You've heard the term Magnafluxed. To most, it means taking a part and checking it for cracks. Typically, the teardown guy brings a part to the "Mag" unit, wets it down with solution, presses the button a couple of times, then shines the black light on the part, checking it for cracks. The process seems simple enough, but there is much more to inspection than most racers realize.
Magnaflux is actually the name of the company that began building magnetic particle inspection (MPI) equipment in the '30s, and is still the leading manufacturer of the equipment.
The process of magnetic particle inspection is straightforward: An inspector places a part on the bed of the unit, either clamps the part in the contact plates or places it in the coil, wets the part, magnetizes it, then looks at it under a black light. To do the job properly, you must understand the basics of the inspection process and how to find cracks or defects in parts.
Magnetic particle inspection is one of several inspection methods used in the nondestructive testing industry. Other methods are X-ray, ultrasonic, dye penetrant, eddy current, and visual inspection, to name a few. Each inspection method is used to find defects or flaws in components. The method of choice depends on the type of parts being tested and the location and type of defect being sought. Magnetic particle inspection is only used to test parts made of a ferromagnetic metal, which can be magnetized and retain magnetism. Magnetic particle inspection is capable of finding flaws at the surface and those up to 1/4 inch below the surface of a part.
This is a typical setup for a horizontal bath magnetic particle inspection unit. It is large enough to inspect an engine block. Currently, it is set up to inspect a crankshaft for cracks running parallel to the line of the crank.
Magnetic particle inspection is based on the fact that a crack in a magnetized ferromagnetic part creates a small magnetic field on the surface of that part. When iron powder is sprayed onto the part, the field acts as a little magnet to attract the powder to the crack. You may not be able to see a crack, but you are able to see the iron powder collecting at the magnetic field in and around the crack. The iron powder used in wet horizontal units is contained in water or oil bath. In fluorescent inspection, the iron powder is encapsulated in a fluorescent pigment. When the part is sprayed with the solution, the iron dust settles in the cracks and brightly shines when viewed under a black light.
The Process The first step in MPI is magnetizing the part. In this case, only wet horizontal inspection units will be discussed. The unit produces an electric current that flows directly through a part or around a part. Electrical current is measured in units called amperes. When the electrical current flows, a magnetic field is created. When current flows through or around a ferromagnetic part, the part remains magnetized after the current stops flowing. The magnetic field left in a part is called the residual field. The residual field is measured in units called gauss. The material and the size of the part determine the amount of electrical current required to create the magnetic field. Typically, more electrical current (or amperes) is needed to magnetize bigger parts. Typical wet units have two contact plates and a coil. One of the contact plates moves on a rail, while the other moves in and out by means of a pneumatic cylinder. The first way to magnetize is to place the part between the contact plates, then clamp the part in place by extending the pneumatic plate. When the energize button is pushed, an electrical current flows to one of the contact plates through the part and returns to the unit through the other contact plate. The electrical current that flows through the part creates a magnetic field contained within and around the part.
The second way to magnetize the part is to place it in the magnetizing coil. In the coil, the electrical current flows up one of the coil cables, around the coil, then back to the unit through the other cable. Electrical current that flows around a part induces a magnetic field lengthwise through the part. The direction of the magnetic field depends on the direction of the current flow, and a part can only be magnetized in one direction at one time.
Because they are smaller and easier to handle, connecting rods can often be magnetized quickly by hand.
Inspection
The goal of MPI is to find cracks or defects in metal parts. There are a few rules to be followed when inspecting parts. The first rule is that a crack is easily detected if it lies in the same direction as the flow of electrical current. When the coil is used, electrical current flows around the part, which means cracks lying radially around the part are easily seen. When the part is clamped between the contact plates and current flows through a part, a crack oriented in the direction of the current flow can be seen. To find every possible crack in a part, it must be inspected at least twice--once after being magnetized in the contact plates, and again after being magnetized in the coil.
The second rule concerns the strength of the magnetic field. Remember, each crack produces a small magnetic field that attracts the iron powder. If there is not enough electrical current, the magnetic field created at a crack may not be strong enough to attract the iron powder. Conversely, if the electrical current is too strong, the resulting magnetic field may be so strong that small, tight cracks are overlooked. Ideally, the electrical current should be just strong enough to create a magnetic field to find the smallest crack one wants to find.
Keith Dorton wets the magnetized connecting rod with a solution containing magnetic particles that will show up under a black light. He puts his safety glasses on before checking the part.
It is important to determine the current necessary to properly inspect a part. There are several gauges and tools on the market to help an operator arrive at the proper current level.
The best tool is actually a part with small, tight cracks. Every racer has either been in a wreck or had an engine or transmission fail. The parts that come off these cars should not be thrown away. Instead, they can be used to set the current levels for MPI. For example, let's use a pinion gear from a bad transmission.
Take the pinion gear and clean it, removing any grease and oil. Clamp the gear between the contact plates and set your unit at mid-range. (Always wear safety glasses when performing magnetic particle inspection!) Coat the part with the bath, and then apply the current. Note the current level displayed on the amp meter. Take the black light and look at the part. Note the location of the smallest crack. Demagnetize the part and clean it. Repeat the process, but now turn the current level down. Find the smallest crack from the previous trail and see if there are any cracks smaller than the one found earlier. Repeat the process again at lower current levels. The best current setting is the lowest amperage that exposes the smallest crack.
After inspection is complete, the part must be demagnetized. Using a gauss meter is a simple way to make sure a part no longer has magnetic properties.
Next, move on to the coil. Place the part in the coil, set the unit at mid-range, wet it with the bath, apply current, and look for the smallest crack. Demagnetize the part and repeat the process, writing down the best current level for finding small cracks in this process. The best current level for the contact shot and the coil shot will now serve as the baseline or inspection procedure for future inspection of pinion gears. Keep the old gear and mark it as a test piece for magnetic particle inspection. The test piece can be used to verify the current settings and to train others.
With a procedure in hand, the operator is ready to test. Before any inspection, the part should be clean and free of any grease or grit. The operator must make sure the inspection unit is running correctly and that the proper amount of iron particles or dust are in the bath solution. The black light should be bright enough to illuminate the smallest of cracks at a distance of 15 inches. The inspection area should be as dark as possible to minimize white light that interferes with fluorescent inspection.
The part should first be clamped between the contact plates, and the unit should be set to the correct electrical current. Spray the part with the bath solution while the unit is energized (while current is flowing through the part). Most units have timed current shots. You will need to energize the unit two or three times to fully magnetize the part while applying the bath. Once the part is fully covered with the bath, stop spraying the part and energize the unit one last time.
Inspection
As you can see, the crack in this gear (at the top) can be easily seen when the part is held under a black light.
Take the black light and inspect the part. Part edges, tool marks, and scratches and cracks appear as bright lines on an otherwise smooth surface. Surface cracks are typically jagged and do not follow a straight line (like a tool mark). Cracks can cross over an edge and into smooth welds.
After completely inspecting the part in the contact plates, place it in the coil and repeat the process. If the part does not exhibit any defects in the coil, it can be returned to service after it is properly demagnetized. The part does not need to be demagnetized before testing in the coil. If the electrical current is set to the correct level, the part is remagnetized in the proper direction once the coil current is applied.
Magnetic particle inspection is a process. Knowing and understanding the basics is the foundation on which to build. To become a good inspector, one needs to build on these basics, try different techniques, use inspection tools, and prepare an inspection procedure to test a part. Whether you're a weekend warrior or member of a Cup team, you need to make certain that the parts you put in the car are good from the start. It is much safer and much less expensive to find a cracked part during magnetic particle inspection than to find the part in pieces after a wreck.