Backstep and Skip-Pass Weld Sequences for Distortion Control
How backstep and skip-pass weld sequencing reduces distortion in structural steel fabrication under AWS D1.1:2025 — when to use them, how to document them on the WPS, and CWI oversight.
Weld distortion is one of the most common quality and rework problems in structural steel fabrication. A long fillet weld deposited in a continuous pass from one end to the other heats the plate unevenly — the trailing edge of the weld shrinks while the leading edge is still molten — and the result is a bowed, bent, or twisted member that requires flame straightening, mechanical correction, or rejection.
Backstep welding and skip-pass (block) sequencing are the two primary weld sequencing methods for managing this thermal distortion. Both are described in welding engineering references and routinely referenced in fabrication quality plans, but their relationship to AWS D1.1:2025 documentation requirements is frequently misunderstood. This article explains when each technique applies, how to document it on the WPS, and what the CWI needs to verify.
The Mechanics of Weld Distortion
Distortion during welding has two root causes: thermal gradient and weld metal shrinkage. As a weld bead is deposited, the heat-affected zone (HAZ) expands. The cooler surrounding base metal restrains this expansion. When the weld cools, the weld metal and HAZ contract — but they contract from a position the surrounding metal is now holding. The result is residual tensile stress in the weld and compressive stress in adjacent base metal, producing angular distortion (rotation of connected plates relative to each other) and longitudinal bowing.
The longer the weld and the thinner the base metal relative to the weld, the worse the distortion. Fillet welds on plate girder webs, long gusset connections, and continuous attachment welds are the most distortion-prone joints in structural fabrication.
The goal of sequencing is to spread the thermal input so that shrinkage forces partially cancel each other rather than adding up in one direction.
Backstep Welding: How and When
In a backstep sequence, the welder deposits each successive bead segment in the opposite direction to the overall joint travel. The joint is divided into segments — commonly 6 in to 18 in long for structural welds — and each segment is welded from right to left (or vice versa) while the overall joint advances from left to right.
The practical effect is that each segment starts in a zone that has already been heated by the previous segment and is slowly cooling, rather than starting cold. The temperature gradient across the joint is more uniform; the shrinkage at each segment partially offsets the shrinkage of adjacent segments. The result is less net longitudinal bowing and less angular rotation than a continuous single-direction weld.
When backstep is most effective:
Long fillet welds on plate girder webs, box columns, and fabricated beams
Flange-to-web T-joints where angular distortion pulls the flange out of plane
Continuous attachment welds where cumulative bowing would exceed drawing tolerances
When backstep is less effective:
Short welds (under 12 in) where thermal gradients are small regardless
Groove welds in restrained joints where distortion is dominated by transverse shrinkage, not longitudinal bowing
Overhead and vertical positions where bead restart geometry is harder to control
Skip-Pass (Block) Sequence: How and When
Skip-pass sequencing — sometimes called block sequence or skip sequence — deposits short weld segments at evenly spaced intervals along the full joint length in the first pass, then fills in the gaps in a second pass after the first-pass segments have cooled. Some applications use three or more passes with progressively closer spacing.
The thermal logic is different from backstep: rather than reversing bead direction, skip-pass spreads the initial heat input over the full joint length immediately. No single end of the joint runs persistently hotter than the other.
When skip-pass is most effective:
Very long welds (over 36 in) on thin-to-medium plate where continuous backstep becomes impractical
Symmetrical assemblies (box sections, plate girder flange-to-web joints on both sides simultaneously) where balanced heating is the goal
Production settings where multiple welders working simultaneously can each take alternate segments
Common skip-pass intervals: For structural fillet welds on 3/8 in to 3/4 in plate, typical segment lengths of 12 in to 18 in with equal skip gaps are used in practice. The exact interval is calibrated to the weld size, plate thickness, and desired thermal balance — experience and measurement of distortion on the first piece inform the next.
When AWS D1.1:2025 Requires Sequence Documentation
AWS D1.1:2025 does not prescribe backstep or skip-pass sequencing for standard structural welds. The code sets essential variables, acceptance criteria, and qualification requirements — it does not dictate weld sequence on routine joints. However, sequence documentation becomes mandatory in several scenarios:
High-restraint joints: When the fabricator or engineer identifies a joint as high-restraint — thick plate CJP groove welds, T-joints welded simultaneously on both sides under heavy fixturing — the WPS or companion welding procedure document should specify the sequence. AWS D1.1 gives the engineer authority to require specific procedures for unusual joints, and sequence documentation is part of that authority.
CVN-qualified tested WPS: When a tested WPS is qualified with CVN toughness requirements under AWS D1.1:2025 Table 6.8, the heat input range is an essential variable (tied to the pass sequence). A change in weld sequence that alters heat input outside the qualified range requires requalification. The sequence used during the PQR test is the reference — production welding must stay within the heat input range demonstrated during testing.
Seismic demand-critical welds: AWS D1.8 (the seismic supplement to D1.1) imposes tighter controls on weld heat input and sequence for demand-critical welds in moment frame connections. If the project references AWS D1.8, the WPS for demand-critical joints must address sequence explicitly, and the CWI must verify compliance during production.
Owner and engineer-specified requirements: The structural engineer or owner may specify backstep or skip-pass sequences in the project welding specification or on the fabrication drawings. When the specification is explicit, it becomes a contractual requirement and must be followed and inspected.
For standard structural work where the fabricator voluntarily uses backstep or skip-pass sequencing for quality reasons, the most practical documentation approach is a brief note on the WPS form (Annex J of AWS D1.1:2025) or in a companion weld procedure document:
Weld sequence: backstep; deposit in segments of 12–18 in, each segment welded opposite to overall direction of progression. Restart each segment at the leading end of the previous segment. Allow minimum 30 s cooling between segments or confirm interpass temperature is within limits before restart.
Or for skip-pass:
Weld sequence: skip-pass; deposit alternating 12 in segments at 24 in centers on first pass; fill between segments on second pass after first-pass segments have cooled to touch. Minimum preheat to be re-established at each restart if ambient temperature is below 40°F [4°C].
This language:
Tells the welder exactly what to do
Gives the CWI an observable basis for inspection
Establishes the expected sequence as part of the shop's quality record
When the engineer specifies sequence explicitly in the contract documents, that sequence language flows directly into the WPS, and any deviation requires written approval from the engineer of record.
Preheat and Interpass Temperature in Sequential Welding
A practical complication of backstep and skip-pass methods is that each bead restart occurs on metal that may have cooled below the minimum preheat temperature specified on the WPS. The minimum preheat temperature must be re-established at each restart — this is not optional. The welder should confirm with a contact pyrometer or temperature-indicating crayon that the restart zone is at or above the minimum preheat before striking the arc.
The maximum interpass temperature is not a restart restriction — it is a ceiling on how hot the base metal can get before depositing the next pass. A cooled restart does not violate the interpass maximum; it may violate the minimum preheat. These are different limits with different compliance requirements.
For A36 and A572 Grade 50 in typical structural thicknesses (up to about 1 in) with low-hydrogen processes, minimum preheat at ambient temperature is generally not required by AWS D1.1:2025. In cold weather (ambient below 32°F [0°C]), minimum preheat applies regardless of thickness. The WPS must specify both limits, and the CWI must verify them during in-process inspection.
The CWI's role in distortion-control sequencing is verification — confirming that the sequence specified in the WPS or the shop work order is actually followed, and identifying quality issues created by improper sequencing:
What to watch for in production:
Welders skipping segments and going back to them after significant cooling — creating potential cold-lap starts at the restart point
Inadequate interpass cleaning at skip-pass gap fill-in positions (slag inclusions can form at the cold restart zone if the slag from the previous segment is not completely removed)
Preheat not re-established at cold restarts during cold-weather welding
Skip-pass gap fills deposited while the adjacent first-pass segments are still hot enough to cause overlap distortion (opposite of the intent)
Post-weld checks:
Dimensional verification that distortion is within the drawing tolerance before final assembly. For plate girders, AWS D1.1:2025 specifies limits on camber, sweep, and web flatness — the weld sequence plan should be calibrated to keep the as-welded member within these tolerances, not just near them.
If a member is outside tolerance after welding, flame straightening may be required. Flame straightening must follow a documented procedure consistent with AWS D1.1 — heating above 1,200°F for A36 and A572 Grade 50, with verification that the HAZ properties are not degraded.
Measure first. Before implementing backstep or skip-pass on a new part, run a test piece with a continuous weld, measure the distortion, then trial the sequence and compare. This calibrates the approach to your specific plate, process, and fixturing.
Sequence both sides simultaneously on symmetric joints. For plate girder flange-to-web T-joints, welding both fillet welds simultaneously with two welders (one on each side) is more effective at eliminating angular distortion than any single-side sequencing technique. Document the simultaneous welding requirement on the WPS.
Use fixturing before sequencing. Tack welding or strong-back clamping to hold the assembly in shape before the main welds go in is the first line of defense against distortion. Sequencing reduces residual distortion; fixturing prevents it from starting.
Train welders on the specific sequence. A written procedure that the welder has never practiced is not a quality control measure. Train on scrap pieces; verify that the welder can identify and maintain the correct sequence under production conditions.
Tie the sequence to the WPS when it matters. For CVN-qualified tested WPS, for demand-critical seismic welds, and for any joint where the sequence is specified contractually, the sequence is a quality variable that must be controlled and documented. See essential variables under AWS D1.1 Table 6.6 for which variables require new qualification if changed.
Rule library based on AWS D1.1:2025; verify against your governing edition. Project specifications and the structural engineer of record's weld notes may impose additional sequence requirements beyond code minimums.
What is backstep welding and when should it be used?
Backstep welding is a technique where each successive bead is deposited in the opposite direction to the overall weld progression. Heat builds up behind each deposit rather than accumulating ahead of it, reducing the temperature gradient that drives longitudinal distortion. It is most effective on long fillet welds and groove welds where thermal accumulation causes bowing or angular distortion.
Does AWS D1.1 require documentation of backstep or skip-pass weld sequences on the WPS?
AWS D1.1:2025 does not specifically mandate backstep or skip-pass sequencing for most structural welds. However, if the fabricator uses sequencing as a distortion-control method, the WPS or a companion weld procedure document should describe the sequence so that welders and inspectors follow the same approach consistently. On high-restraint joints, the engineer may specify sequencing explicitly.
How does the skip-pass technique differ from backstep welding?
Skip-pass (also called block sequence or skip sequence) deposits short weld segments at intervals along the joint, then fills in the gaps after the first sequence cools. This spreads heat input along the length of the joint rather than concentrating it at one end. Backstep refers to the direction of individual bead deposition; skip-pass refers to the spatial distribution of bead placement across the joint.
Can distortion-control sequencing affect weld quality or cause code violations?
Yes, if not managed carefully. Depositing weld beads on a joint that has partially cooled can leave slag inclusion traps if interpass cleaning is incomplete, or create cold laps if the previous bead surface is too cool. The WPS maximum interpass temperature does not restrict restart on a cooled joint, but minimum preheat must be re-established. The CWI must verify that the sequence does not create incomplete fusion at restart positions.