passing through the metal in a direction at right angles
do not determine the seriousness of the indication, they
to the direction of the first test. One of the advantages
do serve to identify the kind of defect indicated.
of the prod method is that the current can be easily
A crack is indicated by a sharp, well-defined pattern
passed through the metal in any direction. Therefore,
of magnetic particles with a definite buildup. This
if you think a given area is defective, you can conduct
magnetic field tests in various directions to locate the
indication is produced by a relatively low magnetizing
defect.
current. Seams are revealed by a straight, sharp, fine
indication. The buildup of particles is relatively weak,
The prod method is used by adjusting the unit for a
and the magnetizing current must be higher than that
current output suitable for the magnetizing and testing
required to detect cracks. Small porosity and rounded
to be performed for any particular kind of metal. The
indications or similar defects are difficult to detect if
amperage setting will depend on the distance between
you are inexperienced. A high magnetizing current
prod contact points. The prod kit supplied with the unit
continuously applied is usually required. The particle
has a space between prod contact points of 4 to 6 inches.
patterns for these defects are fuzzy in outline and have
For this space and a material thickness of less than 3/4 a medium buildup.
inch, a current setting between 300 and 400 amperes is
satisfactory. When the material thickness is 3/4 inch Defect Repair
and over, use 400 to 600 amperes. You can get the same
magnetic field force with less amperage if the prod Whether or not an indicated defect is to be chipped
contact points are closer together. When you hold the or ground out and repaired by welding depends on the
prods constantly at the same spacing, you can use a specifications governing the job. Surface cracks are
greater amperage to induce a field of greater strength. always removed and repaired. Indications of
After adjusting the unit, place the prods in position.
subsurface defects are evaluated by the inspector. If the
Hold them in firm contact with the metal and turn on
indication is positive, it is usually best to grind or chip
the current. Then apply magnetic particles to the test
down to the solid metal and make the repair. Unless you
area with the duster bulb and observe any indicator
have had considerable experience and can differentiate
patterns. With the current still on, remove the excess
accurately between true and false indications, it is best
particles from the test area with a blower bulb and
to use the magnetic particle inspection only to locate
complete the inspection. Do not move the prods until
surface defects. The magnetic particle inspection is
after the current has been turned off; otherwise the
almost foolproof for this purpose. After the defects
current will arc, and you would have a flash similar to
have been repaired, the areas should be reinspected to
that occurring in arc welding.
ensure that the repair is sound.
Demagnetization
DEFECT IDENTIFICATION AND REPAIR
The final step is to demagnetize the workpiece.
MT inspection will help you locate hairline cracks
This is especially important when the workpiece is
that are otherwise invisible. The particles form an made of high-carbon steel. Demagnetization is
unmistakable outline of the defect. Large voids beneath
essential when direct current has been used to induce
the surface are more easily detected than small voids,
the magnetic field. It is not as necessary when
but then, any defect below the surface is more difficult alternating current has been employed in the test. In
to detect than one that extends through to the surface. fact, the usual demagnetization procedure involves
Since false indications occur frequently, you must be placing the workpiece in an ac coil or solenoid and
able to accurately interpret the particle indications to slowly withdrawing it while the current passes through
make correct repairs to the weld.
the coil.
Demagnetization can be done with the portable unit
Defect Identification
if a special demagnetizer is not available. To
demagnetize with the portable unit, form a coil of
Some of the factors that help you to interpret the flexible cable around the workpiece. Be sure that the
test results include the amount of magnetizing current cable is plugged into the unit for the delivery of
applied, the shape of the indication, the sharpness of the alternating current. Set the current regulator to deliver
outline, the width of the pattern, and the height or a current identical to that used for the inspection, and
buildup of the particles. Although these characteristics turn on the unit. Then gradually decrease the amperage
11-10
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