Visible penetrants for liquid penetrant testing (PT) were evaluated at low temperatures. Previous testing of visible penetrant materials has been done using cleaners and developers with chlorinated solvents (now obsolete), which worked well down to 35°F. With the switch to the nonchlorinated solvents, users are finding more inconsistent results when doing penetrant inspections at lower temperatures. Difficulties arise because of the slower-drying solvents in these cleaners and developers.
At temperatures of 32°–50°F, the cleaner/remover takes longer to dry. If 75°F is taken as the standard operating temperature, the drying times are 2x usual at 50°F, 3x usual at 40°f and 4x usual at 32°F.
It is critical to make sure the solvent has evaporated during the precleaning step either by wiping with clean, lint-free cloths or by using air dryers.
Likewise, penetration into the defects is affected. As temperature decreases, the viscosity of penetrants increases, which slows down penetration significantly where 75°F is the usual penetration dwell time, 45°F is 2x the usual penetration dwell time and 32°F is 2.5x the usual penetration dwell time.
The effect of drying becomes more significant in the developing step when using nonchlorinated, slower-drying solvents. Tests showed the developing action is impeded because of the slower evaporation of the solvent. When the developer dries, the indications become fuzzy or show excessive bleed out. Good results could not be obtained at 32°F because the developer remained wet and did not dry within a practical time period. At 38°–40°F, and with some air movement, developer film dries at five times the usual time at 75°F.
One good note is that at low temperatures, good indications with extremely small and tight defects were found. The slow drying rate of the developer film allows greater bleed out of the penetrant from the defect. For large defects, longer development time causes excessive bleed out making interpretation more difficult.