Magnetic particle inspection (MT) is a nondestructive method used for locating surface or near surface discontinuities in ferromagnetic materials. Magnetic

particle inspection is based on the principle that magnetic lines of force will be distorted by a change in material continuity; i.e., a discontinuity creating magnetic field leakage. See Figure M-1. Magnetic particles, scattered on the plate, will be retained at the location of magnetic flux leakage. The accumulation of particles will be visible under proper lighting conditions.

A weld can be magnetized by passing an electric current through the weld (direct magnetization), or by placing it in a magnetic field (indirect magnetization).

 

Direct Magnetization. The direct magnetization method is illustrated in Figure M-2. This method is  normally used with direct current (dc), half-wave  direct current (HWDC) or full-wave direct current  (FWDC).These types of current have penetrating abilities that generally enable slightly subsurface discontinuities to be detected. Direct magnetization can also be used with alternating current (ac), which is limited to the detection of surface discontinuities only.

Indirect Magnetization. Detection of subsurface discontinuities depends on several different variables–the magnetizing method, the type of current, the direction

and density of the magnetic flux, and the material properties of the weld to be inspected.

When evaluating surface discontinuities only, ac is preferred with the indirect magnetization method. See Figure M-3. Alternating current has a very low penetrating ability, which allows the magnetic field to be concentrated at the surface of the weld.

The alternating nature of the current provides continuous reversal of the magnetic field. This action provides greater particle mobility, and, in turn, aids the detection of surface discontinuities.

When the magnetic field has been established within the weld, magnetic particles (medium) are applied to the inspection surface. After the excess particles are removed, the residual particles trapped in the leakage field of a discontinuity reveal the location, shape and size of a detectable discontinuity. These indications are usually distinguishable by their appearance as sharp, well defined lines of medium against the background of weld surface.

Advantages of MT Inspection. Magnetic particle inspection is considerably less expensive than radiography (RT) or ultrasonic inspection (UT). Magnetic particle inspection equipment is relatively low in price compared to equipment required by the RT and UT

methods of nondestructive inspection. Less training time is generally required for personnel to become competent in performing magnetic particle inspection and evaluating discontinuities.

Using the MT method, the inspector obtains an instant visual indication that assists in locating a defect. Compared to penetrant inspection (PT),the MT method has the advantage of revealing discontinuities that are not open to the surface (i.e., cracks filled with carbon, slag or other contaminants) and therefore not detectable by penetrant inspection. Magnetic particle inspection is generally faster, requires less surface

preparation, and is usually more economical than penetrant inspection.

Disadvantages of MT Inspection. The MT method is limited to ferromagnetic material. This method cannot be used to inspect non-ferromagnetic materials such as aluminum, magnesium or austenitic stainless steel. Difficulties may arise when inspecting welds where the magnetic characteristics of the weld differ appreciably from those of the base metal, e.g., austenitic steel surfacing on a low-carbon steel weld. Welded joints between metals of dissimilar magnetic characteristics may create magnetic particle indications even though the welds themselves are sound. Most weld surfaces are acceptable for magnetic particle inspection after the removal of slag, spatter, and other extraneous material that may mechanically hold the medium.

 

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