The growth of partitioned pearlite in Fe-C-Mn steels

The isothermal growth of partitioned pearlite in a series of high purity Fe-C-Mn alloys over the temperature range 575-650 degreesC has been investigated. Two types of behavior were observed. For those alloys transformed within the (alpha + M3C) two phase field of the respective Fe-C-Mn isothermal section, growth occurred under steady-state conditions, with constant rate and interlamellar spacing. Those compositions transformed within the (gamma + alpha + M3C) three phase field invariably transformed under non-steady-state conditions with a growth rate which decreased and an interlamellar spacing that increased (divergent) in time. Detailed analytical transmission electron microscropy (ATEM) measurements of the Mn concentration profiles across the gamma/alpha and gamma/M3C growth interfaces were made as a function of time for two alloys transformed at 625 degreesC. For the alloy transformed within the (alpha + M3C) two phase field, the Mn contents inherited by the growing a and M3C were remarkably constant for much of the reaction and are well approximated by the local equilibrium values. In the case of the alloy transformed within the (gamma + alpha + M3O) three phase field, the Mn contents inherited by the growing alpha and M3C both increased with time. The Mn contents of the growing M3C were at all times observed to fall within the bounds expected by the local equilibrium (LE) model, whereas a Mn enrichment over and above the LE values was observed for the growing ferrite. The effect of this enrichment on the expected pearlite growth rate is shown to be very small and it is concluded that the LE approximation for the interfacial conditions during pearlite growth under the conditions studied is a good approximation. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.