Magnetic Particle Testing (MT) is an effective method for detecting linear discontinuities such as cracks, seams, laps on or near the surface of ferromagnetic materials such as carbon steel, cast iron, nickel and certain alloys. Rounded indications may be detected as well but sensitivity dimishes as diameter decreases.
Magnetic Particle Testing starts by inducing a magnetic field into the object under examination. If any discontinuities or flaws are present in the object they will interrupt the magnetic field within the piece and cause an external magnetic field to be created. This external magnetic field will attract the “magnetic particles” which in turn creates an indication that is easily detected with the human eye.
The particles may be dry which can be used on clean, dry surfaces, or they may be suspended in a liquid carrier such as water or an oil-based solution depending on the technique.
Due to the visual inspection involved with this method, magnetic particles are manufactured in several highly visible colors designed to contrast with a variety of backgrounds to optimize detectability. It is very important to have magnetic particles that contrast sharply with the test surface to ensure the human eye can quickly and accurately identify all indications present. Magnetic particles are primarily used in three different forms that include:
Dry powder - Very small iron oxide particles that are shaped elliptically and covered in a dye to provide contrast against the test surface, e.g. red, yellow, blue, gray, black.
- Black on White - In this technique, black particles are suspended in a liquid carrier and applied to a surface where a white contrast paint has been applied to optimize the contrast and therefore detectability of indications.
Fluorescent - Very small iron oxide particles dyed with a fluorescent pigment and suspended in a liquid carrier. Fluorescent particles greatly enhance the sensitivity of the examination.