Do plug and slot welds require a separate WPS from fillet welds?

Yes. Plug and slot welds involve a different joint geometry — welding through a hole or slot in one plate to the surface of the plate beneath — and require a WPS that explicitly covers the process, filler, preheat, and technique. A fillet weld WPS does not automatically qualify plug or slot welds.

What dimensions does AWS D1.1:2025 specify for plug welds?

AWS D1.1:2025 sets minimum hole diameter based on the thickness of the plate through which the hole is made. The minimum diameter is the plate thickness plus 5/16 in [8 mm], with a maximum of 2¼ times the plate thickness. Hole spacing has minimum edge and center-to-center requirements as well.

Is FCAW a permitted process for plug welds?

Yes. FCAW, SMAW, and GMAW are all used for plug and slot welds. The WPS must specify the process, and the filler metal, current settings, and technique are essential variables. Self-shielded FCAW is particularly common because wind sensitivity is lower and access with a flux-cored wire gun into a confined hole is practical.

How deep must a plug weld be filled?

AWS D1.1:2025 requires that plug welds be filled flush with the surface of the plate — or to a depth specified on the design drawings — in a single pass or layered passes depending on hole depth and process. Underfilled plug welds are a common inspection finding because visual access is limited once the welder is past the first pass.

Plug and Slot Weld WPS Requirements Under AWS D1.1:2025

Plug and slot welds have distinct fit-up, size, and qualification rules under AWS D1.1:2025. Here's what your WPS must address and the inspection checkpoints CWIs use.

Plug welds and slot welds appear on structural fabrication drawings less often than fillet welds or groove welds, but when they appear, they generate disproportionate documentation problems. Shops that have qualified WPS procedures for SMAW and FCAW fillet welds frequently discover — during an audit or third-party inspection — that they have no qualified procedure covering plug or slot welds, despite having made them in production.

AWS D1.1:2025 treats plug and slot welds as distinct joint types with their own design provisions, dimensional requirements, and qualification rules. Applying a fillet weld WPS to a plug weld is not permitted and is not conservative — it is simply noncompliant.

What plug and slot welds are

A plug weld is made through a circular hole in one plate — called the "upper plate" — to fuse it to the plate beneath. The hole is drilled, punched, or flame-cut in the upper plate, the two plates are fit up in contact, and the welder fills the hole with weld metal from the bottom up. The finished weld is roughly circular in plan and extends the full depth of the hole.

A slot weld is the same concept with an elongated hole — typically with straight sides and rounded ends. Slot welds are used where a longer weld length is needed in a small width, or where edge clearance prevents running a fillet along the full plate length.

Both joint types are used to connect lapped plates, attach cover plates to flanges, connect gussets or diaphragms to web plates, and join built-up sections where access for fillet welding is restricted to one side. They are common in bridge fabrication, plate girder construction, and built-up column sections.

Design provisions under AWS D1.1:2025

Plug weld dimensions

AWS D1.1:2025 specifies minimum and maximum hole dimensions for plug welds based on the thickness of the plate containing the hole:

Minimum hole diameter: thickness of the plate through which the hole is made plus 5/16 in [8 mm]
Maximum hole diameter: 2¼ times the thickness of the plate through which the hole is made, or the minimum plus 5/16 in [8 mm], whichever is larger

These limits exist to ensure that the weld can be made with adequate fusion to the hole walls and to the faying surface. A hole too small prevents the welder from achieving sidewall fusion. A hole too large increases the risk of lack of fusion on the base plate beneath.

Spacing: Minimum center-to-center spacing for plug welds is four times the hole diameter. Minimum edge distance from the center of the hole is three times the hole diameter. These requirements mirror those for bolts, reflecting the load transfer mechanics of plug weld groups.

Slot weld dimensions

For slot welds, the width is subject to the same minimum and maximum as plug weld hole diameter, based on the plate thickness through which the slot is made. The length of a slot weld is specified by design and not subject to an AWS D1.1 maximum — though it is limited to the available length of the contact area and the structural design intent.

Rounded ends are required on slot welds made through metal. Square-ended slots produced by torch cutting require grinding or machining of the ends to avoid stress concentration at the corner radii.

Depth of filling

When the upper plate thickness is 5/8 in [16 mm] or less, plug and slot welds are filled flush with the surface of the plate in a single layered sequence. When the plate is thicker than 5/8 in [16 mm], the weld must be filled to a depth of at least half the thickness of the plate through which it is made — or as specified by design — with the weld deposited in multiple passes.

The distinction matters for WPS drafting: a deep plug weld in 1-inch plate requires a multi-pass technique specification, with pass sequence, interpass temperature control, and minimum preheat maintained through all passes.

WPS requirements for plug and slot welds

Process selection

SMAW, GMAW, and FCAW are all commonly used for plug and slot welds. SAW is generally not practical due to access constraints. GTAW root passes are rare but occasionally used on tight-tolerance applications.

Process selection is driven by:

Hole depth and diameter — a small hole in thick plate restricts electrode angle and nozzle access
Wind exposure — gas-shielded processes (GMAW, FCAW-G) require wind protection; self-shielded FCAW-S does not
Deposition rate requirements — larger holes in thicker plates are more efficiently filled with FCAW or GMAW than covered electrode

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

Essential variables that apply

For plug and slot welds, the essential variables in AWS D1.1:2025 Table 6.6 apply to the same processes as for groove and fillet welds. Additionally, the WPS must specify:

Hole or slot dimensions — at minimum the nominal diameter or width and the plate thickness range the WPS covers
Number of passes — whether the weld is completed in a single pass or requires multiple layers, based on depth
Current, polarity, and electrode diameter — a narrow-bore plug weld may require a smaller electrode than the shop's standard to achieve proper fusion at the bottom
Technique — whether the welder moves the electrode in a circular pattern, a straight-in pattern, or a layered sequence with each pass leaving a concave bead before the next pass starts
Preheat — same requirements as for the base metal group and thickness under the preheat provisions; plug welds in 1-inch A572 in cold weather require the same treatment as a CJP groove weld in the same material

See: Preheat and interpass temperature on a WPS

Qualification path

Plug and slot welds are not covered under the prequalified WPS provisions of AWS D1.1:2025. A prequalified WPS covers certain groove and fillet weld joint configurations meeting the dimensional requirements in the prequalified joint tables — plug and slot welds are not in those tables. This means a PQR test is required to qualify a WPS that covers plug or slot welds.

The PQR test specimen for plug welds is made per the qualification provisions of AWS D1.1:2025, with the test hole made in material of the thickness being qualified. Required tests are macroetch specimens sectioned through the weld to verify fusion to the faying surface and sidewalls. Tensile and bend tests are not typically required for plug and slot weld qualification; fusion verification by macro is the primary acceptance criterion.

See: PQR vs prequalified WPS: cost and timeline comparison

Common production and inspection problems

Lack of fusion on the faying surface

This is the most common failure mode for plug welds. The welder completes the hole fill and the top surface looks fused and flush, but the weld metal did not bond to the base plate beneath — particularly in the center of a large-diameter plug. The root cause is usually inadequate current for the hole size, or starting the arc too high in the hole before the base plate is hot enough to support fusion.

Macro examination of a production plug weld is the only reliable way to confirm bottom fusion without NDE. For critical applications, UT of plug welds is used but is technically difficult; MT or PT checks only the surface.

Underfill

An underfilled plug weld — where the weld metal does not reach the specified depth or does not flush up to the plate surface — reduces the effective weld area and the strength of the connection below design capacity. Visual inspection from above can miss underfill in deep holes. Depth gauging with a straight rod is a quick field check.

Oversized holes from torch cutting

Holes flame-cut in the field to accommodate a late-design plug weld often exceed the maximum size limits or have ragged walls with cut oxide. Both conditions compromise fusion and are not code-compliant starting geometry. Drilled or punched holes in the shop setting reduce this risk; field cuts require dimensional verification and often wall cleanup by grinding.

Wet or dirty faying surfaces

Because the faying surface between two lapped plates is enclosed once the joint is assembled, any moisture, mill scale, paint, or debris trapped at the interface beneath the plug weld area can prevent complete fusion. Joint preparation at the faying surface before assembly — particularly cleaning the area around each hole location to bare metal — is part of WPS compliance, not just good practice.

CWI inspection checkpoints

Before welding:

Verify hole dimensions (diameter, edges, wall condition) against the WPS and design drawing
Verify plate fit-up — no gap between upper and lower plate at the weld location
Verify preheat at the joint
Confirm filler classification and current settings match the WPS

During welding:

Observe the first pass for evidence of arc starting position and fusion technique
Verify interpass temperature maintenance on multi-pass fills

After welding:

Depth measurement of fill (flush or to specified depth)
Visual inspection of surface profile and edge fusion to the upper plate surface
Document on the weld inspection record tied to the applicable WPS

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

Keeping plug and slot weld WPS qualifications in the same procedure library as your groove and fillet procedures, with linked PQR records, ensures auditors can trace every weld type without hunting across separate files. WPS Welding organizes the full qualification library with PQR linkage and revision control built in.

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