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PT control for penetrating liquids

Inspection with penetrating liquids is a particularly suitable method for detecting and locating superficial discontinuities such as cracks, porosity and folding. In a fast and economical way and with great accuracy, it is possible to treat all types of materials: steel, copper alloys, aluminum alloys, glass, plastic and other, without any limitation as to the shape of the components themselves.

It’s basically based on a visual examination of the surface after the application in sequence, of a penetrating product and then of a detector that makes visible discontinuities surface not visible or hardly visible to the naked eye.

A widespread method also for the immediacy and simplicity of the equipment in use, which provides special expertise at each stage described below, to avoid the risk of incorrect application resulting in failure in the detection of imperfection.

  • Preparation of the surface
  • Application of the penetrating liquid
  • Hold (Penetration time)
  • Removal of the excess penetration
  • Drying
  • Application of the detector
  • Development time
  • Inspection and evaluation of indications
This technique takes advantage of the ability of some liquids to penetrate by capillarity and not by gravity, within superficial defects (cracks, cavities, etc.).
The low surface tension and the good wettability of these liquids ensure the penetration even inside very thin discontinuities.
The operation requires a variable time depending on the type of product used, the type of material to be inspected and the type of discontinuity to be detected.
Subsequently, the liquid is removed from the surface by washing with cold running water.Since the water has higher surface tension and lower wettability than the penetrant, it will not be able to remove the liquid from the cracks into which it is penetrated by capillary.
After washing, the liquid remaining inside the defects is extracted; the operation is carried out by spreading on the surface of the piece a layer of white powder of appropriate thickness (detector). The penetrating liquid “ascended” by capillarity, will leave in the detector a signal much larger than the defect that created it.

Depending on the technique applied, the defect can be highlighted in various ways: as a red stain (visible technique or color contrast) or as a fluorescent stain (technique with fluorescent liquid) easily detectable by irradiation, in the dark or with Wood’s light.                                                                    The method is not without difficulties even in evaluation of the defect, in fact sometimes the mechanical workings determine “indications” of defectiveness that can be resolved only by a particularly experienced operator.

… The technician, after running PT, makes the recording of the results after 10 minutes from the application of the detector. On the same detail, a second technician repeats the operation by performing the characterization after 20 minutes. The results related to the present imperfection are different. They could both be right, only one of the two or wanting no one. Why??

Not sure about the reliability of your applied technique? Talk to us.