Microstructure – Properties Relationships in Carbide-free Bainitic Steels

TLDR

The study correlated microstructural features to tensile properties. Two carbide‑free bainitic steels with distinct microstructures were produced via alloy design and austempering, and characterized by EBSD and synchrotron diffraction to assess phase fractions, carbon content, and microtextures. Both steels achieved >1250 MPa strength, up to 14 % uniform elongation, and 46 % area reduction, with MA block volume fraction being a key factor controlling work‑hardening and high‑strain damage.

Abstract

We elaborated two carbide-free bainitic steels with different microstructures through specific alloy design and austempering process. Microstructural characterizations were performed by means of EBSD analysis and in-situ high energy synchrotron diffraction in order to evaluate the phase fractions and the carbon content in the retained austenite, as well as the microtextures. These microstructural features were correlated to the tensile properties. Both steels exhibited an excellent compromise between high strength (above 1250 MPa), good ductility (uniform elongation up to 14%) and high fracture strain (reduction of area up to 46%). The volume fraction of MA blocks (blocks of retained austenite partially transformed into fresh martensite during the final cooling at room temperature) was a key relevant parameter that strongly influenced the work-hardening at the expense of the damaging processes at high strain.