Design of novel high strength bainitic steels: Part 1

TLDR

Fine upper‑bainitic ferrite plates separated by thin retained austenite films, with minimal martensite and carbides avoided by silicon, are widely used; optimizing such microstructures requires avoiding large unstable high‑carbon retained austenite. The study aims to extend these concepts to achieve unprecedented strength and toughness in bulk samples under continuous cooling transformation while meeting hardenability and processing constraints. The authors employ phase‑transformation theory to design bainitic alloys within industrial constraints, with a subsequent experimental verification in Part 2. With careful design, high‑silicon bainitic steels have achieved impressive combinations of strength and toughness.

Abstract

AbstractMixed microstructures consisting of fine plates of upper bainitic ferrite separated by thin films of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental verification of the design process.