Fit-Up, Tacks & the Moving Plate
Weld metal shrinks as it cools and pulls the assembly with it, so distortion is controlled before the first full bead — by fit-up, tack sequence, restraint, and weld order. · 12 min
Every folio so far assumed the plate holds still while you weld it. It does not. Weld metal goes in near 1,500 °C and cools to room temperature, and cooling metal contracts. The weld cannot shrink freely — it is fused to the plates on every side — so it pulls them instead. Welders call the result distortion: the assembly that was square when you tacked it comes off the bench bowed, tipped, or twisted. This folio is about winning that fight before the first full bead goes down.
Guess before you learn
You run one fillet along one side of a tee joint and let it cool. Which way does the flange move?
The cooling weld contracts and pulls the flange toward itself, tipping the plate up on the welded side. If you guessed away from the weld, you are in good company — the intuition says heat pushes. But the lasting movement comes from cooling, and cooling pulls.
9–12
3–5
A cooling weld gets shorter and narrower, but it is fused to the plates on both sides, so it cannot shrink alone — it drags the plates with it. The pull is always toward the weld.
You cannot stop the shrinking. You can balance it: weld a little on one side, then a little on the other, so each pull cancels the last. And you can hold the parts with clamps and small tack welds while you work.
6–8
Shrinkage pulls in more than one direction. Transverse shrinkage draws the plates together across the weld; longitudinal shrinkage shortens the assembly along it; and because a weld is wider at its face than at its root, the face shrinks more — tipping the plates in angular distortion.
Control comes in four moves, all before or during welding: fit the parts with correct, consistent gaps; hold them with tack welds — short welds placed in a balancing order; clamp or pre-set the parts against the expected pull; and sequence the full welds so each shrink cancels the last.
9–12
Restraint is a trade, not a free win. Clamp everything rigid and the parts stay put — but the shrinkage strain must go somewhere, so it stays behind as residual stress, locked in the metal: sometimes high enough to crack a brittle weld, sometimes released as a sudden spring the moment the clamps come off.
Pre-setting spends that knowledge in advance: tilt the parts a few degrees against the coming pull and let shrinkage draw them square. The amount is learned from practice pieces — experienced fabricators keep notes on how far each joint type moves at each size.
K–2
Weld metal goes in very hot. Hot metal takes up more room than cold metal. As the weld cools, it shrinks.
The weld is stuck fast to both plates. When it shrinks, it pulls them toward it. The plates tip and bow. This is called distortion.
Undergrad
The mechanism is plastic, which is why the movement is permanent. Metal near the weld is heated while confined by cold surrounding steel, so it yields in compression; on cooling it contracts from that upset state and ends up too short for its surroundings. Elastic strain would recover; this does not.
Magnitudes are worth carrying: thermal contraction across a 1,000 °C fall is near 1.2 percent in steel — millimetres per hundred, easily visible across a weldment. Heat input is the lever: smaller welds, fewer passes, and faster travel all shrink the shrink.
Postgrad
Computational welding mechanics predicts distortion with inherent-strain methods: the plastic strain field near the fusion zone, applied as an eigenstrain, reproduces global deformation without simulating every pass. Thin panels add a failure mode of their own — compressive residual stress buckles them, which is why ship decks ripple between stiffeners.
Mitigation research spans process and metallurgy: low-heat-input variants and balanced simultaneous welding on one end; on the other, low-transformation-temperature fillers that time the austenite-to-martensite expansion to cancel thermal contraction, buying residual-stress relief right at the fusion line where fatigue cares most.
distortion
The permanent movement of a welded assembly as cooling welds contract and pull the parts toward themselves.
Control starts with fit-up — how the parts sit before any arc strikes. Gaps come first: a root gap that wanders from 2 mm to 5 mm along one seam will shrink unevenly and pull the assembly crooked, and the wide spots invite burn-through while the tight spots invite lack of penetration. Fit to the called-out gap, keep it consistent end to end, and check square with a framing square before you commit a single tack.
Why is this true?
Why tack the ends before the middle?
The end tacks fix the joint's overall geometry — gap and alignment — so nothing later can swing the plates. A middle-first tack leaves both free ends to scissor closed as it cools.
The last lever is the order of the full welds, and two habits do most of the work. Alternate sides: on a tee with fillets both sides, weld a stretch on one side, then the matching stretch on the other, so each pull cancels the last. Backstep: divide a long seam into short segments and weld each one against the direction of overall progress — the seam advances left to right while every individual bead travels right to left. Each segment's shrink stays small, local, and partly cancelled by the next.
Plan the welds for a tee stiffener, fillets both sides, 600 mm long — the steps fade as you master them
Each fillet pulls the flange up toward its own side as it cools
Alternate: 150 mm on side A, then the matching 150 mm on side B
Backstep — lay each 150 mm bead against the direction of progress
After the work cools to hand-warm — the pull is not finished until then
Fit, tack, restrain, sequence — four quiet decisions that decide whether the finished piece is square. Next folio is the whole course on one plate of steel: fit it, tack it, weld it, and then judge it the way an inspector would.
Practice — new ink and old, interleaved
1.A 600 mm seam is backstepped in 150 mm segments. How many beads will you lay?
2.Wide, high-crowned, piled bead. Verdict?
3.Order the restart, from arc-out to traveling again.
- Strike half an inch ahead of the crater
- Travel on at the normal pace
- Chip and brush the crater
- Swing back and refill the crater
4.What makes a good tack weld?
5.Match each kind of movement or stress to its description.
6.A square tube post welded upright onto a flat baseplate?
7.Match each defect to its cure.
8.You must weld all four corners of a rectangular frame. Which order fights distortion best?
9.From memory: name the four moves that control distortion before and during welding.
Fit with correct, consistent gaps; tack in a balancing sequence; clamp or pre-set against the pull; and order the welds — alternating sides and backstepping.
How close were you? Grade yourself honestly — it sets your review date.