bevel at a harmless, microscopic sensing current (<10 A). Upon lifting the torch, the machine senses the voltage break and instantaneously ramps up to full welding current. Prevents tungsten sticking and transfer.
3. Scratch Start: The welder strikes the tungsten directly against the bevel like a match. Strictly prohibited on ASME Section I and Section VIII pressure welds. Scratch starting breaks off brittle tungsten fragments into the steel, causing instant non-destructive examination (NDE) reject indications on radiographic (RT) and ultrasonic (UT) film.

Backing Gas Purging and Root Oxidation Prevention ("Sugaring")

When performing an open-root weld on stainless steel (P-No. 8), chrome-moly steels (P-No. 4, P-No. 5A/5B/5C including P91), or nickel alloys, the ambient air inside the pipe bore contains 21% oxygen. At molten welding temperatures (>2,500°F), oxygen aggressively attacks the unshielded back side of the root puddle.

This rapid oxidation creates a severely defective metallurgical condition known in the trade as "sugaring" (or root cauliflower). Sugared roots exhibit extreme porosity, deep black oxidized crusts, micro-cracks, and total loss of alloying elements (chromium and molybdenum burn out as volatile oxides). A sugared root cannot pass visual or radiographic inspection and suffers rapid stress corrosion failure in service.

               TIG TORCH (100% Ar SHIELD)
                         │
                         ▼
           ════════════[ROOT]════════════
             PIPE WALL       PIPE WALL
           ──────────────────────────────
             PURGE VOLUME (100% ARGON)
             O2 LEVEL < 50 PPM (< 0.5%)
           ──────────────────────────────
             ▲                        ▲
             │                        │
         [PURGE DAM]              [PURGE DAM]
      (SOLUBLE PAPER)          (SOLUBLE PAPER)

Purge Containment and Oxygen Monitoring Protocols

To ensure full root protection, boilermakers install purge dams inside the pipe spool on either side of the weld joint:

1. Purge Dam Types:
   • Water-Soluble Paper / Film (Sodium Carboxymethyl Cellulose): Installed 6 to 8 inches away from the bevel, taped with water-soluble adhesive tape. During post-fabrication boiler hydrostatic testing or steam blow, the paper completely dissolves in water, leaving zero residue in the steam line.
   • Inflatable Rubber Bladders / Silicone Baffles: Reusable mechanical dams used for repetitive shop pipe spooling. Must be heat-resistant and fully vented.
2. Purge Gas Ingress and Venting:
   • Pure welding-grade Argon is introduced at the lowest point of the dammed chamber.
   • An exhaust vent is located at the highest point of the pipe joint (often through the top of the root gap covered with heat-resistant masking tape) to allow air to escape without turbulent mixing.
3. Purge Gas Flow Rates:
   • Initial Purge: High flow rate (20 to 35 CFH) to rapidly displace 5 to 7 volume exchanges of air.
   • Welding Purge: Once target oxygen levels are achieved, flow is dropped to a gentle trickle (5 to 10 CFH). High purge flow during welding creates positive back-pressure, blowing the liquid root puddle outward and causing concave root suck-back.
4. Oxygen Concentration Limits:
   • An electronic optical / electrochemical oxygen purge analyzer must be connected to the exhaust line.
   • Code Standard: Welding must not begin until the oxygen content inside the purge chamber drops below 50 parts per million (ppm)—or less than 0.5% max depending on client WPS specifications (P91 and high-alloy stainless frequently mandate <20 ppm).
   • Duration: The internal purge must be maintained through the completion of the root pass and the hot pass (second pass). Once at least 3/16-inch of solid weld metal is deposited, the joint is thick enough to shield the backside from atmospheric oxidation, allowing the purge to be safely removed.

Flux Cored Arc Welding (FCAW) and Submerged Arc Welding (SAW)

In addition to manual SMAW and GTAW, high-deposition continuous-wire processes are widely deployed in structural boiler erection and heavy vessel manufacturing.

Flux Cored Arc Welding (FCAW)

FCAW utilizes a continuous, hollow tubular steel sheath filled with granulated flux, metal deoxidizers (silicon, manganese, aluminum), and slag-forming compounds. It operates on a Constant Voltage (CV) power source, typically in DCEP mode.

                       FCAW-G (GAS-SHIELDED) vs. FCAW-S (SELF-SHIELDED)

          [ FCAW-G: "DUAL SHIELD" ]                    [ FCAW-S: SELF-SHIELDED ]
        External Shielding Gas Nozzle                      No External Shielding
              │  [ TUBULAR WIRE ]  │                             [ TUBULAR WIRE ]
              │        │        │                                       │
              ▼        ▼        ▼                                       ▼
          (GAS)      (ARC)    (GAS)                                   (ARC)
        75% Ar / 25% CO2 or 100% CO2                        Flux core generates all
        High toughness, low spatter                          gas; high wind tolerance

1. Gas-Shielded FCAW (FCAW-G / "Dual Shield"):

   • Requires both the internal core flux and an external shielding gas supply (typically 75% Argon / 25% $\text{CO}_2$ or 100% $\text{CO}_2$).
   • AWS Classification Example: E71T-1M (E = Electrode, 7 = 70 ksi min tensile, 1 = All positions, T = Tubular wire, -1 = Usability/polarity designator, M = Mixed gas $75/25\text{ Ar}/\text{CO}_2$).
   • Characteristics: Exceptionally high deposition rates (8 to 14 lbs/hr compared to 3 to 5 lbs/hr for SMAW), deep penetration, low spatter, and high Charpy V-notch impact toughness.
   • Application: Pressure vessel nozzle attachments, boiler casing buckstay stiffeners, skirt-to-shell attachment welds, and structural support steel in enclosed shop environments.

2. Self-Shielded FCAW (FCAW-S):

   • Operates without any external shielding gas cylinder. The core flux contains high concentrations of aluminum and magnesium deoxidizers that vaporize in the arc, reacting with atmospheric nitrogen and oxygen to form protective slag compounds.
   • AWS Classification Example: E71T-8.
   • Characteristics: Highly portable and immune to weld porosity caused by windy outdoor drafts (up to 30 mph).
   • Application: Heavy structural field erection, ductwork framing, boiler buckstays, and temporary rigging lugs. Generally restricted or prohibited on ASME code pressure boundaries due to lower impact ductility and higher diffusible hydrogen levels.

Submerged Arc Welding (SAW)

Submerged Arc Welding (SAW) is a fully automated or semi-automated heavy-fabrication process. A continuous solid or tubular wire is fed into the joint while the entire electric arc remains completely buried under a deep, blanket layer of granular fusible flux.

• No Visible Arc Flash: The flux blanket completely conceals the arc, eliminating arc radiation, spatter, and the need for protective welding helmets in the work cell.
• Unmatched Deposition Rates: Operates at extremely high currents (300 to 1,500+ Amperes) using large diameter wires (1/8" to 1/4"), achieving deposition rates of 15 to 45+ pounds of weld metal per hour.
• Primary Boiler / Vessel Application: Longitudinal and circumferential (girth) butt welds on heavy-wall boiler steam drums, deaerator storage tanks, catalytic reactor shells, and large-diameter downcomer piping. SAW is limited strictly to the 1G (Flat) and 2G (Horizontal) positions.

Welding Process Selection Matrix & Field Trade Comparison

The following matrix summarizes the operational parameters, shielding methods, and primary pressure vessel applications for each welding process encountered by boilermakers: