Magnetic and electrical properties of the ternery alloys Fe-Si-C.

The variations in the magnetic properties of iron and iron-alloys, even of supposedly constant composition, have been puzzling to the users and investigators of ferro-magnetic materials ever since the introduction of such materials for electrical apparatus. The author started to investigate this problem over ten years ago at the University of Illinois, and has continued it at the Westinghouse Research Laboratory since 1916, concentrating on iron and iron-silicon alloys. While the results obtained do not eliminate 100 per cent of the difficulties, they go a long way in that direction. It has been found that carbon is largely responsible for the variations, because of the fact that amounts so small as previously to be regarded as traces — less than 0.01 per cent — remain dissolved in the iron in the solid state, even after slow cooling, and have a tremendous influence on the magnetic properties. Of much less effect is carbon precipitated as pearlite, free cementite and graphite, the effect being in the order named. If the effect of dissolved carbon be represented by 100, the effect of carbon as pearlite is 16.5, of carbon as Fe
3
C 2.25, and of carbon as graphite nearly nil. The form assumed by carbon — aside from the carbon in solution — depends largely on the silicon content, and can best be explained by referring to Fig. 9. Besides carbon, it has been found that the grain size has a large and definite influence on the magnetic properties, due to the accompanying inter crystalline amorphous cement that may be regarded as an impurity similar to other inter crystalline impurities. The detrimental effect of sulphur, phosphorous and manganese on pure iron is in the order named, while phosphorous has a beneficial effect on high silicon alloys. The evidence obtained is to the effect that the increased reluctivity, coercive force, or hysteresis loss, due to carbon and other impurities that are precipitated combined with iron — including in this class the inter-crystalline cement — is caused by the inherent corresponding property of these precipitated impurities. Regarding the tremendous effect of carbon in solution, it is suggested that this is due to the entering of carbon into the more or less stable equilibrium arrangement of the ferro-magnetic structure, upsetting this equilibrium arrangement.