How Magnetic Particle Testing Works

Magnetic particle examination (MT) is a very popular, low-cost method to perform nondestructive examination (NDE) of ferromagnetic material. Ferromagnetic is defined in ASME Section V as “a term applied to materials that can be magnetized or strongly attracted by a magnetic field.” MT is an NDE method that checks for surface discontinuities but can also reveal discontinuities slightly below the surface.

How Magnetic Particle Examination Works

When ferromagnetic material (typically iron or steel) is defect-free, it will transfer lines of magnetic flux (field) through the material without any interruption.

But when a crack or other discontinuity is present, the magnetic flux leaks out of the material. As it leaks, magnetic flux (magnetic field) will collect ferromagnetic particles (iron powder), making the size and shape of the discontinuity easily visible.

However, the magnetic flux will only leak out of the material if the discontinuity is generally perpendicular to its flow. If the discontinuity, such as a crack, is parallel to the lines of magnetic flux, there will be no leakage and therefore no indication observed. To resolve this issue, each area needs to be examined twice. The second examination needs to be perpendicular to the first so discontinuities in any direction are detected. The examiner must ensure that enough overlap of areas of magnetic flux is maintained throughout the examination process so discontinuities are not missed.

History of Magnetic Particle Examination

Magnetism was first used as early as 1868 to check for cannon barrel defects. Cannon barrels were first magnetized and then a magnetic compass was moved down the length of the barrel. If a discontinuity was present, the magnetic flux would leak out and cause the compass needle to move. Defects could be easily located with this technique.

In the early 1920s, William Hoke noticed metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns that followed the cracks in the surface of parts he was machining. He also found that by applying fine ferromagnetic powder to the parts, there was a build-up of powder at the discontinuities which formed a more visible indication.

By the 1930s, MT was quickly replacing the oil and whiting method of NDE (liquid penetrant [PT]) in the railroad industry. It was quicker and did not leave behind the white powder that required clean-up. After an MT evaluation, only iron powder was left behind, which could easily fall off the part or be blown away.

Different Techniques

There are many different techniques and combinations of techniques of MT. The ASME Boiler and Pressure Vessel Code, Section V, Article 7, recognizes five different techniques of magnetization:

• Prod technique
• Longitudinal magnetization technique
• Circular magnetization technique
• Yoke technique
• Multidirectional magnetization technique

There are two different ferromagnetic examination media: dry particles and wet particles. Both forms can be either fluorescent or non-fluorescent (visible, color contrast) and come in a variety of colors to contrast with the tested material.

The following illustrate basic steps to use with the dry powder, non-fluorescent, yoke technique. Prior to the start of examination, all equipment and meters shall be calibrated in accordance with ASME Section V, Article 7.

by Jim Worman; Senior Staff Engineer-National Board

source : http://www.nationalboard.org/index.aspx?pageID=164&ID=377