5.2 Selecting the Proper Magnetization Method: Coils, Yokes, Current Type

Official MT general topic 5 on the outline administered prior to 15 December 2026 is Selecting the Proper Method of Magnetization. The exam is matching field direction, current waveform, continuous versus residual, particle form, and hardware to the part and the discontinuity. A Level II who always grabs the AC yoke, or always runs the same bench recipe, will miss both the indication and the item.

Field direction first

Magnetic particle indications form where a discontinuity cuts flux. Select the method that puts flux across the expected crack. If you do not know the orientation, you need two directions approximately 90° apart, or a qualified multidirectional process.

A coil or cable wrap around the part is the standard longitudinal shot. Current in the winding produces a field along the part axis (right-hand rule: fingers follow the turns, thumb is the field). Circumferential cracks leak that field. A cable wrap is simply a flexible coil: wrap the high-current cable around a large casting, nozzle, or vessel section that will not fit the bench coil, count the turns, and compute ampere-turns as current times turns.

A yoke is a portable U-shaped electromagnet. The field runs in the part between the legs. It is local, electrically contact-free (magnetic contact only), and the usual weld-toe and field-repair tool. Articulate the legs so both poles sit flat. Two placements, about 90° apart, with overlap, are the normal complete local exam.

Coil selection and the L/D starting point

Longitudinal coil shots are not "as many amps as the bench will give." Training courses teach an ampere-turn starting point that depends on the part's length-to-diameter ratio (L/D) and on how much of the coil opening the part fills.

Commonly taught classroom formulas — the governing procedure or code table still wins:

• Low fill (the part occupies a small fraction of the coil opening): about NI = 45,000 / (L/D)
• Higher fill (the part more nearly fills the coil): about NI = 35,000 / (L/D)

NI is ampere-turns. Divide by the number of coil or cable turns to get current. If L/D is less than about 2, a single short part in a large coil is an inefficient longitudinal shot; procedures often stack similar parts end-to-end, use a cable wrap that better fits the piece, or choose another method. Very long parts need more than one coil position so the field is adequate along the length.

Position the area of interest inside the coil. A part that only has one end in the winding is not fully longitudinally magnetized. These numbers are textbook starting points used in ASTM E1444/E709-style teaching. If the traveler prints a current, use the traveler.

Current type: AC, HWDC, FWDC, and DC

Waveform controls depth and particle mobility.

Classroom compression the exam expects:

• AC = surface, particle mobility, and a self-demagnetizing tendency when the AC is removed
• HWDC / FWDC = near-surface
• DC = deeper

AC is a poor choice if the target is a subsurface shrink cavity or a slag pocket under a weld cap. DC or rectified current is a poor first choice if you only need a mobile powder on a surface fatigue crack and the procedure allows AC. An AC yoke on a toe crack is not "weaker DC." It is a different tool: skin-effect flux plus particle mobility plus no electrical contact.

AC versus DC also matters on yokes. AC yokes are the surface, self-demagnetizing, high-mobility choice. DC yokes pull flux deeper and leave more residual field; they are chosen when the procedure wants that deeper local field, not because they are automatically "stronger" for every surface crack.

Continuous versus residual

Continuous method: particles are applied while the magnetizing current is on, or while the yoke is energized. Use it for most production wet fluorescent work and for almost all AC work. AC leaves little residual field, so "residual AC" is not a real technique.

Residual method: the part is magnetized, current is removed, and particles are applied to the retained field. It requires a material that holds residual magnetism (high retentivity — many hard or heat-treated steels) and is usually a DC or rectified shot. Residual is common on some automated bench lines and on parts that cannot stay in a bath during the shot.

Do not use residual on low-retentivity alloys just because it is faster. If the residual field cannot leak at the discontinuity, you will miss the crack. Residual current, when the procedure allows the method, is the value that procedure assigns to residual work. Do not assume it is the same number as the continuous shot, and do not demagnetize the part before residual particles are applied.

Dry powder versus wet fluorescent

Choose dry when the surface is coarse and the target is a relatively open weld crack. Choose wet fluorescent when the procedure is looking for tight fatigue or grind cracks on a prepared surface. Particle color and vehicle do not replace waveform: a fluorescent bath on a residual AC shot is still the wrong physics.

Yoke on welds versus bench unit on production parts

An AC electromagnetic yoke is the default mental picture for in-service or field weld examination:

• No electrical current in the part, so no arc burn
• Surface flux from AC skin effect, matched to toe fatigue cracks
• Legs span the weld toe or the local area of interest
• Two orientations, about 90° apart, with overlap
• Particles — often dry, sometimes aerosol wet — applied while the yoke is on (continuous)

A wet-bench unit (headstocks plus coil) is the default for production parts:

• Repeatable amperage from a calibrated current meter
• Circular and longitudinal shots in one fixture
• Wet fluorescent sensitivity on machined surfaces
• Throughput on shafts, fasteners, and small fittings

Do not drag a bench-coil amperage recipe into a vessel-nozzle walkway, and do not pretend a handheld yoke replaces a specified multidirectional bench shot on a high-volume aerospace fitting. Hardware follows access, contact rules, and the discontinuity — not habit.

Multidirectional and swinging field

A multidirectional or swinging-field bench applies two field directions (often circular and longitudinal) in rapid sequence or as a combined vector so particles can show cracks of more than one orientation in a single processing cycle. Conceptually, the flux direction is not frozen on one axis long enough for a randomly oriented crack to hide.

It is a productivity and coverage tool, not a license to skip the procedure's required directions. If the written practice still requires two distinct shots and two inspections, a swinging-field unit must be qualified to that practice. On the exam, "randomly oriented cracks on a production fitting, one processing cycle" is the usual hint that multidirectional magnetization is in play.

Field indicators: verify, do not replace amperage

After you pick a method, you still have to prove the field is in the right direction and adequate.

Use indicators with the procedure amperage, not instead of it. A pie gauge that shows slot indications while you run half the required current does not make the shot legal. A gauss meter reading on the surface does not replace a QQI when the procedure requires a QQI. Place the pie gauge or shim so the artificial slots or notches cross the intended field. If the slots that are parallel to the field light up and the ones that should be crossing it stay dead, the field is pointed the wrong way — fix the setup, do not just raise amperage.

Method-selection checklist for the exam: direction first, waveform second, continuous versus residual third, hardware and particle form fourth, then verify with the indicator the procedure names. The pie gauge is a witness, not a new procedure.