Quantitative assessment of carbon allocation anomalies in low temperature bainite

Low temperature bainite is a mixture of ferrite and austenite with a high dislocation density and nanoscale precipitates produced by isothermal transformation of the austenite in high-carbon high-silicon steels. The mass balance for carbon is systematically unsuitable when considering only ferrite and austenite forming the structure, but no attempt has been made to evaluate the amount of carbon located at linear defects and precipitates. Additionally, bainitic ferrite has been recently shown to have a tetragonal crystal structure, allowing greater amounts of carbon in solid solution than those expected by the paraequilibrium phase boundaries. In order to quantify the contribution of all the carbon sinks, we have followed the evolution of carbon in ferrite and austenite, along with the precipitation of cementite and η–carbide, during the isothermal bainitic transformation at 220 and 250 °C by means of in-situ synchrotron high energy X-ray diffraction and complementary transmission electron microscopy (TEM) and atom probe tomography (APT) analyses. This is the first time that the mass balance for carbon is successfully achieved by considering all the transformation products together with an estimation of the carbon segregated to linear defects.APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. The synchrotron experiments were conducted in December 2014 under the MA2305 grant at the ESRF in Grenoble which is fully acknowledged. The authors thank the support of LabEx DAMAS (ANR-11-LABX-0008-01) from Lorraine. The authors would also like to show their gratitude to Dr. J.C. Hell for his valuable assistance when conducting the experiments at ESRF. This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of a Coordinate Project (MAT2016-80875-C3-1-R); and the Research Fund for Coal and Steel under the Contract RFSR-CT- 2014-00019.Peer Reviewe