Magnetic Particle (MT)

Magnetic particle inspection is used primarily to detect cracks on ferrous materials.

 

Magnetic particle is often called Magnaflux, MPI, MT, yoke check, or coil check. They are all essentially the same thing being that Magnaflux is a name brand.

 

So what is magnetic particle? In simple form, you are taking a part which is ferrous in nature (attractable by a magnet) and magnetizing it. By magnetizing the part you are making any opening or void on the surface or close to the surface its own little magnet, having its own attraction strength due to the leakage field which it has created. This little magnet or leakage field is then found by applying an iron particle (small shavings of iron) to the surface and seeing where they are attracted at.

 

MT – Magnetic particle testing may only be performed on ferrous materials. Keep in mind that a conductive material and ferrous materials are two different things. The selection of the process is normally broken down into circular or longitudinal, visible or fluorescent. When inspecting critical components, the fluorescent method is usually selected because of its contrast value of inspecting in darkness. Visible is normally utilized for field applications of weld inspections.

 

When we say fluorescent we are saying that the inspection needs to be performed in a darkened room with a black light because the iron particles are fluorescent in nature and will light up when excited by a black light. Visible particles are simply visible and the inspection can be performed under normal lighting or a flashlight. A key component of both methods is lighting requirements.

 

Most procedures require coverage of the inspection in two directions and this is usually performed through the use of longitudinal or circular applications. Magnetization of parts using traditional methods occurs in one direction at a time. Cracks or voids that you are seeking can only be found perpendicular to the magnetic field because the crack has to disrupt the magnetism in order to form a leakage field. If your magnetic field is in the same direction as the flaw you will probably not see the crack.

 

When speaking of electrically induced magnetization it is said that cracks are found in the direction of the electrical field. This is easily explained by stating that magnetization occurs at 90 degrees to the electrical field and cracks occur at 90 degrees to the magnetic field.

 

Equipment is available in portable and stationary configuration. Portable instrument is usually much weaker than the stationary counterpart, but they serve an important purpose of making the process portable. Most portable equipment is used for weld inspections.

 

One of the main concerns of the process is ensuring that after the inspection the technician demagnetizes the part. Magnetism on a rotating or moving part like an engine could be detrimental by attracting metal particles and causing wear or catastrophic failures.

 

Note: Magnetic particle is relatively simple to use but can be very easily done inappropriately making the inspection useless or causing harm to the inspector if performed by an untrained person.

 

A critical mistake performed with magnetic particle is not ensuring that the part is appropriately magnetized with a gauss meter, also often called a field indicator. A field indicator is similar to a compass – it has a needle to show the presence of a positive or negative magnetic field. If the inspector does not verify the magnetization, the inspection could be nonexistent since magnetism is not visible to the human eye.

 

Another tool utilized to determine field direction is a pie gage or shim. These are good devices for quick verification of field direction, but are subject to external fields and may be misleading at times. ASTM E1444 covers these devices and how to use them.

 

For magnetic particle testing proper lighting is important. For visible, you usually need 100 fc (foot candle) of white light at the surface of the part that you are inspecting. This is to assure that you can see the flaw adequately. Many people overlook this requirement

 

For fluorescent testing the light requirements are tougher. Hence, the reason most people try to accomplish visible even if it is not appropriate. You usually have to be in a dark area not to exceeding 2 fc (foot candle) and the black light has to be able to produce 1200 µW/cm² (microwatts centimeter squared) at the surface. In simple terms, you have to be in a very dark room with a black light much stronger than what you see at a dance club. If you do not meet these minimums, your sensitivity will be off and you will probably miss what you are looking for. The flipside to this statement is that your black light or white light can be too bright and you can miss cracks because of glare, which they can produce.

 

Remember that when we are talking about missing a crack we are referencing the possibility of an inspector missing a crack on an airplane, railroad wheel, fuel tank, ship hull, pipeline, etc. We are usually talking about the possibility of people getting hurt if the technician is doing their job improperly. This is why NDT magnetic particle technicians should be properly qualified and certified to a Level II when performing examinations. Qualification is referencing classroom training and experience hours. Certification is referencing that the individual has passed a program by testing and demonstration of their capabilities in accordance with a code such as SNT-TC-1A or NAS-410 and has been reviewed by a Level III. Facilities are supposed to have a primary Level III on file for code conformance. If you do not have a Level III on file we can provide that service for you (shameless plug # 2) and keep you code compliant.

 

If you are a quality assurance manager or a company outsourcing NDT, I strongly recommend you ensure that the facility providing your services has not reduced code requirements. You can ensure this by requesting the personnel records of the technician doing the work and verifying them with the applicable standard. This is a normal practice and on request the contractor should provide you with the information within an hour. If they do not, you should question their services and perhaps hire a third party ASNT Level III to provide an onsite audit of their facility. Common mistakes found on certification paperwork include not enough training or experience hours, training facility and person that provided the training not listed, and method or technique not listed.
Is magnetic particle testing the answer for your specific method? It depends on many variables. Keep in mind that nondestructive testing includes many different methods. In conclusion thank you for taking the time to read our article. For any of your nondestructive needs, we hope you consider Aqualified LLC in the future. Feel free to email or call me directly with any questions.