Dye Penetrant Examination is commonly used for the
inspection of surface associated discontinuities in weld components. In this
technique low viscosity liquids are allowed to penetrate in the surface
openings and after penetration, liquid is made to extract out again by means of
some developers. That is why it is also known as Liquid Penetrant Examination
for the evaluation of weldment. The examination includes five steps which are associated with the use of a cleaner, penetrant and developer.
Cleaning
Before Dye Penetrant Examination to start, the test surface
is need to be cleaned with the help of wire brush or some old cloth using a suitable cleaner. The surface must be cleaned from organic compounds (oil and grease),
dust, scale and other pollutants because an even surface is essential part for easy removal
of penetrant in the later stage.
Introducing Penetrant
After cleaning, penetrants are introduced to the surface
which fills the surface openings. The Penetrants are available in different
categories depending on type of examination technique. The Penetrant must be
thin enough that can penetrate into small cracks and also easily removable when
required otherwise it may get entrap into the weldment, which causes adverse
effects in welded equipment in later stages. Normally the penetrants are
available in spray bottles which are used to spray on test surface. After
spraying the penetrants are allowed to remain there, for a particular period of
time (normally referred to as Soak Time), to achieve complete penetration
throughout the test section. Soak time is very essential part of examination so
that no crack miss out by the inspector. Also see the pre-requisites of Visual Inspection. Normally the recommended Soak Time
ranges from five minutes to half an hour. Don’t extend the duration of Soak
Time because in doing so, the penetrant may dry off, reducing the effectiveness
of examination.
Penetrant Removal
After the penetrant completely penetrates into the
cracks/openings by giving suitable soak time, the next step is to remove it
from the test surface. The test section is cleaned with the help of clean piece
of rag/cloth. (Note the rag should not be old or worn out, use a new one). Use
special cleaner on rag to make the surface, free from penetrant.
Introducing Developer
The next step is to introduce developer on the surface and
wait to allow liquid penetrant to come out from the cracks, making an
indication. The time spent during this process is also termed as Dwell Time
which normally ranges from ten minutes to an hour. The developers are also
available in spray cans and are gently sprayed on the parts under examination.
Examination
Indications must be examined within the time proposed in
examination procedure. The indications help in evaluation of discontinuities (nature,
shape, depth) due to which they are accepted or rejected according to
acceptance criteria proposed by codes and standards.
After examination, it is recommended to clean the test
section by removing developer. As it is stated earlier substances (penetrant
and developer) may cause defects in weldment in later stages.
Important Points to Follow
Dye Penetrant is an easy examination technique but operator
must heed the following advice.
- Before examination clean the weld part with the use of cleaner as proposed in procedure.
- Give suitable soaking time to the penetrant and developer.
- Dye penetrant requires strict post-cleaning. Penetrants and developers after inspection must be removed.
- Observe the weld part within proposed time after applying developer because any delay may lose the effectiveness of examination.