Magnetic particle inspection (MPI)
MPI is basically a combination of two NDT methods: Visual inspection and magnetic flux leakage testing. Developed in the USA, the magnetic particle inspection is extensively used to detect defects in casting, forging, and welding industry. MPI is simple, easy, fast, and very effective. This is the reason Magnetic particle test is used in a variety of industries like automotive, oil & gas construction, chemical, and petrochemical plant construction, structural steel, aerospace, offshore structures, power generation industries, and pipeline industries. This is also known as magnetic particle test or magnetic particle examination in NDT.
Basic Principle of Magnetic Particle Inspection
MPI uses magnetic fields and magnetic particles for detecting defects in ferromagnetic components. The basic principle of this inspection method is that the component specimen is magnetized to generate magnetic flux in the material which travels from north pole to south pole (magnetic flux exits at the north pole and enters at the south pole). Now if there is any discontinuity or flaws in the component, secondary magnetic poles are produced in the cracked faces. In this location, the magnetic field spreads out due to the air gap in the defect causing a magnetic flux leakage field. Such regions can be detected easily by using magnetic particles (iron powder), or a liquid suspension on the surface. Due to the magnetic effect, such particles are attracted to the flux leakage and make a cluster around the flaw making it visible. Refer to Fig. 1 showing the basic principle of magnetic particle inspection.
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The magnetic particles can be dry or wet. Normally, dry particles can be used up to a temperature 316 Deg C wet particles can be used up to a temperature of 50 Deg C.
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Steps for Magnetic Particle Inspection
The magnetic particle inspection (MPI) is performed in the below-mentioned six steps.
1. Surface Preparation:
All surfaces and adjacent areas (within 1 inch) that will be examined must be free from rust, scale, sand, grease, paint, slag, oily films, or other interfering conditions. Unusually rough or non-uniform surfaces may interfere with magnetic particle cluster formation making interpretations of the magnetic particle inspection method’s indications difficult.
2. Inducing a Magnetic Field:
This is the most important step in the magnetic particle inspection procedure. In this step, place the equipment on the area to be tested and induce a magnetic field. Various types of magnetic particle inspection equipment are available. Widely used industrial equipment are Permanent magnet, Electromagnetic Yoke, Current flow probes, Magnetic Flow, Flexible coil, Threading Bar, Adjacent cable, etc. Magnetization technique can be Longitudinal, Circular, or Multidirectional Magnetization. Equipment spacing in the inspection area is normally kept in between 3 inches to 8 inches. An ASME Pie Gauge or Burmag Castrol strip can be used to verify adequate magnetization of the part.
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3. Applying Magnetic Particles on the Test Surface:
Both dry and wet magnetic particles can be either fluorescent or non-fluorescent (visible, color contrast) and are available in a variety of colors to contrast with the tested material. So accordingly choose the required particles for the magnetic particle inspection and apply on the surfaces when the specimen is in magnetized condition.
4. Examine the component surface for defects
Remove the excess particles using light airflow and inspect the component for defects as per acceptable standards.
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5. Repeat the test by changing the magnetic field
Two separate examinations are carried out on each area to be tested. The second examination is performed with the lines of flux perpendicular to those used for the first examination in that area.
Refer to Fig. 2 that shows the above 5 steps. An electromagnetic Yoke is used in the test to inspect welding of two plates.
6. Demagnetization and Cleaning:
The presence of Residual magnetism in the component may interfere with the subsequent usage. Hence, the demagnetization shall always be performed on the parts once the magnetic particle inspection is over. The presence of residual magnetism can be verified using a calibrated Gaussmeter, Magnetic Field Meter, or a hall Probe Gauss meter. Residual magnetism must not exceed (+/-) 2 gausses.After that, the parts shall be cleaned to remove all residual magnetic particle materials. If wet fluorescent MPI was performed, the part shall be scanned with the backlight to assure that the cleaning is adequate.
Advantages of Magnetic Particle Inspection/Test
The main advantages of magnetic particle inspection/testing are
- Find flaws on the surface and near surfaces
- Fast examination method with an immediate result
- An easy method as compared to other NDT methods
- Portable and low-cost equipment.
- Defects are visible directly on the surface.
- Relatively safe method.
- Hot testing can be performed using dry particles.
- The shape and size of the cracks are indicated.
- Less training requirements.
Disadvantages of Magnetic Particle Inspection
The major drawbacks of magnetic particle inspection/examination are
- MPI is limited only for ferromagnetic materials like steels, cast irons, etc. Non-ferrous materials, cannot be inspected.
- The inspection is limited to small sections only. The examination of large parts may require the use of special equipment.
- Equipment must be calibrated, with no permanent record of the result.
- Before inspection thick paints (>0.005″) shall be removed.
- Post cleaning and demagnetization are normally required.
- Magnetic flux and indications must be aligned for proper results.
- Access may be a problem for the magnetizing equipment.
- Testing in two perpendicular directions required.
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