Magnetic particle testing (MT) detects surface and near-surface defects in ferromagnetic welds by disrupting a magnetic field at the flaw location. Iron particles (dry powder or wet suspension) collect at the disruption and form a visible indication. MT finds cracks, lack of fusion, and near-surface porosity that visual inspection might miss, and it detects subsurface flaws that dye penetrant testing can’t reach.
MT is faster than PT for ferromagnetic materials and more sensitive to tight cracks that don’t open fully to the surface. It’s the standard NDT method for structural steel weld inspection after VT, and it’s used on everything from building frames to pressure vessels to heavy equipment.
How MT Works
A magnetic field is established in the part being inspected. If a defect exists at or near the surface, it disrupts the magnetic field and creates a “flux leakage” at the defect location. Magnetic particles (iron powder or iron oxide in liquid suspension) are applied to the surface. The particles are attracted to the flux leakage and collect at the defect, forming a visible indication.
The key principle: defects oriented perpendicular to the magnetic field lines produce the strongest indications. Defects parallel to the field may not be detected. This means you need to magnetize in at least two directions (typically 90 degrees apart) to detect defects in all orientations.
Magnetization Methods
Yoke Method (Most Common for Welds)
An electromagnetic yoke (or permanent magnet yoke) is placed on the weld surface with its legs straddling the area to be inspected. The magnetic field flows between the legs through the part.
AC yoke:
- Produces a surface-concentrated field
- Best for surface-breaking defects
- Most common for weld inspection
- Lifting power requirement: 10 lbs minimum at the maximum leg spacing (ASTM E1444)
DC yoke:
- Produces a deeper field (better for near-surface defects)
- Less sensitive to surface conditions
- Lifting power requirement: 40 lbs minimum at maximum spacing
Permanent magnet yoke:
- No electricity required (field work without power)
- Lifting power: 40 lbs minimum
- Less adjustable than electromagnetic yokes
Prod Method
Two copper electrodes (prods) are pressed against the surface and current flows between them through the part. The magnetic field forms circular loops around the current path, detecting defects between and around the prods.
Caution: Prods can create arc strikes on the surface if contact is poor. Arc strikes are defects themselves under most codes. Many specifications prohibit prods on finished welds for this reason.
Coil Method
The part is placed inside a coil that generates a longitudinal magnetic field. This method works for small components and piping. It’s not practical for large structural weldments.
Particle Types
Dry Particles
Iron powder dusted onto the magnetized surface. Available in several colors (gray, red, yellow, black) to contrast against the surface being inspected.
Best for:
- Rough, as-welded surfaces
- Outdoor/field work
- Hot surfaces (up to 600F)
- Detecting subsurface defects (with DC magnetization)
Application: Puff the powder onto the surface while the field is active. Let excess particles fall away. Remaining particles at defect locations form the indication.
Wet Particles
Iron oxide particles suspended in a petroleum-based or water-based carrier fluid. Applied by spraying or flowing over the magnetized surface.
Visible wet particles:
- Black iron oxide in light carrier fluid
- Viewed under white light
- Good sensitivity for most structural work
Fluorescent wet particles:
- Coated with fluorescent dye
- Viewed under UV-A light (blacklight) in a darkened area
- Highest sensitivity
- Standard for aerospace, nuclear, and critical applications
| Particle Type | Sensitivity | Surface Condition | Typical Application |
|---|---|---|---|
| Dry (gray/red) | Good | Rough OK | Structural steel, field work |
| Wet visible (black) | Better | Smooth preferred | Shop inspection, machined surfaces |
| Wet fluorescent | Best | Smooth preferred | Aerospace, nuclear, critical welds |
MT Procedure for Weld Inspection
Step 1: Surface Preparation
Clean the weld surface of slag, spatter, oil, and loose scale. The surface doesn’t need to be ground smooth for dry particle MT (unlike PT), but heavy deposits will interfere with particle mobility.
For wet fluorescent MT, the surface should be clean and reasonably smooth. Dark backgrounds help (avoid bright paint).
Step 2: Establish Magnetic Field
Yoke placement: Position the yoke legs on either side of the area to be inspected. For weld inspection, place the legs perpendicular to the weld axis first (to detect longitudinal defects). Then rotate 90 degrees (legs parallel to the weld, spanning the weld width) to detect transverse defects.
Overlap: Move the yoke in overlapping steps to cover the entire weld length and HAZ. Each placement should overlap the previous one by at least 1 inch.
Step 3: Apply Particles
Apply particles while the magnetic field is active (continuous method) or just after deactivating the field (residual method). The continuous method is more sensitive and is standard for weld inspection.
Dry: Puff powder gently across the surface. Don’t dump it. The particles need to be free to move toward flux leakage points.
Wet: Spray or flow the suspension over the magnetized area. Apply enough to coat the surface evenly.
Step 4: Interpret Indications
Examine the surface for particle accumulations. Defects show as sharp, defined lines or clusters of particles.
| Indication Pattern | Likely Defect | Typical Location |
|---|---|---|
| Sharp, well-defined line along weld toe | Toe crack or undercut | HAZ at weld/base metal boundary |
| Line along weld centerline | Centerline crack or incomplete fusion | Weld metal center |
| Transverse lines across weld | Transverse crack or hydrogen crack | Weld face or HAZ |
| Star or branching pattern at weld end | Crater crack | Weld termination point |
| Rounded, fuzzy accumulation | Near-surface porosity or subsurface defect | Weld face |
| Broad, diffuse line at weld edge | Possible non-relevant (geometry change) | Weld toe, reinforcement edge |
Step 5: Evaluate Against Acceptance Criteria
The most common acceptance standard is ASME Section V, Article 7, or the project specification. General criteria:
- Linear indications over 1/16 inch are typically rejectable
- Rounded indications over specified diameter are rejectable
- Any indication confirmed as a crack is rejectable regardless of size
- Four or more rounded indications in a line spaced less than 1/16 inch apart are treated as linear
Step 6: Record and Clean Up
Document the results. Photograph indications if required. Clean particles from the surface with a brush or solvent wipe.
Demagnetization
After MT, the part may retain residual magnetism. Demagnetize if:
- The part will be welded next (residual magnetism causes arc blow)
- The part will be machined (magnetic chips cling to the surface)
- The part is near sensitive instruments or electronics
Demagnetization method: Pass the part through a decreasing AC field using a demagnetizing coil, or gradually withdraw the AC yoke while it’s energized.
Verification: Use a gauss meter. Residual magnetism should be below 3 gauss for most applications.
MT Advantages Over Other Methods
MT holds a specific niche in the NDT toolkit. Understanding where it excels helps you choose the right method.
Faster than PT. MT doesn’t require dwell time. Apply the field, apply particles, and read the indications immediately. A weld that takes 30-40 minutes to PT (including dwell and development time) can be MT’d in 5-10 minutes.
Detects near-surface defects. PT only finds surface-breaking defects. MT finds defects up to 1/4 inch below the surface, which catches subsurface cracks and near-surface slag inclusions that PT would miss.
Works on rough surfaces. Dry particle MT works on as-welded surfaces without the sensitivity problems that rough surfaces create for PT. This makes it practical for in-process inspection of multi-pass welds.
No temperature-sensitive chemicals. Unlike PT penetrants that have strict temperature ranges (40-125F), dry particle MT can be performed on surfaces up to 600F, making it useful for in-process inspection on hot weldments.
MT Limitations
- Only works on ferromagnetic materials (not stainless 300 series, aluminum, copper, titanium)
- Defects parallel to the magnetic field may not be detected (requires two-direction testing)
- Prod contacts can leave arc strikes on finished surfaces
- Residual magnetism may need to be removed
- Surface coatings over about 2 mils thick reduce sensitivity
Common Mistakes
Magnetizing in only one direction. Defects parallel to the magnetic field don’t produce indications. Always inspect with at least two field directions, 90 degrees apart.
Using AC yoke for subsurface detection. AC fields concentrate at the surface. If you’re looking for near-surface defects (up to 1/4 inch deep), use a DC yoke or DC prods.
Poor particle application. Dumping a pile of dry powder on the surface buries indications. Dust lightly and let gravity and the magnetic field do the work.
Ignoring non-relevant indications. The weld toe geometry change creates a magnetic flux disruption that attracts particles even without a defect. Learn to distinguish geometry-related indications from defect indications.
Not verifying yoke strength. ASTM E1444 requires regular verification of yoke lifting power. A weak yoke produces a weak field and misses defects.
For surface-only defect detection on non-magnetic materials, see the dye penetrant testing guide. For internal defect detection, read the ultrasonic weld testing guide. For visual inspection basics, visit the visual weld inspection guide. Return to weld inspection or the welding techniques pillar page.