Which Weld Inspection Method to Specify, and on Which Welds
“NDT per drawing” written on a title block is one of the more expensive four word phrases in fabrication. It gives the shop no way to price the work except to assume the worst, and it tells the welders nothing about which joints actually matter.
Here is what each method does, so you can specify the ones you need on the welds that need them.
Visual inspection, VT
Every weld gets looked at. A qualified inspector checks profile, size, undercut, overlap, surface porosity, cracks and spatter against an acceptance standard such as an ISO 5817 quality level or the relevant AWS criteria.
It is the cheapest inspection and it catches most of what goes wrong. It finds nothing under the surface.
Assume VT on all welds unless you say otherwise. It is normal practice and it is already in the price.
Dye penetrant, PT
A coloured or fluorescent penetrant is applied, left to seep into anything open to the surface, wiped off, then a developer draws it back out so the indication shows.
Finds surface breaking defects only. Works on any non-porous material, which makes it the surface method for austenitic stainless and aluminium where magnetic particle will not work.
Modest cost, needs surface preparation, needs the part clean and dry. Slow if applied over large areas.
Magnetic particle, MT
The area is magnetised and iron particles are applied. Where a defect interrupts the magnetic field, the particles gather and show the flaw.
Finds surface and near surface defects in ferromagnetic material, so carbon and low alloy steel, and it is quicker over a large area than penetrant. It will not work on austenitic stainless or aluminium.
For most carbon steel weldments, MT is the surface method of choice.
Ultrasonic testing, UT
A probe sends sound into the material and the return signal is read. Internal flaws reflect sound and show as indications with a depth.
Finds internal defects. Good at planar defects such as lack of fusion and cracks, which are the ones that matter structurally and the ones radiography is worst at. It needs access to one surface, a trained technician and a reference standard.
More expensive than surface methods, and the results depend more on the operator. Phased array variants give a clearer picture of the weld cross section and cost more again.
Radiographic testing, RT
X-rays or gamma rays are passed through the joint and captured on film or a digital detector. Density differences reveal internal flaws.
Finds internal volumetric defects such as porosity and slag very clearly, produces a permanent record that a customer can look at years later, and is poor at tight planar defects lying in the wrong plane.
It is the most expensive of the five, it needs access to both sides of the joint, and it usually needs an exclusion zone which can mean night shift work. Budget schedule for it, not just money.
How to specify it so the quote is real
Put four things on the drawing or in the RFQ.
Which welds. Mark them. A weld map with joint numbers is worth the hour it takes to make. “All primary load bearing welds” is an invitation to guess.
Which method. VT on all, and then the specific method for the specific joints.
What extent. 100 per cent of a named weld, or 10 per cent spot checked, or the first three parts then reduced. The percentage changes the price substantially.
Which acceptance standard. The method finds indications. The standard decides whether an indication is a defect. Without one, nobody knows what passes.
What it costs you beyond the invoice
NDT is a scheduling item as much as a cost item. Surface preparation, technician availability, an exclusion zone for radiography, report turnaround and then repair and re-test on anything rejected. On a tight programme, the re-test loop is what hurts.
It also has to happen before coating. Surface methods cannot see through paint, so the sequence is weld, inspect, stress relieve where required, machine, then coat. Adding an inspection requirement late, after the part is painted, means stripping it.
In house or subcontracted
Most fabrication shops do VT themselves. Many do PT and MT in house. UT and RT are frequently subcontracted, and there is nothing wrong with that as long as you know, because a subcontracted step adds transport, a queue and a handoff to your lead time.
Ask directly at supplier selection: which methods are in house, which are bought in, and who holds the qualifications. It is one of the more useful questions in a supplier assessment.
Send your weldment RFQ with the inspection scope marked and you will get quotes you can actually compare.
Frequently asked questions
Do I need NDT on every weld?
Almost never. Visual inspection of every weld is normal. Volumetric methods such as ultrasonic and radiographic testing are applied to specific welds chosen because they carry load or because a code requires it. Blanket NDT on a whole frame is an expensive way to inspect welds nobody was worried about.
What is the difference between PT and MT?
Dye penetrant works on any non-porous material and finds only defects open to the surface. Magnetic particle works on ferromagnetic material and finds surface and slightly subsurface defects. PT is used on stainless and aluminium where MT will not work.
Can NDT be done after painting?
Surface methods cannot be done through paint, because the coating hides and blocks the very thing they detect. NDT belongs before coating, which is one more reason the process sequence has to be settled at quote time.
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