Effect of Si on bainitic transformation kinetics in steels explained by carbon partitioning, carbide formation, dislocation densities, and thermodynamic conditions

Abstract

The effect of Si addition on the evolution of bainitic transformation, carbon diffusion, carbide formation, and dislocation density in steel was investigated using in-situ high-energy X-ray diffraction (HEXRD). Alloys Fe-0.4C-1.7Mn (in wt%) with 1–4 wt% Si were austenitized at 1273 K and then isothermally heat treated at 573, 623, and 673 K. According to the HEXRD results, increasing Si content reduces the bainitic transformation kinetics and causes the incompleteness of the bainitic transformation to occur at lower bainite volume fraction. This is because i) Si retards carbide formation, impeding the eutectoid bainitic transformation, and leads to the accumulation of carbon at the migrating interface; ii) Si leads to higher strain energy and more dislocations in the austenite that also hinders the migration of the interface. Carbide formation was observed to occur prior to the incomplete transformation stage. During further isothermal holding, the decrease in dislocation density due to dislocation annihilation had little effect on carbide formation or carbon diffusion. Finally, the Si content has a minor effect on the calculated T 0 , T 0 ’, and WBs lines. The measured carbon content in carbon enriched austenite agrees well with WBs and T 0 but not with T 0 ’. • Bainitic transformation was systematically studied by in-situ HEXRD and microscopy. • Si affects transformation by solute hardening and retardation of carbide formation. • Carbide formation is observed prior to incomplete transformation stage. • Dislocation annihilation has insignificant effect on carbide formation. • The final carbon content in retained austenite agrees with T 0 and WBs predictions.