Residual Magnetism
Residual magnetism is the magnetization remaining in a ferromagnetic part after the magnetizing current is removed, dependent on the material's magnetic <a href="/glossary/hysteresis">hysteresis</a> behavior and coercive force.
Definition
Residual magnetism enables the so-called residual MT method (especially on hardened tool steels) but can also cause downstream problems such as weld-pool deflection, bearing damage, or compass interference, requiring demagnetization.
The principle
The magnetisation left in a ferromagnetic component after the applied field is removed, determined by the material's coercivity and by how it was magnetised.
How it shows up in practice
It is useful in the residual technique of magnetic particle testing, and a nuisance almost everywhere else: it deflects arc welding, attracts swarf into machined surfaces and bearings, and disturbs instrumentation. Demagnetisation is specified where any of those follow.
Where the codes address it
ASME Section V Article 7 addresses demagnetisation requirements; ISO 9934-1 covers the practice.
Where practitioners get caught
Skipping demagnetisation before welding. Arc blow from residual magnetism produces weld defects that are then attributed to the welder rather than to the inspection that preceded them.
Where Residual Magnetism fits in an inspection programme
A term is only useful when it connects to a decision. Residual Magnetism appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Demagnetization — Demagnetization is the process of reducing the residual magnetic field in a part after magnetic particle testing, performed by passing the part through a decaying AC field or by AC current with reducing amplitude.
- Hysteresis (B-H Curve) — Hysteresis is the lag of magnetization (B) behind the magnetizing force (H) in ferromagnetic materials, producing the characteristic B-H loop whose width is governed by coercive force and whose intercept is residual magnetism.
- Magnetic Permeability — Magnetic permeability (μ) is a material property describing how easily it can be magnetized, equal to the ratio of magnetic flux density (B) to magnetizing force (H), and is a primary factor in MT current selection.
More physics terms
Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.