Welding Processes, Discontinuities and Hydrogen Control

Welding is the primary manufacturing method used to assemble modern waterwall panels, boiler steam drums, superheater headers, and high-energy steam piping. Because a welded joint becomes an integral, continuous part of the pressure containment boundary, welds must possess mechanical properties, chemical purity, and corrosion resistance meeting the applicable design and procedure requirements.

Power engineers must understand the operating characteristics of industrial arc welding processes, identify critical weld defects, and evaluate welds using standardized Non-Destructive Examination (NDE) methodologies governed by ASME Section V.

Electric Arc Welding Processes in Power Plants

The common arc processes below utilize an electric arc struck between an electrode and the workpiece, generating temperatures between $3,000^\circ\text{C}$ and $6,000^\circ\text{C}$ to melt base and filler metals into a localized weld pool.

1. Shielded Metal Arc Welding (SMAW / "Stick")

• Operating Principle: An electric arc is maintained between a flux-covered consumable metal electrode and the work. The metallic core wire melts to supply filler metal while the chemical flux coating decomposes under the intense heat.
• Flux Functions:
  1. Gaseous Shielding: Generates a cloud of carbon dioxide ($CO_2$) and carbon monoxide ($CO$) that displaces atmospheric oxygen and nitrogen, preventing weld puddle contamination.
  2. Deoxidation & Alloying: Introduces manganese and silicon to deoxidize the molten pool and adds alloying elements.
  3. Slag Formation: Forms a lightweight molten slag blanket that floats over the weld pool, protecting it from atmospheric oxidation, shaping the bead profile, and slowing the cooling rate.
  4. Electrical Stabilization: Ionizes the arc stream to stabilize arc voltage.
• Low-Hydrogen Electrode Classification (AWS / ASME):
  • Example: E7018
    • E: Electric arc welding electrode.
    • 70: Minimum tensile strength of $70\text{ ksi}$ ($485\text{ MPa}$).
    • 1: Approved for all welding positions (flat, horizontal, vertical, overhead).
    • 8: Low-hydrogen potassium flux coating operating on AC or DC electrode positive (DCEP).
  • Storage: Follow electrode-maker and qualified-procedure requirements for unopened packaging, exposure time, holding and reconditioning.
• Applications: Outstanding portability and wind resistance make SMAW the workhorse for structural framing, piping tie-ins, and field boiler repairs.

2. Gas Tungsten Arc Welding (GTAW / "TIG")

• Operating Principle: An arc is struck between a non-consumable tungsten electrode (pure, $2\%$ thoriated, or ceriated) and the workpiece. The weld zone and electrode are shielded from atmospheric contamination by a continuous stream of $100\%$ inert argon gas (or argon-helium mixtures).
• Characteristics: If filler metal is needed, a bare wire rod is fed manually or mechanically into the leading edge of the puddle. No flux is used; therefore, no slag is produced.
• Power Plant Application: GTAW permits close control and clean weld deposition when properly performed. It is commonly selected for root passes where the qualified procedure requires its control and cleanliness. Once the root pass provides an internal seal, subsequent fill and cap passes may be completed with SMAW or GMAW.

3. Gas Metal Arc Welding (GMAW / "MIG")

• Operating Principle: A continuous, consumable solid wire electrode is fed automatically from a spool through a welding gun cable. An external shielding gas flows through the nozzle to protect the puddle.
• Shielding Gases: Inert argon-rich mixtures ($Ar + 2\% - 5\% O_2$) for stainless steel; active mixtures ($75\% Ar + 25\% CO_2$) or pure $CO_2$ (MAG welding) for carbon steels.
• Advantages & Limitations: High deposition rate, continuous operation without electrode changes, and minimal slag removal. However, equipment is bulky, less portable than SMAW, and highly susceptible to breeze drafts dispersing shielding gas, limiting its use outdoors.

4. Submerged Arc Welding (SAW)

• Operating Principle: A continuous bare wire consumable electrode is fed into the joint while the electric arc is completely submerged beneath a deep blanket of granular, fusible mineral flux.
• Characteristics: Because the arc is completely buried under the flux layer, there is no visible arc flash, no spatter, and virtually no smoke. Shielding, eye/face protection and other PPE must follow the process assessment and approved procedure; a hidden arc does not remove all hazards. Unfused granular flux is recovered via vacuum systems for reuse.
• Power Plant Application: Operates at very high electrical currents ($300\text{ to }1,500\text{ A}$), delivering tremendous heat input and exceptionally high metal deposition rates. Conventional SAW is generally applied in flat or suitable horizontal positions. SAW is the primary process used in fabrication shops for the longitudinal and circumferential main seams of boiler steam drums and heavy reactor shells.

Welding Discontinuities & Critical Defects

A discontinuity is any interruption in the normal physical structure of a weld. When a discontinuity exceeds code-mandated size or severity thresholds established by the applicable construction code, it becomes a defect requiring gouging, repair, or joint rejection.

1. Incomplete Root Penetration (Lack of Penetration - LOP)

Occurs when weld metal fails to extend completely to the root of the joint, leaving an un-fused gap between the root faces. In high-pressure steam lines, LOP acts as a severe internal stress concentration notch and a nucleation site for fatigue cracking.

2. Incomplete Fusion (Lack of Fusion - LOF / Cold Lap)

Failure of the molten weld metal to fuse properly with the adjacent base metal or previously deposited weld beads. Caused by insufficient heat input, improper electrode angle, or presence of heavy mill scale/oxides on the bevel face.

3. Porosity

Spherical, cylindrical (piping/wormhole), or clustered gas pockets trapped in the weld bead as the molten metal solidifies. Caused by atmospheric nitrogen and oxygen entering the pool due to shielding gas loss, wet electrodes, oil/grease on base metal, or excessive arc length.

4. Slag Inclusions

Non-metallic solid entrapment trapped within the weld metal or between weld passes. Occurs in SMAW and SAW when an operator fails to thoroughly chip and wire-brush the brittle slag layer from a weld pass before depositing the next bead.

5. Undercut

A groove or channel melted into the base metal adjacent to the weld toe or root that remains unfilled by weld metal. Undercut reduces the effective wall thickness of the pipe or shell and creates an aggressive notch that concentrates cyclic stresses.

6. Weld Cracking & HAZ Hydrogen-Induced Cold Cracking

Cracking is the most dangerous defect; crack indications require evaluation against the applicable acceptance criteria; they must not be dismissed as harmless surface marks.

• Hot Cracking: Occurs at elevated temperatures during final puddle solidification along the weld centerline, caused by low-melting-point sulfur and phosphorus compounds.
• Heat-Affected Zone (HAZ) Hydrogen-Induced Cold Cracking (Underbead Cracking): Occurs in the parent metal adjacent to the fusion line hours or days after the weld cools to room temperature. It requires three simultaneous conditions:
  1. A susceptible hard, brittle microstructure (e.g., martensite in the HAZ).
  2. High residual tensile stresses locked into the joint.
  3. The presence of diffused atomic hydrogen (absorbed from damp flux, humid air, or organic grease/moisture).
• Prevention: Eliminate hydrogen sources by using low-hydrogen electrodes (E7018) maintained in rod warming ovens, preheating the joint to slow the cooling rate, and using the specified preheat, interpass, hydrogen-control and post-weld treatment provisions.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2.