Magnetic particle testing (MT), also called magnetic particle inspection, is the quickest way to find cracks at or just below the surface of carbon and low-alloy steel. The part is magnetized and fine iron particles are applied. Any crack that interrupts the magnetic field gathers particles into a visible indication.
- Finds
- Surface and near-subsurface discontinuities in ferromagnetic material
- Typical use
- Weldments, crankshafts and axles, lifting devices, structural steel, oilfield tubulars
What magnetic particle inspection finds
MT finds discontinuities that break the surface or sit just beneath it, provided they lie across the magnetic field. Typical indications include:
- Fatigue cracks in parts that carry cyclic load
- Weld cracks and surface lack of fusion
- Quench and grinding cracks from heat treatment and finishing
- Seams, laps and forging bursts near the surface
- Heat-affected-zone and stress-corrosion cracking found in service
Because the field has a direction, each area is magnetized in two directions at roughly right angles. The technique is chosen to suit the part: wet or dry particles, fluorescent or visible, direct or alternating current, and a yoke, prods, coil or bench unit depending on the size and location of the part.
Magnetic particle or dye penetrant?
MT only works on ferromagnetic material. Austenitic stainless steel, aluminum, titanium, copper alloys and nonmetals cannot be magnetized, so they are examined with liquid penetrant testing or eddy current instead. On carbon steel, MT is usually faster than penetrant, is less sensitive to surface cleanliness, and can find tight cracks just below the surface that penetrant cannot reach.
Where it’s used
Anywhere steel carries load and a crack would matter:
- Structural steel, pressure vessel and piping welds
- Crankshafts, axles, gears and other rotating parts
- Crane hooks, shackles and other lifting gear
- Oilfield tubulars and drilling equipment
- Steel structure, pins and shafts on amusement rides
- Defense and automotive components, bridges and maritime structure
Codes and standards
MT is performed to a written procedure that follows an ASTM practice or the ASME Code. Acceptance criteria come from the construction or product code that applies to the part. Common documents include:
- ASTM E1444/E1444M, the standard practice for magnetic particle testing, widely used in aerospace and defense work
- ASTM E709, the guide to magnetic particle testing
- ASME Boiler and Pressure Vessel Code, Section V, Article 7, with acceptance in the referencing section, such as Section VIII, or in ASME B31.3 for process piping
- AWS D1.1, Structural Welding Code — Steel, for structural welds
- Customer and prime-contractor specifications
Tell us which code or specification governs your part and we will work to it. If you are not sure, a Level III can help you work it out.
Who does the work
Every Midgard technician who performs magnetic particle testing holds current certification in that method, issued under a written practice. For most industrial work the written practice follows ASNT’s SNT-TC-1A; for aerospace and defense work it follows NAS410. A Responsible Level III oversees the program: approving procedures and technique sheets, examining and certifying technicians, and answering the technical questions that come up on the job.
If your contract calls for a different scheme, such as ANSI/ASNT CP-189 or NAVSEA requirements, tell us when you ask for a quote.
Training in magnetic particle testing
Midgard also trains in magnetic particle testing, along with all the other major methods. Instruction is given at Level I, II and III by certified Level III instructors, to SNT-TC-1A, CP-189, NAS410, NAVSEA or whichever code you certify to, with general, specific and practical examinations at your facility or ours. It suits new hires working toward first certification as well as technicians due for re-examination. See training and certification, or ask about a class.
Common questions
Can magnetic particle testing be used on stainless steel?
Only on the magnetic grades, such as the ferritic and martensitic stainless steels and precipitation-hardening grades like 17-4. Common austenitic grades such as 304 and 316 are not magnetic enough, so they are tested with liquid penetrant or eddy current instead.
How deep can magnetic particle testing find a flaw?
MT is a surface and near-surface method. It reliably finds cracks open to the surface and can detect some just beneath it, and direct current reaches a little deeper than alternating current. For flaws buried deeper in the part, ultrasonic or radiographic testing is the right method.
Does the part need to be cleaned or stripped first?
The surface must be dry and free of loose scale, grease and heavy dirt. Thin nonmagnetic coatings can sometimes stay in place when the procedure allows it and sensitivity has been demonstrated through them. Thick coatings reduce sensitivity and usually have to come off in the area being examined.
Is the part demagnetized afterward?
When its use requires it, yes. Residual magnetism can attract debris to bearings and moving parts or interfere with later welding and machining, so the procedure says whether demagnetization is needed and what residual field is acceptable.
Send us the part or weld, the material, the quantity and the code you answer to, and we will scope the inspection. Call 1-844-MIDGARD, email contact@midgardscientific.com, or use the contact page.