The basics of magnetic particle inspection

Training requirements, ease of inspection, and surface preparation are among factors that can affect whether this method is right for detecting cracks.

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Magnetic particle inspection is a technique that is commonly used for ferromagnetic alloys for detecting surface and subsurface discontinuities. This method is extensively used in a variety of industries to detect cracking in primary metal production, manufacturing, welding, and in service (Table 1).

Table 1: Applications of magnetic particle inspection [1].

Theory and principles of operation

When a steel or other ferromagnetic material is magnetized, it has a magnetic pole at each end. There will be magnetic lines of force that flow from the south pole to the north pole. The lines of force will flow through the material and through the space around the magnet (Figure 1, left).

Figure 1: Example of the magnetic lines of force in a magnetized steel bar without a flaw (left) and with a crack (right) [2].

When a crack or other defect is present, an additional field is formed around the defect, with local north and south poles (Figure 1, right). This local disruption of the magnetic field is called field leakage. When iron particles are applied, the particles will accumulate at the disruption of the magnetic field and will outline the crack. The number of particles attracted to discontinuity will be a function of the local magnetic poles created. This is a function of the air gap at the surface, and the depth of the crack.

The lines of magnetic force are always at right angles to the direction of current flow. Because the detection of discontinuities is always more sensitive perpendicular to the discontinuity, it is important to inspect at different angles by changing the magnetic field.

Equipment

It is necessary to properly magnetize a part prior to magnetic particle inspection. Generally, this is accomplished using electromagnets, although permanent magnets are also used. There are two difference methods of applying a magnetic field to a part: yokes and coils.

Yokes

Both permanent magnet and electromagnet yokes are available for magnetic particle inspection. These yokes are readily carried to the site and can be used for a variety of applications. An example of an electromagnetic type yoke is shown in Figure 2.

Figure 2: Typical handheld electromagnetic yoke. (Courtesy: Magnaflux Corp., Glenview Illinois)

Permanent magnet yokes are used where arc is not permissible. For instance, during inspection of landing gear, the chance of arcing can result in a localized overheated region and can cause local microstructure changes. Another application would be where arcing is not permitted because of the potential for explosion due to a flammable atmosphere.

Permanent magnet yokes are limited in the size of parts that can be inspected due to the limited magnetic field. This can result in inadequate strength to produce adequate indications. If the magnet is too strong, then it is difficult to separate the yoke from the part.

Electromagnetic yokes produce their magnetism by wrapping a ferrite or soft iron “U” shape with a wire coil. Often the legs are adjustable to allow for irregular shaped parts. Because the coil is an electromagnet, the field can be turned “OFF” and “ON,” allowing easy removal of the part from the coil.

Electromagnetic coils can either be AC or DC. AC coils are useful in detecting surface discontinuities due to skin effect. DC coils have greater depth penetration. The expected indication should be perpendicular to a line drawn between the two yoke poles.

Coils

Coils are used for longitudinal magnetization of ferromagnetic parts. The field strength is dependent on the number of turns in the coil and the current (amperes). A rule of thumb for determining the necessary field for inspection is given by [3]:

Where (L/D) is the length to diameter ratio of the part, N is the number of turns in the coil, and I is the current in amperes. Figure 3 shows a typical coil type magnetic particle inspection.

Figure 3: Stationary coil used for longitudinal magnetization of a part for magnetic particle inspection [2].

This type of stationary equipment is often used in a manufacturing setting as part of quality assurance. It is used for a variety of parts such as camshafts, crankshafts, and other parts that have a high length to diameter ratio.

One problem with this type of inspection is that transverse discontinuities at the end of the parts may be missed due to end effects. In this case special techniques are used to deliver a proper magnetic field.

Magnetic particle and liquids

There are many different types of magnetic powders and carrier liquids. Dry powders can also be used. These particles can be of any ferromagnetic material. The particles should have a high magnetic permeability so that they can be readily magnetized. They should also not retain the magnetic field once it is removed. If the particles retain their magnetism, then they will tend to clump together, or attach to any steel part contributing to a high background. Small indications may be obscured.

Small particles can be held by weak magnetic fields and can detect very fine indications. Large particles are not likely to be held by small indications. Large particles may adhere to surfaces where there are no discontinuities.

The preferred part shape for magnetic powders is long and thin. This is because long particles would tend to have distinct magnetic poles, which allow particles to chain together to form an indication. Round or spherical particles do not have distinct poles. However, round particles tend to flow freely and not tend to clump. Usually, a mix of particle shapes is desired for flowability and sensitivity.

There are different types of liquid particle carriers used for magnetic particle inspection. Oils can be used if they have a low viscosity with a high flash point. Typically, the recommended viscosity is 3-5 cSt (40°C). As a comparison, a cold quench oil will have a kinematic viscosity of 18-20 cSt at 40°C. Water is used to reduce flammability as well as reduce cost. These water-based magnetic particle carriers will also have corrosion inhibitors, wetting agents, and antifoam chemicals added to improve the performance and wetting characteristics of the carrier fluid. If water is used with electromagnetic coils or yokes, adequate protection of the operator is necessary to prevent electrical shock.

In either type of carrier, a chemical is added to the carrier to make it fluoresce under ultraviolet light of a frequency of approximately 365 nm (3650 Å). This wavelength of ultraviolet light was chosen to pose no hazard to the operator and make the detected discontinuities readily visible. An example is shown in Figure 4.

Figure 4: Magnetic particle indications on gear teeth due to abusive broaching.

Advantages and disadvantages of magnetic particle inspection

The advantages and disadvantages of magnetic particle inspection are summarized in Table 2.

Conclusions

In this short article, the basic principles and technique of magnetic particle inspection were discussed. Advantages and disadvantages to this method of NDT were provided.

Should there be any comments, or suggestions for further articles, please contact the editor or myself. 

References

  1. C. J. Hellier, Handbook of Nondestructive Evaluation, New York, NY: McGraw-Hill, 2003, p. 5.50.
  2. The National Board of Boiler and Pressure Vessel Inspectors, “Magnetic Particle Inspection,” [Online]. Available: https://www.nationalboard.org/Index.aspx?pageID=164&ID=377.
  3. American Society for Materials, Nondestructive Evaluation and Quality Control, vol. 17, Materials Part, OH: ASM, 1998, p. 190.