Partitioning of Co during pearlite growth in a eutectoid steel

Abstract

Partitioning of cobalt between ferrite and cementite during the isothermal decomposition of austenite to pearlite has been studied for a 2.1 wt-%Co eutectoid steel using analytical electron microscopy of two stage extraction replicas. Cobalt partitioned preferentially to ferrite at the transformation front for temperatures down to 580°C. Although the extent of partitioning decreased as the reaction temperature was decreased, a no-partition temperature could not be identified experimentally for the alloy. However, calculations predicted that the no-partitioning temperature would be close to the eutectoid temperature. The inflexion in the plot of interlamellar spacing v. transformation temperature previously reported for the alloy is not inconsistent with the calculated no-partition temperature. For most of the pearlite transformation region the observed partitioning of Co could have accompanied the rate controlling step for pearlite growth which, for other than low undercoolings, would be carbon diffusion. Differential thermal analysis studies showed that the addition of 2.1%Co raised the eutectoid temperature to 742 ± 2°C, but the presence of a three phase (α + γ + Fe₃C) field for the alloy could not be detected.