Atomic-scale investigation of epsilon and theta precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations

Abstract

Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both epsilon and theta carbides precipitate in lower bainite isothermally held at 260 degrees C and only theta precipitates form in upper bainite isothermally held at 500 degrees C. epsilon and theta precipitate under paraequilibrium condition at 260 degrees C in lower bainite and theta precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 degrees C. In order to theoretically study epsilon and theta precipitation and the epsilon -> theta transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that epsilon and theta carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol(-1) at room temperature and still by 4 kJ mol(-1) at 500 degrees C. Hence, the thermodynamic predictions agree well with the atom probe tomography results. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved