INTRODUCTION

Magnetic particle testing is a highly sensitive method used for locating:

• Cracks
• Porosity
• Slag inclusions
• Other discontinuities in materials and welds which can be magnetized

Picture 1: Cracking due to Hydrogen Cavitation

A simple bar magnet can simulate this testing method. When a small amount of magnetic particles are placed on a bar magnet, the particles are attracted only to the ends, which are referred to as the north and south poles.

Picture 2: Magnetic field of magnet

If the magnet is notched, the magnetic lines of force pass through the air, creating new north and south poles on each side of the notch. Magnetic particles will be attracted to the new poles.

When a part being tested is magnetized, the part will act like a magnet. The magnetizing is done either by passing a current through it or by placing it in a strong magnetic field.

Picture 3: A leakage field on surface of the discontinuity.

A surface flaw or one slightly under the surface willattract and hold the magnetic particles in a patternsimilar to the outline of the flaw. Magnetic particles can be applied by a dusting bag, atomizer, spraygun, or brush depending on whether a wet solution or dry powder is used. The particles in dry powder form must be applied evenly and excessive particles should be removed.

WET SOLUTION FOR SURFACE DISCONTINUITIES

The wet solution provides a greater sensitivity for fine surface cracks because it consists of a much smaller particle size.

DRY POWDER FOR SUBSURFACE DISCONTINUITIES

The dry powder remains more sensitive for flaws located below the surface.

Picture 4: Dry particle buildup at leakage field

This testing method is highly sensitive todiscontinuities open to the surface and becomes less sensitive as the flaw depth increases. This method is sensitive to discontinuities down to one quarter inch below the surface.
 

Picture 5: Some type of indication 

The two types of magnetic fields used are circular magnetization and longitudinal magnetization. The size and shape of the weldment to be tested determines the type to be used.

Circular magnetization either passes a current through the piece to create a field, or creates a field in which a cylindrical part may be placed. Circular magnetization is provided by using a set of prods. Walking the prods along the weld provides full coverage. Extreme caution should be practiced to insure that no arcing takes place between the test surface and the prods.

Picture 6: The headshot create a cirular magnetization in object

Longitudinal magnetization passes a current around the piece to create a field through the piece. Longitudinal magnetization can be applied by a coil, a wrapped coil, or a yoke.

Picture 7: An Yoke make a Longitudinal magnetization in plate 

The yoke method is the most practical and widely used method. It is also the safest because no current comes in contact with the test surface eliminating arcing on the surface.

Magnetic particle testing uses the different characteristics of alternating current (AC) and direct current (DC). Equipment availability and degree of sensitivity required determine the type of current to be selected.

Alternating current provides the least sensitivity and magnetizes only the surface of the specimen, instead of the entire thickness of the part. Therefore, it should be limited to locating surface discontinuities.

Direct current penetrates throughout the thickness of the test specimen. It is the more sensitive of the two currents, and is recommended for use on welds when looking for subsurface discontinuities. Visual  valuation is performed on all indications represented by accumulated particles on the surface.

The inspector evaluates the shape, sharpness, width and height of the particle pattern to determine the type of discontinuity or defect.

Picture 8: AC and DC current detect a discontinuos

EVALUATION OF TEST RESULTS

Many factors should be taken into consideration in order to make precise judgments. Careful evaluation by a trained inspector is important. Decisions should be based on requirements specified through applicable specifications, procedures, or codes. Further nondestructive testing, such as ultrasonic or radiographic may be required on a questionable indication.

Picture 9: A crack is detected by fluorescent praticle mangetic

REPORTS AND RECORDS

A concise record of the test results should be developed once all inspection has been completed. The clear transparent tape method uses clear tape placed over the indication. This allows the particle indication to be transferred to a test report form. The tape with the particles stuck to the back is placed over a simple sketch of the weldment. A photograph of the tested surface may also be taken and attached to the final report.

CLEAN-UP OF TEST SURFACE

Demagnetizing a part may be necessary after all reports are finished and visual inspection has been performed. Some components may retain enough agnetization to attract metal shavings, causing excessive wear during service operations. It may also make it difficult to have quality welds.

CONCLUSION

Remember that proper application of this test is based on having qualified personnel to perform the testing procedures and carefully following the steps previously discussed.