Porosity is one of the most frequently encountered weld discontinuities in structural steel production, and one of the most preventable. Unlike cracks or incomplete fusion—which require investigation and often repair—porosity within acceptance limits does not threaten structural performance and does not require corrective action. Outside those limits, it signals a root cause that will recur unless the WPS or the production conditions change. Understanding the distinction between the two situations, and knowing which controls in the WPS prevent porosity in the first place, is fundamental CWI and QC knowledge.
What Porosity Is
Porosity is gas trapped in solidifying weld metal, forming rounded or elongated voids. The arc decomposes atmospheric gas, flux chemicals, and surface contaminants into reactive species during welding; if gases cannot escape before the weld pool solidifies, they remain as discrete pores. Porosity is classified by distribution and morphology:
Scattered porosity — Individual pores distributed throughout the weld cross-section, not in a pattern. The most common form in production FCAW-G and GMAW.
Cluster porosity — Pores concentrated in a localized area, often at a weld restart, a stop/start, or a zone of excessive shielding gas disturbance. Cluster porosity is more likely to affect weld integrity than scattered porosity of the same total void volume.
Linear porosity — Pores aligned along the weld axis, often at the weld toe or along the fusion boundary. Linear porosity patterns may indicate a consistent contamination source at a specific joint location.
Piping porosity (wormholes) — Elongated, tube-shaped voids oriented perpendicular or at an angle to the weld surface, typically caused by gas evolving from the base metal surface during solidification. Often associated with galvanized, primed, or heavily contaminated base metal surfaces.
Crater porosity — A single pore or void at the weld crater (arc stop location), formed when the arc terminates too quickly and the solidifying pool gas cannot escape. Addressed by proper crater fill technique—running the arc back over the crater before breaking the circuit—which should be specified in the WPS.
Root Causes by Process
SMAW with low-hydrogen electrodes: Porosity from SMAW E7018 and similar low-hydrogen classifications is almost always moisture-related. These electrodes are designed to suppress hydrogen gas generation, but their hygroscopic flux absorbs atmospheric moisture within hours of exposure. Exposed electrodes must be returned to a holding oven at 250–300°F (120–150°C) or reconditioned by re-baking per the manufacturer's recommendation and AWS D1.1 requirements. The H-suffix on the electrode classification (H4, H8, H16) indicates the maximum diffusible hydrogen content in mL/100g of deposited weld metal—lower numbers mean stricter moisture control requirements. See Low-Hydrogen Electrode Storage and Control Under AWS D1.1 for oven and exposure time requirements.
FCAW-G: Shielding gas disruption is the primary porosity cause in flux-cored gas-shielded wire. Causes include inadequate gas flow rate, clogged or spatter-blocked nozzles, excessive contact tip extension, wind disruption at field weld stations, and incorrect shielding gas composition. A porosity cluster that appears when a welder moves to a new position often indicates wind exposure at the new location. Shielding gas flow rate is a nonessential variable in most FCAW-G WPS frameworks but must remain within the range proven during PQR testing.
FCAW-S: Self-shielded FCAW generates its shielding from flux decomposition rather than external gas, making it resistant to wind porosity but susceptible to moisture absorption in the wire's flux core. Electrode angle is also critical—significant deviation from the specified travel angle in the WPS reduces the effectiveness of the slag-gas shielding system. FCAW-S porosity in production often traces to a bad wire spool or to welders using electrode angles outside the qualified range.
GMAW: Similar to FCAW-G for shielding gas causes. Short-circuit transfer GMAW is additionally sensitive to wire feed speed and voltage stability—erratic short-circuit conditions prevent complete fusion and can trap gas in the semi-molten pool.
SAW: Submerged arc welding is inherently low-porosity because the arc is fully submerged under flux and protected from atmospheric contamination. SAW porosity, when it occurs, typically traces to wet or contaminated flux, insufficient flux coverage depth, or flux that has been recycled too many times and accumulated fine particles that disrupt shielding. SAW flux handling procedures should specify moisture content limits and prohibit use of flux that has been on the joint for more than a specified time without protection.
Surface Contamination: The Joint Cause Across All Processes
Regardless of process, surface contamination is the single most common cause of porosity in production structural welding. Mill scale on A36, A572, or A992 sections decomposes in the arc and contributes to gas generation. Rust entraps moisture that vaporizes during welding. Primer and paint on pre-coated or mismarked sections decompose into organic vapors. Oil and grease from grinding wheels, marking crayons, and handling leave hydrocarbons that the arc decomposes into CO and CO₂ within the weld pool.
AWS D1.1 Clause 5 requires that base metal be clean and free of mill scale, rust, and paint within the weld joint area. The CWI's pre-weld inspection should verify that the weld area is cleaned to the preparation condition specified in the WPS. Grinding, wire brushing, and solvent wiping are typical preparation methods; the WPS should state which preparation method is acceptable and to what condition (bare metal in the weld zone vs. light mill scale permitted at distance from the joint line).