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CHAPTER 4 MAGNETIC PARTICLE INSPECTIONS

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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. 4-1

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