Magnetic Particle Inspection: Comprehensive Guide to MPI Methods

Magnetic particle inspection (MPI) is a non-destructive testing method that finds surface and near-surface defects in ferromagnetic materials using magnetic fields and fine magnetic particles. It helps spot cracks, seams, and inclusions that weaken metal parts in aerospace, automotive, and manufacturing. Since MPI relies on magnetic flux leakage, it often reveals flaws you just can't see with the naked eye, which is pretty crucial for safety and reliability.

This process blends physics, materials science, and a bit of inspection tech to deliver a reliable and affordable quality control tool. Engineers and inspectors turn to MPI for parts made from steel, iron, and nickel alloys—especially when ultrasonic or eddy current testing just isn't practical.

Automation and image processing have really sped things up, making defect detection both faster and more accurate than ever. So what exactly is MPI, how does it work, and what kinds of defects can it uncover?
Let's look at magnetization methods, inspection steps, and where MPI shines—or falls short—in real-world use. Knowing these basics helps professionals use MPI better and make smarter calls about whether a material is truly up to the task.

What Is Magnetic Particle Inspection

Magnetic Particle Inspection (MPI) is a nondestructive testing (NDT) method for spotting surface and near-surface cracks in ferromagnetic materials like carbon steel, nickel, and iron. You magnetize the part and then sprinkle fine ferromagnetic particles over it—those particles pile up wherever the magnetic field leaks, making flaws visible.

MPI works best in a magnetic flux density range of 0.8–2.4 Tesla, depending on the material's thickness and shape. That range lets the field get through the surface without overdoing it and hiding tiny defects. Only materials with relative magnetic permeability above 100 make the cut for this test.

The magnetic particles are usually tiny—between 3–8 micrometers—and float in either oil or water. For low-light inspections, fluorescent particles that glow under 365 nm UV light make flaws much easier to spot. The usual particle concentration runs from 0.1 to 0.4 grams per liter, which affects how sharp and clear the defect images look.

People use either direct current (DC) or alternating current (AC) to magnetize parts. DC fields dig deeper, so they're better for finding subsurface flaws. AC fields, thanks to the skin effect, highlight surface cracks more clearly. Yokes, coils, or prods get used depending on the part's shape.

You'll see MPI in automotive, aerospace, and pressure vessel manufacturing because it quickly finds fatigue cracks and weld issues. But since it only works with magnetic materials and needs access to the surface, it doesn't do well with nonferrous metals or parts with tricky shapes where you can't control the field direction easily.

Fundamentals of Magnetic Particle Inspection

Magnetic particle inspection (MPI) spots surface and near-surface flaws in ferromagnetic stuff using magnetic fields and fine magnetic particles. It's a mix of electromagnetic theory and visual detection, hunting down discontinuities that could mess up a part's strength.

The process relies on both how you magnetize something and the properties of the material you're testing. It's not just science—there's a bit of art in getting it right, too.

Principles of Operation

To run MPI, you magnetize the test part and then apply magnetic particles. Those particles pile up at leakage fields caused by cracks or other discontinuities.

You can induce a magnetic field with either direct current (DC) or alternating current (AC), using coils, prods, or central conductors. AC fields, usually at 50–60 Hz, are great for surface cracks. DC fields (0–10 A mm⁻²) reach deeper and help find subsurface defects.

The field strength should hit at least 2 kA m⁻¹ for proper magnetization. Too weak, and you miss defects; too strong, and you get false positives. The magnetic particles are usually iron oxide (Fe₂O₃) or magnetite (Fe₃O₄), anywhere from 1 µm to 50 µm in size.

Wet suspensions keep those particles moving in oil or water with a viscosity under 3 mPa·s. That way, they flow evenly and don't clump up.

How well you spot a flaw depends on the angle between the defect and the field lines. A 90° angle gives you the strongest leakage flux, while anything under 45° can drop visibility by more than 60%. Since it's all visual, you need good lighting—over 1000 lx for visible particles or 100 µW cm⁻² for fluorescent types under UV-A light.

Ferromagnetic Materials in MPI

Only ferromagnetic materials work for MPI because they have high magnetic permeability (µᵣ > 100) and can hold on to residual magnetism. Common choices: carbon steels (AISI 1020–1045), low-alloy steels (AISI 4140), and nickel-based alloys with permeability up to 5000 H m⁻¹.

MPI's effectiveness comes down to the right balance of permeability and coercivity. If a material's coercivity is below 80 A m⁻¹, it's easy to magnetize and demagnetize, so you don't get leftover fields messing up later work.

Austenitic stainless steels, with µᵣ ≈ 1.05, just don't cut it—they can't hold stable magnetic domains.

Temperature matters, too. Most steels stay ferromagnetic below 700 °C, but go past the Curie temperature (770 °C for pure iron) and they lose magnetic properties, turning paramagnetic. In practice, folks stick to inspections between 10 °C and 50 °C—keeps things predictable.

In aerospace and automotive, MPI checks shafts, bearings, and landing gear parts where fatigue cracks love to hide. The method’s reliability hangs on a consistent material response—a uniform magnetic field in a ferromagnetic part gives you clear, repeatable signs of surface flaws.

What Are Types of Defects Detected?

Magnetic particle inspection finds flaws that mess with magnetic fields in ferromagnetic materials. These flaws—breaks, inclusions, voids—change the magnetic flux and pull magnetic particles right to the defect.

Surface Discontinuities

Surface discontinuities break through to the material’s exterior and cut the magnetic field right at the surface.

Think fatigue cracks, weld undercuts, laps, and seams. They’re usually 0.05 mm to 1.0 mm wide and can stretch several millimeters.

The smoother the surface, the better the detection. Ground steel surfaces (Ra ≤ 1.6 µm) give you crisper particle indications than rough cast ones (Ra ≥ 6.3 µm). Since the magnetic field leaks right at the defect, the flux density can top 50 mT—that’s plenty for visible particle buildup, wet or dry, even with fluorescent methods.

In practice, surface flaws show up as sharp, continuous lines when you use wet fluorescent magnetic particles (particle size 3–7 µm). That size flows well in oil-based suspensions and sticks to leakage fields. Rail, aerospace, and pressure vessel industries keep an eye on these flaws to avoid fatigue failures in the field.

Near-Surface Flaws

Near-surface flaws hide just below the surface, usually within 0.5–2.0 mm deep. These include subsurface cracks, nonmetallic inclusions, and shrinkage cavities.

Since magnetic flux leakage drops with depth, signals from these flaws are weaker—often just 30–60% of what you’d get from surface cracks. Magnetizing current and field orientation matter a lot here.

AC magnetization (50–60 Hz) boosts surface detection, while DC or half-wave rectified current digs deeper for near-surface flaws. Field strength usually sits between 800 and 1,200 A/m—it’s a balancing act between depth and clarity.

Steels like AISI 1045 or AISI 4140, with relative permeability over 200, generate stronger leakage fields, making flaws easier to find. Near-surface flaws matter in bearing rings, gears, and shafts, where tiny discontinuities can grow under repeated stress.

Subsurface Discontinuities

Subsurface discontinuities run deeper than 2 mm and include voids, deep cracks, and inclusions from casting or forging. MPI struggles a bit here because magnetic flux leakage fades quickly with depth.

If you use DC magnetization (steady current), the field can reach up to 6 mm deep in medium-carbon steels.

But at that depth, leakage flux might dip below 10 mT, so particle attraction gets pretty weak.

For those deep flaws, people often bring in ultrasonic testing to double-check. Flaw geometry matters, too—elongated flaws running parallel to the field lines are harder to spot than ones that cut across. That’s why techs often magnetize in two perpendicular directions to cover all bases.

Subsurface defects are a big deal in heavy forgings, crankshafts, and weldments where internal strength is everything.

What Are Methods of Magnetization?

Magnetization methods create a magnetic field in a ferromagnetic material so you can hunt down surface or near-surface defects with magnetic particle inspection (MT). The method you pick depends on the part’s shape, size, conductivity, and the magnetizing equipment at hand.

Direct Magnetization

Direct magnetization runs an electric current right through the part you’re testing. That current, measured in amperes, creates a circular magnetic field around its path (thanks, Ampere’s law). Typical current levels? Anywhere from 100 A to 5,000 A, depending on size and permeability.

Steel parts with diameters between 10–150 mm often get AC current for surface flaws or DC current for subsurface ones up to 3 mm deep. Since the current flows through the part, you want contact resistance below 0.02 Ω—otherwise, you risk arcing and burns.

Copper or brass contact pads help cut down on heating, thanks to their high conductivity (58 MS/m). But if the clamping force tops 50 N, you might bend thin rings or lighter parts, so it's not ideal for delicate stuff.

I’d say direct magnetization works best for shafts, bolts, and forgings where you have good electrical contact and consistent cross-sections. It gives you strong field penetration, but you need to watch your current density—keep it around 1–3 A/mm² to avoid overheating.

Indirect Magnetization

Indirect magnetization creates a magnetic field with an external conductor—think a coil, solenoid, or yoke—instead of running current right through the test piece. The field strength? It all hinges on the ampere-turns (NI) of the coil.

Let’s say you’ve got a 5-turn coil carrying 500 A. That’s 2,500 ampere-turns, which can magnetize a steel bar up to 300 mm long. The magnetic flux density (B) inside the part usually sits between 0.8 and 1.5 tesla, though it depends on the material’s relative permeability (μr, somewhere around 500–2,000 for carbon steels).

Since no current runs through the part itself, you don’t need to worry about arcing or damaging the surface.

Coil diameters typically measure out to 2–3 times the part diameter, which helps ensure the magnetization stays uniform.

People tend to choose indirect magnetization for odd shapes, hollow pieces, or assemblies where direct electrical contact just isn’t practical. The downside? You’ll get a lower field intensity than with direct methods, so you need more current or extra coil turns to reach the same effect.

Permanent Magnet vs Electromagnet

Permanent magnets and electromagnets both supply the magnetic field for MT, but they’re not the same when it comes to control and strength. Permanent magnets—often made of NdFeB (Neodymium Iron Boron, grade N42)—deliver fields around 0.3–0.5 tesla without needing any external power.

They work reliably from -40°C to 120°C, but you can’t adjust their field intensity. Electromagnets, like AC yokes, use coils powered by 110–240 V to create adjustable fields that go up to 2 tesla.

Because you control the current, you can fine-tune magnetization with electromagnets. The catch? They need cooling and regular calibration to stay in line with ASTM E1444 standards.

Permanent magnets shine in field inspections where you want portability and simplicity. Electromagnets are better in labs or controlled environments, especially if you need to change field direction or strength to spot cracks in any direction.

So, which is better? Permanent magnets give you stability and convenience, while electromagnets offer flexibility and stronger, tunable fields. **It really depends on what you need for the job.**

Application of Magnetic Particles

Magnetic particles make surface and near-surface flaws visible by clustering at leakage fields. Their size, shape, and coatings all play a role in how they behave—whether you’re using them dry or in a wet suspension—affecting sensitivity and clarity during inspection.

Dry Method

The dry method uses finely divided magnetic particles averaging 45–150 µm. Usually, they’re made of iron or iron oxide, and you apply them to the surface of a magnetized part with air pressure.

Since the particles are dry, they line up right along the leakage fields. No carrier liquid means you can inspect parts at temperatures up to 315°C, making this method a go-to for hot welds or castings fresh out of heat treatment.

These particles respond to field gradients over 10⁻³ T/mm, forming clear marks at discontinuities. But here’s the thing: the dry method doesn’t catch fine cracks under 0.05 mm wide very well, since the larger particles can’t squeeze into tiny gaps.

Operators usually reach for portable yokes or prods that generate fields between 1.5 and 2.5 kA/m. High-temperature tolerance comes at a price—you lose some detail when it comes to tiny surface flaws.

Wet Method

With the wet method, you suspend magnetic particles in a liquid like light mineral oil or water with a surfactant. The particles are much smaller—about 1 to 10 µm—so they move easily and can pick up even weak leakage fields.

You keep the suspension at 1.0 to 2.5 g/L to spread the particles out evenly. The liquid cuts down friction, letting particles make crisp, high-contrast marks, even for cracks as tight as 0.01 mm.

You’ll get the best results at 20–40°C. Higher temps thin out the fluid, which can mess with how the particles flow. Most folks use the wet method with stationary bench units that apply fields up to 5 kA/m.

It’s a consistent, repeatable process, but you’ve got to keep the suspension stirred and work in controlled conditions to stop the particles from settling.

Fluorescent Particles

Fluorescent magnetic particles have dyes that glow under ultraviolet radiation at 365 nm. Each particle gets a fluorescent resin coating about 0.5–1 µm thick—that keeps the dye safe and boosts brightness.

With UV-A light, you’ll see bright yellow-green lines with a contrast ratio over 5:1 compared to the background. The fine size (usually 2–6 µm) lets them spot microcracks as tiny as 10 µm wide.

These are mostly used in wet suspensions for even coverage. You need to keep the inspection area below 20 lux ambient light to make the glowing marks stand out.

The downside? Fluorescent setups need dark rooms and UV gear, which adds some hassle, but they do boost detection accuracy for critical aerospace or automotive parts.

Iron Oxide Particles

Iron oxide particles—usually magnetite (Fe₃O₄)—offer stable magnetism and resist corrosion. Their magnetic saturation hits around 480 kA/m, and they’re pretty dense at 5.2 g/cm³.

Thanks to their fine grain size (often 1–5 µm), these particles stay suspended in water-based carriers. The oxide coating keeps them from rusting or clumping, so you get a steady magnetic response every time.

Iron oxide’s high coercivity (30–50 kA/m) helps it keep its magnetization long enough to form visible marks, even with weak leakage fields. In fluorescent systems, you’ll find iron oxide particles coated with organic phosphors to add optical punch.

Automated magnetic particle inspection systems love these for their predictable behavior and machine vision compatibility. The only real catch? Their dark color can make marks harder to see in non-fluorescent setups, so you’ll need stronger lighting.

Inspection and Interpretation Process

Inspectors find and interpret surface flaws by watching where magnetic particles cluster over defects. They rely on steady lighting, consistent fields, and careful recording to keep results reliable and defects classified correctly.

Lighting and Visibility

Lighting makes all the difference in how well inspectors see magnetic particle patterns. Standards like ASTM E1444/E1444M call for at least 1000 lux of visible light for color inspections and 100 µW/cm² of UV-A for fluorescent checks. That’s enough to spot defects as small as 0.1 mm wide.

The usual viewing distance is 300 to 600 mm—close enough to focus, far enough to cut glare. Surfaces need to be clean, dry, and free from oil or dust, since grime scatters light and kills contrast.

For fluorescent methods, keep ambient light below 20 lux. That makes glowing particle marks really pop along magnetic field lines at cracks or seams.

Indication Evaluation

After particles pile up, inspectors look over the indications to decide if they’re real flaws or just noise. The magnetic field strength usually lands between 2.0 and 6.0 kA/m for steel, which saturates the part without overdoing it.

Indications break down as relevant or nonrelevant. Relevant ones run about 90° to the magnetic field—right where flux leakage peaks at the defect edge. Nonrelevant marks? They’re usually fuzzy or odd-shaped, often from geometry changes, not cracks.

Inspectors measure indication dimensions using calibrated gauges accurate to ±0.1 mm. Interpretation follows standards like ISO 9934-1, which set acceptance levels based on flaw size and part use. Since field distribution shifts with part shape, cylindrical parts might need a spin to check magnetization before making the final call.

Reporting and Documentation

Every inspection needs a permanent record: test settings, results, and acceptance calls. Reports include current type (AC or DC), magnetizing current amplitude, field direction, and particle type (wet fluorescent or dry visible).

Digital systems log magnetic field values using Hall sensors with 10 mV/mT sensitivity. That way, you can prove the field hit the right levels during testing.

People often save images or videos of indications at at least 1024×768 pixels for later review. Documentation also covers environmental details like ambient temperature (20–25°C) and humidity (30–60%), since those affect particle movement and sticking. Good records keep things traceable and up to speed with ISO 9712 certification for both staff and process control.

Advantages and Limitations of MPI

Magnetic particle inspection (MPI) finds surface and near-surface discontinuities in ferromagnetic materials with high precision. It’s fast and cheap, but it does depend on the material’s magnetism and surface condition to deliver accurate results.

Sensitivity and Reliability

MPI can catch cracks as small as 0.1 mm wide and 1 mm long on materials like AISI 1045 steel or ASTM A36. Since the method relies on magnetic flux leakage, any break in the field draws in particles, making flaws visible under white or UV light.

Using wet fluorescent particles at 1.0–2.5 mL/L bumps sensitivity up by about 30% compared to dry powder. That’s because the suspended particles line up more easily with leakage fields, giving better contrast and visibility.

Reliability hangs on keeping the field strength between 2.4 and 4.8 kA/m. Go too low, and the field won’t reach the surface; too high, and you’ll get background noise. Calibration with ASTM E1444/E1444M keeps things uniform and consistent.

But let’s be honest—subsurface detection only works down to about 2 mm. Deeper than that, flux leakage fades, so you won’t spot deep flaws. That’s why MPI mostly sticks to surface-critical stuff like shafts, gears, and welds. **It’s not a magic bullet, but it gets the job done for what it’s built for.**

Material and Surface Requirements

MPI only works on ferromagnetic materials like carbon steels, low-alloy steels, and certain cast irons with relative permeability values over 100 μr.

Nonferromagnetic alloys—think aluminum or austenitic stainless—just don't hold magnetic flux, so this method won't work on them.

Surface finish makes a big difference. If roughness goes above Ra 3.2 μm, particles can get trapped, which leads to false readings.

Smoother, cleaner surfaces without much oil or paint make indications easier to spot and cut down on background noise. Once coatings are thicker than 50 μm, magnetic flux gets blocked and sensitivity drops off.

Temperature plays a role too. Inspections should stay between 10°C and 50°C, since higher temps reduce magnetic strength due to lower permeability.

Because of these constraints, MPI really works best when you have direct metal access and not much in the way of coatings.

Speed and Cost-Effectiveness

With automated wet systems, you can check medium-sized components—like axle shafts up to 1.5 m long—in under 5 minutes per part.

Manual dry methods take a bit longer, but they're handy when you need to control the field direction for odd shapes.

Consumables, such as iron oxide or nickel-coated particles, are cheap—usually around $0.05–$0.10 per test.
A 5 kA AC yoke draws about 0.8 kWh per hour, so it's way more energy-efficient than something like radiographic testing.

Operators certified to ASNT Level II can get up to speed after about 40 hours of training. Demagnetization and post-cleaning do add 10–15% to the overall process time, but for high-volume production or maintenance, MPI is still a cost-effective NDT method.

Applications and Industry Use Cases

Magnetic particle inspection (MPI) shows up all over industries where ferromagnetic materials like carbon steel, low-alloy steel, and iron are in play.

It finds surface and near-surface discontinuities that could mess with mechanical strength. You'll see it in manufacturing, maintenance, and safety checks—pretty much anywhere these metals matter.

Weld Inspections

Welded joints are trouble spots for cracks or incomplete fusion, thanks to stress concentrations. MPI spots these flaws by applying a magnetic field of 1.0–2.5 Tesla across the weld and adding fluorescent magnetic particles sized between 3–8 µm.

When there's a discontinuity, magnetic flux leaks out, and the particles pile up—giving you a visible indication under 365 nm UV-A illumination. This wavelength is what ISO 3059:2012 calls for.

Common welds checked include fillet welds, groove welds, and butt joints in things like pressure vessels, pipelines, and car frames. MPI is a favorite here because you get instant visual feedback and can catch cracks as tiny as 0.1 mm wide and 1 mm deep.

But remember, it's only for ferromagnetic materials (like ASTM A36 or AISI 1020). If you're working with aluminum, you'll need something like dye penetrant testing instead.

Castings and Forgings

Cast and forged parts can hide shrinkage cavities, laps, or cold shuts from the manufacturing process. MPI can pick out these defects on gray iron, ductile iron, and 4140 steel surfaces.

You usually need a magnetic field between 800 and 2000 A/m, depending on the size and thickness of the part. For big forgings, a longitudinal magnetization setup works well since it lines up the field with the part's length—better for finding long cracks.

People use MPI on crankshafts, connecting rods, gears, and turbine blades. Since these parts handle repeated stress, catching cracks early can prevent failure. The downside is, rough casting surfaces can trap particles and give you false positives if they aren't cleaned or demagnetized first.

Large Structural Components

Think about big steel structures—bridge girders, crane hooks, rail axles. Inspectors check weld seams and heat-affected zones with portable MPI systems.

These systems use yokes with lifting capacities of 4.5–6.0 kg and create magnetic fields of 1.2–1.8 Tesla, covering up to 300 × 300 mm per scan.

Dry magnetic particles are the go-to for fieldwork, since they handle temperatures from -10°C to 50°C and aren't easily contaminated.

Because these structures are huge, technicians break up the testing into sections and keep defect maps for records. MPI's portability and real-time results make it great for on-site maintenance of heavy equipment and infrastructure, though it does slow down when you're dealing with really large areas.

Frequently Asked Questions

Magnetic particle inspection (MPI) finds surface and just-below-the-surface flaws in ferromagnetic stuff using magnetic fields and fine iron particles. The process depends on magnetization technique, field strength, and inspection conditions to get accurate results and keep things safe.

What are the two main techniques used in magnetic particle testing?

The two main techniques are dry powder and wet fluorescent methods. Dry powder uses finely milled iron particles between 45–150 µm, sprinkled on surfaces magnetized with a DC or half-wave rectified current. It's good for rough surfaces and field jobs since the particles are bigger.

Wet fluorescent testing suspends smaller particles—2–6 µm—in a carrier fluid at 1.0–2.5 mL/L, and uses AC current for magnetizing. The dye and tiny particles make it easier to spot tiny cracks under 365 nm UV-A light.

Dry methods work up to 200°C, but wet methods top out at 50°C because the fluid gets volatile. Wet fluorescent inspection is more sensitive, but you need controlled lighting and careful handling.

What is the standard procedure for conducting a magnetic particle inspection?

A standard MPI procedure follows ASTM E1444/E1444M. You clean, magnetize, apply particles, inspect, demagnetize, and document. Magnetizing current usually sits between 800 and 1500 A for medium-sized steel parts.

You have to make sure the field direction is perpendicular to where you expect defects, so leakage flux will show up. Sensitivity depends on field strength—2–6 kA/m is ideal for most carbon steels.

After inspection, demagnetization drops residual magnetism below 2 Gauss so it won't mess with later machining or assembly.

How deep can flaws be detected using magnetic particle inspection methods?

MPI can catch flaws up to 2–3 mm below the surface with DC magnetization.

AC current gives you a skin effect, so you're only seeing surface cracks as small as 0.1 mm wide. Half-wave rectified DC goes deeper, sometimes up to 5 mm depending on the metal's permeability (µr = 200–800).

Detection depth drops off in alloys with low permeability, like austenitic steels (µr < 10). So, MPI really shines with ferromagnetic stuff like carbon steels and irons.

What hazards should be considered when performing magnetic particle inspections?

**Inspectors need to watch out for electrical, chemical, and optical hazards.**

High magnetizing currents over 1000 A can cause burns if the contact points aren't insulated. Carrier fluids with petroleum distillates might release vapors above 50 ppm, so you need ventilation that meets OSHA 29 CFR 1910.94.

Fluorescent inspections under UV-A light (365 nm) can hurt your eyes—wear goggles rated to EN 170 UV 400. Skipping protection can lead to corneal irritation over time.

Residual magnetism can also pull in metal debris, which gets risky during handling.

What equipment is essential for magnetic particle inspection?

**You'll need a magnetizing unit, particle applicator, UV-A lamp, and demagnetizer.**

Bench units pump out controlled currents from 500 to 6000 A, using copper contact heads spaced 150–600 mm apart. Portable yokes create fields of 2.5–4.0 kA/m for spot checks.

Wet systems use circulating pumps rated at 5–10 L/min to keep particles moving. UV lamps offer 1000 µW/cm² at 38 cm distance so you can see those fluorescent indications.

Demagnetizers run at 50–60 Hz and gradually drop the field strength to avoid leaving more than 2 Gauss behind.

How does the cost of magnetic particle testing compare to other NDT methods?

MPI costs usually fall between $3 to $10 per part. The price depends a lot on the part's size and how much time you need to set things up.

If you look at ultrasonic testing (UT), that runs about $15–$30 per part. So, **MPI is definitely cheaper for ferromagnetic components**.

Why's it less expensive? You don't have to spend much time calibrating equipment, and inspections move pretty fast—think 2–5 minutes per part. That's a big deal if you're working with lots of parts.

But here's the catch: **MPI can't test nonmagnetic materials**. That limits what you can actually use it for.

Radiographic testing can spot flaws deeper inside a part, but the equipment's pricey and dealing with radiation controls is a hassle. So, **for routine surface defect checks, MPI just makes more sense**.