Magnetic particle inspection

Magnetic particle inspection (MPI) is a nondestructive testing process where a magnetic field is used for detecting surface, and shallow subsurface, discontinuities in ferromagnetic materials. Examples of ferromagnetic materials include 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. 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). Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source.
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.
To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action, if any.
There are several types of electrical currents used in magnetic particle inspection. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity one is seeking, and how far the magnetic field needs to penetrate into the part.
An AC electromagnet is the preferred method for find surface breaking indication. The use of an electromagnet to find subsurface indications is difficult. An AC electromagnet is a better means to detect a surface indication than HWDC, DC, or permanent magnet, while some form of DC is better for subsurface defects.
A wet horizontal MPI machine is the most commonly used mass-production inspection machine. The machine has a head and tail stock where the part is placed to magnetize it. In between the head and tail stock is typically an induction coil, which is used to change the orientation of the magnetic field by 90° from the head stock. Most of the equipment is built for a specific application.
Mobile power packs are custom-built magnetizing power supplies used in wire wrapping applications.
Magnetic yoke is a hand-held device that induces a magnetic field between two poles. Common applications are for outdoor use, remote locations, and weld inspection. The drawback of magnetic yokes is that they only induce a magnetic field between the poles, so large-scale inspections using the device can be time-consuming. For proper inspection the yoke needs to be rotated 90 degrees for every inspection area to detect horizontal and vertical discontinuities. Subsurface detection using a yoke is limited. These systems used dry magnetic powders, wet powders, or aerosols.
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. A popular method to record residual magnetism is by using a Gauss meter.
A common particle used to detect cracks is iron oxide, for both dry and wet systems.
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 dangerous to use kerosene or mineral spirits as a carrier due to the risk of fire.
The following are general steps for inspecting on a wet horizontal machine: