Bench and Specialist Magnetisation Methods

Outcomes

After this chapter, the learner should be able to:

• distinguish magnetic-flow, axial-current, central-conductor and induced-current methods;
• select the method for the expected discontinuity orientation;
• calculate source-derived current for perimeter methods;
• manage stepped sections and multiple shots;
• sequence circular and longitudinal bench fields; and
• identify burn and demagnetisation risks.

10.1 Magnetic flow between bench heads

In magnetic-flow mode, the component completes a ferromagnetic path between magnetising heads. The field runs longitudinally through the component and favours transverse discontinuities. Properly shaped extenders can reduce air gaps and flux crowding at the ends.

The source does not give a simple current formula for this arrangement. Field adequacy is established by measurement or demonstration, including the least favourable central region. Whole-part coverage is possible only when the component length, section and contact geometry support a suitable field.

10.2 Axial current flow through contact heads

In a head shot, low-voltage high current passes along the component, producing a circular field. Longitudinal discontinuities are favoured. Clean, compliant contact pads are essential to prevent arcing and heat damage.

The source-derived relationship is:

I = H x P

where I is amperes on the stated basis, H is kA/m and P is component perimeter in mm.

For a circular component, P = pi x diameter.

Example: A 50 mm round bar, H = 2 kA/m, using an AC RMS training basis:

I = 2 x pi x 50 = 314 A RMS approximately.

Component length does not enter this perimeter model, but geometry, current path and equipment limits still affect actual field distribution.

10.3 Stepped components

The source model permits one current value only when the calculated currents for the largest and smallest cross-sections are within a ratio of 1.5:1. The larger section controls that common current. If the ratio is greater, test sections separately, beginning with the smallest and applying demagnetisation between shots where required.

Example: Diameters 30 mm and 50 mm give a ratio of 1.67:1, so two AC RMS head shots are needed:

• 30 mm section: I = 2 x pi x 30 = 188 A RMS approximately;
• 50 mm section: I = 2 x pi x 50 = 314 A RMS approximately.

10.4 Central conductor or threading bar

An insulated non-ferromagnetic conductor through a bore produces a circular field in the surrounding hollow ferromagnetic component without sending current through it. The technique can examine inner, outer and end surfaces for discontinuities broadly parallel to the conductor.

For a centred conductor, the same perimeter relationship is used. Choose the perimeter of the surface being evaluated:

• use inside perimeter for the bore surface;
• use outside perimeter for the external surface.

Example: Ring OD 40 mm, ID 32 mm; external surface objective; H = 2 kA/m; source-example HWR RMS basis:

I = 2 x pi x 40 = 251 A RMS approximately.

If the conductor is offset, the field is local and non-uniform. Demonstrate the effective circle and use multiple overlapping placements around a large ring. Keep separate parts from touching when several are loaded on one conductor.

10.5 Induced-current technique

In an induced-current system, a ring-shaped component acts like a transformer secondary. AC in a primary circuit induces current around the ring. The secondary magnetic field encircles that induced current path and can reveal circumferentially oriented discontinuities on the ring surfaces.

The method avoids direct electrical contact with a finished component, reducing burn risk. It is limited to suitable closed-loop geometry and AC surface examination. A clamp meter suitable for the induced waveform may measure secondary current. The primary current cannot be assumed to equal or directly predict the induced current.

The source applies I_induced = H x P to the measured secondary current, with the same stepped-section ratio concept. If current measurement is unavailable, a qualified field/performance indicator and representative part are needed.

10.6 Multi-directional systems

Some benches apply two fields in rapid sequence. Their relative magnitude, phase or timing must be balanced so a strong field in one direction does not suppress indications in the other. Circular artificial-flaw shims can help visualise balance, but the controlling method must define how adequacy is accepted.

Do not call a system multi-directional merely because it has a coil and contact heads. If shots occur separately, each direction remains a separate magnetisation and needs its own verified settings.

10.7 Bench process sequence

1. Inspect contacts, cables, bath circulation and emergency controls.
2. Confirm ammeter, timer and media checks.
3. Load the component without metal-to-metal damage.
4. Select and document current waveform and basis.
5. Apply the circular head or central-conductor shot as specified.
6. Inspect and record before the pattern is disturbed.
7. Apply the longitudinal coil or magnetic-flow shot.
8. Inspect and record the second direction.
9. Demagnetise when required, commonly leaving the easier-to-measure longitudinal field last.
10. Verify residual field, post-clean and release.

10.8 Common traps

• Confusing magnetic-flow heads with current-flow contact heads.
• Calculating head-shot current from component length.
• Using OD when the bore is the target surface.
• Allowing rings to touch on a central conductor.
• Treating primary current as induced secondary current.
• Applying one current to a stepped component outside the permitted ratio.
• Forgetting that a head shot can leave a difficult circular residual field.

Chapter summary

Bench equipment can create complementary longitudinal and circular fields with repeatable media application. Magnetic-flow heads favour transverse discontinuities; axial current and central conductors favour longitudinal features. Perimeter calculations, stepped-section rules, contact controls and field verification turn the bench from a powerful box into a controlled examination system.