Magnetic Particle Testing (MT) - Level II Study Guide
Core concepts
Magnetic particle testing detects surface and near-surface discontinuities in ferromagnetic materials only, such as carbon steel, cast iron, and some nickel alloys. The part is magnetized and ferromagnetic particles are applied; they collect at leakage fields where a discontinuity interrupts the field. Master magnetization direction: longitudinal magnetization runs along the part axis and finds transverse defects, while circular magnetization runs around the part and finds longitudinal defects. Also know current types (AC for surface-breaking flaws, DC and half-wave rectified DC for deeper penetration), continuous versus residual technique, wet versus dry and fluorescent versus visible particles, demagnetization, and field strength verification.
Traps to expect
- A discontinuity roughly parallel to the field lines produces little leakage and is missed. Questions often state a flaw orientation and ask which method applies.
- AC yokes are surface-only; DC gives more subsurface penetration.
- A central conductor or coil produces circular magnetization in a ring or tube; prods produce a circular field around each contact.
- Thick paint or coatings reduce sensitivity.
- Austenitic stainless steel and aluminum are nonferromagnetic and cannot be inspected by MT.
- Excess particle concentration causes background that hides indications.
Rule of thumb
An AC electromagnetic yoke must lift at least 10 lb (4.5 kg) at its maximum rated pole spacing, typically 4 in (100 mm), verified with a lifting test on a steel plate before each use or shift.
Worked example: a longitudinal seam in a pipe runs along the axis. Circular magnetization (central conductor or prod technique) puts the field across the seam and gives the strongest indication.