Magnetic particle testing in NDT

magnetic particle testing
Written by writerchetan

Magnetic particle testing is a type of non-destructive testing method in which discontinuities of a ferromagnetic material, the specimen on the surface or near to surface (subsurface) are detected without damaging it, ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys,

There are many types of non-destructive testing methods in non-destructive inspection, including the MPI inspection method, magnetic testing is a method performed by magnetic energy, so it is called magnetic particle inspection,

magnetic particle testing procedure

In this method a ferromagnetic material, the specimen is directly or indirectly magnetized, which creates a magnetic field in the material, after which magnetic iron particles are applied to the surface of the material,

After applying the particles, the iron particles deposited on the surface of the materials are accepted and rejected by detecting discontinuities and defects according to the acceptance criteria and code of magnetic particle testing, ASME sec. V, article 7 and asme sec.viii.

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When magnetic particle examination on the material is completed, after that the material is demagnetized,

Note: Most of the demagnetize in mpi testing is done only when there is a possibility of problems in any further process of that material,

When doing this mpi testing on any material, the NDT magnetic particle testing procedure is followed at that time, this procedure is made by ndt level 3 in any company, organization, etc. keeping in mind the ASME codes and standards, etc.,

magnetic particle testing principle

The principle of mpt testing is magnetic lines of force and flux leakage by which discontinuities are detected on the surface of the specimen,

In this, if there is any kind of discontinuities between the magnetic lines of force on the surface of a specimen, then on applying dry magnetic powder and wet magnetic particles,

those magnetic particles get deposited at the place of that discontinuities and there are continuities in the magnetic lines,

In this method, there are discontinuities between the magnetic lines of force where dry powder and wet magnetic particles get deposited and the NDT inspector conducts a magnetic particle examination of that place,

NDT technician recognizes the shape (indications) of the accumulated magnetic particle and accepts and rejects it according to the ASME code,

magnetic particle testing equipment

In ndt magnetic particle testing, some equipment is used to magnetize any ferromagnetic materials, specimens, in which portable and fixed equipment are used like coils, yokes, prod, headshots, central conductors, etc.

With the help of this equipment, a magnetic field is created in a specimen by alternating current (AC) and full-wave DC (HWDC), FWDC,
In this, magnetic testing is done according to the equipment mentioned above and mainly mpi testing is done in two ways –

  • Dry powder particle testing
  • Wet particle testing

Dry iron magnetic particle is used in dry powder magnetic testing, this magnetic powder comes in different colors, in general, red and black dry magnetic particle powder is more commonly used everywhere,
Wet ndt magnetic particle testing is done using liquid iron magnetic particle, in which wet particle testing is done in two ways, in which wet visible or wet fluorescent method comes,
In the wet visible method of magnetic non-destructive testing, mostly black magnetic particles are used and in the wet fluorescent method, magnetic particles are used according to UV light, these particles appear in yellow, green, etc. Due to UV light (ultraviolet light),
mpi testing equipment which is used in magnetic nondestructive testing

  • Magnetic field generators like – Yoke, coil, headshots, prod, central conductor, etc.,
  • Iron particles like – dry powder and wet particles,
  • Ultraviolet light (UV lamp),
  • Ultraviolet light meter
  • Lux meter
  • Temperature gun
  • Lifting block like – 18.1 kg, 4.5 kg block and others as per requirement, code and standards,
  • Gauss Meter, Field indicator (pie-shaped magnetic field indicator),
  • centrifugal tube,
  • MPI sensitivity block etc.

magnetic particle testing advantages and disadvantages

This magnetic nondestructive testing has its advantages and disadvantages which are as follows-

advantages of mpi inspection

  • Low cost and time-saving method,
  • Portable method,
  • Easy to found surface and subsurface (near to surface) defects,
  • Easily inspection method in Large and small object,
  • Indications and defects present fast,
  • High sensitivity method
  • Pre-cleaning is not necessary etc.

disadvantages of magnetic particle testing

  • the skilled person required in non-relevant indications interpretation time,
  • Method Use on only ferromagnetic materials and austenitic stainless steel material can not be inspected in MPI,
  • In mpi, most the techniques need a supply of electricity,
  • If thick paint is available on the specimen surface the mpi testing method cannot be used,
  • Equipment calibrations required if damage and repair,

magnetic particle testing welding

magnetic yoke mpi inspection on welding – Some important information related to magnetic particle examination and magnetic yoke inspection on welding –

If you do yoke magnetic particle inspection, then you should do internal yoke calibration from time to time, in which you should know yoke lifting, magnetic field accuracy,
While doing mpi on any ferromagnetic materials, you should find out whether the magnetic field in that object is equalizing or not by placing a pie-shaped magnetic field indicator on that material.
do magnetic testing only by keeping the yoke in AC mode, because in this you will easily get the discontinuity,
If you use the DC mode in the magnetic yoke method, the magnetic iron particles will collect in large quantities near the legs of the yoke and at other places, which can cause problems to interpret,

A perfect mpi test for welding is one in which you keep the yoke in four directions while doing magnetic yoke inspection, you can easily catch any crack as in the example given below,

magnetic yoke

magnetic yoke

yoke magnetic particle inspection

yoke magnetic particle inspection

mpi inspection

mpi inspection

mpi testing

mpi testing

By the way, to save time everywhere, the weld from magnetic yoke is checked by two methods, perpendicular and parallel,

With the magnetic yoke method, you should position the yoke legs in such a way that the HAZ part is in between the yoke legs along with the weld, as most cracks are found around the weld as well as the HAZ part,

You can inspect the HAZ part on either side of the weld as in the example below,

mpi test for welding

mpi test for welding

If the NDT technician and NDT inspector are confused in any non-relevant indications interpretation, then you should again inspect the magnetic particle to clear that confusion until you are confirmed, you can ask your senior person if there is any confusion in the interpretation.

Final word

If you want other educational information related to ndt method and updates of the latest NDT jobs then you can go to the nondestructive testing page of our website,
This magnetic particle testing article is written for educational purposes, you can get more information about NDT from the websites of Wikipedia, NDT organization, NDT institute, etc.

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