PSI - Issue 68

Renata Latypova et al. / Procedia Structural Integrity 68 (2025) 1115–1120 Author name / Structural Integrity Procedia 00 (2025) 000–000

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instance, in MS1500 (0.19% C) steel, HE susceptibility decreases after tempering at 180°C, while in MS1700 (0.24% C) steel, it controversially increases under similar conditions. However, MS1500, which exhibited improved HE resistance, also demonstrated faster H diffusion, like T250. In MS1700, slower H diffusion can be caused by additional H-trapping by transition carbides, which form with high enough C content (Venezuela et al., 2020b).

Figure 4. All measured threshold stress levels curves.

4. Conclusions This study examined the effect of low-temperature tempering (LTT) on hydrogen (H) concentration, diffusion, and hydrogen embrittlement (HE) susceptibility in direct-quenched (DQ) auto-tempered martensitic steel. The tempering treatment was conducted for DQ at 50°C (T50), 150°C (T150), and 250°C (T250), all with 1-hour holding time, leading to carbon (C) segregation at all temperatures with additional decomposition of retained austenite (< 1%) for T250. H concentration measured with melt-extraction was highest in DQ, and it decreased progressively with increasing tempering temperatures. This reduction is attributed to C diffusion and the formation of Cottrell atmospheres that reduce H-trapping at dislocations. Electrochemical permeation (EP) tests confirmed these findings, showing that H diffusion is slowest in DQ and fastest for T250. HE susceptibility was evaluated with tuning-fork tests using an incremental step loading technique, which demonstrated that T250 has the highest threshold stress, indicating superior resistance to HE. The higher HE resistance in T250 is likely due to the pronounced C diffusion and pinned dislocations, and increased cementite fraction, which change the HE cracking mechanism. However, the exact cracking mechanisms involved in the improved performance of T250 still require further investigation. Acknowledgments We express our gratitude to Business Finland for funding this research. The projects HYDROMAT - Towards the design of hydrogen-resistant material solutions: hydrogen diffusion, trapping, and inhibition in steels, and MASCOT – Materials for CO 2 -Neutral Processes in Resource-Intensive Industries are acknowledged. The Research Council of Finland grant JustH2Transit (#358422) is thanked as well. References ASTM F1624-12., 2018. Bhadeshia, H., Honeycombe, R., 2006. Steels Microstructure and Properties, 3rd ed, Steels: Microstructure and Properties. Elsevier, Oxford, United Kingdom. https://doi.org/10.1016/B978-0-7506-8084-4.X5000-6 Chan, S.L.I., Lee, H.L., Yang, J.R., 1991. Effect of retained austenite on the hydrogen content and effective diffusivity of martensitic structure. Metallurgical Transactions. A, Physical Metallurgy and Materials Science 22 A, 2579–2586. https://doi.org/10.1007/BF02851351 Kömi, J., Karjalainen, P., Porter, D., 2016. Direct-Quenched Structural Steels. Encyclopedia of Iron, Steel, and Their Alloys. CRC Press, 1109– 1125. https://doi.org/10.1081/E-EISA-120049737 Krauss, G., 2014. Quench and Tempered Martensitic Steels: Microstructures and Perfromance. Comprehensive Materials Processing. Elsevier,