MAGNETIC PARTICLE TESTING (MT)
Magnetic particle testing is used to detect surface, surface-breaking or near surface discontinuities (in some conditions, up to a few millimetres deep) in ferromagnetic materials exclusively.
Although magnetic particle testing is more “restrictive” than penetrant testing, it is nevertheless preferred when applicable as it is much faster, among other things.
Nowadays, the growing use of magnetic particle testing can be explained by its reliability along with many recent major technical breakthroughs.
Although non-ferromagnetic alloys have been used more and more over the years, an increasing number of users have also been requesting multi-functional magnetic benches able to process both large and small parts. To achieve this, manufacturers’ design offices must be at their best and show enormous ingenuity.
Magnetic particle testing involves magnetising the part to be tested using a sufficiently strong magnetic field. The magnetic field lines distort when there is a discontinuity which generates a “magnetic leakage field”, also known as “magnetic flux leakage”.
A detection medium is applied onto the surface to be tested during magnetisation (simultaneous technique) or after magnetisation (residual technique). The black coloured and/or fluorescent detection medium is attracted in line with the defect by the magnetic forces, thereby forming indications.
These indications can be seen, under appropriate conditions, either under white artificial light or daylight or under ultraviolet radiation (UV-A) or blue actinic light according to the type of detection medium used.
The indications are detected even better when they are perpendicular to the magnetic field force lines. Two magnetisations orthogonal to each other are necessary to detect all surface discontinuities of a part. Longitudinal magnetisation highlights the transverse discontinuities (± 45°) and transverse magnetisation highlights the longitudinal discontinuities (± 45°).
2. Test method
There are miscellaneous techniques for magnetic particle testing:
- simultaneous technique or residual magnetisation technique;
- magnetic flow technique or current flow technique;
- longitudinal, transverse or multidirectional magnetisation;
- successive application technique of transverse or longitudinal magnetisation, i.e. a combined technique (transverse and longitudinal magnetisation applied one after the other, without intermediate observation);
- direct current (DC) or half-wave (HWDC) or full-wave (FWDC) rectified alternating current, alternating current (AC), full-wave rectified direct current (FWDC) (three phases), etc.
- or dry powder technique or wet technique.
The equipment used for magnetisation includes permanent magnets, portable electromagnets (yokes), current generators and magnetic benches.
Parts are demagnetised, when necessary, using demagnetisation equipment or any other appropriate device or technique.
Magnetic particle testing has been perfected significantly many times:
- In 1985: magnetic particle testing bench with continuous adjustment thyristor control and digital display and timers;
- in the mid-1990s: multidirectional magnetisation technique using alternating magnetic heads on a magnetic bench;
- in 1993: non-contact multidirectional magnetisation technique in 2D or 3D chamber;
- in 1995: appearance of the first alternating magnetic heads in France and growing use of the multidirectional magnetisation technique on magnetic bench;
- in 1997: first magnetic bench offering a computer-controlled process system (used to make the process reliable);
- in 1999: magnetic bench with interface via operator console with creation of “pre-recorded parameters” and automatic adjustment by PLC;
- in 2002: non-contact magnetisation by induced current flow technique;
- in 2004: computerisation of the operations room. User-friendly, adaptable touch screen. Creation and storing of “receipts”. Adaptive control.
Without entering into detail, let us clarify the following points:
Multidirectional magnetisation: a technique that can obtain a vector on a part from magnetisation rotating very quickly. This is normally obtained using a three-phase current: one phase magnetises in one direction whilst the second phase magnetises in another, noticeably perpendicular direction. The difference in phases causes the magnetisation vector to sweep all directions (360°).
This process can be implemented on a magnetic bench or in a non-contact magnetisation chamber.
Non-contact magnetisation by induced current flow: This technique involves generating a current in a normally “closed” part forming an electric circuit with itself (annular or tubular part, for example) and assimilating this circuit with a transformer secondary. The transformer primary is generally the magnetic circuit or the electromagnet from the magnetic bench.
Today, multidirectional magnetisation on a magnetic bench remains the most common technique (basically for reasons of cost), but multidirectional magnetisation in a chamber is preferred in some applications. Induced current remains expensive and therefore is used infrequently, but this situation will more than likely change in the years to come.
Magnetic particle testing is thus a method used widely in NDT and more especially for both manufacturing and maintenance in such sectors as transport (aerospace, automotive, railway, marine, ski lifts), energy (petroleum, thermal, hydraulic, nuclear), boiler-manufacturing, metallurgy (foundry, forging), mechanics, agri-food (sugar refineries, etc.), cement works, chemical complexes, defence, thrill rides, etc.
Magnetic particle testing is used to test iron, cast iron, forged steel parts, welds, metal sheets, tubes, in fact, all sorts of parts with simple or complex geometry, provided that their constituent material is ferromagnetic.
The method complements the ultrasonic testing or eddy current methods. Where ultrasounds can detect shallow near surface discontinuities, magnetic particle testing reveals all surface-breaking discontinuities (just a few micrometres wide) and some shallow near surface discontinuities. Unlike eddy currents, it is not particularly sensitive to the geometric effects; it is not limited to local testing.
Magnetic particle testing is, in fact, one of the so-called “global” methods, that enables the inspection of an entire part to be performed in a single operation. The relatively quick tests can be conducted equally well on 10 mm-long screws, electric diesel locomotive crankshafts or aircraft landing gear.
4. Advantage of the method
- Global method;
- detection mainly of surface-breaking discontinuities;
- testing of parts of a few millimetres to several metres long;
- relatively fast and inexpensive inspections;
- significant resolution;
- robust equipment, can be used in difficult environments.
- Testing limited to ferromagnetic parts;
- method cannot be entirely automated;
- sometimes difficult to detect near surface discontinuities (depending on their size, depth, etc.);
- need to use chemical products.
5. Related standards
Standards currently in force
- NF EN ISO 12707
Non-destructive testing - Magnetic particle testing - Vocabulary
- NF EN ISO 3059
Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions
- NF EN ISO 9934-1
Non-destructive testing - Magnetic particle testing - Part 1: General principles
- NF EN ISO 9934-2
Non-destructive testing - Magnetic particle testing - Part 2: Detection media
- NF EN ISO 9934-3
Non-destructive testing - Magnetic particle testing - Part 3: Equipment
Text prepared by COFREND in conjunction with Pierre Chemin.