In-situ investigation on the morphological evaluation of bainitic microstructure of medium C-steel

Abstract

The bainitic transformation behavior and microstructural evolution in C–Mn–Si steel were investigated using in-situ Laser Scanning Confocal Microscopy (LSCM) and dilatometry. The study focused on three distinct isothermal bainitic transformation (IBT) temperatures (400 °C, 350 °C, and 300 °C), revealing morphological transitions from granular and upper bainite to refined lower bainite with decreasing IBT. During rapid cooling from the austenitization temperature, bainitic ferrite nucleation at the free surface commenced approximately 60 °C higher than the bainite start (Bs) temperature measured by dilatometry. This discrepancy was attributed to reduced strain energy requirements at the free surface, facilitating easier plastic relaxation of the surrounding austenite. The IBT300 sample exhibited the finest microstructure, characterized by a predominant lower bainite morphology and thin retained austenite films, significantly improving strength and impact toughness. Sympathetic nucleation of bainitic ferrite was frequently observed, especially in lower IBT-treated samples, contributing to microstructural refinement. Additionally, free energy calculations indicated a correlation between isothermal holding temperatures and carbon partitioning, influencing the extent of bainitic transformation. This work underscores the critical role of transformation kinetics and morphological evolution in achieving optimized bainitic microstructures, providing a pathway for enhancing the mechanical properties of high-strength steels.