Weld inspection and NDT: what “all welds inspected” really means
At the handover of the hall the client is given a folder, and in it a sheet of paper: “all welds inspected, no objections”. It sounds reassuring — until you ask who looked, with what, at how many welds and against which criterion. Because the difference between “the welder looked over his own work” and “a certified NDT technician tested 20 % of the butt welds by ultrasound to level B” is not paperwork: it is the difference between a hall that is trusted on its word and a hall that is trusted on the proof.
Why a weld cannot simply be looked at
The most dangerous weld defects are not the ones that can be seen. An undercut or poor geometry catches the eye; but lack of fusion in the root, a slag inclusion or a crack at depth look from the outside like a perfect weld — and it is precisely those that cut away the resistance of the cross-section. That is why the standard combines two things: visual testing on everything, and deeper methods on a sample whose size grows with the execution class.
There is also a trap in timing that a layman would never guess: some cracks — hydrogen-induced ones in particular — do not appear at once but with a delay, after the weld has cooled. That is why the standard prescribes a minimum waiting time before the inspection, from a few hours to more than a day, depending on the strength of the steel, the thickness and the heat input. An inspection carried out “immediately after welding” may have been carried out too early — and is then formally invalid.
Four methods you will meet in the report
Visual testing
The surface: weld geometry, spatter, undercut, a root that has not been welded through
The eye, a weld gauge and good lighting — on 100 % of the welds, always
Penetrant testing
Surface cracks open to the outside, on any material
A coloured liquid is drawn into the crack and the developer pulls it back out as a visible indication
Magnetic particle testing
Surface and shallow sub-surface cracks in ferromagnetic steel
A magnetic field + iron powder that gathers at the crack
Ultrasonic testing
The inside of the weld: lack of fusion, inclusions, cracks at depth
The sound wave is reflected off the flaw; the standard for butt welds in thicker plate
The division is simple: VT, PT and MT see the surface, UT sees the inside. That is why for butt welds that carry tension — the most critical place in the structure — ultrasonic testing is required as a rule, and penetrant or magnetic particle testing for fillet welds. The personnel carrying out the testing must be qualified to EN ISO 9712 — that is, a certified NDT technician, not a welder looking over his own work.
What the inspection actually covers on your hall
Visual testing — every class, without exception
100 % of the welds
Additional NDT (transverse butt welds)
10 %
Required weld quality (EN ISO 5817)
level C
Welding quality system (EN ISO 3834)
EN ISO 3834-3 (standard requirements)
Documentation you are entitled to ask for
WPSs, welder qualifications, NDT reports, traceability by marking
* The percentages refer to the additional NDT of transverse butt welds under Table 24 of EN 1090-2; for other types of weld the standard prescribes separate extents. The execution class is determined by the designer, and EXC4 requires everything from EXC3 plus special criteria for design connections.
What “10 %” really means — and when it becomes 100 %
The percentage is not a random sample for the sake of statistics. It is a check on the confidence in the process: if production is under control, the sample confirms it. But if an unacceptable defect is found in the sample, the inspection is extended— the adjacent welds by the same welder and the same process are tested until the extent of the problem has been established. A bad result on one weld never means “repair that one and move on”.
There is also a detail that quotations like to skip over: a repaired weld is tested again, by the same method. A repair is not the end of the story but a new weld which has to pass the same criterion. In the report it shows up as a second mark for the same weld — and that is exactly how you recognise documentation that has been kept honestly.
A second detail that is often passed over in silence: the percentages in the table refer to transverse butt welds— the ones that carry tension across the whole cross-section. Fillet welds, longitudinal welds and welds of low load-bearing capacity have their own, generally smaller extents. A quotation that says “NDT 20 %” without stating what the percentage refers to has in fact said nothing verifiable.
A report that is worth something has these six things
- Which welds were tested — the marks on the drawing, not a “representative sample” without a list.
- By which method and to which testing standard.
- Against which acceptance criterion — level B, C or D to EN ISO 5817; without the level the result cannot be compared with the contract.
- Who carried out the testing — the name and the certificate to EN ISO 9712, with the level and the method.
- When — the date in relation to the date of welding, because of the waiting time.
- What happened with the repairs — the finding, the repair and the retesting of the same weld.
And the other way round — what is notin serious documentation: the sentence “all welds inspected, no objections” without a single one of the details above. That is not a report, that is an impression. Along with the NDT reports go the WPSs (welding procedure specifications) and the welder qualifications — which document proves what, we wrote about in the article on EN 1090-2 and the EXC classes.
What to ask before signing
- Which execution class applies — because everything else follows from it?
- Which quality level to EN ISO 5817 applies to the welds of my hall?
- Who does the NDT — an in-house certified technician or an outside company — and to EN ISO 9712?
- Which welds does the stated NDT percentage refer to?
- Do I get reports weld by weld in the handover documentation, with the repairs?
- What happens when a result is not good — how is the inspection extended and who bears the cost?
Weld inspection is the only place where the quality of a hall can be proved with a figure instead of an impression. Everything else — the tidiness of the workshop, the certificates on the wall, the years of experience — speaks about probability; the NDT report speaks about this particular hall. How what has been welded is protected we set out in the article on anti-corrosion protection, and what about the bolts that hold it all together in the article on bolted connections.
We make steel structures and prefabricated halls in our own production in Đakovo. If you have the dimensions, the use and the details of the crane, we can start from there: about steel structures and an enquiry with dimensions.
Frequently asked questions
100 % of the welds are inspected visually, in every execution class. Additional NDT of transverse butt welds depends on the class: EXC1 0 %, EXC2 10 %, EXC3 20 % (EN 1090-2, Table 24), while EXC4 requires everything from EXC3 plus special criteria for design connections. For other types of weld the standard prescribes separate extents.
English version of the Croatian original: Kontrola zavara i NDT: što „svi zavari pregledani“ stvarno znači.