New York Paper - Chemical Equilibria During Solidification and Cooling of White Cast Iron (with Discussion)

Of the outstanding investigators of the system iron-carbon-silicon Gontermann,1 Charpy and Cornu-Thenard,= and Honda,3 only the first touched on the chemical composition of the solid and liquid phases in equilibrium with one another. He concluded that his data did not permit a decision as to which of an indefinitely large number of pairs of compositions of liquid and solid would be found in any given case. If through the three dimensional equilibrium diagram of the iron-carbon-silicon system an isothermal plane is passed in the region of partial solidification, two lines of intersection with the liquidus and solidus, respectively, are obtained, also a point marking the alloys' composition. If, in this plane, a straight line is drawn through the point, and intersecting both the liquidus and solidus isothermals, the two intersections mark a liquid and a solid alloy that may be chemically in equilibrium at the chosen temperature. From thermal data alone, Gontermann could not assign any particular direction to the line and, hence, could not determine which pair of conjugate points to choose, A separation of the liquid and solid for analysis, obviously, presents grave experimental difficulties. During the solidification of white cast iron, austenite of progressively higher carbon will separate as the temperature falls and cementite will appear suddenly at the freezing point of the eutectic. This cementite will be accompanied by a corresponding amount of austenite which, for our purpose, will be chemically inseparable from the primary austenite. We have attempted to decide on the conditions at the end of the freezing by a chemical separation applied to the solid metal.