clause5.io/ welding/ weld-porosity

What Porosity Is

Porosity is a weld discontinuity caused by gas becoming trapped in the weld pool as the metal solidifies. The gas forms spherical or elongated voids inside the weld metal. Small, scattered pores are called distributed porosity. Elongated or cylindrical voids, often oriented perpendicular to the weld surface, are called piping porosity. Clustered pores concentrated in one area are called cluster porosity.

D1.1:2025 Clause 8.9 requires visual inspection of all welds. Table 8.1 lists the acceptance criteria for visual inspection, and item (8) specifically addresses piping porosity. The limits vary by weld type (groove vs fillet) and by loading condition (statically loaded vs cyclically loaded nontubular connections).

Table 8.1 Porosity Acceptance Criteria

Table 8.1 item (8) divides piping porosity acceptance into three sections. Here is what the code requires for each:

Statically Loaded Nontubular Connections

These three limits work together — a fillet weld must satisfy the per-inch limit AND the per-12-inch limit (or proportional limit for shorter welds) simultaneously.

The critical distinction: CJP groove welds carrying computed tensile stress allow zero visible piping porosity. Any visible piping pore on these welds is a reject. For fillet welds and groove welds not transverse to tension, the code permits some porosity within the aggregate limits above.

Cyclically Loaded Nontubular Connections

Cyclic connections face stricter limits on general fillet and groove welds: one pore per 4 in with a maximum diameter of 3/32 in. The exception is fillet welds connecting stiffeners to webs, which follow the same aggregate limits as statically loaded connections.

What the Numbers Mean in Practice

For a fabricator welding a statically loaded fillet weld, the per-linear-inch limit of 3/8 in aggregate piping porosity for pores 1/32 in and larger means you could have several small pores in each inch of weld and still pass visual inspection. A single pore of 1/32 in diameter is barely visible to the unaided eye.

For a cyclically loaded fillet weld, one pore per 4 in with a max diameter of 3/32 in means the inspector is counting individual pores and measuring them. If your 12 in fillet weld has four visible pores of 1/16 in diameter, all four are under the 3/32 in diameter limit, but you need at least 4 in between each one.

Beyond Visual: RT and UT Acceptance Criteria

Table 8.1 covers visual inspection only. When the contract documents require radiographic testing (RT), additional acceptance criteria in Clause 8.12 apply. RT can detect subsurface porosity that visual inspection cannot reach.

For statically loaded connections, RT criteria per Clause 8.12.1 require no cracks and limit rounded discontinuities to a maximum size of S/3 (weld size divided by three), not to exceed 1/4 in. When S exceeds 2 in, the maximum rounded indication may be 3/8 in. The minimum clearance of rounded discontinuities greater than or equal to 3/32 in to an acceptable discontinuity or to an edge shall be three times the greatest dimension of the larger discontinuity.

Ultrasonic testing (UT) acceptance criteria are defined in Tables 8.2 and 8.3, which use a severity class system (Classes A through D) based on weld size and search unit angle. UT acceptance criteria for statically loaded connections follow Table 8.2, while cyclically loaded connections in tension follow the stricter Table 8.3.

What Causes Porosity

Porosity forms when gas dissolves into the liquid weld pool and cannot escape before solidification. The common sources are:

Moisture: Water on your base metal, in your electrode coating, or in your gas lines breaks down into hydrogen and oxygen at arc temperature. For SMAW with low-hydrogen electrodes like E7018, D1.1 Clause 7.3.2.1 requires oven storage at 250°F minimum after opening the hermetically sealed container. Table 7.1 limits atmospheric exposure to 4 hours for E70XX electrodes.

Surface contamination: Oil, grease, paint, rust, and mill scale on your base metal introduce carbon, hydrogen, and oxygen into the weld pool. Clause 7.14.4.1 requires surfaces to be welded to be cleaned of water, oil, grease, and other hydrocarbon-based materials.

Shielding gas problems: Insufficient gas flow, excessive gas flow (which creates turbulence and draws in atmosphere), wind, and drafts all allow atmospheric nitrogen and oxygen into the arc zone. For GMAW and FCAW, a flow rate that is too high is just as problematic as one that is too low.

Excessive arc length: Running a long arc increases the distance between your electrode and the weld pool, allowing atmosphere to penetrate the gas shield. This is especially common with SMAW in overhead and vertical positions.

Porosity vs Other Discontinuities

Porosity is one of several weld discontinuities addressed in Table 8.1. Understanding where it falls in the severity hierarchy matters for prioritizing inspection focus:

Item (1) Cracks: Any crack is unacceptable, regardless of size or location. This is the most severe discontinuity in D1.1.

Item (2) Incomplete fusion: Complete fusion shall exist between adjacent layers of weld metal and between weld metal and base metal. No exceptions.

Item (7) Undercut: For material less than 1 in thick, undercut shall not exceed 1/32 in in depth. For welds 12 in or longer, undercut up to 1/16 in is permitted for accumulated lengths up to 2 in in any 12 in.

Item (8) Piping porosity: Variable limits depending on weld type and loading, as detailed above. Porosity is the only common discontinuity where the code allows measurable amounts under specific conditions.

For the complete list of all eight Table 8.1 discontinuity categories — including cracks, incomplete fusion, and undercut — see the weld defects overview.