Surface Cleaning
until the ammeter indicates zero. If the piece is large, it
may be necessary to demagnetize a small portion of the
After surface preparation, you should clean the
work at a time.
surface of the material, including areas adjacent to the
You can use a small compass to check for the
inspection area, very carefully. You can clean the
presence of a magnetic field. When the compass is held
surface by swabbing it with a clean, lint-free cloth
near the workpiece, deviation of the needle from its
saturated in a solvent, such as acetone or isopropyl
normal position indicates the presence of a magnetic
alcohol, or by dipping the entire piece into a solvent.
field. If a magnetic field is present, the workpiece will
After you have cleaned the surface, remove all traces of
require demagnetization.
the cleaning materials. It is extremely important that all
dirt, grease, scale, lint, salt, and other materials are
removed. Make sure that the surface is completely dry
LIQUID PENETRANT INSPECTION
before the liquid penetrant is used.
Liquid penetrant (PT) inspection is used to inspect
Application
metals for surface defects similar to those revealed by
the MT inspection. Unlike the MT inspection, which
You must maintain the temperature of the liquid
can reveal subsurface defects, the PT inspection reveals
penetrant and the workpiece between 50 and a
only those defects that are open to the surface. Both
maximum of 150. Do NOT attempt to use liquid
ferrous and nonferrous metals can be inspected by the
penetrant when this temperature cannot be maintained.
use of the PT inspection.
Do NOT use an open flame to increase the temperature,
Seven groups of penetrant are listed in
since liquid penetrant materials are flammable.
MIL-STD-271. According to MIL-STD-271, group 1
When the material is clean and dry, coat the surface
penetrant material should be used for all welds. The use
with the liquid penetrant. The penetrant may be sprayed
of a group other than group 1 penetrant material for
on, brushed on, or the entire piece may be immersed in
welds requires approval of the authorized representative
the penetrant. You must allow time for the penetrant to
of NAVSEA. The application of all PT inspections
get into all cracks, crevices, or other defects that are
must be according to the appropriate MIL-STD or
open to the surface. Wet the surface of the piece with
NAVSEA document. Instructions prescribed for each
the penetrant and keep it wet for a minimum of 15 to 20
penetrant should be followed carefully according to the
minutes. The time limits will vary depending upon
applicable procedure, since there are some differences
which group of penetrant is being used. Follow the
in the procedures and safety precautions required for
instructions prescribed by the appropriate MIL-STD or
various penetrants.
NAVSEA document concerning the length of time the
surface must be kept wet.
PT INSPECTION PROCEDURES
When the surface has been kept wet with the
penetrant for the required length of time, remove the
The following procedures should be followed when
excess penetrant from the surface with a clean, dry,
you use liquid penetrants to inspect a weld.
lint-free cloth or absorbent paper towel. Then, dampen
a clean, lint-free cloth or absorbent paper towel with
penetrant remover and wipe the remaining excess
Surface Preparation
penetrant from the test surface. Dry the test surfaces
after removal of excess penetrant only by normal
First, you must prepare the surface. Remove all
slag from the surface. Except where a specific finish is
absorbent paper towel previously mentioned. When
required, it is not necessary to grind the weld surface as
you are drying the surface, be very careful so that you
long as the weld surface is in accordance with applicable
do not contaminate the surface with any oil, lint, dust,
specifications and as long as the weld contour blends
or other materials that would interfere with the
into the base metal without undercutting. If a specific
inspection.
finish is required, PT inspections may be made before
the finish is made. This will detect defects that extend
After the surface has been dried, you must apply
beyond the final dimensions, but a final liquid penetrant
another substance called a "developer." The developer
must he made AFTER the specified finish is made.
(powder or liquid) must be allowed to stay on the
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