Preparing the metal surface
This step determines the effectiveness in getting the final result of defects . The surface of the metal to be inspected may contain a lot of impurities such as oil, water, grease, dust and many other impurities that prevent the Penetrant which is applied onto the surface in the coming step. These impurities may prevent the Penetrant to get into any defects . so usually the metal surface is cleaned off by using a liquid called as solvent. The solvent is usually available in pressurized spray can which is to be sprayed at an angle of 45 degrees keeping a one feet distance from the metal surface, hence the diluted impurities are cleaned off using a lint free towel in a single direction.
Application of Penetrant
The cleaned metal surface is then applied with Penetrant. Penetrant is a high contrast colored dye which is usually available in black, brown and red in color. The Penetrant is applied by any of the following methods which are spraying, brushing, immersing, and pouring.
Dwell time
The applied Penetrant is let on the specimen for a specific time so that the Penetrant gets inside any open defects at the surface; which is called as the dwell time. The dwell time varies upon the type of defects. a gross defect may take a time between 3 to 10 minutes where as a tight crack may take a time from 10 to 30 minutes. The dwell time also depends upon the temperature, humidity and also size and shape of a discontinuity.
Cleaning the excess Penetrant
The Penetrant all over the surface except the Penetrant which got inside the defect area is an excess Penetrant. This Penetrant at the surface should be cleaned off for the further step. The excess Penetrant is cleaned off using a clean lint free towel and wiped in one side direction which is called as wipe -on or wipe-off technique.
Application of developer
The developer is a white contrast colored dye; it is applied to the metal surface where the excess Penetrant is cleaned off. The application mode involves spraying, dipping, and dusting. The main property of a developer is reverse capillary action or bleed out action or absorption property.
Developing time and development of an indication
After applying the developer, it needs a time to absorb the Penetrant from the defect area. This time is referred to as the developing time. Normal developing time can be calculated as 1.5 * dwell time. After this particular time it can be noted that the Penetrant from any defect will be absorbed off by the developer to the surface and hence a clear indication can be seen.
Inspection
Any defects present which is shown at the metal surface, is to be inspected under proper lighting.
Post cleaning
Finally the metal surface with Penetrant and developer should be cleaned off using solvent or cleaner
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