The Magnetic Properties of the Ternary Alloys Fe-Si-C

The variations in the magnetic properties of iron and iron-alloys, even of supposedly constant composition, has 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. 29.