Magnetic particle testing NDT – Mt
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Magnetic particle testing (MT) (also called magnetic particle inspection – MPI) is a non-destructive test (NDT) tool for detecting surface or subsurface (near-surface) discontinuities. This nondestructive research method can be used on metals that are easily magnetized (ferromagnetic). Metals are categorized as ferromagnetic, paramagnetic, or diamagnetic depending on their magnetic properties.
Ferromagnetic metals are those that are highly attracted to a magnet and can be magnetized quickly. Iron, nickel, and cobalt are examples of ferromagnetic metals.
Paramagnetic metals, such as austenitic stainless steel, are those that are only weakly attracted by magnetic forces of attraction and cannot be magnetized.
Diamagnetic metals: These are metals that are slightly repelled by a magnet but cannot be magnetized. Bismuth, gold, and antimony are examples of diamagnetic metals.
Magnetic Particle Scanning can only be used to inspect ferromagnetic metals.
Magnetic Particle Testing or Mag Particle Testing (MT) for industrial Nondestructive Testing applications.
In magnetic particle inspection, a variety of electrical currents are used. To choose the right current, take into account the component geometry, material, form of discontinuity desired, and how deep the magnetic field needs to penetrate into the part.
Surface discontinuities are often observed using alternating current (AC). Due to the skin effect, where the current runs along the surface of the element, using AC to detect subsurface discontinuities is restricted. Since the current alternates polarity at a rate of 50 to 60 cycles per second, it does not penetrate much beyond the test object’s surface. This means that the magnetic domains will only be aligned to the degree that AC current penetrates the component. The depth of penetration is determined by the frequency of the alternating current.
Full wave DC (FWDC) is a technique for detecting subsurface discontinuities where AC cannot penetrate far enough to magnetize the component to the required depth. The amount of magnetic penetration is proportional to the current flowing through the part. In terms of how effectively DC magnetizes very large cross-sectional sections, it is also constrained.
Half wave DC (HWDC, pulsating DC) is similar to full wave DC in that it can detect surface breaking signs and has more magnetic penetration into the component than full wave DC. HWDC is beneficial to the inspection process because it aids in the movement of magnetic particles when the test sample is bathed. The half-wave pulsating current waveform enables particle mobility. There are 15 current pulses in a standard mag pulse of 0.5 seconds while using HWDC. This increases the chances of the particle colliding with areas of magnetic flux leakage.
For surface breaking indication, an AC electromagnet is the preferred process. Finding subsurface signs with an electromagnet is difficult. Surface indications are better detected with an AC electromagnet than with a HWDC, DC, or permanent magnet, whereas subsurface defects are better detected with any kind of DC. For information on the most common volumetric examination, please see Radiography Testing here.