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Field direction for flaw detection :
For maximum detection sensitivity the magnetic field should be 90 Degree to the major dimension of the flaw. However magnetic field direction up to 45 Degree to the major dimension may produce detectable indications.
1. A suitable technique of magnetization is chosen.
2. The part is magnetized in the desired direction.
3. While the magnetizing field is present, iron powders
are applied dry or wet.
4. Iron powders bridge the leakage fields, if present, to produce
indications.
5. Powder patterns produced by flaws are identified and
accept reject decision is made.
Advantages Of MPT:
1.Detects flaw below ¼” below surface.
Limitation of MPT :
1.Applicable to only ferromagnetic material only.
2.Limited depth of detection.
3.Depth & size is not indicated.
4.Sensitivity varies with surface roughness & position.
5.Sensitivity rapidly diminishes with depth.
Detection of leakage field :
Finely divided iron particles are used for detection of leakage field. Leakage field at the surface collects and hold the iron particles together to form visible indication patterns.
Discontinuities at or near the surface produce magnetic leakage field on the surface :
Magnetic flux in a magnetized part pass through the body of the part. Discontinuities at or near the surface of the part disturb the normal flow of magnetic flux and leakage flux fields are produced on the surface above the discontinuities. These leakage fields attract iron particles and are detected by applying iron powders.
post emulsifiable liquid penetration testing: It is divided in to two category:
Water-Washable, is the most economical method & it has not replaceable for rough casting surface, threaded & key surfaces.
5. Inspection:
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.
Penetrant Classification System
Penetrants Type:
Type 1 Fluorescent Dye
Type 2 Visible (Red) Dye
Removal Method:
Method A Water Washable
Method B Post Emulsifiable, lipophilic (oil base)
Method C Solvent
Method D Post Emulsifiable, hydrophilic (water base)
Removers
Class 1 Halogenated (non-flammable)
Class 2 Nonhalogenated (flammable)
Class 3 Special Application
Developers Form:
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
Fluorescent Sensitivity*
Level 1/2 Ultra Low
Level 1 Low
Level 2 Medium
Level 3 High
Level 4 Ultra High
