Many applications such as bearings, ballistic shields, and railways require low-cost, high-strength, and wear-resistant materials. One of the ways to achieve these goals is to use ultra-fine-grained (UFG) materials, as the reduced grain size aids in hindering the dislocation movement, thereby increasing the strength. Recently, carbide free nano-Bainitic steels are being widely used, owing to their ease of production. UFG can be generated in these steels with the help of heat treatments, without the aid of expensive high-energy processes. There is an ongoing argument over the mechanism underlying the formation of bainite. Diffusion-less transformation and transformation incorporating diffusion are the two perspectives used to examine the genesis of bainitic ferrite. The viability of both pathways has been demonstrated using thermodynamic and experimental methods. According to the diffusion-less transformation theory, bainite grows by displacive transformation similar to martensite while carbon is partitioned after the growth. The diffusional theory supports a transformation that is governed by paraquilibrium model (PE) in the beginning when the interface velocity is fast enough to transition towards local equilibrium non partitioning (LENP). The substitutional alloying elements partition across the interface based on solute drag effect, thereby consuming the driving force that leads to transformation stasis.
In the present study, two special designed compositions, with added Si (>1.5wt%) and different amount of Mn (which contributes to transformation stasis) are used to examine the transformation mechanism of bainite formation. Dilatometric studies are carried out to find the transformation stasis (transformation saturation) of steels with two different compositions. The Gibbs Energy Balance (GEB) approach is used to model the transformation stasis in which the driving force is balanced by dissipation of Gibbs energy by solute drag effect. The compositions also differ by the addition of Al and Co which is known to accelerate the transformation. In this study the role of Al and Co on the kinetics of bainite formation is also studied. The bainite formation is also looked upon using in-situ TEM to see the nucleation behavior of bainitic ferrite. The effect of cold rolling and cryo-rolling on the retained austenite distribution and variant selection of bainitic steel with its effect on mechanical properties is also observed. Finally, SEM-EBSD was carried out to correlate impact of Taylor factor distribution on texture evolution and tensile deformation behavior of bainitic steel held at different isothermal holding temperatures is studied.
Keywords: Bainitic steel, paraequillibrium model (PE), local equilibrium non partitioning (LENP), cryo-rolling, Taylor factor.