Magnetic particle testing (MPT) can be used for testing of materials (ferromagnetic materials) that can be easily magnetized. This method is capable of detecting flaws open to the surface and just below the surface. Ferromagnetic materials are made of iron, nickel and cobalt alloys. These materials lose their ferromagnetic properties above a characteristic temperature known as Curie point, which is approximately 1033 K for most ferromagnetic materials.
When a specimen is magnetized, the magnetic lines of force are predominantly inside the ferromagnetic material. The magnetic field introduced into the specimen is composed of magnetic lines of force. Whenever there is a flaw, which interrupts the flow of magnetic lines of force, some of these lines must exit and reenter the specimen. The points of exit and reentry form opposite magnetic poles. When minute magnetic particles are sprinkled onto the specimen these particles are attracted to these magnetic poles and create a visual indication approximating the size and shape of the flaw. The magnetic particles can be applied as powder or more commonly as liquid suspension usually known as magnetic ink. The color of the magnetic particles should be in good contrast to the color of the surface of the specimen for easy detection.
Procedure for testing
The following stages are necessary for satisfactory detection of defects:
Surface preparation of component before testing: Machined or plated surfaces do not require any preliminary surface treatment other than degreasing. Loose rust and scale should be removed from the component to prevent contamination of ink. On painted parts, paint must be removed locally to provide adequate contact areas for current flow.
Initial demagnetization: Components, which have been machined on magnetic chucks or handled in the vicinity of a magnetic field, could have been magnetized wholly or partially. It is necessary to remove this residual magnetism to avoid false indications.
Degreasing and Cleaning: The component should be thoroughly cleaned before testing because adhering dirt and grease particles can mask defects and also contaminate the magnetic ink. Degreasing is done by a spirit or trichloroethylene bath.
Magnetization of the component: It is necessary to choose suitable operating values of electrical parameters to obtain optimum magnetization of the part being inspected. Direct current (DC) or alternating current (AC) can be used. Magnetization of a component can be done in two ways (1) continuous method or (2) residual method. Continuous method is the application of magnetic particles to the component when the current is flowing. If magnetic particles are applied after the magnetizing current is shut off, the technique is called residual method. The residual method is less sensitive than the continuous method.
Application of magnetic particles: There are two classes of magnetic particles depending on the vehicle or carrying agent used. In the wet method particles use a liquid vehicle and in the dry method particles are carried by air. The particles used in the wet method are suspended in oil and water and are obtained from the manufacturer as a powder or heavy thick paste. In the dry powder method, since dry particles depend on air to carry them to the surface of the part, care must be taken to apply them correctly.
Viewing: The paste or powder indications are viewed under proper illumination. Paste and powder particles must be viewed under black light and the equipment or the inspection area must be darkened with a curtain to cut off normal light. The inspection area must also be equipped with an adequate black light source.
Marking of defects: All relevant indication should be marked after allowing the ink to drain. For permanent record, apart from television recording and photography, the area under inspection can be covered with a transparent adhesive film. When the film is peeled off it comes out with magnetic particles adhered corresponding to the indications.
Demagnetization: All ferromagnetic materials retain some residual magnetism after magnetic particle inspection. In most cases demagnetization can be achieved when the material is heated to 1033 K. Electrical demagnetization can be carried out by placing the component in the field of an AC coil or by subjecting it to a reversing, diminishing DC.
Removal of ink from the component: Ink particles can pose problems in using the component for assembly. A paraffin oil wash by hand brush must be administered, to remove the ink.
Equipment used for MPT
To suit various needs the following types of equipment are used:
Simple equipment: For occasional testing of small castings or machine parts for detection of surface cracks, small and easily portable equipment is the most convenient. Magnetic yokes are used for inspection of welds.
Large potable equipment: Large portable equipment is used when higher power is required or heavier duty cycles make small kits inadequate. Large units with output up to 6000 amperes are available.
Stationary magnetizing equipment: A large variety of stationary bench type units are available for different testing requirements. The smaller sized equipment is used for small parts that can be easily transported and handled manually. Larger ones are used for heavy parts such as long diesel engine crankshafts, where handling must be made by crane.
Large heavy duty DC equipment: Heavy-duty stationary equipment units are powered through direct current. They are designed for application of MPT for the inspection of very large and sophisticated castings. Current values as high as 20,000 amperes are delivered. Such high currents are needed to magnetize an entire casting which may weigh tons.
There are certain limitations using MPT methods.
It can detect only surface openings and subsurface defects in ferromagnetic materials.
Magnetic fields must be in a direction that is perpendicular to the defects. This requires two or more sequential inspection steps with different magnetization directions.
Care is necessary to avoid local heating and burning of finished parts or surfaces at the points of electrode contact.
It is necessary for complete demagnetization of the part after completion of MPT.
Presence of thin coatings of paint and non-magnetic surface layers such as plating adversely affect the sensitivity of MPT method.
Figure 1: Portable Magnetic Particle Tester Figure 2: Stationary Magnetic Particle Tester