TM 1-1500-204-23-7
CHAPTER 4
MAGNETIC PARTICLE INSPECTIONS
4-1. General. This chapter explains what magnetic particle inspection is and what its purposes and capabilities are.
Magnetic field characteristics are described as well as the various methods and techniques of magnetization and
demagnetization used in magnetic particle inspection in conjunction with the magnetizing and demagnetizing equipment.
4-2. Theory of Magnetism. Magnetism is defined as the property of an object to attract certain metallic substances. In
general, these substances are ferrous metals; that is, metals composed of iron or iron alloys, such as soft iron, steel, and
alnico. These metals, sometimes called magnetic metals, today include at least three nonferrous elements: nickel,
cobalt and gadolinium, which are magnetic to a limited degree. All other substances are considered nonmagnetic, and a
few of these nonmagnetic substances can be classified as diamagnetic since they are repelled by both poles of a
magnet.
4-3. Basic Terminology. To discuss the magnetic particle inspection process, certain terms and the essential principles
of magnetism must be defined and understood. The following paragraphs define these terms.
a.
Ferromagnetic Metals. The attraction or repulsion of most metals when under the influence of a magnet is very
slight. A few metals, particularly iron, steel, cobalt and nickel are strongly attracted. These metals, permeable to
magnetic flux, are called ferromagnetic. In magnetic particle testing, we are concerned only with ferromagnetic metals.
b.
Leakage Field. The magnetic field forced out into the air by the distortion of the field within a part caused by the
presence of a discontinuity or change in section configuration is the leakage field.
c.
Magnetism. The property of some metals, chiefly iron and steel to attract other pieces of iron or steel is called
magnetism. While most metals are magnetic to some degree, only iron and steel and some of their alloys are sufficiently
affected for the application or use of magnetic particle inspection.
d.
Magnetic Substances. Magnetic substances are those which are attracted by magnetism, or which are
permeable to magnetic flux.
e.
Magnetic Flux. Magnetism may be considered a force which tends to produce a magnetic field. Magnetic flux is
a condition in this magnetic field which accounts for the effect of the field on magnetic objects. To picture a magnetic
field in a diagram, magnetic flux is commonly represented by flux lines that form a pattern or series of curved lines within
the magnetic field flowing through the magnet and air around the magnet. The stronger the field, the greater the number
of flux lines. These lines are also called lines of force.
f.
Permeability. The ease with which a metal or metallic part can be magnetized is called permeability. A metal
that is easy to magnetize is said to have high permeability or to be highly permeable. A metal that is difficult to
magnetize is said to have low permeability. Soft iron and iron with a low percentage of carbon are usually easy to
magnetize and are highly permeable. Hard steel with a high percentage of carbon content is usually hard to magnetize
and, therefore, is usually lower in permeability. Permeability and retentivity are inversely related characteristics. The
higher the permeability the lower the retentivity and the lower the permeability the higher the retentivity.
g.
Residual Magnetism. The magnetic field that remains in the parts when the magnetizing force has been reduced
to zero or the magnetizing current is shut off is called the residual field. The magnetism which remains is called residual
magnetism.
h.
Retentivity. The property of any magnetic metal to keep or retain a magnetic field after the magnetizing current
is removed is called its retentivity. Metals such as hard steel with a high percentage of carbon which keep a strong
magnetic field have high retentivity or are said to be highly retentive. Those metals, such as soft iron or iron with a low
percentage of carbon, which lose most of their magnetism as soon as the magnetizing current is removed have poor
retentivity.
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