Acta Materialia. Volume 61, Issue 20, December 2013, Pages 7582–7590.
Song, W.1, Appen, J. von 2,Choi, P. 3, Dronskowski, R. 2, Raabe, D. 3,Bleck, W.1.
1 (Department of Ferrous Metallurgy, RWTH Aachen University, Aachen (Germany));and
2(Institute of Inorganic Chemistry, RWTH Aachen University, Aachen (Germany)); and
3(Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung, Düsseldorf (Germany));
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 °C and only theta precipitates form in upper bainite isothermally held at 500 °C. Epsilon and theta precipitate under paraequilibrium condition at 260 °C in lower bainite and theta precipitates under negligible partitioning local equilibrium condition in upper bainite at 500°C. In order to theoretically study epsilon and theta precipitation and the epsilon to 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 mol1 at room temperature and still by 4 kJ mol 1 at 500 °C. Hence, the thermodynamic predictions agree well with the atom probe tomography results.