9.1 Magnetic Particle Testing

Principle and the Ferromagnetic-Only Rule

Magnetic particle testing (MT) detects surface and slightly subsurface discontinuities in ferromagnetic materials — iron, most carbon and low-alloy steels, nickel, cobalt, and their alloys. The part is magnetized so that magnetic lines of force (flux) travel through it. Where a discontinuity interrupts or distorts that flux, some of it is forced out of the surface, creating a local flux leakage field with north and south poles straddling the flaw. Fine ferromagnetic particles applied to the surface are attracted to and held by that leakage field, forming a visible indication far wider than the crack itself. This magnification is why MT reveals cracks far too tight to see with the unaided eye.

Because the entire mechanism depends on the material concentrating and leaking magnetic flux, MT works only on ferromagnetic materials. Austenitic stainless steels, aluminum, copper, titanium, and magnesium are effectively non-magnetic and cannot be magnetized enough to produce a leakage field, so MT does not apply — a Level III selects penetrant (surface) or eddy current (surface/near-surface, conductive) instead. This ferromagnetic-only limit is one of the most frequently tested Basic-exam facts, and it explains why MT fails on an austenitic weld even though it excels on the adjacent carbon-steel base metal.

Field Direction Must Be Perpendicular to the Discontinuity

A leakage field forms only when the flux lines are cut by the discontinuity. Maximum sensitivity occurs when the magnetic field is oriented approximately perpendicular (about 90 degrees) to the length of the discontinuity. A crack lying parallel to the flux barely disturbs it and may produce no indication at all. Since real parts contain cracks of unknown orientation, MT is performed in at least two directions roughly 90 degrees apart to catch discontinuities in any orientation. This single rule drives the choice between circular and longitudinal magnetization.

Magnetization Methods

• Yoke: a hand-held electromagnet (or permanent magnet) with two legs placed on the surface produces a longitudinal field between the poles. Yokes are portable, need no direct electrical contact (no arc-burn risk), and are the standard for field weld inspection. Per ASME Section V, an AC yoke must have a lifting power of at least 10 lb (4.5 kg) and a DC or permanent-magnet yoke at least 40 lb (18 kg) at the maximum pole spacing used — a common numeric check.
• Prods: two hand-held electrodes press onto the surface and pass current through the part for a circular field, useful on large weldments and castings. Typical practice is 90 to 125 amperes per inch of prod spacing, with prod spacing kept roughly between 3 and 8 inches. The main hazard is arc burn at the contact points, so prods are prohibited on finished or fracture-critical surfaces.
• Coil (solenoid): wrapping a coil around the part creates a longitudinal field for transverse cracks; effective field length is limited near the coil ends.
• Central conductor: a conductor threaded through the bore of a ring, tube, or hollow casting produces a circular field on both the inside and outside diameters without electrical contact, avoiding arc burn entirely — the preferred method for hollow parts.

Continuous vs Residual; Particles and Contrast

• Continuous technique: particles are applied while the magnetizing field is present, so the field is at maximum and sensitivity is highest. It is the standard for most work and is required for low-retentivity materials.
• Residual technique: the part is magnetized, the current removed, and particles applied using only the remanent (residual) field. It works only on materials with high retentivity and coercivity (hardened, high-carbon steels) and is generally less sensitive than continuous.
• Dry vs wet particles: dry powder suits rough surfaces, subsurface flaws, and portable yoke/prod work; wet particles suspended in a water or oil bath give higher sensitivity to fine surface cracks on smooth, machined parts.
• Visible vs fluorescent: visible particles are viewed under white light against a contrasting background; fluorescent particles are viewed under ultraviolet (black) light in a darkened area and provide the highest sensitivity for fine cracks.

Demagnetization and Field Indicators

After MT, residual magnetism can attract machining chips, deflect instruments, cause arc blow during later welding, or interfere with service, so demagnetization is often required — typically by applying a reversing field of steadily decreasing amplitude (AC step-down or withdrawing the part through a coil). Field adequacy and direction are verified with field indicators: a pie gauge, QQI (quantitative quality indicator) shims, or a Hall-effect gaussmeter. A common mistake is assuming that current flow alone proves the part is properly magnetized; the indicator confirms a field of adequate strength and the correct direction actually reaches the inspection surface.

Depth Limits and Common Basic-Exam Traps

MT reveals surface-breaking discontinuities with high sensitivity and can detect discontinuities lying slightly below the surface, but its subsurface reach is shallow and drops off quickly with depth. Deep internal flaws are outside its range; a Level III routing a thick weld for internal lack of fusion selects ultrasonic or radiographic testing instead. Two traps recur on the Basic exam. First, a non-relevant indication can arise from an abrupt geometry change, a permeability boundary such as a heat-affected zone, or magnetic writing from part-to-part contact — these must be distinguished from real flaws, often by demagnetizing and re-testing. Second, the yoke's legs must straddle the expected crack: because a yoke produces a longitudinal field between its poles, a crack running parallel to the line between the legs is nearly invisible, so the yoke is rotated about 90 degrees for a second look. Remembering that MT is fundamentally a flux-leakage method — not a surface-coating or conductivity method — keeps these scenario questions straight.