Magnetic particle testing

Applicability and measured evidence

Magnetic particle testing (MT or MPI) detects surface and some near-surface discontinuities in ferromagnetic materials. Field leakage at a discontinuity attracts particles. Verify the alloy and condition before selecting the method. ASNT, magnetic particle testing principles and limitations

A discontinuity is most readily detected when it cuts across magnetic flux. One parallel to the field can produce little or no useful indication. The National Board explains the need to examine an area with two perpendicular magnetization directions and maintain overlapping coverage. For complex geometry, demonstrate that the selected arrangements establish useful fields in the required locations; moving the yoke without a coverage plan is insufficient. National Board, field direction and overlap

Detection depends on the discontinuity, field and surface condition. MT is not a general volumetric examination, and a particle pattern does not directly establish crack depth. Coatings or plating may need removal to obtain the required sensitivity. A negative result concerns the defined examination area and qualified process; it cannot establish that all internal material is free of discontinuities. National Board, magnetic particle examination, originally published Summer 2011

The procedure should identify the component, inspection stage and area, governing method and edition, permitted magnetization technique, consumables, process checks, viewing conditions and acceptance criteria. Material eligibility, a working magnetizer and a visible particle deposit are necessary considerations, but none alone demonstrates a valid examination.

Field direction and coverage

Circular magnetization, produced by current through a part or central conductor, is useful for longitudinal discontinuities. Longitudinal magnetization from a coil or yoke is useful for transverse discontinuities. AC is principally associated with surface examination; DC provides greater penetration for near-surface work. These descriptions guide technique selection, not a universal detectable-depth limit or current setting. ASNT, field types and magnetizing current

Original planning example: for a ferromagnetic shaft, a circumferential field can reveal an axial crack that crosses it. A circumferential crack requires another suitable field orientation. This explains why a single magnetization direction cannot support a claim of coverage for both flaw directions. The example does not prescribe amperage, coil turns or acceptable crack dimensions.

Current direction and magnetic-field direction are different. In a straight conductor arrangement, the field encircles the current path. Magnaflux's reference guide illustrates both direct-current-through-part and central-conductor arrangements. Select the setup for the required field, rather than aligning an electrical connection with the desired flux direction. Magnaflux reference guide, copyright 2011, printed p. 4

Equipment and field verification

A portable yoke is useful for localized examinations; coils, central conductors and bench equipment support other shapes and production needs. Direct electrical contacts and prods introduce additional concerns. TWI identifies arcing, overheating and electrode contamination as disadvantages of prod techniques. Check whether the product specification permits the chosen contact method and how contact condition will be controlled. Do not infer that a portable technique is harmless to every finished surface. TWI, prod inspection advantages and disadvantages

Magnaflux describes yoke performance checks using standard lift weights or field-strength measurement between the legs under the applicable method. Follow the required check, leg spacing and equipment instructions. A successful lift check addresses equipment performance; it does not by itself establish coverage, direction and adequate magnetization throughout a complicated part. Magnaflux, magnetic yoke FAQs

Artificial-flaw shims such as quantitative quality indicators (QQIs) help demonstrate field direction and relative strength at selected locations. Magnaflux describes their use in setting up part-specific techniques and multidirectional fields. They are aids to verifying the setup, not a calibration of real-crack depth or a guarantee that every discontinuity will appear. Use the specified shim type, placement and interpretation procedure. Magnaflux, magnetic particle test pieces

Particles and application sequence

Dry versus wet application and visible versus fluorescent observation are separate choices. Select an approved particle system suited to the surface, component and procedure. Do not assume every wet suspension is fluorescent or every visible particle is dry. The required field must interact with mobile particles under the selected application sequence.

A bounded example is Magnaflux 7HF, an oil-based visible wet-method ink. Its European data sheet is revised October 2026 and identifies WCP-2 white contrast paint for dark surfaces. It instructs users to clean the component, apply a thin contrast layer where needed, and shake/agitate the suspension to maintain uniformity. The product's particle-size listing is not a minimum detectable flaw size. 7HF product data sheet, pp. 1–2

For 7HF's wet continuous method, the sheet instructs users to stop ink flow before switching off magnetizing current, avoiding wash-off of indications. Its residual method magnetizes the part before ink application and is described as generally less sensitive. These are distinct routes, not interchangeable steps. Use the route allowed by the governing procedure and appropriate to the component's magnetic behaviour. 7HF application instructions, p. 3

Control recirculating bath condition. Magnaflux explains that particle depletion, carry-out and contamination alter bath performance; water evaporation can change a water-based formulation differently from particle loss. Check concentration and contamination using the prescribed method and product limits. An indiscriminate addition of concentrate is not a reliable correction for every low bath level. Magnaflux, magnetic particle inspection FAQs

Viewing and interpretation

Visible systems rely on white-light contrast; fluorescent systems need suitable UV-A illumination and controlled ambient visible light. Inspector vision, adaptation and background brightness affect useful contrast. Measure the conditions required by the inspection procedure rather than deciding that a lamp looks sufficiently bright. Magnaflux, brightness and contrast in NDT

Not every particle accumulation is a rejectable defect. TWI distinguishes relevant indications from spurious deposits such as lint, dirt and drainage lines, and nonrelevant magnetic patterns associated with geometry or material structure. Abrupt section changes, threads and magnetic-property changes can create patterns. Evaluate location, shape and repeatability with product/process knowledge and the approved procedure; do not dismiss an indication solely because it occurs near a geometric feature. TWI, subclasses of magnetic particle indications

Completion and reporting

Determine whether demagnetization is required and verify the applicable residual-field limit. Magnaflux describes demagnetization as applying a reversing field that progressively decreases, followed by a residual-magnetism check. Its older reference guide's example settings are explanatory, not universal acceptance limits. Clean off inspection media and restore any required surface protection. Magnaflux reference guide, printed p. 11

Retain part identity, material, inspected area and stage, surface/coating condition, procedure revision, magnetization arrangements and settings, field directions and coverage, equipment/process checks, consumable batches, viewing conditions, indication maps and disposition. Keep observations distinct from acceptance under the applicable product rule. Document post-inspection condition where required, including residual field and cleaning. Completion of MT does not independently authorize release of nonconforming material.

Related reference

Inspection and quality documents