Setting a Yoke Technique on a Flush-Ground Butt Weld
A 25 mm thick carbon steel butt weld, 800 mm long, is ground flush and must be inspected 100% by magnetic particle, covering 25 mm of base metal each side. The only tool on site is an AC electromagnetic yoke rated to lift 4.5 kg at pole spacings up to 150 mm; the legs are adjustable. The technician proposes 200 mm spacing to cut down the number of shots, wet fluorescent particles, and a single pass per direction. Service history for this joint type includes toe cracks running longitudinally along the weld. The customer wants a technique sheet with the number of magnetizations, field directions, and the process control that supports them.
1. How many magnetizations does your technique need, and how do you justify the number?
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Build it from coverage geometry rather than habit.
- Required length coverage: 800 mm of weld.
- Effective inspected band per placement: at 150 mm pole spacing the field spreads roughly 25-40 mm beyond each pole, giving about 200 mm of usable coverage across the legs.
- With 50% overlap, advance 100 mm per placement, giving positions at 0, 100, ... 800 mm: 9 placements for one direction.
- Two directions at roughly 90 degrees to each other: 18 magnetizations.
Cross-weld coverage: legs 75 mm either side of the weld centreline clears the 25 mm HAZ requirement with margin.
Why two directions: an indication is only reliably detected when the field is within about 45 degrees of perpendicular to it. A single direction leaves an insensitive band 45 degrees either side of the field lines. The yoke straddling the weld gives a field across the weld, which is close to ideal for the longitudinal toe cracks in the service history; the second, rotated pass covers transverse indications.
Apply particles continuously for each shot, and state the overlap and directions explicitly in the technique sheet so the number is auditable.
2. Can the technician use 200 mm pole spacing? What must be verified before that is written into the procedure?
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Not on the strength of the 150 mm rating. Lifting power falls steeply as the poles separate, and the field between the poles is what magnetizes the part, not the field at the pole faces.
Before 200 mm spacing can be used:
- Verify lifting power at 240 mm (the actual maximum spacing plus margin) with a calibrated weight or spring scale, and confirm it meets the 4.5 kg minimum your procedure and the governing code require at maximum spacing. If it does not lift, the spacing is not permitted.
- Confirm adequate field at the far side of the 25 mm plate, not just at the surface, with a field indicator or by a demonstration on a reference weld containing known indications.
- Check that the wider spacing still gives the assumed coverage band; if the field spreads less than assumed, the 100 mm advance is wrong and the magnetization count must increase.
If the yoke fails the lifting test at 200 mm, keep 150 mm and budget the extra shots. Document the verification value, the date, and the instrument in the technique sheet. A spacing that has not been verified is a change to the technique, not a shortcut.
3. The customer asks why the yoke's daily lifting-power check is necessary if the yoke was new last year. How do you answer?
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Because lifting power is not a fixed property of a new tool; it degrades with the things that happen to yokes on a site, and it is the only field measurement most crews can make quickly and repeatably.
Degradation routes: cracked or frayed coil leads and damaged cable connectors increase resistance; a partially shorted coil or a failing rectifier/transformer in the controller reduces output; pole faces worn or burred lose contact area and add air gap; and the wrong voltage tap or an extension lead with a long, small-gauge cord drops the current. All of these reduce the field with no outward sign, and the yoke still buzzes and still attracts particles.
A daily check with a known 4.5 kg weight at the maximum spacing actually used is a direct, traceable measurement of the capability that the technique relies on. Do it at the start of each shift or whenever the yoke is dropped, rewired, or used with a different cord. Record the result, the weight identification, and the spacing. If the yoke fails, the technique is invalid from the previous successful check onward, and the parts inspected in that window need re-inspection. That re-inspection exposure is what the customer is actually buying with the daily check.