1 CARBON SUPERSATURATION OF FERRITE IN A NANOCRYSTALLINE BAINITIC STEEL F.G. Caballero 1 , M.K. Miller 2 and C. Garcia-Mateo 1 1 Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalúrgicas (CENIM-CSIC), Avda. Gregorio del Amo, 8. E-28040 Madrid, Spain 2 Oak Ridge National Laboratory (ORNL), Materials Science and Technology Division; P.O. Box 2008, Oak Ridge, TN 37831-6136, USA Keywords: Steels, bainite, three-dimensional atom probe (3DAP) Abstract The extremely slow transformation kinetics of a nanocrystalline bainitic steel, allow the carbon content of the bainitic ferrite away from any carbon-enriched regions such as dislocations and boundaries to be determined by atom probe tomography (APT) as the bainite transformation progresses at 200 ºC. A high level of carbon, well above that expected from paraequilibrium with austenite, has been detected in solid solution in bainitic ferrite at the early stage of transformation. Results provide strong evidence that bainite transformation is essentially displacive in nature so that the newly formed bainitic ferrite retains much of the carbon content of the parent austenite.