Magnetic Particle Inspection(MPI)- All you need to know
Magnetic Particle Testing uses one or more magnetic fields to locate surface and sub-surface discontinuity in ferromagnetic materials.
Magnetic Particle Inspection(MPI) is one of the NDT methods where magnets are used to detect flaws in ferromagnetic materials.
In this article, you are going to get detailed knowledge about Magnetic particle Inspection(MPI), from basics to advanced in an understandable manner at your fingertip, right in front of you.
So, Stay with me and I will take you on the journey of knowledge. Without further delays, let’s get straight to the mainstream.
What is Magnetic particle inspection(MPI)?
An NDT method that detects flaws in Ferromagnetic particles(Ex. Iron, Nickel, cobalt, and their alloys) through magnetic field lines, is called magnetic particle testing(MPT).
It is the oldest and most widely used test in NDT methods.
Principle of Magnetic Particle Inspection-
A strong magnetic field is applied to the test object, if there is any discontinuity on the surface of the test object, magnetic particles get stuck inside the flaws due to magnetic leakage.
The flaw can be seen with the help of magnetic particles.
How does Magnetic Particle Testing work?
When a test object is placed inside the magnetic poles, it releases a strong magnetic field that magnetized the whole test object, flowing magnetic lines from one pole to another(ie. north pole to south pole).
If any flaws or discontinuity occurs between the magnetic field, magnetic leakage start to happen, because magnetic lines lose their path in the air.
Then a powder form of magnetic particles is sprinkled on the surface between the magnetic poles. These magnetic particles will get attracted inside the flaw, due to magnetic leakage.
Now, Rest powered will be cleaned and those attracted particles will be left alone. Now, you can easily identify the shape, area and depth of the discontinuity.
This figure will help you to understand more clearly, Have a look…
Fig. Diagram of How Magnetic particle Inspection Works-
➤ Here is the video guide to understanding deeply How MPI works. Have a look…
Techniques of Magnetic Particle Inspection-
There are mainly Two techniques used When Magnetic Particle Testing is done. These two methods of MPI is given below-
Dry Powder Technique
1. Dry powder Technique-
The Dry MPI usually chooses to detect flaws on the flat surface. But it is not advisable for depth inspector or sub-surface inspections due to the low range of penetration and sticking capacity. Dry particles can hold temperatures up to 316℃.
2. Wet Technique-
The wet method is mostly and widely used in MPI, due to the flexibility of position selection. The wet technique is suitable for depth penetration and sub-surface discontinuity inspection. Water-based magnetic particles are advised rather than petroleum-based particles in the wet method due to reduced magnetic permeability.
Methods of Magnetization-
There are two types of methods by which a test object can be magnetized. Those methods are-
1. Direct Magnetization-
When current passes directly to the test object and gets magnetized through the coil on the object surface, known as the direct magnetization method.
2. In-Direct Magnetization-
The magnetic field is induced by using a strong external magnetic field without passing a current through the test object, The in-direct method can be done by using a permanent magnet or an electromagnet device like a yoke. Known as In-Direct magnetization.
➤ Although a Permanent magnet produced a fixed magnetic field and cannot be increased or decreased according to the need, Electromagnets are preferred use.
Electromagnet gives flexibility in producing magnetic fields.
The procedure of Magnetic Particle Testing-
There are several magnetic particle testing steps by which MPI go through-
The ferrous material test object is get cleaned properly.
Now, The test object is placed in the MPT test area, If possible.
The working status of the Electromagnetic yoke is checked through a field indicator(Mostly used Pie gauge)
After checking the working status, Yoke is placed between the determined test area.
A contrast is sprayed on the determined test area to get better contrast and visibility during the inspection.
Yokes induced the Magnetic flux in the test object.
Yoke is placed cross-sectionally side by side, maintaining 45 degrees of each move to avoid parallel magnetic field lines in the testing area.
Now, a wet or dry magnetic particle(Mostly used iron powder) is sprinkled with the help of a blower or container(if, using the wet technique).
Now, After completed inducing, the excess powder is blown off through a special blower, which blows air in a controlled manner to prevent attracted particles from blown off.
Now, the Rest of the magnetic particles stick inside the flaw of the test object due to the magnetic flux leakage.
Then, According to the shape and formation of particles, an inspection is done.
As shown in the figure above, the stuck particles indicate an undercut in the weld test piece.
Equipment used in Magnetic Particle Inspection-
The various equipment are used in MPT, Given below-
Cleaning Agent- Cleaning agent is used for cleaning the surface of the object.
Contrast Coating Agent- Contrast coating is used to give the surface a clearer base to see the magnetic particle during the inspection. The mostly white coating agent is used in MPI.
Field indicator- A field indicator is used to test the workability of an Electromagnetic device(ie. yoke). A pie gauge is also used to test the workability of the EM device.
Magnetic particles- Magnetic particles are used to attract inside the flaw when magnetic leakage happens in the discontinuity path. Mostly iron powder is used to apply as the magnetic particle. These particles can either be dried or wet.
Electromagnetic Yoke- Electromagnetic Yoke is used to inducing a magnetic field in the test object.
Blower- Low-pressure blower is used to subside the excess magnetic particles or powder on the surface.
Advantages of Magnetic Particle Inspection-
Here are some Advantages of MPI, Given below-
1. Simple Process-
MPI is the simplest method of Non-destructive Testing, Where little training is required to perform the general test of MPT.
2. Cost-Effective Process-
Equipment used in MPI is less costly than other NDT methods. So, MPT is the most cost-effective method rather than other NDT methods.
3. Surface and Sub-Surface Detection-
MPT have the ability to detect Surface flaws as well as Sub-surface or near-surface flaws, detailing with shape, size and length of flaws.
4. Complex Casting Inspection-
MPI can easily detect discontinuity in complex object. All thanks to the flexibility of the testing position in MPI.
5. Large Object Inspection-
MPT has the ability to inspect larger and complex castings due to less equipment and flexibility in the move.]
We all know that all NDT methods have their own disadvantages over advantages. MPI is one of them.
So, here are some Disadvantages of MPI-
1. Ferromagnetic Materials Inspection Only-
MPI has major disadvantage over other NDT methods is that MPI can inspect only Ferromagnetic Material(ie. iron, cobalt, nickel, and most of their alloys). Non-ferrous materials(ie. zinc, lead, copper, aluminium, brass etc.) are not advised to inspect due to less conduction of magnetic field lines in these materials.
2. Contamination can affect the result-
The dirty and dustier test object can restrict or reduce the penetration of magnetic particles and reduce the magnetic field propagation. That’s why the object surface is cleaned properly before proceeding with inspection.
3. Magnetic field direction-
Flaws parallel to the magnetic field go undetected. Therefore, Vertical and horizontal placement of magnetic fields is suitable to detect flaws accurately.
Limitations of Magnetic particle Inspection-
MPI also has limitations over the other NDT methods, Given below–
ASTM A456 Large crankshaft forgings Specification of MPI
ASTM E709 Guide For MPI testing
History of Magnetic Particle Inspection-
In 1868, Ferromagnetic materials were inspected by magnetizing them and passing a magnetic compass over them. Then also, finding magnetic leakage in the magnetised area is the key point to finding flaws in those objects and at that time no magnetic particles or any type of ferrous powder is used.
In 1920, W.E hoke discovered the uses of magnetic particles in the detection of flaws in magnetised material and from there the base ground of MPI was laid down.
In 1945, A bunch of professors started developing MPI and they refined and improved the overall workability of the method to give more accuracy and reliability among the method.
After much development and research, more reliable and accurate than ever MPI has been now. All thanks to those professors and researchers.
Magnetic particle inspection is one of the oldest and most widely used in various industries due to its simplicity and convenience in use and rest, you already know after reading this article, so I am not going to repeat those again.
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A desire to read more? The following articles are recommendations for you to read further:
Q. What is the basic principle of magnetic particle testing?
Ans. Magnetising of ferrous materials and using magnetic particles to detect flaws, When magnetic leakage is happening. This is the basic and general principle of MPI.
Q. What is a magnetic particle test used for?
Ans. Magnetic particle testing(MPT) is used for detecting flaws in ferrous materials by magnetising them.
Q. How do you test magnetic particles?
Ans. A ferrous material test object is cleaned and applies contrast coating to increase visibility and sharpness overall. The Electromagnetic yoke is placed over the determined area and sprinkled with magnetic particles to get penetrate where magnetic leakage is happening. This is a short and simple procedure of MPI.
Q. Which material can be tested using magnetic particle testing?
Ans. Only ferrous materials can be inspected using Magnetic particle testing, Non-ferrous materials are not advised to inspect due to reduced propagation of magnetic field lines.
Abhishek Tiwary is a blogger by passion and a Quality Engineer by profession. He completed his B.Tech degree in the year 2017. Now working in a reputed firm. He loves to share his knowledge with others.