4.3 Magnetic Particle Testing (MT)
Key Takeaways
- MT detects surface and near-surface discontinuities in ferromagnetic materials only
- Two inspections at 90° are required because discontinuities must be perpendicular to the magnetic field
- Magnetization methods: yoke (longitudinal field), prods (circular field), coil (longitudinal), head shot (circular)
- Wet fluorescent particles under UV-A light provide the highest sensitivity
- Parts must be demagnetized after inspection to prevent arc blow, machining issues, etc.
- MT advantage over PT: detects near-surface flaws; PT advantage over MT: works on all metals
4.3 Magnetic Particle Testing (MT)
Magnetic Particle Testing (MT) detects surface and near-surface discontinuities in ferromagnetic materials only (carbon steel, low-alloy steel, some stainless steels like 400-series). It is the preferred surface inspection method for ferromagnetic weldments because it can find near-surface defects that PT cannot.
How MT Works
- A magnetic field is induced in the part being inspected
- If a discontinuity is present, it disrupts the magnetic field lines, creating a flux leakage at the surface
- Finely divided magnetic particles (iron powder) are applied to the surface
- Particles are attracted to and accumulate at the flux leakage site, forming a visible indication
Magnetization Methods
| Method | Type | Magnetic Field Direction | Best Detects |
|---|---|---|---|
| Yoke (electromagnet) | Longitudinal | Between pole pieces | Transverse discontinuities (perpendicular to field) |
| Prods (direct contact) | Circular | Around the current path | Longitudinal discontinuities (parallel to prods) |
| Coil/cable wrap | Longitudinal | Through the axis of the coil | Transverse discontinuities |
| Head shot (central conductor) | Circular | Around the conductor | Longitudinal discontinuities |
Critical Rule: Magnetic fields detect discontinuities oriented perpendicular to the field direction (within about 45°). To ensure full coverage, two inspections at approximately 90° to each other are required.
MT Media Types
| Type | Application | Viewing |
|---|---|---|
| Dry powder | Applied to surface as dry particles (gray, red, yellow) | White light |
| Wet visible | Particles suspended in liquid carrier (oil or water) | White light |
| Wet fluorescent | Fluorescent particles in liquid carrier | UV-A (black light) — most sensitive |
MT Advantages and Limitations
| Advantages | Limitations |
|---|---|
| Detects surface AND near-surface discontinuities | Ferromagnetic materials only (no aluminum, copper, austenitic SS) |
| Fast — results are immediate | Must magnetize in two directions for full coverage |
| More sensitive than PT for near-surface flaws | Parts must be demagnetized after inspection |
| Relatively portable (yoke method) | Prod method can cause arc burns on the surface |
| Can inspect through thin coatings | Geometry can affect magnetic field and create false indications |
Demagnetization
After MT inspection, parts must be demagnetized because residual magnetism can:
- Interfere with subsequent welding (arc blow)
- Attract metal chips during machining
- Affect sensitive instruments
- Cause compass errors in maritime applications
For the Exam: MT works ONLY on ferromagnetic materials. Two inspections at 90° are needed for full coverage. Fluorescent wet particles are the most sensitive MT media. Know the difference between yoke (longitudinal field) and prod (circular field) magnetization methods.
MT can be used to inspect which type of material?
Why are two MT inspections at approximately 90° to each other required?
Which advantage does MT have over PT?