Magnetic Particle NDT MT and MPI
Magnetic Particle (MT) can be used to test welds, castings, and forgings for surface or subsurface defects; however, it can only be used on ferromagnetic metals. Magnetic Particle is also known as MPI Magnetic Particle Inspection, yoke check, magnetism check, Magnaflux a brand name of a popular Magnetic Particle and Liquid Penetrant chemical.
Magnetic Particle is a fairly quick inexpensive Nondestructive Testing Method compared to other NDT methods such as Ultrasound, Eddy Current and X-Ray. The principle of Magnetic Particle testing is quite simple, the main concern is ensuring that the choice of instrumentation yoke, coil, bench, cables, prods is adequate for the application.
Most codes require magnetism coverage in two directions to cover 100% of the part. In order to achieve this you need to understand the equipment that you are using; as well as, the direction of the magnetic field, amperage and number of turns that you are using. If you do not know and are currently performing magnetic particle you are probably missing flaws during your inspections.
An electro magnet (yoke) is a cost effective electromagnet utilized to perform longitudinal magnetization. When using a yoke you want to make sure that the legs are oriented to sit flat on the surface of the part being magnetized. You should have the legs 4-6 inches apart and ensure coverage by rotating the yoke 90 degrees to ensure 100% coverage. A yoke sometimes called a parker probe (name brand) is actually a high intensity probe so it can be beneficial where other methods fail. Another advantage of the yoke is that it is an induced field which keeps the product from arcing, making the process safer than others.
AC yokes should be calibrated by lifting a 10 lb weight and DC yokes should be calibrated by lifting a 30 lb weight. The lifting test gives you a correlation of the adequacy of magnetism coming from the yoke, but it is not the same as determining the amperage value when utilizing a coil. Yokes usually require power and can be powered from a small generator or a vehicle inverter. Just make sure that you calibrate it with the same type of power you plan to use in the field. Modern yokes are available with a battery pack for greater portability.
A coil is similar to an MT Yoke in that it provides a longitudinal induced field that will not cause arcing. Coils are usually a part of a bench but can be purchased as standalone units to utilize in the field. The one major drawback to a coil is the necessity to have an adequate L/D ratio. This can cause some issues when trying to magnetize ring and square shaped parts. This is why they are usually a part of a bench ensemble with headstocks and cables to complete the system.
Headstocks induce circular magnetization and are capable of causing arcing if not utilized correctly. Common causes or arcing include not compressing the headstocks, misaligning the part or utilizing the improper amperage. The process is known for causing a few technicians their fingers and is perhaps the biggest cause of damaging perfectly good parts because of arcing. The benefit of the headstocks is that the math required for determining the amperage is quite simple and it induces a secondary magnetic field complementing the coil process.
Prods have the same arcing concerns as the headstock. Yet, the advantage with them is that the cables utilized can often be converted to be used in a coil wrap process. Thus, allowing the technician to have both circular and longitudinal methods in the field or on the shop floor for heavier parts.
Cable wrap or coil wrap is another form of longitudinal magnetization with the added benefit that you make the coil the size and number of turns you want to make it. The cable wrap method is often used with a portable power source or plugged in directly to inputs on a bench for the inspection of larger parts difficult to maneuver onto the bench. The cable wrap method uses short flexible insulated copper cables usually 4 feet in length that can be connected together to produce a larger coil. These same cables can be connected to prods for the introduction of a circular field into a part. Any cable and cable leads used with and for cable wrapped coils must have good quality electrical connections. Poor connections result in overheating and reduced coil amperage.
To keep cable wrap areas low, cables should be wrapped directly on a test object or on some insulating material only a little larger than the test object. The insulation assists in keeping the localized poles effects from interfering with the inspection. Multiple tests along a long test object, using a coil of only a few turns is preferable to using a coil of many turns over the length of the test object. The field is theoretically zero in the coil center and increases to a maximum at the inside edge of the conductor. Cable wraps usually incorporate the high fill factor formula of longitudinal magnetization. Therefore, the coils should not be more than 2 times the part diameter.
When using the cables in a coil shape the NDT inspector should ensure that the cable does not walk or slide down the part when energizing, this may demagnetize the part during energizing. To assist the process an energizing remote is usually provided with the unit. Most cable wrap units come with a demagnetizing option. The inspector simply switches a switch to demagnetize and it demagnetizes the part. Portable units although equipped with an amp meter that reads 2,000 they are usually only effective to 1,000 amps on DC and 2,000 on AC.