Process guide
Weldment Fabrication Processes
A machined weldment passes through eight or nine processes before it ships, and each leaves something for the next to deal with. Welding leaves shrinkage and residual stress. Stress relief leaves scale. Machining needs datums that survived all of it.
This page walks the route in order. What a weldment is and when to use one is in the machined weldments guide. How to dimension it is in weldment tolerances.
1. Cutting plate and section
The cutting method sets the edge quality, the heat affected zone and how well parts locate at fit-up. Most shops run two or three and choose by thickness.
| Process | Typical range | Edge | Used for |
|---|---|---|---|
| Fibre laser | Up to about 20 to 25 mm carbon steel on most job shop machines | Narrow kerf, small heat affected zone, fine accuracy | Gussets, brackets, holes and tab and slot features that locate parts at fit-up |
| Plasma | About 3 to 50 mm, carbon steel, stainless and aluminium | Moderate heat affected zone, slight edge bevel | Medium plate, with weld bevels cut in the same pass |
| Oxy-fuel | From about 25 mm to several hundred mm, carbon and low alloy steel only | Widest heat affected zone, hard edge on higher carbon grades | Thick bases, ribs and bosses |
| Band or cold saw | Tube, box section, beams, angle and bar | No heat affected zone, square or mitred cut | Tube and section frame members |
| Waterjet | Any material, slow on thick plate | No heat affected zone | Edges where heat is not allowed |
Ranges are typical and depend on the machine. Holes that will be bored later are cut undersize, so the boring tool has metal to clean up. A thermally cut edge on a high strength grade can be hard enough to chip an insert, so a face that crosses one gets extra stock.
2. Fit-up and fixturing
The parts are set in a fixture, checked against the drawing and tacked. Root gaps, alignment and the order of the tacks are fixed here, and they decide most of the distortion the shop fights later.
A one off frame is usually built on a modular welding table with stops and clamps. A repeat part justifies a dedicated fixture. The trade-offs are in fit-up decides the weldment.
- Self locating partsLaser cut tabs and slots put members in position without measuring, which is the cheapest accuracy a shop can buy.
- PresetMembers are set slightly out of position so weld shrinkage pulls them back to the drawing.
- PreheatThick plate and higher carbon equivalent grades are preheated before tacking as well as before welding. The procedure sets the temperature.
3. Welding processes and when each is used
Every shop runs a written welding procedure for each joint type. The process is chosen by material, thickness, position and the length of the weld. The numbers are the ISO 4063 process codes that appear on a WPS. The physics of each arc welding process is well documented.
| Process | ISO 4063 | Used on a machined weldment for | Limits |
|---|---|---|---|
| GMAW, MIG and MAG | 131, 135, 138 | The shop default for carbon steel frames. Pulsed MIG for aluminium. | Shielding gas is disturbed by draughts, so it stays indoors. |
| FCAW, flux cored | 136, 114 | Large fillets and multi pass joints on thick plate, and out of position work. | Slag to remove between passes. |
| SMAW, stick | 111 | Repairs, site work and joints a torch cannot reach. | Slow, lowest deposition rate, most dependent on the welder. |
| SAW, submerged arc | 121 | Long straight seams on thick plate: beam flanges, box sections, spliced base plates. | Flat and horizontal only, on a carriage or manipulator. Highest deposition. |
| GTAW, TIG | 141 | Stainless, aluminium, thin wall tube and root passes on critical joints. | Slow and expensive per metre. |
A typical machine frame is MAG or flux cored throughout, with submerged arc on long seams and TIG only where the material asks for it. The paperwork behind each process is in WPS, PQR and welder qualification. Weld callouts are in the weld symbols chart, grade notes in the steel grade cross reference.
4. Distortion control
Every weld shrinks as it cools, along the joint, across it and in angle. A shop cannot stop that. It can balance it, reduce it and plan for what is left.
What survives is straightened or left for the machining allowance to absorb. Flame straightening is limited on quenched and tempered steels, because local heat can undo the mill's heat treatment. More in why your frame came out bowed.
- Weld no more than the drawing asksAn oversized fillet adds metal, heat and shrinkage for no strength the design needed.
- Balance the weldingWelds placed symmetrically about the neutral axis, alternating sides, so one pull cancels another.
- Sequence the jointsBack step and skip welding on long seams, the stiffest joints first, closing welds last.
- Control heat inputThe smallest weld that does the job, at the travel speed the procedure allows, in as few passes as the joint permits.
5. Stress relief: thermal or vibratory
Fixtures keep the shape and lock in the stress. Stress relief releases it before the machining does. The two methods are not interchangeable.
| Aspect | Thermal PWHT | Vibratory |
|---|---|---|
| What it does | Relaxes residual stress across the whole part | Vibrates the part at resonance to reduce the peak stresses |
| Typical cycle | 580 to 650 degrees C for carbon steel, about one hour per 25 mm of the thickest section, controlled heating and cooling | A frequency sweep, then a hold at the resonances found. A fraction of oven time |
| Size limit | The oven, or local heating on one joint | Practically none |
| Metallurgy | Tempers the heat affected zone and lowers peak hardness | No change |
| Where a code requires PWHT | Accepted | Not accepted as a substitute |
| Surface afterwards | Oxide scale, removed by blasting | Unchanged |
| Record | Time and temperature chart from thermocouples on the part | Resonance curves before and after treatment |
Figures are published practice for carbon and low alloy steel. The applicable code governs. Quenched and tempered grades and austenitic stainless need their own cycle. In the oven the frame is supported so it cannot sag at temperature. When a part needs relief at all is covered in when a welded frame needs stress relief.
6. Machining after welding
The machine is chosen by the envelope of the part and by how many sides need a feature. The setup plan is chosen by the datums.
| Machine | Typical work |
|---|---|
| Bed or travelling column mill | Long frames, rails and mounting faces on one or two sides |
| Horizontal boring mill | Bores, faces and tapped holes on several sides in one setup, with a rotary table |
| Gantry or bridge mill | Wide bases and machine beds |
| Vertical turning lathe | Round weldments: rings, drums, flanged housings |
| Surface or Blanchard grinding | Flat plates and pads where flatness and finish beat a milled face |
- Find the part before cutting itThe first setup probes or marks out the as-welded part and balances the stock so every face cleans up.
- Machine the primary datum firstThe part sits on it for every later setup, so it should be a datum face on the drawing.
- Clamp without springing the frameAn as-welded base is shimmed, not clamped flat. A frame pulled flat by clamps springs back when released.
- Rough, release, finishOn tight flatness, rough the faces, release the clamps so the part settles, then finish with light clamping.
- Fewest setupsFeatures tied by a tight tolerance are cut in one setup.
A common starting allowance is 3 to 6 mm on faces and about 3 mm on a bore radius, more on long or flexible sections. Try the allowance calculator, read why one setup is cheaper than three and see weldment tolerances for where datums belong on the drawing.
7. Weld inspection and where it sits in the route
When inspection happens decides cost, because a weld repair after machining puts heat back into a finished part.
| Method | Finds | When in the route |
|---|---|---|
| VT, visual | Profile, size, undercut and surface defects on every weld | At fit-up, on the root, between passes and on the finished weld |
| MT, magnetic particle | Surface and near surface cracks in carbon and low alloy steel | After welding and any hold time the code sets, before stress relief. Again after PWHT or machining only if the specification asks |
| PT, dye penetrant | Surface breaking defects in stainless and aluminium | After welding, and on machined faces where the drawing calls for it |
| UT, ultrasonic | Lack of fusion and cracks inside full penetration joints in thick plate | After welding and before machining, so any repair happens before a face is cut |
Radiography needs access to both sides and is less common on frames. Some codes set a hold time before final inspection on thick or higher strength steel, because hydrogen cracking can appear hours after welding. Acceptance is judged against a quality level such as ISO 5817 B, C or D. The methods in detail are in which weld inspection method to specify, and background on the techniques is on Wikipedia.
8. Blasting and painting
Blasting removes mill scale, weld spatter and the oxide left by stress relief. Paint systems normally call for a blast grade such as Sa 2½ to ISO 8501-1, with the primer applied before the steel flashes back to rust.
The risk is the finished faces. Masking is a line item and galvanising can move a part. Both are covered in coating a weldment without ruining the faces.
- Blast and prime, then machineThe whole part is coated before machining. Machined faces are bare afterwards and get rust preventive or touch up.
- Machine, then blast and paintFinished faces, bores and threads are masked or plugged before blasting. More masking, fewer touch ups.
- Name the systemA corrosivity category from ISO 12944 or a named paint system and dry film thickness, plus which faces stay bare.
How we match the route to a shop
We are a matching service, not a fabrication shop. A shop is only shown a drawing when it can run the route the part needs.
Envelope and weight
We check the size against each shop's machine travels and crane capacity, and the material against what it welds.
What is in house
Shops tell us at registration whether stress relief, machining and inspection are in house or bought in. That answer goes on the match.
Up to 3 shops
You get up to 3 shops that fit, they quote you directly and we reply within 24 business hours.
Go deeper
Machine choice and the materials that change the route.
- What a Horizontal Boring Mill Does for a Welded Frame That a Machining Centre CannotHow horizontal boring mills machine large weldments: table and floor types, spindle reach, rotary table indexing, setup count and what to ask a shop at quote stage.
- When a Large Welded Face Should Be Ground Instead of MilledBlanchard grinding versus milling on large weldment faces: flatness, surface finish, scale and mill scale removal, part size limits and what each route costs.
- Why a Welded 6061-T6 Frame Is Weaker Than the Plate It Was Cut FromWhat welding does to the strength of 6061-T6 and 5083 aluminium, how wide the heat affected zone is, and how to design and machine an aluminium weldment around it.
- What Changes When a Welded Frame Is Stainless Instead of Carbon SteelDistortion, sensitisation, back purging, pickling and passivation on 304L and 316L weldments, and what each adds to the cost of a stainless fabricated frame.
Questions about the process
In what order are the steps of a machined weldment done?
Cut, fit and tack, weld, inspect and repair, stress relieve, machine, blast and coat, then a final dimensional check. Stress relief comes before machining so the part moves in the oven, not after the faces are cut.
Should I specify the welding process on the drawing?
Usually not. Specify the weld size, quality level and code, and let the shop use a process it has qualified. Name one only when the material or code requires it.
Can some features be machined before welding?
Holes in individual plates are often laser cut before fit-up, and small bosses are sometimes pre-machined and welded in. Any feature tied by a tolerance to a feature on another member has to be machined after welding and stress relief.
Do you carry out any of these steps?
No. We are a matching service, not a fabrication shop and not a machine shop. We own no welding bay, no machines and no oven. We read your drawing and connect you with up to 3 shops that weld, stress relieve and machine parts like it themselves.
Know the route your part needs?
Send the drawing with the material, weld quality level, stress relief and inspection called out. We match it with shops that run that route in house. Free for buyers, reply within 24 business hours.