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Sign inDirect circular magnetisation current
Passing current directly through a part sets up a circular field that closes on itself around the current path. Inside a solid conductor the field rises linearly from zero on the axis to a maximum at the surface, which is exactly where surface-breaking discontinuities are; and because the flux runs circumferentially, the technique reveals discontinuities lying longitudinally, parallel to the current. Nothing leaks at the part ends, so unlike a coil shot there is no demagnetising field to fight and no L/D ratio in the formula.
The surface field of a round conductor is H = I / (pi D), so the field per unit of current falls as the part gets bigger. Holding the field constant therefore means scaling current with diameter, and the codes express that as amperes per inch of diameter. Between 300 and 800 A/in covers everything from a light check on a machined surface to a demanding examination of a rough casting; 500 A/in is a common starting point, and values near the top of the band are used where the surface is rough or the expected discontinuities are tight.
For non-round sections, diameter means the greatest cross-sectional diagonal, since that is the dimension that sets the longest flux path. Large-diameter parts quickly demand currents beyond the machine, and that is the point at which the work moves to a central conductor, multiple contact positions or a prod technique.
Contact is the hazard. Every head shot carries burn and arc-strike risk if the contact area is small, dirty or the current is switched while the part is not clamped. Lead or copper braid pads are used on finished surfaces, and the current is always switched on and off with the part held firmly in the heads.
I = (A per inch) x D / 25.4 D in mm Code band: 300 to 800 A per inch of diameter (about 12 to 31 A/mm) Surface field of a round bar: H = I / (pi D) Circular field finds longitudinal discontinuities
- Circular fields find longitudinal discontinuities. A coil or yoke shot in the transverse direction is still required for full coverage.
- For non-round parts use the greatest cross-sectional diagonal as the diameter.
- Direct circular magnetisation puts current through the part - check contact pads and clamping before every shot to avoid burns and arc strikes.
- Switch the current on and off only while the part is clamped in the heads.
- The surface field figure is theoretical for a solid round bar. Verify with a Hall-effect probe (30-60 G tangential) or an artificial flaw shim.
- Hollow parts should normally be magnetised with a central conductor rather than a head shot, because a head shot leaves the bore surface unmagnetised.
Reference: ASME BPVC Section V (2023 Ed.), Article 7 - direct (head shot) circular magnetisation; ASTM E1444/E1444M-22 and ASTM E709-21. The 300 to 800 A per inch of diameter band is taken from those standards.
