Does AWS D1.1:2025 specifically require slag removal between passes?

Yes. AWS D1.1:2025 workmanship provisions in Clause 5 require that each pass of weld metal be cleaned of slag, spatter, and other surface contaminants before the next pass is deposited. This applies to all processes that produce slag — SMAW, FCAW, and SAW. Failure to remove slag between passes is a direct cause of slag inclusion discontinuities.

Does GMAW solid wire require interpass cleaning?

GMAW with solid wire (ER70S-3, ER70S-6, etc.) does not produce slag, so there is no slag to remove between passes. However, AWS D1.1 still requires that spatter, mill scale in the joint area, and any other surface contamination be removed before welding. Inter-pass cleaning on GMAW joints typically means brushing and inspecting for spatter bridges or cold lap at the toes of prior passes.

What inspection check should a CWI perform between passes on a multi-pass weld?

A CWI performing in-process inspection on a multi-pass groove weld should visually verify that all slag has been removed before the next pass starts, that no cold lap or incomplete fusion at the prior pass toe is being covered over, that the interpass temperature has not exceeded the WPS limit, and that the pass width and contour are consistent with the WPS technique requirements.

Can a CWI reject a completed weld for slag inclusions caused by poor interpass cleaning?

Yes. Slag inclusions that exceed the acceptance criteria in AWS D1.1:2025 Clause 6.9 are rejectable regardless of how they formed. RT and UT will reveal slag inclusions inside a completed weld that looked acceptable on the surface. The cost of NDE and repair is always higher than the cost of proper interpass cleaning during fabrication.

Multi-Pass Weld Interpass Cleaning: AWS D1.1:2025 Rules

AWS D1.1:2025 requires slag removal between weld passes. Here's what CWIs check, what welders must do, and why shortcuts cause slag inclusions and rework.

Multi-pass welding is the standard for most structural steel joints heavier than about 3/16 in (5 mm). Every additional pass introduces a new opportunity for trapped slag — and every pass that buries an uncleaned prior bead creates a slag inclusion that does not go away. The NDE finds it. The repair crew opens it. The schedule slips.

AWS D1.1:2025 workmanship requirements are explicit on this point: each pass must be cleaned before the next is deposited. What looks like a minor step in the heat of production welding is a code requirement and a quality gate.

Why slag inclusions form in multi-pass welds

Slag forms in SMAW, FCAW, and SAW welds as the flux or electrode coating reacts with the weld pool and floats to the surface. In an ideal flat or horizontal weld with correct travel speed and arc length, the slag floats cleanly out and solidifies as a thin, easily removed crust on top of the bead.

Conditions that prevent complete slag flotation:

Steep joint sidewalls — in a deep, narrow groove, slag can become trapped against the sidewall as the bead solidifies, particularly if the welder's arc angle does not wash the toes
Undercut at the prior pass toe — a small groove at the weld toe traps slag and prevents it from floating free; the next pass fills over it
Insufficient heat input — a cold bead solidifies before slag has time to rise; slag pockets form in the solidification front
Wrong travel speed — excessive travel speed leaves a convex bead with slag trapped at the side toes; the welder must slow down for the pass shape to allow clean slag release
Poor electrode angle — on vertical or overhead passes, slag control requires specific angles and stringer bead technique to keep slag ahead of or away from the pool

See: Incomplete fusion in structural welds — WPS-level controls

AWS D1.1:2025 workmanship requirements

Slag removal between passes

AWS D1.1:2025 Clause 5 (Workmanship) requires that all slag be removed from each pass before the next pass is deposited. The requirement applies to the full bead surface including the weld toes, not just the center of the bead crown. In a tight groove, this means the welder must get a chipping hammer or needle scaler into the joint corners before running the next pass.

The standard does not prescribe a specific tool. Chipping hammer, needle scaler, and angle grinder (disc or wire brush) are all used in practice. What matters is that the bead surface is visually clean — no visible slag remaining — before the next arc is struck.

Surface condition at the start of welding

Before the first pass is deposited, the joint surfaces must be cleaned per the workmanship requirements. Mill scale in the groove is not specifically prohibited by AWS D1.1 except where it would prevent fusion, but many project specifications require that joint surfaces be cleaned to near-white or bare metal condition. The WPS and project quality plan should specify the required preparation standard.

After fit-up and before welding begins, the joint must be free of loose scale, rust, moisture, oil, paint (in the weld zone), and any other material that would contaminate the weld deposit.

Spatter removal

Spatter adjacent to the weld does not directly cause slag inclusions in the weld metal, but heavy spatter buildup can: (a) mask crater cracks or incomplete fusion at the weld toe; (b) trap moisture; (c) violate the appearance acceptance criteria for finished welds. AWS D1.1:2025 acceptance criteria for visual inspection do not define a specific spatter allowance, but project specifications often require that excessive spatter be removed from the base metal within a specified distance of the weld centerline.

Process-specific cleaning requirements

SMAW (Shielded Metal Arc Welding)

SMAW produces the most slag of any common structural welding process because the entire electrode coating converts to flux during the arc. Slag is continuous across the bead surface and must be removed after every pass without exception. On root passes in a tight groove, a wire brush followed by a thin chipping hammer or a pick hammer for corners is standard.

Common SMAW interpass cleaning failures:

Skipping the toe areas, leaving a thin ribbon of slag that buries under the next bead
Wiping warm slag that appears removed but re-adheres as it cools (the surface must be struck, not wiped)
Ignoring the weld crater at the stop location, which often collects a concentrated slag deposit

FCAW (Flux-Cored Arc Welding)

FCAW also produces slag, though the slag morphology varies by wire classification. Slag from E71T-1 and E71T-9 wires (FCAW-G) tends to be more self-releasing than SMAW slag, particularly on flat and horizontal beads. However, on vertical and overhead passes, and in tight groove geometries, FCAW slag must be removed between passes the same as SMAW.

FCAW-S (self-shielded) wires produce denser, harder slag than gas-shielded types. Interpass cleaning after FCAW-S passes requires more aggressive cleaning — a needle scaler is often more effective than a chipping hammer alone.

See: FCAW-G vs self-shielded FCAW: WPS implications for field welding

SAW (Submerged Arc Welding)

SAW produces slag in the form of a solidified flux crust that typically breaks away in large pieces from the bead surface. On flat, mechanized single-pass SAW, the slag often self-releases. On multi-pass SAW groove welds, the slag must be removed and the bead surface cleaned before the next pass. Because SAW beads tend to be wide and convex, slag can accumulate at the bead edges where the arc energy tapers off. These areas must be examined and cleaned before the next pass covers them.

CWI inspection points

A CWI assigned to in-process inspection of a multi-pass weld should establish hold points at intervals that catch interpass quality before it gets buried:

Before welding:

Verify joint preparation and cleanliness meet WPS requirements
Confirm preheat has been achieved and documented

During welding (after each pass or group of passes):

Inspect that slag has been fully removed — look at toes, not just bead crown
Check for cold lap, undercut, or cracking in prior passes before they are covered
Verify interpass temperature has not exceeded the WPS maximum
Compare bead width and contour to WPS technique requirements

When slag inclusions are suspected:

A thin, dark line visible in the groove sidewall area after slag removal may indicate a slag ribbon from a prior pass that was not fully removed; have the welder grind the area before continuing
Any visual indication of trapped slag should be excavated before the next pass covers it

See: CWI pre-weld inspection: what to verify before arc ignition

What happens when slag inclusions are found by NDE

RT (radiography) reveals slag inclusions as elongated, irregular darker areas on the image — typically rounded or worm-shaped rather than the linear, sharp image produced by cracks. UT detects them as subsurface reflectors, though small dispersed slag inclusions can be difficult to characterize.

AWS D1.1:2025 acceptance criteria for RT and UT both include limits on slag inclusions — total length, individual size, and aggregate limits within a defined length of weld. Individual inclusions or a group exceeding those limits is rejectable and requires repair.

Repair procedure for slag inclusions: the defect must be removed by air carbon arc gouging or grinding to visually clean base metal, then the excavation re-welded per an approved repair WPS. The repair weld is then re-inspected. The cost in time and material for a slug inclusion repair typically runs 5–15× the time saved by skipping one interpass cleaning cycle.

See: Multi-pass weld pass sequence documentation

WPS documentation related to interpass cleaning

The WPS itself does not typically have a field for "interpass cleaning method" — that level of detail belongs in the welding procedure rather than the qualified WPS form. However, the WPS should specify:

Maximum interpass temperature (which creates the discipline to slow down and clean between passes)
Technique requirements (stringer or weave, bead size limits) that affect slag release behavior
Process and electrode classification, which govern what slag type and volume to expect

For shop quality plans and fabrication procedures that reference the WPS, it is good practice to explicitly require interpass cleaning in the traveler or inspection checklist. That document-level requirement, combined with in-process CWI holds, is what actually prevents slag inclusions in production — not just the code text.

WPS Welding tracks WPS revisions and inspection hold points in one place, so QC managers can link interpass inspection requirements directly to the procedure without relying on separate paper checklists.

Rule library based on AWS D1.1:2025; verify against your governing edition.