Magnetic Particle Testing (MT)
Magnetic particle testing (MT or MPI) detects surface and near-surface discontinuities in ferromagnetic materials—carbon steel, low-alloy steel, and ferritic stainless steels. The method works by magnetizing the test piece and applying fine ferromagnetic particles (dry powder or wet suspension) to the surface. Discontinuities create magnetic flux leakage that attracts particles, forming visible indications at defect locations.
MT is fast, inexpensive, and highly sensitive to surface-breaking cracks. It is specified routinely for weld inspection, forging examination, and in-service crack detection on piping and pressure equipment per ASME Section V Article 7.
MT Magnetization Techniques
| Technique | Equipment | Application | Advantages |
|---|---|---|---|
| Yoke (AC/DC) | Electromagnetic yoke | Field weld inspection, small areas | Portable, no burn risk, most common for piping |
| Prods | Current-carrying contact prods | Large surfaces, castings | Covers large areas quickly |
| Coil | Solenoid coil around part | Circumferential flaws on cylindrical parts | Uniform longitudinal field |
| Head shot | Current passed through part | Longitudinal flaws | Full coverage, shop use |
| Central conductor | Bar through hollow part | Outer surface of pipes, rings | Uniform circular field |
For piping weld inspection, the AC yoke is the standard field tool. ASME V requires a minimum lifting force of 4.5 kg (10 lbs) for AC yokes and 18 kg (40 lbs) for DC yokes to confirm adequate field strength.
Particle Types
| Type | Medium | Visibility | Best For |
|---|---|---|---|
| Dry powder | Air-applied colored powder | Visible light (white, yellow, red) | Rough surfaces, hot surfaces, field inspections |
| Wet fluorescent | Particles suspended in carrier fluid | UV-A light (black light) | Smooth surfaces, critical welds, highest sensitivity |
| Wet visible | Particles in carrier fluid | Visible light (black particles on white contrast paint) | General shop inspection |
Wet fluorescent MT under UV-A light provides the highest sensitivity and is specified for critical applications: NACE MR0175 sour service welds, high-pressure piping, and nuclear components.
MT vs PT Comparison
| Parameter | MT | PT |
|---|---|---|
| Material | Ferromagnetic only (carbon steel, low-alloy) | Any non-porous material (including stainless, aluminum, nickel alloys) |
| Defect detection | Surface and near-surface (up to 3-4 mm depth) | Surface-breaking only |
| Sensitivity | Higher for ferromagnetic materials | Lower than MT for steel |
| Surface prep | Light cleaning (remove scale, paint up to 0.5 mm) | Clean, dry surface required (no paint, grease, scale) |
| Speed | Fast (magnetize, apply, inspect, demagnetize) | Slower (apply, dwell 10-30 min, develop 10+ min) |
| Post-test | Demagnetization required | Cleaning only |
| Standards | ASME V Article 7, ASTM E709 | ASME V Article 6, ASTM E165 |
Key Standards and Acceptance Criteria
| Standard | Scope |
|---|---|
| ASME Section V, Article 7 | MT examination procedures |
| ASTM E709 | Standard guide for MT |
| ASTM E1444 | Standard practice for MT |
| ASME B31.3, Table 341.3.2 | Acceptance criteria for piping welds |
| EN ISO 17638 | MT of welds (European standard) |
| EN ISO 23278 | MT acceptance levels for welds |
ASME B31.3 acceptance criteria for MT: no cracks permitted; linear indications >1.5 mm are rejectable for severe cyclic service, >5 mm for normal service. Rounded indications are evaluated per the applicable code table.
MT results are recorded in the inspection and test plan alongside hydrostatic test and mill test certificate data.
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