Published on 02-Dec-2021

Debunking myths related to Magnetic Particle Inspection

Debunking myths related to Magnetic Particle Inspection

Magnetic Particle Inspection (MPI) is a very popular, low-cost method to perform nondestructive examination (NDE) of ferromagnetic material. This method is used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection. 


Also known as Magnetic Particle Testing, it is a fairly simple process with two variations: Wet Magnetic Particle Testing (WMPT) and Dry Magnetic Particle Testing (DMPT). In either one, the process begins by running a magnetic current through the component. Any cracks or defects in the material will interrupt the flow of current and will cause magnetism to spread out from them. This will create a “flux leakage field” at the site of the damage.

How does Magnetic Particle Inspection works

When ferromagnetic material (typically iron or steel) is defect-free, it will transfer lines of magnetic flux (field) through the material without any interruption.

 

But when a crack or other discontinuity is present, the magnetic flux leaks out of the material. As it leaks, magnetic flux (magnetic field) will collect ferromagnetic particles (iron powder), making the size and shape of the discontinuity easily visible.


However, the magnetic flux will only leak out of the material if the discontinuity is generally perpendicular to its flow. If the discontinuity, such as a crack, is parallel to the lines of magnetic flux, there will be no leakage and therefore no indication observed. To resolve this issue, each area needs to be examined twice. The second examination needs to be perpendicular to the first so discontinuities in any direction are detected. The examiner must ensure that enough overlap of areas of magnetic flux is maintained throughout the examination process so discontinuities are not missed.
Magnetic Particle Inspection Testing methods

The magnetic particle inspection can be performed using two methods, wet as well as dry one, depending on the application. In both these methods, the product is electrically charged to create a magnetic field. The next step involves applying ferrous iron particles to the part in a dry or wet suspension.

The next step is detection of any flaws or other defects in the product. The particles will be attracted to the flaw and build up around the leakage giving us an “indication.” The indication is then evaluated and the cause of the defect is determined. 

Metals that can be inspected using Magnetic Particle Inspection

Magnetic Particle Inspection can be used to inspect a lot of metals. However, it is very important to know which metal the job is made of. Is the metal ferrous or nonferrous? A point to take into consideration is that we can only do MPI on metals which are ferrous in nature. 

To avoid the confusion, one should carry a magnet to check the nature of the metal. Examples of ferromagnetic materials are iron, nickel, cobalt and some of their alloys.

There are certain myths related to ferromagnetic particles which we’ll be busting in this article.

Myth: Bath does not have a life; it can be used for a long time

Bath gets contaminated due to a lot of things like oil, sand, dirt which can give us false indications. According to ASTM-E-1444 if the total volume of the contaminants, including bands or striations, exceeds 30 % of the volume of magnetic particles the bath must be adjusted or replaced.

Myth: Current should always be more

More is not always better. If we pass more current through a job, then the fluorescent particles will not only attract the flaws but the whole job will glow. So, this can mask the defects and may give us false results.

Myth: All material magnetizes the same way

No, permeability and retentivity is different for different materials. 

Myth: Wet and dry MPI are the same

Wet and dry MPI are very different. While wet MPI is used for shallow and fine surface crack, dry MPI is good for locating surface and sub- surface indications. Wet MPI can test a variety of different geometry types while dry MPI is easy to use on large objects with portable systems.

Myth: Pre-cleaning and post-cleaning of the job isn’t much important

Pre-cleaning is the first step in MPI. It comprises cleaning the job of all the contamination as it could give false indications. This step can save a lot of time for future tasks. Post cleaning is generally not necessary for a visible method but for wet methods it is advised to dry the part being inspected as it can cause corrosion.

Myth: Magnetizing isn’t related to part geometry

Discontinuities are difficult to detect by the magnetic particle method when they make an angle less than 45° to the direction of magnetization. To ensure the detection of discontinuities in any direction, each part must be magnetized in a minimum of two directions at approximately right angles to each other. Depending on part geometry, this may consist of circular magnetization in two or more directions, multiple circular and longitudinal magnetization, or of longitudinal magnetization in two or more directions.

Myth: If we add more particles, it will be easier to detect indications

The particle concentration should be in proportion. It shouldn’t be more or less. Otherwise it will give excess background. Particle concentrations outside of the range of 0.1 to 0.4 mL in a 100 mL bath sample for fluorescent particles and 1.2 to 2.4 mL in a 100 mL sample for non-fluorescent particles shall not be used. Just keep in mind fluorescent particles and non-fluorescent particles shall not be used together.

Myth: You can magnetize through coating

As per the ASTM-E709, thin non-conducting coatings like paint (0.02 to 0.05 mm) will not normally interfere with the formation of indications, but they must be removed at all points where electric contact is to be made for direct magnetization. If non-conducting coating/plating is left on the area to be examined that has a thickness greater than 0.05mm, it must be demonstrated so that discontinuities can be detected through the maximum thickness applied.

Now we are sure you must have got a better understanding of the myths surrounding Magnetic Particle Inspection. 

Conclusion:

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