The changes of magnetic properties of mild steel containing 0.06∼0.385% C caused by plastic elongation were investigated. With the increase of the degrec of cold work the permeability of specimen decreasee hyperbolically, the st epness of the slope of the magnetization curve decreasss also and the intensity of external field required to saturate the specimen increases. Increase of magnetic induction due to the action of elastic tension is observed and it reaches to a maximum and then decreases slowly. Ifdenote by σ
c the elastic tension under which magnetic induction of the specimen we takes a maximum value, when the external tension σ
c works on the specimen all the magnatization curves of cold worked specimen coincided with that of the annealed speeimen. The value of σ
c increases by cold working and we obtained the following relation between σ
c and the degree of cold working, ε. σ
c=αlogε+β where α and β are constants depending mainly upon the carbon content of the specimen. Both α and β increased proportionally with the carbon content. If μ
0 is the initial and μ
max the maximum permeability, the following relations were obtained, μ
0σ=
k1, μ
maxσ
c=
k2 where
k1 and
k2 are constants independent of the carbon content. These results coincide qualitatively with the theoretical relation between the initial permeability and internal stress which is deduced by Becker and Kersten, Since there are such regular relations between σ
c and the degree of cold working and between σ
c and the permeability deduced from our results, it may by possible to conclude that σ
c is a measure of residual stress caused by plastic deformation.
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