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S. Ghosh et al. / Procedia Structural Integrity 68 (2025) 1329–1336 S. Ghosh et al. / Structural Integrity Procedia 00 (2025) 000–000

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3.5 Fractography Figure 4 shows the fracture surfaces at the junction between the low ΔK and high ΔK regimes in the failed FCGR CT specimen of H-Si DQP steel. In the low ΔK regime, no striations were observed due to the significantly lower stress intensity ranges. However, in the high ΔK regime, distinct striations were visible in the high magnification fractographs, which were recorded on specimens at select elevated ΔK values within this increasing ΔK range. 4. Conclusion In this study, the FCG behaviours of H-Si (containing martensite and RA) and M-Si (containing martensite, bainite and RA) DQP steels with varied volume fractions of martensite/bainite and RA, were investigated. Research results indicate that M-Si specimens with ~8 vol.% bainite content, alongside martensite and RA (13 %), exhibited superior FCG resistance compared to H-Si specimens devoid of bainite, but containing nearly same level of RA (15 %). In M Si steels, the combination of bainite and RA can promote plasticity near the crack tip, aiding in crack closure under lower stress cycles and inhibiting further propagation. This effect is particularly pronounced in M-Si DQP steel with multiphase structures, where both bainite and RA interact with martensite to enhance toughness. Furthermore, the multiphase structure of M-Si DQP steel (martensite, bainite, and RA) creates more beneficial interphase boundaries that cause the crack path to deviate, forcing the crack to take a longer, more tortuous route, thereby reducing the crack growth rate. As the crack propagates, the increase in cyclic plastic zone suggests more RA transformation, while the monotonic plastic zone growth implies that multiphase microstructure consisting of austenite-bainite-martensite would enhances plasticity and retard crack propagation. Acknowledgements Authors gratefully acknowledge financial support from Jane ja Aatos Erkon säätiö (JAES) and Tiina ja Antti Herlinin säätiö (TAHS) on the Advanced Steels for Green Planet project, and the Business Finland funded project HYDROMAT (Dnro 5651/31/2023). References Speer, J. G., Edmonds, D. V., Rizzo, F. C., & Matlock, D. K. (2004). Partitioning of carbon from supersaturated plates of ferrite, with application to steel processing and fundamentals of the bainite transformation. Current Opinion in Solid State and Materials Science, 8(3-4), 219-237. Li, H. Y., Lu, X. W., Li, W. J., & Jin, X. J. (2010). Microstructure and mechanical properties of an ultrahigh-strength 40SiMnNiCr steel during the one-step quenching and partitioning process. Metallurgical and Materials Transactions A, 41(5), 1284-1300. Kumar, G., Ghosh, S., Pallaspuro, S., Somani, M.C., Kömi, J., Mishra, S.K. and Gokhale, A.A. (2024), Stress Intensity Range Dependent Slowing Down of Fatigue Crack Growth under Strain-Induced Martensitic Transformation of Film-Like Retained Austenite. steel research int., 95: 2300578. De Moor, E., Lacroix, S., Clarke, A. J., Penning, J., & Speer, J. G. (2008). Effect of retained austenite stabilized via quench and partitioning on the strain hardening of martensitic steels. Metallurgical and Materials Transactions A, 39(11), 2586. Speer, J. G., Assunção, F. C. R., Matlock, D. K., & Edmonds, D. V. (2005). The" quenching and partitioning" process: background and recent progress. Materials Research, 8(4), 417-423. Clarke, A. J., Speer, J. G., Miller, M. K., Hackenberg, R. E., Edmonds, D. V., Matlock, D. K., ... & De Moor, E. (2008). Carbon partitioning to austenite from martensite or bainite during the quench and partition (Q&P) process: A critical assessment. Acta materialia, 56(1), 16-22 Ghosh S, Kaikkonen P, Javaheri V, Kaijalainen A, Miettunen I, Somani M, Kömi J, Pallaspuro S, Design of tough, ductile direct quenched and partitioned advanced high-strength steel with tailored silicon content, J Mater. Res. Technol., 17, 2022, 1390-1407. Ghosh S, Rakha K, Devi A. A.S., Reza S, Pallaspuro S, Somani M, Huttula M, Kömi J, A combined 3D-atomic/nanoscale comprehension and ab initio computation of iron carbide structures tailored in Q&P steels via Si alloying, Nanoscale, 2023, 15, 10004-10016. Miettunen I, Ghosh S, Somani M, Pallaspuro S, Kömi J, Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels, Journal of Materials Research and Technology, 11, 2021, 1045-1060.