Guide
How do you control weld distortion on a weldment that is machined after welding?
Weld distortion is the permanent change in shape and size that a welded part takes on as the hot weld metal and the metal beside it shrink while they cool: plates pull shorter, flanges tilt, frames bow and box sections twist. On a weldment that is machined after welding it is controlled in two places at once. The fabrication side keeps the as-welded shape small and repeatable with fit-up, balanced weld sequence, fixturing and, where needed, stress relief. The drawing side makes sure every precision feature is machined after welding, from datums that are themselves machined, with enough stock left to clean up whatever distortion remains.
By Saola Manufacturing engineering team · Published · Updated
This guide starts from a situation many engineers and buyers know. A welded base, bracket or frame comes back, and the bored holes do not line up with the mating part, a mounting pad will not sit flat, or one face never cleaned up and still shows mill scale. Sometimes the parts measured fine at the supplier and moved afterward. The fix is rarely “weld more carefully.” It is usually a combination of a realistic as-welded tolerance, a stock allowance that matches it, the right order of welding, stress relief and machining, and datums that the machinist can actually find on a part that is not the shape of the CAD model. The sections follow that order: recognize the problem, rank the causes, measure it, fix it, write it down, and hand it to a supplier.
On this page
What does weld distortion look like on a machined weldment?
It shows up as features that are correct relative to each other but wrong relative to the rest of the part, faces that will not clean up, and parts that pass on the machine but move once they are unclamped or cut further.
The welding institute TWI lists six basic forms of distortion: longitudinal shrinkage, transverse shrinkage, angular distortion, bowing and dishing, buckling, and twisting (TWI, distortion types and causes). On a part that is machined afterward, each one has a recognizable symptom on the inspection report:
- Shrinkage makes the overall frame shorter or narrower than drawn, so hole patterns spread across two welded members come out of position even though each hole is round and on size.
- Angular distortion tilts a flange or gusseted plate, so a face that should be square to a base needs more stock on one edge than the other, and a thin edge may not clean up at all.
- Bowing and dishing leave a long base or rail curved, so the machined pads are flat individually but not coplanar, and the part rocks on a surface plate.
- Twisting in box or channel frames puts the four corner pads out of one plane, which looks like a flatness failure but is really a warped section.
- Residual stress release is the delayed version: the part measures correctly while clamped, then springs once it is unclamped, or moves after a later operation removes a lot of metal from one side.
The last symptom matters most for machined weldments, because it makes a good supplier look careless and a careful inspection look wrong. Record when each deviation appears: on arrival from welding, on the machine, after unclamping, or after a later operation. That timing is the fastest clue to the cause.
What causes weld distortion, and which causes are most likely?
The root cause is uneven expansion and contraction around the weld, and on machined weldments the practical causes to check, in this order, are too little machining stock, locked-in stress released by machining, unbalanced or oversized welds, and poor fit-up.
The physics is simple to state. TWI explains that the weld and heat-affected zone contract on cooling while the cold surrounding metal holds them back. For carbon-manganese steel it puts the volume loss at roughly 3% as the molten weld metal solidifies, then roughly another 7% for the solidified weld metal and heat-affected zone as they cool to room temperature (TWI). Where those stresses exceed the yield strength of the parent metal, it deforms plastically and the shape change is permanent. What remains below yield is locked in as residual stress. A practical order in which to check the causes when a machined weldment misses its features:
- Machining stock that does not cover the as-welded variation. The model shows a nominal weldment; the real one is shorter, tilted and bowed within its fabrication tolerance. If the drawing leaves only a thin skin on each machined face, some faces will not clean up.
- Residual stress released by machining. Facing a pad or boring through a welded member removes stressed metal, and the frame relaxes into a new shape. Heavy, one-sided cuts make this worse, and so does finishing in the same clamping as roughing.
- Unbalanced or oversized welds. TWI’s design guidance points out that weld shrinkage is proportional to the amount of weld metal, and that on a fillet designed with a 6 mm leg, depositing an 8 mm leg adds about 57% more weld metal without adding design strength (TWI, prevention by design). The same guidance recommends intermittent welds where strength allows and welds placed close to the neutral axis or balanced on opposite sides, and notes that in an unrestrained joint angular distortion is roughly proportional to the number of passes. A drawing that asks for a larger fillet than the load needs buys distortion, not strength.
- Fit-up gaps and inconsistent tacking. TWI notes that an excessive joint gap increases the weld metal needed and therefore the distortion, and that joints should be tacked so they cannot move during welding.
- Restraint released after welding. A part welded in a rigid fixture holds its shape while clamped; TWI cautions that once the clamps come off, the locked-in stresses still move it somewhat, and suggests a slight preset or stress relief before release (TWI, pre-setting and restraint).
- Material. Stainless steel expands more with heat than plain carbon steel does, so TWI rates it as more prone to distortion; a design that worked in mild steel can fail when the material changes.
Clamping distortion during machining is a separate suspect worth ruling out early. A bowed frame clamped flat on a machine table will machine flat, then spring back when released. That looks like weld distortion but is fixed in the machining setup, not at the welding bench.
How do you diagnose and measure weld distortion before blaming the machining?
Measure the weldment as welded, before any machining, against a stated general tolerance class, then measure again after machining both clamped and unclamped. Comparing those three readings tells you whether the problem is fabrication, residual stress or setup.
The usual reference for an as-welded part is ISO 13920, which gives general tolerances for welded constructions in classes A to D for lengths and angles and E to H for straightness, flatness and parallelism (ISO 13920:2023 sample pages). The standard measures flatness by aligning the surface to a measuring plane so the largest gap is as small as possible, for example with surface plates, floor plates, machine beds or optical devices, and it lists straightedges, squares, calipers and dial gauges as measuring devices. A simple diagnostic sequence that any inspector can run:
- As-welded layout. Set the weldment on supports, locate it from the intended machining datums, and check that every face to be machined has positive stock everywhere. A face with stock missing on one corner is a fabrication problem, not a machining one.
- Compare with the class. Check the measured deviations against the ISO 13920 class on the drawing. Out of class means the welding process needs correcting; inside class but still short of stock means the drawing allowance is too small.
- Clamped versus free. After machining, measure the critical features in the fixture, then again after releasing it. A large change points to clamping distortion or released residual stress.
- Time and later operations. If features drift after rest, after a second operation or after painting, residual stress is still in the part and the routing needs a stress relief step or a rough-rest-finish sequence.
Write the readings on a sketch of the part with the measuring points marked. That record is what lets the supplier change the right thing, and it is the evidence a corrective action needs.
How much distortion does a weldment tolerance allow, and how much stock should you leave?
A general weldment tolerance is measured in millimeters, not hundredths, so the machining stock has to cover at least the position and flatness tolerance of the class on the drawing plus a cleanup cut.
The table below reproduces part of ISO 13920 for common machine-frame sizes. Read it as the shape a conforming weldment may legally have before any machining. For an 800 mm frame called out as ISO 13920-BF, per ISO 13920 a length may be off by up to 3 mm either way and the face may be 3 mm out of flat, and both can occur on the same face. A machined pad on that face therefore needs stock for the worst position, the worst flatness and a cleanup cut, or the class has to be tightened, which costs more fixturing and slower welding.
There is no single published stock figure for weldments, so set it from the class you specify and agree it with the supplier at RFQ. Avoid the opposite mistake as well: piling extra stock onto every face adds machining time and removes more stressed metal, which can increase the movement you were trying to absorb.
| Nominal size l (mm) | Linear class B, plus or minus (mm) | Linear class C, plus or minus (mm) | Flatness class E (mm) | Flatness class F (mm) |
|---|---|---|---|---|
| 30 < l ≤ 120 | 2 | 3 | 0.5 | 1 |
| 120 < l ≤ 400 | 2 | 4 | 1 | 1.5 |
| 400 < l ≤ 1,000 | 3 | 6 | 1.5 | 3 |
| 1,000 < l ≤ 2,000 | 4 | 8 | 2 | 4.5 |
| 2,000 < l ≤ 4,000 | 6 | 11 | 3 | 6 |
How do you fix or prevent weld distortion on a part that is machined afterward?
Reduce and balance the heat at the welding stage, hold the shape with fixtures or presetting, relieve stress where the part will move, and machine every precision feature after welding in a rough, release and finish sequence.
TWI’s fabrication guidance gives the welding-side tools: tack welds to set and hold the joint gap, welding identical parts back to back so the heat is balanced about the combined neutral axis, longitudinal stiffeners along a seam, depositing weld metal quickly with high-deposition processes, and short back-step or skip runs instead of long seams in one direction (TWI, prevention by fabrication techniques). Its restraint guidance adds presetting or pre-bending members so shrinkage pulls them back into line, noting that the right amount is hard to predict and usually needs trial welds. TWI’s rule of thumb for assembly-stage shrinkage allowances in steel is about 0.8 mm of transverse shrinkage per fillet weld (where the leg does not exceed three quarters of the plate thickness), 1.5 to 3 mm per butt weld in a 60° V joint depending on the number of runs, and longitudinally about 0.8 mm per 3 m of fillet weld and 3 mm per 3 m of butt weld (TWI, weld shrinkage allowance). That is why a long frame with many cross members comes out short unless the cut lengths allow for it. Match the fix to the symptom:
| Symptom on the machined part | Most likely cause | Control at welding | Control at machining or on the drawing |
|---|---|---|---|
| A machined face will not clean up on one edge | Angular distortion or bow larger than the stock | Preset or balance welds; double-sided joints | Raise the stock to match the ISO 13920 class; locate from a fixture that centers the stock |
| Hole pattern across two members out of position | Transverse and longitudinal shrinkage | Allow for shrinkage in cut lengths; tack and weld in a fixed sequence | Machine the whole pattern after welding in one setup |
| Pads flat individually but not coplanar; part rocks | Bowing or twist of the frame | Balance welds about the neutral axis, back-step long seams, add more tacks against twist | Machine all pads in one setup; control them with one profile or flatness callout |
| In tolerance clamped, out after unclamping | Clamping distortion or residual stress | Reduce weld size and heat input | Shim to the free shape before clamping; rough, release, re-clamp and finish |
| Part moves after a later operation or in storage | Residual stress still in the weldment | Balanced sequence; stress relieve before the part leaves its fixture | Add stress relief after welding and before machining |
| Cracks near welds after heavy restraint | Too much restraint for the material | Use wedged strongbacks or presetting instead of full restraint; preheat if the procedure calls for it | Inspect welds before machining time is spent |
Stress relief deserves a careful decision rather than a reflex. It is the strongest single fix for a frame that moves after machining, but it adds a heat-treatment step, can scale the surface, and needs a defined temperature and hold; the stress relieving guide covers when it belongs on the drawing and where in the routing it goes. Where the weldment is a pressure part, postweld heat treatment is governed by the code, such as ASME BPVC Section VIII Division 1, not by preference.
On the machining side, the most reliable sequence for a precision weldment is: check stock on the as-welded part, rough all faces, release the clamps so the part can relax, then re-clamp lightly and finish the datum faces and critical features in as few setups as possible. Supporting the part on its free shape with shims or adjustable supports, instead of forcing it flat, keeps clamping from hiding distortion that reappears later.
What should the drawing, RFQ and PO say so weld distortion does not recur?
State the as-welded tolerance class, which features are machined after welding, where the machining stock and datums are, and whether stress relief is required. A weldment drawing that shows only finished dimensions leaves the supplier to guess all four.
ISO 13920 itself says a drawing whose dimensions carry no individual tolerance is incomplete without a reference to general tolerances, and it designates the class on the drawing as, for example, “ISO 13920-BE.” Build the notes around that:
- General tolerance. “Unless otherwise specified, as-welded dimensions per ISO 13920-BF.” Choose the class from function, not habit.
- Machined-after-welding features. “Machine after welding” on every bore, pad and tapped hole that has a tight tolerance, or a general note that all toleranced machined features are finished after welding and any stress relief.
- Stock and datums. Show the as-welded envelope or the stock allowance on each machined face, and place datum features on machined surfaces, as ASME Y14.5 datum practice allows. Give the machinist a temporary set-up datum on the raw weldment if needed; the datum reference frame guide explains how to choose them.
- Weld size and sequence. Call welds no larger than the load requires, using AWS A2.4 weld symbols, and allow intermittent welds where they are structurally acceptable.
- Stress relief. Either “stress relieve after welding, before machining” with the specification, or “stress relief at supplier’s option to meet machined tolerances,” so the requirement is visible in the quote.
- Inspection. Ask for an as-welded stock check and a free-state final inspection of critical features, recorded in the first article report described in the first article inspection guide.
On the PO, add that any change to weld sequence, fixturing, weld size or stress relief after first article approval must be notified before shipment, because each of those changes the as-welded shape.
Who does what on a Saola order for a welded and machined part?
Welding and the machining that follows can both be done at our lead CNC plant, T.K Viet Nam Precision Company Limited, so the welding sequence and the machining stock can be planned together. Painting, plating and other finishes go to partner plants we manage, and the quote states each production site and the country of origin.
The lead plant has run since 2017 and has machining centers and conventional CNC lathes, with EDM, welding, and grinding and polishing alongside. Welded parts quoted there are welded and then machined on the CNC milling line, with turned details from CNC turning. Inspection tools on site include a 2D vision measuring machine, a height gauge and a hardness tester, so stock and free-state readings on critical features can be recorded on the inspection report. Stress-relief heat treatment is not on the lead plant’s equipment list; if your drawing requires it, say so on the RFQ and the quote will state where it is done, as it will for any step, such as cutting plate blanks, that is not done at the lead plant.
Coatings after machining are handled through the surface finishing partners. Partner plants are not named and are not necessarily in Vietnam. ISO 9001 certification is in progress, not held. Send the drawing with the general tolerance class, the machine-after-welding notes and any stress relief requirement through the quote form.
Frequently asked
Can a distorted weldment be straightened instead of remade?
Often, yes, by mechanical straightening or flame straightening, but both add stress or heat of their own. Straighten before machining, never after, and check with the designer whether straightening is allowed on the part at all.
Should holes be drilled before or after welding?
Any hole that locates another part, or that has a position tolerance, should be machined after welding. Clearance holes in a single plate that do not relate to other members can be cut before welding to save setups.
Does stress relief remove distortion that is already there?
No. Stress relief lowers residual stress so the part stops moving when it is machined; on its own it does not return a bowed frame to straight. Correct the shape first, then relieve, then machine.
Is vibratory stress relief a substitute for thermal stress relief?
It is used for dimensional stability on some weldments, but it is not accepted where a code or the drawing requires postweld heat treatment. Treat it as an alternative only when the drawing allows it by name.
Why do stainless and aluminum weldments distort more than steel?
Stainless has higher thermal expansion than carbon steel, and aluminum expands even more and conducts heat away quickly, which needs higher heat input. Both need more deliberate sequencing, fixturing and machining stock.
Sources
- ISO 13920:2023, Welding: General tolerances for welded constructions, dimensions for lengths and angles, shape and position (sample pages), ISO, second edition, June 2023
- Distortion: types and causes (Job Knowledge 033), TWI, accessed October 2026
- Distortion: prevention by pre-setting, pre-bending or use of restraint (Job Knowledge 035), TWI, accessed October 2026
- Distortion: prevention by fabrication techniques (Job Knowledge 036), TWI, accessed October 2026
- Distortion: prevention by design (Job Knowledge 034), TWI, accessed October 2026
- How much shall I allow for weld shrinkage? (FAQ), TWI, accessed October 2026
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)
- AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination, American Welding Society, 2020
- BPVC Section VIII, Rules for Construction of Pressure Vessels, Division 1, ASME, 2025 edition