Assessment of the compositional requirements to form Fe-Mn-C austenite-martensite composites

Recently, a new generation of high strength steels was introduced by utilizing a lateral chemical pattern of an austenite stabilizer to create microstructures of austenite γ and martensite α’ after quenching. A ternary Fe-Mn-C pearlite is a suitable initial state for this if Mn effectively partitions into the cementite. In the present study, two model Fe-Mn-C alloys were pearlite treated outside the well-established local equilibrium, Mn partitioning regime (P-LE). A complete pearlite formation was achieved not only for Fe-3.0Mn-3.0C (at.%, Alloy A) at high pearlite formation temperature but also for Fe-6.9Mn-3.2C (at.%, Alloy B) at low transformation temperature. The morphology of the pearlite included fine-scaled fibers and lamellae. Even though outside the P-LE region, significant Mn partitioning into cementite was obtained for both alloys. Pearlite was formed at approximately the overall Mn content, while growing either enriched or depleted in C for most of the reaction. The successful application of a short time austenitization treatment was proven for both alloys transforming the pearlite into α’ + γ microstructures while retaining the initial pearlite morphology. Thus, fine-structured α’ + γ can be synthesized from pearlite processed well outside the established Mn partitioning regimes, opening a much larger compositional and processing space.

This work has been supported via personal grants by the Landesgraduiertenförderung (LGF) by the local state of Baden-Württemberg (Germany) and the GRAFÖG funding by the German Academic Exchange Service (DAAD). The authors gratefully acknowledge Karlsruhe Nano Micro Facility (KNMFi) for providing advanced instruments (proposal number: ha032044). The synchrotron XRD experiments (proposal no. 2024B1779) at SPring-8 (Super Photon ring-8 GeV) were conducted with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). Furthermore, the authors thank Professors Christopher Hutchinson and Sebastian Weber for valuable discussions on the contents of this article.