How Dye Penetrant Examination Works in Welding
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.
See the Comparison of Dye Penetrant with Magnetic Particle Inspection.
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.
