CEMP-E
TI 809-26
1 March 2000
3. MAGNETIC PARTICLE TESTING (MT).
a. Method Description. Magnetic particle testing uses the relationship between electricity and
magnetism to induce magnetic fields in the steel. Magnetic particles, commonly in the form of iron
powder colored for better visibility, are dusted onto the magnetized surface. Cracks and other
discontinuities on or near the surface disturb the lines of magnetic force, essentially acting as poles of a
magnet, attracting the magnetic particles. After the area has been magnetized, the particles are applied,
then removed with gentle dusting or application of air. Particles attracted to discontinuities remain on the
surface at the discontinuity, attracted to the magnetic poles. The MT technician then evaluates the
location and nature of the indicating particles. Tight lines are indicative of surface cracks or other
discontinuities. Subsurface cracks and slag inclusions would show a broader indication. A permanent
record of detected discontinuities can be made with the use of transparent adhesive tape or photography.
(1) The magnetic fields can be induced using either prods, which directly magnetize the steel
through direct contact with the steel and the induction of current flow in the steel, or with a yoke, which
does not transfer electrical current but provides magnetic flux between the two elements of the yoke.
(2) MT equipment may be operated either DC (rectified AC) or AC. DC provides higher
magnetization levels which allows for inspection for discontinuities somewhat below the surface.
Inspection with AC is generally limited to surface-breaking and very near-surface discontinuities, and is
considered more effective for surface discontinuities because the particles are more mobile.
b. Advantages and Disadvantages.
(1) MT is relatively fast and economical.
(2) The equipment is relatively inexpensive, compared with ultrasonic or radiographic equipment.
(3) A source of electric power is necessary.
(4) Inspection costs are generally equal to or slightly more than PT, but considerably less than UT
or RT.
(5) More training is necessary for MT, compared to PT, but substantially less than that required for
UT or RT.
(6) MT can be performed effectively while the joint is still warm from welding or postheating.
(7) After inspection, removal of magnetic particles is quick and thorough, not delaying repairs or
affecting coating application.
(8) Existing coatings may reduce the effectiveness of MT.
(9)The depth of inspectability depends upon the equipment, selection of current, and the type of
particles used. Although opinions vary as to the maximum depth that can be effectively inspected using
MT, 8 mm (5/16 in.) is generally considered the deepest discontinuity that can be detected under good
conditions.
(10) MT is effective for detecting surface-breaking discontinuities such as cracks and laminations.
It is also effective for cracks, laminations, incomplete fusion, slag inclusions, and incomplete penetration
D-5
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