Fillet Indications and Stubborn Residual Field on a Cast Lever
A normalized cast carbon-steel lever (0.30% C, 300 mm long) is inspected on a wet fluorescent bench. The arm tapers from 25 mm down to 12 mm through a 3 mm fillet radius. The technique uses an AC electromagnetic yoke at 150 mm pole spacing, lifting power verified at 4.5 kg, continuous wet application, and two perpendicular directions. On both the top and bottom faces, two 8 mm linear indications lie exactly along the fillet radius; nothing is seen anywhere else on the part. The written procedure requires demagnetization to 3 G maximum. After coil demagnetization the 12 mm end still reads 6 G on a Hall-effect gauss meter. Production wants the lot shipped today, and the Level II asks you whether the indications and the residual field are acceptable.
1. Are the two indications along the fillet relevant? How would you prove your answer on the shop floor?
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Start from the physics: flux lines crowd into the 12 mm section, refract at the section change, and exit/enter at the fillet, producing leakage poles with no discontinuity present. Three features point that way: the indications are symmetric on both faces, they are confined exactly to the radius, and they do not appear on the adjacent straight sections.
Symmetry alone does not prove it, because fillets are also the classic location for casting laps and hot tears. Confirm with independent evidence:
- Visual examination at 5x and a PT of the two fillets for a surface break.
- Re-magnetize at a reduced current (just above the level needed to give the normal background) and change field direction. Geometry leakage tracks the section change and does not care about field direction; a real crack gives its strongest indication when the field is closest to perpendicular to it.
- Check for magnetic writing from the handling magnet or the previous part on the bench.
Disposition: record them as nonrelevant geometry indications, with the PT result and the reduced-current re-test as supporting evidence, and note that an AC yoke cannot rule out a small subsurface defect in the fillet. If PT shows any break, treat as relevant and evaluate against the drawing's acceptance criteria.
2. The 12 mm end reads 6 G after coil demagnetization against a 3 G limit. What are the likely causes and what is your corrective sequence?
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AC coil demagnetization works by driving the material around a shrinking hysteresis loop, so residual magnetism that survives usually means the loop was not driven small enough or not in all directions. Ranked causes:
- Withdrawal too fast or the part not carried far enough from the coil (aim for a slow, steady withdrawal of at least 1 m, or use the step-down current method).
- Thin sections (12 mm) saturate easily and hold flux along their length; a single pass in one orientation leaves the component of magnetization that the coil field did not oppose.
- Stray field from the yoke left standing on the bench, or the part resting against steel furniture.
Sequence: re-demagnetize at a higher current, withdraw slowly, then repeat with the part rotated 90 degrees, then 180 degrees, measuring after each with the gauss meter in three orthogonal orientations (a single-orientation reading is not a valid measurement). If AC still leaves 6 G, switch to a reversing DC step-down on a coil or use a yoke with reversing polarity and decreasing current. Only sign off when all three orientations read at or below 3 G; document the final readings and the meter identification. Residual field matters here because it will cause arc blow on any subsequent welding and will attract particles during machining.
3. A casting engineer asks whether this same AC yoke technique would find a 2 mm gas pore sitting 3 mm below the surface in the 25 mm section. What do you tell them?
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No, and the answer is fundamental rather than a matter of turning up the current. An AC yoke field is concentrated in a thin surface layer (skin effect), so the flux density 3 mm deep is far too low to produce detectable leakage from a 2 mm pore. Increasing the amperage does not fix this; it only raises surface flux and background.
If subsurface porosity is a design concern, the options are:
- Direct-current or half-wave DC magnetization (yoke, coil, or prod) to push flux deeper, accepting that the part then requires more careful demagnetization.
- In practice, use RT or UT for internal soundness. MT is a surface and near-surface method, and the 3 mm depth plus 2 mm size is marginal even with DC.
Also note the geometry penalty at the fillet: the section-change leakage already masks small indications there. The correct action is to record the limitation in the technique sheet and route the internal-soundness requirement to the NDT method the drawing specifies for volumetric examination.