What is this Method?
Magnetic particle examination (MT) is a very popular, low-cost method to perform nondestructive techniques of examination of ferromagnetic material. Magnetic Particle Examination is accomplished by inducing a magnetic field into a ferromagnetic material and applying iron particles to the surface of the item being examined. Ferromagnetic is defined in ASME Section V as “a term applied to materials that can be magnetized or strongly attracted by a magnetic field.” MT is an NDE method that checks for surface discontinuities but can also reveal discontinuities slightly below the surface.
Surface and near-surface discontinuities affect the flow of the magnetic field within the part causing the applied particles to gather at locations of flux leakage, thus producing a visible indication of the irregularity on the surface of the material.
Why is this method?
By the 1930s, MT was quickly replacing the oil and whiting method of NDE (liquid penetrant [PT]) in the railroad industry. It was quicker and did not leave behind the white powder that required clean-up. After an MT evaluation, only iron powder was left behind, which could easily fall off the part or be blown away.
When is this method used?
Two of the most-used methods are the stationary horizontal system, using longitudinal and circular magnetization techniques, and the very portable yoke technique.
ASME requires the magnetic particle visible method (color contrast) be evaluated with a minimum light intensity of 100 foot-candles on the part surface. The proper quantity of light must be verified using some type of calibrated light meter and witnessed and accepted by the inspector. If fluorescent magnetic particles are being used, a black light shall achieve a minimum of 1,000 microwatts per square centimeter on the examined surface. If alternate wavelength light sources are used to provide ultraviolet light, causing fluorescence in the particles, it shall be qualified in accordance with ASME.
Roles and responsibilities:
- Visually examine materials, structures, or components using tools and equipment such as endoscopes, closed circuit television systems, and fiber optics for signs of corrosion, metal fatigue, cracks, or other flaws.
- Interpret or evaluate test results in accordance with applicable codes, standards, specifications, or procedure.