Study PCN Level 2 MT by rehearsing two recurring decisions: orienting the magnetic field correctly for each suspected flaw orientation, and classifying each indication as relevant, non-relevant, or false. Work through paper scenarios, compare magnetisation techniques against part geometry, and self-mark against a rubric. Administrative details such as eligibility, exam booking, and certificate requirements are set by BINDT; confirm them at bindt.org rather than relying on secondhand summaries.
Why field direction decides whether a discontinuity shows at all
Magnetic particle testing only produces an indication where flux lines are distorted and leak from the surface. A discontinuity that runs parallel to the field creates little or no leakage, so the same crack can be visible or invisible depending solely on how the part was magnetised.
Build your understanding around the chain of cause and effect: magnetisation establishes flux in the part, a discontinuity lying roughly perpendicular to that flux forces lines to bridge the gap, leakage fields form at the surface, and particles collect there. When you revise any technique, ask first which direction the flux will run, not which equipment is being used. This habit turns scattered facts about yokes, prods, and coils into one coherent model you can apply to any sketch or scenario question.
The practical consequence is that a competent MT procedure always starts from expected discontinuity orientations. Longitudinal weld cracks run along the seam and need a field across the seam; transverse cracks need a field along it. If you internalise that the flaw must cut across the flux lines, you can reason out an answer even when the scenario describes equipment you have never handled. Practise stating, for any given discontinuity type, the field direction required before you consider which current or device produces it.
- Flux runs through the part; indications appear where flux leaks at a discontinuity cutting across it.
- Parallel flaw-and-field alignment means minimal leakage and a likely miss, regardless of equipment quality.
- Two magnetisation directions at roughly right angles are generally needed when discontinuity orientation is unknown.
Choosing between magnetisation techniques for a given part geometry
Each magnetisation method creates flux in a specific direction and suits specific part shapes and access conditions. Level 2 decisions require matching the method to the geometry, the expected discontinuity, and whether the part can be electrically contacted or must be examined indirectly.
Compare the methods on three axes: field direction produced, contact requirements, and portability. An AC yoke is portable and contacts the surface through its poles, producing flux mostly between those poles in the surface layer. Prods contact the part directly and drive current through it, producing a circular field around the current path, so the flaw direction that shows depends on where you place the contacts. A coil surrounds the part and induces a longitudinal field along its length; a central conductor threads a bar or tube so the bore surface is magnetised circumferentially without electrical contact on the part itself.
Confusing current direction with field direction is the classic reasoning trap with prods and central conductors. With prods, the current flows between the contact points and the useful field arcs around each conductor path, so the field lies roughly perpendicular to the current line. A discontinuity running along the line between the prods cuts the circular field and is well detected, while one crossing that line lies roughly parallel to the field between the contacts and may produce little leakage. With a central conductor, the current runs through the bar and the field circles the bore, so longitudinal discontinuities on the bore surface are the target. When you meet a scenario, sketch the current path first, then draw the flux it creates, then judge whether the suspected flaw cuts across that flux.
| Technique | Field direction relative to setup | Typical use in scenarios |
|---|---|---|
| AC yoke | Flux between the poles, across the gap placed on the surface | Portable weld inspection; orient poles across the expected flaw |
| Prods | Circular field around the current path between contacts, roughly perpendicular to the contact line | Large welds where contact is permitted; move contacts for coverage |
| Coil | Longitudinal field along the part inside the coil | Shafts and bars; find transverse discontinuities |
| Central conductor | Circumferential field around the bore | Tubes, bolt holes; no contact on the part surface |
AC, DC, and rectified current: what each actually buys you
The current waveform shapes where the field sits in the part. Alternating current concentrates the field near the surface, while direct and rectified currents penetrate deeper, at the cost of somewhat harder particle mobility and different demagnetisation behaviour.
Keep the comparison qualitative rather than chasing numbers. With AC, the field reverses continuously and remains strongest near the surface, which suits detecting surface-opening discontinuities and pairs naturally with mobile yokes. With DC or rectified current, the field extends deeper into the section, which is why scenarios involving near-surface discontinuities call for DC-type magnetisation, often with the continuous or residual method chosen separately. Distinguish two independent choices in every scenario: the waveform, and the timing of particle application relative to magnetisation.
The continuous and residual methods are frequently conflated with the current choice, so separate them explicitly in your notes. Continuous means particles are applied while the magnetising field is applied, giving maximum sensitivity on low-retentivity materials. Residual means the part is magnetised first and the remanent field holds the particles, usable only on materials with sufficient retentivity. A scenario that describes a low-carbon weldment examined by residual magnetisation contains a mismatch you should be able to identify and justify in writing, because retentivity depends on the material, not on preference.
Interpreting indications: relevant, non-relevant, and false
Interpretation is the Level 2 skill of classifying what particles reveal. Relevant indications come from real discontinuities, non-relevant ones from harmless flux leakage at geometry or material features, and false ones from particles held by surface condition rather than any leakage field.
Non-relevant indications arise wherever the design itself forces flux to leak: abrupt section changes, threads, keyways, corners, and the edges of attachments. They are often sharp, repeatable, and located exactly where the geometry predicts, which is the clue. A sound interpretation habit is to predict leakage points from the part drawing before examining, so that when particles collect at a section change you can recognise the pattern as expected rather than treating every sharp line as a crack. Relevant indications at real discontinuities tend to align with weld features or service stresses instead.
False indications are surface artefacts: particles held by rough scale, grease, or by gravitationally settled powder in excess, none involving a leakage field. The discriminating tests you should know for scenario questions include re-examining after surface cleanup, checking whether the pattern follows an actual geometric feature, and verifying particle application technique. A Level 2 answer that simply says 'crack' without classification and evidence is weak; a strong answer names the class, cites the observation supporting it, and states what re-check would confirm it.
- Relevant: follows weld or stress features; supported by leakage at a discontinuity cutting the field.
- Non-relevant: predicted by geometry such as threads, keyways, section changes, attachment edges.
- False: surface artefacts from roughness, contamination, or excess powder; disappears after cleanup or re-application.
Worked scenario 1: orienting the examination of a butt weld
A scenario asks you to inspect a butt weld where transverse cracking is suspected. The better decision is to orient the field along the weld so transverse flaws cut across the flux, using a technique that achieves that direction on the available part.
The plausible mistake: the candidate places yoke poles so the flux runs across the weld, reasoning that 'the field must cross the weld'. For longitudinal toe cracks that direction works, but for transverse cracks it is exactly wrong, because those cracks then lie parallel to the flux and produce negligible leakage. The better decision follows the first-principles chain: expected flaw is transverse, so flux must run longitudinally along the weld, so the yoke poles are placed on the weld surface spaced apart along the seam axis, making the flux path between them run along the weld, or the technique is changed to one that yields a longitudinal field on that geometry. It matters because a wrong field direction hides the discontinuity entirely, and the paper answer must show you understood why, not just name a device.
Extend the reasoning to complete coverage. Since a weld can contain both longitudinal and transverse discontinuities, a single magnetisation direction is not a complete examination; the scenario expects you to recognise that two roughly perpendicular field directions are needed, and to describe how the technique is repositioned between passes. Add the reporting elements a Level 2 would record: technique, field directions used, and the location and classification of any indication. In your revision, redraw this scenario with the flaw orientation rotated, and re-derive the field direction each time until the derivation is automatic.
Worked scenario 2: sharp line at a section change or crack
Particles form a sharp line at a machined step in a component. The plausible mistake is reporting a crack; the better decision is classifying it as non-relevant by matching the location to the drawing, then re-examining to confirm no discontinuity is present.
The plausible mistake: the candidate sees a crisp, strongly held line and records a relevant linear indication, recommending rejection. The better decision is to check the line against the part geometry: it lies precisely at a section change where flux must concentrate and leak by design. Steps toward confirmation include comparing the indication location with the drawing, assessing whether the line continues beyond the geometric feature, and re-examining that area from a different magnetisation direction, since a real crack generally persists across orientations while geometric leakage follows the feature. It matters because the two outcomes differ enormously in consequence, and the classification reasoning is what the scenario is testing.
Contrast this with a false indication so the categories stay distinct. Suppose particles also form a loose, diffuse patch on a rough, scaled area away from any geometric feature. After surface cleanup and re-application the patch does not return, which identifies it as false rather than non-relevant: the geometry was never the cause, the surface condition was. Practise writing one or two sentences for each classification that name the evidence, because scenario answers are marked on the reasoning chain: observation, candidate class, confirming check, and the resulting decision within the written procedure.
A sketch-based exercise and a self-check rubric
Draw three simple part sketches, mark suspected discontinuity orientations, and for each write the field direction, the technique achieving it, and the classification checks for any indication. Mark yourself against a five-point rubric.
Exercise: sketch (a) a butt weld with both longitudinal and transverse crack suspects, (b) a shaft with a suspected transverse discontinuity, and (c) a tube whose bore needs examination. For each, derive the required flux direction, select a technique from the comparison table, and note where non-relevant leakage is predicted by the geometry. Expected observations: the weld needs two field directions, the shaft needs a longitudinal field such as a coil provides, and the tube needs a central conductor so the bore is magnetised circumferentially without contact. If your derivations disagree with these, revisit the field-direction chain in the first section.
Self-check rubric, scored per sketch: one point for the correct flux direction relative to the flaw; one for a technique that genuinely produces that direction; one for identifying at least one non-relevant leakage location from the geometry; one for a correct classification check described in words; one for the coverage question, meaning whether you noted the need for a second field direction where flaw orientation is uncertain. A learning milestone to aim for before moving on is consistent full marks across the three sketches, re-derived from first principles without notes. This is a study milestone, not a prediction of any exam outcome.
- Point 1: flux direction correct relative to suspected flaw.
- Point 2: named technique genuinely produces that field direction.
- Point 3: geometry-driven non-relevant locations predicted in advance.
- Point 4: classification check stated with evidence and a confirming re-check.
- Point 5: second field direction identified where orientation is uncertain.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.