ASNT NDT Level III MT cheat sheet

Magnetic flux density is B = μH, where B is in tesla, H is in amperes per meter, and μ = μ0μr is the permeability of the material.

Ferromagnetic materials such as iron, cobalt, nickel, and many alloys are strongly attracted to a magnet and can support high flux density; paramagnetic and diamagnetic materials are not suitable for MT.

A discontinuity produces a leakage field best when it is oriented approximately perpendicular to the flux lines; defects parallel to flux may go undetected.

Longitudinal magnetization by coil or yoke produces flux along the part axis and detects circumferential defects; circular magnetization by current through the part detects longitudinal defects.

AC magnetization concentrates current near the surface because of skin effect, favoring surface discontinuities; DC or rectified current penetrates more deeply.

Retentivity is the residual flux density after the magnetizing force is removed; coercive force is the reverse field needed to reduce residual flux to zero.

1 tesla = 10,000 gauss; 1 A/m is approximately 0.01257 Oe, and 1 Oe is approximately 79.58 A/m.

Heating a ferromagnetic material above its Curie temperature destroys ferromagnetism, so hot parts may be untestable by magnetic particle testing.

An AC electromagnetic yoke should lift a 10 lb (4.5 kg) steel weight at the poles; DC yokes commonly must lift 30–40 lb (13.6–18.1 kg) or as specified by the governing standard.

Continuous method applies particles while magnetizing current flows; residual method applies particles after current is removed and relies on retained magnetism.

Wet fluorescent examination uses fine fluorescent particles in liquid viewed under UV-A; dry powder is often used with portable yokes and on rough surfaces.

Prod magnetization passes current through the part between contact prods, producing circular magnetization; contact pads and adequate spacing reduce arc burning.

For prod magnetization, a common rule is 90–110 A per inch of prod spacing for 1/4–1/2 in. prods; follow the applicable standard.

A central conductor passed through a hollow part produces circular magnetization around the conductor and is useful when direct contact is impractical.

AC demagnetization is effective for surface residual fields; DC demagnetization penetrates more deeply. Reversing and progressively reducing the field improves results.

Particle concentration, contamination, UV-A intensity, and current-meter calibration are process-control items that must be checked at specified intervals.

For circular magnetization by head shot or central conductor, a typical rule is 1,000 A per inch of part diameter; verify against the governing code or specification.

For coil longitudinal magnetization, a common ampere-turn formula is NI = 45,000/(L/D), valid for length-to-diameter ratios from about 3 to 15.

Fluorescent wet particle concentration is typically 0.1–0.4 mL settled solids per 100 mL; visible wet concentration is typically 1.2–2.4 mL per 100 mL.

Fluorescent MT typically requires at least 1,000 µW/cm² UV-A at the examination surface and white light not exceeding about 20 lux.

Use two magnetization directions approximately 90° apart because a discontinuity is most detectable when it is perpendicular to the flux lines.

System performance checks with a known-defect reference piece or yoke lift test verify that equipment, particles, and viewing conditions remain adequate.

Residual magnetism after demagnetization is commonly limited to 3 gauss (0.3 mT) or less unless a stricter specification applies.

Remove grease, paint, scale, and loose rust before examination; surface condition can create nonrelevant indications or mask leakage fields.

ASTM E709 is a standard guide for magnetic particle testing covering equipment, techniques, and general procedures.

ASTM E1444/E1444M is a widely referenced standard practice for magnetic particle testing, including equipment and process-control requirements.

ASME BPVC Section V Article 7 specifies magnetic particle examination method requirements; acceptance criteria are found in the applicable construction section.

ISO 9934 is the international series for magnetic particle testing, covering general principles, detection media, equipment, and component examination.

ASNT SNT-TC-1A is a recommended practice for personnel qualification; the employer's written practice defines training, experience, and certification requirements.

The construction code or contract specification sets acceptance limits; the MT method standard usually defines how to perform the examination, not whether indications are acceptable.

Calibration of current meters, light meters, and UV-A meters is required at intervals specified by the code or written procedure, often annually.

Codes may specify current type, magnetization technique, particle type, demagnetization, and reporting requirements in addition to acceptance criteria.

An indication is a visible particle accumulation; it is relevant only when caused by a discontinuity that must be evaluated against acceptance criteria.

Nonrelevant indications arise from geometry, section changes, edge effects, or permeability variations and are not caused by defects.

False indications can result from surface roughness, paint, contamination, or magnetic writing; cleaning, demagnetizing, and re-examining help distinguish them.

Magnetic writing is a false indication caused by residual magnetism from magnets or current-carrying conductors; demagnetization usually removes it.

Surface discontinuities generally produce sharp, tight indications; subsurface discontinuities produce broader, fuzzier indications because the leakage field spreads.

A linear indication has length at least three times its width; rounded indications are evaluated by size and distribution under the applicable code.

Reports should include part identification, material, technique, current, particle type, coverage, indications, acceptance status, examiner, date, and standard.

Evaluation compares indication type, size, orientation, and location with the acceptance criteria; reporting records the results and disposition.