PSI - Issue 69

R. Surki Aliabad et al. / Procedia Structural Integrity 69 (2025) 69–75

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Zhao et al. [11] observed the transformation of RA into pearlite during tempering in a 0.1C-5Mn steel, while Ribamar et al. [25] detected a pearlite-like constituent after tempering 0.99C-1.0Mn-1.5Cr-1.82Si steel at 500°C. Similarly, Sadeghpour et al. [7] identified the same decomposition in a 4Mn–0.31C–2Ni–0.5Al–0.2Mo steel. Moreover, in this region, the carbon content of RA decreases, a trend also previously reported by Zhao et al. [11]. This decline could be attributed to the depletion of carbon in the martensitic matrix, which results from several carbon consuming phenomena occurring in the earlier stages. At this point, for further growth of cementite, the only available carbon source at high temperatures could be RA. This applies to cementite located near the RA or farther away, as the BCC matrix could act as a pathway for carbon diffusion [26]. 4. Conclusions This study investigated the microstructural evolution and carbon redistribution in MMnS with a nominal composition of Fe-0.40C-1Si-6Mn-2Al-0.05Nb (wt.%) during the heating stage prior to IAT. The initial microstructure of the hot-rolled and air-cooled steel consisted of tempered martensite containing nano-sized plate-like η-carbides and 7 vol.% RA. Through HEXRD and SEM observations, the overlapping phenomena over various temperature ranges were deconvoluted as follows: 1. Low temperatures (165°C - 375°C): Carbon enrichment in RA from 0.4 to 1 wt.% and coarsening of carbides occurred simultaneously. Despite the high fraction of carbides in the microstructure, they were undetectable by HEXRD, likely due to their nano-sized nature. 2. Mid-range temperatures (375°C - 530°C): Cementite nucleation occurred. 3. Higher temperatures (530°C - 640°C): Decomposition of RA resulted in a pearlite-type microstructure and at the same time spherical carbides nucleated from transitional carbides in the matrix grew. This process reduced the carbon content of the remaining RA. 4. Above 640°C: An increase in the austenite volume fraction was observed, identifying this temperature as the Ac1 point for the studied steel. Acknowledgements The authors would like to thank Jane and Aatos Erkko (J&AE) Foundation and Tiina and Antti Herlin (TAH) Foundation for their financial supports on Advanced Steels for Green Planet (AS4G) project. References [1] Y.K. Lee, J. Han, Current opinion in medium manganese steel, Materials Science and Technology (United Kingdom) 31 (2015) 843– 856. https://doi.org/10.1179/1743284714Y.0000000722. [2] J.H. Nam, S.H. Yu, Y.K. Lee, Effect of auto-tempering on the cold roll-ability of medium-Mn steel, Materials Science and Technology (United Kingdom) 35 (2019) 2069–2075. https://doi.org/10.1080/02670836.2018.1547474. [3] D. Kumar, I. Sen, T.K. Bandyopadhyay, A Systematic Review of Medium-Mn Steels with an Assessment of Fatigue Behavior, Steel Res Int 95 (2024). https://doi.org/10.1002/srin.202300375. [4] D. Raabe, S. Sandlöbes, J. Millán, D. Ponge, H. Assadi, M. Herbig, P.P. Choi, Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite, Acta Mater 61 (2013) 6132–6152. https://doi.org/10.1016/j.actamat.2013.06.055. [5] A. Kwiatkowski da Silva, G. Inden, A. Kumar, D. Ponge, B. Gault, D. Raabe, Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography, Acta Mater 147 (2018) 165–175. https://doi.org/10.1016/j.actamat.2018.01.022. [6] S. Lee, S.H. Kang, J.H. Nam, S.M. Lee, J.B. Seol, Y.K. Lee, Effect of Tempering on the Microstructure and Tensile Properties of a Martensitic Medium-Mn Lightweight Steel, Metall Mater Trans A Phys Metall Mater Sci 50 (2019) 2655–2664. https://doi.org/10.1007/s11661-019-05190-4. [7] S. Sadeghpour, M.C. Somani, J. Kömi, L.P. Karjalainen, A new combinatorial processing route to achieve an ultrafine-grained, multiphase microstructure in a medium Mn steel, Journal of Materials Research and Technology 15 (2021) 3426–3446. https://doi.org/10.1016/j.jmrt.2021.09.152. [8] Q. Ye, H. Dong, Q. Guo, Y. Yu, L. Qiao, Y. Yan, Tailoring the austenite characteristics via dual nanoparticles to synergistically optimize the strength-ductility in cold rolled medium Mn steel, J Mater Sci Technol 169 (2024) 158–171. https://doi.org/10.1016/j.jmst.2023.07.002. [9] M. Enomoto, K. Hayashi, Simulation of Austenite Formation During Continuous Heating from Low Carbon Martensite with Poly dispersed Cementite, Metall Mater Trans A Phys Metall Mater Sci 51 (2020) 618–630. https://doi.org/10.1007/s11661-019-05569-3.