PSI - Issue 69

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ScienceDirect

Procedia Structural Integrity 69 (2025) 69–75

13th European Symposium on Martensitic Transformation 2024 (ESOMAT 2024) Carbon Redistribution in a Martensitic Medium Mn Steel During Heating to Intercritical Region: An In-Situ Synchrotron XRD Study R. Surki Aliabad a, * , S. Sadeghpour a , P. Karjalainen a , J.Kömi a , H. Singh b , V. Javaheri a a Materials and Mechanical Engineering, Centre for Advanced Steels Research, University of Oulu, Oulu 90570, Finland b Nano and Molecular Systems Research Unit, University of Oulu, Oulu 90570, Finland Abstract This study investigates the microstructural evolution and carbon redistribution during the heating stage prior to intercritical annealing treatment (IAT) in a medium manganese steel (MMnS) with the nominal composition of Fe-0.40C-6Mn-2Al-1Si-0.05Nb (wt.%). The material was characterized using high-energy X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The initial microstructure primarily consisted of tempered martensite containing nano-sized plate-like η carbides and 7 vol.% retained austenite (RA) with thicknesses of 10–20 nm and 300 nm in average, respectively. During heating, carbon partitioning caused an increase in carbon content within the RA up to 530 °C, rising from 0.4 wt.% to 1 wt.%. η-carbides initially coarsened and subsequently transformed into cementite with an average diameter of ~ 20 nm. Above 530 °C, RA began to decompose, resulting in the formation of a pearlite-type microstructure. Concurrently, the carbon content in the remaining RA decreased, facilitating further growth of cementite formed in the earlier stages. The microstructure at the onset of IAT at 640 °C consisted of tempered martensite with nano-sized spherical cementite, 9 vol.% RA with >1 wt.% carbon and a small fraction of pearlite-type decomposed RA. The study highlights the complex interplay between carbon redistribution, carbide formation, and RA stability during the heating stage of MMnS and emphasizes the importance of accurately characterizing the initial microstructure to tailor the properties of these advanced high-strength steels. © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Guest Editors

Keywords: Medium Mn Steel; Synchrotron XRD, Carbon Partitioning, Auto-Tempered Martensite

* Corresponding author. Tel.: +358-505649693. E-mail address: Roohallah.surkialiabad@oulu.fi

2452-3216 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of Guest Editors 10.1016/j.prostr.2025.07.010