piece is magnetized, finely divided iron powder is
Gas cavities are indicated by round dark areas.
applied to it. As long as the magnetic field is not
disturbed, the iron particles will form a regular pattern
Inclusions may appear as regular or irregular
on the surface of the test piece. If the magnetic field is
light or dark areas, depending on the density of
disturbed by a crack or some other defect in the metal,
the included material.
the pattern is interrupted and the particles cluster around
the defect.
Shrink porosity is indicated by a lacy,
honeycombed, discontinuous pattern, while
CIRCULAR AND LONGITUDINAL
individual shrink cavities appear as localized
MAGNETIZATION
dark spots, usually with a branching, or
tree-like, pattern.
The test piece may be magnetized either by passing
an electric current through it, as shown in view A of
figure 11-7, or by passing an electric current through a
Cold shuts appear as dark lines or bands that
coil of wire that surrounds the test piece, as shown in
tend to be curved. When two streams of molten
view B of figure 11-8. When an electric current flows
metal that are significantly different in
in a straight line from one contact point to the other,
temperature meet and do not fuse together, the
magnetic lines of force are in a circular direction, as
flow is known as a cold shut.
shown in view A of figure 11-7. When the current flow
is through a coil around the test piece, the magnetic lines
Cracks are shown as dark lines of various
of force are longitudinal through the test piece. In order
widths, while hot tears are seen as dark lines
for a defect to show up as a disturbance in the pattern
containing many branches.
of the iron particles, the direction of the magnetic field
Standard radiographic plates are valuable guides in
must be close to a right angle to the major axis of the
determining the acceptability of a weld. However, the
defect. Since the orientation of the defect is unknown,
individual who views the plates must depend a great
a minimum of two current directions must be used
deal on training and experience. Further, the individual
during the test. In figure 11-7, circular magnetism is
must be sure that the radiograph plate was properly
induced in the test piece so that the piece may be
exposed and developed. If it is not, a radiograph
inspected for lengthwise cracks. Longitudinal
showing characteristic defects may, in fact, be sound.
magnetism (fig. 11-8) is induced so that the piece may
If there is doubt, the part should be rejected or submitted
be inspected for transverse cracks. In general, magnetic
for further radiographic inspection.
particle inspection is satisfactory for detecting surface
cracks and subsurface cracks that are not more than 1/4
Frequently, an unacceptable weld can be salvaged
inch below the surface.
by chipping or grinding out the defects and rewelding.
When this is done, the area must be re-radiographed to
MT INSPECTION EQUIPMENT
determine the acceptability of the repaired area. Before
you start to grind or chip, though, be sure that the defects
are real to avoid grinding into or through a perfectly
The type of MT inspection unit most commonly
good section.
used in the Navy is the portable unit shown in view B
of figures 11-7 and 11-8 It is a high-amperage,
In addition to the use of radiographs to determine
low-voltage unit having a maximum magnetizing
acceptability, an analysis of radiographs can lead to a
current output of 1,000 amperes, either alternating or
correction of the conditions leading to the defects. By
direct current. It is ready to operate when it is plugged
comparing the results of the inspection procedures with
into the voltage supply specified by the manufacturer.
your knowledge of the causes of defects, you can
The unit consists of magnetizing current source,
improve the overall quality of the product and rejects
controls, indicating meters, three 10-foot lengths of
can be reduced or eliminated.
flexible cable for carrying the current to the test piece,
and a prod kit. The prod kit includes an insulated prod
MAGNETIC PARTICLE INSPECTION
grip fitted with an ON-OFF relay or current control
switch, a pair of heavy copper contact prods, and two
Magnetic particle (MT) inspection can be used
5-foot lengths of flexible cable. Cable fittings are
designed so that either end of any cable may be fitted to
for the detection of weld defects in metals or alloys
in which magnetism can be induced. While the test
the unit, to the prods, or to any other cable. The unit has
11-8