Magnetism, Materials and the B-H Response

Outcomes

After this chapter, the learner should be able to:

• describe magnetic domains without relying on a misleading bar-magnet analogy;
• distinguish ferromagnetic, paramagnetic and diamagnetic response;
• use flux, flux density, field strength and permeability correctly;
• interpret the main features of a hysteresis loop;
• relate permeability, retentivity and coercivity to MT practice; and
• predict how local material changes can create non-relevant indications.

2.1 From atomic moments to domains

Magnetism arises from moving electric charge and the magnetic moments associated with electrons. In ferromagnetic material, atomic moments interact strongly enough for regions called domains to form. Within a domain, many moments share a preferred direction.

An unmagnetised component does not contain "no magnetism" at the microscopic level. Instead, its domains are arranged so their effects largely cancel at the component scale. An applied field favours domains aligned with it; domain walls move and moments rotate. As alignment increases, the component develops a stronger net flux. Near saturation, a large increase in magnetising force produces only a small additional rise in flux density.

When the applied force is removed, some materials return close to their initial state, while others retain substantial alignment. That retained condition is central to residual testing and demagnetisation.

2.2 Material classes

Iron, nickel and cobalt are familiar ferromagnetic elements, but engineering response depends on alloy, phase, grain structure, stress and heat treatment. Low-carbon ferritic steel commonly magnetises more easily than hardened high-carbon steel. Austenitic stainless steel is usually not suitable, while ferritic and martensitic grades can be. Dissimilar weld metal, a hard-faced region or a heat-affected zone may therefore produce a different response from the parent metal.

2.3 Magnetic circuits and lines of flux

Magnetic field lines are a model used to visualise direction and relative density. They form closed paths, do not cross, and concentrate in routes of lower magnetic reluctance. Outside a simple magnet they are conventionally shown travelling from north to south; within the magnet the return path completes the loop.

An air gap has much higher reluctance than a ferromagnetic path. A crack, lack of fusion, sharp section change or material boundary can therefore divert flux. Some flux leaves the component and returns nearby, creating the leakage field used by MT.

Consequent poles are additional pole regions that may appear along a long or irregularly magnetised component. They can collect particles and imitate discontinuity response. Their presence is a reason to evaluate field distribution rather than assuming a simple two-pole model.

2.4 Quantities and relationships

Magnetic flux, Phi, represents the total magnetic field passing through a selected surface. Flux density is:

B = Phi / A

where A is area normal to the flux. The unit of B is tesla. Magnetising field strength, H, describes the applied magnetising influence. For a simplified linear region:

mu = B / H

Real ferromagnetic materials are non-linear, so permeability changes with field strength and magnetic history. Relative permeability is the material permeability divided by the permeability of free space. A single handbook value should not be treated as permanent truth for a fabricated component.

Reluctance is the magnetic-circuit counterpart of resistance: a higher reluctance path carries less flux for a given magnetomotive force. This analogy is useful, but magnetic and electrical circuits are not identical.

2.5 Hysteresis loop

A B-H loop plots material response (B) against applied field (H):

1. From the demagnetised state, the initial or virgin curve rises as domains reorient.
2. The curve approaches positive saturation.
3. Reducing H to zero leaves remanent or residual flux density.
4. A reverse field is needed to reduce B to zero; its magnitude is associated with coercivity.
5. Further reverse field produces negative saturation.
6. Reversing the process completes the loop.

A narrow loop commonly represents a magnetically soft material: relatively high permeability, low coercivity and easier demagnetisation. A wide loop commonly represents a magnetically hard material: lower permeability, higher coercivity and stronger retained magnetisation. "Soft" and "hard" here describe magnetic behaviour, although they often correlate with metallurgical condition.

2.6 Practical consequences

• High permeability: Flux is established readily, but local changes in permeability can create strong boundary patterns.
• Low permeability: More magnetising force may be needed for a comparable surface field.
• High retentivity: Residual techniques may be possible, but post-test demagnetisation can be difficult.
• High coercivity: A stronger reverse field is needed during demagnetisation.
• Saturation: More current produces little useful gain and can increase particle background at geometry changes.
• Mixed microstructure: Welds, cold-worked regions and heat-treated zones can produce non-relevant linear or diffuse patterns.

2.7 Practical: reading an unknown response

If a component produces weak indications despite adequate equipment output:

1. confirm the field direction at the test surface;
2. verify the material and local microstructure;
3. measure or demonstrate field adequacy with an approved method;
4. check coating, surface condition and media performance;
5. avoid simply increasing current past a useful range; and
6. compare a representative known discontinuity or approved reference piece.

2.8 Common traps

• Calling magnetic flux density "field strength" without stating which quantity was measured.
• Describing diamagnetic relative permeability as a negative number; it is normally slightly less than 1, not negative.
• Assuming permeability is constant through a hysteresis cycle.
• Equating magnetic hardness directly with mechanical hardness in every alloy.
• Driving the component into excessive magnetisation to compensate for poor orientation.

Chapter summary

MT works because a ferromagnetic component provides a controllable magnetic circuit. Domains, non-linear permeability and hysteresis explain why materials magnetise differently, why residual fields remain and why local material changes can create legitimate leakage fields that are nevertheless non-relevant to service integrity.