The magnetic particle test is used for magnetizable materials. The detection sensitivity is usually significantly higher compared with the dye penetrant test. The test is cheaper compare to PT but only limited to ferromagnetic materials.
Furthermore, it is a practical way of testing fillet welds in steel construction. The test can detect only surface and limited sub-surface discontinuities.
Magnetic particle testing (MT) would technically fall within the topic of electromagnetic testing, but given its widespread usage and historic separation as a method of its own, this book will follow suit by considering it independently. The closely related technique of magnetic flux leakage testing, which mainly differs from MT in the detector—a hall effect sensor versus magnetic particles—will be included in a separate section.
How Magnetic Particle Examination Works
During the magnetic particle test, a strong magnetic field is introduced into the component. Manual yoke magnets are often used. The conductivity for magnetic fields (permeability) is very high in steel which are commonly used for machine, steel plant construction. Any discontinuities, such as lack of fusion or cracks, create significant resistance for a magnetic field. At such discontinuities, the magnetic field escapes at the surface of the test piece (leakage flux). Extremely fine magnetic particles, which are mixed to a paste in water or oil, show the location of the leakage flux and/or the cracks or lack of fusion.
Fig. below shows schematically the pattern of the field lines and the crack indication when testing a weld.
Magnetic Particles (MT) Techniques
Various techniques of Magnetic testing are used in different applications. The ASME Section V, Article 7, specified five MT methods or techniques as given below:
- Prod Method
- Longitudinal magnetization method
- Circular magnetization method
- Yoke method
- Multidirectional magnetization method
For testing, two different examination media are used which are Wet Particle type and Dry particle type.
They are either fluorescent or non-fluorescent (visible, color contrast) and come in different colors to contrast with the material to be inspected.
Most Commonly used MT method
1. Stationary horizontal system, using longitudinal magnetism and
2. circular magnetization Method.
3. Portable yoke Method.
No discontinuities would be detected in MT without ferromagnetic particles; the particles may be used dry or wet. Ferromagnetic particles may also be colored such that inspections are performed under ambient visible light, under ultraviolet radiation, or under a combination of the two, referred to as dual-use. Dry powders often utilize iron filings, while wet particles generally employ some form of iron oxide. Wet fluorescent oxide particles are encapsulated within an epoxy shell; a fluorescent pigment in this shell absorbs excitation energy, often UV-A radiation, and emits visible light. Typical yellow-green particles emit visible light with a wavelength of approximately 520 nm.
MT inspection of long seam using Yoke Technique
Field-flow longitudinal magnetization may be induced with a coil or using an electromagnetic yoke. A rigid five-turn coil or a flexible conductor may be wrapped around the specimen. Sample geometry, the relative diameter of a specimen versus the magnetizing coil (fill factor), and sample position within the coil—each plays a significant role. Specimen shape is generally described in terms of an aspect ratio, which is length (L) divided by diameter (D). When magnetic poles formed at the ends of a coil-magnetized part are very close together, as would be the case when the aspect ratio (L/D) is less than 3, their opposing fields work fiercely against the magnetizing force, and this opposition decreases the overall field strength within the part. The other field-flow longitudinal magnetization option for wet horizontal benches is a set of flux flow coils, which also act as the headstock and tailstock. This type of field-flow longitudinal magnetization uses two copper-covered laminated cores surrounded by induction coils as pole extenders, which
induce the magnetic field into the test object.
Before use, the magnetizing power of MT Yoke needs to be verified within the last year. An AC yoke must have a lifting power of a minimum of 10 pounds and a DC yoke of at least 40 pounds (lb).
Steps for MT (Yoke Method)
The below steps shows the basic ways to perform MT with the dry powder, non-fluorescent, yoke method. Before the start of the inspection, all MT equipment and meters need to be calibrated as per ASME Section V, Article 7 requirements.
Before the inspection, light intensity needs to be verified as per the ASME Section V, Article 7 for the visible MT technique. The minimum light intensity shall be 100 footcandles on the part surface. This must be checked and verified with a calibrated light meter. For fluorescent MT, a black light intensity of a minimum of 1,000 microwatts per square centimeter is required on the part to be tested.
Advantages and Disadvantages of Using Magnetic Particle Examination
MT is a widely used method that may be applied to ferromagnetic parts of nearly every size and shape. With the proper magnetizing waveform, MT may be used to inspect complex shapes, such as gears, splines, and crankshafts. This method is sensitive to surface and near-surface discontinuities, and the inspection process is quite rapid. Inspections may be performed in nearly any location and across a wide temperature range. Unlike many other NDT methods, MT indications are visible directly on the surface.
MT is limited to ferromagnetic parts and may miss large discontinuities if the magnetic flux and discontinuity are not properly oriented. Coatings on the sample also limit test sensitivity; a coating may need to be removed prior to testing if it is too thick or if the highest sensitivity possible is required. Post-inspection demagnetization is generally required, as strong residual magnetism can cause problems later during machining or during end-use. Post-inspection cleaning is sometimes required to remove the particles and also to apply corrosion inhibitors.