Weld Porosity Causes — D1.1:2025 Accettazione Criteria

Quali sono le cause della porosità di saldatura

Porosity forms when gas becomes trapped in the solidifying weld pool. The arc generates extremely high temperatures that cause chemical reactions in the Metallo Base, Metallo d'Apporto, and surrounding atmosphere. When the weld pool solidifies faster than trapped gas bubbles can escape to the surface, those bubbles freeze in place as pores — either visible on the surface (piping porosity) or hidden within the weld cross-section (subsurface porosity).

Le sei cause principali sono:

Moisture — Water in any form is the most common cause. Moisture on base metal surfaces, in filler metal flux coatings, or absorbed into Filo animato introduces hydrogen into the arc. The hydrogen dissolves into the weld pool at high Temperatura and is released as the pool cools, forming hydrogen porosity. This is why E7018 and other Basso Idrogeno electrodes require controlled storage and redrying after exposure.

Surface contamination — Oil, grease, paint, rust, mill scale, and zinc (galvanized coatings) all release gases when they enter the arc zone. Even thin contamination layers on joint faces can generate enough gas to cause visible porosity. Joint faces must be clean and dry for a Minimo of 1 in each side before Saldatura.

Shielding gas loss — GMAW, FCAW-G, and GTAW processes depend on an inert or semi-inert gas shield to exclude atmospheric nitrogen and oxygen from the arc. Drafts, insufficient flow rate, damaged nozzles, or excessive contact-tip-to-work distance all allow atmospheric contamination. Nitrogen porosity is particularly difficult to eliminate once the shield is compromised.

Umidità dell'Elettrodo (SMAW) — Gli Elettrodi Basso Idrogeno (E7016, E7018) sono fabbricati con rivestimenti di flusso progettati per mantenere l'Idrogeno Diffusibile al di sotto di 4–16 mL/100g di Metallo di Saldatura depositato. L'esposizione all'aria umida reintroduce rapidamente l'umidità. Gli Elettrodi esposti all'aria aperta per più di 4 ore richiedono tipicamente una risecatura a 500–800°F secondo le istruzioni del produttore.

Excessive travel speed — When the welder moves too fast, the weld pool does not have time to degas before solidification. Bubbles that would otherwise float to the surface become trapped. Reducing Velocità di Saldatura allows more time for outgassing and typically reduces porosity frequency.

Arc instability — Incorrect polarity, excessive arc length, or improper Tensione settings can destabilize the arc and disrupt the gas shield. An unstable arc also produces inconsistent Apporto Termico, leading to areas where the weld pool solidifies before degassing is complete.

D1.1:2025 Table 8.1 Item (8) — Piping Porosity Acceptance Criteria

D1.1:2025 Table 8.1 governs Esame Visivo of all production welds under Clause 8.9. Item (8) applies specifically to piping porosity — porosity visible on the weld surface. Subsurface porosity is evaluated by Radiografico or Ultrasonico Prova when specified in the contract documents.

Prevenzione e Guida sul Campo

Effective porosity Prevenzione starts with the WPS. The procedure should specify Preriscaldo requirements (which reduce moisture and hydrogen uptake), filler metal storage conditions, joint preparation requirements, and shielding gas flow rates. A WPS that does not address these parameters leaves prevention entirely to the welder.

In the field, the most reliable controls are: (1) inspect joint faces for contamination before welding and clean as needed; (2) Verifica shielding gas flow rate at the start of each shift and after any equipment changes; (3) check Elettrodo storage — low-hydrogen electrodes should be in a rod oven at manufacturer-specified temperature, not sitting on the deck; (4) for FCAW, inspect wire spools for moisture or damage, especially after rain or overnight exposure.

When porosity is found during VT, identify the probable cause before resuming production. Continuing to weld with an unaddressed cause will produce more rejectable welds. Common cause-to-fix mappings: clustered surface porosity → check electrode storage; uniform scattered porosity → check shielding gas; porosity concentrated at weld start/stop → adjust crater fill technique and preheat.

Riferimenti Correlati

■ Weld Defects HubTable 8.1 all 8 categories ■ Weld PorosityTable 8.1 criteria overview ■ Visual Weld InspectionClause 8.9 VT procedure ■ Weld DiscontinuityDiscontinuity vs defect ■ Weld CracksZero tolerance under D1.1 ■ Preheat CalculatorReduce hydrogen cracking risk