Page 249 - CITS - Welder - Trade Theory

WELDER - CITS

Location of surface flaws and sub-surface flaws is essential for many of the industrial components since the failure
to do so may lead to catastrophic situations. For locating gross surface flaws, visual inspection with the aid such as
magnifying glasses is adequate. However, location of minute cracks may not be possible even with the aid of magnifying
devices. In such cases, Liquid Penetrant Inspection can come to the assistance in the location of defects. For locating
only surface flaws, Liquid Penetrant Inspection is sufficient Penetrant examination is generally considered to be one
of the easiest methods of surface inspection to locate discontinuities that are open to the surface. Consequently the
need for an accurate application and competent personnel may be under-estimated. To obtain optimum results, the
method should be applied with care and accuracy. PT can be used on any material except when it is extremely porous.
Principle
DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking
discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate
penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps
to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed
under ultraviolet or white light, depending upon the type of dye used - fluorescent or non-fluorescent (visible).

Surface discontinuities, such as cracks or other separations, as well as porosity open to the surface can be detected by
‘bleeding’ after the surface has been treated with a penetrant. A developer is generally used to increase the evaluation
efficiency.
Types of Penetrant
Since the main feature of penetrant testing is the visibility of the indications, the liquid penetrant contains a coloured
dye (usually RED eye) easily seen in white light or a fluorescent dye visible under black or ultra violet light. Hence, the
penetrant can be classified as visible dye penetrant or fluorescent dye penetrant

Penetration Time or Dwell Time
The period of time during which the penetrant is permitted to remain on the specimen is a vital part of the test. The
minimum time required for the penetrant to enter into the discontinuity is determined by the following factors:

Manufacturer’s recommendation.
Type of material tested.
Type of discontinuity expected.
Temperature of the specimen.
It may be pointed out here that tight crack like discontinuities may require as high as 30 minutes while gross discontinuity
may require 3 to 5 minutes.
The temperature of the specimen is most important. Usually at 10 to 45°C the usual liquid penetrants work normally
well and at elevated temperature more than 45 , the normal penetrants will become dry and hence will not penetrate
into discontinuities. Similarly, at temperature below 10, the mobility of the particles will be very slow and hence, there
will not be any penetrating action. For use at higher temperature special penetrants have been developed. However,
in case of low temperature, the article has to be heated to a temperature of 10 to 45°C.

This test is based on the principle that colored liquid dyes and fluorescent liquid penetrate into the cracks and are used
to check for surface defects in metals, plastics, ceramics and glass. A solution of the colored dye is sprayed on the
clean welded joint and allowed to soak. Then the dye is washed off using a cleaner, and the surface dried with soft
cloth.
A liquid developer (white in colour) is then sprayed on the weld. The coloured dye comes out in the shape of
surface defects into the white developer coating. The defect can be seen in normal light with naked eyes. (Fig 1)

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CITS : C G & M - Welder - Lesson 83 - 97 CITS : C G & M - Welder - Lesson 83 - 97