Abstract
Partitioning of chromium between ferrite and cementite during the isothermal decomposition of austenite to pearlitic or pearlitic/bainitic decomposition products has been studied in a 1.4 wt pct Cr eutectoid steel using analytical electron microscopy on two-stage extraction replicas. Chromium was observed to segregate preferentially to cementite at the pearlite reaction front for temperatures in the range 730 to 550 °C. Although the extent of partitioning decreased with decreasing reaction temperature, a no-partition temperature could not be identified for the steel. It is clear that previous studies on thin foils have underestimated the temperature range over which partitioning of chromium can occur. At high reaction temperatures measured values of pearlite growth rates were found to be in excellent agreement with those calculated, using the assumption that phase boundary diffusion of chromium was rate controlling. At lower reaction temperatures models based on volume diffusion of carbon and on phase boundary diffusion of chromium both gave reasonable predictions of measured growth rates. However, it seems likely that solute drag effects influence pearlite growth at temperatures in the austenite bay region which the chromium addition produces in the T.T.T. diagram. Measurements made on upper bainite which co-existed with pearlite following transformation at 500 and 550 °C showed that preferential partitioning of chromium to cementite did not occur during this reaction.
Published Version
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