Partitioning-driven heat treatments for the microstructural tailoring of additively manufactured medium manganese steels

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The 3rd generation of Advanced High Strength Steels (AHSS) offers excelling mechanical properties, contributing to the end of the strength-ductility tradeoff. A notable class of AHSS is composed of Medium Manganese Steels (MMnS), which attain their microstructure via intercritical annealing. This heat treatment relies on partitioning phenomena to adjust the chemical composition of austenite, and tailor its mechanical response under load. Conventionally, these steels are cast, submitted to thermomechanical processing and are finally heat treated to obtain their final properties, but processing can be difficult in particular due to macrosegregation of Mn. Currently, new opportunities rise with the possibilities of Additive Manufacturing (AM), which can be used to manufacture MMnS. This project aims to investigate the potential and the specific mechanisms for tailoring the microstructure of MMnS manufactured by PBF-LB/M. In order to achieve different austenite deformation mechanisms, the local chemical composition will be adjusted by subsequent heat treatments which target the enrichment of austenite by partitioning from the martensitic phase. These heat treatments, here denominated as “partitioning driven heat treatments-PDHT”, are possible over a broad range of temperatures activating interstitial and/or substitutional partitioning leading to varying austenite fraction and stability, as well as further microstructural modifications. The main objective of the project lies on establishing a link between microstructural evolution throughout AM, the subsequent PDHTs, and the resulting mechanical properties. Advanced characterization techniques such as atom probe tomography and transmission electron microscopy will be employed to analyze elemental partitioning and accumulation between martensite, austenite and also at their interfaces. Additionally, in-situ X-ray diffraction experiments will be performed to follow precisely the microstructural changes during PDHTs and subsequent mechanical testing. The targeted results will provide an in-depth understanding about the ongoing mechanisms of microstructure evolution during PDHTs and answers two main questions: I- how can the proposed heat treatments be optimized regarding the time/temperature cycle to achieve tailored chemical compositions without compromising the properties of the martensitic matrix and II- how can the austenite deformation mechanisms be preferentially triggered by specific heat treatment conditions to achieve the desired mechanical properties. The microstructure will be tailored to promote one (TRIP or TWIP), or two different deformation and strengthening mechanisms in combination (TRIP + TWIP), achieving tunable mechanical properties.