Magnetic Particle Testing (MPT)

supplies the component with a magnetic field. Direct or indirect magnetization can be used to magnetize the material. A magnetic field is produced in the material as a result of direct magnetization, which arises when an electric current is conducted through the test object. When a magnetic field is produced from an external source but no electric current is provided to the test object, indirect magnetization takes place. The direction of the electric current, which might be either alternating current (AC) or some kind of direct current (DC), is perpendicular to the magnetic lines of force (rectified AC). Magnetic Particle Testing is a non-destructive test method based on the principle that magnetic lines of force, when present in ferromagnetic materials, will be distorted by a change in material continuity, such as a sharp dimensional change or a discontinuity. A discontinuity lying open to or close to the surface of a magnetized test part, causes the distortion of the flux lines at the surface. This condition is termed as “flux leakage”. When fine magnetic particles are distributed evenly over the area of discontinuity where flux leakage exists, they will be held in place. The accumulation of particles at points of flux leakage will be visible under proper lighting conditions and are characteristic of the type of discontinuity that is detected. Maximum sensitivity exists for discontinuities at the surface. It diminishes rapidly with the increasing depth of subsurface discontinuities. Discontinuities oriented parallel to the lines of force will remain essentially undetected. Therefore, to detect discontinuities of random orientation, it usually necessary to magnetize the part twice or more to induce magnetic lines of force in a suitable direction in order to perform an adequate examination.

Surface temperature: Before performing the examination it shall be verified that the temperature of the part shall not exceed 600°F for dry particle examinations or 135° F for wet particle examinations. The temperature shall not exceed 600ºF (316ºC) for dry particles and 122ºF (50ºC) for wet particles.

Lighting: For dry particle and wet non fluorescent magnetic particle examination, adequate visible light shall be available. It shall be a minimum of 100 foot candles (1000 lux) and measured by a lux meter on a daily basis. For fluorescent particle examination, the testing shall be performed in a darkened area with an ambient light intensity of 2 foot candles (20 lux) maximum. The black light used shall be allowed to warm up for a minimum of 5 min. prior to use or measurement of the intensity of the ultraviolet light emitted. The black light intensity shall be measured with a black light meter prior to use, whenever the light’s power source is interrupted or changed, and at the completion of the examination or series of examinations. A minimum of 1000μW/cm² on the surface of the part being examined shall be required. The black light intensity shall be measured once in 4 hr, whenever the workstation is changed, or after change of filter or bulb. The readings shall be recorded on a log sheet and kept with the black light unit at all times and monitored for intensity fading before each usage. As specified by the client, required black light level may be as much as 4000μW/cm² for applications such as detection of hydrogen induced cracking