Magnetic particle inspection

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Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).

A technician performs MPI on a pipeline to check for stress corrosion cracking using what is known as the "black and white" method. No indications of cracking appear in this picture; the only marks are the 'footprints' of the magnetic yoke and drip marks.
A close-up of the surface of a (different) pipeline showing indications of stress corrosion cracking (two clusters of small black lines) revealed by magnetic particle inspection. Cracks which would normally have been invisible are detectable due to the magnetic particles clustering at the crack openings. The scale at the bottom is numbered in centimetres.

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.

A popular name for magnetic particle inspection is or used to be magnafluxing.

Contents

Types of electrical currents used

There are several types of electrical currents used in MPI. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity one is looking for, and how far the magnetic field needs to penetrate into the part.

Each method of magnetizing has its pros and cons. AC is generally the best for discontinuities on the surface, while some form of DC is better for subsurface defects.

Equipment

A wet horizontal MPI machine with a 36 in (910 mm) coil
Using a similar machine, a U.S. Navy technician sprays magnetic particles on a test part under ultraviolet light.
An automatic wet horizontal MPI machine with an external power supply, conveyor, and demagnetizing system; it is used to inspect engine cranks.

Demagnetizing parts

A pull through AC demagnetizing unit

After the part has been magnetized it needs to be demagnetized. This requires special equipment that works the opposite way of the magnetizing equipment. The magnetization is normally done with a high current pulse that reaches a peak current very quickly and instantaneously turns off leaving the part magnetized. To demagnetize a part, the current or magnetic field needed has to be equal to or greater than the current or magnetic field used to magnetize the part. The current or magnetic field is then slowly reduced to zero, leaving the part demagnetized.

Magnetic particle powder

A common particle used to detect cracks is iron oxide, for both dry and wet systems.

After applying wet magnetic particles, a U.S. navy technician examines a bolt for cracks under ultraviolet light.

Magnetic particle carriers

It is common industry practice to use specifically designed oil and water-based carriers for magnetic particles. Deodorized kerosene and mineral spirits have not been commonly used in the industry for 40 years. It is very dangerous to use kerosene or mineral spirits as a carrier due to their low flash points, and inhalation of fumes by the operators.

Inspection

The following are general steps for inspecting on a wet horizontal machine:

  1. Part is cleaned of oil and other contaminants
  2. Necessary calculations done to know the amount of current required to magnetize the part. See ASTM E1444-05 for formulas.
  3. The magnetizing pulse is applied for 0.5 seconds during which the operator washes the part with the particle, stopping before the magnetic pulse is completed. Failure to Stop prior to end of the magnetic pulse will wash away indications.
  4. UV light is applied while the operator looks for indications of defects that are 0 to +/- 45 degrees from path the current flowed through the part. Indications only appear 45 to 90 degrees of the magnetic field applied. The easiest way to quickly figure out which way the magnetic field is running is grab the part with either hand between the head stocks laying your thumb against the part (do not wrap your thumb around the part) this is called either left or right thumb rule or right hand grip rule. The direction the thumb points tell us the direction current is flowing, the Magnetic field will be running 90 degrees from the current path. On complex geometry like an engine crank the operator needs to visualize the changing direction of the current and magnetic field created. The current starts at 0 degrees then 45 degrees to 90 degree back to 45 degrees to 0 then -45 to -90 to -45 to 0 and repeats this for crankpin. So inspection can be time consuming to carefully look for indications that are only 45 to 90 degrees from the magnetic field.
  5. The part is either accepted or rejected based on pre-defined accept and reject criteria
  6. The part is demagnetized
  7. Depending on requirements the orientation of the magnetic field may need to be changed 90 degrees to inspect for indications that can not be detected from steps 3 to 5. The most common way to change magnetic field orientation is to use a Coil Shot. In Fig 1 a 36 inch Coil can be seen then steps 4, 5, and 6 are repeated

Standards

International Organization for Standardization (ISO)
European Committee for Standardization (CEN)
American Society of Testing and Materials (ASTM)
Canadian Standards Association (CSA)
Society of Automotive Engineers (SAE)
United States Military Standard

Further reading

Liquid Penetrant and Magnetic Particle Testing at Level 2, International Atomic Energy Agency, 2000 (pdf, 2.5 MB).

References