Magnetic particle examination is a very popular, low-cost method to perform non-destructive 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.

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:

1. Prod technique
2. Longitudinal magnetization technique
3. Circular magnetization technique
4. Yoke technique
5. 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.

 

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.

 

Typical Examples of ASME Code-Required Inspections

In the ASME codes of construction, magnetic particle examination or liquid penetrant examination is specified many times to detect the possibility of surface defects. If material is nonmagnetic, the only choice is liquid penetrant examination. However, if material is ferromagnetic, magnetic particle examination is generally used. Some typical examples of ASME Code-required inspections include, but are not limited to:

• Castings for surface defects
• Plates for laminations in corner joints when the edge of one plate is exposed and not fused into the weld joint
• Head spin hole plug welds
• Weld metal build-up on plates
• Areas where defects have been removed before weld repair

 

Once boilers and pressure vessels are in service, MT can be a widely-used examination method. MT may be used for the inspection of items such as:

• Internal and external surfaces of boiler and pressure vessels
• Vessels in liquid ammonia service
• Components subjected to fire damage
• Locomotive and historical boilers
• Yankee dryers
• Cargo tanks
• Vessels in LP gas service
• Weld repairs and alterations to pressure-retaining items