In the last two decades, extensive research has been conducted on medium carbon steels with carbide-free bainitic microstructure, primarily due to their impressive combination of tensile properties. These steels demonstrate an ultimate strength higher than 1500 MPa and reaching elongation up to 20%. High silicon contents are included to prevent cementite precipitation from the untransformed austenite during the isothermal treatments for the bainitic microstructure formation. The exceptional mechanical performance exhibited by these steel grades are attributed to a multiphase composite microstructure consisting of ferritic bainite and carbon-enriched retained austenite. Bainitic ferrite forms during isothermal treatments via a displacive mechanism and thanks to the low isothermal temperatures (Tiso) guaranteed by the medium-high carbon content, it is characterized by submicro-nano scale thickness. This results in both, high yield and tensile strengths. On the other hand, the carbon-enriched austenite, which plays a crucial role in influencing the ductility through the TRIP effect, assumes a dual morphology: i) forming films sandwiched between ferrite subunits and ii) creating blocks positioned between the sheaves of ferritic bainite. In this work the effect of Tiso (300, 325 and 350 °C) on the microstructure and the bainitic transformation has been investigated in two medium carbon- Si rich Al bearing steels (0.38C-3.2Si-2.56Mn-0.1Al and 0.45C-2.8Si2.7Mn-0.56Al wt.%). The bainitic transformation and its kinetic was investigated by dilatometry, while a complete microstructural characterization was performed using optical microscopy, SEM, XRD and HV10 measurements. It emerged that the kinetic of the bainitic ferrite formation, the volume fraction and the morphology of the microstructural constituents is affected by both the chemical composition and the Tiso.

4th Mediterranean Conference on Heat Treatment and Surface Engineering (MCHTSE 2024) - Lecce, Italy