Liquid Penetrant in the Welding Industry
There are two different types of liquid penetrant inspections that can be used for the inspection of welds, visible red dye and fluorescent. While fluorescent penetrant inspection can be used, visible red dye penetrant is most common because it does not require UV-A lighting and dark conditions, making it ideal for field and shop testing environments. Both penetrants can be removed with solvent cleaners that eliminate the need for water to remove excess penetrant on the surface after the dwell time.
Typically, a liquid penetrant inspection procedure will be required after the first weld pass to check if any cracks have formed. This is accomplished by using VP-30, VP-31A, VBP-300, FP-923, or FP-95A(M) penetrant and E-59, E-59A, or R-502 removers on the weld area. Most specifications instruct to provide at least 2” of penetrant coverage adjacent the root of the weld so as to include the heat-affected zone (HAZ) of the weld. Excess penetrant is removed by wiping the surface with a clean dry rag so as to only remove penetrant that is on the surface. After the excess penetrant has been removed, D-70 or D-702 is applied by spraying a fine, even coat onto the surface to draw out penetrant retained in flaws and to expand this retained penetrant into visible flaw indications.
If defects are present, they will show up as white or green marks on the white developer coating. Some may appear immediately and some may take a little longer to develop. A wide, shallow defect shows up almost at once, while narrower, deeper flaws may not assume final patterns for some time.