Liquid Penetrant Testing (PT)
Penetrant testing based on the properties of capillary action, or the phenomenon of a liquid rising or climbing when confined to a small opening due to surface wetting properties of the liquid. Penetrant testing is used for finding surface breaking discontinuities on relatively smooth, nonporous surfaces.
The types of defects that can be found with penetrant inspection are:
Rolled Products: penetrant identifies anomalies (cracks, seams or laminations)
Castings: cold shuts, hot tears, porosity, blow holes or shrinkage
Forgings: illuminating cracks, laps or external bursts
Welds: to identify cracks, porosity, undercut, overlap, lack of fusion or lack of penetration
There are two main types of penetrant; fluorescent or visible. Within each method there are several methods including water washable, postemulsifiable-lipophilic, solvent removal and postemulsifiable-hyperdrophilic. The type and penetrant method are chosen based on sensitivity levels 1-4 and are based on job site conditions and other variables.
There are six main steps involved with penetrant testing:
1. Pre-Clean: Parts must be free of dirt, grease, rust, scale, oil or grease.
2. Application of Penetrant Material: The penetrant material may be applied by brushing, spraying, dipping/immersing or flow on the material.
3. Dwell Time/Penetrant Removal: The solution must be allowed to "dwell" on the surface to allow the penetrant to fill any defects that are present. Dwell times vary according to penetrant type, temperature and material types and finishes. Removal technique depends upon the type of penetrant used i.e. Solvent Removable, Water Washable or Post-Emulsifiable.
4. Developer Application
5. Inspection/Evaluation: In almost all cases the inspector evaluates the penetrant indications a specified accept/reject criteria and attempts to determine the origin of the indication.
6. Post Clean: The final step is to remove all penetrant processing materials from the component.
The main advantages of Penetrant testing are:
Relatively easy to use
Used on a wide range of material types
Large areas or large volumes of parts/material can be inspected rapidly and at low cost
Parts with complex geometries can be inspected easily
Indications are produced directly on the surface of the part providing a visual image of the anomaly
Aerosol spray cans can make equipment very portable