Fine structured austenitic-martensitic composites in the Fe-Mn-C system are promising candidates to achieve outstanding strength and ductility combinations. A novel approach to achieve such a microstructure involves a multistep processing scheme consisting of pearlite formation, a short time austenitization and subsequent quenching. The basic premise of this processing route is the formation of a chemical Mn pattern from a Fe-Mn-C pearlite to finally achieve an ultra-fine microstructure consisting of martensite and metastable austenite, due to the austenite stabilizing effect of the Mn.
The feasibility of this processing route is strictly determined by the initial microstructure, e.g. entirely pearlitic with strong partitioning of Mn between both phases. Eutectoid decomposition accompanied by Mn partitioning is believed to be compositionally limited, due to a transition in partitioning regimes, where Mn partitioning does not occur or only to a negligible degree. In this work the alloy design in the Fe-Mn-C system is re-visited, disregarding this "partitioning vs no- or negligible partitioning" concept. Following our alloy design, the feasibility of the processing route was validated on two alloys with the nominal compositions Fe-3Mn-0.67C (Alloy 1) and Fe-7Mn-0.7C (Alloy 2) (both in wt.-%). It was shown, that despite the relatively low overall Mn content in Alloy 1, the Mn content within cementite after pearlite formation at 600 °C was sufficient to achieve the novel ultra-fine austenitic-martensitic microstructure. In case of Alloy 2, pearlite formation was carried out at 540 °C, where, based on previous literature, no- or negligible partitioning of Mn would be expected. However, our APT (atom probe tomography) analysis revealed that Mn partitioning did take place, therefore significantly increasing the field of parameters applicable for this processing route. Additionally to the general feasibility, the influence of the simultaneous occurrence of different carbide morphologies, which were observed in both alloys investigated, will be discussed in the presentation.