How magnetic particle testing works (Level II view)

The physics in plain terms

A magnetized steel part carries lines of magnetic flux through it. A crack across those lines is a gap that flux struggles to cross, so some of it spills out into the air above the crack. That spill is the leakage field. Particles that land nearby are pulled into it and pile up, and the pile is the indication you see.

Three conditions make the picture work:

• The part is ferromagnetic. Carbon steel qualifies. Most austenitic stainless steel and aluminum do not, so MT is the wrong method for them.
• The discontinuity is at or near the surface. Deep flaws leak too little field to attract particles.
• The field crosses the discontinuity. A field running along a crack barely disturbs it. A field running across it is disturbed strongly.

That last condition is the heart of MT, and the field direction and technique guide turns it into a sketching routine you can reuse on any part.

Two kinds of field, and why you need both

Passing current through the part, or through a conductor inside it, creates a circular field around the current path. That field is good at showing flaws that run along the length of the part. Wrapping the part in a coil, or placing it between the poles of a yoke, creates a longitudinal field along the part. That field is good at showing flaws that run across it.

Because a flaw can sit in any direction, a thorough examination checks in more than one direction. A bar with a crack along its length and a crack around its girth needs both fields. Skipping one is the easiest way to pass a part that should fail. When an exam question gives you a flaw orientation, draw the part, the flaw and each field before you read the answer options.

Particles, light and process

Particles come dry or suspended in a liquid, and in visible or fluorescent form. Visible particles are viewed in white light, and fluorescent ones glow under UV-A light in a darkened area, which makes fine indications easier to see. Wet methods generally suit fine surface flaws, and dry powder suits rough surfaces and field work.

The process is short, but each step affects the result. You prepare the surface, magnetize, apply particles while the field is present or as the procedure directs, examine, record, and demagnetize if the procedure requires it. Procedures set the limits for current, coverage and lighting, so those values come from your written procedure and not from memory. The MT amperage calculator is a way to practice the arithmetic with practice numbers, while the procedure for your job supplies the real ones.

Reading an indication

Not every line of particles is a defect. Level II work means sorting indications into three groups:

1. Relevant. Caused by a discontinuity that could affect the part, judged against the acceptance criteria your code or contract names.
2. Nonrelevant. Caused by something real but harmless, such as a change in section or the edge of a weld.
3. False. Caused by the process, for example particles trapped at rough surface features or a wet surface.

Describing what you see, classifying it and then evaluating it against a reference are three separate steps. Merging them is how acceptable parts get rejected and rejectable parts get passed.

A study sketch to try tonight

Take a plain round bar and draw two flaws on it: one running along its length and one running around its girth. Now draw a circular field around the bar, as if current ran through it end to end, and ask which flaw it crosses. Repeat with a longitudinal field, as if the bar sat in a coil. You should find that each field shows one flaw well and the other poorly, which is exactly why a single setup is rarely enough.

Then change the part. Make it a hollow ring and ask how you would get a circular field through it, and what a central conductor adds. Five minutes with a pencil on three or four shapes builds more intuition than rereading a table. Add a short log of what you got wrong and why, so the pattern in your misses is visible by the end of the week.

Where MT sits in your exam prep

The ASNT MT General exam is 50 questions in 2 hours, and ASNT posts a new MT topic list for exams taken after 15 December 2026. Read the MT outline change guide to find the one that matches your date. For a short plan around either outline, see the 2 to 4 week MT and PT study plan, and the MT practice routine for building an error log.

MT also rewards sketching more than memorizing. A rough drawing of the part, the flaw and the field catches most direction errors before they cost you a point.

The free 10-question check shows exactly which areas to study first, so your first sessions go to the topics you actually miss. Take the free MT check before you build your plan.

FAQ

How does magnetic particle testing work?

It magnetizes the part, so a surface or near-surface discontinuity leaks magnetic field into the air. Fine magnetic particles collect at the leak and show the discontinuity as a visible line. The method works only on ferromagnetic materials.

Which materials can MT inspect?

Ferromagnetic materials such as carbon steel and many low-alloy steels. Most austenitic stainless steels and nonferrous metals such as aluminum are not magnetic enough, so penetrant testing is the usual surface method for them.

Why does field direction matter so much?

A flaw disturbs the field most when the field runs across it. If the field runs along the flaw, little leakage forms and the particles may not show it. That is why examiners check in more than one direction.

Is MT harder than PT?

They test different skills. MT leans on sketching field and flaw directions and on process judgment, while PT leans on choosing among penetrant, removal and developer options. Read the PT guide and compare the topic lists before deciding which fits your work.