Compositional design of high strength nanostructured bainite

Strength of carbide-free bainitic steels primarily depends on the thickness of bainitic ferrite and its volume fraction in the microstructure. The morphology and volume fraction of phases formed in bainitic transformation are governed by the steel’s composition and transformation temperature. Higher carbon concentration in bainitic steels is expected to refine the bainitic microstructure even when the austempering temperature is kept to be the around 300 °C due to the enhanced strength of austenite. However, there is a concomitant loss in the volume fraction of bainite formed. Therefore, predicting the optimal composition for getting the highest strength and hardness in nanostructured carbide free bainitic steels is not possible without systematic experimental studies. We have made steels with different carbon concentrations and have demonstrated that after austempering at 300 °C, maximum hardness as well as tensile strength are attained in a wide range of carbon concentration ranging from 0.5–0.9 wt% C but drops significantly outside this range. The experimental results have been compared with the empirical strength in terms of lath thickness and volume fraction formulated from data extracted from several past studies with different carbon concentrations.