Application of welding technology
Dye penetrant inspection (DPI)
This is typically used for finding small cracks or surface pores not visible to the naked eye. The equipment consists of 3 aerosol tins, cleaner, penetrant dye & developer. The instructions on the tins must be followed carefully. The area to be tested is thoroughly cleaned by wiping with a lint free rag moistened with cleaner. All cleaner must be wiped off and any entrapped (in the fault) residue given time to evaporate. The dye penetrant is sprayed on & left for the stipulated time. It is then wiped off with dry rag followed by wiping with a rag very slightly moist with cleaner. The developer is the sprayed on the surface. If any of the dye has been trapped in pores or cracks it will gradually spread out into the developer showing the extent of the weld fault. The total size of the dye patch on the developer gives some indication of the extent of the fault.
By looking at the weld its width, height & profile can be assessed. Faults such as surface porosity, spatter, surface cracks, lack of penetration & undercut can be seen. Specialised measuring equipment and magnifying glasses are also frequently used.
The weld is cut in half and then polished & etched sample using only the naked eye or low magnification to inspected after applying a solution to the fusion zone. The surface is then etched using one of the following solutions. Either 10 to 20% Nitric acid in distilled water. Alcohol may be used instead of distilled water. An alternative solution is 10% iodine, 20% potassium iodide, 70% distilled water. Immediately the solution is dripped / run onto the surface the outlines of the weld metal, fusion zone & any impurities will begin to appear. The usual time for etching is between a few seconds & a few minutes. Too short or too long a time for etching will reduce definition. When etching is complete wash the solution off with clean water and dry....