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Weld Porosity Causes — D1.1:2025 合否基準 Criteria
Per D1.1:2025 Table 8.1 item (8), visible piping porosity 1/32 in or greater shall not exceed 3/8 in per linear inch of fillet or 開先溶接 on statically 読み込み済み connections. CJP butt joints transverse to tensile stress require zero visible piping porosity.
What Causes Weld Porosity
Porosity forms when gas becomes trapped in the solidifying weld pool. The arc generates extremely high temperatures that cause chemical reactions in the 母材, 溶加材, 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).
The six primary causes are:
Moisture — Water in any form is the most common cause. Moisture on base metal surfaces, in filler metal flux coatings, or absorbed into flux-cored wire introduces hydrogen into the arc. The hydrogen dissolves into the weld pool at high 温度 and is released as the pool cools, forming hydrogen porosity. This is why E7018 and other 低水素 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 最小 of 1 in each side before 溶接.
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.
Electrode moisture (SMAW) — Low-hydrogen electrodes (E7016, E7018) are manufactured with flux coatings engineered to maintain 拡散性水素 below 4–16 mL/100g of deposited 溶接金属. Exposure to humid air rapidly re-introduces moisture. Electrodes exposed to open air for more than 4 hours typically require redrying at 500–800°F per manufacturer instructions.
Excessive travel speed — When the 溶接士 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 溶接速度 allows more time for outgassing and typically reduces porosity frequency.
Arc instability — Incorrect 極性, excessive arc length, or improper 電圧 settings can destabilize the arc and disrupt the gas shield. An unstable arc also produces inconsistent 入熱, 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 外観検査 of all production welds under 条項 8.9. Item (8) applies specifically to piping porosity — porosity visible on the weld surface. Subsurface porosity is evaluated by 放射線透過 or 超音波探傷試験 when specified in the 契約図書.
| Connection Type | Weld Type | Porosity Limit |
|---|---|---|
| Statically loaded (A) | CJP 突合せ継手 transverse to tensile stress | No visible piping porosity |
| Statically loaded (A) | Fillet welds and other groove welds | Sum of pores ≥1/32 in dia: ≤3/8 in per linear inch; ≤3/4 in per 12 in (for welds ≥12 in); ≤weld length × 0.06 (for welds <12 in) |
| Cyclically loaded (B) | Fillet welds (except stiffener-to-web) | Frequency: ≤1 pore per 4 in; max diameter: ≤3/32 in |
| Cyclically loaded (B) | Fillet welds connecting stiffeners to webs | Sum of pores ≥1/32 in dia: ≤3/8 in per linear inch; ≤3/4 in per 12 in; ≤weld length × 0.06 |
| Cyclically loaded (C) | CJP butt joint transverse to tensile stress | No piping porosity |
| Cyclically loaded (C) | All other groove welds | Frequency: ≤1 pore per 4 in; max diameter: ≤3/32 in |
Inspector note: The distinction between “piping porosity” and general “porosity” matters for Table 8.1. Item (8) applies only to piping porosity visible at the weld surface. The table counts individual pore diameters 1/32 in or greater — pores smaller than 1/32 in are not counted toward the limit. When measuring, use a calibrated weld gauge or magnifier and count only pores meeting the minimum diameter threshold.
Prevention and Field Guidance
Effective porosity 予防 starts with the WPS. The procedure should specify 予熱 要求事項 (which reduce moisture and hydrogen uptake), filler metal storage conditions, joint preparation requirements, and シールドガス 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) 確認 shielding ガス流量 at the start of each shift and after any equipment changes; (3) check 電極 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.
Frequently Asked Questions
The six main causes of weld porosity are: (1) moisture — in the base metal, filler metal, or atmosphere, which decomposes in the arc and releases hydrogen or oxygen; (2) surface contamination — oil, rust, paint, mill scale, or zinc coating on the joint faces that release gases as they burn off; (3) shielding gas loss — insufficient flow rate, drafts, or damaged gas lines that allow atmospheric nitrogen and oxygen into the weld pool; (4) electrode moisture — undried SMAW electrodes (especially E7018 low-hydrogen) that introduce hydrogen into the arc; (5) excessive travel speed — the weld pool solidifies before trapped gases can escape; and (6) wrong polarity or voltage — arc instability that disrupts the gas shield and allows atmospheric contamination.
Per D1.1:2025 Table 8.1 item (8)(A), for fillet welds and groove welds (except CJP butt joints transverse to computed tensile stress) on statically loaded connections, the sum of visible piping porosity 1/32 in or greater in diameter shall not exceed 3/8 in in any linear inch of weld. For welds 12 in or greater in length, the sum shall not exceed 3/4 in in any 12 in of weld length. For welds less than 12 in in length, the sum shall not exceed the weld length multiplied by 0.06.
No. Per D1.1:2025 Table 8.1 item (8)(A)(1), CJP groove welds in butt joints transverse to the direction of computed tensile stress shall have no visible piping porosity. This is a zero-tolerance criterion for that specific joint and loading condition. For other groove welds and fillet welds on statically loaded connections, limited porosity is acceptable per the frequency and size limits in Table 8.1 item (8)(A)(2).
Porosity reduces the effective cross-sectional area of the weld, lowering its load-carrying capacity in proportion to the volume of voids. Scattered porosity in small amounts has minimal effect on static strength, which is why D1.1 permits limited porosity in fillet welds under static loading. However, porosity under cyclic loading is more damaging because each pore acts as a stress concentration that can initiate fatigue cracks. D1.1:2025 Table 8.1 item (8)(B) and (C) apply tighter porosity limits to cyclically loaded connections for exactly this reason.