The advantages of magnetic particle testing using indirect magnetism may be offset by the part’s coating thickness.
Magnetic particle testing (MT) is one of several nondestructive testing (NDT) methods used to test ferromagnetic metals. The test method utilizes a magnetic field and flux leakage to detect surface-breaking and slightly subsurface discontinuities such as cracks, incomplete fusion, laps, seams, etc., in castings, forgings, and weldments. The item being tested can be magnetized by direct magnetism (passing electrical current through the part) or indirect magnetism using leeches (permanent magnets), a coil, or electric solenoid.
When using direct magnetism, direct electrical contact must be made between a source of high current and the part surface. The surface where the electrical contact is made must be bare metal to ensure positive electrical contact when the system is energized and deenergized or damaging arc burns can result. The arc burns are such a concern that some welding standards will not permit use of direct magnetism if prods are used.
The possibility of an arc burn is eliminated if indirect magnetism is used as an alternative to direct magnetism. There is no need to pass electrical current through the part if indirect magnetism is used. There are three common methods of magnetizing the part using indirect magnetism: a magnetic yoke, leeches, or a coil. Two common methods of testing welds are to use a yoke or leeches. While most yokes are powered with alternating current (AC), occasionally a battery is used when AC power is unavailable. Leeches (strong permanent magnets) have the advantage of being self-contained, i.e., no external power source is needed.
Another advantage of using the indirect method of magnetizing the test piece is the test surface can be painted or hot-dipped galvanized. The coating does not have to be completely removed before performing the magnetic particle test. However, there is a concern that coatings can diminish the sensitivity of the magnetic particle test, causing relevant discontinuities to be missed. The concern is justified because the strongest magnetic field is induced into the part when the legs of the yoke or the leeches are in direct contact with the metal surface of the part being tested. If there is a space, i.e., an air gap or coating between the end of the legs of the yoke or the leeches and the part, the intensity of the magnetic field drops off rather quickly.
Regardless of whether direct or indirect magnetism is used, the density of the magnetic flux in the part must be sufficient to produce a flux leakage at the location of the discontinuity to collect and hold the magnetic particles. When the space between the ends of the yoke and the part or the coating thickness is too great, the magnetic flux leakage can be insufficient to hold the magnetic particles in place. There is the nagging question of whether a small but critical discontinuity is undetectable because of the coating thickness.
The following experiments showed that there is a practical limit to the coating thickness before a loss of sensitivity makes magnetic particle testing impractical. A test plate consisting of a 1 x 6 x 12” low-carbon steel plate contained two slots that were saw cut into the plate. Each slot was tested with the slot centered between the yoke legs 6” apart. One slot was cut 0.250” deep and the second slot was cut with a taper that went from 0.375” down to 0.085” (The taper measured 0.375” deep on one end and only 0.085” on the opposite end.) Once the slots were cut, the plate was heated and placed under a 30-ton compressive load using a hydraulic press to close the slots to 0.030” wide or less to resemble a crack. The magnetic particle test was conducted with the slotted steel plate in the vertical position.
The magnetic particles were applied while the yoke was energized and the excess dry magnetic particles were allowed to fall away. Only those magnetic particle indications held in place by the remnant field after the yoke was deenergized were considered.
Details of the Experiment
The following is the equipment used for the demonstrations.
• 120-V AC MagnaFlux Model Y-6 magnetic particle yoke
• ArcMetal red magnetic particles
• 50-ft-long heavy-duty extension cord
• 1” thick ASTM A-36 steel plate measuring 1 x 6 x 12”
• 0.006” thick yellow plastic electrician’s tape.
As mentioned previously, two slots were tested. One was 0.250” deep and tight. The other was 0.085” deep and 0.024” wide.
Plastic electrical tape was applied to the test plate surface to simulate a coating such as paint or hot dipped galvanizing. The upper limit of coating thickness was determined by adding layers of plastic tape until the magnetic particles no longer remained. Ultimately, five layers of electrician’s tape were applied for a total thickness of 0.030” as measured by a dial caliper.
The tapered slot provided an opportunity to see the interaction of the crack depth and coating thickness on the magnetic particle indication. Again, the magnetic indication was judged too weak if the remnant field would not hold the magnetic particles in place after the yoke was removed from the test surface.
You must decide whether the magnetic particle test performed on the coated surface using indirect magnetism provided sufficient sensitivity for the application.
Conclusions
When possible, it is best to conduct magnetic particle tests on uncoated surfaces to optimize the probability of detecting small discontinuities. However, NDT technicians are often faced with situations where it is impractical to strip all coatings from the surface of the component being examined. When it is necessary to perform magnetic particle testing on a coated component, the NDT report should note the part’s surface conditions and that there are practical limitations to what is detectable due to the coating.
NDT technicians do not work in a vacuum. There are resources available that can be used when the situation warrants it. Those resources include the NDT Level III, the client, and the engineer. If an NDT technician is concerned a coating is too thick and the test results compromised, the client must be informed and a decision made whether the coating must be removed or a different NDT test method used. The decision whether to remove the coating should be made by the owner, since the owner will be paying the cost of removing the coating and replacing it once the magnetic particle test is completed.
Magnetic particle testing, like other NDT methods, has limitations. NDT technicians using the magnetic particle test method need to have a working knowledge of its limitations and its advantages. One advantage of the magnetic particle test method is that it can detect surface-breaking and slightly subsurface discontinuities. An additional advantage is the magnetic particle test method can detect discontinuities that may be concealed by a coating. However, the coating thickness can be a limitation. The size of the discontinuity of interest is one factor that limits the thickness of the coating and the probability of detection.