Magnetic Analysis

MAGNETIC ANALYSIS, broadly, the art of determining the constitutional and structural state of ferromagnetic materials through study of the co-existing magnetic characteristics. In the United States the use of the term is restricted to denote the pro cess of interpreting the magnetic characteristics of ferrous or other magnetic materials in terms of their physical characteristics that have a bearing on their qualifications for a given service.

The practicability of magnetic analysis rests upon the hypoth esis that in a ferrous material there is a definite connection between the magnetic and the mechanical properties. Variations in chemical composition, inhomogeneities in structure, the presence of cracks and internal strains, processes of mechanical and thermal treatment, etc., all result in variations in magnetic characteristics which are more or less open to measurement and interpretation.

Magnetic analysis predicates a test procedure which is non destructive and may be applied at any stage in manufacture.

Both direct and alternating current are used as the primary means of magnetization. Various types of indicating means are employed, such as instruments of the deflected pointer type, galvanometers or oscillographs reflecting a light ray, the latter permitting of photographic recording. Magnetization by direct current offers advantages where it is desired to make tests at high inductions, or where the cross-section of the material is large. The alternating current method permits of greater speed in measure ment. It also allows of obtaining separate indications representing different summations of permeability and watt loss characteristics, from which more than one property of the article under test may be inferred. Thus far its application in practice has been confined

mainly to the examination of relatively small articles.

T.

Spooner (Proc. Am. Soc. Testing Materials, pt. ii., 26, 5926) has carried out an extended investigation on high speed steel which consolidates much of the information on the subject of testing under alternating magnetization. Fig. i shows a series of oscillo grams obtained by means of a cathode ray oscillograph of speci mens of high speed steel of one composition which have undergone different heat treatments. The ordinates represent the differen tial of the magnetic flux for individual specimens of stated heat treatment and a standard specimen, both being subjected to the same magnetizing force. The abscissae represent time on a har monic scale which is varied in phase with respect to the cycle of magnetization, so as to give prominence to that portion of the loop which has been found to carry special significance. Each vertical column, reading downward, shows for a given quenching tempera ture the changes in form as the drawing temperature is increased.

See Proc. Am. Soc. Test. Mat. (1913-27), R. L. Sanford, "Magnetic Analysis," Trans. Am. Inst. Elect. Eng. (1929) ; and bibliography of the Congres International pour L'essai des Materiaux (Amsterdam, Sept. 1927).