It is NOT recommended that either dye-penetrant inspection (DPI) or fluorescent-penetrant inspection (FPI) be used for inspecting brazed joints, particularly on any parts for which subsequent braze repair may be required! DPI and FPI have long been used in the welding industry and should certainly continue to be since weld cracks and surface imperfections can readily be seen by these techniques and subsequently repaired without difficulties. Not so in brazing!
Braze fillets (especially those from nickel brazing) can contain surface roughness and porosity that would readily show up in DPI or FPI, but such surface indications have virtually no relationship whatsoever to the quality or soundness of the braze joint itself – between the faying surfaces. Thus, DPI and FPI should never be used to draw any conclusions about the quality of the joint as a whole. If the goal of DPI or FPI is to merely identify the presence of any tiny surface cracks, please note that any such cracks can readily be found using 10X or 20X visual magnification without contaminating the surface with DPI or FPI chemicals.
DPI and FPI chemical residues are virtually impossible to remove from surface porosity and small cracks in brazed joints that might subsequently need to be repaired. This is not a problem in welding, since weld repair implies grinding out the weld metal in the area of a crack all the way down to clean, fresh base metal (which would also get rid of all the DPI or FPI chemical residues). This would be followed up in welding by putting in a new weld fillet bead.
That is totally impractical in brazing! Surface imperfections and/or cracks in a brazed joint cannot be removed to the point of exposing only fresh new base metal for re-brazing. Therefore, it is NOT recommended to use DPI or FPI on parts containing braze joints that may require any subsequent brazing.
Some inspectors choose to use FPI or DPI on large surface areas, even though they contain braze joints, in an effort to locate any cracks over a large surface area. If cracks are found, especially if they are in any brazed joint areas, then (because braze repair cannot usually be made on such DPI- or FPI-contaminated joints) the parts would need to be scrapped.
Alternative inspection technique: visual and/or pressure-leak testing or ultrasonic inspection of joints.
Dye- and Fluorescent-Penetrant Inspection in Brazing?
By Dan Kay
Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute in 1966 and his MBA from Michigan State University in 1982.
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