Welding inspection and traceability for sheet metal

A genset base frame leaves the welding cell at 09:00 and reaches the assembly line at 16:00 with no record of which coil it came from, which procedure was running, or who checked the 6 mm fillet. The weld may be sound. The record is not, and the first field failure turns into a week of sorting because nothing links the part back to the process that made it.

Welding inspection and traceability work as one system. The inspection plan defines what is measured and at which stage; the traceability record proves which material, procedure and operator produced the result. On sheet metal assemblies from 1.5 mm to 6 mm, three controls do most of the work: a declared quality level, a numbered weld map, and a marking method that survives pretreatment and powder coating.

The bending and welded assemblies hub collects the rest of this cluster, from fixture design to assembly tolerances.

Which quality level should the weld symbol carry?

Most sheet metal assemblies belong at ISO 5817 quality level C. Level B is for joints that carry fatigue load, are visible on a premium surface, or sit on a safety-critical interface. Level D is adequate for brackets hidden inside an enclosure where the weld only has to hold.

The scope matters more than the number. ISO 5817 sets quality levels for imperfections in fusion-welded joints in steel, nickel, titanium and their alloys, applies to material from 0.5 mm upward, and covers fully penetrated butt welds and all fillet welds. It does not decide how much non-destructive testing to perform, which method to use, or how many parts to sample. Those are contract decisions, and if the drawing is silent, the supplier will apply the level it finds easiest to meet.

Two consequences follow. First, put the quality level on each joint, not in a general note, because the base of a frame and its visible front rail rarely need the same treatment. Second, state the inspection extent next to the level. A level B callout with no agreed method usually means the part is inspected to level C and argued about later.

A fillet weld also needs a size, and the size that matters is the throat. A full-strength fillet on a 3 mm sheet needs roughly 0.7 × thickness, or 2.1 mm of throat. A 3 mm leg is the number most drawings carry, and it is more weld than the sheet needs — extra heat, more distortion, and a larger cap to grind.

The limits worth putting in the inspection plan

The table below is the working acceptance set for 1.5–6 mm sheet. It follows ISO 5817 level C where the standard defines the imperfection, and adds shop limits where the standard leaves the decision to the drawing.

The 25 mm per 100 mm figure comes from the definition of a short imperfection in ISO 5817 and is worth knowing because it separates a local defect from a systematic one. A single 12 mm porous run in a 600 mm seam is a repair. Porosity repeated every 80 mm is a gas supply or wire condition, and repairing the welds without fixing the cause guarantees the next lot fails the same way.

Inspection before, during and after the arc

ISO 3834-2 defines comprehensive quality requirements for fusion welding in workshops and at installation sites. Its structure is a useful checklist even where certification is not required, because it separates the checks that happen before a torch is lit from those that cannot be recovered later.

Before welding

Verify the material certificate against the drawing grade, check that the WPS covers the thickness and joint type, confirm the welder or operator qualification is current for the position, and check consumable storage. A spool of MIG wire left open in a humid shop absorbs moisture, and moisture in the wire shows up as porosity that no amount of parameter tuning removes.

Fixtures belong in this stage too. The fixture sets the datum for every hole and diagonal in the assembly, so a clamp pad worn by 0.5 mm moves the part more than any weld parameter will. The fixture design guide covers clamp placement; the inspection point is that the fixture itself is checked against its drawing at a defined interval, not only when a part fails.

During welding

The useful in-process checks are the ones that catch a drift before a lot is finished: wire feed and gas flow at the start of a shift, the program revision loaded on the robot, and the first part of each lot checked against the weld map. On a manual station, the operator's travel speed varies through a shift, and the first symptom is cap height moving outside the limit on one side of the joint.

After welding

Visual inspection at 100% is the baseline. Dimensional inspection follows the assembly drawing, not the individual part drawing, because distortion moves the interfaces that matter. Then coating, then a final check of threads, masking and earth points, which are the features most often destroyed by pretreatment rather than by welding.

The traceability chain, and where it breaks

A workable chain has five links, and each one needs a marking that survives the next process.

1. Coil or sheet — heat number and certificate, recorded against the sheet serial at goods-in.
2. Nest and cut part — laser program revision and sheet serial, with the part marked before it leaves the bed.
3. Formed part — press or brake program, first-off result, and the operator or cell.
4. Weld — WPS number, quality level, wire batch, gas, cell or welder, and the inspection result per joint.
5. Coating and dispatch — pretreatment batch, powder batch, cure record, DFT readings and the packing list.

The break almost always happens at link two. A 12 kW fibre laser with a 40 ft × 16 ft bed nests several parts from one sheet, and if the sheet serial is recorded only at the nesting software, the parts lose their identity the moment they are skeletoned. Marking the part — laser etched serial, a durable tag, or a stamped code on a non-critical tab — costs seconds and is the difference between a traceable lot and a plausible story.

The second break is the consumable batch. Wire and gas change far more often than coils, and a porosity investigation that cannot name the wire batch has to presume the worst and quarantine everything welded in that window.

Welding inspection and traceability on a repeat order

The honest answer is that inspection volume should follow the failure modes, and sheet metal weld failures are mostly visible. A plan built around that fact is cheaper and more effective than one built around equal sample sizes.

• 100% visual on every part, against the weld map.
• Weld gauge on 5 parts per shift, or every 50 parts, whichever comes first.
• Dimensional check on the first part of every lot and 1 in 50 thereafter, on the datums that control assembly.
• Full first article on 3 parts from production tooling, with every dimension on the drawing and the weld map signed off.
• Macro section at procedure qualification and once a year per WPS, to confirm penetration that a gauge cannot see.
• Destructive or third-party non-destructive testing only where the drawing names it.

If a customer asks for a capability study, run 30 consecutive parts and calculate Cpk on the two or three characteristics that actually drive fit. Reporting Cpk on a dimension that nobody measures at assembly impresses no one and consumes the sample budget that should have gone to the diagonal.

The Ganesh welding route

Ganesh runs three CLOOS robotic MIG cells and more than 30 manual MIG/MAG, spot and SPM stations, with fixtures designed and built in the in-house tool room. The robotic cells matter for traceability because the program revision and the fixture define the process, so the record can point to a controlled document rather than an individual's habit.

Welded work in this shape includes elevator frames for TKE and Eros Elevators, genset base frames and canopy structures for Mahindra Powerol, and assemblies for Indian Railways. The route sits inside ISO 9001:2015 and ISO 14001:2015, with PPAP and first article inspection available, and the welded assemblies capability describes the stations and the finishing path.

Parts that are welded and then coated pass through seven-tank pretreatment and an 18 ft × 14 ft powder booth, which is why the weld record has to include the coating batch as well as the weld. A porosity defect that only appears after curing is a coating failure as often as a welding one, and the distortion control guide covers the sequence that keeps the frame square before that stage.

What to put on the drawing and in the inspection plan

• Weld symbols to ISO 2553, with the quality level per joint rather than a general note.
• The WPS reference where the customer specifies one, and the process (135 for solid-wire MIG/MAG) where it does not.
• Fillet throat or leg, and whether the weld is full strength or a sealing run.
• The inspection method and extent per joint, especially where radiography, ultrasonic or penetrant testing is required.
• Datums for post-weld inspection, taken from the assembly interface.
• Traceability level: serial, lot, or batch, and the marking method that survives coating.
• Records to ship: material certificate, weld map, inspection report, coating DFT and adhesion result.
• Any joint that must not be ground, and any face that must stay free of spatter.

The RFQ checklist covers the commercial fields; the weld map and the quality level are the two technical fields most often left out of it.

When this route is not the right one

Phosphating and powder coating hide a great deal, and a weld that is structurally adequate can still fail a salt spray test because of a pinhole at the toe. Where the assembly is exposed outdoors for a decade, the joint design and the coating system need to be reviewed together, not weld-by-weld.

Ganesh does not build pressure-retaining vessels, and the procedure qualification and non-destructive testing regime for that class of work sits outside this route. If a joint needs radiography or ultrasonic testing to an acceptance level, say so at enquiry: those methods use an external agency, and they change both the inspection plan and the programme.

Send the drawing with the joint list, the material and the quality level you need. The engineering team will return a weld map, an inspection plan and the record pack that ships with each lot.