Inspecting Parts - Magnetic Particle Inspection

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?

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.

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.

The Sunnen DCM has a 14-inch internal diameter. The advantage of having this large size allows cylinder heads, crankshafts, and other large parts to be fed through the machine. Smaller parts such as connecting rods can be hand-held. Holding a small part with your hand allows you to turn the part around and in several directions. This way flaws in any area of the part can be found.

MPI machines can be either an alternating current (AC) or a direct current (DC). AC will concentrate the current on the surface of a part. Furthermore, AC will let the magnetic particles (iron dust) spread evenly across a part, thus assisting in the search for flaws. The AC process is used when the primary goal is to find surface defects. It is the type commonly used in automotive parts inspection.

The use of DC electricity allows a skilled operator to actually look under the surface. This can expose hidden defects in castings, forgings, and even in welded parts. Use the DC process on critical parts. Operator skill and having the correct machine are important here as there are no clearly defined fault lines such as surface cracks.

The electrical current in the magnetic field should flow in the direction that a crack is suspected to lie. We will use the crankshaft as an example. With it supported in the machine, the magnetic field will be passed from end to end. This will reveal cracks that are parallel to the centerline of the shaft. Next, the crankshaft will be passed through a circular magnetic field. This will reveal cracks around the journals as well as any flaws perpendicular to the shaft.

This is the reason a connecting rod should be exposed to the magnetic field in several positions. Using only one pass with the magnetic coil could cause you to miss cracks in other directions.

After a part is magnetized for inspection, it must then be demagnetized. If not, the part will attract steel and iron dust. With a crankshaft or a rod, it would be virtually impossible to reassemble an engine while keeping these parts clean. On the other hand, it might make little difference on an A-frame.

For the most part, you can't perform MPI at home. Machines such as the Sunnen are a tad expensive for the home-shop engine builder. However, if you call around, you should be able to find a shop with this type of equipment. The fee for checking some of your parts is reasonable considering the costs incurred if one of the parts were to break.

You can usually obtain one of the U-shaped permanent magnet yokes locally. With this you can check cylinder heads for cracks on their outer surfaces, deck, combustion chamber, and so on. Your nearest auto machine shop should be willing to part with some of the cast-iron dust gathered under its machinery. While a little bit coarse, this dust will get you started and you'll see how the process works.

MPI testing can find the beginnings of part failures before they actually fail. Iron dust will cling to an otherwise invisible surface flaw on a magnetized steel or iron part. Be sure to demagnetize an internal engine part. AC magnetics are used to detect surface flaws. DC magnetics can find internal flaws. MPI is a form of insurance. If it is important, then protect it.