Solute Trapped at Defects during the Displacive Formation of Bainitic Ferrite

New developments with bainite have resulted in a steel with an ultimate tensile strength of 2500 MPa, a hardness of 600-670 HV and a toughness in excess of 30-40 MPa m [1]. The novel microstructure is generated in a high carbon (0.98 wt.%) high silicon (1.46 wt.%) steel austenitized for 15 min. at 1000 oC, and then isothermally transformed at temperatures of ∼200 oC. Iron does not diffuse during the transformation to bainite at this low temperature. As a result, 20-40 nm thick plates of ferrite are generated, giving rise to the exceptional properties. Extensive transmission electron microscopy of this microstructure has failed to reveal carbides within the bainitic ferrite. This is an interesting observation, since at these temperatures, steels with such high carbon levels should transform to a lower-bainitic microstructure. Similarly, analysis of X-ray diffraction results indicated that the carbon concentration in the bainitic ferrite was much higher than that expected from paraequilibrium thermodynamics between austenite and ferrite. This supersaturation is explained by the trapping of carbon at the dislocations in the bainitic ferrite. The relatively high dislocation density in bainite is attributed to the fact that shape deformation accompanying the displacive transformations is accommodated partially by plastic relaxation. The goal of this research is to evaluate the hypothesis of carbon trapping at dislocations within bainitic ferrite by atom probe tomography.