Well, no, we are not inspecting magnetic particles. However, the way magnetized iron particles act under certain conditions can tell us a lot about the condition of a steel or iron part that we are inspecting.
Magnetic Particle Inspection (MPI) is a commonly used method of detecting flaws such as cracks, pits, seams, and other surface imperfections. For this article, I turned to Jerry Lawson of Sunnen Products for advice. Jerry is an expert on MPI.
There is a reason to look closely at a flaw on the surface of a part. If the flaw is visible on the surface, how deep does it go? Not only that, but how deep will the flaw go if the part is used in racing service?
This is the Sunnen DCM Magnetic...
This is the Sunnen DCM Magnetic Particle Inspection (MPI) machine. Its size and abilities can detect flaws in most automotive parts. Sunnen makes a number of sizes and types of these machines. Some have wet particle flow. Others are AC or DC, or both. Each type serves a specific purpose.
A surface flaw will often act as a stress point for a crack to develop. This is the reason highly stressed parts such as connecting rods are often "surface enhanced." If you have ever seen a polished set of connecting rods, you probably realize this work was not done to make the inside of an engine more beautiful. A smooth surface is less susceptible to stress points than a cast or forged part with a grainy finish. For this reason, highly stressed parts of an engine are often hand-worked and polished.
Properly used and with the right equipment, MPI can detect flaws even deep in the interior of the part. Such a situation might occur when the metal used in a forging or extrusion contains a contaminant particle. It's also possible that the part was forged at too low of a temperature where the metal could not flow together. This type of flaw would not be apparent on the surface.
The magnetic field that allows this process to work can be generated in two ways. It can be done with a permanent magnet or an electromagnet.
The permanent magnet is the most common and least expensive. A refrigerator magnet is an example of a permanent magnet. These magnets are too weak to be useful in MPI testing, but you get the idea. Many shops have a U-shaped magnet that is very useful for finding cracks in the combustion chamber area. U-shaped electromagnets are also used for this purpose. They are usually stronger.
Magnetic fields will show...
Magnetic fields will show cracks in the direction the field flows. For this reason, a part such as a connecting rod should be examined at several angles to the magnetic field. This photo shows the rod being hand-held so it can be rotated.
With either of these magnets, iron powder is blown or brushed onto the part to be tested. Then a magnetic field is applied to the part. This magnetic field will run uninterrupted across the surface if there is no flaw. If a crack is present, the magnetic field will be interrupted, making the magnetic field stronger at that point. When this happens, the iron powder will align itself along the edges of this crack. This gives a clear view of a crack that may be too small to be seen.
These U-shaped magnets, while adequate for cylinder head checking, leave something to be desired when testing other parts. Larger machines such as Sunnen's DCM are made with a large-diameter ring electromagnet. These units have the ability to grasp parts and feed them slowly through the magnetic field. This allows an operator to carefully inspect a part. Machines with this capacity are quite useful on cylinder heads that have been ported and perhaps overheated. Proper MPI procedures can detect flaws in thin port areas where water might be seeping out under pressure.
The Sunnen DCM has a recirculating fluid pump where the magnetic particles are held in suspension in a liquid. When a flaw shows up, the particles in the fluid will be immediately attracted to it. The fluid captures any free particles, reducing dust. Additionally, it washes away free particles, making it easier for the operator to identify a flaw.