The Phosphide Eutectic in Cast Iron

Abstract

THE accompanying photomicrograph (Fig. 1) shows the phosphide eutectic in a grey cast iron at 5000 diameters. This shows that the constituent which appears as dark markings at × 1000, and is still commonly described as iron carbide, is actually duplex, and in fact very similar to pearlite, consisting of alternate laminæ of iron carbide and iron. It is recognised from the research of Stead that there would be present the three constituents of the triple eutectic, namely, iron phosphide, iron carbide, and austenite, or their transition products. Thus the presence of laminated pearlite, from the transformation of austenite, is accounted for. The pearlite thus associated structurally within the phosphide masses may conceivably be richer (it probably is, but may even be poorer) in carbon content than obtains in the normal pearlite ratio, Fe₃C₂₁Fe. Prof. Desch, to whom I have shown several similar photographs, confirms the foregoing explanation. The photograph was taken at 5000 diameters by means of the super microscope.