Non-destructive testing or NDT is a set of inspection and analytical methods utilising specific tools and equipment to assess the properties of a materials, systems or components without damaging it or affecting its serviceability or functionality.
There are different types of NDT methods, and one of the more commonly used techniques is magnetic particle testing or MPT (aka Magnafluxing or magnetic particle inspection or MPI).
Just like other NDT methods, magnetic particle testing is widely used in the mining sector. If you’re wondering whether MPT is something that could prove useful to your business, then you’re sure to find this comprehensive guide helpful.
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What Is Magnetic Particle Testing?
First, let’s answer the question: what is magnetic particle testing?
As mentioned earlier, magnetic particle testing is a non-destructive testing method. Similar to other NDT methods, MPT or MPI is used to detect surface cracks or flaws in ferromagnetic (easily magnetised) materials, such as steel, nickel, cobalt, iron and their alloys.
Magnetic particle testing makes use of tiny magnetic particles and magnetic fields to reveal flaws or discontinuities in various products, components or parts, such as castings, weldments and forgings. The primary objective of the process is to determine whether the components being tested are fit for use.
Since the process does not require the same level of surface preparation needed for other NDT methods, conducting MPT is relatively faster and easier – and this is also why it is one of the most preferred and commonly utilised NDE techniques.
Magnetic Particle Testing Diagram
The above magnetic particle testing diagram is a simple illustration of the basics of MPT.
Magnetic Particle Testing Steps/Procedure
How does magnetic particle inspection work?
Before learning the steps and procedure in MPT though, it’s crucial to know the role of magnetic particles in the process.
The Role of Magnetic Particles
From the name of the testing method itself, you already have a fair idea that magnetic particles play a significant role in the application of MPT. The magnetic particle to be used is crushed iron or iron oxide. The magnetic particles are pigmented to make it easy to identify any defects found in a specimen.
Moreover, the particles used must demonstrate a high level of magnetic permeability so they are quickly attracted to the tiny magnetic leakage fields of the cracks. The particles should also have a low level of retentivity, so they don’t stick to the surface or with each other. Since iron has low retentivity and yet is easily magnetised, it is an ideal material for MPT.
Magnetic Particle Testing Types and Methods
The magnetic particles used in MPT can be used in the form of a dry mix or a wet solution.
The two types of magnetic particle testing methods are named after the type of magnetic particles used: wet magnetic particle testing (WMPT) and dry magnetic particle testing (DMPT).
The magnetic particle testing steps or procedure depends on whether dry or wet magnetic particles are used.
Dry Magnetic Particles
Dry magnetic particles are a mixture of fine and coarse particles ranging from 50 µm to 150 µm. They come in different colours like yellow, red, black, and others. Dry MPT is recommended for testing rough surfaces. It is also capable of detecting shallow subsurface flaws.
Below are the steps in performing a dry magnetic particle test/inspection:
- Surface Preparation: The ultrasonic cleaning method is used to ensure the surface of the test component or specimen is clean and free from moisture, oil, or grease.
- Introduction of Magnetising Force: A magnetic field or flux is introduced by using permanent magnets, a coil, an electromagnetic yoke, or something similar.
- Application of Dry Magnetic Particles: A thin layer of the ferromagnetic medium (crushed magnetic particles) is sprinkled on the specimen while it is still magnetised.
- Removal of Excess Particles: While the magnetising force is being applied, any excess powder should be removed by applying low-pressure dry air from a syringe, bulb, or a similar source. Note that the force of air should be controlled to ensure it doesn’t blow off the particles that have adhered or are stuck to the magnetic flux leakage field.
- Termination of the Magnetising Force: The magnetising force is terminated; however, the permanent magnets may be left in place if they are still being used.
- Inspection and Interpretation: The magnetic particles cluster in the faulty section/s. Observations are recorded, interpreted and evaluated according to acceptance standards.
Wet Magnetic Particles
Wet magnetic particles are more sensitive than their dry counterpart. They are also smaller (10 µm and below), which makes them more mobile and adherent. Wet particles come in fluorescent shades in the form of an oil or water suspension. Wet MPT is recommended for detecting minute discontinuities on smooth surfaces. It is also ideal for testing larger areas, as the wet particles are easy to apply.
To perform a wet magnetic particle inspection, follow these steps:
- Surface Preparation: Follow the same procedure in step 1 of dry MPT.
- Application of Wet Magnetic Particles: Carefully spray on or pour the magnetic particle suspension on the component or specimen.
- Introduction of Magnetising Force: Quickly apply the magnetising field after the suspended magnetic particles are poured or sprayed on the specimen.
- Inspection and Interpretation: Just like in dry MPT, the magnetic particles will cluster in the faulty area. While there is a sharp indication of surface discontinuities, subsurface flaws tend to be less defined. Infrared lights are sometimes used to get a better vision of the faults on the surface of the specimen. Observations are then recorded, interpreted, and evaluated according to acceptance standards.
What NDT Magnetic Particle Testing Equipment Is Required?
There are various types of NDT magnetic particle testing equipment available.
- Adjacent cables
- Current flow probes
- Demagnetisation equipment
- Electromagnetic yokes
- Flexible coils
- Magnetic particle test blocks
- Permanent magnets
- Power packs
- Threading bars
- Wet benches
What Is Demagnetisation in Magnetic Particle Testing?
Demagnetisation in magnetic particle testing involves the removal of residual magnetism after MPT is done. There are several reasons why demagnetisation may be required after MPT:
- To prevent interference with subsequent machining operations
- To prevent interference with ionised plasma
- To prevent interference with moving parts
- To make it easier to clean parts with magnetised particles
- To prevent small parts from sticking together during the wash cycle
- To avert any potential effects on local instruments
- To avoid any effects on the subsequent magnetisation of components
However, if the magnetisation is not expected to have any effects on subsequent operations, demagnetisation may no longer be necessary.
Applications of Magnetic Particle Testing
There are several industry applications of magnetic particle testing. Below are just some examples:
- Mining: To monitor equipment for damage and flaws, preventing failures, injuries, potential deaths and lost productivity. Often MPT is conducted on drill rigs, excavators, tyre handlers, lift or pad eyes and crane hooks.
- Aerospace: To inspect the integrity of aircraft propeller hubs and turbine blades.
- Engineering and Fabrication: To inspect electroslag (ES) welds on network-critical structures
- Marine: To check lift or pad eyes that have been welded onto a module
- Oil and Gas: To investigate the internal blistering of oil pipelines
- Power Generation: To inspect gas turbine blades
Advantages of Magnetic Particle Testing
Below is a list of advantages of magnetic particle testing that make it a popular NDT method:
- Can be used on stationary and mobile components
- Defects can be detected directly on the surface
- Easier to apply compared with other NDT methods
- Fast and reliable
- Flexible application, as there is a specific MPT method for either rough or smooth surfaces
- Indicates the shape and size of the cracks
- Makes use of portable and low-cost equipment
- Relatively safe to perform
- Results are immediate
- Very sensitive, so it can detect even the finest surface flaws
Disadvantages of Magnetic Particle Testing
While MPT comes with several advantages, it also has its drawbacks.
Below are the disadvantages of magnetic particle testing:
- Access can be a problem for the magnetising tools.
- Application is limited to ferromagnetic materials (e.g., steels, cast irons, etc.).
- Equipment must be calibrated and does not keep permanent records of results.
- Magnetic flux (or field) and indications should be aligned to get accurate results.
- Post cleaning and demagnetisation are usually required.
- Recommended for inspecting small sections only; specialised equipment may be required to examine large parts.
- Testing should be done in two perpendicular directions.
- Thick paints (>125µm) have to be removed prior to inspection.
Magnetic Particle Testing vs Eddy Current Testing
Eddy current testing (ECT) vs magnetic particle testing (MPT) is a fair comparison between two commonly used non-destructive testing techniques.
MPT is a very cost-effective NDT method. On the other hand, eddy current inspection requires very little setup and clean-up. And while MPT can detect infinitesimal flaws and reveal subsurface defects effectively, ECT can be used to probe multi-layered structures. Also, MPT is recommended for ferromagnetic materials, whilst ECT can be applied to conductive materials, including metals and alloys.
In any case, magnetic particle inspection has its own best uses, just like the ultrasonic testing procedure – an older method that has remained useful to this day. In choosing between MPT and EDT, you may find one to be more advantageous than the other after considering your budget and the component or specimen being tested.
Choosing the Best NDT Method
At Asset Management Engineers, we can help you identify the best NDT method for root cause analysis (RCA) and other testing requirements based on the specific circumstances of the equipment, system or component failure.
Our Perth and Kalgoorlie NDT services have assisted hundreds of clients in making decisions that impact workplace safety, productivity and regulatory compliance.
Get in touch today!