When a ferromagnetic material is magnetised the flux lines flow inside the component
When there is a discontinuity in the material there is flux leakage
The leaking flux attracts a magnetic medium resulting in an indication.
What is magnetic particle testing?
This method is used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection. The specimen is magnetised either locally or overall, and if the material is sound the magnetic flux is predominantly inside the material. If, however, there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage flux is displayed by covering the surface with very fine iron particles applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually even when the crack opening is very narrow. Thus, a crack is indicated as a line of iron powder particles on the surface.
Post emulsifiable Penetration System:
Post emulsifiable lipophilic/ hydrophilic methods are formulated to maximize penetrating and visibility characteristics. They do not contain any emulsifying agent in penetrant and cannot be completely removed with plain water.
Post emulsifiable lipophilic: Removal is made possible by applying an emulsifier in a separate process step after dwell time for penetrant. This converts the excess surface penetrant into an water washable penetrant . This emulsifier uses diffusion method to convert penetrant in to water washable.
Post Emulsifiable Hydrophilic Method: The hydrophilic postemulsifiable, method also uses penetrants requiring a separate emulsifier. The penetrants are the same as those used in the lipophilic method. The difference between hydrophilic and lipophilic methods is in the emulsifiers. Hydrophilic emulsifiers are water soluble emulsifiers and actually remove excess surface penetrant by means of a scrubbing action rather than diffusion action.
post emulsifiable liquid penetration testing: It is divided in to two category:
- Method B: Post-Emulsifiable, Lipophilic
- Method D: Post-Emulsifiable, Hydrophilic
Water-Washable, is the most economical method & it has not replaceable for rough casting surface, threaded & key surfaces.
- Water-washable or self-emulsifiable penetrants contain an emulsifier as an integral part of the formulation.
- The excess penetrant is removed from the object surface with a simple water rinse.
- Penetrant materials have the property of forming relatively viscous gels upon contact with water, which results in the formation of gel-like plugs in surface openings.
- While they are completely soluble in water, given enough contact time, the plugs offer a brief period of protection against rapid wash removal. Thus, water-washable penetrant systems provide ease of use and a high level of sensitivity.
Inspection with visible dye penetrant with minimum whate light intensity 100 foot-candles or 1100 lux shall follow. Ultraviolet (UV-A) radiation of minimum intensity 1,000 micro-watts per centimeter squared is common, along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after 10- to 30-minute development time, and is dependent on the penetrant and developer used. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye. It is also good practice to observe indications as they form because the characteristics of the bleed out are a significant part of interpretation characterization of flaws.
6. Post Cleaning:
The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are recommended as per PT procedure.
General formulas used in Ultrasonic flaw detection.
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Penetrant Classification System
Type 1 Fluorescent Dye
Type 2 Visible (Red) Dye
Method A Water Washable
Method B Post Emulsifiable, lipophilic (oil base)
Method C Solvent
Method D Post Emulsifiable, hydrophilic (water base)
Class 1 Halogenated (non-flammable)
Class 2 Nonhalogenated (flammable)
Class 3 Special Application
Form a Dry Powder
Form b Water Soluble
Form c Water Suspendable
Form d Nonaqueous Type 1 Fluorescent (solvent based)
Form e Nonaqueous Type 2 Visible (solvent based)
Form f Special Application
Level 1/2 Ultra Low
Level 1 Low
Level 2 Medium
Level 3 High
Level 4 Ultra High