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newSchoolWorks.com › VoTechWorks › Building & Construction › Welding › Fabrication: Layout, Fitting, and Structural Assembly › Lesson 5
BLD-WELD01-11 · Lesson 5 · 10 clock hours · theory

Distortion Control: Prediction, Pre-Setting, and Sequencing

Objective. Students will predict the direction and approximate magnitude of weld-induced distortion on common joint configurations and select the correct control technique—pre-setting, back-step, intermittent welding, or balanced welding—for a given assembly.

Why Welds Move Metal

Every time you strike an arc, you pump intense heat into a small zone of metal. That metal tries to expand freely as it heats, but the surrounding cool base metal resists the expansion. The hot zone is forced to yield—it upsets, or compresses plastically. When the weld cools, it tries to contract back, but because it was upset during heating, it ends up shorter than it started. The result is a net shrinkage force that bends, twists, or shortens the assembly. Controlling distortion means either reducing those shrinkage forces or arranging the structure so the forces cancel each other out.

Distortion takes three main forms you will see on structural assemblies:

The amount of distortion depends on heat input, the number of passes, joint restraint, and base-metal thickness. Thinner plates distort more easily; greater heat input (slower travel speed, higher amperage) produces more shrinkage. Predicting the direction is straightforward: metal always moves toward the weld.

Predicting Distortion Direction

Before you can control distortion you must visualize it. Follow this rule: the part will rotate or bow toward the side that received more heat. For a single-pass fillet weld on a T-joint, the weld is on one side of the flange, so the flange tips toward the weld. For a butt weld made from one side only, the plate will bow concave on the weld face. For a multi-pass groove weld, whichever side accumulates more passes will tend to pull the joint toward it.

Practice sketching distortion vectors before welding an assembly. Draw an arrow at each weld showing the direction of expected shrinkage. Arrows that oppose each other mean the joint may self-balance; arrows that add together mean distortion will be significant and you need a control strategy.

Control Strategy 1: Pre-Setting (Preset Camber)

Pre-setting means deliberately tilting or bowing the parts in the opposite direction of expected distortion before welding. After the weld cools and the part distorts, it arrives at the correct geometry. This technique is especially effective for T-joints and beam flanges.

How to determine the pre-set angle: Weld a sample joint under the same conditions you plan to use on the production piece. Measure the angular distortion with a bevel protractor. If a T-joint fillet weld reliably produces 3° of angular pull, pre-set the next joint 3° open on the weld side before tacking. After welding, the flange should be within ±1° of 90°.

Tack welds must be strong enough to hold the pre-set position during heating. Use at least two tacks on each side, and make them roughly the same size as the planned fillet so they carry comparable stress.

Control Strategy 2: Back-Step Welding

In a standard continuous sequence you start at one end and travel to the other. All shrinkage forces accumulate in the same direction, and longitudinal distortion can be significant on long seams. Back-step welding breaks the seam into short segments, typically 75–150 mm (3–6 inches), and welds each segment in the direction opposite to the overall progression. You move the starting point forward for each new segment, but the arc travels backward.

The key effect: each segment's heat source starts in metal that is already partially warm from the previous pass, and the shrinkage of each segment partially offsets the overall progression. Longitudinal bowing and end-pull are noticeably reduced compared with a straight continuous bead.

Control Strategy 3: Intermittent (Skip) Welding

Intermittent welding places short weld segments with unwelded gaps between them. It is used when a continuous fillet is not required by the design. Less total weld metal means less total heat input and less shrinkage. Intermittent welding is common on non-structural gussets and stiffener plates where code permits it. Do not use intermittent welding in corrosive environments or where the gap would trap moisture unless the design explicitly allows it.

Control Strategy 4: Balanced Welding

Balanced welding places welds symmetrically about the neutral axis of the assembly so that each weld's shrinkage force is offset by a weld on the opposite side. Welding a box column by alternating passes on opposite corners, or welding both sides of a double-bevel groove in balanced passes, are classic examples. The order matters: complete a pass on one side, flip or reposition, complete a matching pass on the other side. Do not finish all passes on one side before starting the other.

Jigs, Fixtures, and Tack Sequences

Fixtures clamp the work rigidly so distortion is mechanically restrained. The part springs back slightly when released, so experienced fabricators account for springback. Heavy tack sequences—tacking from the center of a seam outward rather than from end to end—distribute residual stress more evenly and reduce end-pull.

Safety Notes

Key terms9

Transverse shrinkageContraction of a weld joint perpendicular to the weld axis as the bead cools, pulling the joint faces together across the width of the weld.
Longitudinal shrinkageContraction along the length of the weld bead, which bows the workpiece concave on the weld side.
Angular distortionRotation of a plate or flange toward the weld caused by unequal heating through the thickness; common in fillet-welded T-joints and lap joints.
Pre-settingIntentionally positioning parts at an angle opposite the expected distortion before welding so that, after cooling, the assembly reaches the correct geometry.
Back-step weldingA sequencing technique in which short weld segments are deposited in the direction opposite to the overall weld progression, reducing cumulative longitudinal shrinkage.
Intermittent weldingA pattern of short weld segments separated by unwelded gaps; reduces total heat input and distortion where a continuous weld is not required by design.
Balanced weldingDepositing welds alternately on opposite sides of a member's neutral axis so that each pass's shrinkage force is counteracted by the next pass on the other side.
Heat inputThe amount of thermal energy delivered to the base metal per unit length of weld, governed by amperage, voltage, and travel speed; higher heat input generally increases distortion.
Plastic upsettingPermanent compressive deformation of the weld-zone metal during heating when surrounding cool metal prevents free expansion; the root cause of weld shrinkage on cooling.

Worked examples4

1. A T-joint fillet weld on 3/8-inch A36 plate consistently produces 4° of angular distortion (flange tips toward the weld). What pre-set angle should you set on the next identical T-joint, and on which side?
  1. Identify the direction of distortion: the flange rotates toward the weld side, so the included angle on the weld side closes by 4°.
  2. To compensate, open the flange on the weld side by 4° before tacking. If the target final angle is 90°, set the flange at 90° + 4° = 94° (measured on the weld side) or equivalently tilt the flange 4° away from the web on the weld side.
  3. After welding and cooling, the expected 4° pull should bring the flange back to 90°.
Answer: Pre-set the flange 4° open (away from the weld side), placing the initial included angle at 94° on the weld side. After the weld cools, angular distortion of 4° returns the joint to 90°.
2. You weld a 24-inch butt seam using a continuous straight-ahead sequence and measure 3/16-inch of longitudinal bow (concave on the weld face). You reweld an identical coupon using back-step welding in 4-inch segments. How many back-step segments are needed, and in what order are they deposited relative to the overall progression?
  1. Divide the total seam length by the segment length: 24 in ÷ 4 in = 6 segments.
  2. Label the starting points of the segments 1 through 6 from left to right across the seam.
  3. Overall progression is left to right, but each segment is welded right to left within itself. Deposit: segment 1 (arc travels from point 1 toward the start), then move starting point to position 2 and weld segment 2 from position 2 back toward position 1, and so on through segment 6.
  4. Each segment's arc direction is opposite the overall left-to-right advance. The result is that shrinkage forces partially cancel across the seam.
Answer: Six 4-inch back-step segments are needed. Each segment is welded in the direction opposite the overall progression (e.g., right to left within each segment while the start points advance left to right). This reduces cumulative longitudinal bow compared with the continuous straight-ahead sequence.
3. A box column is fabricated by welding four corner fillet welds. If a fabricator welds all passes on corners A and B completely before touching corners C and D, what distortion problem is likely, and how should welding be sequenced instead?
  1. Corners A and B on the same side of the column accumulate all their shrinkage first. The column will bow toward the A–B side because those welds shorten that side of the section before C and D can provide an offsetting force.
  2. The correct approach is balanced welding: deposit one pass at corner A, then one pass at the diagonally opposite corner C (or the mirror corner D), alternating so that each new pass offsets the previous pass's shrinkage.
  3. A common balanced sequence for a four-corner box is: A → C → B → D (diagonally opposite pairs), repeating this rotation for each additional pass layer.
Answer: Welding all passes on two adjacent corners first causes the column to bow toward those corners. The correct sequence alternates passes on diagonally opposite corners (e.g., A → C → B → D per layer) so shrinkage forces balance about the section's neutral axis.
4. Two identical 12-inch A36 cover plates are each welded with four longitudinal beads. Plate 1 is welded bead 1 through 4 in straight continuous sequence, all in the same direction. Plate 2 is welded using back-step in 3-inch segments. A student measures plate 1 has bowed 1/4 inch over 12 inches; plate 2 bows 1/16 inch. Express each bow as a fraction of plate length and identify which control technique produced the smaller distortion.
  1. Plate 1 bow as a fraction: 0.25 in ÷ 12 in = 0.0208, or about 1/48 of the plate length.
  2. Plate 2 bow as a fraction: 0.0625 in ÷ 12 in = 0.0052, or about 1/192 of the plate length.
  3. Compare: 1/192 < 1/48, so plate 2 has approximately 75% less longitudinal bow than plate 1.
  4. The control technique that produced the smaller distortion is back-step welding.
Answer: Plate 1 bowed 1/48 of its length; plate 2 bowed 1/192 of its length—roughly 75% less. Back-step welding produced the smaller distortion.

The math in this lesson1

Learners set a 4° pre-set angle and compare measured bow between straight-ahead and back-step sequences.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor lecture and whiteboard demonstration of thermal expansion/contraction mechanics, angular distortion in fillet welds, and longitudinal shrinkage in butt welds; students complete a guided-notes worksheet and sketch distortion vectors on five joint diagrams. · Pre-setting exercise: using instructor-prepared data from a previous T-joint weld (measured angular distortion), students calculate the required pre-set angle, then set up and tack-weld a new T-joint at that pre-set angle before welding, then compare final angle to target. · Sequencing comparison lab: students weld two identical four-bead cover plates—one using continuous straight-ahead sequence, one using back-step sequence—then compare length distortion with a steel rule and discuss findings in a short written entry in the fabrication log.
Materials: A36 plate coupons (3/8-inch), SMAW or GMAW welding station, bevel protractor, steel rules, tape measure, guided-notes worksheet, fabrication log; Lincoln Electric Welding Handbook or AWS Welding Handbook Vol. 1 (Welding Science and Technology) as reference
Check: Worksheet scored by instructor: pass = correctly identifies distortion direction and control method for at least 8 of 10 scenarios; pre-set T-joint final angle within ±1° of target; written sequencing comparison entry demonstrates correct cause-and-effect reasoning

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Fabrication and Distortion Control
1. A single-pass fillet weld is deposited on one side of a T-joint flange. In which direction will the flange distort as the weld cools?
Fabrication and Distortion Control
2. A fabricator measures 5° of angular distortion on a sample T-joint fillet weld. To achieve a final angle of 90° on the production piece, the flange should be pre-set to:
Fabrication and Distortion Control
3. Which welding sequence reduces longitudinal bow on a long butt seam by depositing each short segment in the direction opposite to the overall weld progression?
Fabrication and Distortion Control
4. A designer specifies intermittent fillet welds on a non-structural interior stiffener plate. The primary benefit of intermittent welding compared with a continuous fillet in this application is:
Fabrication and Distortion Control
5. When welding a four-corner box column, the best sequence to minimize bowing of the section is:
Welding Science and Metallurgy
6. Plastic upsetting is the root cause of weld-induced distortion. It occurs because:
Fabrication and Distortion Control
7. A student welds two identical 12-inch plates: one with back-step sequence, one with straight-ahead sequence. The straight-ahead plate shows 3/16-inch of longitudinal bow. Which conclusion is best supported by this result?
Welding Safety and PPE
8. Before measuring the pre-set angle of a freshly tacked T-joint with a bevel protractor, the safest immediate action is to:
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