TL;DR: MPI weld testing, short for magnetic particle inspection, is a non-destructive test that reveals surface and just-below-surface cracks in steel welds. You magnetise the part, dust it with fine iron particles, and any crack pulls those particles into a clear line you can see with your eye. It is fast, portable, low cost and ideal for checking welds on ships, structural steel, cranes, hoppers and gangways. The catch is that it only works on magnetic metals like carbon and low alloy steel, not aluminium or austenitic stainless. If you need welds checked on a marine or steel fabrication job, talk to EAP Ltd about weld inspection here.

What is MPI weld testing?

MPI weld testing is a non-destructive testing (NDT) method that finds cracks and other flaws breaking the surface of a steel weld without cutting, loading or damaging the part. MPI stands for magnetic particle inspection, and you will also see it written as magnetic particle testing or simply MT.

The idea is simple. A sound weld carries a magnetic field smoothly through the metal. A crack interrupts that field and forces some of it to leak out at the surface, and that leaked field grabs hold of fine iron particles and holds them in place. The particles pile up right along the flaw, so an invisible hairline crack turns into a dark line you can actually see and measure.

Because it is quick, cheap and can be done on site with a hand held unit, MPI is one of the most widely used weld inspection methods in shipbuilding, ship repair, structural steelwork and heavy plant fabrication. It is the first line of defence for the kind of surface breaking cracks that fail welds in service.

What does MPI stand for in NDT and welding?

In both NDT and welding, MPI stands for magnetic particle inspection. Some standards and inspectors call the same method magnetic particle testing (MT) or magnetic testing. They all describe the identical process: magnetising a ferromagnetic part and using iron particles to reveal surface breaking and slightly subsurface defects.

It sits alongside the other common NDT methods you will hear about on a fabrication or ship repair project:

MT or MPI, magnetic particle inspection, for surface breaking flaws in magnetic steel
PT or DPI, dye penetrant inspection, for surface flaws in any material including stainless and aluminium
UT, ultrasonic testing, for flaws deep inside the weld
RT, radiography, an X-ray style image of the full weld thickness
VT, visual testing, the trained eye that comes before all of them

How does magnetic particle inspection work?

The process is straightforward and, on a portable job, takes only minutes per weld. A qualified technician works through five steps:

Clean and prepare. The weld and the metal either side are cleaned so paint, scale, slag and dirt do not hide or mimic a flaw. A thin white contrast paint is often applied so indications stand out.
Magnetise the weld. A portable electromagnetic yoke is placed across the weld to push a magnetic field through the metal. The yoke is the tool you will most often see used at a dockside or on structural steel.
Apply the particles. Fine iron particles are dusted or sprayed onto the surface. These can be dry powder or wet particles suspended in a liquid, and many are fluorescent so they glow under an ultraviolet lamp for extra sensitivity.
Read the indications. Where a crack breaks or sits just under the surface, the leaked magnetic field pulls the particles into a sharp line. The technician inspects, measures and records every indication against the acceptance standard.
Demagnetise and clean. The part is demagnetised where needed and the particles are cleaned off so it is ready for coating or return to service.

A key point is direction. A flaw shows up best when it lies across the magnetic field, so the technician magnetises the weld in two directions at 90 degrees to each other to make sure cracks running any way are caught.

What weld defects does MPI detect?

MPI is built for surface breaking and near surface flaws, which is exactly where welds tend to fail under fatigue and load. On a typical weld it reliably picks up:

Cracks, including fine fatigue and hydrogen cracks in the weld and the heat affected zone
Lack of fusion and lack of sidewall fusion that reaches the surface
Surface breaking porosity and gas pores
Undercut along the weld toe
Laps, seams and laminations in the parent steel
Overlap and other surface profile faults

What it will not find are flaws buried deep inside a thick weld with no connection to the surface. For those you need ultrasonic testing or radiography, which is why inspectors often pair MPI with one of those methods on critical joints.

MPI vs other NDT methods

Choosing the right test comes down to the material, where the flaw is likely to be, and how fast and cheaply you need an answer. Here is how MPI compares with the other methods you are most likely to be offered.

Method	Best at finding	Works on	Speed and cost	Main limitation
MPI (magnetic particle)	Surface and near surface cracks	Magnetic steels only	Fast and low cost	Ferromagnetic metals only, surface flaws
Dye penetrant (PT)	Surface breaking flaws	Any non porous material	Fast and low cost	Surface flaws only, needs clean surface
Ultrasonic (UT)	Flaws deep inside the weld	Most metals	Moderate	Skilled operator, harder on complex shapes
Radiography (RT)	Internal flaws, full weld image	Most metals	Slower and costlier	Radiation safety, access to both sides
Visual (VT)	Obvious surface faults	Everything	Fastest and cheapest	Misses tight and subsurface cracks

For surface cracks in steel, MPI is usually the fastest and most sensitive option, which is why it is the default on ship repair and structural steel welds. For a wider view of how welding fits into a fabrication project, see our guide to what marine fabrication involves and our comparison of marine steel and stainless steel fabrication.

Which materials can be tested with MPI?

This is the question that catches people out. MPI only works on ferromagnetic materials, meaning metals that can hold a magnetic field. That covers most of the steel used in marine and industrial fabrication:

Carbon steel and low alloy structural steel
Cast iron and cast steel
Ferritic and martensitic stainless steels

It does not work on non magnetic materials, so it cannot be used on aluminium, copper, or austenitic stainless steels such as 304 and 316. If your fabrication uses those, the equivalent surface test is dye penetrant inspection instead. Knowing which grade you are welding decides which test you can use, and it is one more reason material choice and inspection should be planned together at the quoting stage.

What standards cover MPI weld testing?

Reputable weld inspection is done to a written standard so results mean the same thing to everyone. For magnetic particle testing of welds in the UK and EU the key documents are:

BS EN ISO 17638, the method standard for magnetic particle testing of welds
BS EN ISO 23278, the acceptance levels that decide whether an indication passes or fails
BS EN ISO 9934, the general principles of magnetic particle testing

Technicians themselves are usually certified to a scheme such as PCN, run by the British Institute of Non-Destructive Testing (BINDT), or an equivalent ISO 9712 qualification. Independent, authoritative background on the method is published by TWI if you want to read further. When you commission MPI, ask which standard the work is inspected to and what certification the technician holds, because that is what gives the result weight with a client, class society or insurer.

Why MPI matters for marine fabrication and ship repair

On a vessel or a piece of dockside steel, a surface crack in a weld is not a cosmetic problem. It is the starting point for a fatigue failure that can take a gangway, a crane pedestal, a hopper or a hull repair out of service, sometimes with people standing on it. MPI is how you catch that crack while it is still a line of iron dust rather than a failure.

That is why weld inspection runs through good marine and industrial contracting from start to finish. When EAP Ltd fabricate and repair steel across sites like Rosyth, Hull, Peterlee and Chatham, coded welding and weld quality assurance are part of the same job, because a weld is only as good as the inspection that signs it off. The same discipline applies to structural steel and to on site repairs, where our mobile plant teams carry out and check structural welds in the field.

If you have welds that need checking on a fabrication, ship repair or structural steel project, EAP Ltd can help. Get in touch for a quote on your marine or steel fabrication work here.

Frequently asked questions

What does MPI mean in welding?

In welding, MPI means magnetic particle inspection, a non-destructive test that finds cracks and other flaws on or just under the surface of a steel weld. It is also called magnetic particle testing or MT.

Is MPI destructive or non-destructive?

MPI is non-destructive. The weld is magnetised and dusted with iron particles, then cleaned off afterwards, so the part is fully usable once testing is complete. Nothing is cut, loaded to failure or damaged.

What is the difference between MPI and dye penetrant testing?

Both find surface breaking flaws, but MPI works only on magnetic steels and can also catch flaws just below the surface, while dye penetrant works on any non porous material including aluminium and stainless steel but only finds flaws that break the surface. The material usually decides which one you use.

Can MPI find cracks inside a weld?

Only near the surface. MPI detects surface breaking and slightly subsurface flaws. For cracks buried deep inside a thick weld you need ultrasonic testing or radiography, which is why the two are often used together on critical joints.

How long does MPI weld testing take?

On a portable yoke inspection it usually takes only a few minutes per weld once the surface is prepared, which is a big reason it is the go to test for on site and dockside weld checks.