1987 Volume 5 Issue 3 Pages 403-409
The microsegregation of phosphorus in steel caused by a rapid heating was investigated using Fe-P-C alloys. The specimens were heated by an electric furnace or oxy-acetylene flame with heating rates ranging from 0.056 to 200 K/s. The phosphorus concentration at the prior-austenite grain boundary was examined quantitatively by the EDX analyses or the grain boundary etching method. The process of phosphorus segregation was clarified as follows. (1) Phosphorus is concentrated at the α/γ interface during the α/γ transformation. Just before completing the transformation, the phosphorus concentration at the α/γ interface reaches a maximum value, and this final interface becomes the austenite grain boundary. (2) The maximum phosphorus concentration at the grain boundary brought by the transformation is reduced by heating the austenite in a higher temperature range. (3) The grain boundary of the rapid heated alloy begins to ilquate at a lower temperature than its equilibrium liquidus temperature. The higher the phosphorus concentration at the grain boundary is, the lower the temperature of beginning the liquation becomes. The phosphorus concentration at the grain boundary is largely increased by this liquation process. (4) There is a critical value of heating rate above which the temperature of beginning the liquation falls adruptly for each alloy; for example 40 K/s for Fe-0.6P-0.3C alloy.
