How to write AWS D1.1:2025-compliant WPS documents and CWI inspection plans that prevent burn-through on thin-wall HSS structural sections from 3/16 in down to 1/8 in wall.
Structural engineers have embraced hollow structural sections for their efficiency, clean appearance, and excellent axial-load-to-weight ratio. HSS bracing members, roof truss chords, canopy supports, handrail columns, and equipment-support frames appear in nearly every structural steel project. As the demand for lighter, more slender members grows, sections with wall thicknesses of 3/16 in or less—and sometimes as thin as 1/8 in—appear with increasing frequency in shop and field fabrication.
Thin-wall HSS creates a welding challenge that heavier plate and wide-flange sections do not: burn-through. When the arc melts through the wall entirely, the result is a hole in the base metal, a disrupted weld profile, and a likely rejectable discontinuity under AWS D1.1:2025's workmanship requirements. Prevention starts with the WPS.
Why Thin-Wall HSS Burns Through
A fillet weld deposited on the face of an HSS wall must fuse into the base metal without exceeding its heat absorption capacity. For thick sections this is not a problem—the mass of the steel draws heat away from the weld pool faster than the arc can deliver it. For thin-wall HSS the equation inverts: the wall section is small, thermal conductivity to the adjacent metal is limited, and a momentary reduction in travel speed or increase in amperage produces a weld pool that overruns the available material.
Several factors compound the risk on thin-wall HSS compared to plate welding:
HSS corners. At the formed corners of rectangular or square HSS sections, the arc is working around a tight radius with varying joint geometry. Travel speed naturally slows as the welder navigates the corner, increasing local heat input at precisely the location where the cross-section is most susceptible.
Interior inaccessibility. Unlike plate, the interior of an HSS member cannot be visually inspected for burn-through without borescope equipment or surface NDE. A through-hole in the wall may not be obvious from the outside, particularly if weld metal bridges over the perforation.
ERW seam location. HSS sections produced by electric resistance welding carry a longitudinal seam that may have different hardness or microstructural characteristics from the parent material. While the seam itself should not affect burn-through risk significantly, positioning the structural weld adjacent to the ERW seam is generally avoided.
Fit-up gaps. A gap at the root of a fillet weld on thin-wall HSS reduces the effective wall backing for the arc and increases burn-through risk. Fit-up control is inseparable from burn-through prevention.
WPS Heat-Input Controls
The most direct WPS control for burn-through risk is limiting heat input to a maximum value appropriate for the wall thickness being welded. Heat input (HI) in AWS D1.1 is calculated from the standard formula using voltage, amperage, and travel speed:
HI = (60 × V × A) / (1000 × TS)
where V is arc voltage in volts, A is current in amps, and TS is travel speed in inches per minute. The result is in kJ/in.
For thin-wall HSS the maximum heat input documented on the WPS should be verified during the PQR test at the minimum wall thickness being qualified. The test should demonstrate that the maximum HI value (corresponding to maximum amperage, maximum voltage, minimum travel speed) does not produce burn-through in the test coupon. Any WPS that lists only a minimum heat input or only a minimum travel speed without also bounding the maximum values on the high-heat end is not adequate for thin-wall work.
Specific amperage and voltage ranges should be tighter than the defaults used for 3/8-in or 1/2-in plate work. A WPS written for heavier fabrication and simply stated as "qualified down to" a thin-wall thickness without testing at the actual thin-wall condition is a documentation shortcut that creates real production risk.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Process Selection and Its Effect on Burn-Through Risk
FCAW-G (gas-shielded flux core) is the dominant process for structural HSS fabrication. It delivers good deposition efficiency and works well in the 1F and 2F positions common for HSS-to-plate and HSS-to-HSS connections. For thin-wall HSS, FCAW-G requires tight parameter windows on the WPS—E71T-1 wires at the smaller available diameters (0.035 in or 0.045 in) with controlled wire feed speed ranges to avoid hot spots in the weld pool.
GMAW (spray arc) at higher wire feed speeds produces a smooth, well-tied weld but runs at higher heat input than short-circuit transfer. For walls above 3/16 in, spray arc is usually manageable with adequate travel speed. For 1/8-in walls, spray arc heat input is difficult to control within safe limits.
Short-circuit GMAW provides the lowest heat input of the gas-shielded processes and produces good results on thin-wall HSS in test conditions. AWS D1.1:2025 restricts the use of short-circuit transfer GMAW in structural groove welds without an approved WPS qualification test, and some contract specifications further restrict its use even for fillet welds. Where the contract allows it and a PQR has been run, short-circuit GMAW is worth considering for the thinnest walls.
SMAW with small-diameter electrodes (1/8 in or smaller) can be used for thin-wall HSS tack welds and short runs but is generally slower and more welder-dependent than wire processes. The stop-start nature of SMAW creates more arc restart events, each of which is a burn-through risk on thin walls until the arc re-establishes stable conditions.
GTAW (TIG) is the benchmark process for thin-wall precision welding but is not commonly used in structural fabrication because of its low deposition rate. For critical small-bore HSS connections where burn-through risk is extreme and wall thickness is under 3/16 in, GTAW root passes with GMAW or FCAW fill may be specified.
For a discussion of how the essential variable rules govern process changes on structural WPS documents, see GMAW transfer mode as an essential variable in structural WPS.
Electrode Diameter as an Essential Variable
Under AWS D1.1:2025 Table 6.6, electrode diameter is an essential variable for certain processes. Increasing electrode diameter during production above what was qualified in the PQR requires requalification. For thin-wall HSS, this essential variable has practical importance: a welder accustomed to running 0.045-in wire on heavy structure may default to that diameter on thin-wall work, while the PQR and WPS specify 0.035-in wire because the lower current range enabled by the smaller diameter is what keeps heat input within the burn-through prevention window.
The WPS must state the qualified electrode diameter range explicitly. The CWI should verify that the wire spool loaded at the welding station matches the WPS-specified diameter before production welding begins on thin-wall sections.
Position Qualification and Its Effect on Travel Speed
Welding position affects travel speed and welder technique in ways that directly influence heat input. In the flat position (1F for a fillet weld on the top of an HSS cap plate), gravity helps the weld pool flow and allows faster travel speeds. In the vertical position (3F), the weld pool must be controlled against gravity, which often means lower wire feed speed and more arc time per inch of weld—increasing heat input.
For thin-wall HSS in the vertical position, a WPS that was tested and optimized in the flat position may not have been evaluated at the reduced travel speeds possible in the vertical. The PQR test should be conducted in the most heat-intensive position to be used in production, or separate WPS documents should be qualified for each position class with separate heat-input limits.
Fit-Up Requirements on the WPS
As noted above, fit-up gaps increase burn-through risk on thin-wall HSS. The WPS should state a maximum root opening consistent with the wall thickness qualified in the PQR. For HSS-to-plate fillet welds with no intentional root opening, specifying a maximum gap of 1/16 in and requiring corrective action above that threshold is a reasonable and enforceable standard.
Pre-weld CWI inspection must verify this dimension before the arc is struck. A gap that opened up after tack welding because of thermal distortion of a light-weight member is a common discovery that is better addressed before the first production pass than after a burn-through interrupts the work.
Welder Qualification at the Production Thickness
A welder who has qualified on 3/8-in or thicker plate has demonstrated the skill to control the weld pool in heavy sections. That qualification does not automatically transfer to thin-wall HSS in terms of practical technique. AWS D1.1:2025 does not prescribe a separate qualification test for thin-wall work—the standard WPS essential variable rules apply—but many QC managers require a production mock-up weld at the minimum wall thickness before releasing a welder to thin-wall HSS production.
A mock-up weld on the actual wall thickness being produced, inspected visually and by MT or PT for burn-through evidence, is a low-cost quality gate that catches technique issues before they cause non-conformances in the work. This practice aligns with the CWI's authority under AWS D1.1 to require production qualifications when there is reason to doubt a welder's ability to perform a specific application.
For more on how production mock-up testing is used as a quality control step in structural fabrication, see weld mock-up and pre-trial testing for critical structural joints.
Inspection Plan for Thin-Wall HSS Connections
An inspection plan for thin-wall HSS weld work should include at minimum:
Pre-weld:
- Verify HSS wall thickness against CMTR (actual wall tolerances can be wider than nominal)
- Verify fit-up gap is within WPS allowable
- Confirm preheat is correct per WPS (thin walls cool quickly; for low-alloy material, verify preheat holds when ambient is cold)
- Confirm electrode diameter and wire spool diameter match WPS
During welding:
- Monitor travel speed (slow travel is the primary burn-through mechanism)
- Watch for welder stop-start practices at corners; require smooth technique with no long hesitations
- Spot-check WFS and voltage settings against WPS range
Post-weld visual (before paint or coating):
- Inspect full weld length for profile issues: undercut, irregular bead width, pinholes at toes
- Examine both weld toes and the accessible portions of the HSS wall for signs of heat discoloration or surface distortion indicating near-burn-through
- For connections where the interior of the HSS is accessible (open ends, knockouts), visually confirm no through-penetration
NDE:
- MT or PT for surface-breaking discontinuities when specified by contract or when visual inspection raises concern
- For CJP groove welds on HSS (rarer but present on heavily loaded moment-frame HSS chord splices), UT or RT per the connection's inspection category
For the broader framework of how to structure your CWI inspection plan for structural steel work, see structural welding quality plan under AISC Chapter N, and for guidance on managing fillet weld WPS records across multiple structural projects see the WPS library management guide.
Writing the WPS for Thin-Wall HSS
A WPS for thin-wall HSS fillet welding should specify all of the following, beyond the standard fields:
- Minimum and maximum amperage, voltage, and wire feed speed ranges (most WPS documents only provide minimums)
- Maximum heat input calculated from those maximum electrical parameters at the minimum travel speed
- Electrode diameter as a range, not just a minimum
- Fit-up root opening maximum
- Position class and confirmation that the PQR was tested at the most heat-intensive position in that class
- Preheat minimum appropriate for the wall thickness and base-metal group per Table 5.11
- Maximum interpass temperature (overheating on subsequent passes is as much a risk as the first pass)
A WPS that checks all these boxes is straightforward to enforce during CWI inspection and gives the welder the specific guidance they need. If your current shop WPS for HSS fabrication was developed for heavier sections and has been informally extended to thin-wall work, consider whether a PQR test at the minimum production wall thickness is needed to establish a fully compliant qualified range. wpswelding.com/pricing lets you build that WPS with process-specific parameter ranges and the heat input calculation built into the form.